basic corrosion.pdf

8/13/2019 Basic Corrosion.pdf

1/30

1

Introduction to Corrosion


2/30

2

Corrosion

Even in modern technology, corrosion still remains a very destructive

problem. It not only costs the economy billions of dollars by affecting theindustry, businesses and homes, but also endangers the health and safety of thegeneral population. Electrochemists and materials scientists continue to investigate

the causes of corrosion and develop methods for preventing but new challengesare always emerging.

With the exception of gold, metals are naturally unstable. Corrosion occurswhen metals react with oxygen, water, and other substances in the environment.

Even metallic gold will corrode under the certain conditions. Fortunately, mostmetals form protective films in reaction to their initial exposure to a normal,benign environment, that slow down the corrosion process.

Most corrosion reactions are electrochemical in nature. From anodic sites,where oxidation of the metal occurs, electric currents flow to cathodic sites, wherereduction of oxygen dissolved in water, hydrogen ions in acids, or SO2 in theatmosphere takes place. The electrochemical nature of corrosion is very similar,

whether the reaction takes place in an aqueous or dry environment. For example,aluminum corroding in sea water or silver tarnishing in air both contain traces ofsulfur compounds.


3/30

3

Corrosion is a major obstacle to overcome when developing many new technologies,

such as in the area of energy generation. The use of controlled nuclear fusion willrequire the development of alloys able to withstand corrosion at high temperatures. Thegeneration of energy from coal by magnetohydrodynamic systems, which pass hotionized gas through magnetic fields, involves temperatures and environments that can

destroy materials in contact with the gas. Solar energy systems require alloys towithstand hot circulating heat-transfer fluids. Geothermal systems must resistconcentrated solutions of corrosive salts at high temperatures and pressures. In order todrill for previously unrecoverable oil, technology must overcome stress corrosioncracking, microbiological degradation, and the highly corrosive marine environment.

Corrosion is accelerated by pollution. Priceless works of art, such as the gilded worksof art in Venice, Italy, the stained glass in many churches, and other valuable objectshave been affected.

However, corrosion can have some beneficial effects and is responsible for thebeautiful patinas that form on bronze statues and copper roofs. The corrosion ofweathering steel produces a dark purple-brown protective and decorative layer. Artistsand architects use it in sculptures and the construction of buildings. Curiously, the most

beautiful patina is formed in polluted atmospheres.


4/30

4

Types of Corrosion

When a metal is exposed to a corrosive environment, its protective film may bedestroyed so that the metal can be damaged. General corrosion is when the entire protectivefilm is dissolved and the entire metal is subjected to uniform attack. On the other hand,

breakdown of the protective film at specific defective sites results in a highly localizedform of corrosion called pitting, which can penetrate thick sheets of metal. Solid corrosion

products form around those sites and allow damaging chemical species that accelerate

pitting to accumulate underneath. A pile of sand, a barnacle, a gasket, or a collection ofleaves in a gutter act as a similar cover that allows a form of degradation called crevicecorrosion to occur.

Protective films may also fail when metals are exposed to high temperatures in the

presence of corrosive species. Mechanical processes such as tensile stress can also crackthe protective film and expose the metal to the corrosive environment. This type of attack iscalled stress corrosion cracking.

Galvanic corrosion results when a more active metal like steel or aluminum comes intocontact with a more noble metal like copper or gold. The more active metal has a greatertendency to become the anode and the more noble metal to become the cathode. Togetherthey form a corrosion cell. Therefore, when the aluminum comes into contact with copperin a corrosive environment, it will corrode more easily than it would by itself under the

protection of its naturally formed oxide film. The cathodic reaction takes placesimultaneously on the copper.


