Corrosion Resistance of Two Phase Ferrite-Austenite Stainless Steels in Boiling 5% Sulfuric Acid

Structural investigation was made to explain corrosion resistance of two phase ferrite-austenite stainless steels SUS329J1 and DSD1H containing Nb+B which were reheated over the temperature range of 400∼750°C for 2hr after solution treatment.The results are as follows:(1) The corrosion rate of SUS329J1 and DSD1H after solution treatment was 1.24g/m2·hr and 2.36g/m2·hr respectively.(2) The corrosion rate of SUS329J1 increased extremely at around reheating temperature of 600°C and 800°C. In the case of DSD1H the peak of corrosion rate was at around only 800°C.(3) In the reheated specimen at 600°C of SUS329J1, film-like carbide Cr23C6 was observed at ferrite-austenite interface, but at the ferrite-ferrite and austenite-austenite interface, no chromium carbide was observed. Preferential corrosion was observed near the ferrite-austenite interface of the reheated specimen at 600°C of SUS329J1, and then corrosion proceeded to the austenite phase.(4) Corrosion resistance of the reheated specimen at 600°C of SUS329J1 was recovered by heating at 700°C.(5) In the reheated specimen at 600°C of DSD1H containing Nb+B, chromium carbide was not observed at the ferrite-austenite interface and also decrease in corrosion resistance was not observed.