Corrosion Control Methods

8/6/2019 Corrosion Control Methods

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CORROSION CONTROL METHODS

Some of the commonly used methods to control/prevent corrosion include:

HUMIDITY

Control the level of humidity around the metal. For example, Water reacts

with iron to form carbonic acid, which breaks down the bonds of the iron. Atthe same time, at least some of the water is broken up into hydrogen and

oxygen. When iron comes in contact with the oxygen from the water, theoxygen and iron bond to form iron oxide (rust). By keeping the humidity level

low around an object containing iron, the amount of corrosion that will occuris minimized. Dehumidifiers are a simple, inexpensive way to control the

humidity in a confined environment.

PURITY OF METAL

Corrosion resistance of a metal increases on increasing its purity. This isbecause impurities help in creating tiny electrochemical cells and to promote

corrosion due to heterogeneity.

PROTECTIVE COATINGS

Materials preferred as coatings should be chemically inert and must becapable of preventing the penetration of the environment to the base metal.

It should also be wear-resistant, hard, oxidation resistant and thermallyinsulated.

PAINTING

Coat the surface of the metal with a corrosion-resistant paint. The paint will

serve as a barrier between the metal and the moisture and other corrosiveagents that can come in contact with it. With such a barrier in place, the

chemical reactions that leads to the formation of oxides, sulphides,carbonates etc:- on the surface of the metal are deterred. But since paint

wears away over time, the surface must be repainted regularly.

OIL / GREASE / TAR / PLASTIC



Like paint, oil also acts as a protective barrier between the metal surfaceand corrosive agents. This method works best for small objects, such as irontools, but it also can be used for small parts of larger structures, such asiron bolts.

METALLIC COATINGS

ANODIC COATING

Base metal is protected sacrificially Electrode potential of the coating metal is lower than that of the

base metal. The base metal will not undergo corrosion until all the coating

metal is consumed even if some pores or breaks appear in such

coating. Example; Coating of zinc on iron

CATHODIC COATING

Here a noble metal of higher corrosion resistance protects thebase metal.

Electrode potential of the coating metal is higher than that of thebase metal.

The coating should be continuous. Any pores or breaks in thecoating will speed up corrosion.

Example; Coating of tin on iron.

PROPER DESIGNING

Figs 3.10, 3.11, 3.12

CATHODIC PROTECTION

In this method the metal to be protected from corrosion is made to act as acathode. Its most commonly used to protect steel, water, and fuel pipelinesand tanks.

SACRIFICIAL ANODIC PROTECTION



This process involves the enclosement of a metal to be protected with amore reactive anodic metal which will corrode first, thus protecting thelower layer. The more active metal so used is called “sacrificial anode”.'Rusting' can be prevented by connecting iron to a more reactive metal (e.g.,

zinc or magnesium). This is referred to as sacrificial protection or sacrificialcorrosion, because the more reactive protecting metal is preferentially

oxidized away, leaving the protected metal intact.(fig 3.13 from page 3.14)

IMPRESSED CURRENT CATHODIC PROTECTION (ICCP)

For larger structures galvanic anodes can’t deliver current economically for

complete protection. ICCP systems use anodes connected to DC power

source. Anodes for ICCP systems are tubular and solid rod shapes of variousspecialized materials like graphite, mixed metal oxide etc:-.(3.14)

ANODIC PROTECTION

Anodic protection impresses anodic current on the structures to be

protected (opposite to the cathodic protection). It is appropriate for thosemetals that exhibit passivity (stainless steel) and suitably small passive

current over a wide range of potentials. It is used in aggressiveenvironments eg:- solutions of sulphuric acids.

ALTERING THE ENVIRONMENT

It involves removal of the problem making constituents or by adding

substances capable of nullifying the activity of the corrosive constituent.Different methods included are:

Deaeration – removal of dissolved oxygen by mechanical agitation.Deactivation – Addition of chemicals which rapidly combine with the oxygenpresent in aqueous solution.

Dehumidification – reduce water content in airAlkaline neutralization – acidic character of the corrosive environmentneutralized using alkaline media.

ALLOYING



Metals can also be protected by 'alloying' or mixing with other metals (e.g.,chromium) to make non-rusting alloys. Alloys are homogenous solid solution of2 or more metals. Corrosion resistance of most metals increases when it is

alloyed with suitable elements. Stainless steel is an example of a non-rustingalloy of iron and carbon. Brass, an alloy containing copper is another metal

alloy which is less expensive and non reactive.

