2002 Volume 42 Issue 1 Pages 80-85
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 a-Al was finer than that of the Zn–6Al coating. Corrosion in a 5 % NaCl solution commenced from the a-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–0.1Si coating more anti-corrosive.
