Cr, Mo and W alloying additions in Ni and their effect on passivity

The passive corrosion properties of a series of Ni–Cr–Mo alloys were investigated. The alloys studied were C22, C2000, C276, C4 and 625. Potentiostatic experiments at potentials within the passive range were obtained as a function of temperature (25–85 °C) for each alloy. Each specimen was subsequently analyzed by time-of-flight secondary ion mass spectrometry (TOF SIMS) and X-ray photoelectron spectroscopy (XPS). Results indicated that much lower passive dissolution currents, and a much slower attainment of steady-state conditions, were observed on those alloys with >20 wt.% Cr content. These alloys also consistently showed only a minor temperature dependence of the passive current. The surface analyses showed that the high-Cr alloys were able to build thicker oxides with a layered structure consisting of an inner Cr–Ni oxide layer and an outer Mo/Cr oxide. By contrast, such a high-Cr content inner layer and structured elemental distribution were not achieved with alloys having a lower bulk Cr content.For low potentials (200 mV), when Cr dissolution can only occur as Cr(III), additional alloying elements, specifically Mo and W, exert little influence on passive current densities. At 500 mV, a potential at which Cr(VI) release appears to start, the presence of Mo, and especially W, in the outer regions of the oxide suppresses passive dissolution. This may be due to the low dissolution rate of the W-containing surface oxide layer, but some influence of W on the defect density within the oxide cannot be ruled out. The observed temperature dependence for high-Cr alloys (C22, C2000) reflects the lack of steady state and cannot be interpreted as an accurate activation energy. [Copyright &y& Elsevier]