kompedium frosio

FROSIO-COURSE TEXT BOOKLET

06/13

The National Institute of Technology Postal address: P.O.Box 141 Økern NO-0509 Oslo Visiting address: Kabelgaten 2 NO-0580 Oslo

INSPECTION OF CORROSION PROTECTIVE COATINGS

©Teknologisk Institutt as www.teknologisk.no Inspectors duties and behaviour 09/11 Materials Technology 1

Inspectors duties and behaviour



THE DUTIES AND BEHAVIOUR OF THE INSPECTOR The coating inspector has a huge responsibility and is often a part of the quality assurance system. The main objective is to assure that the corrosion protection provides the best possible result and is according to the specification. By use of knowledge, instruments and experience a qualified inspector will fulfil this very important task. Sometimes inspectors may be used as experts for determining surface treated areas e.g. performing surveys, and in some cases used for evaluating specifications.

Knowledge To be able to perform the job as a paint inspector a lot of basic knowledge is required. Among these:

• Corrosion • Materials • Pre-treatment requirements • Paints and coatings • Application requirements • Health and safety issues • Specifications • Standards for surface preparation and inspections • Other corrosion preventing methods; hot dip galvanising, thermal spray, passive fire

protection Often, as is the case with this course, the inspector must be certified. This involves a general course on corrosion protection and an examination. There are different levels of paint inspectors often based on relevant experience.

Duties The inspector must be updated in standards, specifications and new developments in the field of corrosion protection. A paint inspector has many duties and must be able to perform the following tasks.

Specification • Be able to read and understand the specification and make sure that the correct specification

is used. • Be able to evaluate a specification with regards to suitability

Procedure • Is the procedure suitable, and according to the paint specification?



Standards • Must know and understand the relevant standards required by the specification • Use of normative references in standards (these are the additional standards required)

Equipment • Be able to have some basic knowledge of paint pumps, blast cleaning equipment, water

jetting equipment a swell as other equipment and to be able to determine if it is suitable • Confirm that the used equipment is according to the specification (e.g. brush for primer

coat) • Log and monitor the equipment used (e.g. type on nozzle and pressure during airless spray)

Pre-treatment • preparation of steel surfaces, e.g. rounding of sharp edges, grinding of surface defects,

removal of weld spatter etc. is in accordance with the specification • Using the standards to determine cleanliness, surface profile, any contaminations • Checking the climatic conditions • Logging results of inspection • Make sure that it is according to specification at the time the application starts.

Application • Are the correct products used • Checked shelf life? • Correct colour • Is suitable equipment used? Roller or brush? Airless spray? • Are the climatic conditions acceptable? • Are base materials, paints/coating, thinners being used are in accordance with the

specifications, and are being used correctly • Are recoat interval followed

Final inspection • Check final result for paint defects, film thickness, adhesion and so on according to

requirements in the specification • Calibration of instruments

Reporting • Make report from findings or from surveys • Make a non-conformance report • Make daily logs • Make progress reports • Make inspection plan

Surveys • Perform surveys to judge the overall corrosion protection • Performing measurements



Maintenance • Is the maintenance specification suitable • Is the pre-treatment suitable • Is the paint suitable for pre-treatment (e.g. for water jetted surfaces) • Are the edges feathered?

Health and safety • Know the local rules and regulations • Know if the personal protective equipment is suitable (if not the operator should be warned,

but the inspector usually does not have authority to demand)

Behaviour How an inspector behaves is of utmost importance. It is an advantage to work together with the operators and earn their respect. This respect can be earned by proving that you have the skills and knowledge required. There will always be some difficulties during corrosion protection and we have to make the best of it. This may involve some compromises, but an inspector should never deviate from the specification. Earning trust from the operators will usually increase the quality, the job is after all a joint effort by many parts who all want the best possible result. The inspector should be frank and straightforward and try to establish good communication with the people he/she will be working with. Presence when the work is being carried out will enable the inspector to draw the correct conclusions, thus avoiding many delays. The way an inspector behaves on site is another way to gain respect. Attend meetings and be constructive, be organized and be polite. The inspector is often a “lonesome wolf”, and very often on his/her own, and has few people within the company present to discuss things with. As an inspector it is important at all times to take notes of ongoing events. Not necessarily when present on the worksite - this can affect the working relationship with the operators. Everything should be recorded in logs e.g. daily or weekly reports, progress reports. It is also important that the inspector is capable of understanding what is actually possible to achieve and what is not. During painting operations you will always measure variations in DFT over a construction - the variations may be caused by several things, not only the operators. All instruments we use have uncertainties - this should be kept in mind during inspection.

Common sense is a key word in inspection.


©Teknologisk Institutt as www.teknologisk.no Material selection 09/11 Materials Technology 1

MATERIAL SELECTION



MATERIAL SELECTION Engineers in the construction of bridges, cars, pipelines, ships, power plants, process plant equipment etc use a large variety of materials. To some extent the inspector should also have some knowledge of the different materials.

Carbon steel and low alloyed steel Pure iron is seldom used commercially. But iron alloyed with carbon and small amounts of other elements such as manganese and silica is our most common construction material known as carbon steel.( Iron has a density on 7,87 g/cm3.) In carbon steel and low alloyed steel the mechanical properties are determined by the carbon content.

• The carbon content in carbon steel is up to 0.2 % • The carbon content in cast iron can be as high as 2 – 4 %.

By alloying iron with small amounts of other elements such as chromium, nickel, phosphor, molybdenum, and vanadium we will achieve a wide variety of steels, called low-alloyed steels. The total percentage of other elements than iron in low-alloyed steel is seldom higher than a few percent (2-3 %). The corrosion resistance of carbon steels and low-alloyed steels are more or less the same. The mechanical properties of carbon steels are good, but without protection they corrode under atmospheric conditions when the relative humidity rises above approximately 60 – 70 %. The corrosion rate of steel depends upon time of exposure in the atmosphere, humidity, pH-value and air pollution. During production of the steel in the hot rolling mill, the steel reaches high temperatures and mill scale is created on the steel surface. The mill scale can be seen as a blue-black coloured metallic coating on the steel surface. The mechanical properties are poor. It is very brittle and easily starts to flake of. Another negative property with the mill scale is that it is nobler than the steel itself. If there become cracks in the mill scale there can be problems with galvanic corrosion. Therefore it is often removed, either with blast cleaning or during the cold rolling process. As long as it is intact on the steel surface it protects the steel. After the coarse rolling process, the steel can either be warm or cold rolled. Usually steel rolled at temperatures above 600 oC is hot rolled (mill scale is formed) and those below 600 oC are cold rolled. The cold rolling process provides the steel with greater hardness and strength as well as a smooth surface, the mill scale is also removed. The thickness of the cold rolled steel are normally below 4 mm thick, while the hot rolled steal plates are thicker then 4 mm.



Weathering steels The weathering steels are special low-alloyed steels that during atmospheric exposure provide better corrosion resistance than ordinary low-alloyed steels. The improvement is due to small amounts, 2-3 %, of alloying elements of chromium (Cr), phosphor (P) and copper (Cu). The weathering steels were developed in the USA in the 1930s. When exposed in the atmosphere weathering steel will corrode at approximately the same rate as low-alloyed steel for the first 1 ½ to 4 years. During the exposure a dark brown / violet patina (rust) is created, the rust being more dense, which slows down the corrosion rate considerably. After about 4-5 years the corrosion rate reaches a stable state. And as the rust layer that has formed during the first 4-5 years protect the steel; this corrosion rate is lover than for unalloyed steel. The rust layer is left for decorative purposes and is not blast-cleaned or painted. Today there are a fairly large number of weathering steels with different names; COR-TEN, Patinax 37, Corox, Atmofix and Mayari R.

Stainless steels Unlike the low alloyed steels, the amount of alloying elements in the stainless steels is high and often in the range of 15 – 30 %. There are quite a variety of stainless steels. Sometimes you will hear terms like;

• ferritic stainless steel • austenitic stainless steels • martensitic stainless steels. • duplex stainless steels

These terms ferritic. austenitic and martensitic refer to the crystalline structure of the steels. Duplex stainless steels have a combination of austenitic + ferritic crystalline structure with high strength, toughness and corrosion resistance. The main alloying element in stainless steels is chromium. When iron is alloyed with chromium a very thin film of chromium oxide is created on the metals surface. In order to achieve this thin protecting film the steel must have a chromium content of minimum 13 % and it is this passive layer that makes the steel stainless. Therefore stainless steel is also referred to as a passive material. The corrosion protection of stainless steels is in addition to chromium very dependent on the content of nickel and molybdenum. Stainless steels containing molybdenum are in some countries referred to as “acid proof stainless steels”. Molybdenum makes it more sea water resistant.



Typical stainless steels that you might have experienced with are AISI 304 or AISI 316. (AISI is the abbreviation for The American Iron and Steel Institute).

• AISI 304 contains approximately; 18 % Chromium, 8 % Nickel • AISI 316 contains approximately 18 % Chromium, 8 % Nickel and 3 % Molybdenum

An increase in the molybdenum to 6 % betters the corrosion resistance dramatically. Stainless steels will corrode in environments that are very corrosive, for example environments that contains chlorides, e.g. seawater. High temperatures can also make different high alloyed steel suffer from corrosion.

Copper and copper base-alloys Copper is a quite noble, but soft metal. There are a large number of copper base-alloys available for different areas of use. The best known ones are brass (copper + zinc), bronze (copper + tin), copper-nickel (copper+nickel). The corrosion rate of copper is low and seldom more than 0.5 – 2.5 µm / year during atmospheric exposures. This is because the green patina that forms on copper, the verdigris, is an oxide that to some extent protects the copper material. Therefore copper has been used for many purposes as roofing, statues, piping etc. For freshwater piping normal copper can be used, but for seawater piping systems, the piping is usually made from more corrosion resistant alloys of aluminium brass (copper + zinc+ aluminium) or copper-nickel. The main reason for this is that sea water is more corrosive than pure water. In addition, the internal velocity of the seawater in pipes is often higher, and therefore requires a stronger material. Plain copper alloys will suffer from turbulence corrosion (Erosion) at velocities above approximately 1 m/s but copper-nickel alloys can be used up to a velocity of 4 m/s. As a comparison stainless steels can withstand above 20 m/s. The tendency of fouling is less on copper-alloys than on other metals, which is one of the reasons to the widely used copper-piping systems world-wide. This has to do with the anti-fouling properties for the copper material.

Aluminium As a construction metal aluminium is also widely used, especially in the atmospheric environments. It is a light material with a density on 2,7 g/cm3.When aluminium is exposed in the atmosphere it reacts with the oxygen in the air, and a passive oxide film is created. The oxide film is very thin, only about 0.01 µm thick, but it provides the metal with a very good protection against corrosion. Aluminium is therefore also a passive material. Due to the thin oxide film aluminium corrodes very slowly in the atmosphere. The average corrosion rate in industrial atmosphere will seldom be higher than 1 µm/year. In a severe marine



atmosphere, the maximum depth in pits has been measured to be 85 – 265 µm after 20 years of exposure. The pits are formed due to the salt environment. Chlorides destroy the passive film at weak points, resulting in pitting corrosion. In other environments the corrosion rate is much lower. Aluminium is considered to be an ignoble metal. In contact with noble metals such as steel or copper it will sacrifice itself to protect the metal(s) in contact (galvanic corrosion). Due to this it is important that metals in contact with each other are insulated from each other to avoid unnecessary corrosion problems.

Zinc Zinc is seldom used as a construction material, it has a density of 7.13 g/cm3. Instead it is used in combination with iron and steel. Around 40 % of the total amount with zinc on earth is used this way- as cathodic protection. Ether as hot dip galvanized steel or as sacrificial anodes on ship hulls or submerged steel areas of offshore platforms etc. Both ways it is used for corrosion protection of steel. The application / hot dip galvanising process is usually done in plants where old paint, grease etc. has been removed by alkaline cleaning, and rust and mill scale is removed by pickling. Several water rinsing baths are available and the steel is also fluxed prior to dipping in the molten zinc having a temperature of 460 – 470 oC. The corrosion rate of zinc under atmospheric conditions is low and usually not more than 1 – 10 µm/year. When zinc corrodes a white salt is formed as corrosion product. This is called white rust or zinc salts. The corrosion rate of zinc in submerged solutions is on the other hand high. This has to do with the nobility of the material. Zinc is an ignoble metal (which is why we use it as sacrificial anodes/cathodic protection) and in a solution or in coupling with other materials it will suffer from corrosion.

Titanium Titanium is the heaviest of the light metals that we have, and weigh 4.5 g/cm3. It is therefore much lighter than iron and steel, and heavier then aluminium. From corrosion point of view titanium reacts with the oxygen in the air, forming a fantastic passive layer. This layer protects the material very well, making it a passive material with excellent corrosion properties. Titanium is therefore an expensive material, but it is worth to consider, because of it excellent mechanical properties. As its corrosion resistance is so good, it can among other things be used in those cases where stainless steels are not sufficiently resistant to corrosion. Unalloyed titanium is very resistant to corrosion in damp chlorine gas, chlorine compounds etc. In these media’s titanium is superior to most metals. In dry chlorine gas, however, titanium is severely attacked.



Concrete The construction industry uses large quantities of steel reinforcement bars (re-bars) to strengthen concrete structures. The alkaline environment that exists in the concrete protects the steel from corrosion, by maintaining a passive film on the surface of the steel. This is the case if the pH in the concrete is higher than 13. If the concrete over time becomes more acidic, the steel bars may corrode. When this happens the re-bars must be pre treated, before it is painted. In some cases the corrosion process has come so far that it must be reinforced before it is painted. When the steel is in good repair, the damaged construction can be reinforced with more concrete. The concrete is produced from: • cement • fine aggregate e.g. sand • coarse aggregate – gravel or crushed rock • water The cement is the binder in the concrete. The cement reacts with the water and a hydratisation process occurs. The most common cement types are Portland cement and modified Portland cement. The water-cement ratio is connected to the mechanical strength of the concrete. Approximately 0.4 kg of water is needed to bind 1 kg of cement, but to get a workable, free flowing paste we must higher w/c ratios, in the range of 0.5 or higher. There are different kinds of concrete, depending on where the concrete is used. The curing time of the concrete prior to painting is generally thought to be minimum 28 days.


©Teknologisk Institutt as www.teknologisk.no Corrosion 06/13 Materials Technology 1

CORROSION



INTRODUCTION Corrosion comes from the Latin word corrode and means gnaw (eat) away. Whit knowledge of the types of corrosion that can occur, and an understanding of what causes the corrosion and degradation, it is possible to take measures to prevent them from occurring. For example, we may change the nature of the environment, select a material that is relatively nonreactive, and/or protect the material form deterioration with coatings. As a motivation to why this chapter is important, we have to have a wider perspective. Corrosion costs the society and is extremely expensive financially. It has been estimated that approximately 4% of an industrialized nation’s income is spent on corrosion prevention and maintenance. It is also a wasteful way to handle our natural resources and on top of that, it can cause considerable inconvenience to humans and sometimes loss of lives. To many people the word corrosion is synonymous to the read-brown rust layer on old surfaces. For others working with corrosion and corrosion protection, corrosion is a general deterioration process, taking place on materials. In our case the material is normally a metal, but we can also have corrosion on ceramic materials and polymers. All metals will form a type of oxide layer, or corrosion product. This mainly occurs when metals react with oxygen in the environment. Rust is the corrosion product that forms when iron or steel degrades. Other examples are the green patina (verdigris) that forms on copper/copper alloys and the white products forming on zinc alloys. All three are corrosion products, but only iron and steel rusts.

CORROSION THEORY OF METALS

Definition In Metals Handbook, Volume 13, the definition of corrosion is the following:

“Corrosion = The chemical or electrochemical reaction between a material, usually a metal, and its environment that produces a deterioration of the metal and its properties”.

Other sources might give other definitions like: Unintentional attack on a material through reaction with the environment. As the definition of corrosion states, corrosion is deterioration of metal because it reacts with the environment. But why does this happen, and which factors must be present to get corrosion? We know that most metals are not found in the nature as pure metals, with exception of the noble metals as gold and platinum and others. The other metals are present as ores or oxides because this is the most stable state. To get the pure metal, the ore is reduced in a process using a high consumption of energy. When the ore is melted, the pure metal is separated from the slag, and cooled down to metal as we know it. The stable state is no longer present, so the metal is vulnerable. It “wants to be stable again”. This is the driving force in many corrosion processes. The corrosion process is just a natural process, and under the right circumstances, the metal convert back to its most stable state ore, which is very similar to the oxide. The process is also shown in the figure below.



Summing up, corrosion can be viewed as the process of returning metals to their natural state- the ores from which they were originally obtained. Metals will degrade under many different conditions, but we normally talk about aqueous corrosion, which reduce the temperature range to below 100°C. Under these circumstances there are three things that must be present to get corrosion. This is metal, electrolyte and oxygen. An electrolyte is any substance containing free ions that make the substance electrically conductive. The most typical electrolyte is a solution consisting of water and salts; here the salts make the water conductive. Other examples of electrolytes are sea water, soil, polluted fresh water, acids, alkalis. Pollution from the air can also dissolve in water and make it conductive. If the presence of one of the three elements in the triangle can be eliminated the corrosion process stops. This we will get back to in the chapter about avoiding corrosion.

Galvanic series / Electro chemical series Metals have a tendency to release electrically charged atoms (ions) when it is in contact with solutions. For noble metals (gold and platinum) this tendency is weak, while for ignoble metals (Magnesium, Sink and Aluminum) it is strong. Consequently different metals will have different

Electrolyte/Humidity

Metal Oxygen



corrosion resistance, and hence different ability to withstand corrosion in a given environment. In all metals or alloys there is a certain amount of energy stored. This can be measured in volts using a voltmeter and these measurements are usually referred to as potential measurements. By using a voltmeter we can easily measure the potential difference between two metals in an electrolyte. However if we want to determine the potential of a metal alone or in contact with another metal, we must use a reference electrode. The reference electrode acts in a way as a zero point. The Standard Hydrogen Electrode (SHE) is by definition defined as 0 volts, but the most commonly used reference electrodes are the following:

Table 1 Potentials of different reference electrodes

Type of reference electrode Electrolyte Potential [volts] Zinc Seawater - 0.78 Silver / silver chloride (SSC) Seawater + 0.25 Copper / copper sulphate (CSE) Seawater + 0.32 Calomel (SCE) KCl + 0.245

As can be seen from the table above, the measured value must be corrected if the potential should be compared with the literature. An example of a widely used table from the literature is the Galvanic series in sea water. Here, the potential of many kinds of metals and alloys have been measured and arranged in a table. As can be seen from the table, the materials that are in the same potential range are grouped together. These materials may be used together without significant risks of galvanic corrosion.



Table 2 The galvanic series in seawater1

Ignoble - 1.6 V - 1.0 V - 0.6 – 0.7V - 0.1– 0.2V + 0.1 V + 0.2-0.3 V Noble

Magnesium Zinc Aluminium alloys Mild steel, cast iron Low alloy steels Austenitic nickel cast iron 18Cr 8Ni stainless steel (active) 18Cr 8Ni 3Mo stainless steel (active) Lead soldering Lead Tin Nickel (active) Inconel (active) Hastelloy B(60 Ni 30 Mo 6 Fe 1 Mn) Chlorimet 2 (66 Ni 32 Mo 1 Fe Admirality brass, aluminium brass Copper Manganese bronze Cu-Ni alloys (60Cu 40Ni), (90Cu 10 Ni) Monel (70Ni 30Cu) Silver solder Nickel (passive) Inconel (80Ni 13Cr 7Fe) 18Cr 8Ni stainless steel (passive) 18Cr 8Ni 3Mo stainless steel (passive) Hastelloy C(62Ni 17Cr 15 Mo) Chlorimet 3 (62Ni 18Cr 18 Mo) Silver Titanium Graphite Gold Platinum

The corrosion process As we have already stated, corrosion is a natural occurring process that will take place under certain conditions on most of the metals that we know. Depending on the material and the environment, some times the degree of degradation is small and almost unessential, but other times it can be severe. Independent on this, the corrosion process is the same; it is just the corrosion rate that differs. In order to explain the corrosion process, let us look at some examples. If we place iron in contact with copper in an electrolyte, the iron will start to corrode and send of energy to the copper. If you don’t know which material that will corrode, you can use the galvanic series above. Looking in the table, you can se that iron is placed above copper. This means that iron 1 Olsen, A: Korrosion 1 - Korrosjonsformer og, Universitetsforlaget, 1983



is the most ignoble material and copper is the noble material. The ignoble material will sacrifice it self to protect the most noble material. This happens as the ignoble material (iron) degrades to its oxide by sending electrons to the noble material (copper), forming rust on the iron surface. At the same time another reaction that involves oxygen or water occurs at the cathode, making sure the system is in balance. When we talk about corrosion, we usually talk about the anode and the cathode instead of talking about noble and ignoble metals. The metal that degrades is called an anode; here we have corrosion, while the material that is protected/most noble is called the cathode. The anode is negative and the cathode is positive, making the electrons released at the anode is attracted to the

cathode. As shown in the figure, a voltmeter can be placed between the two metals to measure the potential difference (shown by a ‘V’ in the figure). The anode degrades while the cathode is unaffected. It may therefore look like there is nothing happening at the cathode, but this is not the case. Parallel to the degradation happening at the anode, the solution around the cathode becomes alkaline (OH-). Consequently two processes happen at the same time, one at the anode and one at the cathode. The reaction occurring at the cathode contains oxygen: Cathode: O2 + H2O +4e- 4OH- The reaction on the anode contains the metal that is degraded. (In this example we used iron, and iron ions (Fe2+) are made): Anode: 2Fe 2Fe2+ +4e- When the iron ions react with the hydroxyl ions (OH-) rust is formed (Fe(OH)2: Fe2+ + 2 OH- = Fe(OH)2 The reactions above are shown to explain what is happening, but is not important to remember.

