The influence of water vapour and silicon on the long term oxidation behaviour of 9Cr steels

In order to provide oxidation resistance to Cr-steels, the Cr content in the alloy must be above a critical limit. Since recently developed 9% Cr steels are close to that limit their oxidation behaviour is a critical issue concerning service lifetime and reliability of structural components. In the present paper the influence of water vapour and the alloying element Si on the oxidation behaviour of 9% Cr steels was investigated. Seven different alloys of the ferritic-martensitic 9% Cr steel type were discontinously oxidised at 650degreesC for more than 7 000 h in dry air and in air containing 4% and 10% water vapour. The results show that water vapour significantly accelerates oxidation of these steels and that the Si-content of the alloys is critical for the oxidation performance, especially under wet conditions. The influence of growth stresses and Cr depletion on the long term oxidation behaviour is also discussed.