ADDITION OF CORROSION INHIBIORS

Substances which when added in small quantities to the corroding solution,decrease the corrosion of the metal are called corrosion inhibitors. There

are 2 types of inhibitors : Anodic inhibitors and cathodic inhibitors.

PASSIVATION

GALVANISING

Coating iron or steel with a thin zinc layer is called 'galvanising'. This layer isproduced by electrolytic deposition. Dipping the iron/steel object in molten

zinc and using it as the negative cathode zinc is coated on it. Zincpreferentially corrodes or oxidizes to form a zinc oxide layer that does not

flake off like iron oxide rust. This process resembles sacrificial protection.Also, if the surface is scratched, the exposed zinc again corrodes before

the iron and continues to protect it. But this process is not good for theprotection of cooking utensils because zinc dissolves in dilute acids to give

highly poisonous compounds.

TINNING

This method consists of coating tin over iron or steel articles. Excellentcorrosion resistance and non toxic nature of tin makes it a highly suitablematerial to coat containers made of steel, copper, etc. used for cooking and

storing food stuffs.

METAL CLADDING



This metal consists of bonding firmly and permanently a dense, homogeneouslayer of coating metal on the base metal on one or both sides. Corrosionresistant metals like silver, copper, etc. and their alloys are generalpreferred. The base metal sheet is kept between two thin sheets of the

coating metal and passed through rollers under high temperature andpressure. For example, ‘alcad’ produced in this method is used in aircraft

industry.

ELECTROPLATING

It is the process by which metals are deposited on metallic surfaces by

electrolysis. The surface is coated with metals like tin, chromium, nickel,

gold, silver, etc. The method involves passing a direct current through asoluble salts solution of the plating metal; the object to be plated is madethe cathode whereas the anode is either the coating metal itself or an inert

material of good electrical coCORROSION CONTROL METHODS

Some of the commonly used methods to control/prevent corrosion include:

HUMIDITY

Control the level of humidity around the metal. For example, Water reactswith iron to form carbonic acid, which breaks down the bonds of the iron. Atthe same time, at least some of the water is broken up into hydrogen and

oxygen. When iron comes in contact with the oxygen from the water, theoxygen and iron bond to form iron oxide (rust). By keeping the humidity level

low around an object containing iron, the amount of corrosion that will occuris minimized. Dehumidifiers are a simple, inexpensive way to control the

humidity in a confined environment.

PURITY OF METAL

Corrosion resistance of a metal increases on increasing its purity. This isbecause impurities help in creating tiny electrochemical cells and to promote

corrosion due to heterogeneity.

PROTECTIVE COATINGS



Materials preferred as coatings should be chemically inert and must becapable of preventing the penetration of the environment to the base metal.It should also be wear-resistant, hard, oxidation resistant and thermallyinsulated.

PAINTING

Coat the surface of the metal with a corrosion-resistant paint. The paint will

serve as a barrier between the metal and the moisture and other corrosiveagents that can come in contact with it. With such a barrier in place, the

chemical reactions that leads to the formation of oxides, sulphides,carbonates etc:- on the surface of the metal are deterred. But since paint

wears away over time, the surface must be repainted regularly.

OIL / GREASE / TAR / PLASTIC

Like paint, oil also acts as a protective barrier between the metal surfaceand corrosive agents. This method works best for small objects, such as iron

tools, but it also can be used for small parts of larger structures, such asiron bolts.

METALLIC COATINGS

ANODIC COATING

Base metal is protected sacrificially Electrode potential of the coating metal is lower than that of the

base metal. The base metal will not undergo corrosion until all the coating

metal is consumed even if some pores or breaks appear in suchcoating.

Example; Coating of zinc on iron

CATHODIC COATING

Here a noble metal of higher corrosion resistance protects thebase metal.

Electrode potential of the coating metal is higher than that of thebase metal.



The coating should be continuous. Any pores or breaks in thecoating will speed up corrosion.

Example; Coating of tin on iron.

PROPER DESIGNING

Figs 3.10, 3.11, 3.12

CATHODIC PROTECTION

In this method the metal to be protected from corrosion is made to act as acathode. Its most commonly used to protect steel, water, and fuel pipelines

and tanks.

