XPS studies on passive film formed on stainless steel in a high-temperature and high-pressure methanol solution containing chloride ions

The composition and structure of passive film formed on 316L SS immersed in an anhydrous methanol solution (water content < 0.05 wt%) containing 0.42 wt% LiCl at 323–473 K were investigated by X-ray photoelectron spectroscopy (XPS), and compared with those of film formed in an aqueous solution. The passive film formed in the methanol solution was mainly composed of Fe and Cr oxides, and it possessed a double-layered structure consisting of an Fe oxide-rich outer layer and a Cr oxide-rich inner layer. Dissolution of the Fe-rich layer and densification of the Cr-rich inner layer were observed, especially at high temperatures. However, these were suppressed in an aerated methanol solution at 423 K or below, probably due to the barrier effect of adsorbed oxygen. No Ni compound contributed to composing the passive film, even at higher temperatures. The ratio of OH− to O2− was small and decreased with an increase in temperature (the presence of oxygen suppressed the decrease, especially at 423 K or below). The chloride ions were concentrated in the Fe-rich outer layer, and they penetrated more deeply than that in the aqueous solution into the passive film formed in the methanol solution.