Effects of Tungsten on Corrosion and Kinetics of Sigma Phase Formation of 25% Chromium Duplex Stainless Steels

Duplex stainless steels (DSS) of 25% Cr-7% Ni-x% Mo-y% W-0.25% N (x = 1.5 to 3, and y = 0 to 3) were designed with the same pitting resistance equivalent (PRE) value of 42 by varying the contents of Mo and W. Effects of W on pitting and stress corrosion of the solution-annealed or aged alloys in chlorides were examined using anodic polarization testing, critical pitting temperature measurements, and slow strain rate testing. Influences of W on degradation of mechanical properties of the alloys caused by precipitation of secondary phases during aging at 850°C were investigated using the Charpy impact and tensile tests, x-ray diffraction, and back-scattered scanning electron microscopy. Resistance to pitting and stress corrosion increased with the ratio of W to Mo content. During aging, the alloys were embrittled rapidly by precipitation of the sigma (σ) phase, with the rate of embrittlement delayed significantly by increases in W content. Thus, the alloy containing 3% W-1.5% Mo exhibited the highest resistance to pitting and stress corrosion in the solution-annealed condition and the highest resistance to embrittlement induced by aging. The degree of degradation in corrosion and mechanical properties of the alloys during aging was associated closely with the amount of σ precipitates. Addition of W to 25% Cr DSS retarded nucleation and growth of the σ phase during aging, thereby delaying degradation of the corrosion and mechanical properties of the alloys. Retardation of precipitation of the σ phase by W during aging resulted from its inherently slower diffusion rate compared to that of Mo. W in the alloys caused preferential precipitation of the chi (χ) phase along the grain boundaries and, hence, inhibited nucleation and growth of the σ phase by depleting W and Mo around the χ precipitates. This beneficial effect of W on retardation of σ phase precipitation was most dominant in the alloy containing 3% W and 1.5% Mo.