SACRIFICIAL ANODIC PROTECTION

This process involves the enclosement of a metal to be protected with a

more reactive anodic metal which will corrode first, thus protecting thelower layer. The more active metal so used is called “sacrificial anode”.

'Rusting' can be prevented by connecting iron to a more reactive metal (e.g.,zinc or magnesium). This is referred to as sacrificial protection or sacrificialcorrosion, because the more reactive protecting metal is preferentiallyoxidized away, leaving the protected metal intact.

(fig 3.13 from page 3.14)

IMPRESSED CURRENT CATHODIC PROTECTION (ICCP)

For larger structures galvanic anodes can’t deliver current economically forcomplete protection. ICCP systems use anodes connected to DC power

source. Anodes for ICCP systems are tubular and solid rod shapes of variousspecialized materials like graphite, mixed metal oxide etc:-.

(3.14)

ANODIC PROTECTION

Anodic protection impresses anodic current on the structures to be

protected (opposite to the cathodic protection). It is appropriate for thosemetals that exhibit passivity (stainless steel) and suitably small passive



current over a wide range of potentials. It is used in aggressiveenvironments eg:- solutions of sulphuric acids.

ALTERING THE ENVIRONMENT

It involves removal of the problem making constituents or by adding

substances capable of nullifying the activity of the corrosive constituent.Different methods included are:

Deaeration – removal of dissolved oxygen by mechanical agitation.Deactivation – Addition of chemicals which rapidly combine with the oxygen

present in aqueous solution.Dehumidification – reduce water content in air

Alkaline neutralization – acidic character of the corrosive environment

neutralized using alkaline media.

ALLOYING

Metals can also be protected by 'alloying' or mixing with other metals (e.g.,chromium) to make non-rusting alloys. Alloys are homogenous solid solution of2 or more metals. Corrosion resistance of most metals increases when it is

alloyed with suitable elements. Stainless steel is an example of a non-rustingalloy of iron and carbon. Brass, an alloy containing copper is another metal

alloy which is less expensive and non reactive.

ADDITION OF CORROSION INHIBIORS

Substances which when added in small quantities to the corroding solution,decrease the corrosion of the metal are called corrosion inhibitors. There

are 2 types of inhibitors : Anodic inhibitors and cathodic inhibitors.

PASSIVATION

GALVANISING

Coating iron or steel with a thin zinc layer is called 'galvanising'. This layer isproduced by electrolytic deposition. Dipping the iron/steel object in molten

zinc and using it as the negative cathode zinc is coated on it. Zincpreferentially corrodes or oxidizes to form a zinc oxide layer that does not



flake off like iron oxide rust. This process resembles sacrificial protection.Also, if the surface is scratched, the exposed zinc again corrodes beforethe iron and continues to protect it. But this process is not good for theprotection of cooking utensils because zinc dissolves in dilute acids to give

highly poisonous compounds.

TINNING

This method consists of coating tin over iron or steel articles. Excellentcorrosion resistance and non toxic nature of tin makes it a highly suitable

material to coat containers made of steel, copper, etc. used for cooking andstoring food stuffs.

METAL CLADDING

This metal consists of bonding firmly and permanently a dense, homogeneous

layer of coating metal on the base metal on one or both sides. Corrosionresistant metals like silver, copper, etc. and their alloys are generalpreferred. The base metal sheet is kept between two thin sheets of thecoating metal and passed through rollers under high temperature and

pressure. For example, ‘alcad’ produced in this method is used in aircraftindustry.

ELECTROPLATING

It is the process by which metals are deposited on metallic surfaces byelectrolysis. The surface is coated with metals like tin, chromium, nickel,

gold, silver, etc. The method involves passing a direct current through asoluble salts solution of the plating metal; the object to be plated is made

the cathode whereas the anode is either the coating metal itself or an inertmaterial of good electrical conductivnductivity.

CORROSION



Corrosion is the deterioration/disintegration of materials by chemicalinteraction with their environment. The term corrosion is sometimes alsoapplied to the degradation of plastics, concrete and wood, but generallyrefers to metals.

