CORROSION CONTROL METHODS Some of the commonly used methods to control/prevent corrosio n 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. At the 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, the oxygen 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 occur is minimized. Dehumidifiers are a simple, inexpensive way to control the humidity in a confined environmen t. PURITY OF METAL Corrosion resistance of a metal increases on increasing its purity. This is because 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 be capable of preventing the penetration of the environment to the base metal. It should also be wear-resistan t, hard, oxidation resistant and thermally insulated. 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 corrosive agents 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
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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.
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.
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.
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
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
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 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."
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
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.
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,
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
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
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
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
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.
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.
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
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.