Influence of Cooling Rate on Corrosion Resistance of Rapidly Solidified Mg-Zn-Y Alloys with Long Period Stacking Ordered Phase

Magnesium alloys have recently attracted a great deal of attention as structural materials because of their lightweight and high specific strength. In general, Mg alloys produced by traditional ingot metallurgy (I/M) processing have lower strength, ductility, creep resistance and corrosion resistance. The main reason for the low corrosion resistance of Mg alloys is internal galvanic attack due to impurities and/or alloying elements. In the initial corrosion stage, filiform corrosion occurs in neutral aqueous solution. Therefore, it is important to clarify the mechanism of occurrence and propagation mechanism of filiform corrosion and to prevent filiform corrosion. The application of rapid solidification processing to Mg alloys results in reduction in grains and intermetallic particle sizes, extension of solid solubility, formation of a non-equilibrium phase, and improving chemical homogeneity. Enhancement of chemical homogeneity by rapid solidification may bring about suppression of internal galvanic corrosion [1]. Recently, several nanocrystalline Mg-Zn-Y alloys with high strength, high temperature strength, large elongation and high corrosion resistance have been developed by rapid solidification processing [2]. However, corrosion mechanism of the rapidly solidified (RS) Mg-Zn-Y alloys has not been examined in detail. Mg-Zn-Y alloys are composed of alpha-Mg matrix phase and long period stacking ordered structure (LPSO) phase. The LPSO phase acts as cathodic site during corrosion. In order to enhance corrosion resistance of the Mg-Zn-Y alloys, it is important to control the LPSO phase morphology. In this study, we investigated the corrosion behavior of rapidly solidified Mg-Zn-Y alloys that were fabricated using various cooling rate. Particular attention was paid to the relationship between occurrence of the filiform corrosion and change in the morphology of the LPSO phase according to the cooling rate change. Several Mg97.25Zn0. 75Y2 alloys with different cooling rate when solidifying were fabricated. Conventionally cast alloy was prepared by high frequency induction melting. Cooling rate of conventional casting was estimated 10 K/s. RS alloys were prepared by copper mold injection casting and rapidly solidified flakey powder metallurgy (RS FP/M) processing. Cooling rates of them are 5.5 x 10 K/s and 1.8 x 10-1.4 x 10 K/s, respectively. The preparation of the RS FP/M Mg alloys was described in detail elsewhere [1]. Microstructure of the Mg97.25Zn0. 75Y2 alloys with different cooling rate was observed using TEM and SEM techniques. The corrosion tests were carried out in 0.17 M NaCl aqueous solution at 298 K pH is 6.3-6.8. The dissolution rates were calculated from hydrogen evolution during immersion. Anodic polarization was performed in 0.1 M NaCl aqueous solution at 298 K after immersed for 5 min at open-circuit potential. Corroded surface was observed using optical microscopy and SEM. Conventionally cast Mg-Zn-Y alloys consist of alpha-Mg matrix and LPSO phases. Matrix grain size is around 500 μm and coarse block-like LPSO phase is formed in grain boundaries and grain inside. RS Mg-Zn-Y alloys also consist of two phases. However, morphology of RS alloys is significantly different from that of the conventionally cast alloy. Matrix grain size is reduced to less than 500 nm. Morphology of LPSO phase was changed from block-like shape in boundary to finely plate-like shape in matrix grains. Increase in the cooling rate brings about grain refinement and high dispersion of LPSO phase simultaneously. Mg-Zn-Y alloys as well as commercial Mg alloys exhibit two stages of corrosion manner, that is, initial corrosion stage and filiform corrosion stage. Microstructure refinement by rapid solidification is effective in prevention of corrosion in both stages. Especially, the fine dispersion of the LPSO phase that is the second phase in the alloys seems to improve electrochemical homogeneous, resulting in suppression of filiform corrosion. In the filiform corrosion stage, propagation speed of filiform corrosion is decreased. The highest corrosion-resistant Mg97.25Zn0. 75Y2 RS FP/M alloys exhibited obvious passivity in the anodic polarization curve when immersed in NaCl neutral aqueous solution. This result suggests that electrochemical homogeneity is enhanced by changing substructure of the alloys.