Analysis of the acid passivation of stainless steel

Passivation of 304 and 316 stainless steels in various acid solutions was studied as a function of exposure time and acid concentration. Nitric acid, citric acid, and the commercial Citrisurf (a commercial citric acid-based passivating solution, Stellar Solutions, USA) were compared. The materials were studied by low-angle PXRD (powder x-ray diffraction), XRF (x-ray fluorescence), SEM (secondary electron microscopy), and XPS (x-ray photoelectron spectroscopy). As might be expected, the measurements showed increased Cr:Fe ratios at the surface following acid passivation. Using the combination of characterization methods, it was possible to generate concentration-depth profiles, and these suggest that chromium enrichment can penetrate several micrometers into the surface for nitric acid treatment, and this is related to some surface damage. The low-angle PXRD work illustrated that complex phases are formed at the passivated surface, and these phases exhibit a rich structural chemistry. It is concluded that citric acid-based passivating agents result in more coherent oxide surfaces that are more resistant to corrosion.