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Abstract: Corrosion products are identified on Zn, ZnMg and ZnAlMg coatings in cyclic corrosion tests with NaCl or Na2SO4 containing atmospheres. For Mg-containing alloys the improved corrosion resistance is achieved by stabilization of protective simonkolleite and zinc hydroxysulfate. At later stages, the formation of layered double hydroxides (LDH) is observed for ZnAlMg. According to thermodynamic modeling, Mg2+ ions bind the excess of carbonate or sulfate anions preventing the formation of soluble or less-protective products. A preferential dissolution of Zn and Mg at initial stages of corrosion is confirmed by in situ dissolution measurement. The physicochemical properties of different corrosion products are compared. [Copyright &y& Elsevier]

Abstract: The polarization behaviour of zinc in alkaline solution has been investigated using atomic emission spectroelectrochemistry. By independently measuring the oxidation rate of zinc (electrical current) and the rate of Zn2+ dissolution (partial elemental current) it is possible to calculate the amount of insoluble zinc cations produced at any instant. Assuming the insoluble cations are present as a zinc oxide film, the growth of this film as a function of potential and time was determined. On the basis of kinetic evidence, it was found that at least three forms of zinc based oxide/hydroxide films form during polarization experiments. Type I oxide formation occurs when the metal/electrolyte interface becomes locally saturated with Zn2+ ions. Type II oxide forms on the metal surface underneath the film of Type I oxide but has little inhibiting effect on zinc dissolution. Type III oxide is produced in much smaller quantity and results in a transition to the passive state. This may be due to a potential induced transition of Type II→Type III oxide. [ABSTRACT FROM AUTHOR]

Abstract: This study explores the initial release of zinc and aluminum from non-treated Galvalume and the parallel formation of corrosion products when exposed to synthetic seawater and rainwater of different chloride content. Comparisons were made with long-term field exposures at non-sheltered marine conditions. Observed release rates from short-term conditions agree qualitatively with the long-term findings with a selective release of zinc over aluminum. The release and corrosion processes were intertwined through the formation of corrosion products with properties that influence the long-term release process. Prior to exposure, Al2O3 dominated the entire surface, and was subject to local destruction upon interaction with chloride ions. As a consequence Al2O3 was gradually replaced and covered by zinc-rich corrosion products primarily in interdendritic areas during the first year of marine exposure. This was followed by the gradual formation and integration of aluminum-rich corrosion products, reflected by an increased zinc release rate during the first year, followed by a gradually decreased rate during subsequent years. The importance of Al2O3 was also evident in deaerated synthetic rainwater or seawater, where the formation of Al2O3 was presumably hindered. In synthetic rain water this resulted in a higher ratio between released aluminum and zinc compared with non-deaerated conditions. [Copyright &y& Elsevier]

Abstract: The effect of a silicate conversion coating, which was applied on electrogalvanized steel (EG) and hot dip galvanized steel (HDG), on zinc passivation and dissolution is investigated by atomic emission spectroelectrochemistry (AESEC) using anodic/cathodic cycles in a neutral borate buffer solution. We are able to decompose the total zinc oxidation rate into a soluble (dissolution) and insoluble (passivation) component. It is found that the silicate conversion coating reduces the soluble component with no measurable effect on the insoluble component. The results imply that zinc passive film formation occurs unhindered underneath the silicate film but the film effectively blocks zinc dissolution. [Copyright &y& Elsevier]

The corrosion inhibiting and accelerating effect of l -cysteine on Zn were correlated with the formation of Zn oxides by means of a novel coupling of impedance spectroscopy (EIS) with atomic emission spectroelectrochemistry (AESEC). A plateau of anodic Zn dissolution was discovered in the cathodic branch of the polarization curve, hidden by the large cathodic current. The effect of l -cysteine at all pH values and all concentrations was to increase the plateau current. The effect was most pronounced at high pH suggesting a synergy between hydroxide and l -cysteine. Due to the presence of Zn plateau current, Tafel extrapolation resulted in a wrong estimation of corrosion currents. The contribution of the anodic Zn oxidation in the plateau region to the total current was inversely proportional to the frequency of the AC perturbation. [ABSTRACT FROM AUTHOR]

Effect of l -cysteine on the Zn dissolution in 0.5 M NaCl with initial pH of 4–12 was measured in continuously renewed electrolyte and in immersion tests. At short exposures 1 mM l -cysteine inhibited dissolution. The initial inhibition decreased with pH because of pH-enhanced dimerization of l -cysteine, confirmed by UV–vis spectroscopy. In long exposures at pH = 6–8 the inhibition turned into the acceleration. This was not observed at pH = 12. 10 mM l -cysteine accelerated Zn dissolution. The effects were explained by a competition between strong complexation of l -cysteine with Zn-ions and its weak physisorption on the substrate. The hypothesis was confirmed by surface characterizations. [ABSTRACT FROM AUTHOR]

The inhibitive action of soluble Na 2 MoO 4 on the spontaneous reactivity of hot dip galvanized steel in 0.5 M NaCl was studied at pH 4–13 by a direct measurement of Zn dissolution rate in the flowing electrolyte and postmortem surface analysis. The stability and the composition of Mo-rich films depended on the solution pH and flow conditions. The inhibition efficiency of soluble Mo(VI) correlated with the composition of the films: Mo(V)-rich films were immediately formed under uniform flow at pH 6–12, and the inhibition efficiency of Mo(VI) in these conditions (>92%) was comparable with the efficiency of Cr(VI). [ABSTRACT FROM AUTHOR]