- +



However, in order for corrosion to take place, it is not necessary that dissimilar metals are joined in the electrolyte. As you probably have seen, blast cleaned steel corrodes when you leave it outdoors e.g. in rain. The reason is that steel (and other metals) is not microscopically uniform. It contains a number of small grains with small differences in the potential. Or said in another way, some of the grains are nobler and some are less noble. This result in some anodic- and some cathodic areas: Therefore we will have corrosion in the exact same way as described above. As the material corrodes, the grains potential varies, so a place that was cathodic can change and be anodic. We will therefore see a uniform corrosion attack on the surface. The reason for the small potential differences can be alloy particles, different carbon contain, mill scale or graphite material.

Corrosion rate If two metals are submerged in a liquid that conducts electricity (an electrolyte), both metals will corrode at a certain corrosion rate. If we connect these two metals with each other, the corrosion rate will increase on the most ignoble metal and decrease on the most noble metal The corrosion rate e.g. in air, fresh water and seawater will vary for the different metals. The corrosion rate will amongst other things depend on the following: • Type of metal • Environment • Contact with other metals • Surface films on the metal • Temperature • Submerged in a liquid Often the corrosion rate is measured in µm/year. In the table below there is shown an example with four metals: Steel, aluminium, copper and zinc. All materials were exposed at a test rig close to the sea on the west coast of Sweden. The corrosion rate (in µm/yr) was determined after 2, 5 and 10 years. As can be seen from the table, there is a great difference in the corrosion rates of the four



metals. Some of the materials with low corrosion rate offer good long-term protection due to the protective oxide layer.

Table 3 Corrosion rates in µm/year for different metals after exposure for 2, 5 and 10 years to marine atmosphere on the West Coast of Sweden.

Material 2 years 5 years 10 years Steel 51.1 µm 32.8 µm 20.7 µm Aluminium 0.48 µm 0.76 µm 0.35 µm Copper 1.8 µm 1.1 µm 0.71 µm Zinc 3.6 µm 2.6 µm 1.7 µm

The differences in corrosion rate relates to the properties of the different materials. All the metals will form oxide layers, but some will form a more protective layer then others in reaction with oxygen / during the corrosion process. The oxides are the corrosion product in any case. Rust is the oxide formed on iron, while white rust is the oxide on zinc. These oxides will not protect the material, so it will continue degrading. On titanium, aluminum and stainless steel the oxide layers are more protective. That’s because the oxide formed is a passive layer, much more protective than rust or white rust. The materials that forms this passive layer is often called passive materials. If it is damaged, the fresh surface will react with the oxygen in the air and reveal itself. The corrosion rate is also influenced by the environment. The different types of environment or corrosivity classes are listed in ISO 12944 – Part 2; Classification of environments. This standard will be discussed in a separate chapter. A short summary is that the environment are divided in two:

• Atmospheric corrosivity categories • Submerged corrosivity categories

In both cases, there are definitions on different categories based on the corrosivity in the environment. In general we can say that for the atmospheric categories the corrosivity increases as the salt/humidity/contamination level increase. For the submerged categories, corrosivity will also increase with the salt level. One material can behave differently in the two categories.

Corrosive environment and pH values In dry places, areas with relative humidity less than 60 % and small amounts of contaminations like sodium chloride and sulphur dioxide, there is little risk of corrosion occurring. This is due to the fact that no continuous electrolyte is created on the surface. Outdoors the corrosion rate will be very dependant on the surrounding environment. The higher the content of sulphur pollution and salts (chlorides) in the air, the higher the corrosion rate is. Some of the gasses in the air will form either alkaline or acidic solutions. In addition to this, the temperature influence must be considered. Normally corrosion rate increase with the temperature. Whether or not a solution is alkaline or acidic can be decided by measuring the pH value of the solution. A pH value is determined on a scale from 0 to 14. On the pH scale, solutions with values



less than 7 are considered as acidic, a value of 7 is neutral and solutions above 7 are considered to be alkaline.

Acidic Neutral Alkaline 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 The pH scale is a logarithmic scale, meaning that a decrease from e.g. 7 to 6, indicates that the value is 10 times more acidic than at pH=7. If the value had increased from 7 to 9, the value will be 10 x 10 = 100 times more alkaline than at pH = 7. Examples on acidic solutions

• Hydrochloric acid (HCl), • Sulphuric acid (H2SO4) • Citric acid

Examples on alkaline solutions

• Sodium hydroxide, • Caustic soda (NaOH) • Cement • Epoxy hardener

Corrosion attacks of some metals will be affected by the pH. In some cases the metal will corrode in both an acidic and alkaline solutions (e.g. zinc and aluminium), whilst others metals (e.g. steel) will corrode in acidic solution, but be passivated in alkaline solutions. This is why steel reinforcement bars are used in concrete; it is passivated and will not corrode in the alkaline cement. Two know if a material is passivated or not is dependent on the nature of it. This has been carefully mapped bye a lot of experimental testing, and fortunately summarized in books.

Different types of corrosion In this chapter we will talk about the following corrosion types:

• General corrosion • Galvanic corrosion • Pitting corrosion • Crevice corrosion • Erosion corrosion • Cavitation corrosion • Selective corrosion • Stress cracking corrosion • Microbiological



General corrosion

General corrosion is as the name says a general attack over large areas on a metal surface. This is one of the most common types of corrosion, and can occur at all metals to some extent. It is normally characterized by a chemical or electrochemical reaction, which proceeds uniformly over the entire surface or over a large area; therefore it is also called uniform corrosion. As the metal suffers from corrosion, it becomes thinner and thinner, ant finally it fails, but since the degradation is predictable, it is easy to handle. With this type of attack on a surface, both the anodic and the cathodic processes are evenly distributed over the metal surface. The location of anodic and cathodic areas shifts from time to time, as discussed in earlier chapters. A general corrosion attack leads to a reduction of the materials thickness linear with time. The rate of penetration can be calculated i.e. from corrosion data. This makes it easy to predict the lifetime of a material, as long as you know the corrosion rate in the given environment. If you have a specification that requires a lifetime of at least 15 years, you can calculate the extra thickness of the material you need to add to the required construction thickness. This extra material is the corrosion allowance for the construction. The extra material, the corrosion allowance is the material that can be gone while the equipment is still capable of working as designed. For example a corrosion rate of 0.13 mm/yr (5 µm) would result in a metal loss of 1.52 mm (0.060") in a twelve-year period. A corrosion allowance of 1.59 mm (1/16 ") is often adequate to provide 12 years of service in process exposures, perhaps 25 years in storage tanks.



On the other hand heat exchanger tubes with a wall thickness of 2.11 mm (0.083") can probably tolerate no more than a 50 % loss of thickness (e.g. 1.02 mm (0.040 ") in 8 years at a rate of 0.13 mm/yr (5 mpy).

General corrosion may be reduced / prevented by:

• Change of materials • Use of paints or coatings • Use of inhibitors (in systems with liquids)

Galvanic (bimetallic) corrosion

Galvanic corrosion occurs when a metal or alloy is electrically coupled to another, or to a conducting non-metal (e.g. carbon or mill scale), in the same corrosive environment (electrolyte). The different metals are in metallic coupling with each other, and this will result in corrosion problems. The rate of attack of one metal or alloy is usually accelerated, while the corrosion rate of the other decreases. I.e. the metal with the most negative corrosion potential (most ignoble) in the uncoupled state (the active member of the couple) will show enhanced corrosion.

Original thickness

Anode

Cathode



To know the corrosion potential to the different metals, we need the galvanic series as shown previous. If you go back and look in this table, you will see that the materials with the most negative potential, is listed first. This is the ignoble metals. As the potential increase the metals become more noble. Generally the following factors will influence the galvanic corrosion process:

• The difference between the corrosion potentials of the uncoupled metals or alloys • The surface areas of the cathodic and anodic zones • The distance between the cathode and the anode • The electrical resistance of the galvanic circuit (use of paint films)

One method of predicting galvanic corrosion is by immersion testing. The metals that is coupled to each other, is tested in the environment of interest, to see which of them that corrodes. In most cases however the galvanic corrosion is predicted by use of a galvanic series. In the galvanic series metals and alloys are arranged according to their potentials, measured e.g. in seawater. Factors like area, distance and geometric effects affect the galvanic corrosion behavior.

Influence from the difference in potential As discussed previously under the galvanic series, different materials can be used together without problems, as long as the potential difference between them are small enough. In the table that shows the galvanic series materials that can be used together without problems are grouped. If materials with very different potential are coupled, e.g. aluminum and stainless steel, the corrosion rate is increased in the ignoble material, here aluminum. To avoid this problem, we must use materials that are in one group, or we must insulate the different metals such that the galvanic coupling does not exist.

Influence from the surface areas ratio When the surface area of the more noble metal or alloy (the cathode) is large compared to the more active member (the anode), an unfavorable ratio exists producing an accelerated galvanic effect on the most active/ignoble material. RAPID CORROSION OF THE STEEL

Copper

Copper Steel



The opposite area ratio (large active member and a smaller noble member surface) produces only slightly accelerated galvanic effects. SLOW CORROSION OF THE STEEL Serious problems have occurred on welded pipes where the welding electrodes used were more ignoble than the pipes. In this case the welds are more anodic than the surrounding steel pipes which results in a severe attack on the welds due to galvanic corrosion. The area effect accelerates the galvanic effect. Problems have also occurred on the Statue of Liberty in New York City that was erected in 1886. The steel armature was originally insulated from the copper skin using shellac-impregnated asbestos. This of course has broken down during more than 100 years of exposure, causing a metallic coupling between the copper and the steel. The asbestos absorbed water and no longer insulated the two metals caused great damage on the steel armature. During rehabilitation in 1981 -1986 large amounts of the steel was changed with stainless steel and Teflon was used for insulating the parts.

Galvanic corrosion may be reduced / prevented by:

• Avoiding combinations of metals / alloys widely separated in the galvanic series • Change of the environment (inhibitors) • Use of paints or coatings such that the cathode area is reduced • Use of proper welding electrodes (more noble) • Design • Insulating metals from each other • Avoiding deposits from more noble metals on the less noble metal

CATHODIC PROTECTION You may have asked yourself why this section is taken in to the corrosion chapter. The reason is that cathodic protection and galvanic corrosion is more or less the same, from two different points of view. We use the same theory here, as you learned about in the galvanic corrosion section. But this time it is planned so, that the coupling of different materials result in protection of steel, and sacrifice of another material. Consequently, cathodic protection is a way of protecting steels by making the steel we wish to protect the cathode in a galvanic cell. As we have seen previously in connection with the galvanic corrosion -

Steel

Steel

Copper



the cathode will not corrode. The attack takes place at the anode. This method of protection has been known for more than 160 years and is widely used for protection of ship hulls, oilrigs, and pipelines.

How does this kind of protection work? The corrosion rate is very dependent on both the metal itself and the surrounding electrolyte. As we have seen all metals have a certain electrical potential when immersed in an electrolyte solution e.g. seawater. The potential is measured against a reference electrode. As you might be aware of - steel will corrode in acids, but not in alkali solutions. This tells us that the corrosion of steel i.e. the energy level of the steel depends on the environment around it. A Belgian, Mr. Poirbaix studied this carefully and was able to make potential / pH diagrams showing where corrosion would occur. He also found that below a certain potential, corrosion could not occur at all. The purpose of cathodic protection is to place the steel (or another metal) in a position where it will not be able to corrode in the given environment. When steel is put into seawater the steel corrodes. When zinc is put into seawater, the zinc corrodes. When the steel is electrically connected to zinc, zinc will corrode and send off energy. The steel will consume this energy. Since steel is given energy at all times, it is impossible for the steel to send off energy, hence the zinc protects the steel. In this case, zinc will be the anode and steel the cathode. Cathodic protection can be achieved in two ways: • Sacrificial anode systems • Impressed current systems



Sacrificial anode systems In this system the protective current is supplied by external anodes of a metal which are ignoble (i.e. more anodic) than the steel we want to protect. When the steel is in contact with the anode, the anode will supply the steel with sufficient electric energy and protect the steel. Normally the sacrificial anodes are made of zinc- or aluminium alloys. Both of these can be used for protection of steel in seawater.

Impressed current systems In this system the protective current is supplied from the alternating current system of the ship or an offshore structure and the current is transformed to direct current through a rectifier. The anodes used are more or less chemically resistant, i.e. very noble materials. They can be made of platinized titanium or graphite. They will not be attacked or consumed and are inert anodes. When impressed current systems are used, automatic control is usually required. This control will at all times check the potential of the steel against a reference electrode. When changes occur the current will be altered. The numbers of anodes are much fewer when using impressed current systems, because they are designed for high current outputs. This will result in overprotection of areas close to the anodes and

Steel

Zinc

Protective current

Steel Cathode Platinized titanium

anode

Seawater

DC rectifier - +

Reference electrode

Protective current



cathodic disbonding (disbonding, loss of adhesion). To prevent damage/cathodic disbonding of the coating in these areas, a thick coating (an anode shield) is applied around the anodes. The thickness may be up to 1 to 1.5 mm. It is common to apply the shield in a diameter of 2 to 3 meters around the anodes. The impressed current systems can be used both on permanent constructions such as oil-rigs, or on ships. The use of sacrificial anodes will result in an increased drag in the sea. However, using impressed current systems, the anodes are usually flush mounted and drag is negligible.

Pitting corrosion

Pitting corrosion is a form of localized attack that results in holes in the metal. These holes may be small or large in diameter, but in most cases they are relatively small. They may result in perforation of a metal or alloy, which is very risky, because it is difficult to se the seriousness of the corrosion attack without looking at the cross-section of the material. Pits are sometimes isolated or so close together that they look like a rough surface. Generally a pit may be described as a cavity or hole with the surface diameter about the same as, or less than, the depth. Pitting is one of the most insidious forms of corrosion. It causes equipment to fail because of perforation with only a small percent weight loss of the entire structure. Seen from above Seen from the side



Pitting corrosion occurs frequently at passive metals and alloys when these are exposed in an environment that contains salts as chlorine, bromine (halogens) and sea water. High temperature will also increase the risk of getting pitting corrosion. The theory of why pitting corrosion occurs is not entirely understood. This makes it difficult to generalize. Mostly it is salt/halogens that destroys the passive oxide film on places where this is weak. When the pit is initiated, the further growth is the same as for crevice corrosion, and will be explained in the next chapter. Pitting corrosion often occurs on stainless steel alloys, aluminium and may also occur on titanium under special conditions. In addition to the salt concentration and the temperature, flow conditions and pH may also influence the pitting mechanism. Due to the fact that pitting looks like small holes, it can be confuses with galvanic-, crevice- , erosion- and cavitation corrosion. It is therefore important to evaluate the surroundings and the type of material you have, to be sure that it is pitting. The other corrosion types are characterized by other requirement, which pitting don’t have, e.g. galvanic coupling to a more noble metal, high velocity on liquids or gas bubbles.

Pitting corrosion may be reduced / prevented by: • Change of materials • Reduction in the temperature of the electrolyte (e.g. seawater) • Avoiding stagnant conditions in electrolytes

Crevice corrosion

Crevice corrosion is a type of intense localized corrosion frequently occurring within crevices and other shielded areas on metal surfaces exposed to corrosive liquids. The type of attack is usually associated with small volumes of stagnant solution caused by holes, gasket surfaces, lap joints, and crevices under bolt and rivet heads. As a result, this form of corrosion is called crevice corrosion or, sometimes, deposit corrosion.



As for pitting corrosion, crevice corrosion occurs frequently at passive metals and alloys that are exposed in a saline environment. The corrosion mechanism is also quite similar. For both types the salt attacks the oxide layer and increases the corrosion. For crevice corrosion to occur the crevice must be wide enough to permit entry of the solution, but sufficiently narrow to maintain a stagnant zone of solution within the crevice, limiting the transport processes of diffusion and migration of ions. Solutions containing chloride ions/salts are most conductive to crevice corrosion.

The corrosion starts in small crevices with stagnant water. Over time the oxygen level decrease because it is used to maintain the oxide film. When there is no oxygen left, the oxide film can not repair itself and therefore becomes vulnerable. The salt in the liquid starts to destroy the oxide film and at the same time, the environment inside the crevice becomes acidic, so the environment get’s even more aggressive, hence the material corrodes. It is not always necessary to have crevices - this type of corrosion attack may also occur on a metal surface covered with sand, mud or dirt. This also results in low oxygen contents underneath the sand, mud e.g.

Metal surfaces

Sand, mud etc.

Crevice corrosion attack

Narrow gap

Metal

Mental

Electrolyte with salts High oxygen content No corrosion Acidic environment

Low oxygen content Crevice corrosion



Crevice corrosion may be reduced / prevented by: • Change of materials • Change of environment • Use of paints or coatings • Design • Welding instead of using flanges and bolting • Avoiding areas with stagnant water • Cathodic protection

Erosion corrosion

Erosion corrosion is a form of corrosion that occurs when a metal is attacked because of a relative motion between an electrolyte and a metal surface. Metal is then constantly removed from the surface or its corrosion products are removed by the electrolyte. Soft metals are particularly vulnerable to this form of attack, for example copper, brass, pure aluminium and lead, but most metals are susceptible to erosion corrosion in particular flow situations. If the corrosion products or sand particles follow the flow further, the probability to get corrosion problems is larger, because of the solids.

Narrow gap

Crevice corrosion attack



Some factors, which are likely to cause this form of corrosion attack, are: Sudden change in the bore diameter or direction of a pipe A badly fitting gasket or joint which introduces a discontinuity in the otherwise smooth metal surface A crevice which allows liquid to flow outside the main body of fluid The presence of a corrosion product or other deposits which may disturb the laminar flow Erosion corrosion is characterized in appearance by grooves, gullies, waves, rounded holes, often in the same direction as the flow. Problems with erosion corrosion arise when the flow velocity is too high for the given material. Different materials withstand different flow velocity, so here you must look at the material certificate. Copper materials is normally soft and have a maximum flow velocity below 1 m/s, its alloys some were below 3 m/s, while stainless steels can withstand velocities as high as 20 m/s Often a very specific pattern is created called horseshoe attack. Seen from above Seen from the side

Erosion or turbulence corrosion may be reduced / prevented by:

• Change of materials • Change of environment (reduction in the water flow) • Avoiding angular bends

Corrosion film

Metal - tube wall

Impingement corrosion pits Original metal surface



Cavitation corrosion

Cavitation is a particular form of erosion corrosion caused by the formation and collapse of vapor bubbles on the surface. This type for corrosion is often encountered on parts being driven at high velocity through a liquid, rather than in pipes or tanks where fluid flow occurs across stationary metal surfaces. The collapsing vapor bubbles produce shock waves at very high pressures, which in turn produce deformation of many materials. The appearance may look very similar to erosion corrosion, but it is possible to distinguish between them. The depression formed here is deep, have a small diameter. The depressions are often close to each other which give the surface large roughness. The surface may look like a sponge. Water turbines, propellers, impellers and hydraulic turbine gear are the most common instances for encountering corrosion by cavitation. Cavitation damages have been attributed to both corrosion and mechanical effects.

Collapsing bubbles destroying the film.



Dealloying corrosion / Selective corrosion

This kind of corrosion is also called selective leaching or parting. One element, generally the most active one/most ignoble metal, is selectively removed from a solid alloy. As a result the components of the alloy react in proportions, which differ from a solid alloy. When one component of an alloy is removed the construction in this way, it may look intact, but it is important to remember that the strength and properties are changed. To recognize this type of corrosion you must look after a color change in the material. Apart from the general term the process is often named after the removed element in the specific cases, e.g. dezincification of brass, dealuminification of certain aluminium-bronzes. However not in the case of graphitization of grey cast irons (here the removed element is iron). A well-known example of dealloying is dezincification of brass. Dezincification is readily recognized as the alloy assumes a red copper color, i.e. in contrast to the original yellow. There are two general types of dezincification. One is uniform, or layer type, and the other is localized, or plug-type dezincification. No attack, yellow brass colour Local attack, red spots on surface Uniform attack on surface, red colour Uniform attack over whole area, surface

becomes brittle and red



Selective corrosion may be reduced / prevented by:

• Change of materials • Change of environment • Cathodic protection

Stress Corrosion Cracking (SCC)

Stress corrosion cracking can be defined as crack formation/unexpected sudden failure of normally ductile metals subjected to a tensile stress in a corrosive environment. It is especially vulnerable at elevated temperature. The chemical environment that causes SCC for a given alloy is often one which is only mildly corrosive to the metal otherwise. Hence, metal parts with severe SCC can appear bright and shiny, while being filled with microscopic cracks. This factor makes it common for SCC to go undetected prior to failure. SCC often progresses rapidly, and is more common among alloys than pure metals. The specific environment is of crucial importance, and only very small concentrations of certain highly active chemicals are needed to produce catastrophic cracking, often leading to devastating and unexpected failure. Well-known materials, which may show susceptibility to SCC in chloride environments, are austenitic stainless steels containing chromium and nickel (e.g. 316 SS) and a number of aluminium alloys. Generally the susceptibility to SCC increases with increasing temperature. For a number of alloy / environment combinations a safe temperature can be indicated, below which the susceptibility to SCC is practically nil. This corrosion type is recognized on the characteristic cracks that will appear on the surface:

Stress corrosion cracking



In the off-shore industry a lot of the hot stainless steels tubes are blast-cleaned with a fine non-metallic abrasive and then applied an epoxy coating. This is done to minimize the tendency of SCC, especially on insulated stainless steels with an operating temperature of 60 °C or higher. Elimination of tensile stresses, in order to reduce the risk of stress corrosion cracking (SCC), can be accomplished by stress relief annealing. The annealing conditions (temperature, time) should be such that a satisfactory stress relief is obtained without substantially reducing the strength of the material.

