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PREVENTION OF CORROSION BY

MODIFICATION IN ENVIRONMENT

To avoid corrosion various protective method are commonly used.1) Control of the physical, mechanical, chemical characteristics of metals andalloys.2) Control of environment3) Control of external source of electric potential.

Control of the physical, mechanical, chemicalCharacteristics of metals and alloys.

The various method based on the above principle are listed below.1) Protective coatings.2) Purification.3) Heat treatment.4) Alloying.

5) Change in design.

1) Protective coatings.The protective coatings applied to prevent corrosion can be either metallic ornon-metallic type. Both metallic and non-metallic coatings applied to metalsfor the prevention of corrosion should have the following characteristics.

a) Good corrosion resistance against medium with which it comes incontact.

b) Perfect adherence to the underlying metal.c) Continuous, so as to cover the surface completely.

A) METALLIC COATINGS.The metal film coated on surface of the base metal may act to prevent

corrosion of base metal in two ways being either cathode or anode.

a) Cathodic coating: It provides physical barrier between base metaland corrosion media. E.g.Coating of gold, aluminum, chromium, brassand iron.

b) Anodic coating: It provides better solvating cage or moreanodic surface than base metal. E.g. coating of zinc,aluminum and cadmium on steel.


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ADVANTAGES OF METAL COATINGS

1) More durable than non metallic coatings.2) More decorative than most of other cladding.

3) Best shielding against corrosion as compared to all othermetals.

METHOD OF APPLYING METALLIC COATINGS.

A) Hot dipping: In this method case is dipped in molten bath ofmetal to be coated. Although the method is simple but manycomplications are involved. Method is not very good due tofollowing reasons.

a) Uneven coating is obtained.b) Control of proper thickness is difficult.

B) Spraying: In this method the liquid metal is sprayed oversubject. The surface is grit blasted and metal is applied as ashower of tin, globules, which on striking the surface flatten togive splay porous coating.

C) Sherardizing: The coating metal in this process is takenin powder form and base metal pieces to be coated are

revolved in cylinder with thatpowder.

D) Mechanical cladding: This method is used for copperAnd steel plates or lead on copper sheet. This method can beused only for sheets and plates.

E) Cementation: This process differ from cladding only inSense that in this case object is heated to sufficiently hightemp, so that coating metal diffuses into it form a layer ofalloy on surface.

ELECTROLYTIC COATINGS

This is most commonly employed method of coating metalnowadays. This method can be used to coat almost every metal and wide rangeof alloys. The object to be coated is cleaned, pickled, washed and then dippedinto suitable bath. Another plate of coatingmetal or noble metal dipped into it.These plates are connected to DC power source supply making the object plate


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as cathode. By varying c.d. distance between electrodes, temp, concentration ofbath, addition of various additives various properties such as brightness,thickness, adhesiveness, hardness can be varied.

Some of the most common used metals for coatings and

properties of these coatings:

A) NICKEL:

Properties: 1) good wear and corrosion resistance.2)Highly resistance to attack by water

and most aqueous solution atdifferent pH and concentration.

3) Most resistance to alkaline.4) Nickel coating is cathodic to iron and

Brass.

USES: For protection, decoration and wear resistance oniron, steel and brass. Recently it has been used also in zincbased alloy.

B) CHROMIUM :

Properties: 1) chromium surface when exposed to airis immediately coated with thin Film of oxide which istransparent, hard and corrosion resistance.

2) Chromium deposits has characteristics

bluish white tint.

Uses: chromium coating are done on house holdObjects to large machine tools, bearing of cylinder.

C) ZINC:Properties: 1) Zinc is anodic to steel and

hence if the coating is non continuous and has holes etc.., steel is not corroded.2) Zinc coating provides effective

protection to steel under sea water and atmospheric condition.

Uses: zinc has been coated on base metalsespecially on iron plates extensively.

D) COPPER: Properties: moderately good corrosion resistance toatmosphere and salt solutions.


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Uses: copper plating usually done only as undercoats before nickel orchromium plating.

E) ALUMINUM:

Properties: owing to fact that aluminum reacts

with oxygen and superficial inert film is formed and brings aboutprotection. It offers little if any electrochemical protection.

Uses: as a protective coating for iron and steel.

B) NON METALLIC COATING:Non metallic coatings applied to prevent corrosion are of two types.

