Investigation Of The Corrosion Behaivoure For Mg-9Al-2Sn-Mn Magnesium Alloy

Mg-9Al-2Sn alloy is a new alloy developed recently, which has manyadvantages of good mechanical property, low cost, excellent castingproperties. However, its poor corrosion resistance is a major limitation forits application. So studying the corrosion mechanism of magnesium alloyshas an important value for promoting their application. At present,controlling the alloying elements and applying the heat-treatment to thealloys are important ways to improve the corrosion resistance ofmagnesium alloys.The influence of Mn content on the microstructure, corrosion rates,eletrochemical behavior and corrosion morphologies of Mg-9Al-2Sn wasinvestigated by immersion tests, electrochemical tests, OM, XRD andSEM ect. on the Mg-9Al-2Sn alloys with different Mn content. At thesame, applying solution treatment (T4) and aging treatment (T6) indifferent time on the Mg-9Al-2Sn-xMn (x=0,0.1,0.3,0.6) alloys.Analyzing the influence of different heat treatments on the microstructureand the corrosion behavior of alloys. SEM and EDS analysis showed that after adding manganese element,Al8Mn5and Al8Mn5(Fe) phase have formed and with the increase of Mncontent, the volume ratio of Al8Mn5and Al8Mn5(Fe) phase increased.The alloys’ grain size decreased a little after adding0.1%and0.3%Mnelement while the grain size increase greatly after adding0.6%Mnelement. Corrosion rates measured by weight loss test and hydrogenevolution test showed corrosion rate of as cast Mg-9Al-2Sn-xMndecreased with an increasing of Mn content. When the Mn content was0.1%, the corrosion rate decreased by70%, and then, when the Mncontent increased up to0.3%, the corrosion rate decreased by95%. Whenthe Mn content increased up to0.6%, the corrosion rate decreased but to asmall extent. That Mn element can reduce the corrosion rate can beattributed to the following three reasons that:(a) Mn element couldeffectively remove harmful element of Fe;(b) Al8Mn5and Al8Mn5(Fe)had lower cathodic activity which is mainly due to the thick and denseoxide film formed on the surface of the Al8Mn5and Al8Mn5(Fe) phase sothat the effect of galvanic corrosion between them was weakened. c. WhenMn was added, and due to the Al8Mn5and Al8Mn5(Fe) phase, theredistribution of Al element caused the decrease of the potential difference between a-Mg phase and β-Mg17Al12phase, and therefore the galvanic corrosion between the two phases were weakened.The different heat treatments had great influence on the microstructure of alloys. In Mg-9Al-2Sn alloy, most Mg17Al12and Mg2Sn were dissolved in to a-Mg after T4treatment, producing supersaturated a-Mg and remaining a small amount of undissolved β-Mg17Al12phase. After T6treatment for different time, Al elements diffused to the grain boundary and precipitate eutectic (α+β) phase. With the increase of aging time, although precipitation (α+β) eutectic phase volume ratio has increased, but they were not continuous. For Mg-9Al-2Sn-xMn alloys containing Mn element, when the aging time was same, the volume of (α+β) eutectic phase increased with increasing Mn content and the volume ratio of residual β-Mg17Al12phase decreases with an increasing of Mn content. But the heat treatment had no influenced on the Al8Mn5and Al8Mn5(Fe) phases. It was found that the corrosion rate of alloy without Mn element had relationship with heat treatment by immersion test and eletrochemistry test. Compared to the corrosion rate of alloys in three conditions of F, T4, T6, corrosion rates decreased with the following order: T4> T6> F. This was because that in the F conditon, β-Mg17Al12phases in alloys were in continuous distribution within the alloy. During the corrosion process, it has good protection effect to retard corrosion. In T4condition, most β-Mg17Al12was disspolved into the α-Mg so that residual β-Mg17Al12can not play a protective role. When the corrosion started, it would continue to deepen, leading to a maximum corrosion rate. In the T6condition alloy, although the alloy precipitated (α+β) phase, but they were not continuous, and in the corrosion process, the residual β-Mg17Al12has fell off so that the remaining β-phase was not as protective as that of in F condition. This leads that the corrosion rate T6alloys was lower than F alloys. in the alloy Mg-9Al-2Sn-xMn (x=0.1,0.3,0.6) alloy, the corrosion rate of the alloy in T4condition was biggest and then the alloy in F condition was lower. The corrosion rates of alloys in T6condition were smallest. For the alloys in T6conditions, the corrosion rates decrease with the increase of aging time. This is because with the growth of the aging time, the volume retio of eutectic (α+β) phases increased and became continuous. The residual β-phase can play a better protective role than the β-Mg17Al12in F condition, which makes the corrosion resistance of the alloy got better with an increasing of aging time. But after adding Mn element into Mg-9Al-2Sn, Mn content in the alloy did not affect the influence of heat treatment on the corrosion behavior of Mg-9Al-2Sn-xMn (x=0.1,0.3,0.6). Compared to AZ91D alloy, the corrosion rates ofMg-9Al-2Sn-xMn (x=0.1,0.3,0.6) were lower than its rate.