| Magnesium alloys has been wildly applied in the fields of aerospace,electronic information,automobile,and home appliances due to their low density,high specific strength,high specific stiffness,good damping capacity,castability and mechanical properties. However, its relatively lower corrosion resistance behavior has severely limited its further development and application. In this paper, the effect of Mn addition on microstructure and corrosion resistance property of AZ91 alloys was investigated to find the optimal amount of Mn addition.The effect of Mn addition on the microstructure and corrosion resistance property of as-cast and heat-treated AZ91 alloys was investigated by using OM,SEM and XRD testing method. Results show that the addition of Mn can greatly refine the grain size of as-cast and heat-treated AZ91 alloys. AIMn particle mainly exists in the form of angular blocky morphology. Al2Mn particle was observed in the as-cast alloy, while Al6Mn particle was found in heat-treated AZ91 alloys which can be attributed to the fact that heat treatment can transform the particle Al2Mn into Al6Mn. The grain sizes of alloys with Mn addition were remarkably decreased compared with the alloy without Mn addition, among which, AZ91-0.8Mn exists the most refined grain size and the highest level of microstructure homogenization.The influence of Mn on the corrosion resistance property of AZ91 alloys was investigated with the static weight loss test, electrical chemical test as well as the salt fog test. Results demonstrate that the as-cast and heat-treated AZ91 alloys show the similar corrosion tendency with alloy AZ91-0.8Mn possesses the lowest corrosion rate, and the overall corrosion rate of heat-treated alloys was much lower than that of the as-cast alloys.The Static weight loss test results show that the average corrosion rate of as-cast AZ91-(0.4-1.2) Mn alloys is 7%,78% and 71.8% lower than that of the alloy without Mn addition,while for the heat-treated alloys the corresponding number is 51.1%,69.6% and 68.3%. AZ91-0.8Mn alloy show the lowest corrosion rate and the beat corrosion resistance.Electrical chemical polarization curves of the as-cast and heat-treated alloys show the similar tendency. Corrosion current density of the as-cast alloys with Mn addition are 15.8%,55.3% and 52.6% lower than that of Mn-free alloy, while the number was 65.3%,76.5% and 25.5% for heat-treated alloys, respectively. Therefore, it can be concluded that Mn addition can increase the corrosion electric potential and decrease the corrosion current density of AZ91 alloys. In addition, Mn addition also makes the polarization curves move towards the positive direction and reduces corrosion rates. Therefore, the corrosion resistance behavior of alloys is improved. Alloy AZ91-0.8Mn shows the best corrosion resistance capacity.Salt fog test results show that the corrosion resistance property of Mn content as-cast alloys is lower than that of alloy without Mn content. The corrosion resistance property of heat-treated alloys is better than that of the as-cast alloys with the AZ91-0.8Mn alloy shows the slightest corrosion behavior. Conclusion can be reached that addition of Mn can significantly improve corrosion performance of AZ91. The test results of the three testing methods mentioned above are basically correspondent with each other.The relationship between microstructure and corrosion resistance property of AZ91-xMn was demonstrated. Results show either for the as-cast or heat-treated AZ91 alloys element Mn can greatly refine the a-Mg phase and makeβ-Mg17Al12phase more dispersive in the matrix. The corrosion products, AlMn particle andβ-Mg17Al12 phase which exist in the grain boundary can impede the further development of corrosion behavior during the corrosion process, consequently, corrosion current is reduced and the corrosion property of alloys greatly improved. The effects of Mn on the grain refinement and distribution of the secondβ-Mg17Al12 phase is the influential mechanism of Mn on the corrosion resistance property of AZ91 alloys.
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