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A combination of electrochemical measurements, H2 volume measurements and surface analysis using Time-of-Flight SecondaryIon Mass Spectrometry (ToF-SIMS) and X-ray Photoelectron Spectroscopy (XPS) was used to investigate the role of the metallic Fe impurities (60 ppm) in high purity (99.9 wt%) magnesium and the effect of sulfur on the hydrogen evolution. The Mg corrosion was studied in NaCl solutions containing dissolved hydrogen sulfide. At OCP and under anodic polarization the hydrogen evolution reaction and the growth of dark corroded areas were significantly inhibited in the presence of H2Saq. ToF-SIMS and XPS analyses showed the presence of sulfur adsorbed on Mg and on Fe segregated at grain boundaries. The observed inhibition of hydrogen evolution at OCP and above is assigned to a poisoning effect by adsorbed S of the H adsorption sites on segregated iron which, in the absence of sulfur, provides catalytic sites for excess HER. The equilibrium Pourbaix diagram for the Mg-Fe-S system including the adsorbed species predicts that S is adsorbed on Mg and Fe in the E-pH conditions of this work, which confirms therole assigned to adsorbed sulfur in the inhibition of excess hydrogen evolution at OCP and under anodic polarization in the presence of H2Saq. [ABSTRACT FROM AUTHOR]

Abstract: An equilibrium potential–pH (Pourbaix) diagram is presented for sulfur adsorbed on Ag(111). It allows prediction of the E–pH conditions of adsorption of sulfur and hydroxyl on silver surfaces immersed in water containing dissolved sulfur species such as sulfides. The E–pH relations associated with the equilibria between water and adsorbed hydroxyl, and between dissolved sulfur species and adsorbed sulfur are given at 25°C. The E–pH adsorption diagram is calculated for a sulfur concentration of 10−3 molkg−1 and superimposed to the usual Pourbaix diagram for the Sph name="sbnd" />H2O system. It is shown that an adsorbed sulfur monolayer may form on the silver surface in E–pH domains in which usual Pourbaix diagrams predict no stability of the bulk silver sulfide Ag2S. The calculation of stability domains of adsorbed hydroxyl and sulfur allows prediction of the electrocatalytic and corrosion behavior of Ag in water with sulfur species. [Copyright &y& Elsevier]

The principle of equilibrium potential–pH (Pourbaix) diagrams can be extended to the case of bidimensional layers of species adsorbed on metal surfaces. It allows us to predict the E–pH conditions of adsorption of sulfur and hydroxyl on copper surfaces immersed in water containing dissolved sulfur species such as sulfides, sulfites or thiosulfates. The E–pH relations associated with the equilibria between water and adsorbed hydroxyl, and between dissolved sulfur species and adsorbed sulfur are given at 25 °C. The E–pH adsorption diagrams are calculated for a sulfur concentration of 10−4 mol kg−1 and superimposed to the usual Pourbaix diagrams for the S–Cu–H2O system. It is shown that an adsorbed sulfur monolayer may form on the copper surfaces in E–pH domains in which usual Pourbaix diagrams predict no stability of the bulk copper sulfide. The calculation of stability domains of adsorbed sulfur allows prediction of the corrosion risk for copper-based materials in sulfur-containing aqueous environments. [Copyright &y& Elsevier]