Influence of dissolved oxygen concentration on the oxide film formed on Alloy 690 in high temperature water

Characteristics of the oxide films formed on Alloy 690 under different dissolved oxygen (DO) concentrations in 290 °C water were examined. Under 3 ppm DO, the oxide film mainly consisted of outer spinel particles and porous NiO base layer. A thin Cr-rich film gradually develops with decreasing DO. Fe and Ni levels in the film decrease while Cr level increases concomitantly. The film preformed under high DO changes slightly when further immersed under low DO. The film preformed under low DO becomes much thicker when immersed under high DO, with gradual development of spinel particles and NiO base layer.

► Thinner Cr rich oxide film forms on fresh Alloy 690 as dissolved oxygen decreases.
► The oxide film preformed under 3 ppm DO is slightly changed as DO drops.
► Cr rich oxide preformed at low DO is replaced by Spinel and NiO as DO rises up.
► The film preformed under low DO gets thicker and Cr-depleted as DO rises to 3 ppm.