Intergranular corrosion annealing effect

These steels can also be liable to intergranular corrosion. In addition to Cr carbides, Cr nitrides may be precipitated and thus contribute to the depletion of Cr at the grain boundaries. The mechanism is in principle the same as for austenitic steels, but in the ferritic steels the attacks can occur for a wider spectrum of environments and lower contents of C + N. The critical range of temperatures that makes the metal sensitive is higher, namely above 925°C, and the attack occurs therefore close to the weld or in the weld metal itself. The material becomes resistant again after annealing for 10-60 minutes at 650-815°C. It is noted that the effects of the different temperature ranges are opposite to what is the case for the austenitic steels. 

The most common microstructural effect on the corrosion resistance of nickel alloys is intergranular sensitization, as previously mentioned. This is the result of chromium carbide precipitation in many Ni-Fe-Cr alloys but can result from intermetallic Mu-phase precipitation in low-carbon highly alloyed materials such as alloy C-276 (UNS N10276). Several standard IGA tests (discussed in the Intergranular Corrosion section) are available for determining (1) if stabilized alloys have been properly annealed to prevent subsequent sensitization, and (2) if nonstabilized alloys are free from significant sensitization as produced. 

HG. 2—Corrosion of stainless steel in ferric sulfate-sulfuric acid solution. Effect of carbide precipitation in 18Cr-8Ni steel on corrosion In boiling 50 % sulfuric acid containing ferric sulfate inhibitor. Sensitized chrcanium c arbide precipitate at grain boundaries. Annealed no intergranular precipitate, all carbides dissolved in solid solution. 

---