Effect of Micro-alloying and Microstructure on the Corrosion Behavior of As-Cast Mg-6.2 wt.% Zn Alloy
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JMEPEG https://doi.org/10.1007/s11665-020-05133-w
Effect of Micro-alloying and Microstructure on the Corrosion Behavior of As-Cast Mg-6.2 wt.% Zn Alloy Monalisa Mandal and K. Mondal (Submitted June 30, 2020; in revised form August 17, 2020) The corrosion behavior of the Mg-6.2 (wt.%) Zn alloys with the micro-addition of Zr (0.6 wt. %) and a combination of Ag (0.4 wt.%), Ca (0.2 wt. %), and Zr (0.6 wt. %) was investigated in freely aerated 3.5% NaCl solution using immersion testing for 20 h. The microstructural observation before and after the immersion test helped to comprehend the effect of grain size and second-phase precipitate (cathodic phase) area fraction along with the chemical composition on the corrosion behavior of the alloys. Fine grain size with a homogeneous distribution of the precipitates in the Mg-6.2Zn-0.2Ca-0.4Ag-0.6Zr (wt.%) was found to have greater corrosion resistance by forming micro-galvanic couples in a more uniform way. The corroded surface after immersion test was characterized using X-ray diffraction and Fourier transformed infrared spectroscopy. Insoluble complex carbonates of Mg and Ca in the corrosion products were found to protect the metal surface of the Ca containing alloy from further corrosive attacks. Keywords
corrosion products, immersion test, Mg-Zn alloy, micro-alloying, microstructure, XRD
1. Introduction Researchers have grown immense interest in the synthesis, and thermo-mechanical treatments of the magnesium (Mg) alloys, which are considered to be the potential replacement as structural materials in both cast and wrought forms in the automobile sectors due to their high specific strength. Particularly, Mg alloy products used in the automobile industries, as engine blocks, transmission casings, steering column components, clutch, brake pedal brackets, extension, and pistons substantially reduce the weight of a vehicle (Ref 1). The reduction in automobile body weight helps to reduce the consumption of fuel, which further diminishes the hazardous gas emissions. It has been found that 10% reduction in automobile body weight can increase the fuel efficiency by 6.6%–8% (Ref 2). However, pure magnesium cannot be used as a structural material because of its very poor formability and mechanical properties. Therefore, extensive studies have been reported on the improved specific strength and ductility of the Mg alloys imparted by precipitation hardening, solid solution strengthening and grain refinement (Ref 3-9). The Mg-Zn system has shown the excellent age-hardening responses among a few Monalisa Mandal, Department of Materials Science and Engineering, Indian Institute of Technology, Kanpur, UP 208016, India; and Department of Metallurgical and Materials Engineering, Indian Institute of Technology, Kharagpur, WB 721302, India; and K. Mondal, Department of Materials Science and Engineering, Indian Institute of Technology, Kanpur, UP 208016, India. Contact e-mail: [email protected].
Journal of Materials Engineering and Performance
existing precipitation hardenable Mg alloys (R
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