Effect of Mg and Si on the microstructure and corrosion behavior of Zn-Al hot dip coatings on low carbon steel

Coatings formed on a low carbon steel by a two step hot dipping, primarily in a Zn bath and secondarily in a Zn-6Al bath with or without 0.5 mass% Mg and 0.1 mass% Si addition. The effects of the Mg and Si addition on the surface morphologies, microstructures, and corrosion characteristics of the coatings were investigated using a scanning electron microscope, a laser scanning microscope, and an electron probe microprobe analyzer. The coating formed in the Zn-6Al bath showed flaws as pores and cracks in the outer adhered layer, while there were few flaws in the Zn-6Al-0.5Mg-0.1Si bath coating. The Zn-6Al-0.5Mg-0.1Si coating has a lamellae structure developed well in the outer adhered layer and the initially formed alpha-Al was finer than that of the Zn-6Al coating. Corrosion in a 5 % NaCl solution commenced from the alpha-Al phase at a very early stage and the Zn phase corroded preferentially. The Zn-6Al-0.5Mg-0.1Si coating corroded slowly and relatively homogeneously, while the Zn-6Al coating degraded locally due to a preferential corrosion along flaws. The Zn-6Al-0.5Mg-0.1Si coating incorporates Mg and Si in the outer adhered layer and Si in the inner alloy layer, making the Zn-6Al-0.5Mg-0.1 Si coating more anti-corrosive.