Iridescent rings around cavitation erosion pits on surface of mild carbon steel

Special rings with iridescent color were found around erosion pits on mild carbon steel surface in rotate disk cavitation erosion experiments. The EDS and XPS examinations proved that the ring was an oxidation film mainly composed of Fe2O3. The mean diameter of iridescent rings was 200–300 μm, and the thickness was 200–500 nm. The rings have four main kinds of the shapes, named as O-shaped, U-shaped, pies and comet rings. Their special shapes and chemical compositions indicate that the iridescent rings are products of a kind of local oxidizations related to the collapsing bubbles above them. Based on numerical and experimental results, it is explained that the hot gas in a collapsing bubble is possible to contact the metal surface to cause the high temperature oxidization, and the shape of the bubble at its final stage of collapsing is responsible for the special shape of the ring.