Thiosulfate accumulation on 304 stainless steel in near neutral solutions (pH -5.6) was studied using in situ techniques: electrochemistry and radiochemistry, as well as by Auger electron spectroscopy depth profiling and angle-resolvedx-ray photoelectron spectroscopy in ultrahigh vacuum. It was found that thiosulfate accumulation is an irreversible process and occurs over a broad electrode potential range. Thiosulfate surface concentration is very small, below -1.0 V vs. Ag/AgCl reference. In the potential range from -1.0 to -0.50 V the surface concentration increases linearly with potential, reaches a maximum at -0.30 V, and at even more positive potentials, decreases to a slightly lower level. Ultrahigh vacuum spectroscopic measurements indicate that the irreversible surface behavior can be attributed to thiosulfate incorporation into the substrate passive film. The present data obtained with 304 stainless steel are compared to previous results published from this laboratory on thiosulfate adsorption on 316 stainless steel, and the role of molybdenum surface enrichment in the thiosulfate accumulation reversibility is discussed. The effect of chloride on thiosulfate accumulation was also investigated. At high concentrations of chloride, thiosulfate is desorbed from the surface due to chlorideinduced dissolution of the stainless steel. At very negative potentials, the thiosulfate surface concentration increases upon chloride addition, most probably due to the surface microroughening caused by chloride adsorption.
We report on thiosulfate adsorption on a 316 stainless steel electrode in neutral solutions by radiochemical, electrochemical, and Auger electron spectroscopy (AES) measurements. In contrast to previous work on thin metal films or metal electrodeposits to model real-life metal (alloy) samples, our radiochemical approach permits coverage measurements of labeled adsorbates on smooth solid electrodes of convenient size and thickness. A dedicated electrode preparation yields a reproducible surface for fast and reversible thiosulfate adsorption from a perchlorate solution. However, on the same time scale, reversible adsorption is impeded in sulfate media. Further, no thiosulfate adsorption occurs in borate buffer solution, and complex thiosulfate adsorption behavior predominates in solutions containing varying amounts of chloride anions, initially increasing with bulk chloride concentration, maximal at 10 -4 to 10-SM CI-, and decreasing at higher chloride concentration. We conclude that small amounts of surface chloride assist the thiosulfate adsorption, but increasing surface C1-coverage becomes competitive to that of thiosulfate. The chloride adsorption enhancing mechanism probably results from a destabilization of the locally ordered thiosulfate surface structure. Radioehemistry and AES show that long exposure to thiosulfate solutions results in permanent thiosulfate immobilization on the stainless steel surface.
We have applied radiotracer, electrochemical and ultrahigh vacuum techniques to study sulfate accumulation in passive films on pure aluminum and Al 2024 alloy in 0.1 M NaC104 containing 0.1 mM Na2SC>4. We have found that the sulfate coverage is pH and electrode potential dependent and that sulfate is bonded to the surface in two distinctively different ways. While the breakdown of the passive film results in sulfate removal, the subsequent repassivation reintroduces the sulfate anion into the passive film. There is a strong tendency of sulfate to remain in the passive film which explains the inhibitive properties of sulfate in aluminum corrosion. Our data reveal that the anomalous sulfate accumulation during the negative-going polarization can be attributed to the copper content of the alloy surface. The formation of copper-containing nodules determines the electrode potential threshold below which sulfate anions desorb.
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