Influence of sample treatment methods on the determination of trace oxygen in iron and steel

Removing the oxide film from the sample surface is an important process for analyzing trace oxygen in iron and steel. The depth profiles of oxygen on the surface after electro-polishing in CH3COOH-HClO4, CrO3-H3PO4, and chemical polishing by HF-H2O2 were investigated. As for the high-purity iron, thickness of the oxide layer after electro-polishing in CrO3-H3PO4 was thicker than any other pretreatment methods. This is because Fe(III) hydroxide was formed on the surface of sample. This could be removed by rinsing with an acid after electropolishing. As for the bearing steel, oxygen containing species were found around carbides on the surface of the sample after preteatment except that the pretreatment was made by electro-polishing in CrO3-H3PO4. Those precipitates influenced the analysis of oxygen. Analytical value obtained by conventional methods were 1 or 2 mass ppm higher than those obtained by charged particle activation analysis that is not influenced by surface contamination. This difference corresponds to the oxide layer formed on the sample surface.