Corrosion of Metals can be described as the slow process of deterioration /

destruction of the metal resulting in the loss of solid metallic materialthrough unwanted chemical or electrochemical changes taking place at its

surface. It is a chemical process by which the metal is oxidized. A wellknown example of an electrochemical corrosion is the rusting of iron, which

involves the process of formation of an oxide of iron due to oxidation of theiron atoms in solid solution. The tendency to corrode in a given environment

varies with the particular metal. Other common examples of corrosion

include tarnishing of silver, copper etc:-

The international standard definition of corrosion is as follows:"Physicochemical interaction between a metal and its environment which

results in changes in the properties of the metal and which may often leadto impairment of the function of the metal, the environment, or the

technical system of which these form a part". (ISO 8044-1986)

A broader, but widely accepted alternative definition, from the

International Union of Pure and Applied Chemistry (IUPAC) encompasses the

degradation of non-metals as well as metallic materials, as follows:"Corrosion is an irreversible interfacial reaction of a material (metal,

ceramic, polymer) with its environment which results in consumption of thematerial or in dissolution into the material of a component of the

environment. Often, but not necessarily, corrosion results in effectsdetrimental to the usage of the material considered. Exclusively physical or

mechanical processes such as melting or evaporation, abrasion or mechanicalfracture are not included in the term corrosion."

FACTORS INFLUENCING CORROSION



NATURE OF CORRODING ENVIRONMENT

a) Temperature: The rate of corrosion tends to increase with rising

temperature. As temperature increases, rate of reaction, diffusion

etc:- also increases. Temperature also has a secondary effect throughits influence on the solubility of air (oxygen), which is the most

common oxidizing substance influencing corrosion.

b) Humidity: Rate of corrosion increases with increase in humidity in theatmosphere. This is because atmospheric gases like CO2, O2 etc:- have

the tendency to dissolve in the moisture to form an electrolyte, givingrise to an electrochemical corrosion cell. The oxide film on the metal

surface may also absorb moisture leading to corrosion. Criticalhumidity is the relative humidity above which the atmospheric

corrosion rate increases sharply.

c) Effect of pH: Generally an acidic media (pH<7) is found to be more

corrosive than a basic (pH>7) or neutral media (pH=7). Therefore,

corrosion due to attack of an acid can be minimized by increasing thepH of the system.

1. Acid-soluble metals such as iron have a relationship as shown in Fig. a

below. In the middle pH range (≈4 to 10), the corrosion rate iscontrolled by the rate of transport of oxidizer to the metal surface.

Iron is weakly amphoteric. At very high temperatures such as those



encountered in boilers, the corrosion rate increases with increasingbasicity, as shown by the dashed line.

2. Amphoteric metals such as aluminum and zinc have a relationship as

shown in Fig. b. These metals dissolve rapidly in either acidic or basicsolutions.

3. Noble metals such as gold and platinum are not appreciably affectedby pH, as shown in Fig. c.



d) Existence of corrosive impurities in atmosphere: Increase in thepresence of corrosive gases like CO2, H2S, SO2 and fumes of HCl,

H2SO4 etc:- can lead to an increased corrosion rate. As already

mentioned, corrosion increases with decrease in pH. So increase inacidity of the liquid surrounding the metal leads to an increase in theelectrical conductivity.

e) Flow velocity: Flow velocity of process streams increases the diffusionand corrosion rates. Therefore to decrease the corrosion rate, in caseof non-passivating type corroding metals, minimization of flow velocity

helps.

f) Formation of oxygen concentrating cell: Waterline corrosion, crevice

corrosion etc:- are attributed to the formation of oxygenconcentration cells. Oxygen concentration cell formed due to

differential aeration promotes corrosion particularly at regions of lowoxygen concentration. The less oxygenated part becomes anodic and

the more oxygenated part becomes cathodic, thus setting up anoxygen conc. resulting in corrosion.

g) Nature of surrounding ions: The ions surrounding the particular metal

also has an influence on its corrosion pattern. There may be ions,

which in the presence of that metal lead to an increased corrosionrate. Iron for example undergoes rapid corrosion in a medium ofammonium salts. But there are also ions in whose presence corrosion isprevented by formation of a protective coat on the metal. Silicateions help in formation of silica gel on the metal surface, preventing

further corrosion.

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h) Conductance of corroding media: If the surrounding media is soilcontaining clay and minerals, the metallic structures buried insidethem undergo severe damage due to corrosion than that buried underdry sandy soils because the former is more conducting.

i) Polarisation of electrodes: Substances capable of dissolving in the

corroding medium to develop a protective layer either at the anodic orcathodic area called corrosion inhibitors can make irreversible

changes around the electrodes, opposing the direction of corrosioncurrent flow. This polarisation of electrodes decreases the potential

at both electrodes. Hence corrosion rate also decreases.

