Corrosion and Its Prevention CHANGD

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Corrosion and its

prevention



IntroductionIntroduction

Corrosion is the deterioration of materials by chemical interaction with their Corrosion is the deterioration of materials by chemical interaction with their

environment. The term corrosion is sometimes also applied to theenvironment. The term corrosion is sometimes also applied to the

degradation of plastics, concrete and wood, but generally refers to metals.degradation of plastics, concrete and wood, but generally refers to metals.The most widely used metal is iron (usually as steel) and the followingThe most widely used metal is iron (usually as steel) and the following

discussion is mainly related to its corrosiondiscussion is mainly related to its corrosion..



Why do metals corrode?Why do metals corrode?

Any spontaneous reaction in the universe is associated with a lowering

in the free energy of the system. All metals except the noble metals

have free energies greater than their compounds. So they tend to

become their compounds through the process of corrosion.

Except noble metal, all metals are unstable to varying degrees in a

terrestrial atmosphere. The most widely used metals, namely, Iron,

aluminium, copper, nickel, silver and alloys of these metals all decay

and lose good mechanical properties.



Types of corrosionTypes of corrosion

Corrosion may be classified in different waysCorrosion may be classified in different ways

Wet / aqueous CorrosionWet / aqueous Corrosion

Temperature CorrosionTemperature Corrosion



Wet / aqueous corrosion is the major form of corrosion. Based on the appearance of

the corroded metal, wet corrosion may be classified as

Uniform or General

Galvanic or Two-metal

Crevice

Pitting

Intergranular

Velocity-assisted

Environment-assisted cracking

Type I

Wet / Aqueous corrosion



UNIFORM CORROSION

Corrosion over the entire exposed surface at a uniform rate. e.g..Atmospheric

corrosion.

Maximum metal loss by this form. Not dangerous, rate can be measured in the laboratory.

GALVA NIC CORROSION

When two dissimilar metals are joined together and exposed, the more active of the two

metals corrode faster and the nobler metal is protected. This excess corrosion is due to the

galvanic current generated at the junction.



Prevention of galvanic corrosion:

(1) Do not have the area of the more active metal

smaller than the area of the less active metal.

(2) If dissimilar metals are to be used, insulate them.

(3) Use inhibitors in aqueous systems whenever

applicable and eliminate cathodic depolarizers



Crevice corrosion :

Intensive localized corrosion within crevices &

shielded areas on metal surfaces Small volumes of

stagnant corrosive caused by holes, gaskets, surface

deposits, lap joints



PITTING

1)A form of extremely localized attack causing holes in the metal

2) Most destructive formAutocatalytic nature

3) Difficult to detect and measure Mechanism

Prevention of pitting corrosion :

(1) Use materials with appropriate alloying elements designed to minimize

pitting susceptibility. e.g. molybdenum in stainless steel.

(2) Provide a uniform surface through proper cleaning, heat treating and surfacefinishing.

(3) Reduce the concentration of aggressive species in the test medium, such as

chlorides, sulfates, etc.

(4) Use inhibitors to minimize the effect of pitting, wherever possible.

(5) Make the surface of the specimen smooth and shiny and do not allow any

impurities to deposit on the surface.



Intergranular corrosion:

The grain boundaries in metals are more active than the grains because of

segregation of impurities and depletion of protective elements. So preferential

attack along grain boundaries occurs. e.g. weld decay in stainless steels

Method of Prevention

The following are the methods of prevention of austenitic nickel chromium

stainless steels from intergranular corrosion:

(a) Purchase and use stainless steel in the annealed condition in which there is

no harmful precipitate. This only applies when the steel is not to be exposed to

the sensitizing temperature.

(b) Select low carbon grade steel with a maximum of 0.03% C, such as 304 L.

This would prevent the formation of harmful chromium carbide during

fabrication



Velocity Assisted corrosion :

Fast moving corrosives causea) Erosion-Corrosion,

b) Impingement attack , and

c) Cavitation damage in metals

Cavitation Damage :

Cavitation is a special case of Erosion-corrosion. In high velocity

systems, local pressure reductions create water vapour bubbles which

get attached to the metal surface and burst at increased pressure,

causing metal damage



EnvironmentAssisted Cracking:When a metal is subjected to a tensile stress and a corrosive medium, it may experience

EnvironmentAssisted Cracking. Three types:

1) Stress Corrosion Cracking

2) Hydrogen Embrittlement3) Liquid Metal Embrittlement

Stress Corrosion Cracking:

Static tensile stress and specific environments produce cracking

Examples:

1) Stainless steels in hot chloride2) Ti alloys in nitrogen tetroxide

3) Brass in ammonia



Hydrogen Embrittlement:High strength materials stressed in presence of hydrogen crack at reduced stress levels.

