XPS depth profiling of oxide scales of stainless steels formed in high-temperature aqueous conditions

In this work, we studied the oxide scales of AISI 304 and AISI 316 stainless steels formed under hightemperature aqueous conditions using X-ray photoelectron spectroscopy (XPS) together with sputter depth profiling. Two samples of SS 304 and 316 were prepared by exposing them to water at 300 °C containing 100 ppm boric acid and 7 ppm lithium hydroxide in an autoclave. In the case of sputtering with 3 keV argon ions, the elemental distribution as a function of the sputtering time was obtained. The formation of two oxide layers was observed – one mainly iron-based on top, and a second at a greater depth, consisting mainly of chromium-iron oxides. The depth scale of the profiles was calibrated by measuring the oxygen profile using Rutherford backscattering spectroscopy (RBS). On the basis of the distribution of the three oxidised alloying constituents (Fe, Cr, Ni) with respect to depth and elemental state, a relative increase of Fe2+ at the surface was found, while the chromium was depleted at the surface and Cr3+ tended to increase at the oxide/substrate interface. In order to estimate possible ion-beam-induced effects due to the ion sputtering, the profiles were compared to the computer-simulated ones obtained by a dynamic TRIM computer code. The simulations included only collisional effects during the sputtering process.