Many manufacturing processes require the passivation of the material in order to ensure the surface is inert or non-reactive. With stainless steel, for example, the passivation of the surface helps prevent corrosion or rust. For Nitinol (a nickel-titanium alloy), the passivation of the materials helps prevent corrosion, as well as aids biocompatibility. The passivation of Nitinol will usually deplete the surface of nickel, which can cause severe allergic reactions in the human body. For materials including stainless steel and Nitinol, whose passivation layers may be quite thin, surface analysis is one of the few techniques capable of providing a chemical analysis of the layer. One of the most commonly used techniques is electron spectroscopy for chemical analysis (ESCA, also called x-ray photoelectron spectroscopy, XPS). ESCA has a sampling depth of approximately 30 Angstroms and can provide the chemical composition and thickness of the passivation layer. This technique is also cited by the semiconductor industry in specifications for testing the passivation of stainless steels. The method utilizes an x-ray beam to excite a solid sample, resulting in the emission of photoelectrons. An energy analysis of these photoelectrons provides both elemental and chemical bonding information about a sample surface. The principal advantage of ESCA is its ability to look at a broad range of materials - including polymers, glasses, fibers, metals, semi-conductors and paper - and identify surface constituents as well as their chemical state. Electron Spectroscopy for Chemical Analysis of Stainless Steel The characteristics used to evaluate passivated stainless steel are the chromium-to-iron and the chromium oxide to iron oxide ratios. Both of these ratios, as well as the thickness of the passivation layer, can be measured using ESCA. Electron Spectroscopy for Chemical Analysis of Nitinol Nitinol is a shape memory alloy with superelastic properties. However, to be used in medical devices, the alloy must be passivated to prevent corrosion and any possible leaching of nickel into the human body. ESCA is a useful technique for evaluating the passivated Nitinol surface for the presence or non-presence of nickel and determining the thickness of the passivation layer. Nitinol will usually passivate by forming a titanium dioxide layer on the surface.
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