BREAKDOWN OF PASSIVE FILM ON NI CR ALLOYS IN HIGH-TEMPERATURE HIGH-PRESSURE WATER CONTAINING SULFATE-IONS

A rapid straining electrode behavior was examined for the nickel-chromium alloys in a high-temperature and high-pressure aqueous solution containing sulfate ions. The new surface created by rapid elongation readily repassivated in a range from the corrosion potential to a critical potential. Above the critical potential, complete repassivation of the newly created surface was not achieved, and the surface exhibited localized attack. The critical potential is found to shift to a noble value with increasing the chromium content. Furthermore, the breakdown occurs at the more noble potential as temperature increases above 403 K, but is suppressed below 393 K. Sulfate ions included in the solution are responsible for the breakdown phenomena.