(Figure 3.9 from page 3.11)

Evans diagrams are obtained when potentials of polarized electrodesare plotted against current in the corrosion circuit.

TYPES OF CORROSION



It is convenient to classify corrosion by the forms in which it manifestsitself, the basis for this classification being the appearance of the corrodedmetal. Some of the various forms of corrosion are quite unique, but all ofthem are more or less interrelated. The ten main forms of corrosion are:

1) Dry corrosion

2) Wet corrosion

3) Rusting of iron

4) Galvanic corrosion

5) Pitting corrosion

6) Intergranular corrosion

7) Water line corrosion

8) Stress corrosion

9) Concentration cell corrosion

10) Microbiological corrosion

Other forms of corrosion include uniform corrosion, crevice corrosion,

graphitic corrosion, selective leaching, erosion corrosion etc:-



Uniform or General Corrosion: The reaction starts at the surface and

proceeds uniformly. The metal loss is uniform from the surface. Itsoften combined with high-velocity fluid erosion, with or without

abrasives.

Pitting Corrosion: The metal loss is randomly located on the metal surface.

The basis metal is eaten away and perforated in places in the mannerof holes, the rest of the surface being affected only slightly or not at

all. Its often combined with stagnant fluid or in areas with low fluidvelocity.

Galvanic Corrosion: Occurs when two metals with different electrode

potential is connected in a corrosive electrolytic environment.Increased corrosion in crevices or cracks or at contact surfaces

between two metal articles. The anodic metal develops deep pits andgroves in the surface.



Crevice Corrosion: Occurs at places with gaskets, bolts and lap joints wherecrevice exists. Crevice corrosion creates pits similar to pittingcorrosion.

Graphitic Corrosion: Cast iron loosing iron in salt water or acids. Leaves the

graphite in place, resulting in a soft weak metal.

Wide pitting corrosion: The corrosion causes localized scarring.

Intergranular corrosion: Imperceptible or barely perceptible from outside,

since the corrosion proceeds at the grain boundaries.

Transgranular or intragranular corrosion: The grain boundary material isretained, since the corrosion proceeds preferentially within the grain.

Selective corrosion: Corrosive attack on structural constituents

Exfoliation corrosion: Occurs in deformed articles. Corrosion follows "fiber

orientation".



Interfacial corrosion: Frequently observed at water-air interfaces.

Galvanic or Two-Metal Corrosion

A potential difference usually exists between two dissimilar metals when

they are immersed in a corrosive or conductive solution. If these metals areplaced in contact (or otherwise electrically connected), this potential

difference produces electron flow between them. Corrosion of the lesscorrosion-resistant metal is usually increased and attack of the more

resistant material is decreased, as compared with the behavior of thesemetals when they are not in contact. The less resistant metal becomes

anodic and the more resistant metal cathodic. Usually the cathode orcathodic metal corrodes very little or not at all in this type of couple.

Because of the electric currents and dissimilar metals involved, this form ofcorrosion is called galvanic, or two-metal, corrosion.

Pitting

Pitting is a form of extremely localized attack that results in holes in the

metal. These holes may be small or large in diameter, but in most cases theyare relatively small. Pits are sometimes isolated or so close together that

they look like a rough surface. Generally a pit may be described as a cavityor hole with the surface diameter about the same as or less than the depth.



Pitting is one of the most destructive and insidious forms of corrosion. Itcauses equipment to fail because of perforation with only a small percentweight loss of the entire structure. It is often difficult to detect pitsbecause of their small size and because the pits are often covered with

corrosion products. In addition, it is difficult to measure quantitatively andcompare the extent of pitting because of the varying depths and numbers of

pits that may occur under identical conditions. Pitting is also difficult topredict by laboratory tests. Sometimes the pits require a long time-several

months or a year-to show up in actual service. Pitting is particularly viciousbecause it is a localized and intense form of corrosion, and failures often

occur with extreme suddenness.

Intergranular Corrosion

Grain boundary effects are of little or no consequence in most applicationsor uses of metals. If a metal corrodes, uniform attack results since grain

boundaries are usually only slightly more reactive than the matrix. However,under certain conditions, grain interfaces are very reactive and

intergranular corrosion results. Localized attack at and adjacent to grainboundaries, with relatively little corrosion of the grains, is intergranular

corrosion. The alloy disintegrates (grains fall out) and/or loses its strength.

