Effects of water chemistry transients on crack growth rate of nickel-based weld metals

Stress corrosion crack growth rates of Alloy 182 and 82 weld metals in thermally aged (400°C/200 h) conditions have been measured in simulated BWR conditions. The effects of water chemistry impurity concentration transients on crack growth rate and cracking morphology have been studied. The time for onset of accelerated crack growth due to impurity / conductivity transient was measured when the conductivity of the coolant was adjusted by additions of sulphate as H2SO4 in the range of 10-100 ppb. Crack growth was monitored using potential drop method during slow rising displacement testing applied to 10x10 mm2 cross section, fatigue pre-cracked, three point bend specimens. The delay in crack growth rate slow down after returning to high purity water was also measured. In the tests an increase in the crack growth rate became visible in Alloy 182 within 25-45 h after sulphate was introduced into the bulk water. Although there is no reason or consistent data to support a permanent increase in crack growth rate caused by a sulphate transient, the measured crack growth rate did not return back to the level prevailing prior to the transient. In Alloy 82 no crack growth was observed with any of the applied conditions of this study. Crack growth rates were measured also for cold deformed AISI 316NG and furnace sensitised AISI 304 stainless steels, as reference.