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Abstract

A mechanically alloyed austenitic stainless steel (MA304LZ) was produced from pre-alloyed SUS304L powder with a small amount of zirconium addition. Coupon-type specimens of MA304LZ and SUS304L steels were subjected to hot water at 300 °C/25 MPa and supercritical pressurized water (SCW) at 500 °C/25 MPa for 1000 hr. MA304LZ is significantly less susceptible to corrosion weight gain in SCW than SUS304L which follows the parabolic rule between weight gain and elapsing time. The reduction of weight gain in MA304LZ can be attributed to much smaller grains which enhance chromium diffusion through grain boundaries and consequently accelerate the formation of a protective chromium oxide layer. Nickel oxides were observed in SUS304L but not in MA304LZ after the test at 500 °C. It is considered that zirconium addition suppresses nickel diffusion as well as oxygen diffusion because of the strong interaction of zirconium/nickel and zirconium/oxygen. Electrochemical potentiodynamic reactivation (EPR) measurement of the degree of sensitization (DOS) of annealed and non-annealed samples indicates that both steels were resistant to sensitization of grain boundary corrosion. However, the annealing of MA304LZ at 1050 °C diminishes the beneficial grain size effect of MA304LZ.