Stress corrosion cracking (SCC) may be reduced / prevented by:

• Correct operation temperature • Reduced tensile level • Change of materials • Use of barrier paints or coatings • Annealing to reduce stresses within the metal • Cathodic protection using sacrificial anodes

Microbiological (Bacteria) corrosion

Microbial corrosion, also called bacterial corrosion, bio-corrosion, microbiologically-influenced corrosion, or microbially-induced corrosion (MIC), is corrosion caused or promoted by microorganisms. It can apply to both metals and non-metallic materials. Although in deaerated water steel does not corrode too much, the corrosion rate in some natural environments is found to be abnormally high. The high corrosion rates have been due to presence of sulphate-reducing bacteria (SRB). They thrive only under conditions of poor or no aeration in water and soils and in the pH range of about 5.5 to 8.5. The sulphate reducing bacteria easily reduce inorganic sulphates to sulphides in presence of hydrogen or organic matter, and are aided in this process by the presence of an iron surface. The anaerobic corrosion of iron and steel has been identified in bottom mud of rivers, lakes, marshes, under marine fouling and in various offshore industrial environments.



Microbiological (Bacteria) corrosion may be reduced / prevented by: • Use non-corrodible materials e.g. fiberglass, PVC, polyethylene, concrete • Create a non-aggressive environment around the steel by backfilling with gravel or clay free

sand or using biocides • Cathodic protection of –0.95 V versus Copper/copper sulphate • Using various barrier coatings, with biocides.

Corrosion under insulation2

CUI is a particularly severe form of localized corrosion that has been plaguing chemical process industries for many years. The key problem in CUI is the intruding of water. Therefore it is particularly important during design not to promote corrosion by permitting water to enter the system directly or indirectly by capillary action. Moisture may be external or may be present in the insulation material itself. Corrosion may attack the jacketing, the insulation hardware, or the underlying equipment. For high temperature equipment, water entering an insulation material and diffusing inward will eventually reach a region of dry out at the hot pipe or equipment wall. This will lead to a zone next to the dry out area where pores of the insulation are filled with a saturated salt solution. When a shutdown or process change occurs, and the metal-wall temperature falls, the zone of saturated salt solution moves into the metal wall. When the temperature then rises again, stress-corrosion cracking may begin. The drying/wetting cycles in CUI associated problems accelerates the corrosion damage. The most common and straightforward way to inspect for corrosion under insulation is to cut plugs in the insulation that can be removed to allow for ultrasonic testing. However, many times such plugs can be the source of moisture leakage. The main problem with this technique is that corrosion under insulation tends to be localized and unless the inspection plug is positioned in the right spot, the sites of corrosion can be missed. Other techniques that are available include special eddy current techniques, x-ray, remote TV monitoring, and electro-magnetic devices.

CUI may be reduced / prevented by: • One of the best but most expensive options to prevent corrosion under insulation is the use

of protective coating systems. Special coating system must be utilized that have proven 2 Reference: http://www.corrosion-doctors.org/Forms-crevice/CUI.htm



performance. NORSOK M-501 has recommendations on both pre-treatment and paint systems for carbon- and stainless steel that is acknowledge.

• Inhibitors • Water proofing • Drain holes • Careful selection of insulation materials to prevent those that contain high levels of

corrosive impurities such as chlorides is critical to reducing corrosion under insulation.

How to avoid corrosion? During the chapters about the different corrosion types, some recommendations on how to reduce or prevent them are made. In addition to this, we will make some general advises in this chapter.

Choice of material First of all, it is important to do the correct choice of material in a given environment. Some materials, like aluminum, zinc, steel and stainless steel will be attacked to some extent by acidic gases and chlorides/sea water. Aluminum and zinc will also be affected by alkalis, while steel and stainless steel will be passivated. Hence it is no problem to use this material as re-bars in cement. In addition to the chemical environment, it is also important to remember the temperature aspect. Some materials have temperature limitations, which make them unusable. E.g. 316 stainless steel can not be used above 50-60 °C.

Corrosion “triangle” Another approach is to look at the corrosion process in water and the definition of corrosion. Here there are three requirements that must be fulfilled to get corrosion: We need to have a metal in an electrolyte that contains oxygen, and these three must react with each other. If one of these three is removed, the corrosion process will stop. Remove the electrolyte To remove the humidity (electrolyte) there are different systems that can be used to dehumidify the environment. If the relative humidity is kept below 40 % the corrosion will stop. This is a typical way to protect the inside of bridges, pipes i.e. without painting it. Remove the oxygen In closed systems oxygen is eliminated. As long as the oxygen level is low the inside of the system oxides can not form, hence there is no corrosion. If the system is opened and the solution/electrolyte is changed, there will be corrosion until the oxygen level again decrease to a minimum. This is the case in radiators. They are normally painted on the outside, while the inside is unpainted. “Removing” the metal Removing the metal is not possible, but it is possible to make a barrier between the metal and the environment. This is a common way to avoid corrosion, and can be done in several ways.



Painting or surface treatment is another way to make a barrier between the environment and the metal. This is may be the most common way to reduce corrosion problems and will be discussed in separate chapters. A corrosion inhibitor is a chemical compound that, when added to a liquid or gas, decreases the corrosion rate of a metal or an alloy. The effectiveness, or corrosion inhibition efficiency, of a corrosion inhibitor is a function of many factors, including (but not limited to) fluid composition, quantity of water, and flow regime. If the correct inhibitor and quantity is selected then it is possible to achieve high, 90-99%, efficiency. Some of the mechanisms of its effect are formation of a passive layer. This is a thin film on the surface of the material that stops access of the corrosive substance, either inhibiting the oxidation or reduction part of the corrosion system (anodic and cathodic inhibitors). Contact between different types of metals was described in the chapter galvanic corrosion. It can not be clarified enough how important this subject is. The area relationship is of vital importance and will affect the lifetime of a construction. Choosing wisely the materials that are coupled together, the construction will not suffer form degradation. But if materials with very different corrosion potential are coupled together in the same electrolyte, and the area between the anode and the cathode is unfortunate, corrosion will occur. As long as the anode are large compared to the cathode, the corrosion rate will below.

Cathodic protection is another common way to avoid corrosion. This will also be described in another chapter. Design is also of vital importance. This subject will also be described in other chapters, i.e. in ISO 12944. Generally constructions where design is handled leniently results in severe corrosion problems. As we have seen during this chapter, surfaces that are kept wet constantly will suffer from corrosion. Analogous is seen if a surface is covered with dirt and filth. These problems are related to design, and can in many cases be changed.

GALVANIC SERIES


©Teknologisk Institutt as www.teknologisk.no Surface preparation 06/13 Materials Technology 1

SURFACE PREPARATION and ABRASIVES



STEEL PREPARATION Prior to the actual surface preparation, steel surfaces must be thoroughly cleaned. This is important because a poorly surface prepared substrate will reduce coating lifetime, in worst case it will flake off.

Steel materials For structures we usually use carbon steel and stainless steel. Many carbon steels as well as other types of metals like Titanium, Copper, Aluminium and Nickel compounds may have satisfactory corrosion protection without coating, but in many cases we also paint or coat these types. Carbon steel will always corrode in uncontrolled environments and the mill scale from the hot rolled steel will start to flake of when exposed to weathering. All carbon steel will degrade. The ISO standard 8501-1 divides the steel into rust grades, depending on the amount of mill scale that has been removed and or how much the steel has corroded. For newbuilding of ships / offshore constructions a criteria listed in many specifications is the steel subject to surface preparation on site shall as a minimum requirement be rust grade B in accordance with ISO 8501-1. To protect the steel from degrading, a shop primer is often applied. Shop primers shall be regarded as temporary corrosion protection and shall be removed unless approved by the company. Often zinc ethyl silicate shop primers are approved for use below the specified system, somewhat depending on where and what kind of system is used.

Pre-blasting preparations In order to assure good quality of a coating all over, it is important that all sharp edges, fillets, corners and welds are rounded or smoothened by grinding (minimum radius = 2 mm). Failure to grind these areas will lead to failure and is one of the most common causes of defects. Hard surface layers e.g. resulting from flame cutting shall be removed by grinding prior to blast-cleaning. Any oil and grease shall be removed prior to blasting operation, in accordance with SSPC SP-1. Major surface defects, particularly surface laminations or scabs detrimental to the protective coating system shall be removed by suitable dressing. Where such areas have been exposed during blast-cleaning, and dressing has been performed, the dressed area shall be re-blasted to the specified standard. All welds shall be inspected and if necessary be repaired prior to final blast cleaning of the area. Surface pores, cavities etc. shall be removed by suitable dressing or weld repair.

Steel design Preferable openings, notches in structural steel should be so large in diameter that water and debris traps are voided. The dominating issue of corrosion is the wetting time. The longer the wetting time, the longer the time the corrosion process is talking place. Construction design should be



carried out in such a way that both possibilities of achieving the correct surface preparation initially and for maintenance are possible. Using the correct type of profiles is recommended. Using RHS and bulb profiles are preferred to I-beams and angle iron. Welds All (especially hand) welds should treated prior to painting in order for the paint to provide the appropriate protection. The welding work should be carried out in a proper way as to avoid porosity in the welds, undercuts and cracks. If located, such defects should be repaired. Grind welds smooth if uneven. Coating failure is very common along welds. Is it the welder or the painter’s responsibility to make sure that there is no weld spatter, porosity etc. present? This is not always obvious and problems are likely to arise if workers are not from the same company or if this responsibility is not clearly defined in the procedures. The welders and painters have their own opinion of how a weld shall appear prior to painting, but these views are not necessarily the same. Some welders will after welding leave behind both welding flux and weld spatter. In order to paint weld spatter must be chipped off / grinded and the area washed prior to painting. There is a standard recommended practice provided by NACE “ fabrication details, surface finish requirements and proper design considerations for tanks and vessels to be lined for immersion service” – NACE Standard RP0178-91 visualising 5 different weld preparations (A -E) on butt weld, fillet welded tee joint and lap weld. Provided with the NACE RP 0178-91 is also a visual comparator, which illustrates the various degrees of surface.



SURFACE PREPARATION The objective of designing a structure is to ensure that the structure is suitable for its function, has the necessary stability, strength and durability after corrosion protection As described earlier, sharp edges and corners should be rounded or smoothened by grinding (minimum radius of 2 mm). Hard layers (e.g. resulting from flame cutting) should also be removed by grinding prior blast-cleaning. Foreign matter on the surface like weld flux, residues, oil, grease and salt shall be removed prior to surface preparation. Areas where these contaminations have not been removed prior to painting will most likely be the first to suffer coating degradation and eventually corrosion.

Surface wetting Most people have seen how water “pearls” on a waxed or fatty surface. Water will make pearls on waxed surfaces on polished cars or oil on surfaces. The purpose of wax / oil is to reduce/avoid water accumulating on the surface. If we clean these surfaces well, i.e. we remove all wax and fat, and then apply water we will see that the water makes a continuous film. This is due to the surface tension and can be explained as follows: between all solids, liquids and gases in contact with each other, there are pulling forces. These forces are also present when we apply water on a waxed surface. However the forces between the molecules in the water are greater than between the water and the waxed surface. The surface tension of the water pulls the water film apart and the water has a higher surface tension than the waxed surface. In order to obtain good wetting, the surface tension of the substrate must be higher than our paint coating. When the wax has been removed from the surface the water “wets” the surface better. The surface tension of the substrate increases when we clean surfaces. This implies that the lower surface tension we have on our paint/coating, the better adhesion will be achieved for the coating. We want a clean surface so that our paint or coating will flow smoothly over the entire surface and adhere, or in other words we want our paint/coating to wet the surface.



Means of degreasing surfaces General A perfect clean surface is not always possible to achieve, especially under field conditions. During manufacture, fabrication and service, surfaces become dirty, often covered with soil and oil. It is often advisable that such objects are cleaned prior to surface preparation by blast-cleaning. The types of degreasing/cleaning agents available are usually divided into categories:

• Solvent cleaning • Emulsifying cleaning • Alkali cleaning • Water cleaning

Solvent cleaning Surface preparation specification no. 1 from the Steel Structures Painting Council (SSPC) in the USA covers solvent cleaning. SSPC-SP 1 “Solvent cleaning” is a method for removing all visible oil, grease, soil, drawing and cutting compounds, and other soluble contaminants from steel surfaces. It is intended that solvent cleaning be used prior to the application of paint and in conjunction with surface preparation methods specified for the removal of rust, mill scale or paint. In workshops, during dry-docking etc. it is common practice to use rags saturated with solvents used to wipe surfaces clean from oil and grease. If this is not done properly all you achieve is a smearing of the oil and grease onto the surface and making it penetrate even more, thus, the new paint system that might be applied has little or no adhesion to the steel or previous coat. It is very important to use plenty of rags and changing them often when using this method. Prior to solvent cleaning, foreign matter (other than grease and oil) should be removed by one or a combination of the following; brush with stiff fibres or wire brushes, abrade, scrape, or clean with solutions or appropriate cleaners, provided such cleaners are followed by a fresh water rinse. After solvent cleaning, remove dirt, dust and other contaminants from the surface prior to application of paint. Acceptable methods include brushing, blow off with clean, dry air or vacuum cleaning. If there are heavy oil and grease on the surface remove these using a scraper. Then use one of the following methods:

• Wipe or scrub with rags or brushes wetted with solvent. Use clean solvent and clean rags or brushes for the final wiping.

• Spray the solvent onto the surface. Use clean solvent for the final spraying. Due to environmental concern this method is often used only on small areas, e.g. spot repair.



Emulsifying solvents There are alternative methods of degreasing a surface that does not involve solvent cleaning. The most common one is to use emulsifying solvents - these are solvents that can be mixed with water. Common emulsifiers often smell like paraffin or white spirit. After the emulsifiers have been left on the surface for some minutes they must be removed using fresh water. The oil or grease has now been dissolved in the solvent and washed away with water. Best results will be obtained using hot water, or in combination with steam, but good results will also be achieved using cold fresh water.

• Emulsion or alkaline cleaners may be used in place of the methods described previously. After treatment, fresh water rinse or steam to remove residues.

• Steam clean, using detergents or cleaners and followed by steam or fresh water wash to remove residues.

Due to environmental concern alternative methods without the use of solvents are often used.

Chlorinated solvents Chlorinated solvents are generally used in vapour degreasing units for smaller parts in workshops. The items to be degreased are lowered into a specially designed cabinet with solvent. In the bottom of the solvent cabinets or tanks heating coils are used to make the solvent boil. In the top are cooling coils. When “cold” goods are lowered into the steam zone, the steam condenses on the surface, and both the fat and the solvents will run off. The surface then dries and “clean” products can after a while be removed from the cabinet. Chlorinated products such as tri-chlorethylene, per- chlorethylene, also known under names such as chlorotenes are used for vapour degreasing.

• Vapour degreasing using stabilised chlorinated hydrocarbons. • Immerse completely in a tank of solvent.

Due to environmental concern alternative methods without the use of solvents are often used.

Alkaline cleaning By far, this is the most commonly used method of degreasing today. Essentially it is “strong soap” that dissolves the oil and grease followed by thorough water rinsing to remove it. This method is “water based” and more environmentally friendly. When using alkaline cleaning not only will oil and fat be removed from the surface, but also other contamination like dirt on the steel surface. Water-soluble salts and some paints will also be removed. Alkaline cleaning can be carried out several ways:

• Dipping in an alkaline bath • Washing with alkaline solutions



Dipping The method is often used in factories where the steels are dipped into an alkaline bath. The most common types of baths contain sodium hydroxide (caustic soda) and are very alkaline with a pH around 13-14. When a fatty surface is lowered into the bath, the fat reacts with the alkaline and form soaps that dissolve in the water. The baths are often warm and agitated. Washing The alkaline degreasers are often chemical substances like hydroxides, phosphates and silicates dissolved in water. These solutions of contain surface reactive substances as well as wetting agents and emulsifiers. The primary task is to saponify the oil, fat and grease. These products react with the alkali and create soluble soap, which dissolves in the warm water.

Water cleaning Steam cleaning Steam cleaning, using a high pressure of steam, with or without cleaning compound is used to clean both painted and un-painted steel. Steam removes grease, oil and dirt by a utilizing hot water and impact. Using this procedure a stream of steam is directed under pressure through a cleaning gun or guns against the surface to be cleaned. The guns for cleaning may have changeable nozzles. High pressure washing High pressure washing will not be a guarantee that oil, grease and fat are removed. It is however a very good method of removing the emulsifying or alkaline treated surfaces.

Mechanical surface preparation After steel preparation and degreasing the next step is to prepare the steel for painting/coating. A lot of us have to some extent been involved in removal of rust and oil paint by different means. Equipment such as wire brushes, scrapers, needle pickers and rotating equipment are used. The equipment is quite heavy, tiresome and boring after a while. Hand tool and power tool cleaning are methods of preparing steel surfaces by the use of non-power hand tools and power assisted hand tools. These methods remove all loose mill scale, loose rust, loose paint, and other loose detrimental foreign matter. This process is not intended to remove adherent mill scale, rust and paint. Mill scale, rust and paint are considered adherent if they cannot be removed by lifting with a putty knife. Using the steel wire brush will never remove all the rust, perhaps 10 % at the most. For larger work pneumatic equipment may be required. All these types must be smeared with oil before use. Care must be taken so that the surface is not contaminated during the cleaning operation. There is a large range of equipment available; disk grinders, rotating wire brushes which will normally not damage the surface. However using rotating wire brushes for too long time can polish or burnish a surface so that it becomes too smooth for good paint adhesion due to loss of surface profile. Today special pads which have abrasives baked into them can be used so that a certain roughness can be achieved, for example using clean and strip wheels. The cleaning rate when using hand or power tool operated equipment is lower than for blast-cleaning.



Type

Tool Removes/prepares

Hand cleaning tools

Chipping Hammers Hand scrapers

Slag, laminated rust scale, old paint, rust

Hand wire brushes Sandpaper Plastic fleece with embedded abrasives

For final hand preparation including feathering of the edges,

Power tools

Rotary de-scalers Chipping hammers

Rough and laminated scale

Needle guns

Prepares weld, recessed work and fasteners

Sanding machines and discs Rotary wire brushes Abrasive coated paper wheels

Rust, rust scale and paint

Rotary finishing brushes with abrasives Power grinders

Smoothens welds, edges to give final finish

Table 1. Cleaning tools Rotary power tools are rapid cleaning equipment using different kinds of cleaning media for removal of rust and paint. The most common types are wire brushes, non-woven and woven abrasives.

Wire brushes Wire brushes are available in different shapes, wheel types or cup types and are generally used on equipment operated either by pneumatic or electric motors. Using wire brushes, old paint, rust scale, weld slag and dirt deposits can be removed.

Non-woven abrasives Non woven abrasives come in cup, wheel or disc form. Non-woven abrasives are advantageous in removing coatings, because less tension is added compared to coated abrasives. Using the non-woven abrasives, old paint, rust scale, weld slag and dirt deposits can be removed.



Coated abrasives Coated abrasives are used as discs or flap wheels to remove loose mill-scale, old paint and can also remove base metal.

Impact cleaning tools Impact cleaning tools are those often referred to as chipping and scaling hammers. The chisel (either for scraping/chipping) is operated by an internal piston, which strikes the surface. These tools are useful when there are heavy deposits of rust-scale, welding slag or thick old paint. A needle scaler or needle hammer operates as mentioned above, but with a bundle of steel needles housed in front of the piston so that they work into the surface. Needle hammers are primarily used on brittle and loose surface contaminations. Cleaning surfaces by means of scaling and chipping hammers is very slow, but for areas with heavy rust scale or paint formation it can still be the best and most economical method.

Flame cleaning Some years back it was quite common in many countries to use flame-cleaning for cleaning of steel. However the use of this process for cleaning steel has decreased and is rarely used today. This is mainly due to the great amount of steel that is centrifugal blast-cleaned and shop primed in plants. Previously the method was quite useful for cleaning larger surface areas outdoors for ship hulls made from un-primed steel. However the cleaning rate is quite low compared to blast-cleaning and the blast-cleaned surface is much cleaner. So as time passed by the blast-cleaning process has taken over most of the flame cleaning. The flame cleaning process was a thermal process - the flame and the heating of the surface did the “cleaning” of the surface. The method had the following advantages:

• Remove most of the mill-scale and the rust • Burn off fat and oils from the surface • Dry the surface • Ability to paint on a heated surface

The thermal cleaning process was carried out using a “burner” operating at a certain speed over the surface. During the process the mill-scale and the rust-scale expands more than the steel. The tension that is created cracks the scale and loosens it to some extent. Also smaller amounts of water trapped under the mill- and rust scale contribute to this when heated. The heating process by it self will not be able to remove very much mill-scale or rust-scale and the process always must be finished using power tool wire brushing. Thick layers of rust have to be chipped away.



