Transition metals such as iron are reactive components of environmentally relevant surfaces. Here, dark reaction of Fe(III) with catechol and guaiacol was investigated in an aqueous solution at pH 3 under experimental conditions that mimic reactions in the adsorbed phase of water. Using UV-vis spectroscopy, liquid chromatography, mass spectrometry, elemental analysis, dynamic light scattering, and electron microscopy techniques, we characterized the reactants, intermediates, and products as a function of reaction time. The reactions of Fe(III) with catechol and guaiacol produced significant changes in the optical spectra of the solutions due to the formation of light absorbing secondary organics and colloidal organic particles. The primary steps in the reaction mechanism were shown to include oxidation of catechol and guaiacol to hydroxy- and methoxy-quinones. The particles formed within a few minutes of reaction and grew to micron-size aggregates after half an hour reaction. The mass-normalized absorption coefficients of the particles were comparable to those of strongly absorbing brown carbon compounds produced by biomass burning. These results could account for new pathways that lead to atmospheric secondary organic aerosol formation and abiotic polymer formation on environmental surfaces mediated by transition metals.
Electropolishing as a surface preparation technique is increasing in popularity in industrial applications and for corrosion studies. Electropolished surfaces have shown better resistance to pitting corrosion over mechanical polishing; however, the fundamental reason governing the change in corrosion behaviour remains unclear. This study examined the corrosion behaviour of 13Cr4Ni stainless steel (UNS S41500) after five surface preparation techniques and shows that sulfate is incorporated in the oxide film when it is present in the electropolishing solution. Even after removal from the sulfate-containing solution, the sulfate incorporation increases the material’s pitting resistance by lowering the number of sites available for chloride to induce pitting. This work also demonstrates that, when used as a counter electrode, Pt can dissolve and reprecipitate on the working electrode surface during electropolishing. The deposits result in a more noble open circuit potential, indicating an artificial increase in passivity. These artificial changes to corrosion behaviour due to surface preparation method may result in erroneous conclusions. To establish fair comparisons between surface preparation methods, the counter electrode and the sulfate effect should be strictly considered.
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