Intergranular corrosion can be caused by impurities at the grain boundaries,

enrichment of one of the alloying elements, or depletion of one of theseelements in the grain-boundary areas. Small amounts of iron in aluminum,

wherein the solubility of iron is low, have been shown to segregate in thegrain boundaries and cause intergranular corrosion. It has been shown that

based on surface tension considerations the zinc content of a brass is higherat the grain boundaries. Depletion of chromium in the grain-boundary regionsresults in intergranular corrosion of stainless steels.(back to top)

Stress-corrosion cracking

Stress-corrosion cracking refers to cracking caused by the simultaneouspresence of tensile stress and a specific corrosive medium. Manyinvestigators have classified all cracking failures occurring in corrosive

mediums as stress-corrosion cracking, including failures due to hydrogenembrittlement. However, these two types of cracking failures responddifferently to environmental variables. To illustrate, cathodic protection is

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an effective method for preventing stress-corrosion cracking whereas itrapidly accelerates hydrogen-embrittlement effects. Hence, the importanceof considering stress-corrosion cracking and hydrogen embrittlement asseparate phenomena is obvious. For this reason, the two cracking phenomena

are discussed separately in this chapter.

During stress-corrosion cracking, the metal or alloy is virtually unattackedover most of its surface, while fine cracks progress through it. This crackingphenomenon has serious consequences since it can occur at stresses within

the range of typical design stress. Exposure to boiling MgCl2 at 310°F(154°C) is shown to reduce the strength capability to approximately that

available at 1200°F.

The two classic cases of stress-corrosion cracking are "season cracking" of

brass, and the "caustic embrittlement" of steel. Both of these obsoleteterms describe the environmental conditions present which led to stress-

corrosion cracking. Season cracking refers to the stress-corrosion crackingfailure of brass cartridge cases. During periods of heavy rainfall, especially

in the tropics, cracks were observed in the brass cartridge cases at thepoint where the case was crimped to the bullet. It was later found that

the important environmental component in season cracking was ammoniaresulting from the decomposition of organic matter.

Many explosions of riveted boilers occurred in early steam-drivenlocomotives. Examination of these failures showed cracks or brittle failures

at the rivet holes. These areas were cold-worked during riveting operations,and analysis of the whitish deposits found in these areas showed caustic, or

sodium hydroxide, to be the major component. Hence, brittle fracture in thepresence of caustic resulted in the term caustic embrittlement. Whilestress alone will react in ways well known in mechanical metallurgy (i.e.,creep, fatigue, tensile failure) and corrosion alone will react to produce

characteristic dissolution reactions; the simultaneous action of both

sometimes produces the disastrous results.



EFFECTS OF CORROSION

• Loss of economy and safety

• Reduced Strength

•

Downtime of equipment• Escape of fluids

• Lost surface properties

• Reduced value of goods

The consequences of corrosion are many and varied and the effects of these

on the safe, reliable and efficient operation of equipment or structures areoften more serious than the simple loss of a mass of metal. Failures of

various kinds and the need for expensive replacements may occur even

though the amount of metal destroyed is quite small. This calls on for theneed to protect/control various metals from corrosion.



WAYS TO CONTROL CORROSION

a) HUMIDITY

Control the level of humidity around the metal. For example, Water reactswith iron to form carbonic acid, which breaks down the bonds of the iron. Atthe same time, at least some of the water is broken up into hydrogen and

oxygen. When iron comes in contact with the oxygen from the water, theoxygen and iron bond to form iron oxide (rust). By keeping the humidity levellow around an object containing iron, the amount of corrosion that will occur

is minimized. Dehumidifiers are a simple, inexpensive way to control thehumidity in a confined environment.

b) PAINT

Coat the surface of the metal with a corrosion-resistant paint. The paint willserve as a barrier between the metal and the moisture and other corrosiveagents that can come in contact with it. With such a barrier in place, the

chemical reactions that leads to the formation of oxides, sulphides,



carbonates etc:- on the surface of the metal are deterred. But since paintwears away over time, the surface must be repainted regularly.

c) OIL / GREASE / TAR / PLASTIC

Like paint, oil also acts as a protective barrier between the metal surface

and corrosive agents. This method works best for small objects, such as irontools, but it also can be used for small parts of larger structures, such as

iron bolts.

d) SACRIFICIAL PROTECTION

This process involves the enclosement of a metal to be protected with amore reactive metal which will corrode first, thus protecting the lower layer.

