Characterization of film properties on the Ni-Cr-Mo Alloy C-2000

The passive and transpassive film properties grown at various potentials on a Ni-Cr-Mo alloy, Alloy C-2000 (Ni-23Cr-16Mo-1.6Cu), were investigated by surface-sensitive techniques such as Angle-Resolved X-ray Photoelectron Spectroscopy (AR-XPS), Time-of-Flight Secondary Ion Mass Spectrometry (ToF-SIMS) and SEM. The presence of a layered structure in the passive film has been demonstrated, with the outermost surface being enriched in Cu (or Cu oxide) and Mo oxide, the intermediate region dominated by Cr/Ni hydroxides, and the inner region comprising Cr/Ni oxide. As the potential increases from −0.4 to 0.6 V (vs. Ag/AgCl in saturated KCl), the film thickness increases, and the relative Cr content of the film increases while that of Ni decreases. The passive film (0 V) has a relatively thicker inner oxide layer and higher Cr2O3 content in the inner layer, compared to the near-passive film (−0.4 V). In the transpassive region (0.6 V), the anodic oxidation of Cr(III) to Cr(VI) in the barrier layer of the film and its subsequent dissolution leads to the loss of the Cr2O3-rich inner layer and the destruction of passivity. Destruction of the barrier layer proceeds non-uniformly on the alloy surface