anti corrosive coating

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1.1 I NTR ODUCTIO N

Steel is a suitable construction material for a wide range of structures because of its

favorable mechanical properties and high speed fabrication. Corrosion may lead to

damage of unprotected steel exposed in different environmental conditions. Corrosion is

an electrochemical reaction process which leads to continuous loss of metal or metal

alloys. Energy is needed to extract metals from their mineral state to the pure state. The

same amount of energy is emitted during the formation of its chemical compounds or in

other words during the process of corrosion.

Corrosion returns the metal to its combined state in chemical compounds that are similar

or even identical to the minerals from which the metals were extracted .Thus corrosion

has been called extractive metallurgy in reverse.

1.2 MECHANISM OF CORR OSIO N

Corrosion reactions are electrochemical in nature. Surface of iron goes into solution as

ferrous ions at anodic sites. Iron atoms undergoes oxidation to ions they release electron

whose negative charge will quickly buildup in the metal and prevent anodic reaction, or

corrosion.Corrosion returns the metal to its combined state in chemical compounds that

are similar or even identical to the minerals from which the metals were extracted. Thus

corrosion has been called extractive metallurgy in reverse.

The dissolution will only continue if the electrons released can pass to a site on the

metal surface where a cathodic reaction is possible. At a cathodic site the electrons react

with some reduci ble component of the electrolyte and are themselves removed from the

metal. The rates of the anodic and cathodic reactions must be equivalent according to

Faraday’s Laws, being determined by the total flow of electrons from anodes to cathodes

which is called the “corrosion current”, ICorr . Since the corrosion current must also flow

through the electrolyte by ionic conduction the conductivity of the electrolyte will

influence the way in which corrosion cells operate.

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Corrosion Mechanism

The corroding part of metal is described as a “mixed electrode” since simultaneous

anodic and cathodic reactions are proceeding on its surface. The mixed electrode is a

complete electrochemical cell on metal surface. Since transfer of electrons involves

transfer of charge and is termed as current, the potential/driving force which makes the

reaction happen is given an electrical term of Electrochemical Potential.

1.3 CORROSION CHEMISTRY

The most common and important electrochemical reactions in the corrosion of iron are

Thus

Fe → Fe2+

+ 2e- Anodic reaction (corrosion) (1)

2 H+

→ H2 + 2e- Cathodic reactions (simplified) (2)

H2O + ½ O2 + 2e- → 2 OH ‾ (3)

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Reaction 2 is most common in acids and in the pH range 6.5 – 8.5 the most important

reaction is oxygen reduction 3. In this latter case corrosion is usually accompanied by the

formation of solid corrosion debris from the reaction between the anodic and cathodic

products.

Fe2+

+ 2 OH ‾ → Fe(OH)2 , iron (II) hydroxide

Pure iron (II) hydroxide is white but the material initially produced by corrosion isnormally a greenish color due to partial oxidation in air.

2 Fe(OH)2 + H2O + ½ O2 → 2 Fe(OH)3 , Hydrated iron (III) oxide

Further hydration and oxidation reactions can occur and the reddish rust that eventually

forms is a complex mixture whose exact constitution will depend on other trace elements

which are present. Because the rust is precipitated as a result of secondary reactions it is

porous and absorbent and tends to act as a sort of harmful poultice which encourages

further corrosion.

4 Fe(OH)2(s) + O2(g) → 2 Fe2O3 •H2O(s) + 2 H2O

(Rust)

1.4 TYPES OF CORR OSIO NCorrosion is classified under two categories.

1. Uniform Corrosion

2. Localized Corrosion

1.4.1 UNIFORM CORR OSIO N

Uniform or general corrosion, is a corrosion process without appreciable

localized attack. It the most common form of corrosion and may appear initially

as a single penetration but with thorough examination of the cross section it becomes apparent that the base metal has uniformly thinned. Corrosion is

uniform if the time average of the corrosion current through a unit area of any

macroscopic dimension is independent of the position on the surface.

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1.4.2 LOCALISED CORR OSIO N

The consequences of localized corrosion can be a great deal, more

severe than uniform corrosion generally because the failure occurs

without warning and after a surprisingly short period of use or exposure. Application of the five basic facts needs greater thought

and insight.

1. Galvanic Corr osionGalvanic corrosion, bimetallic corrosion, occurs when two electrochemically dissimilar

metals are metallically connected and exposed to a corrosive environment. The less noble

metal(anode) suffers accelerated attack and the less noble metal (cathode) is cathodically

protected by the galvanic current. The tendency of a metal to

corrode in a galvanic cell is determined by its position in the

galvanic series. On the galvanic scale, aluminum and zinc are

more active than steel and in the presence of a chloride-

containing electrolyte, will corrode preferentially when in

contact with steel. Three special features of this mechanism need

to operate for corrosion to occur:

•

The metals need to be in contact electrically.• One metal needs to be significantly better at giving up

electrons than the other.

