Corrosion Behaviour of a Zn/Zn-Al Double Coating in 5% NaCl Solution

A new hot dip-galvanized Zn/Zn-5Al double coating was prepared by immersing steel into a molten pure zinc bath and then in a Zn-5Al alloy bath. The corrosion behaviour of the double coating was studied in 5% NaCl solution. The results showed that the coating consisted of a typical coating of Zn on the bottom with a Zn-Al layer on the top. The maximum thickness of the alloy coating was approximately 220 mu m. The neutral salt spray (NSS) test showed that the double coating was significantly effective for preventing corrosion. The corrosion resistance of the Zn-5Al alloy-coated steel was almost four times that of the Zn-coated steel according to the NSS test. The electrochemical tests indicated that the corrosion resistance of the Zn/Zn-Al double coating decreased with immersion time. However, the double coating also had a higher corrosion resistance and more negative corrosion potential compared with Zn-coated steel in 5% NaCl solution. SEM and XRD analyses revealed that the improved corrosion resistance of the Zn/Zn-5Al double coating was mainly due to the effect of Al on the corrosion products on the coating surface.