An Element-Resolved Electrochemical Study of the Passivation and Transpassavation States of a Ni-Cr-Mo Alloy in Hot Sulfuric and Nitric Acid

This work demonstrates the use of the AESEC technique for element-resolved electrochemical investigations of Ni-33Cr-8Mo (wt%) in hot nitric and sulfuric acid. A clearly defined active to passive transition with nearly congruent elemental dissolution was detected in 2 M H 2 SO 4 . Selective Mo dissolution was observed at potentials above this transition and was further confirmed by anodic passivation experiments. The quantity of material dissolved during passivation increased markedly with increasing temperature. By contrast, in 4 M HNO 3 , the alloy appeared to be passive throughout the entire potential domain up to the transpassive potential. This behavior was interpreted as resulting from direct chemical oxidation of the alloy surface by nitrate ions, effectively overriding electrochemical control. In the transpassive domain, both electrolytes showed similar behavior with congruent dissolution and a dissolution rate that increased with temperature. These results highlight the capability of AESEC to distinguish electrochemical and chemically driven passivation mechanisms in complex alloys.