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 w as carried out using two parallel high-temperature and high-pressure w ater experimental loops, one with 3 ppb non-active cobalt added and th e other without. The structure and element depth distributions of the corroded specimen surfaces were analyzed by EPMA, XRD, XPS and GDS. Ti le corrosion film of the specimen exposed for 1,000 h had a clear doub le layer structure consisting of an iron-rich outer layer 3 mu m in th ickness and a chromium-rich inner layer 0.5 mu m in thickness. The cob alt distributions in the corrosion film could successfully be determin ed by GDS. The distribution of cobalt was peculiar in the corrosion fi lm and the distribution curves had a peak around the boundary between the two layers. The cobalt concentration tended to increase linearly w ith the exposure time.