Corrosion monitoring of Zn and Zn-Al coated steels under wet-dry cyclic conditions using AC impedance method

AC impedance method has been applied to evaluate the performance of Zn, Zn-5%Al and Zn-55%Al coated steels under wet-dry cycles. The wet-dry cycles were carried out by exposure to the alternate conditions of 1 h-immersion in a 0.5 or a 0.05 kmol/m(3) NaCl solution and drying for various times at 298 K and 60%RH. The impedances at two frequency points, a high frequency (10 kHz) and a low frequency (10 mHz), were monitored during the cyclic wet-dry exposure. The reciprocals of the high frequency and low frequency impedances were taken as the solution conductance of water layer on the coating surface and the corrosion rate of the coated steels, respectively. The impedance monitoring results showed that the Zn-55%Al coating exhibited the most excellent corrosion resistance under the given conditions. This is attributed to the formation of aluminum oxide on the specimen surface. On the other hand, the corrosion resistances of Zn coated and Zn-5%Al coated steels were almost the same under relatively wet and highly chloride-concentrated conditions. An increase in the chloride concentration of the test solution employed during the wet-dry cyclic test decreased the average corrosion rate in all cases. The average corrosion rate of the Zn-55%Al coating under a wet-dry cyclic condition in 0.5 kmol/m(3) NaCl solution was lower than that in 0.05 kmol/m(3) NaCl by one order of magnitude. The combined wet-dry cyclic exposure test and AC impedance corrosion monitoring is very useful for the mechanistic study of the corrosion of coated steels, as well as the evaluation of their corrosion performance, in marine atmospheric environments.