Blast-cleaning Blast-cleaning is the common term for all the methods used where abrasives of different kinds are propelled onto a surface. For new building, this is the most common method of surface preparation. The different methods of blast-cleaning are:

• Compressed air abrasive blast-cleaning • Moisture-injection abrasive blast-cleaning • Wet abrasive blast-cleaning • Centrifugal abrasive blast-cleaning • Water blasting - water jetting

Compressed air abrasive blast-cleaning By this method the abrasive is propelled by means of compressed air. Normally when this type of blast-cleaning is done in the open air, abrasives which are not recycled is used. Using more expensive abrasives such as aluminium-oxide or steel grit / shot, the abrasives will be re-used. They are recycled in a system where dust and contamination is removed. The abrasive may be injected into the air stream from a pressurised container or may be drawn into the air stream by suction from an un-pressurised container. Normal pressures when blast-cleaning is in the range from 7 to 10 bars. The pressure at the nozzle is off course dependant on the compressor size and the hose length. In order to check the blast-cleaning air pressure a hypodermic needle on a pressure gage can be inserted through the blast hose just before the nozzle and the pressure read from the dial.

Moisture-injection abrasive blast-cleaning This method is similar to compressed-air abrasive blast-cleaning, but with addition of a very small amount of liquid (usually clean fresh water) to the air/abrasive stream before the nozzle, resulting less dust. The water consumption can be controlled and is usually 15 liter/hour to 25 liter/hour. Using this method may make the abrasive stick onto the steel, making it difficult to remove. The remedy is to remove by fresh water cleaning. The use of water, to reduce the dust, will create flash-rusting of the surface. This might be accepted for certain types of paints. To decrease flash rust, a suitable rust-inhibitor may be added to the water provided this is approved by the paint supplier.

Wet abrasive blast-cleaning The method is similar to compressed air abrasive blast-cleaning, but with the addition of a liquid (generally clean fresh water) before or after the nozzle to produce a stream comprising air, water and abrasive. This method will create flash rusting of the surface. If this is not acceptable, a suitable rust-inhibitor may be added to the water.



Equipment The common equipment for blast cleaning is:

• Compressor of suitable size • Pressure tank or pot (contains the abrasive) • Air hoses • Abrasive hoses / nozzles • Moisture and oil separators • Dead-mans handle

Pressure pot In many cases the pressure tank or pot is portable. Prior to blast-cleaning the operator must be equipped with suitable safety equipment. When he closes the dead-mans valve the pressure builds up inside the tank causing the pop-up valve to close. The pressure in the tank and in the abrasive hose is equalised. The abrasive falls down into the abrasive valve and is transported through the hose to the nozzle. In the nozzle, the abrasive-air mixture is accelerated and propelled onto the steel surface. The pressure tanks or pots can be of various sizes from approximately 50 litres up to 200 litres. In order to use the equipment a sufficiently large compressor is needed. The size of the compressor will depend on several factors such as the required pressure during the operation and how many outlets will be used at the same time. The abrasive valve There are different types of abrasive valves available, but the most common one used is the Miser valve. Here two plates fitted with holes lie on top of each other and the opening can be regulated as required. Often, but not always, an assistant will assist the operator during the blasting i.e. adjusting the abrasive valve and filling of the abrasive. Hoses and connections The transportation of the abrasive / air mixture from the abrasive valve to the nozzle takes place through the blasting hose. On its way there is a certain pressure loss - increasing with the length of the hose. How large the pressure loss is can be measured by injecting a small needle manometer into the abrasive hose when the abrasive valve is closed. The hoses are normally made from rubber tubing with ¼ -inch thick walls rubber tube with carbon black compounding to earth the static electricity generated by the abrasive flow in the hose. The hoses are also equipped with additional grounding wire between the rubber and the outside ply. The hoses will often be of a length of 30 m, and therefore couplings must be used in order to achieve longer lengths. The couplings used nowadays are always of the exterior type and secured with screws to the hose. Nozzles The nozzles are continually exposed to abrasives. They are often made from tungsten or boron carbide with a life of around 200 - 300 hours. The carbides are very brittle, and in order to absorb some of the shock, the nozzle is fitted with a lead lining and then covered with plastic.



Today mainly venturi nozzles are used. They are conical inside and this increases the speed of the abrasive out of the nozzle from approximately 300 to 700 km/hour at a pressure of 7 bars. Nozzles are available in various sizes, lengths and shapes depending on the objects that are to be cleaned. The blasting pattern from venturi nozzles covers a larger area with a more even pattern than a straight bore nozzle of the same size.

Dead mans handle or valve This handle will ensure the operator that he at all times has control of immediately stopping the blasting operation if something unexpected should happen. By releasing the dead mans handle, the pressure in the tank decreases to zero. The response time can by quite long depending on whether the mechanisms used are pneumatic or electro-pneumatic.

Centrifugal abrasive blast-cleaning Centrifugal abrasive blast-cleaning is carried out in factories where the abrasive is fed to rotating wheels or impellers positioned to throw the abrasive evenly and at high velocity on to the surfaces to be cleaned. The kinetic energy gives the desired effect. Most centrifugal abrasive blast-cleaning units are stationary and the abrasive is circulated in a closed system. The abrasive is most often shot. Some equipment models may be delivered mobile, and therefore useful for cleaning large uninterrupted surfaces, such as ship hulls and oil storage tanks. The method is suitable for continuous operation on structures with accessible surfaces, such as plates, plate girders, castings or rolling-mill products. Centrifugal blast cleaning can be used for all rust grades.

Ultra high pressure water (UHP WJ) Using water at very high pressure is a common way of pre-treatment for maintenance. The water has sufficiently high pressure so that no abrasive is necessary. ISO 8501-4 defines the following pressures:

Venturi nozzle:

Straight nozzle:



• Low-pressure water cleaning (LP WC) Cleaning performed at pressures less than 34 MPa (340 bar) or 5000 psi

• High-pressure water cleaning (HP WC) Cleaning performed at pressures from 34 to 70

MPa (340 – 700 bar) or 5000 - 10000 psi

• High-pressure water jetting (HP WJ) Cleaning performed at pressures from 70 to 200 MPa (700 – 2000 bar) or 10000 - 50000 psi

• Ultrahigh-pressure water jetting (UHP WJ) Cleaning performed at pressures above 200

MPa (2000 bar) or 25000 psi Low-pressure and high-pressure water cleaning at pressures less than 70 MPa removes loose rust, debris and material in depressions and pits, but black oxide (magnetite) remains. High-pressure water jetting at pressures of 70 MPa (10000 Psi) will produce a uniform matte finish which will quickly flash rust unless inhibitors or environmental control is carried out. Black oxides (magnetite) are slowly removed. At pressures of 140 MPa (20000 Psi), a uniform matte finish is obtained that will flash rust quickly unless inhibitors or environmental control is carried out. Black oxides (magnetite), paint, elastomeric coatings, enamel, red oxide and polypropylene sheet lining are removed. Generally chemical contaminants will be removed with varying degrees of effectiveness. At pressures of 235 to 250 MPa (34000 to 36000 Psi), a uniform matte finish is obtained that will flash rust quickly unless inhibitors or environmental control is carried out. Surface material, including most mill scale, is removed from the base metal. Extremely well bonded mill scale may require additional time spent in localised jetting. When an operator carries out this process the amounts of water must be quite low. Otherwise the recoil will be too high for the operator to withstand over a longer period of time. Flash rust can be a problem for paints/coatings and special surface tolerant coatings have been used e.g. surface tolerant epoxy. The process will not result in any additional roughness in the steel i.e. the steel must have the desired roughness in advance (have at some time been blast cleaned), and so far the process has been in use more or less only in connection with maintenance painting. This results in a minimum of problems in connection with the job, due to the fact that no dust is created. At high pressures very small amounts of water are used, maybe only up to 15 -20 litres pr. minute. However the cleanliness of the steel is very good. The water removes all salts and the energy created using water, provides a temperature rise of the steel so that dries it quickly. The operating distance from the object using ultra-high pressure water jetting is small, preferably the nozzle is held 6 - 13 mm from the surface. At distances further apart the cleaning rate will be



much lower. The cleaning rate pr. hour is dependent on the original condition of the coating. Special nozzles that circulate are used.



ABRASIVES A wide variety of both natural and synthetic solid materials and in some cases fluids are used for abrasive blast cleaning. The most common abrasives used in connection with abrasive blast cleaning are divided into two main groups, metallic and non-metallic (mineral) abrasives. The abrasives have three different shapes.

• Grit - angular, irregular (G) • Shot - round (S) • Cylindrical - sharp edged (C)

When selecting an abrasive consider the following abrasive characteristics.

• Particle size Size will influence both the surface profile and productivity. Using as small an abrasive as possible is a good start, but small abrasives may not be able to remove scale and heavy coatings. Coarser abrasives will do this job, but usually leave a less uniform and deeper surface profile. The influence of a particular size on the resulting surface profile is normally greater for metallic abrasives than for non-metallic abrasives. This is because the shatter characteristics differ and because differences in density affect the kinetic energy of the abrasive particles. A balanced mixture of particle sizes may produce the optimum level of cleanliness, cleaning rate and surface profile.

• Hardness As mentioned, hard abrasives (metallic) may shatter on impact and can create a lot of dust. By choosing a softer abrasive, more energy will be transmitted on the surface. Very soft abrasives like sodium bicarbonate, walnut shells, corn cobs, dry ice can be used for surfaces “freshening up” brick or removing coatings on such surfaces.

• Shape The most effective abrasives are usually angular (grit) and they will create a surface profile that is not as uniform, but which will provide a nice anchor pattern for coating adhesion. Using rounded (shot) is more common for removing mill scale or rust, but can also be used to remove coating (e.g. automatic blast cleaning machines like Blastrac).

• Density Higher density will as a rule be more effective and give the best production rate. The high density will transfer more energy to the surface.

In blast cleaning plants where the abrasive is recycled, it is necessary to remove dust and contaminants before the abrasive is re-used and to make up for the abrasive which is lost by wear and adherence to the steel. This is done by adding new abrasive so that the abrasive mixture is maintained within prescribed particle size limits or particle size distribution.



Metallic abrasives These abrasives have a long service life because their particles can resist a large number of impacts before their size is reduced and the abrasive must be discarded. These abrasives are expensive and are more or less used only in installations where they can be re-circulated. Metallic abrasives generally clean more effectively than non-metallic abrasives, particularly on new steel covered by hard, tight mill scale. There are several kinds of metallic abrasives available the most common ones are listed in table 1. Chilled iron grit is the hardest, but also breaks down faster, due to its own brittleness. Steel shot or grit is the most common types. There is limited use of wire cut.

Non-metallic (mineral) abrasives In general, non-metallic abrasives are commonly used for open air-blast cleaning of steel. Most of these abrasives are inexpensive. Their shape is angular with some exceptions (e.g. glass beads). Most of them have a short life i.e. they are usually only used once. Due to health hazards laws in many countries forbids because of the risk of silicosis, the use of silica containing abrasives (especially quartz sand). There are alternatives to quartz sand, e.g. olivine. As shown in Table 1, currently 9 different non-metallic abrasives are standardised in the International Standard ISO 11126. Many abrasives used are slag from metal production e.g. nickel refinery slag, copper refinery slag, iron furnace slag and coal furnace slag. There must be no free metal particles in the slag that can act as sources for corrosion on the blast-cleaned surface and should not contain heavy metals. Even though the name implies that the slag is metallic, they are not, rather oxides from by-products of metal production. Also in certain cases where removal of oxides from aluminium or stainless steels, or where little or no roughness is recommended prior to painting, other types of abrasives than sand or slag may be used, such as crushed nut shells, glass beads, plastics, sodium bicarbonate. For blast cleaning of these metals or hot dip galvanised surfaces only non-metallic abrasives should be used.

Non metallic abrasives Silica sand Silica sand is a hard and low cost abrasive, which is available more or less everywhere in the world. It might not be available in a wide range of particle sizes and there is also a health risk for workers. Silicosis is a chronic lung disease caused by breathing silica dust, which is created upon using the sand for blast-cleaning. In many countries world-wide the use of silica sand is prohibited due to the health reasons mentioned. The use of water will reduce the amounts of dust created, but not below the maximum exposure limit set by the authorities in many countries. The Mohs hardness is approximately 5-6, and it creates light coloured dust upon impact.



Olivine Olivine sand is natural occurring abrasive. Olivine sand is a pale green, silica free abrasive. It is a silicate of iron and magnesium. It has a Mohs hardness of approximately 7, creating a light coloured dust upon impact. Staurolite Staurolite is a common substitute in the North American market for silica sand. It’s a silicate of iron and aluminium. It has a Mohs hardness of approximately 7.5. Staurolite is a dark coloured mineral; the free silica content is lower than the limit set by the authorities. Garnet Garnet is a hard silicate mineral found several places; Australia, India, USA and South Africa and is used extensively around the world. The abrasive has high cleaning efficiency, which results in less use of abrasive. Garnet can also be recycled several times, and is a less expensive alternative to aluminium oxide. The mineral has a Mohs hardness of 7-8 and can be used both for ferrous and non-ferrous substrates. Iron furnace slag Iron furnace slag is a by-product of iron melting. Finer grades of this slag leave the substrate with a light colour. Copper refinery slag This abrasive is also referred to as iron-silicate and is a widely used synthetic abrasive. Copper refinery slag is available in many countries as a by-product from melting of copper. The abrasive is available as a dark coloured slag, more or less black in colour. Upon blast-cleaning the abrasive leaves the substrate with a dark colour. There should be little or no free copper in the abrasive. Nickel refinery slag This abrasive is also an iron-silicate and is a widely used synthetic abrasive. The abrasive is more or less black in colour. After use the substrate has a dark colour. Coal furnace slag This abrasive is often referred to as aluminium silicate, which is a by-product of coal burning power stations. The colour of the abrasive is fairly light brown and the abrasive is widely used on steel. Fused aluminium oxide There are several types available, ranging from brown to pink or white in colour. Aluminium oxide is a very common abrasive used for blast cleaning. The abrasive can be expensive and is often recycled and used in small cabinets. The abrasive is very hard with a Mohs hardness of 9.0 - 9.2. The blasting pressure may be lowered to 3 - 4 Kp/cm2 when using this abrasive.



Type Initial

particle shape

Comparator 1) Remarks

Metallic blast-cleaning

abrasives

Cast iron Chilled G2) G Mainly for compressed air blast-cleaning

Cast iron High-carbon S or G S

Mainly for centrifugal blast-cleaning

Low-carbon S2) S Cut steel wire - C2) S

Non-metallic blast-cleaning

abrasives

Natural

Silica sand G G Mainly for compressed air blast-cleaning

Olivine sand Staurolite S G Garnet G G

Synthetic

Iron furnace slag (Calcium silicate slag)

G G Mainly for compressed air blast-cleaning

Copper refinery slag (Ferrous silicate

slag) Nickel re-finery slag

Coal furnace slag (Aluminium silicate slag)

Fused aluminium oxide G G - 1) Comparator to be used when assessing the resultant surface profile. The method for evaluating the surface profile by comparator is described in ISO 8503-2. 2) Main shapes G angular S rounded C cylindrical Special hardness of the steel can be ordered prolonging the usability. As the abrasives are re-used they will be rounded and become more spherical, the appearance of the surface profile changes and becomes closer to that of the "shot" comparator. Table 2 - Commonly used blast-cleaning abrasives for steel substrate preparation

Checking for contaminated air or abrasive To obtain the best possible surface treatment result the pre-treatment must be done correctly. It is also very important to check that the abrasive or compressed air is not contaminated. If the compressed air or the abrasive is contaminated with oil or grease there is a risk of surface contamination. A thin film of oil or grease will be imbedded in the steel profile, and will likely impair coating adhesion. Salt on a steel surface may lead to osmotic blistering, and checking the abrasives for salts is important.

Checking abrasives and compressed air for oil or grease Abrasives Fill a jar with abrasive, shake well and let stand for 30 minutes. If oil or grease is present it will float to the surface and you will visually be able to see this shiny liquid. Compressed air Let the compressed air run through a white rag and observe any deposits. It is important that there is no humidity in the air, and this can be checked in the same manner at the same time.



Checking abrasives and compressed air for salts (chlorides) Mix equal amount of water and abrasives in a jar. Shake well and let stand for 1 hour. Filter off the water and measure conductivity. High conductivity indicates salt contamination.


©Teknologisk Institutt as www.teknologisk.no Important standards for surface treatment 6/13 Materials Technology

IMPORTANT STANDARDS FOR

SURFACE TREATMENT



Surface cleanliness

ISO 8501-1:2007 Preparation of steel substrates before application of paints and related products - Visual assessment of surface cleanliness

Part 1 Rust grades and preparation grades of uncoated steel substrates and of steel substrates after overall removal of previous coatings.

The standard has four levels of mill scale and rust (designated as "rust grades") that are commonly found on surfaces of uncoated steel. It also identifies degrees of cleanliness (designated as "preparation grades") after surface preparation of uncoated steel surfaces and steel surfaces after overall removal of any previous coating. The various grades are defined, and together with photographs the rust and preparation grade is found. 8501-1is applicable to hot-rolled steel surfaces for painting by methods such as blast-cleaning, hand and power tool cleaning, and flame cleaning. Rustgrades Preparation grades Blast-cleaning Hand tool and power tool cleaning Flame cleaning

A Sa 2½ A Sa 3

B Sa 1 B Sa 2

B Sa 2½ B Sa 3

C Sa 1 C Sa 2

C Sa 2½ C Sa 3

D Sa 1 D Sa 2

D Sa 2½ D Sa 3

B St 2 B St 3

C St 2 C St 3

D St 2 D St 3

A Fl B Fl

C Fl D Fl

A B

C D



Representative photographic examples of the change of appearance imparted to steel when blast cleaned with different abrasives. The last picture in the standard (or supplement 1 of earlier versions) provides representative photographic examples of the colour changes imparted to rust grade C steel that is dry blast-cleaned to preparation grade Sa 3 with different metallic and non-metallic abrasives.

Original steel plate, rust grade C High carbon cast-steel shot, Grade S 100, Vickers hardness 390 HV to 530 HV Steel grit, Grade G 070, Vickers hardness 390 HV to 530 HV Steel grit, Grade G 070, Vickers hardness 700 HV to 950 HV Chilled-iron grit, Grade G 070 Copper refinery slag Coal furnace slag

ISO 8501-2 Preparation of steel substrates before application of paints and related products - Visual assessment of surface cleanliness

Part 2 Preparation grades of previously coated steel substrates after localised removal of previous coatings

This standard specifies a series of preparation grades for steel surfaces after local removal of previous coatings (spot repairs). The various preparation grades are defined by written descriptions and before and after photographs. Photographs showing examples of preparation grades P Sa 2 ½ and P Ma are given. The standard is applicable to surfaces prepared by methods such as blast-cleaning, hand-and power tool cleaning, and machine abrading.



ISO 8501-3 Preparation of steel substrates before application of paints and related products – Visual assessment for surface cleanliness

Part 3Preparation grades of welds, edges and other areas with surface imperfections This part describes preparation grades of welds, edges and other areas, with steel surfaces with imperfections. Such imperfections can become visible before and/or after an abrasive blast-cleaning process. Three preparation grades for making steel surfaces with visible imperfections suitable for the application of paints and related products are as follows: P1 Light preparation; No preparation or only minimum preparation carried out before application of paints P2 Thorough preparation; Most imperfections are removed P3 Very thorough preparation; Surface is free of significant visual imperfections


Part 4 Initial surface conditions, preparation grades and flash rust grades in connection with high-pressure water jetting

This part of ISO 8501 specifies a series of preparation grades of steel surfaces after removal of water-soluble contaminants, rust and paint coatings and foreign matter by high-pressure water jetting. The various grades are defined by written descriptions together with photographs that are representative examples within the tolerance within each grade as described in words. 5 initial conditions are described:

DC A, DC B, DC C, DP I, DP Z.

There are 3 cleaning grades designated: Wa1, Wa2, Wa 2 ½.

The last picture gives flash rust grades: Low, medium and high.



Surface contaminants

ISO 8502 Preparation of steel substrates before application of paints and related products - Tests for assessment of surface cleanliness

Part 3 Assessment of dust on steel surfaces prepared for painting (pressure sensitive tape method) This part of ISO 8502 describes a procedure for the assessment, using a pressure-sensitive tape method, of the quantity and the particle-size of dust on steel surfaces prepared for painting.

Part 4 Guidance on the estimation of the probability of condensation prior to paint application The standard gives guidance on the estimation of the probability of condensation on a surface to be painted. It may be used to establish whether conditions at the job site area are suitable for painting or not. Use either electric or sling psychrometer and determine RH and dew point. Steel should be at least 3 ˚C above the dew point.

Part 6 Sampling of soluble impurities on surfaces to be painted - The Bresle method The standard describes a method of extracting, for analysis, soluble contaminants from a surface by use of flexible cells in the form of adhesive patches which will adhere to any surface, regardless of its shape (flat or curved) or if it is horizontal or vertical. The method is suitable for use in the field to determine the presence of soluble salt contaminants before painting, pre-treatment or other treatment.

Part 9 Field method for the conductometric determination of water-soluble salts This standard describes a field method for the assessment of the total surface density of various water-soluble salts (chlorides) on steel surfaces before and/or after surface preparation by measuring conductivity.



Surface roughness

ISO 8503 Preparation of steel substrates before application of paints and related products - Surface roughness characteristics of blast-cleaned steel substrates

Part 1 Specifications and definitions for ISO surface profile comparators for the assessment of abrasive blast-cleaned surfaces

This part of the ISO 8503 specifies requirements for ISO surface profile comparators which are intended for visual and tactile comparison of steel surfaces that have been blast-cleaned with either shot or grit abrasives. ISO surface profile comparators are for use in assessing, on site, the roughness of surfaces before the application of paints and related products or other protective treatments.

Part 2 Method for the grading of surface profile of blast-cleaned steel - Comparator procedure This part of the ISO 8503 describes a visual and tactile method for assessing the grade of the profile that has been produced by one of the abrasive blast-cleaning procedures described in ISO 8504-2. The method uses ISO surface profile comparators for assessment, on site, the roughness of surfaces before the application of paint or other protective treatments.

