Enhanced oxidation of the 9%Cr steel P91 in water vapour containing environments

The short term (∼100 h) oxidation behaviour of the 9%Cr steel P91 was studied at 650 °C in N₂-O₂-H₂O gas mixtures containing a relatively low oxygen level of 1%. The oxidation kinetics were measured thermogravimetrically and the oxide scale growth mechanisms were studied using H₂¹⁸O-tracer with subsequent analyses of oxide scale composition and tracer distribution by MCs⁺-SIMS depth profiling. The corrosion products were additionally characterised by light optical microscopy, SEM-EDX and XRD. It was found that the transition from protective, Cr-rich oxide formation into non-protective mixed oxide scales is governed by the ratio H₂O^(g)/O₂ ratio rather than the absolute level of H₂O^(g). The results of the tracer studies in combination with the data obtained from experiments involving in situ gas changes clearly illustrated that under the prevailing conditions the penetration of water vapour molecules triggers the enhanced oxidation and sustains the high growth rates of the poorly protective Fe-rich oxide scale formed in atmospheres with high H₂O^(g)/O₂ ratios. The experimental observations can be explained if one assumes the scale growth to be governed by a competitive adsorption of oxygen and water vapour molecules on external and internal surfaces of the oxide scales in combination with the formation of a volatile Fe-hydroxide during transient oxidation. The formation of the non-protective Fe-rich oxide scales is suppressed in atmospheres with low H₂O^(g)/O₂ -ratios, and the healing of any such scale is promoted.