(a) Film rupture model

(b) Slip step dissolution modelHydrogen may be dissolved in the metal or present as a gas outside. Only ppm levels of H

needed

liquid metal embrittlement:

Certain metals likeAl and stainless steels undergo brittle failure when stressed in contact

with liquid metals like Hg, Zn, Sn, Pb Cd etc.

Molten metal atoms penetrate the grain boundaries and fracture the metal.Fig. Shows brittle IG fracture inAl alloy by Pb



Type II

Temperature corrosion:

Generally corrosion rates increase with increases in temperature. This is due to

several interrelated factors:

1. Higher temperatures tend to promote the corrosion reaction kinetics. Therefore

except in cases where oxygen is free to escape, higher temperatures boost the

corrosion rate.

2. Corrosive by products will have a higher diffusion rate at higher temperatures

and thus will be delivered to the corroding surface more efficiently.

(a) High Temperature corrosion

(b)Low temperature corrosion



High temperature corrosion

High temperature corrosion is a form of corrosion that does notrequire the presence of a liquid electrolyte. Sometimes, this type of

damage is called "dry corrosion" or "scaling".

High temperature metals requires neither moisture nor dissolved

electrolytes (salts, acids) to proceed.

Low temperature corrosion:

Low-temperature corrosion appears in the boiler as well as on other

surfaces where the temperature is under approx. 135°C. It is caused

by condensation of the acidic sulphur and chlorine-containing

gases.

This type of corrosion is temperature-dependent. New plants are

being designed differently in order to avoid low-temperature

corrosion



Causes of corrosionCauses of corrosionFailures of various kinds and the need for expensive replacements may occur even though the

amount of metal destroyed is quite small.

Some of the major harmful effects of corrosion can be summarized as follows:

1. Reduction of metal thickness leading to loss of mechanical strength and structural failure or

breakdown.

2. Hazards or injuries to people arising from structural failure or breakdown (e.g. bridges,

cars, aircraft).

3. Loss of time in availability of profile-making industrial equipment.

4. Reduced value of goods due to deterioration of appearance.

5. Contamination of fluids in vessels and pipes (e.g. beer goes cloudy when small quantities

of heavy metals are released by corrosion).

6. Perforation of vessels and pipes allowing escape of their contents and possible harm to the

surroundings.

7. Loss of technically important surface properties of a metallic component.

8. Mechanical damage to valves, pumps, etc, or blockage of pipes by solid corrosion products.



10) Buried gas or water supply pipes can suffer severe corrosion which is not

detected until an actual leakage occurs, by which time considerable damage may be

done.

11) In electronic equipment it is very important that there should be no raised resistance at

low current connections.

12) The lower edge of this aircraft skin panel has suffered corrosion due to leakage and

spillage from a wash basin in the toilet.



Barrier protection :

Here, a thin barrier is developed between the surface of iron and

atmosphere by one of the following methods:a) Painting of the surface

b) Coating the surface with a thin film of oil or grease

c) Developing a thin layer of some non ±corrosive metal like nickel,

chromium copper etc., by electroplating.

Sacrificial protection:

In this case, the surface of iron is covered with a more electropositive metal like

zinc or aluminum. Since this metal loses electrons more readily than iron, rusting

is prevented. As long as metal is present, iron does not get rusted. This type of

protection is called µsacrificial production¶.



Cathodic protection (Electrical protection):

This method is especially used for underground iron pipes. Here, the iron

pipe or tank is connected to a more electropositive metal like magnesium or

aluminum. The more electropositive metal acts like anode (supplies electrons) and

iron acts like cathode (receives electrons). Thus, iron is protected by turning it as a

cathode.Hence, the method is called µcathodic protection¶ .



(b) Alloying the metal

Alloying the metal is 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 Cr2O3

In general, the corrosion behavior of alloys depends on the interaction of:

1. The alloy of specific chemical composition and metallurgical structure.

2. The film on the alloy surface.

3. The environment, whether it is sufficiently aggressive to break down the

protectiveness of the surface film, thereby initiating localized corrosion.

4. The alloy/environment combination, controlling whether the film self

repairs after breakdown and, if not, the type and rate of corrosion that

propagates after initiation has occurred

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