Formation of corrosion products on open and confined metal surfaces exposed to periodic wet/dry conditions - A comparison between zinc and electrogalvanized steel

The formation of corrosion products during atmospheric corrosion on open and confined surfaces of electrogalvantzed steel exposed to periodic wet/dry conditions was studied. The composition of the corrosion products was determined using Fourier transform infrared spectroscopy (FTIR) and extra Information about the phases present was obtained by x-ray diffraction (XRD). Corrosion products that formed consisted mainly of different amounts of zinc oxide (ZnO), hydrozincite (Zn-5[OH](6)[CO3](2)), and simonkolleite (Zn-5[OH](8)Cl-2.H2O). These results were compared to results obtained previously for pure zinc. The wet/dry Pattern of the exposure had a Large Influence on the composition of the corrosion products. Similar proportions of zinc oxide, simonkolleite, and hydrozincite formed on electrogalvanized steel as formed on zinc on open surfaces exposed for shorter drying times. When the drying time was Longer simonkolleite was the dominant compound on electrogalvantzed steel and zinc. In contrast, zinc oxide was the dominant compound on confined surfaces of electrogalvantzed steel exposed for shorter drying times, whereas similar proportions of zinc oxide, simonkolleite, and hydrozincite were formed on confined zinc surfaces. These results are explained as a result of differences in the corrosion rates and in the extent of localized corrosion as a result of different drying rates for the open and confined surfaces. Zinc oxide forms on electrogalvanized steel as a result of the formation of areas of bare iron caused by the high corrosion rate and high degree of localization. The areas of bare Iron act as sites for an oxygen reduction reaction, creating high pH values in the confined space. Conditions that favor a high corrosion rate and high degree of localization of the corrosion, such as higher chloride ion concentrations and shorter drying times, result in the formation of zinc oxide on electrogalvanized steel. The degree of confinement influences more significantly the composition of the corrosion products. At intermediate crevice widths, at which the corrosion rates are highest, zinc oxide dominates the corrosion products.