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The structure of the passive film that protects iron and iron/chromium alloys from corrosion has been investigated in siru in an aqueous environment using X-ray absorption spectroscopy, with fluorescence yield detection. From an analysis of the EXAFS and XANES it has been shown that the film formed on iron consists of FeO, octahedra, linked together by sharing edges to form what are probably sheets, or chains. When the iron is alloyed with chromium, or when it is exposed to a solution containing chromate ions, the passive film becomes more disordered and the distance between neighbouring iron atoms increases slightly. More significantly, from the passivation point of view, it has been shown that the chromium is incorporated into the passive layer as a phase essentially identical to Cr(OH),, i.e. CrO, octahedra linked by hydrogen bonds into an amorphous threedimensional phase. It is this phase that appears to give rise to the enhanced corrosion resistance that chromium imparts to iron alloys.

A quantitative structural study of Na and Rb adsorption on Al(l I l) using normal incidence standingx-ray wave-field triangulation shows that Rb occupies atop adsorption sites and both systems show a constant adsorbate-substrate bond length over the alkali coverage range 0.12-0.33 monolayer. These results indicate that there is directional local bonding in this simple alkali s-p band metal adsorption system, and show that there is no ionic-covalent bonding transition in this coverage range for either system. PACS numbers: 68.35.Bs, 6 l. l o. -i, 82.65.MyThe nature of alkali-atom bonding on metal surfaces and the associated local adsorption geometries, superficially rather simple problems, have recently proved rather controversial. The conventional view of such adsorption is that there is substantial charge transfer from the adsorbed alkali to the substrate metal leading to an essentially ionic adsorbate-substrate bond, with depolarization at high coverage eventually leading to metallic or covalent bonding [1,2]. Evidence believed to support this view includes the measured variation of the work function with alkali coverage [2], and a significant adsorbatesubstrate bond-length extension with increasing coverage as the bond becomes less ionic in the Cs/Ag(111) adsorption system [3]. In addition, quantitative structural studies of several such adsorption systems indicate the occupation of maximally coordinated adsorption sites as might be expected for essentially nondirectional bonding [4-91. By contrast, model calculations of alkali-atom adsorption on jellium lead to a charge redistribution which has been interpreted as characteristic of covalent adsorbate-substrate bonding [10,11], and recent core-level photoemission studies of Cs, K, and Na adsorption of W(110) have been interpreted as consistent with this view [12]. In addition, there are two low-energy electron diffraction (LEED) studies [of Cs on Cu (111) [13] and very recent investigation of K on Ni(111) [14]] which identify atop site adsorption, apparently indicating a more directional form of adsorbate-substrate bonding. All of these results supporting this covalent bonding picture are from nobleor transition-metal substrates.Here we report on quantitative structural studies using a quite different technique [normal incidence standing xray wave-field absorption (NISXW) [15,16]] of the adsorption of both Rb and Na on Al(111), a simple s pband metal, over a wide coverage range including phases both with and without long-range order. In the case of Rb on Al(111) we identify the adsorption site as atop, while we find no evidence in either adsorption system for any significant coverage dependence of the adsorbatesubstrate bond length. Both of these results support the view that no ionic-to-metallic bonding transition occurs, and suggest that the Rb-Al substrate bonding is highly directional, even for this simple s-p band metal substrate.The Na adsorption results differ from those of Rb, however, in indicating that this adsorbate does occupy a high coordination a...