|4. Forms of Corrosion|
4.5 Intergranular Corrosion [2/3]
Mechanism of Intergranular Corrosion
IGC occurs in the following steps
Carbon diffuses to the grain boundary. The grain boundary is a region of high internal energy. The grain boundaries are the preferred sites for impurity segregation.
Carbon forms chromium carbide (Cr23C6) on reacting with chromium which is precipitated in the grain boundary as the zone next to grain boundary becomes depleted of chromium carbide, thus making the zone weaker and anodic to the remainder of the surface.
The chromium content in the region adjacent to grain boundary is depleted. It falls below 12% (the minimum required to resist corrosion).
The above process leads to dislodging of grains.
As the localized attack passes through the grain boundaries, the attack is termed "intergranular attack" or grain boundary corrosion. It is also called weld decay by welders.
Some cases of intergranular corrosion are shown below.
|Microscopic examination of this duplex stainless steel component discloses selective attack to the austenitic phase and intergranular corrosion along the ferrite-austenite grain boundaries.|
|Stainless steel component shows a local attack a short distance from the weld.|
It is not chromium carbide which is attacked, but the area adjacent to chromium carbide which is depleted of chromium that is attacked.
Control of IGC
Use low carbon steels (C < 0.3 %): The use of low carbon steels such as 304 L or 316 L will increase resistance to intergranular corrosion in welded components.
Solution quenching: It consists in heating the alloy in the temperature range to cause dissolution of chromium carbide and attainment of a uniform composition. Rapid quenching from solution temperature is very important as slow cooling may re-precipitate the carbide.
Adding stabilizer: Stabilizers like columbium or titanium are added to stainless steels. Such steels are called 'stabilized' grades of steels. Both elements have a high affinity for carbon, and they form columbium or titanium carbide respectively. The formation of these carbides does not allow any depletion of chromium to take place.