Different Structural Morphologies and Grain Size Effects upon the Corrosion Resistance of Pure Al and Zn Specimens

摘要

Several different structural morphologies may develop due to a wide range of the operational conditions and various growth forms that may exist during solidification of metals. It is well known that mechanical properties depend on solidification structures. Particularly, the literature presents relationship between the materials yield strength and grain size, such as the Hall-Petch's equation, or ultimate tensile strength and secondary dendrite spacings. In general, the corrosion behavior has been shown to depend strongly on the structural morphology and chemical composition. The aim of this study is to investigate the influence of the columnar and equiaxed structures and their grain size upon the corrosion resistance of pure aluminum and zinc samples. In order to obtain the columnar and equiaxed structures both a vertical upward solidification apparatus and a permanent steel mould casting assembly were used. The corrosion behavior was analyzed by the electrochemical impedance spectroscopy technique and Tafel extrapolation method. Corrosion tests were conducted in a 3% NaCl solution at room temperature using a frequency analyzer (Solartron model 1255) coupled to a potentiostat system (EG & G Parc model 273). It was found that both pure Al and Zn equiaxed structures have presented better corrosion resistance than columnar structures. Both grain size and type of structure will contribute to an increase in corrosion resistance. The influence of grain size for each structure is also discussed. The solidification processing variables can be used to gain insight into the corrosion performance of pure metals used by galvanizing and coating methods.