1. School of Physics & Materials Science, THAPAR UNIVERSITY 1
Corrosion in metals
For metallic materials, the corrosion process is normally electrochemical.
- a chemical reaction in which there is transfer of electrons from one
chemical species to another.
Metal atoms characteristically lose or give up electrons is called an
M Mn+ + ne-
Materials Science & Engineering (UES012)
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The electrons generated from each metal atom that is oxidized are
transferred to and become a part of another chemical species.
This is a reduction reaction.
Reduction of hydrogen ions in an acid solution
Reduction reaction in an acid solution
containing dissolved oxygen
Reduction reaction in a neutral or basic
solution containing dissolved oxygen
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The principles of Corrosion
Not all metallic materials oxidize to form ions with same degree of ease.
If the iron and copper electrodes are connected electrically,
reduction will occur for copper at the expense of the oxidation
Cu2+ + Fe Cu + Fe2+
In terms of half-cell reactions:
Cu2+ ions will deposit (electrodeposit) as
metallic copper on the copper electrode,
while iron dissolves (corrodes) on the other side of the cell and
goes into solution as Fe2+ ions. Standard
Fe Fe2++ 2e-
Cu2+ + 2e- Cu
Cu2+ + Fe Cu + Fe2+
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When externally connected, electrons generated from the oxidation of iron
flow to the copper cell in order that Cu2+ be reduced.
In addition, there will be some net ion motion from each cell to the other
across the membrane.
This is called a galvanic couple - two metals electrically connected in a
liquid electrolyte wherein one metal becomes an anode and corrodes,
while the other acts as a cathode.
An electric potential or voltage will exist between the two cell halves.
Cu2+ + Fe Cu + Fe2+ Potential: 0.780 V
Fe2+ + Zn Fe + Zn2+ Potential: 0.323 V
Various electrode pairs have different voltages !
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As a consequence of oxidation, the metal
ions may either go into the corroding
solution as ions or they may form an
insoluble compound with nonmetallic
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This table is for reduction reactions. For oxidation reactions, the
corresponding voltage will change sign.
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Galvanic series: This represents the relative reactivities of a number of metals and
commercial alloys in seawater.
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The corrosion rate, or the rate of material removal as a consequence of the chemical
action, is an important corrosion parameter.
Corrosion penetration rate (CPR), or the thickness loss of material per unit of time.
where W = weight loss after exposure time t ; = density, and A = exposed specimen area, K is a
There is an electric current associated with electrochemical corrosion reactions.
Corrosion rate r can be written as
r = i/nF
i = current density (the current per unit surface area of material corroding)
n = number of electrons associated with the ionization of each metal atom
F = 96,500 C/mol.
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Passivity: Some normally active metals and alloys, under particular
environmental conditions, lose their chemical reactivity and become
extremely inert. E.g., chromium, iron, nickel, titanium, and many of their
This passive behaviour results from the formation of a highly adherent and
very thin oxide film on the metal surface, which serves as a protective barrier
to further corrosion.
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Types of Corrosion:
1. Uniform/general attack Corrosion
2. Galvanic or two-metal Corrosion
3. Pitting Corrosion
4. Crevice Corrosion
5. Intergranular Corrosion
6. Stress Corrosion
7. Erosion Corrosion
8. Cavitation Damage
9. Fretting Corrosion
10. Selective Leaching
11. Hydrogen Damage
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1. Uniform / general attack Corrosion: The chemical reaction proceeds uniformly on the
entire metal surface exposed to the corrosive environment. It often leaves behind a scale
E.g., general rusting of steel and iron and the tarnishing of silverware
Represent greatest destruction of metals!
The most common form of corrosion. It is also the least objectionable because it can be
predicted and designed for with relative ease.
1. Protective Coating
3. Cathodic Protection
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2. Galvanic or two-metal Corrosion: Galvanic corrosion occurs when two metals or
alloys having different compositions are electrically coupled while exposed to an
Galvanized Steel Tin-Plate
The less noble or more reactive metal in the particular environment will experience
corrosion; the more inert metal, the cathode, will be protected from corrosion.
a) steel screws corrode when in contact with brass in a marine environment
b) copper and steel tubing are joined in a domestic water heater, the steel will corrode in the
vicinity of the junction.