5/30

5

When there is an electrical potential difference between two pieces of metal and a paththat a current can flow over, electrolytic corrosion occurs . On thin film or integratedcircuits or on printed wiring boards, adjacent conductor paths are normally at differentapplied potentials; ionic transport occurs if moisture is adsorbed on the substrate to act as atransport medium. Moisture reaches the surface of a device by diffusing through the covercoat or by entering through a faulty seal. Ionic contaminants that increase the conductivity

of the adsorbed moisture layer also greatly increase the corrosion rate .

Thin tarnish films, such as oxides or sulfides, may have sufficient resistance to give aneffective open circuit in electrical contacts. Furthermore, in some applications, e.g.,communications, even films just a few monolayers in thickness can introduce unacceptable

noise into circuits.

The corrosion of many metals and alloys is limited by the initial stages of the

corrosion process itself. Fortunately, the initial reaction of a metal with its environmentresults in a very thin corrosion product film (


6/30

6

Damage

Corrosion has a negative economic impact. It is estimated that in the UnitedStates it is a couple percentage points of the gross national product. Some of the

costs could be avoided implementing proper technology.

Metal implants in the body, such as pacemakers, pins, plates, joints, and otherprosthetic appliances are prone to a variety of corrosion failure mechanisms.

Corrosion can affect our safety and result in injuries and loss of life. Anexample is the collapse of the Silver Bridge over the Ohio River in 1976, whichresulted in 46 deaths. It was attributed to stress corrosion cracking. Certain gas

pipeline explosions have also been attributed to stress corrosion. Recent problems

with aging aircrafts can be categorized as the result of corrosion effects andmany less serious failures, often not recognized as corrosion problems, have causedinjury, maiming, and death. Corrosion must also be considered when using metalliccontainers for waste disposal, especially of nuclear or other toxic waste.


7/30

7

Prevention

Research into the characteristics and causes of corrosion has produced methods forpreventing or slowing it. For example, the addition of alloying elements to steel,

aluminum, etc., has produced many corrosion-resistant alloys, such as stainless steel.

Protective coatings are very useful for controlling corrosion. Among these are:

1. organics, such aspaints,plastics, waxes, andgrease,

2. silicones,

3. inorganic compounds, such as silicon nitride, glasses, and ceramics,

4. metallic coatings, such as chrome plate, which is more corrosion resistant than themetal it covers. Zinc plate or galvanizing, does not act as a barrier, but reduces thetendency of the underlying metal to corrode by forming a galvanic cell. Zinc is moreactive than the steel to which it is coupled so it acts as the anode of the corrosion cell andundergoes galvanic corrosion. In the process, it protects the steel by making it the cathode.The zinc is called a sacrificial anode, the term for this process is cathodic protection.

Corrosion may be prevented by chemicals called inhibitors. Some inhibitors, mainlycertain organic molecules, adhere to the metals surface and form a single layer barrier tothe environment. Other inhibitors control the corrosiveness of the environment by actingas a buffer.

In anodic protection, a protective or passivating film is induced electrochemically onmetals (such as steel) which do not form these films naturally by exposure to theenvironment.


8/30

8

New Approaches for Corrosion Prevention

Life PredictionRapid and early assessment of the rate of corrosion for a material in a given

environment plays a crucial role in corrosion prevention. Due to the electrochemicalnature of corrosion, electrochemical science continues to contribute to the prediction of

corrosion life and the establishment of corrosion mechanisms. Whether performed inthe laboratory or in the field, life prediction methodology helps to improve designs.Corrosion life prediction has become especially important to high-technology areas thatdemand high reliability, such as microelectronics and aerospace. Storage of nuclearand chemical waste also requires accurate life prediction for containment materials.

The introduction of microcomputers has broadened the scope of electrochemicalevaluation of corrosion rates and characterization of mechanism. Electrochemicalimpedance spectroscopy (EIS) provides real-time, in situ rate information and

mechanistic details for corroding metals and is particularly applicable for coated,anodized or slowly corroding metals. Computers allows for the extraction of features inseemingly random electrochemical noise and offer promise as a means of evaluatinglocalized corrosion such as pitting, crevice corrosion, stress corrosion cracking (SCC),and microbiologically influenced corrosion (MIC).


