Use of x-ray absorption techniques to study passive films on alloy surfaces

In this study the films that form on Fe and Fe-Cr alloys and those on Mg and Mg alloys, cast and rapidly solidified, were investigated using x-ray absorption spectroscopic techniques. The x-ray techniques employed were EXAFS (Extended X-ray Absorption Fine Structure) and NEXAFS (Near Edge X-ray Absorption Fine Structure) which have proven to be powerful structural probes. These techniques are especially suitable for vitreous structures because they are sensitive only to short range order. Passive films are generally very thin, about 30{dollar}sim{dollar}100A. To study these films with EXAFS and NEXAFS, the ReflEXAFS technique, which makes use of total external reflection of x-rays from a solid surface, has been applied. For ReflEXAFS the penetration depth of an x-ray beam below or at the critical angle is less than 50A.; The effect of chromium is to increase the interatomic distances in the passive films on Fe-Cr alloys, resulting in a greater degree of disorder and noncrystallinity in the films. The breakdown process involves the chloride ions both penetrating into the passive films and producing a roughening effect on the film-solution interface by chemical reactions. The penetration of chloride ions into the passive films on Fe, Fe14%Cr and Fe24%Cr alloys were shown by the effect of the chloride ions had on the pre-oxygen K-edge absorption feature in the reflectivity spectra. This effect was accounted for by the incorporation of the chloride ions in the passive films.; The oxide films on pure magnesium and AZ61 alloy have structures similar to periclase (MgO) with a greater static disorder in the film on AZ61. The oxide films on Mg15%Al, Mg30%Al and RS magnesium alloys are more like brucite (Mg(OH){dollar}sb2{dollar}). The film on RS magnesium alloy exhibited the greatest disorder and has better corrosion-resistance characteristics.