Breakdown of passive film on Ni-Cr alloys in high temperature and high pressure water containing SO42- ions

The initial process of anodic dissolution and the repassivation on the newly created surface of Ni and the Ni-Cr alloys in a high temperature and high pressure water containing sulphate ions up to 553 K were investigated by means of a rapid straining electrode technique. 1. The passive film of Ni and Ni-Cr alloy are broken down at noble potentials more noble than a critical potential, Eb, which is attributed to the presence of sulphate ions. 2. Eb shifts more noble with an increase in temperature above 403 K, but the breakdown was not observed below 393 K. The chromium alloying also shifts Eb to the more noble potential. 3. It is assumed that a chromium depleted zone in a sensitized Ni-Cr alloy is highly possible to dissolve preferentially, resulting in IGA or IGSCC in sulphate containing high temperature solution. (author)