1) Inorganic coating2) Organic coating

INORGANIC COATING: The principle coatings for metals are described below:a) Vitreous enamels: they form hard glassy outside layer and protectby acting as an efficient barrier to the corrosion environment.

b) Portland cement coating: they are usually several centimeters thickto give adequate protection to steel.

c) Conversion coating: these coatings are formed by reactions withthe metal surface and are of several distinct kinds viz phosphatecoating from acid phosphate on steels, oxide coatings on steel andaluminum formed by the reaction with oxygen and water andstream.

ORGANIC COATING: Organic coatings are inert organic barriers applied on the

surface of materials for both corrosion protection and decoration. Organiccoatings include varnishes, paints, enamels and lacquers.

2) METALLIC PURIFICATION:Pure metals are likely to resist corrosion better than

metals containing impurities. In certain cases very small amountof impurity increases the corrosion resistance by several folds.

Extremely pure metals corrode at a very slow rate,owing to their high hydrogen over potential. Any addition ofmore noble metals possessing somewhat lower hydrogen over

potential will, however behave as a cathode of the local cells.The rate of corrosion therefore rapidly increases with decreaseswith decreasing purity of parent metal.


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PURITY OF AL% LOSS IN WEIGHT (gm/day)

99.998 699.99 11299.97 6500

99.98 360099.2 190000

3) HEAT TREATMENT:The corrosion resistance of metals and alloys can also

be minimized by the proper heat treatment.

ANNEALING: it may be carried out to homogenize themicrostructure, composition and reduce the internal stresses,thereby improving the corrosion resistance.E.g. Elimination of coring, or cast structure or internal stresses.

HARDENING AND TEMPERING: hardening treatmentincreases the corrosion rate of steel because it contains singlephase martensite. The tempering treatment after hardeninghowever affects the rate of corrosion in following ways:

a) Tempering at low temperature: many galvanic cells of ferriteand carbides are formed resulting in a increase in corrosionrate.

b) Tempering at high temperature: the carbide agglomeratecausing reduction in number of galvanic cells which resultsin decrease in the corrosion rate.

3) ALLOYING: An increase in corrosion resistance of most

commercial metals can also be achieved by alloying themwith suitable alloying elements. For maximum corrosionresistance alloy should be from solid solution.

4) CHANGE IN DESIGN: Corrosion rate can also be reducedby proper design and fabrication. In general the designshould be such so as to maintain the uniform condition asfar as possible throughout.

Following steps are taken for minimize the galvanic corrosion.


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a) Material should be as close as possible in galvanicseries.

b) Dissimilar materials should be insulated.c) The anodic material should have large area where as

cathodic area should be much smaller.

MODIFICATION IN ENVIRONMENT

Corrosion is a result of interaction of materials with environment. Corrosionresistance is therefore not an inherent property of materials but depend to largeextent on the nature of the environment, so as to reduce its interaction withmaterial will provide a method of corrosion prevention. Corrosive environmentcan be modified by three ways.

1) Alteration of environment variables such as temp, concentration,velocity, pH etc.

2) Elimination of corrosive ingredients such as humidity, oxygen,Oxidizing agent, solid contaminates.

3) Introduction of inhibitors

1) ALTERATION IN ENVIRONMENT

A) Alteration in temperature:In most of the cases lowering the temperature of the

environment reduces the corrosion rate. The degree of reductiondepends upon the type of environment, metals, and alloys.

Reduction in corrosion rate is large in case such as 18 Cr-8Ni innitric acid, Monel in hydrofluoric acid, as shown in figure. (CurveA).


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Reduction is less in case A) such as 18Cr-8Ni in sulphuric acid,nickel in hydrochloric acid and iron in hydrochloric acid. (In fig.curve B). In certain cases lowering temperature increases thecorrosion rate. E.g. boiling sea water is less corrosive than seawater.

B) Alteration in concentration:The effect of corrosive is complex. Certain cases of corrosion

prevention by altering corrosive concentration.

As shown in figure lowering the concentration of corrosive reducesthe concentration rate. There can be a concentration below whichcorrosion rate remain constant.

Corrosion rate is higher at low concentration of corrosivemedium and it is less at higher concentration.


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C) Alteration in velocity:

Corrosion rate also depend on the velocity of theenvironment. This may increases or decreases the corrosion rate. In mostcases decreases in velocity reduces the corrosion rate. Decrease in velocityto zero can increase corrosion rate. During stagnation period corrosion rate isincreased due to settling of suspended matter, establishment of metal.

