Abstract
The surface structures associated with the very early stages of pitting corrosion on stainless steels have been studied using a set of such surface analytical techniques as XPS, ToF-SIMS and SAM combined simultaneously with EDXA. Two types of 316 stainless steel, having minor variation in chemical composition were prepared for this work and passivated chemically in a solution of 0.5 M H2SO4 + 0.5 M NaCI + 0.08% H2O2. Survey and narrow scan XP spectra were acquired from the samples using a V. G. Scientific ESCALAB (II) in Angular XPS format. The survey scans were examined with emphasis on the essentials of the background slope caused by electron energy losses. The narrow scans of the peaks of significance, i.e., C1s, 01s, Cr2p and Fe2p were used for quantitative analysis in which the compositions of the surface species were determined. An angular and a depth profile were constructed as a basis for obtaining a layer sequence of the surface structures. ToF-SIMS analysis of the passive film was undertaken to validate the Angular XPS data. The program GOOGLY has been employed in fitting the oxide and metallic components of chromium and iron as well as oxygen and carbon, using the intrinsic shape parameter, k. The use of peaks of identical shape, position and background for each electron take-off angle facilitates the dependency of the residual extrinsic background on angle. The use of a polynomial function, that approximates the Tougaard background, has been included in the fitting routine. This function has been utilised in defining a background slope parameter that assigns different background tails to the components in a multiple peak. The rationale behind the use of the extrinsic background is to enable surface structures to be placed in depth-wise order with only a minor extension to the conventional peak-fitting routine. The work has shown also that it is possible to extract information from the background close to the peak, which can be used to develop a useful approach for the estimation of overlayer thickness. Pitting initiation at non-metallic inclusions on the steel surface, i.e., sulphide and mixed oxide particles have been examined by employing the high spatial resolution afforded by SAM in conjunction with the depth analysis offered by EDXA. Many of the multi-oxide inclusions, composed of silicates, remained inert to the solution. A small number that contained calcium silicate and oxide, however, showed evidence of attack. Inference was therefore made that the corrosion pathway, involving insoluble silicate inclusions, is largely dependent on soluble calcium containing phases within the inclusions. Scanning Auger microscopy has been utilised in demonstrating that the relatively high susceptibility of sulphide inclusions to pitting attack in corrosive media stems from the alloy distribution. The data obtained supports the proposal that depletion of chromium, exclusively in the immediate neighbourhood of the sulphide inclusions, leads to favourable ingress of aggressive ions. The GOOGLY program has been used to extend the intrinsic shape and background slope consideration to fit the FeLMM Auger peaks of both air-formed and passive films on stainless steel. Quantification of the peaks was complicated but the fits showed a reasonable correlation with those from Angular XPS. The much-reduced spot size obtainable on a high resolution (V. G. Scientific Sigma Probe) XPS instrument has been utilised in a localised characterisation of the passive film on stainless steel. The spectra acquired from different points on the steel surface have displayed a high degree of similarity to each other. The results obtained have been reasonably satisfactory.