9/30

9

Ultra-microelectrodes hold advantages for probing electrochemical processes in highresistivity media and can perform electroanalysis in small volumes of electrolyte. These

features can be exploited to characterize corrosion in high resistivity media andcharacterizing localized corrosion.

A number of well established surface analytical techniques such as Auger electronspectroscopy (AES), X-ray photoelectron spectroscopy (XPS) and surface ion mass

spectroscopy (SIMS) that probe the first few layers of metallic surfaces remain importanttools for corrosion research. However, these methods are limited because they arerestricted to an ex situ ultrahigh vacuum (UHV) environment. Advances in the use of pre-conditioning cells, have recently been implemented. A number of vibrational

spectroscopies including laser Raman and enhanced Raman spectroscopy can beapplied in situ. Older optical methods such as ellipsometry remain viable tools forin situexamination of corroding surfaces. Newer non-linear optical methods of second harmonicgeneration (SHG) are not yet fully developed as a tool for examining the behavior of acorroding surface. Increased availability of synchrotron radiation now gives the corrosion

scientist a powerful spectral method of obtaining information on the surface chemistry ofmetals with the use of the X-ray absorption near edge structure (XANES) and the surfaceextended X-ray adsorption fine structure (SEXAFS) technoques.


10/30

10

Microscopies such as scanning electron microscopy and transmission electronmicroscopy remain important techniques used in corrosion research. Newermicroscopies such as scanning tunneling microscopy (STM) and surface forcemicroscopy provide the corrosion scientist direct details of the molecular structure.

Acoustic microscopy has also been introduced as an analytical tool for corrosionresearch.

The recently developed electrochemical quartz crystal microbalance allowssimultaneous evaluation of mass change in the sub-nanogram range and

electrochemical current. Therefore, it is an important tool for evaluatingfundamental corrosion and passivation mechanisms. This method relies on thefrequency shift of oscillation modes of a quartz crystal coated with the testelectrode. Mass changes as well as viscoelastic coupling to the electrolyte

influence the response. The method can be quite useful for evaluating slowcorrosion rates critical to passivity and microelectronics and for providingfundamental information on double layer structures. In the future, it may also haveapplicability as an atmospheric and gas phase corrosion sensor, although the

method is now limited to thin metallic films.


11/30

11

Surface Treatments

Surface treatments include the work of surface modification, coating, and alloy

development. Surface modification allows the use of base metals with advantageousstructural properties but unattractive environmental resistance and continues to be a focusof active research. Ion implantation also remains an active area. Its primary objective is toidentify implanted ions that will alter the passive film to make it more protective. Conceptsrelated to the charge state (the Zeta potential or pH of zero change) of the resultingelectrolyte-immersed passive oxide and the susceptibility of attack by the chloride ionhave directed the search for particular elements to implant.

The chemical modification of surfaces through conversion coatings is an importantapproach to corrosion protection, although environmental concerns now make the use ofchromates less attractive. A number of alternative conversion coatings including thosecontaining cerium, tungstate, molybdate, and silicate are currently being investigated.

Adding alloys, such as chromium, to ferrous materials provides improved corrosion

resistance by improving the stability of the passive film. The synergistic influence ofmolybdenum and nitrogen in improving the pitting resistance of stainless steels has beenthe subject of considerable research. Alloy development for non-ferrous materials hasfocused on removing heterogeneous components that provide catalytic sites for thecathodic half of the corrosion reaction, typically the reduction of oxygen. In addition, rareearth elements added to magnesium improve the oxidation resistance of the alloy.However, the search for stainless aluminum and magnesium still continues.


