The Use of SIMS, SEM, EPMA, LRS and X-Ray Diffraction Measurements for the Examination of Corrosive Layers and Protective Coatings on Steels and Alloys in Advanced Power Stations

Abstract.

The development of modern power generation systems with higher thermal efficiency requires the use of constructional materials of higher strength and improved resistance to the aggressive service atmospheres. In this paper the following examples are discussed:

1) The oxidation behaviour of 9% (w/w) Cr steels in simulated combustion gases: the effects of O₂ and H₂O content on the oxidation behaviour of 9% (w/w) Cr steels in the temperature range of 600–800 °C showed that in dry oxygen a protective scale was formed with an oxidation rate controlled by diffusion. However, in the presence of water vapour, after an incubation period, the scale became non-protective as a result of a change in the oxidation mechanism.

Oxidation experiments on 10% (w/w) Cr steel in simulated steam atmosphere at 650 °C have shown that a haematite layer is formed. The investigation suggests that several independent processes, such as oxide dissociation and solid state diffusion of iron cations in the scale, significantly contribute to the overall oxide growth process.

2) The development of light-weight intermetallics based on TiAl-basis: TiAl-based intermetallics are promising materials for future turbine components because of their combination of high temperature strength and low density. These alloys, however, exhibits poor oxidation resistance at temperatures above 700 °C. The experimental results showed that the oxidation behaviour of TiAl-based intermetallics can be significantly improved by minor additions of 1–2 at-% silver.