Relation between Sensitization and Susceptibility to IGSCC for Ni-Base Alloys

Degree of sensitization(DOS) of alloy 600 and 690 aged at 700 .deg. C was evaluated in terms of the standard or modified double loop(DL) EPR tests and correlated with the corresponding susceptibility to intergranular stress corrosion cracking(IGSCC) that were determined under condition of constant strain rate of 0.97 x 10-6/s in deaerated 0.01 M Na2S4O6 solution. The modified DLEPR test, performed in 0.01 M H2SO4 + 10 ppm KSCN, 25 .deg. C, and at a scan rate of 0.5mV/s, was found to provide more enhanced discriminating power for varying degrees of sensitization of alloy 600 than did the standard DL-EPR test in which testing conditions are 0.5 M H2SO4 + 0.01 M KSCN, 30 .deg. C and scan rate of 1.67mV/s. This result can be attributed to both highly selective grain boundary attack and suppression of other types of corrosion such as general corrosion and pitting in the modified EPR testing condition. These characteristics of the modified EPR test caused the maximum DOS of alloy 600 to be achieved in 5 hour-aging that is shorter than 10 hour-aging for the case of the standard test. For alloy 600, IGSCC occurred in samples aged for less than 20 hours with the susceptibility to IGSCC increased with decreasing aging time up to 1 hour. The susceptibility to IGSCC is found to be closely associated with chromium depleted profile across the grain boundary the deeper and narrower chromium depleted zone produced the higher susceptibility to IGSCC. Except for the case of 700 .deg. C-1 hour aging, the DOS measured by the modified DL-EPR test more closely paralleled to the susceptibility to IGSCC than did that by the standard one for the same aged sample of alloy 600. This is attributed to the fact that the modified testing condition attacked predominantly the deeply Cr-depleted region with the shallowly Cr-depleted region unattacked. Alloy 690 aged for 1 hour at 700 .deg. C showed immunity to both intergranular corrosion and IGSCC when tested in the same conditions applied to alloy 600. This resulted from a removal of serious Cr-depletion problems in alloy 690 due to its higher Cr-concentration compared with that of alloy 600