Influence of Sample Pretreatment Methods on the Determination of Trace Oxygen in Iron and Steel

Removing the oxide film from the sample surface is an important process in analyzing trace oxygen in iron and steel. The depth profiles of oxygen on the surface after electropolishing in CH<INF>3</INF>COOH— HClO<INF>4</INF>, CrO<INF>3</INF>—H<INF>3</INF>PO<INF>4</INF>, and HCl, and chemical polishing by HF– H<INF>2</INF>O<INF>2</INF> are investigated. The thickness of the oxide layer after electropolishing is estimated to be about 3 nm, which is approximately equivalent to 1 mass ppm oxygen. This indicates that the currently available surface polishing methods are inadequate for the analysis of mass ppm or sub-ppm oxygen levels. A surface without an oxide layer is obtained by Ar ion sputtering in a vacuum, but the oxide layer reappears within 1 min when the sample comes into contact with air.