Influence of SO2 on the oxidation of 304L steel in O-2+40%H2O at 600 degrees C

Artikel i vetenskaplig tidskrift, 2008

The effect of sulphur dioxide on the oxidation of alloy 304L in O-2 + 40%H2O has been investigated at 600 degrees C. A protective chromium-rich corundum-type oxide forms in clean dry O-2. Exposure to O-2 + 40%H2O environment results in chromium vaporization in the form of CrO2(OH)(2). This causes local failure of the protective oxide and the formation of 10 mu m thick oxide islands on the alloy grain centers. The oxide islands are layered, the outer part consisting of hematite while the inner part is FeCrNi spinel oxide. The addition of 100 ppm SO2 to O-2 + 40%H2O reduces the corrosion rate compared to O-2 + 40%H2O. SO2 is suggested to influence oxidation by two separate effects. Firstly, SO2 forms surface sulfate on the oxide surface that impedes the vaporization of chromium from the protective oxide. This slows down the breakdown of the protective oxide. Secondly, SO2 also influences the rapid oxidation that ensues once the protective oxide has been destroyed. In this case, the presence of surface sulfate interferes with the surface reactions involved in oxidation. In this way, SO2 slows down the growth of the oxide islands