INFLUENCE OF AGING ON INTERGRANULAR CORROSION OF A 25-PERCENT CHROMIUM 5-PERCENT NICKEL DUPLEX STAINLESS-STEEL

Although duplex stainless steels (SS) exhibit good performance in many severe corrosive environments, their corrosion resistance can be hamp ered by precipitation of chromium (Cr)- and molybdenum (Mo)-rich phase s during heat treatments. The influence of aging treatments on intergr anular corrosion (IGC) of a commercial 25% Cr-5% nickel (Ni) duplex SS (UNS S32550) was studied. The SS was given aging treatments in the ra nge 773 K to 1,173 K for durations from 6 min to 100 h. Microstructure study of the aged samples indicated the presence of various phases, a mong them sigma, Cr-rich carbide, secondary austenite, alpha and coppe r (Cu)-rich precipitate. The influence of these precipitated phases on IGC susceptibility of the 55 was evaluated by the ferric sulfate (Fe[ SO4](3))-sulfuric acid (H2SO4) and Cu-copper sulfate-(CuSO4)-H2SO4 tes ts. Results indicated sigma- and Cr-rich carbide increased IGC suscept ibility greatly, as did aging in the range 873 K to 1,073 K.