Abstract: The oxidation behavior of alloy 690 exposed to high-temperature and high-pressure steam and water at 280 °C for 1 h was investigated by scanning electron microscopy (SEM), atomic force microscopy (AFM) and X-ray photoelectron spectroscopy (XPS). In high-temperature and high-pressure steam, the oxide film is composed of an outermost Ni-rich hydroxides layer, an intermediate layer of hydroxides and oxides enriched in Cr, an inner oxide layer. The film formed in high-temperature water is similar to that in steam, except for missing the Ni-rich hydroxides layer. Samples with different surface finishes (electropolished, mechanically polished, ground, and as-received) were prepared for comparison. A general increase of the oxide thickness with the degree of surface roughness is observed. The equivalent oxide thicknesses lie in the range of 100{200 nm for the as-received samples, 150{250 nm for the samples ground to 400# and 10−20 nm for the samples ground to 1500#, mechanically polished, and electropolished.

Key words: Alloy 690, Oxidation, High-temperature water, Surface finish, X-ray photoelectron spectroscopy (XPS)