Wednesday 18 September 2013

Intergranular Corrosion

 We can understand from the name, it s the corrosion between grains (intergranular means between grains),i.e. corrosion at grain boundaries. This type of corrosion occurs under certain environment condition, In which there is localized corrosion at grain boundary with relatively little corrosion of grains. This process results in deterioration of bonding between grains and metal thus disintegrates losing the strength.

Cause of Intergranular Corrosion (IGC):
Under certain environment condition these are the possible cause for IGC.
      (i) Presence of impurities at grain boundary.
      (ii) More content of one alloying element at grain boundary area.
      (iii) Depletion of one alloying element at grain boundary area.

Intergranular Corrosion (IGC) in Austenitic stainless Steel:
 As we know that Chromium is added in ordinary steel to impart corrosion resistance to steel(Why?). Generally minimum 12% Chromium is added to make stainless steel. If by any means Cr content decreases, steel become susceptible to corrosion.

why Chromium content decreases in austenitic stainless steel (SS 304) at grain boundary?

 When austenitic stainless steel are heated in a temperature rage of approximately 480-780 degree centigrade, it forms Chromium carbide Cr23C6 along the grain boundary & this chromium carbide precipitates out of solid solution. thus Chromium is removed from solid solution. This causes depletion of Cr in area adjacent to grain boundary, subsequently corrosion resistance decreases.
This high temperature range may occur during heat treatment and welding.

This is why SS304L is preferred than SS304 where welding is required.

 We know that SS304L has 0.03% max carbon (quite low to that of SS304 which has 0.08% max carbon). Lesser carbon means lesser possibility of  to form Chromium Carbide. Thus lesser possibility IGC.