The activation/depassivation of nickel–chromium–molybdenum alloys: An oxyanion or a pH effect—Part II

The depletion of Cr and Mo in the passive films grown on commercial Ni–Cr–Mo(W) alloys in carbonate/bicarbonate solutions has been studied potentiodynamically and by X-ray photoelectron and Auger spectroscopy. Depletion in these two key alloying elements occurs at lower potentials than expected on all the alloys studied. It has been shown that the key feature leading to this behavior is the buffering of surface pH to >8.6, when the surface deposition (or retention) of insoluble protective Mo(VI) species does not occur as would be expected under the acidic local conditions generated by Cr(VI) and Mo(VI) dissolution. The presence of a small solution concentration of Mg2+ ions leads to the accumulation of a Mg(OH)2 deposit when the surface pH is maintained sufficiently alkaline.