The Effect of Pb-Underpotential Deposition on the Dealloying of Alloy 800 in a Near-Boiling Caustic Environment

This study investigates the influence of Pb-underpotential deposition (Pb-UPD) on the dealloying of Fe–Cr–Ni alloys in a near-boiling conditions (50 wt. % NaOH environment, where the formation of a stable Cr-rich oxide is not thermodynamically favored). SCC testing and electrochemical measurements on Alloy 800 (Fe-32Ni-21Cr) were performed to assess the influence of Pb on dealloying. Post-exposure X-ray photoelectron spectroscopy (XPS) indicated that the Pb is present as Pb(II) at the top surface and, more importantly, deposits as metallic Pb at the metal-film interface. Electrochemical measurements revealed that the addition of Pb shifted the corrosion potential of Alloy 800 toward more positive values and suppressed the dealloying peak. The results were accompanied by lower crack density and shorter crack length compared to Pb-free conditions. The combined electrochemical and XPS results are consistent with the established Pb-underpotential deposition (Pb-UPD) on the metallic surface, where adsorbed Pb substantially suppresses the anodic dissolution of Fe and Cr by blocking active dissolution sites. The present results demonstrate that Pb-UPD can also reduce the susceptibility to dealloying and SCC initiation on a bare metal surface in the absence of passivity. Pb penetrates the surface.Pb deposits as hydroxide/metal.No dealloying peak observed. Pb penetrates the surface. Pb deposits as hydroxide/metal. No dealloying peak observed.