The development of Ni-Cr-W corrosion resistant alloys immune to sensitization

The Ni-Cr-Mo system is the basis of important commercial corrosion resistant alloys (CRA's) such as Inconel 625. These alloys perform very well so long as the Cr and Mo are available in solid solution. One of the limitations of Ni-Cr-Mo based CRA's is that exposure at moderate temperatures (>500°C) can lead to precipitation of Cr and/or Mo containing compounds and this sensitization degrades the corrosion and mechanical performance. This research explores the development of a class of CRA's based on the Ni-Cr-W system. This system has the advantage that compositions can be designed using computational thermodynamics that remain single phase from room temperature to melting, and hence could be immune from classic sensitization. We use atomic emission spectroelectrochemistry, coupled with x-ray photoelectron spectroscopy, to observe the extent to which W interacts with the Ni-Cr-W alloy surface during transpassive dissolution and repassivation in a 0.1 M solution of HCl (pH 1). This is compared with the behaviour of Mo and Nb in a commercial Inconel 625 alloy. The retention and dissolution behaviours of W, Mo and Nb are compared with results of ASTM G48 pitting and crevice testing to relate corrosion mechanisms with alloy performance. W is shown to enrich during transpassive dissolution and dissolve upon repassivation in a similar manner to Mo, suggesting their repassivation mechanisms are similar. The presence of Nb in Inconel 625 is also proposed to increase pitting resistance and reduce crevice propagation. It is shown that Ni-Cr-W alloys can be designed to be immune to sensitization with W playing a similar role in repassivation to Mo in Ni-Cr Mo alloys.