Abstract
Worldwide hot dip galvanising is the most important application of a zinc coating as an efficient method for steel corrosion protection. In southeastern Mexico, in humid tropical climate, the domestic statistics reports that 63% of roofs and walls are made from galvanised sheets, which suffer corrosion damage. When zinc corrosion products are dissolved by dew and rain, zinc ions are released from the corroded surface and dispersed to the environment. This phenomenon, known today as the metal runoff process, is being paid enormous attention. In order to make an accurate estimation of zinc runoff induced by atmospheric corrosion in humid tropical climate, samples of hot dip galvanised steel have been exposed for two years in the Gulf of Mexico environments. The annual zinc runoff rates in both test sites decreased by ∼50% in the second year (2·70-3·28 g m−2/year), as compared to those measured in the first year of exposure (6·52-7·98 g m−2/year). The zinc mass loss has been related to several independent parameters, which control the runoff process. Runoff solutions collected from the samples in exposure after the first flushes of rain, always showed pH values in the range 5·5-7·0, when the rainwater was more acidic (pH 4·7-6·10). The carbonate ion CO2− 3 concentration detected in zinc runoff solutions reached values 1·6-1·7 times higher than those measured in rainwater, due to dissolved zinc carbonate corrosion product. Several chloride containing corrosion products, not released by rain events, were detected as major and minor phases by XRD during the exposure of galvanised steel.
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