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Types of Corrosion
Galvanic Corrosion/ Bimetallic Corrosion
When two different metals are in contact with each other, in presence of an electrolyte
(moisture), the more anodic metal corrodes preferentially. The greater the distance
between the metals in galvanic series greater is the rate of corrosion.
For example, when a brass name plate is fixed on wooden door with nails made of iron, a
tiny galvanic cell is formed at the area of contact and the nail starts corroding.
•
The corrosion reaction initiated due to the potential gradient is called galvanic
corrosion.
•
This type of corrosion proceeds at a much higher rate than concentration gradient
corrosion.
•
It can be prevented by cutting the electrical contact between the metals by way of
painting, using a rubber washer etc.,
Types of Differential aeration corrosion
Pitting corrosion
Selective Leaching
Crevice corrosion
Stress cracking corrosion (SCC)
Water Line corrosion
Intergranular corrosion(IGC)
Microbial corrosion (MIC)
Soil corrosion
Erosion – corrosion
Pitting corrosion
Pitting corrosion is a localized attack on metals / alloys possessing a passive film.
Ex: Metals like Al (Al2O3) , Ti (TiO2) and alloy like SS (Stainless Steel - Passive
CrO3) are prone to this type of corrosion
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A pit may be initiated by a non- homogeneous surface or due to a break in the
protective film.
When the protective film breaks down ( a crack due to excessive thickness/ due to
acid impingement or any such reason), and if the conditions are not favourable for its
replenishment, then that area becomes anodic with respect to the bulk of the metal /
alloy. Pitting is neither uniform nor has a constant rate. Once pitting corrosion is
initiated, the anodicity increases hence, corrosion proceeds at an accelerated rate.
MxOy
Metal-Pitting
Factors which influence pitting
•
Presence of chloride increases pitting
•
Presence of SO42- , NO3-, CrO42- and SiO32- retards corrosion.
•
Pitting of SS is usually reduced by adding 2-4% Mo
Selective Leaching / Dezincification / Parting
•
It occurs in alloys (usually in non-ferrous alloys)
•
Parting is selective leaching of anodic metal from an alloy containing
at least one noble metal.
The anodic metal in an alloy is removed in presence of a suitable corrosive
medium.
Dezincification of ‘Cu’ alloys is the most common example for selective
leaching. In high zinc brass, leaching of Zn occurs even at neutral media.
For example the condenser tubes used in marine boilers (NaCl-seawater
neutral medium) undergo this type of corrosion.
Both ‘Zn & Cu’ are leached out into the solution, but Copper is redeposited.
In most cases selective leaching is accompanied with change in the Colour/
hardness and the mechanical properties are also affected
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Brass
Sponge Copper
Golden yellow
Reddish brown
Stainless Steel
Silvery white
•
Dull grey
Selective leaching of brass can be prevented by adding Sn, Sb or P to
brass containing >15% Zinc.
Crevice Corrosion
The Corrosion resulting due to the presence of a small gap/cleft/ crack on the surface
of the metal is called crevice corrosion.
CREVICE
For example accumulation of dirt, corrosion products, crack in paint can result in the
formation of a concentration gradient cell (differential aeration). This type of corrosion is
observed in bolts, rivets, lap joints and in places where gaskets are used.
The area which is least aerated becomes anodic.
Selection of resistant materials, proper design to minimize crevice and maintaining a
clean surface are essential to control this type of corrosion.
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Intergranular Corrosion (IGC)
•
It is common in stainless steel and other heat treated alloys
The presence of mismatching of grains leads to highly active grain boundaries.
Due to this high activity along the boundaries, the Cr and C present in SS react to give
Chromium Carbide, which precipitates along the boundary. This decreases the % of Cr at
the boundary and hence the boundary becomes anodic with respect to the bulk of the
grain, resulting of corrosion at boundary.
1
4
2
1,2,3 – Grains
3
4- Boundary
Adopting proper and controlled heat treatment methods can minimize IGC
Stress Cracking Corrosion
It is the combined effect of a physical and chemical assault on the metal.
For example caustic embrittlement is a type of SCC occurring in boilers. In highpressure boilers, Na2CO3, used for treating hard water is converted to NaOH.
Na2CO3 + H2O
▬▬▬▬▬►
2NaOH. + CO2 ↑
High pressure
Due to the repeated heating and cooling, the walls of the boiler contain lot of tiny cracks.
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The NaOH formed as above, enters these cracks and evaporates. Thus the cracks contain
higher concentration of NaOH and attack the surrounding area, dissolving iron of boiler
forming sodium ferroate. This causes embrittlement of boiler parts, particularly stressed
parts like bends, joints, and rivets.
Another example is the corrosion of brass in presence of NH3.
magnetite
steam
escapes
Dil. NaOH
Na+
OH Tiny Crack on
the inner
surface
OH Na
+
+
Na
+
OH Na
OH -
+
OH - Na
Con. NaOH
Prevention Methods:
i.
Adopting Phosphate Conditioning as an alternative to
Carbonate conditioning
ii.
By sealing the hair line cracks with Sodium sulphate ,
Tannin , Lignin etc. ( can be done easily by adding little
amount of the sealants in water)
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Water Line Corrosion
Let us consider a zinc rod dipped in a beaker of water. If the rod is partially immersed,
then the part of the rod underwater is less oxygenated compared to the part of the rod
above water. (Dissolved oxygen and atmospheric oxygen are in contact) Similarly the
amount of dissolved oxygen at the surface is much greater than the amount of Dissolved
oxygen at the bottom of the beaker. Therefore, the part, which is in contact with the
surface, is cathodic and the one in contact with the water at the bottom is anodic. Thus
anodic part of rod undergoes corrosion.
OHH
H
F
H2O
H
Fe
Fe2O3
Microbial Corrosion (MIC)
•
Corrosion due to action of microbes
•
It can be due to physical attachment of the micro organisms were the
corrosion is due to differential aeration
•
In some cases it is due to chemical reaction (Sulphur Reducing Bacteria
reduces to sulphates present in water to sulphides) producing H2S which
results in the corrosion of the metal.
Example : Thiobacillus ferrooxidans ,
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SRB-influenced corrosion of carbon
steel
Fi g 15. 2
S
u
l
f
o
l
o
Soil Corrosion:
•
Underground structures and pipelines undergo soil corrosion
•
Buried pipelines or cables passing from one type of soil to another like clay (less
aerated) and sand (more aerated) gets corroded due to differential aeration.
•
In domestic water carrying pipes, large part of the pipeline lies below the surface.
The portion under the ground has less access to oxygen (anodic) and the one
lying on the surface has more access to oxygen (cathodic). The anode undergoes
corrosion.
Erosion – corrosion
•
It is a combination of physical and chemical phenomena.
•
Combined effect of abrasion and corrosion
•
Due to erosion the protective surface film will be lost making the metal anodic
•
Generally occurs in pump impellers, agitators and piping at bends and elbows.
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