12/30

12

Organic coatings are still the most popular choice for protecting base metals in anumber of environments. Organic coatings provide a mass transport barrier, buttheir primary function is to act as a barrier for ionic charge transport and toblock surface reactions. As a result, organic coatings must maintain adhesion incorrosive environments. Methods for pretreating surfaces, including conversioncoating and using adhesion promoters, are very important. Novel approaches,including ion implantation and corona discharge, have been considered as

methods to promote adhesion. Organic coatings can also provide a matrix forcorrosion inhibitors, such as inhibiting pigments. Use of time/stress-released

pigments represents a current area of active research. As with conversion coatings,environmental concerns demand radical reformulations of organic coatings toeliminate heavy metal pigments and most of the volatile organic chemicals (VOC).

The application of conducting polymers is something relatively new and usedvery rarely to prevent corrosion. However, conducting polymers will continue to be

an important area of research, especially since they are able to provide activeprotection rather than just serve as barrier films.

Sol-gel technologies to form protective inorganic oxide films have not beenfully exploited. Since they hold a great deal of promise, they are expected to havethe greatest future impact on the development of coatings that can withstand hightemperatures.


13/30

13

Even though corrosion inhibitors cover a broad range of technology, manystudies have been in the anecdotal realm and barely applied scientific toolsavailable to the electrochemist. One significant exception is the adsorption oforganic compounds to inhibit the corrosion of steel in acidic environments,

where two dimensional films are formed. However, recent work has expanded ourknowledge of neutral media by revealing the detailed role of inorganic and organicinhibitors in neutral electrolytes, where three dimensional films form. Research onnovel approaches using multiphase organic/inorganic inhibitors is still ongoing.

Environmental concerns are now a greater priority for governments around theworld. Older methods for corrosion protection, particularly those that rely on theuse ofhexavalent chromate, are becoming obsolete. It is likely that in the near

future all heavy metal inhibitors, conversion coatings, and pigments willeventually be outlawed from widespread application and the use of the organiccoating VOC will have to be reduced. Changing environmental regulations willcontinue to motivate corrosion research and development in the coming years.


14/30

14

Additional Reading

Transient Techniques in Corrosion Science and Engineering W. H. Smyrl, D. D.Macdonald, and W. J. Lorenz, Editors, The Electrochemical Society Softbound ProceedingSeries, Volume 89-1, Pennington, NJ (1989).

Advances in Corrosion Protection by Organic Coatings, D. Scantlebury and M. Kending,Editors, The Electrochemical Society Softbound Proceedings Series, Volume 89-13,

Pennington NJ(1989).

Corrosion Protection by Organic Coatings, M. W. Kendig and H. Leidheiser, Jr., Editors,The Electrochemical Society Softbound Proceedings Series, Volume 87-2, Pennington

NJ(1987).

Surfaces, Inhibition, and Passivation, E. McCafferty and R. J. Brodd, Editors, TheElectrochemical Society Softbound Proceedings Series, Volume 86-7, Pennington NJ(1986).


15/30

15

Corrosion

D fi iti f C i


16/30

16

Definition of Corrosion

The corrosion of metal is an electrochemical phenomenon in nature.

Corrosion reactions are redox reactions in which a metal is

attacked by some substance in its environment and converted to an

unwanted compound.

All metallic corrosion involves an anodic process, which is an

oxidation reaction that releases electrons into the metal, and a

cathodic process, which is a reduction reaction that consumeselectrons at exactly the same site.

The corrosion of metals may appear as:

1. general material loss with discoloration and scaling;

2. the formation of deep gouges,

3. uncontrolled enlargement of cracks and crevices ;

4. inexplicable pinhole formations.

