The Effects of Heat Treatment Condition on the Structure of Oxide Film on 316L Stainless Steel and Corrosion Resistance against Chlorine Gas
Haruo Tomari, Ikuro Hashimoto, Koji Wada, Hiroshi Satoh
The effect of surface polishing methods and heating atmosphere on the structure of oxide films formed on 316L stainless steel has been investigated. On electropolished surfaces, the oxide films which mainly consist of chromium will form at temperatures above 650°C in vacuum environment of 1Pa. In contrast, the threshold temperature of the formation of such oxide films on mechanically polished surfaces was as low as 450°C. This is attributed to the change of morphology in the surface region caused by polishing. After mechanically polishing, the grain size in the surface region became smaller. This leads to a high diffusion rate of chromium to the surface due to the grain boundary diffusion. As a result, chromium-rich oxide film started to form on mechanically polished surfaces at a relatively low temperature compared to the electropolished surfaces. The thickness of oxide film is increasing with abrasive grit size.
The corrosion resistance of the oxide films to chlorine gas has also been examined. The high chromium-content oxide films which were dense and thick showed superior corrosion resistance to chlorine gas. The chromium-rich oxide films will not corrode in chlorine gas at 250°C if the films are thicker than 300Å.