Steam oxidation and its potential effects on creep strength of power station materials

Abstract

The oxidation behaviour of several commercial alloys in simulated steam was investigated and compared with that of model alloys with systematic variations of selected alloying elements in the temperature range of 550°C to 650°C. Also, creep tests of specimens pre-oxidized in steam or in flue gas were carried out in order to study the interaction between creep and corrosion. The corrosion products were characterised by optical microscopy, XRD, SEM/EDX and Raman spectroscopy. It was found that the oxidation resistance of the materials increased with increasing chromium content. High chromium materials exhibited the best oxidation behaviour, whereas the low chromium materials formed thick, multilayered oxide scales, prone to spallation. Anomalous temperature dependences were found in materials with intermediate chromium contents. Creep tests showed a significant reduction in the creep strength of both alloys was caused by a 1000 h thermal exposure treatment at 650°C for P92 and at 800°C for Alloy 800. An additional, though much smaller reduction in strength was observed for the specimens that had been oxidized for 1000 h at 650 or 800°C prior to testing. Further testing is required for confirmation of this effect.