Types of Localized Corrosion Initially


17/30

17

Types of Localized Corrosion InitiallyAssociated with the Environment

(a) Crevice corrosion (b) Deposit corrosionNarrowgap

Paint film or debris

(d) Filiform corrosion(C) Waterline attack

Low [O2

] Severe

attack

High [O2]

Air

Corrosion products form a

network under a coating

Water

Types of Localized Corrosion Initially


18/30

18

Types of Localized Corrosion InitiallyAssociated with the Environment

(f) Drop corrosion(e) Erosion corrosion FlowHigh [O2] Low [O2] High [O2]

(g) Turbulent-flow corrosion (h) Fretting

Turbulent flow

Very small, frequent movement

Loss of metal via surface disruptioncaused by collapse of gas bubble

cavitation

T f C i


19/30

19

Types of Corrosion

Grain boundary

attack

Crystallinegrains

(b) Intergranular corrosion

Attack

Metal

(a) General, uniform attack corrosion

(c) Selective corrosion, e.g.

dezincification of brass

Porous Cu

Load

Cracks

Stress

(d) Stress corrosion cracking

T f C i


20/30

20

Types of Corrosion

(e) Pitting

Oxide film or noble metal

Pit

(f) Layer corrosion (exfoliation)

Cracks

Stress

(h) Corrosion fatigue(g) Graphitic corrosion

Anodic lossof metal

Graphite inclusions

acting as cathodes


21/30

21

The overall corrosion process is a chemical reactionrepresenting the spontaneous dissolution of a metal (M)

through a reaction with the environment. This can be

described schematically by one of the following processes:

M + n/4 O2 + n/2 H2O Mn+ + n OH- (10.1)

M + n H2O Mn+ + n/2 H2 + n OH

- (10.2)

M + n H+

Mn+

+ n/2 H2 (10.3)M + n H+ + n/4 O2Mn+ + n/2 H2O (10.4)


22/30

22

In reality, these chemical reactions do not occur by

direct interaction between a metal and an oxidizing

agent in the environment (e.g. O2, H2O or H+). Instead,each of these cell reactions is the result of an anodic and

a cathodic reaction occurring simultaneously at

identical rates on the corroding surface, i.e.:

Anode: M - ne-

Mn+

(10.5)Cathode: O2 + 2 H2O + 4 e

- 4 OH- (10.6)Or cathode: O2 + 4 H

+ + 4 e- 2 H2O (10.7)Or cathode: 2 H2O + 2 e

- H2 + 2 OH- (10.8)Or cathode: 2 H+ + 2 e- H2 (10.9)


23/30

23

Thus, the corrosion process represented by reactions(10.1)-(10.4) are overall cell reactions, each being the sum

of the anode reaction (10.5) and appropriate cathode

reaction, e.g.:

Anode: M - ne- Mn+ (10.5)Cathode: n/4O

2+ n/2H

2O + n e- n OH- (10.6)

Cell: M + n/4O2 + n/2H2O Mn+ + nOH- (10.1)

A Simplified Summary of the Step Involved in a


24/30

24

A Simplified Summary of the Step Involved in aCorrosion Process

2

4

2

4

1

5

O2

OH-

OH-O2

1

53B

3B

3A

e-

e-

Fe2+

Fe2+

Fe2+

5

4

Metal surface

(electrodes)Fe

Bulk solution

(electrolyte)H2O, OH

-, O2

Reaction layer

The corrosion of iron via oxygen reduction in an


25/30

25

The corrosion of iron via oxygen reduction in an

alkaline medium is considered

1. Mass transport of reactant O2 to the surface via convection

and diffusion.

2. Adsorption of reactant O2

and H2

O.

3. Electrochemical reactions:

4. Desorption of products (Fe2+ and OH-) or reaction between

products: e.g.

5. Mass transport of products Fe2+ and OH- away from the

surface by migration and convective diffusion.

3A anode Fe - 2e- Fe2+

3B cathode 1/2 O2 + H2O + 2 e- 2 OH-

Cell Fe + 1/2 O2 + H2O Fe2+ + 2 OH-

2Fe2+ + 4OH- 2Fe(OH)2 then2Fe(OH)2 + O2 + (n-2)H2O Fe2O3nH2O

This simplified treatment indicates some of the factors


26/30

26


influencing corrosion; the following points are noteworthy:

1. The anodic process results in metal dissolving from the 0 to the

n+ oxidation state; Mn+ may be a simple, hydrated metal ion

or it may represent a more complex soluble or insoluble species.