Part 3 Method for the calibration of ISO surface profile comparators and for the determination of surface profile - Focusing microscope method The standard specifies the focusing microscope and describes the procedure for calibrating ISO surface profile comparators complying with the requirements of ISO 8503-1.

Part 4 Method for the calibration of ISO surface profile comparators and for the determination of surface profile - Stylus instrument method The standard specifies the stylus instrument and describes the procedure for calibrating ISO surface profile comparators complying with the requirements of ISO 8503-1.



Surface preparation methods

ISO 8504 Preparation of steel substrates before application of paints and related products - Surface preparation methods

Part 1 General principles This part of ISO 8504 describes the general principles for the selection of methods for the preparation of steel surfaces before coating with paints and related products. It also contains information on features that must be taken into account before certain surface preparation methods and preparation grades are selected and specified.

Part 2 Abrasive blast-cleaning The standard describes abrasive blast-cleaning methods. It also contains information on the effectiveness of the individual methods and their fields of application.

Part 3 Hand- and power tool cleaning The standard describes methods for hand-tool and power-tool cleaning of steel substrates. It applies both to new steelwork and to steel surfaces that have been coated previously and that show areas of breakdown requiring maintenance painting. It describes the equipment to be used and the procedure to be followed.

Film thickness

ISO 2808 Paints and varnishes - Determination of film thickness The standard reviews and specifies a number of methods that are applicable to the measurement of the thickness of organic coatings. The standard covers both measurement of wet and dry film thickness.

ISO 19840:2004 Paints and varnishes - Corrosion protection of steel structures by protective paint systems -- Measurement of, and acceptance criteria for, the thickness of dry films on rough surfaces

The standard explains requirements for dry film measurements including correction factor and sampling plan. Also acceptance criteria for measurements below specified dry film thickness. To account for surface roughness a correction value is subtracted, depending on the surface roughness (according to ISO 8503)



Adhesion

ISO 2409 Paints and varnishes - Cross cut test The standard specifies a test method for assessing resistance of paint coatings to separation from substrates when a right-angle lattice pattern is cut into the coating penetrating to the substrate. The test mat be carried out on finished objects and / on specially prepared test specimen.

ISO 16276-2:2007 Corrosion protection of steel structures by protective paint systems – Assessment of, and acceptance criteria for, the adhesion/cohesion (fracture strength) of a coating -- Part 2: Cross-cut testing and X-cut testing The standard contains pictorial references for assessing the adhesion, gives sampling plan and acceptance criteria. Explains both x-cut and cross-cut adhesion.

ISO 4624 Paints and varnishes - Pull off test for adhesion This standard specifies methods for carrying out a pull-off test on a single coating or multi-coat system of paint, varnish or related product. The test result is the minimum tensile stress necessary to break the weakest interface (adhesive failure) or the weakest component (cohesive failure) of the test assembly. Mixed adhesive/cohesive failures may also occur.

ISO 16276-2:2007 Corrosion protection of steel structures by protective paint systems – Assessment of, and acceptance criteria for, the adhesion/cohesion (fracture strength) of a coating -- Part 1: Pull- off testing The standard contains pictorial references for assessing the adhesion, gives sampling plan and acceptance criteria.



Holiday detection

ASTM D 5162 Standard recommended practice - Discontinuity (Holiday) testing of non-conductive protective coating on metallic substrates

This standard covers procedures for determining pores and discontinuities using two types of test equipment. Test method A - Low voltage wet sponge (up to 500 µm) Test method B - High voltage spark tester (above 500 µm).

NACE RP 0188 Standard recommended practice - Discontinuity (Holiday) testing of protective coatings Similar to ASTM D 5162.

MEK tests of inorganic zinc-rich paints

ASTM D 4752 Standard test method for: Measuring MEK resistance of ethyl silicate (inorganic) zinc-rich primers by solvent rub This test method describes a solvent rub technique for assessing if ethyl silicate (inorganic) zinc-rich primers have cured. The technique can be used in the laboratory, field or in the fabricating shop.



Evaluation of degradation of paint coatings

ISO 4628 Paints and varnishes - Evaluation of degradation of paint coatings- designation of intensity, quantity and size of common type of defects

Part 1 General principles and rating schemes This part of the standard is intended to establish a general system for designating the intensity, quantity and size of common types of defects of paint coatings and outlines the basic principles of the system in respect of designation of the intensity, quantity and size of defects, in particular the defects caused by ageing and weathering.

Part 2 Designation of degree of blistering This part of ISO 4628 provides pictorial standards for designating the degree of blistering of paint coatings by use of pictorial references.

Part 3 Designation of degree of rusting This part of ISO 4628 provides pictorial standards for designating the degree of rusting of paint coatings by use of pictorial references.

Part 4 Designation of degree of cracking This part of ISO 4628 provides pictorial standards for designating the degree of cracking of paint coatings by use of pictorial references.

Part 5 Designation of degree of flaking This part of ISO 4628 provides pictorial standards for designating the degree of flaking of paint coatings by use of pictorial references.

Part 6 Designation of degree of chalking This part of ISO 4628 provides pictorial standards for designating the degree of chalking of paint coatings by use of pictorial references.



Corrosion protection of steel structures by protective paint systems

ISO 12944 Paints and varnishes - Corrosion protection of steel structures by protective paint systems

Part 1 General introduction

Part 2 Classification of environments

Part 3 Design considerations

Part 4 Types of surface and surface preparation

Part 5 Protective paint systems (examples)

Part 6 Laboratory performance test methods

Part 7 Execution and supervision of paint work

Part 8 Development of specifications for new work and maintenance


©Teknologisk Institutt as www.teknologisk.no Paints and paint systems 6/13 Materials Technology 1

PAINT AND PAINT SYSTEMS



PAINTS AND PAINT SYSTEMS

Paint composition Paint is composed of several components (raw materials). The main components of paint are:

• Binder(s) • Pigment(s) • Filler/Extender(s) • Solvent(s) • Additive(s)

Many paints can have a combination and several of the main components, typically 7-15 raw materials. Often 1 binder, 1-3 pigments, 1-3 fillers, 2-3 solvents and 2-5 additives. Most paints are mixed in a high speed dissolver. Some pigment requires special mills to decrease the particle size to less than 10 µm.

Binder The binder is the most important component in paints. It provides the main properties, including strength, curing/drying properties, adhesion, weathering properties etc. It “binds” pigments and filler together to provide a uniform film. The binders can be organic, inorganic or a combination, but does not necessarily have to be the largest component. The term generic paint refers to the type of binder. Examples of binders are:

• Acrylic • Vinyl • Epoxy • Polyurethane • Polysiloxane • Latex • Chlorinated rubber • Silicate

Pigments We can divide pigments into two categories:

1. Decorative The decorative pigments give the desired colour and opacity. The opacity is often referred to as the hiding power, the paints ability to reflect light and give a uniform colour.



Some pigments are better than other with regards to UV radiation and fading (discolouration). Pigments are an expensive raw material, specially the bright organic colours. Examples of decorative pigments are:

• Titanium dioxide - white • Iron oxides - red, yellow, black • Organic - bright colours of red, yellow, green, blue (often

Phthalocyanines)

2. Anti-corrosion Anti-corrosion pigments are used to increase the paints corrosion resistance properties. The pigments can also give colour and opacity, depending on the type e.g. aluminium and zinc. We often use metal pigments that are less noble than steel for corrosion protection. These types of pigments are used as primers because they require metal contact. They will work in a similar matter as an anode (it will sacrifice itself and protect the steel), but the metal is spread on the surface as paint. Example of this type of pigment is:

• Zinc

Laminar pigments are used in paints to make them more impermeable. When paint is impermeable it is very dense (watertight) and it is difficult for water, oxygen and corrosive ions to penetrate through it. After application of the paint, the pigments will orient themselves parallel to the surface and overlap, similar to bricks:

Examples of laminar pigments are:

• Micaceous Iron Oxides (MIO) • Aluminium flakes • Glass flakes



Corrosion inhibiting pigments provide corrosion protection by inhibiting the steel. These pigments are usually permeable and when humidity enters the film a reaction occurs between the pigment and steel surface to create a passive layer on the steel. Red lead and zinc chromate type pigments were used for many years and gave excellent corrosion protection, but due to environmental concerns they are rarely used today. Even though they are not used today, we often refurbish structures with the types of systems which require special waste management… Today we use more environmental friendly inhibiting pigments, e.g.:

• Zinc phosphate • Other synthetic types

These pigments have no opacity, even though they usually look white.

Fillers/Extenders Fillers or extenders are usually minerals used as finely grained powders in the paint. They provide the paint with more “body” (consistency) without breaking down the properties of the paint; they are cheap and keep the price of the paint down. The powders are often white or grey, but will usually not provide any hiding power (opacity). Sometimes this component is liquid and we then usually call them extenders. Typically these are tar modification or tar replacement modification (hydrocarbon resins) type products Primers usually have a high amount of fillers and are matt (the surface becomes uneven). Glossy paint on the other hand have little fillers and have high gloss (the surface is smooth). Examples of fillers

• Oxides, mainly silicas • Calcium carbonates (chalk) • Aluminium silicates (china clay, mica) • Magnesium silicates (talcum) • Calcium magnesium carbonate • Barium sulphate (very inert and used for chemical resistance, but expensive)

Solvents The main reason for solvents in paint is to dissolve the binder. If the binder is solid or has high viscosity a solvent is necessary to reduce the viscosity so that we are able to apply the paint. This is more effective if the solvent gives better solvability of the binder. For environmental purposes we like the solvent content as low as possible. Globally we have VOC regulations (volatile organic compounds) and solvents are the main contributor to emissions in paint.



Examples of solvents: • white spirit • xylene • toluene • methyl-ethyl-ketone (MEK) • butyl acetate • Propylene Glycol Methyl Ether • (water in some cases)

The paint manufacturer chooses the solvents for production, and some solvent are more soluble in the binder than others. When we apply the paint, we reduce the viscosity with a thinner. A thinner can be the same as the solvent or a blend of one or several solvents. A thinner usually, but does not have to, dissolve the binder. The blend of thinners can also be a combination of quick and slow evaporating solvents which is important depending on the application temperature. If the temperature is low, we prefer quick evaporating thinners, and if the temperature is high we prefer slow evaporating thinners. The solvents and thinners should evaporate at a rate giving the paint time to flow to an even film, but avoiding any retained solvent. The solvent/thinners must evaporate out of the paint film. A paint film with trapped solvents will become porous and full of voids, thus, the corrosion preventing properties will be reduced. Solvents can be aliphatic (e.g. white spirit) or aromatic (e.g. xylene). This has to do with their chemical structure, aliphatic is linear and aromatic contains a benzene ring. Definitions according to ISO 4618; Vocabulary - Part 1: General terms

• solvent A liquid, single or blended, volatile under normal drying conditions, and in which the binder is completely soluble

• diluent A volatile liquid, single or blended, which, while not a solvent for the non-volatile constituents of a paint or varnish, may be used in conjunction with the solvent without causing any deleterious effects

• thinner A volatile liquid, single or blended, added to a product to lower the viscosity

Additives Additives are different chemicals used to provide or improve a certain property. Most paint use additives to give improved properties, and typically there is between 2 and 5 additives in paint. The dosage is usually small, e.g. between 0-1 %. The additives may improve or provide the following properties, among others:

• air release • catalyst • pigment wetting • driers • surface wetting • surface flow • anti sag • adhesion promotion • thickening • viscosity reduction



HOW DOES A PAINT PROTECT THE SURFACE Corrosion preventing paints will protect steel by three basic principles:

• Barrier protection - reduce ability of water and oxygen to reach the surface • Inhibitive protection - passivate the steel surface • Cathodic protection - sacrifice themselves to protect the steel

Barrier systems use paints that are as impermeable (watertight) as possible. This method tries to prevent oxygen, salt, humidity, ions from passing through the film and reaching the substrate. Even though we want it as impermeable as possible, no paint is completely impermeable and humidity will move in and out of the paint film. Barrier protection is improved by the use of laminar pigments, e.g. MIO, aluminium or glass flakes. Inhibitive system contains pigments that react with the surface of the steel and passivates or inhibits the steel by building up a corrosion protective film. Cathodic protective systems rely on metallic pigments that are less noble than steel. The pigments are in the primer and when the system is damaged and bare steel exposed, the cathodic protective paint will send energy to the steel to protect it. As long as there is enough pigment no corrosion will occur, but when the pigment has been used, corrosion will start. The principles can be used by themselves or in a combination. Typically they are combined, e.g.:

Primer zinc epoxy which gives cathodic protection Midcoat epoxy mastic with laminar pigment which gives barrier protection Topcoat polyurethane which gives barrier protection.



MISSION OF EACH COAT IN A SYSTEM A paint system is typically based on 3 coats, primer, intermediate or midcoat and topcoat.

Primer The primer for the steel must provide good adhesion to the substrate. In addition it often has anti-corrosive pigments. It must form a good basis for the intermediate coat. Different binder may give different adhesion properties and require different surface preparation. Today, epoxy primers are widely used, e.g. zinc epoxy which gives excellent adhesion and excellent corrosion protection. Typical film thickness is 50-75 µm.

Intermediate coat For a paint system to perform well we need minimum film thickness. The film thickness is built up in the mid- or intermediate coat. In this coat we will increase impermeability and often laminar pigments are used. The easiest way of building thickness is with a high solids paint (this will require fewer coats to obtain the same thickness compared with a paint with low solids). Epoxy systems are widely used as an intermediate paint in a heavy duty paint system. A film thickness of 150-200 µm is normal.

Topcoat The top coat or the finish coat provides the paint system with the desired colour and gloss. This coat should withstand radiation from the sun, rain and chemicals. The most widely used topcoat is polyurethane for heavy duty corrosion protection. These products have proven performance over many years and gives good stability against weather, sun radiation and chemicals. Due to their content of isocyanates they are considered hazardous in many parts of the world and special care must be taken. Alternatives to polyurethanes are acrylics and polysiloxanes. Topcoat are usually the most expensive coats and typically only 50-75 µm are applied.



PAINTS Paints can be divided into three groups according to their drying or curing mechanism;

• physical drying paints • oxidative drying paints • chemical curing paints.

Physical drying paints The drying process of this type of paint consists exclusively of the evaporation of solvents and thinners. Many of the solvents are very volatile making the drying process quick. The evaporation is quick at high temperatures, and consequently, slower at low temperatures. Typical for the solvent based physical drying paints is that the dry coat will re-dissolve in similar solvents as the original. When applying the second coat the first coat will soften and to some extent dissolve and the two coats will be mixed to give excellent adhesion. A second coat applied too thick can cause sagging. Due to this re-dissolving phenomena, it is difficult, if not impossible, to measure the wet film thickness of the second coat. Physical drying paints can be applied at low temperatures, even below zero. The substrate must be dry and free from ice. At lower temperatures the evaporation of the solvents will proceed at a slower rate.

Evaporation of solvents

Physical drying paints

One-component water based systems are usually physical drying, but they will neither dissolve the binder nor be applicable below zero degrees.



Oxidative drying paints Oxidative paints are one component but require oxygen to dry. After application and when the solvent has evaporated the binder starts to react with oxygen which is found in the surrounding air.

Steel

Oxygen

Evaporation of solvents

Oxidative drying paints

When oxidative drying paints have dried the original solvent will not dissolve them, but stronger solvents can cause problems such as lifting. Both new and old oxidative drying paints e.g. alkyds may lift when physical drying or chemical curing paints with strong solvents are applied. The underlying coat will not be dissolved, rather destroyed and adhesion to the substrate or previous coats will be lost. As a consequence, removal of the damaged coat(s) must be performed before application of a new coat. The drying process of oxidative drying paints will proceed at a slower rate at lower temperatures. Typically they can be applied and will dry down to +5 °C.

Chemical curing paints The drying of these paints involves a chemical reaction between the binder (base) and a hardener (curing agent). Before use, the base and the curing agent must be mixed thoroughly to achieve the chemical reaction (cross-linking). It is of great importance that the two components are mixed in the correct ratio supplied by the manufacturer. The mixing of the components should take place shortly before application starts. The curing of the paint involves first evaporation of the solvent and then a chemical reaction between the components. After mixing the components the reaction will start, and it cannot be stopped. It is what we call an irreversible reaction.



AB+ Evaporation of solvents

Crosslinking

Chemical curing paints

After mixing the paint is usable only for a certain period of time (from 20 minutes until a few hours) which we call the pot-life. After this time period the chemical reaction has reached a point where the paint becomes very viscous or hard, thus we cannot apply it. Do not add thinner to dilute the paint further. When the temperature increases the pot-life decreases and vice versa. Typical for the chemical curing paints are that they are two component (2-pack) paints (even three pack). When these paints have cured they form a network (cross-linking) so dense that they will not be dissolved by the original solvent. This gives a very strong and chemical resistant film compared to the other types. The drying process of chemical curing paints will proceed at a slower rate at lower temperatures, consequently quicker at higher temperatures (then pot-life is reduced). Generally chemical curing paints require minimum + 10 °C, but there are cold curing types. Epoxy can cure down to - 5 ˚C and polyurethane even lower.

TYPES OF PAINT There are many different paints which have varying properties. The paint manufactures can customise paints for many uses. We will categorize them as follows:

• Prefabrication primers (shop-primers) • Zinc primers • Primers • Physical drying paints • Oxidative drying paints • Chemical curing paints • Topcoats • Special paints and coatings



Prefabrication primers (shop primer) When buying steel, the manufacturer will often blast clean it and apply a thin coat of paint (prefabrication primer or shop primer) to prevent it from corrosion. Not only does this look better, it is also an advantage to the company applying the paint, since they do not have to blast clean themselves. The prefabrication primers are intended to provide the steel with sufficient protection outdoors and depending on the environment they last for approximately 6-12 months before rust appears. The prefabrication primer is applied to protect the steel against corrosion during the assembly of the structure. After cutting and welding these areas are blast cleaned and paint system is applied. This is a common practice, but some companies may require removing all the prefabrication primer prior to painting. The prefabrication primers are applied in an automatic production line and immediately after heating and blast-cleaning of the surface. The thickness of these coatings must be low, preferably between 10 -20 µm. The thickness is important so as not to reduce the speed of cutting and welding during production, as well as the health aspects associated with hot work on coating or zinc.

Iron oxide shop primers The most common types of iron-oxide shop primers are based on polyvinylbutyral (PVB) or epoxy. The colours are often red. Areas of use General used shop primer for onshore application. The PVB based type must not be used submerged in water.

Zinc-rich shop primers The zinc-rich shop primers are based on epoxy- or ethyl silicate binders. The colour of these types of paint is greyish (zinc coloured). Due to health hazards related to the vapours from welding on zinc (zinc fever) these paints are applied in low film thicknesses e.g. 10-20 µm.

Areas of use These types of shop primers are preferred to iron-oxide primers because they last longer and tolerate greater mechanical strength. For offshore application, zinc ethyl silicate types are preferred.

Zinc Primers Primers are very important for a paint system. It will give adhesion to the steel, but some will also give increased anti corrosion properties.



Zinc rich primers Using zinc in the primer will give cathodic protection if the coating is damaged. As long as there is zinc present, it will send energy to the steel to protect it from corrosion. It is essential that the zinc has particle contact with the steel. Zinc primer can be supplied in many binders, e.g. epoxy, silicate, polyurethanes, acrylics, but for heavy duty we will focus on epoxy and silicates. Zinc is an alkaline substance and it is important to avoid oil based coatings due to the risk of saponification (which eventually will lead to flaking).

Organic zinc rich primers (Epoxy) Epoxy is one of the most common binders used today. The epoxy paint is filled with between 65-90 % pure zinc dust. The zinc content must be high enough to achieve cathodic protection and there must be particle to particle zinc contact, in addition to contact with the steel. If there is not enough zinc the epoxy will isolate the particles and you will end up with barrier coating that does not give any cathodic properties. Curing of zinc epoxy is very easy. As long as the components are mixed and the recommended minimum temperature requirements are met they will cure. Curing is temperature dependant, but typically it takes 7 days. Recoating the zinc epoxy primer before full cure is obtained is no problem as long as all the solvents have evaporated.

Inorganic zinc rich primers (Zinc Ethyl Silicate) The zinc ethyl silicate primers are inorganic two-component paints. Zinc ethyl silicate comprise most often of a base (binder) with pure zinc dust as the other component. Unlike the zinc epoxy paints, the binder (ethyl silicate) is conductive and the paint will provide the steel with sufficient cathodic protection even without particle to particle contact providing that there is enough zinc dust in the paint. The zinc content can be up to 90 %. The zinc ethyl silicate paints give excellent cathodic protection and has additional advantages like high heat resistance (400 ˚C), good abrasion resistance and excellent chemical resistance. These paints may be used alone in tanks e.g. in chemical tankers and are resistant to a wide variety of chemicals. The producers have chemical resistance lists. Zinc ethyl silicate paints have a special curing mechanism. To be able to cure the silicate binder requires hydroxyl groups (-OH) which are easily found in humid air. For this reason the relative humidity of the air must be high when curing these paints, preferably above 85 %. Higher temperatures will in addition accelerate cure. The humidity should be introduced within a few hours after application. If possible, steam will provide excellent curing. Due to this curing mechanism, final cure is not very predictable for these paints. In correct climatic conditions they cure within a few hours, but if not favourable, several days. Care must be taken to not apply the next coat before the zinc ethyl silicate has cured, this may lead to flaking of the paint system. The common way to asses curing is by the use the standardised ASTM D 4752 - the MEK solvent test. This test method will be thoroughly described later on, but uses the solvent methyl ethyl ketone to determine the degree of cure of the paint. Other problem associated with these paint are mudcracking and the necessity of a tie-coat or mist-coat.



