Effects of Hydrogen Peroxide on Corrosion of Stainless Steel, (V) Characterization of Oxide Film with Multilateral Surface Analyses

Abstract

In order to understand corrosion behavior of stainless steel in BWR reactor water conditions, characteristics of oxide films on stainless steel specimens exposed to H2O2 and O2 in high temperature water were determined by multilateral surface analyses, i.e., SEM (scanning electron microscope), LRS (laser Raman spectroscope), SIMS (secondary ion mass spectroscope) and STEM-EDX (scanning transmission electron microscope). The following points were experimentally confirmed. 1. Oxide layers were divided into inner and outer layers: Outer layers of the specimen exposed to 100ppb H2O2 consisted of larger corundum type hematite (α-Fe2O3) particles, while inner layers consisted of very fine Ni rich magnetite (Fe3O4). Outer layers of the specimen exposed to 200 ppb O2 consisted of larger magnetite mixture particles, while inner layers consisted of fine Cr rich magnetite. 2. Outer oxide layers consisted of oxide particles. The oxide particles depositing on the specimens exposed to 100ppb H2O2 were divided into two groups, i.e., a larger particle group and a smaller particle group. For other specimens, the diameter distribution of depositing particles was a single peak. Particle density and size were changed by oxidant concentration. The average diameter of the particles (that of the smaller group only for the specimen expose to 100 ppb H2O2) decreased with [O2] and [H2O2]. 3. Total oxide film thickness decreased with [H2O2] and increased with [O2]. 4. A larger dissolution rate at higher [H2O2] resulted in a thinner oxide film with smaller particles and larger hematite particles.