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Area Effect: The rate of galvanic attack depends on the cathode–anode area ratio; that
is, for a given cathode area, a smaller anode will corrode more rapidly than a larger
Reason: corrosion rate depends on current density, the current per unit area of corroding
surface, and not simply the current. Thus, a high current density results for the anode
when its area is small relative to that of the cathode.
A number of measures may be taken to significantly reduce the effects of
galvanic corrosion. These include the following:
1. If coupling of dissimilar metals is necessary, choose two that are close together in the
2. Avoid an unfavorable anode-to-cathode surface area ratio; use an anode area as large
3. Electrically insulate dissimilar metals from each other.
4. Electrically connect a third, anodic metal to the other two; this is a form of cathodic
protection, discussed presently.
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3. Pitting Corrosion: Localized corrosive attack which produces holes or pits in a metal.
Very destructive! leads to perforation of the metal.
Difficult to detect:
May be covered by corrosion products
Depth and no of pits vary
Generally requires an initiation period.
Once started the pits grow at an ever increasing
Grow in the direction of gravity and on lower
surfaces of the equipment.
Pits are initiated at places where local increase in
corrosion rates occurs. Local structural and/or
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O2 + 2H2O + 4e- 4OH- M+Cl- + H2O MOH + H+Cl-
Reaction is autocatalytic!
• Use materials with non-pitting tendency.
• Best resistance (2% molybdenum Steels)
Always do corrosion test before final selection.
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4. Crevice Corrosion: localized Concentration cell corrosion occurs in crevices
and recesses or under deposits of dirt or corrosion products where
the solution becomes stagnant and there is localized depletion of dissolved
Corrosion preferentially occurring at these positions is called crevice corrosion.
The crevice must be wide enough for the solution to penetrate, yet narrow enough
for stagnancy; usually the width is several thousandths of an inch.
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In many aqueous environments, the solution within the crevice has been found to
develop high concentrations of H+ and Cl- ions, which are especially corrosive.
(Stainless steel, Ti, Al and Cu alloys)
Many alloys that passivate are susceptible to crevice corrosion because
protective films are often destroyed by the H+ and Cl- ions.
Crevice corrosion may be prevented by:
• using welded instead of riveted or bolted joints,
• using nonabsorbing gaskets (like teflon) when possible,
• removing accumulated deposits frequently, and
• designing containment vessels to avoid stagnant areas and ensure complete
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5. Intergranular Corrosion: occurs preferentially along grain boundaries for some alloys
and in specific environments. The net result is that a macroscopic specimen disintegrates
along its grain boundaries.
This type of corrosion is especially prevalent in some stainless steels. When heated to
temperatures between 500 0C and 800 0C for sufficiently long time periods, these alloys
become sensitized to intergranular attack. Temp range is called as sensitizing temp range.
treatment permits the formation of small precipitate particles of chromium carbide (Cr23C6)
by reaction between the chromium and carbon in the stainless steel. These particles form
along the grain boundaries
Both the chromium and the carbon must diffuse to the grain boundaries to form
the precipitates, which leaves a chromium-depleted zone adjacent to the grain boundary.
Consequently, this grain boundary region is now highly susceptible to corrosion.
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Intergranular corrosion is an especially severe problem in the welding of stainless
steels, when it is often termed weld decay.
Stainless steels may be protected from intergranular corrosion by the following
• subjecting the sensitized material to a high-temperature heat treatment (heating followed by
water quenching) in which all the chromium carbide particles are redissolved,
• lowering the carbon content below 0.03 wt% C so that carbide formation is minimal,
• Alloying the stainless steel with another metal such as niobium or titanium, which has a
greater tendency to form carbides than does chromium so that the Cr remains in solid
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6. Stress Corrosion: Also known as stress corrosion cracking (SCC).
• results from the combined action of an applied tensile stress and a corrosive environment.
• some materials that are virtually inert in a particular corrosive medium become
susceptible to this form of corrosion when a stress is applied.
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Small cracks form and then propagate in a direction perpendicular to the stress
resulting in failure.
Failure behavior is characteristic of that for a brittle material, even though the metal
alloy is intrinsically ductile.
The cracks may form at relatively low stress levels, significantly below the tensile
Most alloys are susceptible to stress corrosion in specific environments, especially
at moderate stress levels. For example, most stainless steels stress corrode in
solutions containing chloride ions, whereas brasses are especially vulnerable when
exposed to ammonia.
