Surface Oxide Growth Mechanisms on Stainless Alloys and Entry of Hydrogen Studied by Isotopic Tracers and TOF-SIMS Analysis

摘要

The ionic transport is a key factor in the growth of passive films on metals and alloys. A method has been developed to investigate the ionic transport in oxide films, based on a two step process. Firstly the passive film is formed in aqueous solution, then the sample is further oxidized under oxygen 18 in a ToF-SIMS instrument. This in situ oxidation step with an isotopic tracer allows us to locate the growing oxide, at the surface of the passive film or at the oxide-metal interface. This technique was applied to passive films formed in high temperature water (300°C) on the surface of a nickel-base alloy (Alloy 600) of composition Ni-15-Cr-8-Fe (wt%). This alloy has been widely used in the fabrication of steam generators for pressurized water nuclear power plants. The data clearly show that upon further oxidation in oxygen 18, ~(18)O is found at the oxide-alloy interface, giving direct evidence of inward oxygen anions transport in the oxide. The in-depth profiles measured by ToF-SIMS allowed us to determine the diffusion coefficient of oxygen anions in the oxide layer. An apparent diffusion coefficient ≈3.10~(-19) cm~2.s~(-1) was found. Another part of the work was focused on the key question of the fate of the hydrogen produced by the cathodic reaction associated to anodic oxidation of the alloy (water reduction). This was studied by exposing the alloy to deuterated water (D_2O). The signal of deuterium ions was then followed by in-depth profiles measured by ToF-SIMS. The data clearly show that deuterium produced by the cathodic reaction is present in the oxide layer. The above findings are important for a better understanding of the evolution of oxide films on nickel alloys and the initiation of stress corrosion cracking.