EIS evaluation of chromate and organic coating layers on corrosion performance of galvanized steel sheets

EIS results show that corrosion resistance of chromate zinc is mainly due to its passivity behavior. This passivity was not enough to prevent chromate zinc from corrosion in NaCl solution, but in hard and soft waters, it prevented zinc corrosion favorably. This different behavior was attributed to adsorption of a chromate ion associated complex in hard and soft waters. Results showed that chromate zinc behaves more favorably in hard water than soft water by increasing the immersion time. This was related to the increased tendency of the complex adsorption at long times of immersion. The complex adsorption was not occurred in distilled water resulted zinc corrosion. A primer layer applied on chromate zinc acts as a barrier and then electrolyte solution can reach the chromate layer only through the coating pores. In this way, electrical double layer could be build up locally at bottom of the pores. Therefore, high corrosion resistance is expected to achieve only if the chromate zinc can reach to the passivity state at the bottom of the pores. A primer/topcoat layer due to its extremely high pore resistance can prevent zinc corrosion in NaCl solution even after eight weeks exposure.