Radioactive Labeling, Electrochemical, and Ultrahigh Vacuum Study of Sulfate Adsorption on Type 316 Stainless Steel

Adsorption of the sulfate/bisulfate anion on type 316 (UNS S31600) stainless steel (SS) in 0.1 M sodium perchlorate (NaClO₄) solution was studied using electrochemistry, radiochemistry, atomic force microscopy (AFM), ultrahigh vacuum (UHV) depth profiling, and x-ray photoelectron spectroscopy (XPS). Adsorption was shown to depend upon pH and to occur over a broad range of electrode potentials. At pH < 2.0, sulfate surface concentration was shown to be low in the hydrogen evolution region, to increase with anodic polarization up to -0.10 V_Ag-AgCl, but to decrease at even higher potentials. Effect of electrode potential at pH > 2.0 was much weaker. A multilayer sulfate coverage observed at some pH values and electrode potentials was considered evidence that sulfate, under some conditions, was an integral part of the corrosion layer. Adsorption was reversible only in sightly acidic or alkaline solutions. Below pH 2.0, passive film formation enhanced sulfate adsorption irreversibility. Auger electron spectroscopy (AES) and XPS analyses provided evidence about the presence of sulfate in the outer part of the oxide film at the expense of iron content of the film. Data indicated adsorption of sulfate facilitated oxidation of Cr(III) to higher oxidation states.