• An additional path for ion and electron movement is necessary.

The galvanic corrosion mechanism can be used beneficially. It is widely employed as the

Primary protection system for steel. A thin coating of zinc on steel will corrode

preferentially, and this sacrificial action provides long-term protection for the steel

substrate.

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2. Concentration-cell corr osion.

Concentration-cell corrosion occurs because of differences in the environment

surrounding the metal. This form of corrosion is sometimes referred to as “crevice

corrosion,” “gasket corrosion,” and “deposit corrosion” because it commonly occurs in

localized areas where small volumes of stagnant solution exist. Normal mechanical

construction can create crevices at sharp corners, spot welds, lap joints, fasteners, flanged

fittings, couplings, threaded joints, and tube sheet supports. At least five types of

concentration cells exist; the most common are the “oxygen” and “metal ion” cells. Areas

on a surface in contact with an electrolyte having a high oxygen concentration generally

will be cathodic relative to those areas where less oxygen is present (oxygen cell). Areas

on a surface where the electrolyte contains an appreciable quantity of the metal’s ions

will be cathodic compared to locations where the metal ion concentration is lower (metalion cell).

3. Pitting corr osion.Pitting corrosion is a randomly occurring, highly localized form of

attack on a metal surface, characterized by the fact that the depth of

penetration is much greater than the diameter of the area affected10

.

Pitting is one of the most destructive forms of corrosion, yet its

mechanism is not completely understood. Steel and galvanized steel

pipes and storage tanks are suscepti ble to pitting corrosion and

tuberculation by many potable waters. Various grades of stainless

steel are susceptible to pitting corrosion when exposed to saline

environments.

4. Intergranular corr osion

Intergranular corrosion isa localized condition that occurs at, or in narrow zonesimmediately adjacent to, the grain boundaries of an alloy. Although a number of alloy

systems are susceptible to intergranular corrosion, most problems encountered in service

involve austenitic stainless steels (such as 304 and 316) and the 2000 and 7000 series

aluminum alloys. Welding, stress relief annealing, improper heat treating, or overheating

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in service generally establish the microscopic, compositional in homogeneities that make

a material susceptible to intergranular corrosion.

5. Stress corr osion cr ack ing

SCC (environmentally induced delayed failure) describes the phenomenon that can occur when many alloys are subjected to static, surface tensile stresses and are exposed to

certain corrosive environments. Because, it can happen ‘unexpectedly’ and rapidly after a

period of satisfactory service leading to catastrophic failure of structures or leaks in pipe

work. This is specific to a metal material paired with a specific environment. Examples of

well-known material/environment pairs are:

INTERGRANULAR TRANSGRANULAR

6. Dealloying

Dealloying is a corrosion process in which one element is preferentially removed from

an alloy. This occurs without appreciable change in the size or shape of the component;

but the affected area becomes weak, brittle, and porous. The two most important

examples of dealloying are the preferential removal of zinc from copper zinc alloys

(dezincification), and the preferential removal of iron from gray-cast iron (graphitic

corrosion). Graphitic corrosion sometimes occurs on underground cast iron water mains

and leads to splitting of the pipe when the water pressure is suddenly increased.

7. Erosion corr osion.Erosion corrosion refers to the repetitive formation (a corrosion process) and destruction

(a mechanical process) of the metal's protective surface film. This typically occurs in a

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moving liquid. Erosion may be impinging (in the case of a pipe ell) or sliding (pipe wall)

when it occurs. An example is the erosion corrosion of copper water tubes in a hot, high

velocity, soft water environment. Cavitation is a special form of erosion corrosion.

1.5 CORROSING PR EVE NTIO N

In practical terms it is not usually possible to eliminate completely all corrosiondamage

to metals used for the construction. By retarding either the anodic or cathodic reactions

the rate of corrosion can be reduced. This can be achieved in several ways:

1.5.1 Conditioning the Metal

This can be sub-divided into two main groups:

(a) Coating the metal

In order to interpose a corrosion resistant coating between metal and environment.

The coating may consist of:

• Another metal, e.g. zinc or tin coatings on steel,

• A protective coating derived from the metal itself, e.g. aluminum oxide on

“anodised” aluminum,

• Organic coatings, such as resins, plastics, paints, enamel, oils and greases.

The action of protective coatings is often more complex than simply providing a barrier

between metal and environment. Paints may contain a corrosion inhibitor or zinc coating

on iron or steel confers cathodic protection.

