X-RAY PHOTOELECTRON STUDIES OF THE AQUEOUS OXIDATION OF INCONEL-600 ALLOY

Aqueous oxidation of the nickel-based alloy lnconel-600 has been studied using x-ray photoelectron spectroscopy as the primary means of determining the elemental composition and chemistry of the surface films produced. Composition profiles of the entire film thickness back to the metal-oxide interface were determined using ion bombardment for controlled removal of successive surface layers. Samples of the alloy were exposed to aqueous attack in static and continuous autoclaves at 285°C for periods of 1-168 hr. The pH range 7-14 was studied and the dissolved oxygen concentration was varied from <0.005 mg O2/kg H2O to 20 mg/kg using hydrogen, argon, and oxygen overpressures. In hydrogen-saturated water of pH 10, a thin (<10 nm) film, rich in chromium oxide forms. This is shown to result from a solid-state growth process. A similar film, rich in chromium oxide, grows on lnconel-600 in pH 10 water containing higher concentrations of dissolved oxygen, but it is rapidly overgrown by a nonpassivating layer of metal hydroxides whose composition and rate of growth vary with the oxygen concentration of the solution. The chemical stability of the alloy surface is likely determined by the magnitude of the solubility of the chromium oxide layer which is formed on initial oxidation. © 1979, The Electrochemical Society, Inc. All rights reserved.