Abstract
Cobalt based alloy (stellite) has been used for valve seats in the coolant systems of LWR (light water reactor) nuclear power plants. Cobalt dissolved into coolant water during long periods of operation becomes radioactive due to irradiation in the reactor. To decrease the dissolving of Co into the coolant water, we have developed ceramic coatings for the valve seats by using cathodic arc ion plating. Pin-hole density of TiN and CrN films onto austenite stainless steel plates, which was evaluated by the critical passivation current density method (CPCD method), strongly depended on substrate bias voltage and nitrogen pressure during deposition. The radiation with γ-rays during an autoclave test enhanced the corrosion rate of deposited films. The Co elution rate from coated stellite into high temperature and high pressure water was proportional linearly to the pin-hole density of deposited films and it decreased drastically by CrN coating. CrN and TiN coatings improved the galling and wear resistance of stellite in high temperature and high pressure water. Single phased CrN(220) showed excellent performance for both CO elution and galling wear test.
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