The stress that produces stress corrosion cracking need not be externally applied;
it may be:
• a residual one that results from rapid temperature changes and uneven
• for two-phase alloys in which each phase has a different coefficient of
• gaseous and solid corrosion products that are entrapped internally can give rise
to internal stresses.
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Probably the best measure to take in reducing or totally eliminating stress
corrosion is to lower the magnitude of the stress:
• This may be accomplished by reducing the external load
• increasing the cross-sectional area perpendicular to the applied stress.
• Furthermore, an appropriate heat treatment may be used to anneal out any
residual thermal stresses.
Eliminate the corrosive environment.
Change the alloy.
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7. Erosion Corrosion: arises from the combined action of chemical attack and mechanical
abrasion or wear as a consequence of fluid motion.
• All metal alloys are susceptible to erosion–corrosion.
• It is especially harmful to alloys that passivate by forming a protective surface film; the
abrasive action may erode away the film, leaving exposed a bare metal surface. If the
coating is not capable of continuously and rapidly reforming as a protective barrier,
corrosion may be severe.
Usually it can be
and waves having
contours that are
the flow of the
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• The nature of the fluid can have a dramatic influence on the corrosion behavior.
• Increasing fluid velocity normally enhances the rate of corrosion.
• A solution is more erosive when bubbles and suspended particulate solids are present.
Erosion–corrosion is commonly found in piping, especially at bends, elbows,
and abrupt changes in pipe diameter—positions where the fluid changes direction or
flow suddenly becomes turbulent.
Propellers, turbine blades, valves, and pumps are also susceptible to this form of
• One of the best ways to reduce erosion–corrosion is
to change the design to eliminate fluid turbulence
and impingement effects.
• Use materials that inherently resist erosion.
• Removal of particulates and bubbles from the
solution will lessen its ability to erode.
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8. Cavitation Damage: type of erosion corrosion caused by the formation and collapse of
air bubbles / vapor filled cavities in a liquid near a metal surface.
Reason: The rapidly collapsing vapor bubbles can produce locallized pressures as high as
It is sufficient to remove surface films, tear metal particles from the surface resulting in
increased corrosion rates and surface wear.
Occurs where high velocity liquid flow and pressure changes exist.
Pump impellers and ship propellers.
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9. Fretting Corrosion: caused at the interface of materials under load subjected to vibration
The metal between the rubbing surfaces is oxidized. The oxidized film is torn. And the
oxide particles so formed act as abrasive.
Appears as grooves / pits surrounded by corrosion product.
Tight-fitting surfaces: between shafts and bearings or sleeves.
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10. Selective Leaching: preferential removal of one element of the solid alloy by the
E.g.: Dezincifiaction of Brass leaving spongy weak matrix of Cu.
Method: dissolution followed by replating of Cu while Zn remains in the solution
1. Lower zinc content.
2. Change the alloy
3. Change the environment
4. Cathodic protection.
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11. Hydrogen Damage: Load carrying capability of metallic component is reduced due to
interaction with atomic or molecular hydrogen usually in conjunction with residual or
externally applied tensile stresses.
Hydrogen Embrittlement: Hydrogen damages directly related to loss of ductility
1) Hydrogen environment embrittlement: occurs during plastic deformation of metals
such as steels, stainless steels and titanium alloys.
2) Hydrogen stress cracking: brittle fracture in a ductile material such as carbon and low
3) Loss in tensile ductility: significant reduction in elongation capability and reduction
area in steel and aluminium alloys.
Hydrogen attack: High temperature mode of attack in which hydrogen enters metals such
as steels and reacts with carbon to produce methane gas resulting in formation of cracks
Blistering: atomic hydrogen diffuses into internal defects and precipitates as molecular
hydrogen which produces high internal pressure resulting in local plastic deformation and
1. Change the material susceptible
2. Reverse hydrogen contamination: Bakeout.
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Cathodic Protection: Cathodic protection simply involves supplying, from an external
source, electrons to the metal to be protected, making it a cathode.
The process of galvanizing is simply one in which a layer of zinc is applied to the
surface of steel by hot dipping: type of sacrificial protection.
high-conductivity backfill material provides good electrical contact between the anode and
surrounding soil. A current path exists between the cathode and anode through the
intervening soil, completing the electrical circuit. Cathodic protection is especially useful in
preventing corrosion of water heaters, underground tanks and pipes, and marine equipment.