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 (scanning 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.