Grain boundary oxidation and embrittlement prior to crack initiation in Alloy 600 in PWR primary water

The influence of corrosion potential and time of exposure on intergranular oxidation and embrittlement in Alloy 600 has been investigated in simulated primary water. The corrosion potential was observed to have a very significant influence on the proportion of embrittled grain boundaries. A maximum in grain boundary embrittlement was observed at 30 kPa of hydrogen partial pressure while little and almost no embritlement was observed at, respectively, less than 1 kPa and 650 kPa, in good agreement with the known influence of dissolved hydrogen concentration on PWSCC initiation and growth. A significant increase in the depths of intergranular oxide penetration and in grain boundary embritlement was observed with exposure time, the maximum depth of oxide penetrations increasing from 0.6 mu m after 500 h, to 2.2 mu m after 1500h, and up to 5 mu m after 4500h of exposure. Intergranular oxide penetrations were also characterized by TEM in order to get insights into the mechanism(s). Cr-depletion was observed ahead of one of the intergranular oxide penetrations, suggesting that accelerated mass transport may play an important role in the oxide penetration process. Accelerated mass transport may also explain the presence of a hexagonal Cr7S8 particle observed just beyond the tip of an intergranular oxide penetration ending at a Cr carbide.