'Rusting' can be prevented by connecting iron to a more reactive metal (e.g.,zinc or magnesium). This is referred to as sacrificial protection or sacrificialcorrosion, because the more reactive protecting metal is preferentially

oxidized away, leaving the protected metal intact.

e) ALLOYING

Metals can also be protected by 'alloying' or mixing with other metals (e.g.,chromium) to make non-rusting alloys. Stainless steel is an example of a non-

rusting alloy of iron and carbon. Brass, an alloy containing copper is anothermetal alloy which is less expensive and non reactive.

f) GALVANIZING

Coating iron or steel with a thin zinc layer is called 'galvanizing'. This layer isproduced by electrolytic deposition. Dipping the iron/steel object in moltenzinc and using it as the negative cathode zinc is coated on it. Zinc

preferentially corrodes or oxidizes to form a zinc oxide layer that does not

flake off like iron oxide rust. Also, if the surface is scratched, the exposedzinc again corrodes before the iron and continues to protect it.

g) ELECTROPLATING

Coating the surface with metals like tin, chromium, nickel etc. byelectroplating is also utilized to prevent corrosion. Steel cans are protected



by relatively un-reacted tin and works well as long as the thin tin layer iscomplete.

p.s.

Uniform corrosion

The reaction starts at

the surface andproceeds uniformly.

Localized corrosion

(pitting corrosion)

The basis metal is eaten

away and perforated inplaces in the manner ofholes, the rest of thesurface being affected

only slightly or not at all.

Wide pitting corrosion

The corrosion causes

localized scarring.

Intergranularcorrosion

Imperceptible orbarely perceptiblefrom outside, since the

corrosion proceeds atthe grain boundaries.

Transgranular orintragranular corrosion

The grain boundarymaterial is retained, sincethe corrosion proceeds

preferentially within thegrain.

Galvanic corrosion

Increased corrosion increvices or cracks orat contact surfaces

between two metalarticles.

Selective corrosion Exfoliation corrosion Interfacial corrosion



Corrosive attack onstructural constituents

Occurs in deformedarticles. Corrosion follows

"fiber orientation".

Frequently observedat water-air

interfaces.

corrosion



Anodic coating

galvanising

electroplating

Sacrificial protection



TYPES OF CORROSION

Pitting corrosion is a form of extremely localized corrosion that leads to the creation of small

holes in the metal. The driving power for pitting corrosion is the depassivation of a small area,

which becomes anodic while an unknown but potentially vast area becomes cathodic, leading to

very localized galvanic corrosion. The corrosion penetrates the mass of the metal, with limited

diffusion of ions. The mechanism of pitting corrosion is probably the same as crevice corrosion.

Pitting can be initiated by a small surface defect, being a scratch or a local change in

composition, or a damage to protective coating. Polished surfaces display higher resistance to

pitting.

For example, metals like stainless steel and aluminium which are normally protected by a thin

oxide film, are subjected to pitting corrosion in a chloride environment, thus making its unsuitable

for in use in a sea water system.

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Classification of Corrosion

Based on the mechanism of corrosion, it is classified into two types

1. Chemical or dry corrosion2. Electrochemical or wet corrosion

Dry corrosion:

• Corrosion takes place in dry state.

• In occurs due to the direct chemical attack of the metal by theenvironment.

• Corrosion products accumulate on the same spot, where corrosion occurs.

• Dry corrosion is self controlled.

• It follows absorption mechanism.

• Homogenous metal surface undergoes corrosion observed.

• Uniform corrosions are observed. Example: Formation of mild iron oxideon its surface.

Dry corrosion is a form of corrosion, that occurs at elevated

temperature without a liquid phase. It’s a high temperature oxidationreaction.

Wet corrosion:

• Corrosion takes place in the presence of moisture.

• It occurs due to the setting up of large numbers of galvanic cells.

• Corrosion products accumulate on cathode.



• Wet corrosion is continuous process.

• It follows electrochemical mechanism.

• Heterogeneous metals bimetallic surface is the condition for corrosion.

• Pitting corrosions are observed. Example: Formation of rust on iron under moist atmosphere.

Wet corrosion means that there is a liquid phase, for example

condensed humidity, in contact with metal. It’s the most common formof corrosion.

