The role of macrostructural morphology and grain size on the corrosion resistance of Zn and Al castings

Different macrostructural morphologies and grain sizes may develop due to a wide range of the operational conditions that may exist during casting. It is well known that mechanical properties depend on solidification structures. Particularly, the literature presents relationship between the materials yield strength and grain size or ultimate tensile strength and primary and secondary dendrite arm spacings. It is also reported that the corrosion resistance has been shown to depend strongly on the structural morphology and chemical composition. The aim of this study is to investigate: (a) the influence of columnar and equiaxed structures on the corrosion resistance of aluminium and zinc as-cast samples, (b) the influence of longitudinal (columnar structure) and transversal (pseudo-equiaxed structure) sections of the same sample on the corrosion resistance for both metals and (c) the influence of equiaxed grain size of hot-dip galvanized steel sheets on the corrosion resistance. In order to obtain columnar and equiaxed structures both a vertical upward solidification apparatus and a permanent steel mold casting assembly were used. The corrosion resistance was analyzed by electrochemical impedance spectroscopy technique and Tafel extrapolation method. Corrosion tests were conducted in a 3% NaCl solution at room temperature.