Stress corrosion cracking and surface oxidation properties of Ni-base welding alloys exposed to PWR primary water

In-service cracking at the penetration nozzles and surrounding welds of the reactor pressure vessel head (RPVH) of a pressurized water reactor (PWR) has been found after long-term operation. Alloy 182 is widely used as a filler or butter material for dissimilar metal welds to join Alloy 600 with stainless steel and/or low alloy steel. It is now well established that Alloy 600 and Alloy 182 are susceptible to stress corrosion cracking (SCC) in primary water environments, which is known as primary water SCC (PWSCC). The cracking of Alloy 600 and Alloy 182 in most cases was identified as intergranular SCC (IGSCC). The aim of the present study is to compare the PWSCC resistances of Alloy 182 and Alloy 152 welds by measuring the crack growth rate (CGR) using a compact tension (CT) specimen in a simulated PWR primary water condition. Surface oxidation tests are also conducted under identical experimental conditions to obtain clues with which to understand the cracking phenomena. The cracking and surface oxidation characteristics are also examined using microscopic equipment. Finally, the different characteristics of PWSCC of the test welds are discussed in terms of cracking and surface oxidation behavior. From the CGR tests, it was confirmed that the resistance to PWSCC of Alloy 152 weld was much higher that of Alloy 182 weld. Oxygen diffused down along the grain boundary in Alloy 182 from the outer environment, causing IG oxidation. As a result of the oxygen diffusion, Cr oxide formed in the oxidized grain boundary.