Factors affecting primary water stress corrosion cracking initiation of Ni-Cr-Fe alloy

Abstract

Nickel alloys are widely used as the pressure boundary components of pressurized water reactors (PWRs) because it has good mechanical strength and excellent corrosion resistance. However Alloy 600 was revealed as susceptible to primary stress corrosion cracking (PWSCC) and it became the one of the important material degradation mechanism in PWRs. In this study, oxide layer of nickel alloy was investigated and PWSCC initiation test was conducted using 3 heats of Alloy 600 and Alloy 690 each. Nickel alloy’s oxide layer was identified as double layer such as outer layer and inner layer. The morphology and the thickness of oxide layer were varied as dissolved hydrogen concentration and zinc injection. It was expected that chromium rich inner layer played important role on the corrosion resistant performance. Alloy 600 PWSCC initiation test results were discussed with the effects of several factors like chemical composition, mechanical strength and grain boundary characters. In this study grain boundary mis-orientation was considered to be the most dominant factor affecting the Alloy 600 PWSCC resistance. However crack was not developed at Alloy 690 material after the long term test. PWSCC initiation factor of improvement of Alloy 690 to Alloy 600 is suggested to 20.