Effect of Dipping Time on Microstructure and Properties of Zn-1Al-2Mg Coating by Hot-Dipping Galvanizing

The purpose of this study is to investigate the effect of dipping time (30, 60, 90, 120, 150 s) on the microstructure and corrosion resistance of Zn-1Al-2Mg coating under ammonium-free flux. The coatings were prepared at different dipping times, and then the microstructure and composition of the coatings were analyzed by scanning electron microscopy, x-ray diffraction and energy spectrometry; The corrosion resistance of the coatings was evaluated through electrochemical polarization test, neutral salt spray test, and full immersion test. The results show that the Zn-Al-Mg coating consists of Zn-rich phase, Zn/Al/MgZn₂ ternary eutectic, Zn/MgZn₂ binary eutectic phase and Fe-Al alloy phase layer; as the dipping time increases, the coating tends to thicken, and after the 90 s, the coating thickens significantly; while the eutectic structure decreases and the Fe-Al alloy phase thickens, Fe in the substrate continuously diffuses toward the growth direction of the coating and reacts with Al precipitated in the coating to form Fe₂Al₅Zn_x; The corrosion products in the Zn-Al-Mg coating, specifically Mg₂CO₃(OH)₂·3H₂O and MgCO₃, increase the pH of the environment and thus slow down the rate of corrosion. The corrosion resistance performance varies nonlinearly from good to poor and then improves again with increasing dipping time.