A Mechanistic Study on Lead-Induced Passivity-Degradation of Nickel-Based Alloy

The mechanism of lead-induced passivity-degradation in a nickel-based alloy (UNS N06690) was explored in a simulated stream generator alkaline crevice chemistry. The pitting induction test indicated that the detrimental impact of lead contamination on pitting resistance was significantly enhanced by the presence of calcium ions. An X-ray photoelectron spectroscopy analysis revealed the incorporation of lead into the passive film. Calcium ions in the alkaline chemistry entered the passive films with the aid of the lead species but the ingress of calcium did not occur in the solution free of lead contamination. The incorporation of lead reduced chromium and iron content in passive films and hindered the dehydration processes during the passivation. Mott-Schottky and photoelectrochemical measurements showed that passive films on the nickel-based alloy are p-type semiconductors and the incorporation of lead may reduce the acceptors in the passive films. A further analysis suggested that the lead-induced passivity degradation of nickel-based alloys may be related to the increases in M–O and bonds, the electronic structural changes of spinel oxides in the passive film, and reduced ion-selectivity due to the surface adsorption of the lead species.