Mudcracking occurs when the coating is applied above recommended thicknesses and the results looks similar to dry mud. If this occurs re-blasting is required. Modern zinc ethyl silicates have been improved and are less sensitive to mudcracking than before. Zinc ethyl silicate paints are very porous and contain a lot of entrapped air. If a thick coat of paint is applied directly on top a lot of holes appear (pin-holes or popping). To avoid this problem, the use of a tie-coat or mist coat is required (thin coat of approximately 30 µm to drive out the air and seal the film).

Primers There are many other primers available, and they can be based on alkyd, acrylics, epoxies, polyurethane or other binders. For heavy duty corrosion protection, the epoxy based primers are very frequently used. For water based systems the acrylic types are more common.

Etching primers (or wash primers) Etching primers are used to achieve adhesion on “problem substrates” or substrates where it is difficult to achieve adhesion, e.g. aluminium, stainless steel, hot dip galvanized surfaces. Etching primers are two pack primers. The base contains the binder polyvinyl-butyral (PVB) and the curing-agent is an alcohol solution with phosphoric acid. The maximum dry film thickness (DFT) is very low and should not exceed 10 µm, which is impossible to measure on site. If the DFT is above 10 µm, the phosphoric acid may be trapped inside, eventually leading to adhesion problems. This is due to the fact that ingress of water vapour through the coating dissolves the excess of the water soluble phosphoric acid. Since these primers contain a lot of acid and is a rather messy job, their use is very limited. Health and safety issue have made them rarely used. Today these “problem substrates” are lightly blast cleaned (or thoroughly degreased) followed by an epoxy polyamide primer to achieve excellent adhesion.

Epoxy primers Epoxy primers are characterized by their excellent adhesion and barrier properties. They are two-components. The epoxy polyamide cured products provide the best adhesion properties, but other types of hardener will also give adhesion way above the requirements (provided correct surface treatment). Areas of use Epoxy primers are all-purpose primers for corrosion resistant paint systems. Seldom used alone, depending on the exposure. Some of the primers are used as a tie-coat on porous surfaces and can also be used as a holding primer (primer protecting the surface for a limited amount of time).

Acrylic primer Many one- component primers are based on acrylic binders. These primers give good adhesion and have shown excellent performance. The “solvent” is water and this can cause challenges in colder climates, especially during winter since they must not freeze.



Areas for use General primer used for most applications in general industry.

Physical drying paints Traditional physical drying paints have a high content of solvents (60-70 %). Not only does this give low film thicknesses (must apply paint in many coats to build up) but it is not accepted with regards to environmental issues. They will not fulfil the VOC (volatile organic content) requirements since they emit a high amount of solvents into the atmosphere, thus they are seldom used. Water based physical drying paint e.g. acrylics will fulfil these requirements. Even though the paints are seldom used, we often recoat them and it is important to know of some of their characteristics. Many physical drying paints can be formulated as a primer, mid-coat or topcoat.

Chlorinated rubber Chlorinated rubber is a whitish powder produced either synthetically, or as a reaction between natural rubber and chlorine. Chlorinated rubbers are used as binders in paints and are dissolved in aromatic hydrocarbons such as ketones and chlorinated hydrocarbons. The chlorinated rubbers dry by evaporation of the solvents and no chemical reactions take place during the drying process. The paint re-dissolves when a new coat is applied giving excellent inter-coat adhesion. The gloss retention is poor and they have a tendency to yellow in sunlight. To improve the outdoor durability they final coat will often be vinyl-acrylic. Areas of use Chlorinated rubber paints were widely used in chemical, marine and other environments. The paints resistance toward solvents is poor, so is its resistance to vegetable and animal oils, while its resistance to acids and alkalis is better. The chlorinated rubber paints thermoplastics, i.e. they become soft at higher temperatures and get harder at lower temperatures. The paints will withstand temperature up to approximately + 60 °C, at higher temperatures there is a risk that the paint will decompose and create hydrochloric acid.

Vinyls The vinyl paints are similar to chlorinated rubber paints in that they dry only by evaporation of the solvents. The binder is produced synthetically and is dissolved in aromatic hydrocarbons such as ketones. The vinyl paints will re-dissolve and are somewhat more resistant towards solvents than chlorinated rubber paints, but will be attacked by stronger solvents such as xylene and ketones. The percentage of solids in these paints is low and commonly not higher than 30 - 35 %. The gloss retention is low and they have a tendency to yellow in sunlight. To improve the outdoor durability they final coat will often be modified with acrylics. The paints high contents of solvents can cause problems in windy weather, the result of this may be solvent retention and pinholes in the paint.



Areas of use The vinyl paints were widely used for many industrial operations. Their resistance towards organic acids and alkalis is good. They also have good resistance towards water oil, alcohol etc. Previously the vinyls were much in use on the offshore platforms over zinc silicate primers, but nowadays epoxy / polyurethane have replaced them. For use under water, vinyl paints modified with tar (vinyl tar) below the antifouling provides excellent protection. The addition of the tar to the vinyl paints will increase the water resistance and the paint becomes more surface tolerant.

Acrylics (solvent based) The acrylic paints are closely related to the previous mentioned physical drying paints. The binder is produced synthetically and is dissolved in organic solvents such as ketones. Similar to the vinyl and chlorinated rubber paints, they are high in VOC. Acrylics are easy to top coat and repair and have good gloss retention. They have a low film thickness, and must be applied in multiple coats. Areas of use The acrylic paints were widely used for many industrial operations. The durable films have good water and general chemical resistance (especially to acids and alkalis). However being thermoplastic, the paint has poor solvent and heat resistance.

Acrylics (water based) Due to the low solid content of the solvent based, many physical drying paints have been replaced by water based acrylics which are VOC compliant (water is the main ingredient that evaporates). These coatings give excellent corrosion protection and are used for industrial atmosphere. Usually it comprises of a primer (with anti-corrosion properties) and a topcoat with good weathering properties. During application, the RH should be preferably below 70 %. In enclosed areas the paint may not dry if RH is too high since the water has nowhere to evaporate in air that is water saturated. If the RH is very low the water evaporation will be very quick and the paint may not have time to flow. Areas of use Used for general industry applications like bridges, transmission towers, containers. The chemical resistance is weaker than for the solvent based, but sufficient for normal exposure. The solid content is still low so to build up film thickness, multiple coats are required.

Bitumen paints Bitumen paints dry solely by evaporation of solvents. Due to the bitumen content in these paints the colour becomes black or dark brown and is therefore used for underwater application. The paints are not very suitable for outdoor exposure. Due to health hazards associated with bitumen and tar products they have limited use in many parts of the world.



Areas of use Bitumen paints are well known paints for submerged structures due to the increased water resistant properties obtained from the bitumen. These products have been extensively used in ballast tanks for many years, but due to health hazards and the dark colour they have been phased out. Today the requirements in ballast tanks are paints of light colour. These paints remain relatively soft and when recoating by harder paints we typically use today, there is a risk of cracking.

Oxidative drying paints Oxidative drying paints are for the most part oil based. Used as an all-purpose type paint for industrial applications, but seldom for heavy duty corrosion protection. The products are one-component and easy to apply by brush, roller or spray.

Alkyds Alkyds are made by a reaction between an acid and an alcohol. The alkyds are often divided into groups according to their “oil-length”. The oil-length is an expression for the amount of oil related to other raw materials used in the process. You may have heard the expression short, medium and long alkyds. Alkyds can be modified, i.e. other types of binders are added to produce special properties. Most commonly used are binders such as silicone or urethane to give weather resistance (gloss retention and discolouring). Using alkyds on alkaline substrates like galvanised steel or steel applied with zinc-rich coatings may result in saponification and the result will be flaking. Previously linseed oil, red lead and alkyd red lead paints were extensively used on steel structures. The red lead paints used 30 - 40 years ago still protect these structures, but the use of these lead products are banned in many countries to health related issues (heavy metals). Problems are also related to the removal of these types of paints, because they are hazardous to the environment. Areas of use Alkyds show good durability in environments that are not too aggressive. The weather resistance is fair, but it is not very chemical resistant. Since they tend to soften in water they are not recommended submerged.

Epoxy esters This type of paint must not be mixed up with epoxy paints. The epoxy esters are related to the alkyds and dry by a reaction with the oxygen in the air. Areas of use They are quick drying paints and somewhat harder than alkyds. The application can be done using airless spraying, roller or brush. Their use is limited today.



Chemical curing paints The most widely used products today for heavy-duty corrosion protection are chemical curing types, e.g. epoxy, polyurethane, polysiloxane.

Epoxy paints A wide range of epoxy paints exist and the suppliers can formulate these products for most applications like chemical resistance, barrier properties, strength and adhesion. Epoxy paints create a very hard and tough paint film, and these paints are resistant to many environments. They are two-component and must be mixed thoroughly prior to application. Not recommended for outdoor exposure due to chalking and fading. For this reason a more weather resistant topcoat is used e.g. aliphatic polyurethane. The paint is usually applied either by airless spraying, roller or brush. It is important that the paints pot-life is kept in mind if more than one coat is to be applied (there is a maximum recoating interval). If the recoating interval is exceeded the surface needs to be lightly abraded prior to application of the next coat to ensure adhesion. When the paint is applied by airless spraying it is very important that all equipment is cleaned before the work is finished. Failure to clean will result in destroyed hoses and pump equipment du to cured epoxy inside. Areas of use Epoxy paints are widely used offshore, onshore, on ships, bridges and at chemical plants giving excellent corrosion protection in aggressive environments for a long time. They meet the highest requirements in most standards e.g. ISO 12944 corrosivity class C5. Epoxy paints will withstand most solvents, acids and alkalis and many other chemicals. Special formulations increase the chemical and temperature resistance (epoxy phenolic). Epoxy resin Epoxy resins are described according to the molecular weight, low medium and high. Higher molecular weight epoxies have long molecular chain length. Low molecular weight resin has a molecular weight (MW) <700 and above this it is categorized as high. Low molecular weight resin is liquid and with modern curing agents high solid or solvent free paints can be made. The disadvantage is that there is a health risk from low MW resin, namely allergic reactions. Using high MW resin avoids the allergic risks, but since they are solid they need more solvent and then the risks associated with them must be taken into account. Hardeners Various hardeners are used and each will give unique properties. They can also be modified with others or blended. The most common are:

• Polyamides • Polyamines or polyamine adducts • Isocyanate



Polyamides are the first generation hardeners and are easier to work with than the polyamines. They have longer pot-life and not as critical mixing ratio requirement. The manufacturer can vary the mix ratio to obtain the desired property. Often the base and the curing agent need some induction time prior to use. Induction time is a pre-reaction time after mixing before start of application (usually about 20 minutes). The polyamides have higher viscosity which requires more solvent. Another feature of the polyamides is their excellent adhesion properties on many substrates. Polyamide epoxy can experience amine sweating. This phenomenon is a sticky or tacky substance that can occur on the surface. It is the result of the hardener migrating to the surface. Curing at high relative humidity and low temperature may increase the problem. This sticky layer must be removed prior to coating to avoid flaking. It is easily removed with water (warm if possible). Polyamines will give the coating a hard and tough film with high acid and alkali resistance. They are more volatile and reactive i.e. they have a short pot life and cure time. Polyamines have low viscosity and light colour and can be used for formulating high solid paints in bright colours. Even though this is possible, epoxies are not recommended for topcoats outdoors due to chalking. Epoxy is not resistant to the suns UV rays and will fade and loose colour, this phenomenon we call chalking. In addition epoxy tends to become yellow from weather exposure. It will still give excellent corrosion protection, but not look as nice as other more weather resistant topcoats like polyurethane. Polyamines react with carbon dioxide and moisture in the air to form amine carbamate, commonly referred to as amine blushing. These whitish products must be removed before recoating, if possible by water but often solvent is necessary. Failure to remove can result in flaking. Low temperature and high humidity increases the risk of amine blushing. Isocyanates can be used to cure epoxy to obtain a cold curing product. They react quickly, resulting in a short pot life and fast curing. The cured film has good flexibility and barrier protection properties. Their use is limited since many polyamines will cure down to – 5 ˚C.

Pure epoxy paint Epoxy chemistry has been known for many decades and the first paint to arise were the pure epoxy types. They had a solvent content around 50 %, cured with polyamide, gave excellent adhesion and strength. The disadvantage was that they needed blast cleaning, were slow to cure (at low temperatures), had fair chemical resistance and not always possible in very light colours. Today these properties have been improved, but this original technology is still in use, especially for primer requiring excellent adhesion.

Modified epoxy (epoxy mastic) paints Manufacturers wanted to improve the characteristics of the pure epoxy product and this was the emergence of epoxy mastic technology. These products were higher in solid content (75-90 %), were surface tolerant (no blast cleaning necessary), more flexible and the curing agents were lighter in colour. They can be delivered in many colours (white, grey, green…) with regular pigments, but often they are modified with aluminium flakes, glass flakes or micaceous iron oxide (MIO) to improve the barrier effect. As mentioned earlier the flake formed pigments orient themselves like



bricks for more impermeability. The hardeners used are polyamines, polyamine adducts or a combination of polyamines and polyamides. Areas of use Epoxy mastic is most likely the most used product for heavy duty corrosion protection. The epoxy mastics are “surface tolerant paints”, indicating that these paints do not require removal of all rust and other foreign matters. It should however be noted that mastics should not be applied onto surfaces with thick rust layers or grease, oil and fat. The paints will give the best performance over steel that to some extend have been cleaned and are free form water soluble contaminants such as chlorides. Best result will always be obtained when the surfaces have been blast cleaned, but the surface tolerant epoxies are used on water jetted surfaces also.

Epoxy isocyanate cured paints Some epoxy types can be cured using isocyanate (curing agent for polyurethane). The reason for this is to obtain curing at lower temperatures (down to -10 ˚C) not obtainable with regular epoxy curing agents.

Coal tar epoxy coatings Coal tar epoxy (epoxy tar) paints are two pack paints modified with coal tar. The use of tar gives the paint:

• better flexibility • excellent water resistance

Resistance against chemicals and solvents is still maintained. The epoxy coal tar paints do not contain any corrosion preventing pigments and will protect the steel only by barrier protection. Application is usually done using airless spraying. There is a maximum recoating interval. Recoating should be done within the time limit given in the data sheets. If the recoating interval is exceeded the surface should be abraded before recoating. Areas of use Coal tar epoxy paints are often used on submerged areas and for the protection of ballast tanks. The paints are highly resistant to both acids and alkalis. Coal tar epoxy paints are sensible to sunlight, will chalk and become brittle. The most common colours are brown and black although certain bleached types are available in lighter colours. Recoating coal tar epoxy paints with lighter colours will cause bleeding. The tar component in the paint will diffuse into the next coats and cause discolouration (yellowing), especially noticeable in light colours. Coal tar has to be labelled as toxic in many parts of the world, and the use has become limited. In addition, requirements for ballast tanks and many submerged areas require light colour. These requirements cannot be met by traditional coal tar epoxy. Today coal tar epoxy has been replaced by modified epoxy where the coal tar has been replaced by other light coloured and non-hazardous hydrocarbon resins. The performance is similar to coal tar epoxy.



Glass flake epoxy Epoxy paint can be modified with glass to give better barrier protection. The glass will like aluminium and MIO orient themselves like bricks. They have increased abrasion resistance compared to standard epoxy. It is a high solids paint that can be applied with normal airless spray and is supplied in many colours. Areas of use Used in aggressive areas like splash zones, decks, tanks requiring increased protection, often applied in higher film thickness than standard epoxy, typically 500-600 µm.

Solvent free epoxy paint Solvent free (less) epoxy paint are formulated with little or no solvent, thus they are VOC compliant. This makes them an alternative to water based for VOC compliance. They are formulated with low MW resin and modern hardeners so there is a risk of allergy. They can be applied in high dry film thicknesses. The pot-life is usually very short and the recoat intervals may be shorter. Areas of use The solvent free epoxy paints can be used for many applications, often recommended for drinking water tanks, and in some cases for chemical resistant tank coatings. In general they contain little or no flammable solvents, thus reducing the fire and health hazards.

Topcoats (chemical curing) The topcoat must protect the structure, but most of all it must be resistant to weather exposure (if outside). Depending on their chemistry, they may loose gloss, yellow, and fade in colour over time. Even though they may not look as nice upon ageing, they may still give corrosion protection.

Polyurethane Aliphatic polyurethane paints are one of the most proven topcoats with an excellent track record over many years. They have good gloss and colour retention and life expectancy is very long. They can be formulated glossy, semi glossy or matt. To obtain these properties the binder must be aliphatic (linear) as opposed to aromatic (benzene ring) which will yellow. The hardener for polyurethane is isocyanates which have health hazards associated with them. In fire or during hot work they can decompose into dangerous chemicals, among them, cyanide. For this reason they are banned in the North Sea. Areas of use As a topcoat in corrosion protective paint systems giving excellent weathering properties. Polyurethane will have long life expectancy and perform without fading, loosing gloss or yellowing for many years. Used on bridges on- and offshore application, ships, refinery, towers, buildings.

Epoxy acrylic These epoxy paints are modified with acrylics to improve the weathering properties of epoxy. They are much better than epoxy, but not as good as polyurethane.



Areas of use Weather resistant topcoat for heavy duty corrosion protection paint systems. Used on bridges on- and offshore application, ships, refinery, towers, buildings.

Polysiloxane Polysiloxane topcoats have been in the market for many years (since 1990’s) but still do not have the same track record as polyurethane. Due to brittleness there has been experience in flaking of this topcoat in offshore application. They are much more expensive than polyurethanes, but have longer life expectancy. The product gives even better weathering properties than polyurethane. The chemistry is silicone based. The products have high solids and are VOC compliant. Areas of use Topcoat in a coating system used to protect structures from weather and chemical exposure. Properties are better than for polyurethane, but they are more expensive. Used on bridges on- and offshore application, ships, refinery, towers, buildings. Today the initial problems seem to be solved and sales are going up at a high rate.

Special paints and coatings Some paints are made for special purposes, but they have similar build up as traditional paints.

Polyester glass flake paints Unsaturated polyester will dissolve in strong solvents like styrene. When an accelerator is added (peroxide) the coating cures quickly. These types of paints or coatings are often reinforced using glass flakes. These types of coatings have smaller amounts of wax added. The wax floats to the surface to protect the binder from oxygen ingress since this may stop cure. For this reason the recoat interval is short. If it is exceeded the surface must be washed with solvent to remove the wax (within the time limit the second coat will be able to dissolve the wax). The protection offered by polyesters reinforced with glass flakes is purely barrier protection. Application is done by airless spray often with large nozzle (size 0.040 -0.050 inches). Areas of use The use of reinforced polyesters is often connected with the production of smaller vessels. But in connection to corrosion protection these coatings are often used under very tough conditions requiring high abrasion resistance. Typical areas are splash zone and decks, and in special cases chemical resistant tank linings.

Waterborne coatings There are many generic types of water based paints, but for corrosion resistance epoxy, polyurethane and acrylics are most common. The epoxy is used mainly as a primer and top coated with solvent borne. The polyurethane dispersion is often a topcoat. Acrylics have the complete system of primer and topcoat (usually 2 coat system, but may be 3). The binders are often dispersions, meaning that small droplets or spheres are finely dispersed in water by high speed agitation (in the factory). The spheres are usually smaller than 1 µm. After



application, and as the water evaporates, the particles come closer together and “melt” together to form a uniform paint film. Due to VOC requirement for solvent emission waterborne coatings will continue to increase its market share. Waterborne coatings are not completely faultless. There are still some solvents in many of these paints, during spraying a lot of dust will be created and the need for protection is still present. Some the waterborne coatings are based on epoxy resins and will still cause eczema and allergy. Areas of use On blast-cleaned steel these kinds of paints can be used with a good result both indoors and outdoors in mild to aggressive environment in the atmosphere. These paints are not recommended for use in submerged areas or in areas with a high risk of condensation. The paint will normally be applied with a primer containing corrosion preventing pigments and one or more topcoats. The final coat can be acrylic depending on areas of use. The relative humidity is important when working with waterborne coatings. At high relative humidity the evaporation rate of the solvent (water) will be reduced, in worst case trapped inside. Water on steel can cause flash rusting, which will lead to staining of the paint. Small red or brown spots appear in the film. Due to the fact that the solvent is water there is also a lower temperature limit for application, but usually not less than +5 °C.

Coating concrete Although concrete may be a strong and hard substrate, it will also deteriorate. This occurs due to the porous substrate. Chemicals and water can penetrate through the pores and attack the concrete. Water can freeze and will expand when the temperature drops, causing detachment of the concrete. Often it is recommended to paint the concrete. When coated the concrete will be protected from carbonation and chemicals, and the paint will also improve its appearance and ease maintenance. Unlike the paints used on steel, protective coatings for concrete do not in most cases contain inhibitive or sacrificial pigments to provide protection. Typically coatings with barrier protection are used, but at the same time these paints must be open for diffusion. Such a system will let humidity (water vapour) breathe out through the paint, but not let moisture in from the outside. Concrete or cementicious substrates “breathe” all the time, meaning that they are not dense (they are very permeable). Paints for concrete should have a high barrier effect against permeation of carbon dioxide, other acidic gases and against chlorides. Since the concrete contains bonded water some of this is also released. If the water is trapped between the paint and the concrete it will form blisters. Therefore it is often recommended to use a semi-permeable type coating. As a general rule, concrete should cure for at least 28 days at 20 ˚C before painting. When painting concrete, always be aware of the laitance and humidity inside the concrete. Laitance is a layer of fine particles on the surface. This layer is very weak and should be removed (grinding, acid etching or blast cleaning) prior to paint application. If the concrete has a lot of humidity inside and it is covered with a very impermeable coating, the result will likely be blisters.



