Microstructure and corrosion behavior of arc sprayed Zn-xAl (x = 15, 30, 50) alloy coatings in NaCl solution

Abstract

sThe microstructure and long-term corrosion behavior of arc sprayed Zn-xAl (x = 15, 30, 50) alloy coatings in NaCl solution were examined in the research. The microstructure, corrosion product morphology and corresponding phases were observed by SEM and XRD respectively. The corrosion behavior was determined by electrochemical measurements. The obtained results show that three coatings are mainly composed of Zn-rich and Al-rich phase. The cross-section images indicate that Al-rich phase transforms from lamellar, spherical into bulk structure, and corresponding EDS implies an enhancement in the uniformity of chemical composition between Zn-rich and Al-rich phase. Moreover, the compactness of the coatings slightly increases with the increase in the ratio of Al. The corrosion current density of three coatings can be ordered as follows: Zn-15Al > Zn-50Al > Zn-30Al. The evolution of electrochemical impedance spectrum (EIS) confirms that cathodic protection plays a domain role in the corrosion process for Zn-15Al and Zn-50Al coatings. Due to the self-sealing behavior resulting from corrosion product, the charge transfer resistance of Zn-15Al coating is much larger than that of Zn-50Al coatings during 90 days of corrosion. Several cracks are clearly observed on Zn-50Al coating surface, which is mainly responsible for the poor long-term corrosion resisitance. As for Zn-30Al coating, cathodic protection holds a leading position in the early corrosion stage. Electrochemical reaction is not under the control of the mass transfer process until 30 days of corrosion and the barrier properties caused by dense corrosion product layer become the major protection mechanism. In conclusion, Zn-30Al coating shows the most long-term corrosion resistance in the work.