(b) Alloying the metal To produce a more corrosion resistant alloy, e.g. stainless

steel, in which ordinary steel is alloyed with chromium and nickel. Stainless steel

is protected by an invisibly thin, naturally formed film of chromium sesquioxide

Cr 2O3.

1.5.2 Conditioning the Corr osive Envir onment

(a) Removal of Oxygen By the removal of oxygen from water systems in the pH

range 6.5-8.5 one of the components required for corrosion would be absent. The

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removal of oxygen could be achieved by the use of strong reducing agents e.g.

sulphite.

(b) Corr osion Inhi bitor s

A corrosion inhibitor is a chemical additive, which, when added to a corrosive

aqueous environment, reduces the rate of metal wastage. It can function in one of

the following ways:

1. Anodic inhibitors – as the name implies an anodic inhibitor interferes with the

inhi bitors. Examples of anodic inhibitors include orthophosphate, nitrite, ferricyanide

and silicates.

2. Cathodic inhi bitor s– Additives that suppress cathodic reactions called cathodic

inhibitors. They function by reducing the available area for the cathodic reaction. This is

often achieved by precipitating an insoluble species onto the cathodic sites. Zinc ions are

used as cathodic inhibitors because of the precipitation of Zn(OH)2 at cathodic sites as a

consequence of the localized high pH. Cathodic inhibitors are classed as safe because

they do not cause localized corrosion.

3. Adsorption type corr osion inhibitors – Many organic inhibitors work by an

adsorption mechanism. The resultant film of chemisorbed inhibitor is then responsible for

protection either by physically blocking the surface from the corrosion environment or by

retarding the electrochemical processes. The main functional groups capable of forming

chemisorbed bonds with metal surfaces are amino (-NH2), carboxyl (-COOH), and

phosphonate (-PO3H2) although other functional groups or atoms can form co-ordinate

bonds with metal surfaces.

4. Mixed inhibitors – Because of the danger of pitting when using anodic inhibitors

alone, it became common practice to incorporate a cathodic inhibitor into formulated

performance was obtained by a combination of inhibitors than from the sum of the

individual performances. This observation is generally referred to a ‘synergism’ and

demonstrates the synergistic action which exists between zinc and chromate ions.

(c) Electr ochemical Control/Cathodic Pr otection

Since corrosion is an electrochemical process its progress may be studied by measuring

the changes which occur in metal potential with time or with applied electrical currents.

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The principle of cathodic protection is in connecting an external anode to the metal to be

protected and the passing of an electrical dc current so that all areas of the metal surface

become cathodic and therefore do not corrode. The external anode may be a galvanic

anode, where the current is a result of the potential difference between the two metals, or

it may be an impressed current anode, where the current is impressed from an external dc

power source. In electro-chemical terms, the electrical potential between the metal and

the electrolyte solution with which it is in contact is made more negative, by the supply

of negative charged electrons, to a value at which the corroding (anodic) reactions are

stifled and only cathodic reactions can take place. In the discussion that follows it is

assumed that the metal to be protected is carbon steel, which is the most common

material used in construction. The cathodic protection of reinforcing carbon steel in

reinforced concrete structures can be applied in a similar manner. Cathodic protection can

be achieved in two ways:

• Use of galvanic (sacrificial) anodes

• Use of “impressed” current.

Galvanic anode systems employ reactive metals as auxiliary anodes that are directly

electrically connected to the steel to be protected. The difference in natural potentials

between the anode and the steel, as indicated by their relative positions in the galvanic

series, causes a positive current to flow in the electrolyte, from the anode to the steel.

Galvanic effects may be predicted by means of a study of the Galvanic Series which is a

list of metals and alloys placed in order of their potentials in the corrosive environment,

such as sea water. The Galvanic Series should not be confused with the Electrochemical

Series, which lists the potentials only of pure metals in equilibrium with standard

solutions of their ions. Thus, the whole surface of the steel becomes more negatively

charged and becomes the cathode. The metals commonly used, as sacrificial anodes are

aluminum, zinc and magnesium. These metals are alloyed to improve the long-term

performance and dissolution characteristics. Similar protection is obtained when steel is

coated with a layer of zinc. Even at scratches or cut edges where some bare metal is

exposed the zinc is able to pass protective current through the thin layer of surface

moisture.

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Impressed-current systems employ inert (zero or low dissolution) anodes and use an

external source of dc power (rectified ac) to impress a current from an external anode

onto the cathode surface. The connections are similar for the application of cathodic

protection to metallic storage tanks, jetties, offshore structures and reinforced concrete

structures.

Impressed –Current System

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