DISTRIBUTION OF COBALT IN SURFACE OXIDE FILM OF TYPE-304 STAINLESS-STEEL EXPOSED TO HIGH-TEMPERATURE WATER - QUANTITATIVE ANALYSIS BY GLOW DISCHARGE OPTICAL EMISSION SPECTROMETRY

The glow discharge optical emission spectrometry (GDS) was applied to determine the cobalt distribution accumulated in the corrosion film of type 304 stainless steel that was exposed to demineralized water with below 5 ppb of dissolved oxygen content at 561 K. The exposure test was carried out using two parallel high-temperature and high-pressure water experimental loops, one with 3 ppb non-active cobalt added and the other without. The structure and element depth distributions of the corroded specimen surfaces were analyzed by EPMA, XRD, XPS and GDS. The corrosion film of the specimen exposed for 1,000 h had a clear double layer structure consisting of an iron-rich outer layer 3 mu m in thickness and a chromium-rich inner layer 0.5 mu m in thickness. The cobalt distributions in the corrosion film could successfully be determined by GDS. The distribution of cobalt was peculiar in the corrosion film and the distribution curves had a peak around the boundary between the two layers. The cobalt concentration tended to increase linearly with the exposure time.