Typical paints for concrete are epoxy, acrylic, polyurethane, and silicate types.

Antifouling To prevent marine growth an antifouling paint is used. Growth will increase weight on structures and on ships they will increase drag requiring more fuel to transport the ship. By using an antifouling the surface will remain smooth and the ship will move faster with less fuel consumption. Basically antifouling paint is formulated as “normal” paint that contains biocide, often cuprous oxide and other synthetic types. Earlier tin was used, but it has been banned by the International Maritime Organization since 2008. In the future there may be restrictions on copper as well. There are several types:

• conventional where the binder dissolves in water • long-life is the conventional type modified with non dissolving binders to extend lifetime • self-polishing types which slowly wears off by friction.

Today almost all commercial ships use a self polishing tin free antifouling. There is still cuprous oxide and various other biocides. As long as these are on the surface no growth will occur. As the ship is moving the paint is slowly degraded by friction, always leaving fresh biocides at the surface. Lifetime of such a SPC (self polishing copolymer) is 5 years when applied in approximately 300 µm. Non-slip or foul-release coatings may be the future for fouling prevention. They contain no biocide; rather a very smooth and slippery surface prevents growth. The growth will fall of as the ship is in motion. If the ship is not in motion growth can occur, but is easily removed. Special precautions must be taken during application due to silicone content. These coatings have lower abrasion resistance than traditional SPC and are more expensive. Even though more expensive than the self-polishing, many suppliers claim that these expenses may be saved due to low friction, hence, less fuel consumption.


©Teknologisk Institutt as www.teknologisk.no ISO 12944 6/11 Materials Technology 1

ISO 12944-parts1-8

Corrosion Protection of steel structures by protective paint systems



Standards and guidelines

ISO 12944 The International Standard, ISO 12944 - Corrosion Protection of steel structures by protective paint systems consists of 8 parts: Part 1 General introduction Part 2 Classification of environments Part 3 Design considerations Part 4 Types of surface and surface preparations Part 5 Protective paint systems Part 6 Laboratory performance test methods Part 7 Executive and supervision of paint work Part 8 Development of specifications for new work and maintenance

ISO 12944 – Part 1; General Information Unprotected steel in the atmosphere, in water and in soil is subject to corrosion that may lead to damage. Therefore, to avoid corrosion damage, steel structures are normally protected to withstand the corrosion stresses during the service life required of the structure. There are different ways of protecting steel structures from corrosion. ISO 12944 deals with protection by paint systems and covers all features that are important in achieving adequate corrosion protection. In order to ensure effective corrosion protection of steel structures, it is necessary for owners of such structures, planners, consultants, companies carrying out corrosion protection work, inspectors of protective coatings and manufacturers of coating materials to have at their disposal state-of-the -art information in concise form on corrosion protection by paint systems. The standard ISO 12944 is intended to give this information in the form of a series of instructions.

ISO 12944 – Part 2; Classification of environments ISO 12944-2 describes the corrosion stresses produced by the atmosphere, by different types of water and by soil. It defines atmospheric-corrosivity categories and indicates the corrosion stresses to be expected in situations where steel structures are immersed in water or buried in soil. The corrosion stresses to which a steel structure is exposed represent one essential parameter governing the selection of appropriate protective paint systems in accordance with ISO 12944-5.



Table 1 Atmospheric-corrosivity categories and examples of typical environments

Corrosivity

category Mass loss per unit surface thickness loss

(after first year of exposure) Examples of typical environments in a temperate climate (informative only)

Low-carbon steel Zinc Exterior Interior Mass loss

g/m2

Thickness loss µm

Mass loss

g/m2

Thickness loss µm

C 1 Very low ≤ 10 ≤ 1.3 ≤ 0.7 ≤ 0.1

-

Heated buildings with clean atmospheres, e.g. offices, shops, schools, hotels

C 2 Low > 10 to 200 > 1.3 to 25 > 0.7 to 5 > 0.1 to 0.7

Atmospheres with low level of pollution. Mostly rural areas.

Unheated buildings where condensation may occur, e.g. depots, sports halls

C 3 Medium >200 to 400 >25 to 50 > 5 to 15 > 0.7 to 2.1

Urban and industrial atmospheres, moderate sulphur dioxide pollution. Coastal areas with low salinity.

Production rooms with high humidity and some air pollution, e.g. food processing plants, laundries, breweries, dairies

C 4 High >400 to 650 >50 to 80 > 15 to 30 > 2.1 to 4.2

Industrial areas and coastal areas with moderate salinity.

Chemical plants, swimming pools, coastal ship- and boatyards

C 5-I Very high (industrial)

>650 to 1500 >80 to 200 > 30 to 60 > 4.2 to 8.4 Industrial areas with high humidity and aggressive atmosphere

Buildings or areas with almost permanent condensation and with high pollution

C 5-M Very high (marine)

>650 to 1500 > 80 to 200 > 30 to 60 > 4.2 to 8.4 Coastal and offshore areas with high salinity.

Buildings or areas with almost permanent condensation and with high pollution

Table 2 Categories for water and soil immersion

Category Environment Examples of environments and structures Im1 Im2

Im3

Fresh water Sea or brackish water

Soil

River installations, hydro electric power plants Harbour areas with structures like sluice gates, locks, jetties; offshore structures Buried tanks, steel piles, steel pipes



ISO 12944 – Part 3; Design considerations ISO 12944-3 gives information on basic design criteria for steel structures for the purpose of improving their resistance to corrosion. It gives examples of suitable and unsuitable designs, indicating, with the help of diagrams, which structural elements and combinations of elements are likely to cause accessibility problems during surface preparation work and when applying, inspecting and maintaining paint systems. In addition, design features, which facilitate the handling and transport of steel structures, are discussed. In general the overall design shall be planned to facilitate surface preparation, painting, inspection and maintenance. All surfaces of the structure which have to be coated should be visible and within reach of the operator by a safe method. Narrow spaces between elements and gaps should be avoided whenever possible or sealed. Components, which are at, risk to corrosion and inaccessible after erection should either be made of corrosion resistant material or have a protective coating system that shall be effective throughout the service life of the structure. Alternatively, an allowance for corrosion (thicker steel) should be considered

ISO 12944 – Part 4; Types of surface and surface protection ISO 12944-4 describes different types of surface to be protected and gives information on mechanical, chemical and thermal surface preparation methods. It deals with surface preparation grades, surface profile (roughness), assessment of prepared surfaces, temporary protection of prepared surfaces, preparation of temporarily protected surfaces for further coatings, preparation of existing metal coatings, and environmental aspects. Reference is made to existing International Standards on the surface preparation of steel substrates before application of paints and related products. ISO 12944-4 is intended to be read in conjunction with ISO 12944-5 and ISO 12944-7.

ISO 12944 – Part 5; Protective paint systems ISO 12944-5 describes different generic types of paint on the basis of their chemical composition and the type of film formation process. It gives examples of various protective paint systems that have proved suitable for structures exposed to corrosive stresses and corrosivity categories described in ISO 12944-2, reflecting current knowledge on the world-wide scale. ISO 12944-5 is intended to be read in conjunction with ISO 12944-6. The durability of a protective paint system depends on several parameters such as;

• type of paint system • the design of the structure • the condition of the substrate before preparation • the effectiveness of the surface preparation • the standard of the application work • the conditions during application • the exposure conditions after application



The conditions of the paint system applied can be assessed by means of ISO 4628 1-6. It has been assumed in compiling with the tables in the annex A of this standard, that the first major maintenance painting would normally need to be carried out for reasons of corrosion protection once the coating has reached the level Ri 3 as defined in ISO 4628-3. Based on this precondition, durability has been indicated in this part of ISO 12944 in terms of three ranges: Low (L) 2 to 5 years Medium (M) 5 to 15 years High (H) more than 15 years This part of ISO 12944 has annexes where examples of paint systems for the corrosivity categories C 1 to C 5 are listed.

ISO 12944 – Part 6; Laboratory performance test methods ISO 12944-6 specifies laboratory test methods that are to be used when the performance of protective paint systems is to be assessed. It is particularly intended for paint systems for which sufficient practical experience is not yet available and covers testing of paint systems designed for application to steel prepared by blast-cleaning, to hot-dip galvanised steel and to thermally sprayed metallic coatings. Atmospheric environments and immersion in water (fresh, brackish or sea-water) are also covered.

ISO 12944 – Part 7; Execution and supervision of paint work ISO 12944-7 describes how paintwork is to be carried out in the workshop or on site. It describes methods for the application of coating materials. Handling and storage of coating materials before application, inspection of the work and follow-up of the resulting paint system, as well as reference areas, are also covered. It does not cover surface preparation work (See ISO 12944-4).

ISO 12944 – Part 8; Development of specifications for new work and maintenance ISO 12944- 8 gives guidance for developing specifications for corrosion protection work, describing everything that has to be taken into account when a steel structure is to be protected against corrosion. For the convenience of the used, ISO 12944-8 distinguishes between project specification, paint system specification, paint work specification, and inspection and testing specification. Various annexes deal with particular aspects such as planning of the work, reference and inspection, and offer models of forms intended to facilitate the work.



APPLICATION OF PAINTS Paints are commonly delivered in drums or tins from the paint manufacturer and can be regarded as half-fabricates. The paint has to be applied by some method onto the substrate. To achieve this several ways of application are possible:

• Paint glove or paint mitt • Brush • Roller • Spray

Paint glove /-mitt The use of paint gloves or paint mitts is sometimes necessary, but should be used only in areas with very difficult access such as handrails and other round objects. The paint glove is ofte made of lambskin and is then dipped in the paint and applied on to the surface.

Brush application The use of brushes for application of paint is often considered to be good practice when the application is to be done on areas that are difficult or impossible to reach by other means of application. It is generally used for stripe coating along edges, corners, welds, rat holes etc. Brushing will also give good penetration into pits, crevices and pores. The best paint and varnish brushes are generally made from hog bristles. The outer ends of the bristles are split into two or more fine branches, which results in finer brush marks and greater paint holding ability. The size and shapes of brushes vary. Flat brushes are the most commonly employed types of brushes for work on flat surfaces, their width may be up to 100 - 120 mm. The round and oval types are preferred for work on rough surfaces, painting rivet heads or for constricted areas. The use of brushes for application of paint is very time-consuming and therefore expensive. The paint layers obtained by the use of brushing will vary greatly in thickness depending on the painter and the accessibility. The normal thickness achieved by the use of brush will be in the order of 40 - 50 µm. Some of the corrosion protective paints can be applied by the use of brush, but some are very viscous and rapid drying making the application difficult and uneven. In general brushing may be used for all kinds of paints. For physical drying paints, such as chlorinated rubber and vinyl, the solvent evaporation the paint will make it very viscous making the brush marks quite visible. When a second coat is applied onto the primer coat, the fresh coat will soften or dissolve it and the coats will mix or “melt” together.



Roller application The use of roller is particularly suitable for painting large flat surfaces that are impossible to spray. The use of roller does not require he same skill as for spray application. Using the roller enables the operator to work faster and cover larger areas compared to the use of a brush. The hand roller consists basically of a handle with a metal roller core and removable roller cover. The roller covers are made from lambs' wool, mohair or synthetic fabric wound on plastic or fibreboard cylinders. The appearance of the paint after it has been applied using a roller, will differ and depend upon the type of the roller and the length of the hair. There surface tends to become more structured, and small air bubbles into the paint film can occur. In general, rollers are not recommended for the application of primers on steel structures. This is due to entrapment of air and solvents within the primer. There is also a tendency to "roll out" the paint, meaning that the paint may be applied too thin due to excessive rolling operations. Highly viscous coating may make rolling difficult. Typically, application by roller will leave no more than approximately 60 µm dry film.

Spray application The most effective way of paint application is spraying, and there are two common ways:

• Conventional spraying (low pressure air spray) • Airless spraying

When spraying, it is important to keep correct distance (approximately 30 cm) to avoid dry spary, and always holding the gun perpendicular to get an even paint film.

Conventional spraying When the paint is applied by the conventional method, compressed air is used to atomize and transfer the paint onto the steel substrate. This method is gives the best appearance and is used when the surface finish requirements are high e.g automobiles or other general industry objects. The appearance is superior to airless spray. The paint is forced from a small container (cup on top or under the gun) by compressed air. As the paint passes through the nozzle, the paint is mixed with the air and atomized at pressures of about 2 - 5 kg/cm2 (0.2 - 0.5 MPa). The low pressure requires that the paint has low viscosity, and the paint must be thinned considerably to be able to atomize in the gun. Modern high build paints will most likely have to be thinned more than recommended to be able to apply by this method.



The normal working distance will be 40 - 60 cm from and perpendicular to the surface to be coated. When spraying conventional a 50 % overlap is used. During the application it is important that the operator measures the wet film thickness and adjusts his speed accordingly. Although quicker than brush or roller this method is time consuming compared to airless spray.

Airless spraying In airless spraying the paint is forced through the nozzle at high pressure and atomized. This is done with a compressor and a pump. The pumps are usually air driven, but can also be electrically driven. There is no air involved with the atomization and most of the paint will reach the substrate. This makes this method very effective and is the preferred method for application of corrosion protective coatings. The high pressures involve makes it possible to spray paint with high viscosity e.g. modern paints that have high solids. Two factors to be considered when selecting an airless pump is the fluid pressure required and the volume to be pumped. Adjusting the air pressure on the pump varies the pressure required for airless atomization. The maximum pressure obtained again depends upon the size of the air motor and the air pressure used to operate the pump.



The lower part of the pump has ball valves, this will give double action e.g. suction occurs when the piston is going both up and down. The valve enters the pump and the pressure increases and only paint (no air) comes out of the outlet. This hose has much smaller diameter than a typical air hose. The paint comes through the hose and is forced through the small nozzle and is atomized. The pressure is very high 18-30 MPa (180-300 bar) and care must be taken to avoid injury. The pumps pressure ratio gives an indication of how powerful the pump is. This ratio is the area difference between the air and paint piston. The air piston is much larger than the paint piston. A larger ratio indicates a more powerful pump. Common pumps are from 30:1 up to 70:1, very often 50:1. Theoretically, a pump that is 70:1 and has an inlet pressure of 5 bar, gives theoretical pressure of 350 bar at the nozzle (5x70=350). Generally a pump with high ratio will be more effective and will able the operator to apply high viscosity paints. Friction and length of hoses will reduce the output pressure.

It is not always necessary to have high pressure to form a uniform spray pattern as long as the atomization of the paint is satisfactory. If too much paint comes out of the nozzle there is a risk of excessive thicknesses, sag and uneven surface. Insufficient pressure causes uneven paint thickness, stripes and fingering. The nozzles will wear and need to be changed. Lifetime depends on type of paint used, e.g. zinc containing paint will wear the nozzle quicker. The nozzles usually have an oval shape and come in different angles and sizes. The angle decides the spray fan e.g. it may be between 5˚ and 90˚ degrees. For small parts or detailed areas a small angle is preferred to avoid high loss and a lot of dry spray e.g. an angle of 20˚. For huge areas like a ship hull an angle of 85° may be preferred. Often a standard angle is 60˚ degrees. The size is usually found in the technical data sheet of the paint and depends on among others on the viscosity and type of paint. Common nozzles (or tips) are from 0,015 to 0,021 inches.



Application by airless spray requires skilled operators. Information found in the technical data sheets of the paint supplier gives information regarding, nozzle size, atomization pressure, and the amount of thinner allowed. To achieve proper atomization, the operator must change the nozzle, change the pressure, thin the paint or possibly heat the paint (lower viscosity). If heating is chosen, it is important to note that the pot-life of two pack paints will be shorter.

Some paints (primarily those with heavy pigments like zinc that may settle in the bottom of the tin) require agitation (usually automatic) during application. The speed of agitation is important and must not cause entrapment of air in the paint. Incorrect agitation speed can cause porosity in the film. Using airless spray does not only involve chemicals (paint) but also high pressure. Read the safety data sheet carefully and avoid injury.


©Teknologisk Institutt as www.teknologisk.no Instructions-standards and instruments 06/13 Materials Technology 1

INSTRUCTIONS STANDARDS AND INSTRUMENTS



PRACTICAL USE OF STANDARDS AND INSPECTION EQUIPMENT Pre-preparation surface condition (ISO 8501-1) Condition after preparation (ISO 8501-1) Condition after localised removal of previous coatings (ISO 8501-2) Surface defects (ISO 8501-3) Water jetting (ISO 8501-4) Detection of salts and dust (ISO 8502: 2-3) Weather conditions (ISO 8502-4) Sampling of impurities - The Bresle method (ISO 8502-6) Surface roughness (ISO 8503) Wet and dry film thickness (ISO 2808) Film thickness by destructive means (ASTM D 4138) Adhesion - Cross cut test (ISO 2409) Adhesion - Pull-Off test (ISO 4624) Holiday detection (ASTM D5162) Curing of zinc ethyl silicate (ASTM D 4752)



ISO 8501-1 Visual cleanliness of the steel before and after surface preparation Rust grades:

A Steel surface largely covered with adhering mill scale but little, if any, rust.

B Steel surface which has begun to rust and from which the mill scale has begun to flake.

C Steel surface on which the mill scale has rusted away or from which it can be

scraped, but with slight pitting visible under normal vision. D Steel surface on which the mill scale has rusted away and on which general pitting

is visible under normal vision.

Blast-cleaning, Sa Sa 1 Light blast-cleaning The surface shall be free from visible oil, grease and dirt, and from poorly adhering mill

scale, rust, paint coatings and foreign matter (water-soluble salts and welding debris). See photographs B Sa 1, C Sa 1 and D Sa 1

Sa 2 Thorough blast-cleaning The surface shall be free from visible oil, grease and dirt, and from most of the mill scale,

rust, paint coatings and foreign matter. Any residual contamination shall be firmly adhering. See photographs B Sa 2, C Sa 2 and D Sa 2

Sa 2 ½ Very thorough blast-cleaning The surface shall be free from visible oil, grease and dirt, and from mill scale, rust, paint

coatings and foreign matter. Any remaining traces of contamination shall show only as slight stains in the form of spots or stripes. See photographs A Sa 2½, B Sa 2½, C 2½ and D Sa 2½

Sa 3 Blast-cleaning to visually clean steel The surface shall be free from visible oil, grease and dirt, and shall be free from mill scale,

rust, paint coatings and foreign matter. It shall have a uniform metallic color. See photographs A Sa 3, B Sa 3, C Sa 3 and D Sa 3

Hand tool and power tool cleaning, St St 2 Thorough hand and power tool cleaning The surface shall be free from visible oil, grease and dirt, and from poorly adhering mill

scale, rust, paint coatings and foreign matter (water soluble salts and welding debris). See photographs B St 2, C St 2 and D St 2



St 3 Very thorough hand and power tool cleaning As for St 2, but the surface shall be treated much more thoroughly to give a metallic sheen

arising from the metallic substrate. See photographs B St 3, C St 3 and D St 3

Flame cleaning, Fl The surface shall be free from mill scale, rust, paint coatings and foreign matter. Any residues shall show only as discoloration of the surface (shades of different colors). See photographs A Fl, B Fl, C Fl and D Fl

ISO 8501-1 Determination of rust grades and preparation grades

Equipment: The standard ISO 8501-1 Powerful flashlight Procedure: 1. Either in good diffuse daylight or in equivalent artificial illumination, examine the steel

surface and compare it with each of the photographs using normal vision. 2. Place the appropriate photograph close to, and in the plane of, the steel surface to be

assessed. 3. For rust grades, record the assessment as the worst grade that is evident. 4 For preparation grades, record the assessment as that grade nearest in appearance to that of

the steel surface. NOTE . In addition to the type of cleaning method used, for example dry blast-cleaning using a

particular type of abrasive, the following factors can influence the result of the visual assessment:

a) initial state of the steel surface other than any of the standard rust grades A, B, C or D

b) colour of the steel itself c) regions of differing roughness, resulting from differential corrosion attack or

uneven removal of material d) surface irregularities such as dents e) marks from tools f) uneven lightning g) shadowing of the surface profile caused by angled projection of abrasive h) embedded abrasives Note: The basis of the appendix is that many different abrasives are used for blast-

cleaning. Since some of the abrasives are impacted on a blast-cleaned surface, the colour of the abrasives affects the appearance of the surface.



ISO 8501-2 Preparation grades of previously coated substrates after localised removal of previous coatings

Equipment: The standard ISO 8501-2 Powerful flashlight Procedure: 1. In the standard a number of preparation grades, indicating the method of surface preparation and

degree of cleaning, are specified. • Localised blast-cleaning of previously coated surfaces, P Sa • Localised hand - and power-tool cleaning of previously coated surfaces, P St (not

machine abrading) • Localised machine abrading of previously coated surfaces, P Ma

2. The surface is compared with pictures in the standard that show examples within some of the

preparation grades. 3. The photographs show examples from blast-cleaning (P Sa 2 ½ ) and machine abrading (P Ma)


Procedure: By comparing surface with photographs, determine the preparation as follows

• P1 Light preparation; No preparation or only minimum preparation carried out before application of paints

• P2 Thorough preparation; Most imperfections are removed

• P3 Very thorough preparation; Surface is free of significant visual imperfections Describes preparation grades of welds, edges and other areas, with steel surfaces with imperfections. Such imperfections can become visible before and/or after an abrasive blast-cleaning process.




Part 4 Initial surface conditions, preparation grades and flash rust grades in connection with high-pressure water jetting Procedure: In a similar fashion to ISO 8501-1, find the initial condition and compare to the appropriate photograph in the standard. 5 initial conditions are described:

DC A, DC B, DC C, DP I, DP Z.