2. The rate of the anodic process may be significantly altered if

Mn+ builds up in solution or if Mn+ undergoes further reactions,

e.g. hydrolysis.

3. The same anodic process (reaction(10.5)) may be supported by

one or more of the four different cathode reactions (10.6)-(10.9).

4. The cathodic process may determine the nature of the

corrosion products (as well as the rate of reaction), e.g. the local

increase in pH due to reactions (10.6)-(10.9) may result in the

metal being coated by an oxide of hydroxide film.



27/30

27



5. The corrosion products are varied; they include dissolved

species or dissolved ions (e.g. Mn+, OH-); insoluble solids (e.g.M2On, M(OH)n) and gaseous (H2) species.

6. The reactants for corrosion may include a solid (e.g. M),

solvent or dissolved ions (e.g. H2O, OH-, H+) and dissolved

gases, e.g. O2.

7. The cathodic process (and, hence, the corrosion reaction)depends upon the supply of a cathodic reactant. Hence,

reaction (10.6) involves dissolved oxygen and the solvent, water;

reaction (10.8) requires only water; reaction (10.9) necessitates

protons; and reaction (10.7) involves protons and dissolved

oxygen. This has at least two further implications: (1) dissolved

oxygen is often supplied by convective diffusion resulting in the

possibility of mass transport control; and (2) the electrolyte

composition (in particular pH) is very important as it

determines the cathodic reaction which occurs and its rate.



28/30

28



8. The progress of a corrosion process usually results in compositional

changes in the electrolyte; in particular, the local pH near the cathodic

zones usually experiences an increase (due to reactions(10.6)-(10.9)). The significance of this effect depends upon the electrode

and electrolyte conditions; possible implications are the saponification

of paint films or the precipitation of basic metal oxides or hydroxides.

9. So far, reactions between the corrosion products themselves and

between the metal and the corrosion products have been ignored. It will

be seen later that the formation of metal oxides and hydroxides viachemical reaction may be critical in determining the thermodynamics

(section 10.2) and kinetics (section 10.3) of corrosion processes.

Additionally, in the case of acid electrolytes, hydrogen evolution

(reaction(10.9)) may result in the blistering of paint films or hydrogen

embrittlement.

Environmental causes


29/30

29

Environmental causesOils and greases

Fused salts

Liquid metal

Organic liquids

and solutions

Natural

Artificial

Atmospheric

Chemical fumes

Steam

Flue gases

Chemical streams

Distillation columns

Atmospheric

Sea water

River and estuarine water

Potable water

Underground water

Biochemical

Foodstuffs

Cooling water

Chemical process streams

Paint films and polymer

coatings

Oxygen

Nitrogen, CO2Contamination, H2S, SO2,

SO3,NH3, HCl, NH4Cl, etc.

Solid particle

Dry

Wet

Reducing

Oxidizing

Wet

Damp

Engines

Nuclear power stations

PlantAcid

Alkaline

Neutral

Complex-forming

High temperature

Ambient

Ambient or

evaluated

temperature

Non-aqueous

Emulsions

Aqueous (wet)

Aerosols

Gases and vapours

(wet or dry)

Chemicalproperties of

metals and

alloys

(heterogeneities

may be

submicroscopic,

microscopic, or

macroscopic)

Are Some Environments More Corrosive


30/30

30

Than Others?

1. Moist air is more corrosive than dry air.

2. Hot air is more corrosive than cold air.

3. Hot water is more corrosive than cold water.

4. Polluted air is more corrosive than clean air.

5. Acids are more corrosive than bases (alkalies).

6. Salt water is more corrosive than fresh water.

7. Stainless steel will outlast ordinary steel.

8. No corrosion will occur in a vacuum, even at very hightemperatures, etc.

basic corrosion.pdf

Documents