Influence of SO2 on the Oxidation of 304L Steel in O2 + 40%H2O at 600 °C

The effect of sulphur dioxide on the oxidation of alloy 304L in O₂ + 40%H₂O has been investigated at 600 °C. A protective chromium-rich corundum-type oxide forms in clean dry O₂. Exposure to O₂ + 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 μm 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 O₂ + 40%H₂O reduces the corrosion rate compared to O₂ + 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.