There are 3 cleaning grades designated: Wa1, Wa2, Wa 2 ½.

The last picture gives flash rust grades: Low, medium and high.



ISO 8502-2 Laboratory determination of chloride on cleaned surfaces Equipment: The standard ISO 8502-2 Ruler and essentially chloride-free chalk Absorbent cotton pads Metal spatula or knife Gloves of plastic Beakers, of capacity 250 ml Small glass rod Funnel Filter paper Measuring cylinder, of capacity 50 ml Volumetric flasks, of capacity of 50, 100 and 1000 ml Volumetric pipettes, of capacity of 1 and 20 ml Device for titration Procedure: 1. Carry out a blank titration each time on water, retaining the titrated solution for comparison of

end points. See the standard. 2. Mark out a test area measuring approximately 25000 mm2 (e.g. 250 mm x 100 mm) using a

ruler and chalk. 3. Mark two beakers A and B. Pour 45 ml of water into beaker A (corresponding to grade 3 purity

in accordance with ISO 3696). 4. Soak an absorbent cotton pad with the water in the first container and then thoroughly swab the

test area with the soaked pad. Remove the water from the surface with the absorbent cotton pad and squeeze the washings into beaker B.

5. Repeat the swabbing procedure with several portions of water, and if the pad is worn out, use a

fresh one. Continue the swabbing procedure until all the water has been used up. 6. Filter the washings, using the filter paper and the funnel and collect the filtrate in the volumetric

flasks of capacity 50 ml. Wash the absorbent cotton pads with water and squeeze the water into the flasks and make to the mark with water.

7. Shake the volumetric flask and using a pipette, transfer 20 ml of the wash water into a clean

beaker. Determine the chloride content by following the procedure given in the standard. 8. Express the chloride content in milligrams per square meter in nearest 10 mg/m2



ISO 8502-3 Assessment of dust on steel surfaces prepared for painting (pressure-sensitive tape method) Equipment: The standard ISO 8502-3 Colourless, transparent pressure-sensitive tape of width 25 mm, having an peel

adhesion peel strength of at least 190 N per metre measured in accordance with IEC 454-2

Display board e.g. of glass, cards or paper Spring loaded roller Hand lens, capable of magnification x 10 Procedure: 1. Discard the first three turns of tape from the roll and then remove a piece about 200 mm long. 2. Press about 150 mm of the freshly exposed tape on to the surface under test. 3. Place the thumb across one end of the tape and move the thumb, while maintaining a firm

pressure three times in each direction. 4. Remove the tape from the test surface, place it on an appropriate display board and cause it to

adhere to the board by rubbing with the thumb. 5. Assess the quantity of dust on the tape by comparing it visually to an area of the tape with

equivalent areas of the pictorial references shown in the standard. Record the rating corresponding to the reference that is closest match.

6. Assess the predominating dust particle size on the adhesive tape by reference to table 1 in the

standard which defines six dust particle size classes, designated 0, 1, 2, 3, 4 and 5. Report any overall discoloration as size class 1.



ISO 8502-4 Guidance on the estimation of the probability of condensation prior to paint application Equipment: The standard ISO 8502-4 Sling psychrometer Steel thermometer magnetic or digital Dew point calculators Procedure: 1. Check the thermometers prior to wetting the wick on the wet bulb thermometer. 2. Saturate wick with water. 3. Hold the sling psychrometer a little away from your body and whirl it for about 30 seconds

with a rotation of approx. 2 m/s. 4. Observe and make notes of the wet bulb temperature and then the dry bulb temperature. 5. Whirl the psychrometer for another 30 seconds. If the wet bulb and dry bulb temperature differ from your last reading, continue to whirl

until they no longer do. If they do not differ from your last reading, you have determined the dry bulb and wet bulb temperature.

6. Set the wet bulb temperature opposite the dry bulb temperature and determine the relative

humidity at the arrow. 7. When you know the ambient temperature, the wet bulb temperature and the relative

humidity you can determine the dew point temperature by the use of dew point calculators.



ISO 8502-6 Sampling of soluble impurities on surfaces to be painted - The Bresle method Equipment: The standard ISO 8502-6 Adhesive patches (Patch size; A-0155, A-0310, A-0625, A-1250, A2500) Reusable syringe (max. volume: 8 ml, max. needle diameter: 1 mm, max. length

50 mm) Solvent (for water soluble impurities - distilled water) Contact thermometer Procedure: 1. Take an adhesive patch of appropriate size. Remove the protective paper and the punched

out material. 2. Press the patch with the adhesive side on to the test surface, in such a way that the minimum

amount of air is trapped in the sampling compartment. 3. Fill the syringe with solvent - the volume of solvent is proportional to the sampling

compartment area and normally amounts to 2.6 x 10-3 ml / mm2. 4. Insert the needle at an angle near the outer edge of the patch, through the elastomer film and

the body of the patch into the sampling compartment between the elastomer film and the test surface. If the patch is so positioned that access to the sampling compartment is difficult, bend the syringe needle.

5. Inject the solvent, allowing it to wet and rinse all parts of the test surface. If necessary to avoid any trace of entrapped air in the sampling compartment, carry out the

injection in two steps as follows: Inject half of the solvent. Evacuate the air through the needle by reverse operation of the

syringe. Remove the syringe needle from the patch. Holding the syringe with the needle up, expel the air. Re-insert the syringe needle into the sampling compartment, and inject the remainder of the solvent.

6. After a suitable period of time to be agreed between the interested parties, suck the solvent

back into the syringe cylinder 7. Without removing the syringe needle from the patch, re-inject the solvent into the sampling

compartment, and then suck the solvent back into the syringe cylinder. Repeat until at least four cycles of injection and sucking back have been completed.

8. At the end of the last cycle, retrieve and transfer as much as possible of the solvent from the

sampling compartment to a suitable vessel for analysis.



9. It is essential that no solvent is lost from the patch or the syringe, due for instance to inferior-quality materials or improper handling of the materials. If any solvent is lost, the sample obtained shall be rejected.

10. Conduct the necessary analysis e.g. conductivity analysis.

ISO 8502-9 Part 9 Field method for the conductometric determination of water-soluble salts Procedure: Use device to measure conductivity, reference water and injected water. Usually the initial value is subtracted from the value after injection. The given value in µS/cm must be multiplied by 6 to obtain mg/m2.



ISO 8503-2 Method for the grading of surface profile of abrasive blast-cleaned steel –Comparator procedure Equipment: The standard ISO 8503-1 and ISO 8503-2 The comparators ISO 8503-1 for Grit and Shot Hand lens, magnification not exceeding x 7 Procedure: 1. Remove all loose dust and debris from the test surface. 2. Select the appropriate surface profile comparator (either ISO 8503-1 G or S depending on

the kind of abrasive used) 3. Place the comparator against the area of the surface. 4. Compare, in turn, the test surface with the four segments of the comparator, using the hand

lens is necessary. 5 Assess the profiles on the comparator that are nearest to the profile of the test surface and,

from these, determine its grade Fine Profiles equal to segment 1 and up to but excluding segment 2 Medium Profiles equal to segment 2 and up to but excluding segment 3 Coarse Profiles equal to segment 3 and up to but excluding segment 4 Notes: When a mixture of shot and grit is used to blast-clean a substrate, the grit-abrasive reference

comparator G should be used. If visual assessment proves difficult, tactile assessment may provide a useful guide (back of

a fingernail) In case of dispute, a representative sample of the surface shall be provided and measured as

described in ISO 8503-3 or ISO 8503-4.



Testing of abrasives Equipment: The standard ASTM D 4940 or ISO 11127- 6 and 7 Glass equipment, Erlenmeyer flask of 250 ml Distilled water Conductivity meter Procedure: 1. Weigh a test portion of 100 g of the abrasive into a 250 ml flask. 2. Add 100 ml of distilled water. 3. Shake for 5 minutes and allow to settle.

4. Let it stand 1 hour. 5. Shake again for 5 minutes and allow to settle. If the liquid is no completely clear, filter by a

suitable method. 6. Determine the conductivity using a conductivity meter

ASTM D 4285 Detecting oil or water in the compressed air Equipment: A white absorbent material, cloth or filter paper Procedure: 1. Secure in place the white absorbent material at a distance of 60 cm from the nozzle and in the

centre of the air steam. 2. Allow the air stream to flow onto the material for at least one-minute. 3. Visually examine the material for the presence of oil or moisture. Any indication of either is

sufficient cause for not using the compressed air.



Determining the blast-cleaning air pressure Equipment: Hypodermic needle pressure gauge (manometer) Procedure: 1. Start up the blast equipment without the sand. 2. Insert the needle at a 45o angle through the blast-cleaning hose as close as possible to the nozzle

into the air stream. 3. Read the blast-cleaning air pressure

Determining the presence of non-visible grease or oil contamination Equipment: Water Ultraviolet lamp Chalk Procedure: Water break test: Apply a fine mist of atomised water onto the test surface. If the water gathers into droplets within ½ minute, the surface is likely to be contaminated with grease, oil or other water-insoluble matters. Ultraviolet light test: Try to avoid to much light onto the surface when doing the test. Shine an ultraviolet lamp onto the surface. Observe for fluorescence. If you see a bright yellow or green fluorescence this indicates contamination of grease or oil. Chalk test: Draw a line with a piece of chalk through an area that you suspect to be a clean area onto an unclean area. If the line through the suspected area gets thinner, this indicates that the surface is contaminated with oil or grease.



ISO 2808 Determination of film thickness

Comb gauge and wheel gauge Equipment: The standard ISO 2808 Comb gauge or wheel gauge Procedure: Comb gauge 1. Immediately after the application of the paint, place the comb gauge firmly onto the

substrate in such a way that the teeth are normal to the plane of the surface and the gauge does not slip.

2. Remove the gauge, and examine the teeth to determine which is the shortest one to touch the wet paint film.

3. Record the film thickness as lying between the last "touching" tooth and the first "non-touching" tooth as shown on the tooth calibrations marked on the gauge.

4. Take at least two further readings in different places in a similar manner to obtain representative results over the painted area.

5. If none of the teeth or all the teeth are wetted on the comb gauge, either turn the comb or use another comb with a different scale.

Wheel gauge 1. Immediately after the application of the paint, place the wheel gauge into the paint film so

that the two outer rims are in contact with the substrate at the point of maximum gap. 2. Roll the wheel through at least 180° in one direction along the surface and then in the other

direction, and take the mean of the two readings, at the lower scale division, as one reading. 3. Repeat the procedure at least twice in different places in a similar manner to obtain

representative results over the painted area.

Magnetic pull-off principle Equipment: The standard ISO 2808 Film thickness instrument based on the magnetic pull-off principle Calibration standards - for example chromium plated steel Procedure: Place the instrument on the calibration standard (chromium plated steel of a known thickness) preferably with a thickness about the same to be measured. 1. Using the thumb turn the wheel on the pull-off instrument away from you until the magnet

stays in contact with the metal.



2. Hold the instrument to the substrate and carefully turn the wheel against you until you either see or hear the magnet detaches from the painted surface.

3. Repeat step 1 and 2 a few times 4. If the thickness registered on the instrument does not correspond to the actual thickness of

the chromium plated steel, step 5-6 must be carried out. 5. If the instrument shows too high or too low thickness according to the standard: Hold the instrument onto the surface of the chromium plated steel. Carefully turn the scale,

the direction depends on whether the value is too high or too low compared to the chromium plated steel.

6. Repeat step 1 - 2. If the instrument still does not show the correct value, repeat step 5.

Magnetic flux principle

Equipment: The standard ISO 2808 Magnetic flux principle - electromagnetic instrument Unpainted smooth steel for calibration Non-magnetic shims Procedure: 1. Mount the probe to the instrument and turn on the instrument. 2. Place the probe onto the unpainted smooth steel and hold it there. (The instrument shall display 0 µm. If the instrument displays other values than 0, the

instrument must be adjusted. Do this while you hold the probe onto the steel) 3. Take a non-magnetic shim with a defined thickness e.g. 200 µm and place it onto the

smooth steel surface. Place the probe onto the shim and hold it there. (The instrument shall display 200 µm. If the instrument displays other values than 200, the

instrument must be adjusted. Do this while you hold the probe onto the steel) 4. You have now adjusted the instrument for measurements in the area of 0 - 200 µm. But for

your own sake check that you actually get 0 µm on smooth steel and 200 µm on the shim. If you still do not have the correct values displayed repeat step 2 and 3 again until you do.



ASTM D 4138 Determination of film thickness by destructive means Equipment: Paint Inspection Gauge Säberg Thickness Drill Marker pen - red, blue or black Procedure: Paint Inspection Gauge 1. A test area is marked with a marker pen. Use a marker with a different colour than that of

the topcoat. 2. Choose the appropriate angled cutting tool for the paint film (depending on the film

thickness). 3. Place the cutting tool a little above the mark and make a cut across the mark and through all

coats down to the steel. 4. If there is paint left in the cut, remove them by blowing or with a fine brush. 5. Turn the instrument, place the instrument above and perpendicular to the cut (marked area).

Use the microscope with illumination to examine the cut. 6. The width of the cut is examined through the graticule scale in the microscope, and each

coat can then be determined. 7. The distance between the lines on the graticule scale is the same - it does not make any

difference where on the scale you start your measurements. 8. Make notes of the number of lines on the graticule scale. 9 The thickness of each coat depends on which of the cutting tools you have used. Cutting tool 1 Multiply the number of lines with 20 for correct DFT in µm Cutting tool 2 Multiply the number of lines with 10 for correct DFT in µm. Cutting tool 3 Multiply the number of lines with 2 for correct DFT in µm. Säberg Thickness Drill 1. A small hole is drilled into the coating using the cutter. 2. Remove the cutter and examine the width of the cut film through the graticule microscope

to determine the thickness of each coat.



ISO 2409 Cross cut test Equipment: The standard IS0 2409 Multi-blade cutting tool Single-blade cutting tool Instrument with a series of spacing edges Soft brush Transparent pressure-sensitive adhesive tape Viewing lens, magnification of x 2 or x 3 The spacing of the cuts in the coating depends on the thickness of the coating. The spacing of the cuts in each direction shall be equal and depends on the substrate. For hard substrates (steel) the spacing is as follows: 0 µm to 60 µm 1 mm spacing 61 µm to 120 µm 2 mm spacing 121 µm to 250 µm 3 mm spacing (The multi-blade tool is not suitable for thick (> 120 µm) or hard coatings) Coatings with a total thickness of over 250 µm may be tested by means of a single cross-cut. Procedure: 1. Check the film thickness on the panel to be tested and choose the correct spacing of the

knives. 2. Hold the cutting tool with the blade normal to the test panel surface. With uniform pressure

on the cutting tool and using the appropriate spacing guide, make the agreed number of cuts in the coating at a uniform cutting rate. All the cuts shall penetrate to the substrate surface.

3. Repeat this operation, making further parallel cuts of equal number, crossing the original

cuts at 90° to them so that a lattice is formed. 4. Brush the panel lightly with the soft brush. For hard substrates only apply additionally

adhesive tape. Remove an additional length at a steady state and cut a piece approximately 75 mm long.

5. Place the centre of the tape over the lattice in direction parallel to one of the cuts and

smooth the tape over the area of the lattice and for a distance of at least 20 mm beyond with a finger. Rub firmly with a fingertip.

6. Remove the tape by grasping the free end and pulling it off steadily in 0.5 - 1.0 s at an angle

which is as close as possible to 60°. Retain the tape for reference purposes. Examine the cut area and classify the test area according to table 1.



ISO 4624 Pull-off test Equipment: The standard ISO 4624 Pull-off test instrument Test cylinders (dollies) Glue / 2 pack solvent free epoxy or cyanoacrylate Tape Cutting device Procedure: 1. Degrease the dollies and dry them before use. 2. Lightly abrade the dollies using a sandpaper grade 240 – 400 and dry them in dry tissue paper 3. Lightly abrade the painted surface with a sandpaper grade 240 - 400. 4. Remove the abraded paint dust from the surface using a clean paper tissue 5. Mix the 2 pack solvent free epoxy glue in the correct ratio before applying it to the dollies. 6. Press the dollies to the surface, with a pressure so that most of the glue is squeezed out. 7. Use either tape or magnets to hold the dollies to the substrate. 8. The 2 pack solvent free epoxy glue must cure for 24 h at room temperature. Cyanoacrylate glue

must be used according to the instructions. 9. Before the pull-off test are accomplished cut through the cured adhesive and the paint coating to

the substrate, round the circumference of the test cylinder using a cutting device. 10. Record both the breaking strength, in megapascal (MPa) and the nature of failure for each pull-

off test. Express the results as the percentage area and site of fracture in the system under test in terms of adhesive, cohesive or adhesive/cohesive failure. Also list the type of instrument used for the test.

11. For convenience, the following scheme may be used to describe the results observed. A = Cohesive failure of substrate A/B = Adhesive failure between substrate and first coat B = Cohesive failure of first coat B/C = Adhesive failure between first and second coats -/Y = Adhesive failure between final coat and adhesive Y = Cohesive failure of adhesive Y/Z = Adhesive failure between adhesive and test cylinder



ASTM D 5162 Discontinuity (holiday) testing of non-conductive protective coatings on metallic substrates - Low voltage -wet sponge testing Equipment: The recommended practice ASTM D 5162 Low voltage holiday detector Film thickness instrument Water Marker Procedure: 1. Measure the DFT of the paint system using a non-destructive dry film thickness gauge. If

the DFT is lower than 500 µm you can use the low voltage holiday detector, if it exceeds this value use a high voltage holiday detector.

2. Saturate the sponge with water. Hold the sponge and squeeze it, as to avoid dripping. 3. Attach the ground wire from the instrument ground output terminal to the conductive

substrate and ensure positive electrical contact. 4. Contact a bare spot on the conductive substrate to verify that the instrument is properly

grounded. 5. Move the sponge over the surface of the coating at a moderate rate approximately 1 ft/s (30 cm/s), perhaps only 5 - 10 cm /s. 6. If there are discontinuities in the coating, an audible signal will be heard, and the exact spot

shall be identified with a marker. 7. To prevent telegraphing (current travelling through a moisture path to a discontinuity,

giving an erroneously indication), care should be taken to ensure that the solution is wiped dry from a previously detected discontinuity before continuing the test.



ASTM D 5162 Discontinuity (holiday) testing of non-conductive protective coating on metallic substrates - High voltage spark testing Equipment: The recommended practice ASTM D 5162 High voltage holiday detector Film thickness instrument Marker Procedure: 1. Measure the DFT of the paint system using a non-destructive dry film thickness gauge. If

the DFT is higher than 500 µm you can use the high voltage holiday detector, if it below this value use a low voltage holiday detector.

2 Adjust the test instrument to the proper voltage for the coating thickness being tested.

Excessive voltage may produce a holiday in the coating film. 3. Attach the exploring electrode to the test instrument. 4. Attach the ground wire from the instrument ground output terminal to the conductive

substrate and ensure positive electrical contact. 4. Contact a bare spot on the conductive substrate to verify that the instrument is properly

grounded. 5. Move the exploring electrode over the surface of the dry coating at a rate of approximately

1 ft/s (30 cm/s), perhaps only 5 - 10 cm /s. 7. Discontinuities that require repair shall be identified with a marker that is compatible with

the repair coating or one that is easily removed.

8. Suggested voltages are found in the standard



ASTM D 4752 Measuring MEK resistance of ethyl silicate (inorganic) zinc-rich primers by solvent rub Equipment: The standard ASTM D 4752 Test area with inorganic zinc-rich primer Water Cotton cheesecloth Metyl Ethyl Ketone (also called Butanone) Procedure: 1. Measure the dry film thickness of the primer in the selected areas. 2. Clean the surface with a dry tissue, if necessary slightly soaked with fresh water to remove

loose material. 3. Immediately fold cheesecloth into a pad containing four thicknesses of the cloth. Saturate

the cloth to a dripping wet condition with the methyl ethyl ketone (MEK). 4. Rub the test area with the saturated cloth, exerting a moderate stroke pressure with the

thumb, using a 2-in. (50 mm) long stroke that encompasses the test area. 5. Continue rubbing the surface with the MEK saturated pad, wetting the pad as necessary

without lifting it from the surface, until either the metal substrate is exposed or 50 double rubs have been completed. Record the number of rubs when the substrate is exposed.

6. Select an adjacent area to be used as a control. Repeat 1 -5 except use a dry cheesecloth to

establish the effect of burnishing without the influence of MEK 7. Inspect the test area and the cheesecloth. Rate the results in accordance with table 1. Table 1 Scale for resistance rating MEK Test Resistance rating Description 5 No effect on surface; no zinc on cloth after 50 double rubs 4 Burnished appearance in rubbed area; slight amount of zinc on cloth after 50

double rubs 3 Some marring and apparent depression of the film after 50 double rubs 2 Heavy marring; obvious depression in the film after 50 double rubs 1 Heavy depression in the film but no actual penetration to the substrate after

50 double rubs 0 Penetration to the substrate in 50 double rubs or less



Determining coatings hardness by pencil testing Equipment: A series of pencils Sanding paper Procedure:

1. Sharpen the pencils lightly. 2. Flatten the tip of the pencil on a sand paper placed on a flat surface. 3. Hold the pencil at an angle of 45 degrees to the paint and push it downward into the

coating. 4. Repeat the test with the next softer pencil until you find the pencil that no longer

scratches the coating.

Testing for solvent resistance

Equipment: Cloth Methyl-Ethyl-Ketone (MEK) Procedure: 1. Saturate the cloth with MEK. 2. Rub a small area with 50 double rubs. 3. Visually examine the cloth. If paint from thermo-setting coatings is on the cloth after test,

then the coating is not fully cured.

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