Influence of Microstructure on Corrosion Behavior of Zn-Al-Sr Alloys in Sodium Chloride Solution

The present research objective is to evaluate microstructure's influence on corrosion behavior of a new series of Zn-Al-Sr alloys with possible coating applications. Conventional casting procedures prepared four Zn-Al-Sr alloys. The alloys were Zn-1Al-0.055r, Zn-2.5-Al-0.1Sr, Zn-4.5-Al-0.55r, and Zn 10Al 1Sr. Microstructural characterization was performed by scanning electron microscopy. Corrosion behavior was evaluated employing electrochemical techniques such as potentiodynamic polarization (PP) and electrochemical impedance spectroscopy (EIS). The alloys were exposed to 3.5 wt. % NaCl. Results indicated that all samples presented the eta-Zn, alpha-Al, and SrZn13 phases in different arrangements depending on the alloy composition. Electrochemical tests revealed that Al increased alloy corrosion resistance by forming insoluble oxides, whereas Sr improved corrosion product layer compactness and homogeneity of the corrosion product layer. However, the large grain size of the SrZn13 compound favored the corrosion process by galvanic coupling. Compositions between 2.5 and 4.5 wt.% Al and 0.05 and 0.5 wt.% Sr exhibited the highest corrosion resistance of the Zn-Al-Sr alloys in 3.5 wt.% NaCl solution, lower Sr content promoted both homogenous corrosion and a compacted corrosion products layer. A high density of intermetallic particles adversely affects Zn-Al-Sr alloys corrosion resistance, and Sr content must not exceed 0.5 wt.%.