The influence of chromium and molybdenum on the repassivation of nickel-chromium-molybdenum alloys in saline solutions

The electrochemical properties and compositions of passive, transpassive, and re-formed passive oxides on the Ni-Cr-Mo alloys C22 (high Cr/low Mo) and HYBRID BC1 (low Cr/high Mo) have been studied in 5.0M NaCl solutions. At low potentials in the passive region, the oxide was less resistive on BC1 compared to C22, possibly due to the influence of Mo in increasing the number of defects in the oxide. At higher potentials in the passive region, both alloys exhibited similar behaviours and composition of oxide films. Under transpassive conditions, the Cr-III barrier layer (dominantly Cr2O3) was destroyed on both alloys, with a greater accumulation of Mo-VI/Mo-V on the BC1 than on the C22 alloy. Improved passivity was achieved for both alloys on returning to potentials in the passive region. This appears to be due mainly to the re-formation of the Cr-III barrier oxide layer on C22, but to a combination of the reformation of this layer and the accumulation of Mo-VI/Mo-V on the BC1. Copyright (c) 2017 John Wiley & Sons, Ltd.