Study on microstructure, corrosion performance and mechanical properties of hypereutectic Zn-Al alloy

Hypereutectic Zn-Al alloys with different cooling rates and their coatings on mild steel were successfully prepared and characterized. Zn - 6 wt. % Al alloy was cast into different moulds to obtain various cooling rates. It was found that the increasing cooling rates reduced the average grain size. High cooling rate also decreased the decomposition rate of primary γ-ZnAl phase. When the cooling rate increased to 10℃/s, a large number of nano-size eutectic grains were formed in the alloy. Through hot-dipping technique, Zn - 6 wt. % Al was coated on tensile bars made of ASTM CRS 1018 steel in order to study the effect of Zn alloy-based coatings on tensile properties of mild steel. By comparing the same types of steel which experienced the same thermal process without coating, it was found that the yield strength and ultimate tensile strength of CRS 1018 were reduced but the elongation was significantly improved. The change of tensile properties might result from the decrease in dislocation densities, which were estimated by nano hardness measurements. Furthermore, the hypereutectic Zn-Al alloys with various Al contents from 6 to 10 wt.% were prepared to study the Al influence on their corrosion behaviors. In the low voltage corrosion, higher Al content was beneficial to the alloys due to passivity whereas Al increased the risk of pitting corrosion in the high voltage test after passive layers were broken down. The microstructure observed by scanning electron microscopy (SEM) of tested samples also supported the obtained corrosion data