Steamside oxidation behaviour of candidate USC materials at temperatures between 650 and 800°C

Abstract

The Boiler Materials for Ultrasupercritical Coal Power Plants Programme sponsored by the US Department of Energy and the Ohio Coal Development Office has the aim of identifying, evaluating and qualifying the materials needed for the construction of critical components for coal fired boilers capable of operating at much higher efficiencies than current supercritical plants. Operation at ultrasupercritical conditions (steam temperatures up to 760°C) will necessitate the use of new advanced ferritic materials, austenitic stainless steels and nickel based alloys. As well as possessing the required mechanical properties and fireside corrosion resistance, these materials must also exhibit acceptable steamside oxidation resistance. As part of the programme, steamside oxidation testing is being performed. More than 30 ferritic, austenitic and nickel based materials have been exposed for up to 4000 h in flowing steam at temperatures between 650 and 800°C. In addition to wrought materials, steamside oxidation tests have been conducted on weld metals, coated materials and materials given special surface treatments. Exposed specimens were evaluated to determine oxidation kinetics and oxide morphology. High chromium ferritic, austenitic and nickel based alloys displayed good oxidation behaviour over the entire temperature range due to the formation of a dense chromium oxide. With increasing steam temperature, low chromium ferritic materials experienced breakaway oxidation, and low chromium austenitic materials experienced significant oxide exfoliation. Surface treatments, particularly surface alloying and mechanical treatments, appear to have a beneficial effect on oxidation behaviour, but longer term studies are required to confirm their utility.