The effect of phase composition and microstructure of 25Cr-6Ni-3Mo-0.25N duplex stainless steel on corrosion resistance

The effect of phase composition and structure of Fe-25Cr-6Ni-3Mo-0.25N Duplex Stainless steel (DSS) on corrosion resistance in the medium containing chloride ion was investigated. Alloying rare earth metals (RE) with steel had not caused any difficulty and alloying nitrogen (N) using FeCrN was favourable. The optimum δ content attained about 0.237% and RE about 0.062%. The increase of N and RE contents in steel had decreased α-ferrite composition and formed δ-ferrite. The microstructural change of DSS obtained (50% α-phase, 50% ϒ-phase, and without σ-phase) had improved corrosion resistance of 25 Cr-6Ni-3Mo-0.25 N DSS. Phase composition and microstructure of DSS containing N and RE reveals that there to cleanness for grain interior and boundary. The grain boundaries were narrow and the inclusions formed were RE oxysulfides distributing inside the grain and strongly resisting pitting corrosion. Almost all typical specimens containing N and RE were completely passivated. The corrosion phenomena such as pitting, intergranular and stress did not appear, after explosion to 20% NaCl solution.