Role of Chromium in Anomalous Behavior of the Passive Layer in Ni-Cr-Mo Alloys in 1 M HCl Solution

This work seeks to understand the underlying mechanism involved in the passivity of Ni-Cr-Mo alloys in a less concentrated HCl solution (1 M) by systematically varying contents of Cr and Mo solutes in model Ni-Cr-Mo alloys. Corrosion behavior was evaluated based on potentiodynamic polarization tests performed in conjunction with electrochemical impedance and x-ray photoelectron spectroscopies of passive films that formed on alloys during their exposure to the HCl solution. Results have shown that an increase in Mo alone is not sufficient to improve the corrosion resistance of the alloys at lower concentrations of HCl. Optimum concentrations of Cr and Mo solutes have been found to be in the vicinity of ∼17 wt% Cr and ∼19 wt% Mo for superior corrosion resistance of the alloys. This was attributed to the protection of the Cr₂O₃ layer as a consequence of the enrichment of Mo⁶⁺ ions in the passive film in 1 M HCl solution.