As the demand for high performance materials increases, so does the importance of surface engineering. The surface of a solid is the point at which the material interacts with its external environment. Metals found in consumer and commercial use—from non- stick cookware to automobiles, electronics, and medical implants—must be resistant to wear and corrosion to withstand long-term use. Surface modification has a wide variety of applications for improving the performance and behavior of materials.
Material integrity is of particular importance in stainless steel products. If inferior metals are used in the manufacture of airplane parts, for example, costly or even catastrophic consequences may result. A critical factor to consider is the corrosion resistance of the steel.
The surface chemistry of stainless steel is strongly influenced by the chemical or mechanical processing of that surface. Corrosion prevention is a major goal of any steel manufacturing process. This is often achieved through passivation, the process of coating a metal with a protective oxide. This process is critical to the performance of the metal and if performed incorrectly can actually induce corrosion.
Passivation — making the surface more corrosion resistant — typically involves the formation of a chromium-rich ‘passive’ layer on the top surface and there are a number of different methods employed to encourage the growth of such a layer. High-tech industries that require high quality metal building components have relied on passivation technologies for years as a tested and proven step in manufacturing high performance, corrosion-resistant steels. However, the process must be continually assessed to make sure it’s working. The standard metrics used to identify properly passivated steel are the total chromium/iron (Cr/Fe) atomic ratio, the Cr oxide/Fe oxide ratio, and the total oxide thickness.
X-ray Photoelectron Spectroscopy (XPS) is a fundamental characterization tool for investigating a wide range of surface problems on metal and oxide surfaces. With its surface selectivity and quantifiable data, XPS is the ideal tool for measuring composition and thickness of protective oxide films on metals. In addition, sputter profiling the steel surface provides a full understanding of the surface composition and chemistry, which may help to diagnose failures in the passivation process.
How do you identify passivation issues using XPS? Read our next article, which will provide a link to a recording of our webinar: XPS Simplified: Understanding Metal Surfaces and Oxides with X-ray Photoelectron Spectroscopy (XPS). It should answer most of your questions.