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 poin ts, a high frequency (10 kHz) and a low frequency (10 mHz), were monit ored during the cyclic wet-dry exposure. The reciprocals of the high f requency and low frequency impedances were taken as the solution condu ctance of water layer on the coating surface and the corrosion rate of the coated steels, respectively. The impedance monitoring results sho wed that the Zn-55%Al coating exhibited the most excellent corrosion r esistance under the given conditions. This is attributed to the format ion of aluminum oxide on the specimen surface. On the other hand, the corrosion resistances of Zn coated and Zn-5%Al coated steels were almo st the same under relatively wet and highly chloride-concentrated cond itions. An increase in the chloride concentration of the test solution employed during the wet-dry cyclic test decreased the average corrosi on rate in all cases. The average corrosion rate of the Zn-55%Al coati ng 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 moni toring is very useful for the mechanistic study of the corrosion of co ated steels, as well as the evaluation of their corrosion performance, in marine atmospheric environments.