The Role of XPS in Determining the Quality of Stainless Steel Passivation


This blog post was written by Dr. Jerry LaSalle, an expert in XPS surface analysis and physical and powder metallurgy. Dr. LaSalle’s areas of concentration include stainless steel passivation qualification, metallurgical failure analysis, additive manufacturing, and powder metallurgy consultation.

The surface of a bulk material is its window to the outside world, and its first point of contact with the environment around it. This thin layer of surface material dictates a material’s corrosion, adhesion, and friction properties, as well as any chemical reactions. When surface atoms and molecules are illuminated with X-rays, they generate unique electron spectra (fingerprints) that are based on each element’s electron energy and intensity. Modern detectors like the X-ray photoelectron spectroscopy (XPS), also known as Electron Spectroscopy for Chemical Analysis (ESCA), use computer software to quickly and precisely interpret the resulting spectral fingerprints and determine the elemental and molecular states of the top few atomic layers of a material’s surface.

Application

One important application of the XPS is to determine the quality of stainless steel passivation. The alloy, stainless steel, typically contains a minimum of 12% chromium (Cr) in addition to iron (Fe), and owes its corrosion resistance to the formation of a thin, self-healing chromium oxide layer (on the order of 10 atomic layers) on the surface. Passivation, typically achieved by using nitric acid treatments, enhances the corrosion resistance of stainless steel by enriching the chromium oxide to iron oxide ratio at the surface.

The XPS uses a process called sputtering, in which atomic ions gently remove surface atoms. Modern XPS software, such as that used by the K-alpha XPS unit, can then interpret the resulting spectra. Using predetermined sensitivity factors for Fe and Cr, the K-alpha XPS is able to rapidly ascertain the Croxide/Feoxide ratio and provide a pass/fail measurement of the passivation quality. The quality of the software’s algorithm and elemental sensitivity factors is key to a correct assessment. The figure below shows the Cr/Fe ratio as a function of depth. It demonstrates that the Cr/Fe ratio below the surface is in agreement with the bulk chemistry for 316L grade stainless steel, i.e., a nominal ration of Cr/Fe ~0.25. Using this feature, the analyst can document the XPS instrument’s accuracy in detecting the two key elements (Cr and Fe) that define a particular grade of stainless steel.

XPS Steel Passivation Spectrum

XPS Steel Passivation Spectrum

 

RJ Lee Group’s K-alpha XPS unit uses precisely calibrated elemental signatures, giving it the capability to determine a material’s surface chemical constituents and chemical states. This instrument is highly effective in rapid and accurate analysis of a material’s top few atomic layers, and can be employed in diverse applications from finding unknown surface species to rapid quality control applications, such as determining the quality of stainless steel.

For more information on XPS analysis, please contact RJ Lee Group at 1.800.860.1775.