Research On Microstructure And Corrosion Resistance Property Of Mg-Al-Zn-xCa Alloys

Magnesium alloys, one of the lightest metallic structural materials, are widely used in the fields of aerospace, electronic information, automobile, and home appliances because of their low density, high specific strength and high specific stiffness, good damping capacity, castability and mechanical properties. However, its relatively lower corrosion resistance property limits further development and application. In this paper, effect of Ca additions on the microstructure and corrosion resistance property of Mg-Al-Zn alloys was investigated, and the optimal amount of Ca addition was obtained.Effect of Ca additions on the microstructure and corrosion resistance property of as-cast and heatreated Mg-Al-Zn alloys were investigated by using OM, SEM and XRD. Results show that the addition of Ca can greatly refine the grain of as-cast alloys. In addition, a new bone-like Al4Ca phase is found. No new phase is found when the addition of Ca is 0.5%, and when its amount reaches 1.0% and 1.5%, a new bone-like Al4Ca phase, which is dispersively distributed in the matrix, appears.After the solution and aging treatments, theβ-Mg17Al12 was precipitated discontinuously along the grain boundaries as well as inside the grain. The grain sizes of the Ca-added alloys were much finer than as-cast alloys. The highest level of microstructure homogenization and the minimum grain size of 28μm are obtained in the Mg-Al-Zn-1.0Ca alloy, whose grain size is 54.1% less than that of Ca-free Mg-Al-Zn alloy.The influence of Ca on the corrosion resistance property of Mg-Al-Zn alloys was investigated by the electrical chemical test, static weight loss test as well as salt fog test. Results show that the electrical chemical polarization curves of the as-cast and heatreated alloys show the similar tendency. Corrosion electric potentials of the alloys with the Ca addition are higher than that without Ca, while the corrosion current density goes in the opposite direction. Therefore, it can be concluded that Ca addition can increase the corrosion electric potential and decrease the corrosion current density of Mg-Al-Zn alloys. Besides, Ca addition also makes the polarization curves move towards the positive direction and reduces corrosion rates. Consequently, the corrosion resistance property of alloys is improved. The Mg-Al-Zn-1.0Ca alloy shows the best capacity.Static weight loss test results show that the average corrosion rate of as-cast Ca-added alloys is lower than that without Ca and the results also apply to the heatreated samples. The average corrosion rate of heatreated alloys is much lower than the as-cast alloys. The Mg-Al-Zn-1.0Ca alloy shows the lowest corrosion rate. The results of static weight loss test are consistent with that of the electrical chemical test.Salt fog test results show that the corrosion resistance property of Ca content as-cast alloys is lower than that of those without Ca content. The corrosion resistance property of heatreated alloys is better than that of the as-cast alloys with the Mg-Al-Zn-1.0Ca alloy was is the best. The addition of Ca can significantly improve corrosion resistance property. The test results of the three testing methods mentioned above are basically the concordant.Results show that corrosion products of as-cast alloys are Mg(OH)2,MgO andβ-Mg17Al12 phase while that of the heatreated alloys are Mg(OH)2,MgO. It is demonstrated that the solution and aging treatment can change the morphology and distribution ofβphase and improve its corrosion resistance. Ca compounds were found neither in the as-cast nor in the heatreated alloys, which suggests that Al4Ca has relatively good corrosion resistance property.The relationship between microstructure and corrosion resistance property of Mg-Al-Zn-xCa alloys was studied. Results show that the addition of Ca can make theα-Mg phase finer and theβ-Mg17Al12 phase more dispersive in the matrix. During the corrosion process,β-Mg17Al12 phase, the corrosion products and Al4Ca impede the further development of corrosion behavior, the corrosion current is reduced and the corrosion properties of alloys are improved."The effects of Ca on the grain refinement and distribution of the second Al4Ca phase"is the influential mechanism of Ca on the corrosion resistance property of Mg-Al-Zn alloys.