Some Types of Corrosion

1. Galvanic or Bimetalliccorrosion

Example: Zinc and copper, zinc forms the anode and is attacked and getsdissolved, whereas copper acts as cathode.



DRY CORROSION

WET CORROSION

RUSTING OF IRON

GALVANIC CORROSION

When 2 different metals or alloys come in contact with each other, the lessnoble metal corrodes protecting the other cathodically. This phenomenon is

called galvanic corrosion. Galvanic corrosion occurs when 2 different metalsare electrically connected and are immersed in an electrolyte. In order for

galvanic corrosion to occur, an electrically and ionically conductive path isnecessary. This effects a galvanic couple where the more active metal

corrodes at an accelerated rate and the more noble metal corrodes at aretarded rate. Galvanic corrosion is often utilized in sacrificial anodes.

For example zinc is often used as a sacrificial anode for steel structure likepipelines. Factors such as relative size of anode (smaller is preferred), type

of metal and operating conditions (temp, humidity), affect galvanic corrosion.



PITTING CORROSION

INTERGRANULAR CORROSION

Intergranular corrosion is the phenomenon in which there is increased rate

of corrosion along the grain boundaries rather than at the grain interior.Grains are “crystals” usually on a microscopic scale, that constitute the

(micro)structure of metals and alloys. This selective dissolution may lead tothe dislodgement of grains. Some significant examples include Intergranular

corrosion in sensitized stainless steels and exfoliation in aluminium alloysetc:-

At the temperature range of 450-850 oC carbon diffuses to the grainboundary of stainless steel and reacts with chromium to precipitate

chromium carbide. But for stainless steel, the corrosion resistance dependson the Cr content which due to depletion increases the susceptibility tocorrosion.



WATER LINE CORROSION

It is generally found that when water is stored in a steel tank, the maximumcorrosion occurs along a line just beneath the water meniscus. This is

because of the fact that the highly oxygenated area above the waterlineacts as the cathodic part, while the portion just below the waterline act as

the anodic part undergoing corrosion. This type of corrosion is commonlyseen in water tanks, base of ships etc:-

STRESS CORROSION (figure 3.4 from page 3.6)

Stress corrosion is the part of tensile stress (including residual stress

remaining after fabrication) and localized corrosion which combine toproduce a brittle cracking of metal under certain conditions. During stress-

corrosion cracking, the metal or alloy is virtually unattacked over most of itssurface, while fine cracks progress through it. It generally has seriousconsequences.



The stresses can be internal or applied. An example is that of brasscondenser tubes. The reason for this is the moist atmosphere containingammonia. The attack is along the grain boundaries which become more anodicwith respect to grain interior.

CONCENTRATION CELL CORROSION

This corrosion is also known as differential aeration corrosion. It occurs

when a metal is partially immersed in a solution or when partially covered bywater drops, dust, sand etc:- Its often combined with stagnant fluid or in

areas with low fluid velocity. Here the more aerated area acts as thecathode, while the less aerated area acts as the anode undergoing corrosion.

There are 3 general types of concentration cell corrosion:

> Metal ion concentration cell

> Oxygen concentration cell

> Active- passive concentration cell

The common examples in this type of corrosion include corrosion of heater

handles, knives etc:-



MICROBIOLOGICAL CORROSION

Microbiological corrosion is that caused / promoted by microorganisms. Itcan apply to both metals and non-metals, in both presence (aerobic) and in

lack of oxygen (anaerobic). The bacterial activities can -> produce acorrosive environment, -> alter metal film’s resistance, -> create electrolyticconcentration cells on the metal surface, -> affect rate of cathodic andanodic reaction.

Sulphate reducing bacteria are common in lack of oxygen. They producehydrogen sulphide, causing sulphide stress cracking. In presence of oxygen

some bacteria directly oxidize iron to iron oxides and hydroxides, otherbacteria oxidize sulphur and produce sulphuric acid causing biogenic sulphide

corrosion. Microbes like fungi, algae etc:- develop a microbiological film oniron surfaces. Such films can contain dissolved salts, acids etc:- thereby

creating local biological cells to accelerate corrosion.

WET CORROSION



RUSTING OF IRON

GALVANIC CORROSION



PITTING CORROSION

SHAPES OF PITTING

CORROSION

OXYGEN CONCENTRATION CELL

INTERGRANULAR CORROSION



STRESS CORROSION



Concentration cell corrosion

OXYGEN CONCENTRATION CELL



Documents

Corrosion Control Methods