Analysis of the Acid Passivation of Stainless Steel

摘要

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.