In cases where the corrosion process is under diffusion

control and metal is ready passivated. Increase in velocity reduces thecorrosion rate. (In fig curve A)

4) Alteration of pH value: high pH value of environment usually reducesthe corrosion of iron and steel. The curve given in following fig


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illustrates this point. It is clear that solubility of ferrous oxide rapidlyabove pH 9, falling to zero at pH=11.5

Corrosion rate will further increase:a) With further increase in pHb) With increase in velocity of water.c) With presence of oxygen.

2) ELIMINATION OF CORROSIVE INGRADIENTS:

A) DEHUMIDIFICATION: Dehumidification is the process of reducingenvironment moisture to such an extent the amount of water condensed onmetal surface is too negligible to cause corrosion.

B) ELIMINATION OF SOLID CONTAMINATION: Dissolved solidcontent in water increase the corrosion rate. It can be due to two reasons.

a) Solid contamination may increase the electrical conductivity. Thiswill speed up the electrode reaction and hence corrosion rate.

b) Dissolve ions can penetrate the protective coating formed bycorrosion product of inhibitor on the metal.


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C) ELIMINATION OF OXYGEN AND OXIDATION AGENT:Corrosion rate can be reduced in certain cases by reducting the concentrationof oxygen and oxidizing agents from the environment. As shown in abovefigure.

C) INTRODUCTION OF AN INHIBITORS:Inhibitors prevent corrosion by two mechanisms.

1) By formation of protective layer on metal surface.2) By reducing the corrosiveness of the environment towards the

metal.Inhibitors usually retard corrosion rate in three ways.

1) By slowing down the corrosion without suppressing itcompletely.

2) By prolonging the incubation period of the reaction.3) By passivating the metal surface by polymolecular film.

VARIOUS TYPES OF INHIBITORS:1) Filming inhibitors: a) Anodic

b) Cathodicc) Mixed

2) Adsorption inhibitors3) Vapour phase inhibitors.1) FILMING INHIBITORS:

The inhibitors which form a barrier, arecalled filming inhibitors. Filming inhibitors can be classified into

anodic, cathodic, or both aspect of corrosive processsimultaneously.

a) Anodic inhibitors: they operate by producing a passivating oxidefilm mainly at those parts of the surface where metal cations areformed.

b) Cathodic inhibitors: these inhibitors instead of deactivating theactive sides relay upon general coverage of the metal surface.

Cathodic inhibitors are generally less effective due to followingreason;

1) Mechanism of corrosion prevention is indirect while anodicinhibitors directly prevent the metal from mechanism.2) Precipitate form at cathode may not be much in soluble and

may not bond to the metal surface tightly.

c) Mix type: These inhibitors interfere with both anodic and cathodicreaction. An example of an inhibitive ion which may into thiscategory is benzoate.


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2) ADSORPTION INHIBITOR: Adsorption inhibitors are surface activeaddition agent that functions as a result of adsorption to the metal formingan impervious barrier either at anodic sides or indiscriminately over thesurface.

D) VAPOUR PHASE INHIBITOR: Vapour phase inhibitors are essentiallysoluble filming inhibitor. The vapour of which has corrosion inhibitingproperties. Vapour phase inhibitor consist of nitride benzoate andcarbonate anions attach to suitable heavy organic cation.

ADVANTAGES OF VAPOUR PHASE INHIBITORS:1) Easily applicable to articles.2) No need of removal of oil or grease film before application.

DISADVANTAGES OF VAPOUR PHASE INHIBITORS:1) Accelerates the corrosion rate of certain nonferrous metals.2) Requires effective sealing of package against loss of package

against loss of inhibiting vapour.3) This inhibitor discolors certain plastics.

APPLICATION OF INHIBITORS:

Inhibitors are used for protecting ferrous metals and nonferrousmetals such as Al, Sn, Mg, Cu and their alloys.

LIMITATIONS OF INHIBITORS:

1) It is not possible to add inhibitor to all corrosive system becausethey may contaminate the environment.

2) Certain inhibitors are toxic in nature. They cant be used inprotecting food utensils etc.

3) Most of the inhibitors cant be used at high temp.4) Mainly used in closed system.

REFERENCES:

1) AN INTRODUCTION TO ELECTROMETTALURGY BYRAJENDRA SHARAN & SATYA NARAIN.

2) CORROSION ENGUNEERING BYMARS G. FONTANA & NOBERT D. GREENE.


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