| AZ series magnesium alloy has widely used in automobile,aviation,electronics and medicine for its excellent properties,but its further development was limited due to the poor corrosion resistance.In this paper,the effects of rare earth element Sm alloying on the microstructure and corrosion resistance of hot-extruded AZ31 and as-cast,ultrasonic treated AZ91 magnesium alloys were studied.The observation results of SEM and TEM shown that after added the Sm element to the AZ series magnesium alloy,the granular dispersion distribution of Al2Sm intermetallic compound was formed.The results of microregion polarization shown that it not only has high corrosion potential?-1.01mV?,but also has excellent passivation capability.This suggesting Al2Sm intermetallic compound has good corrosion resistance.It as a weak cathode phase will not cause severe galvanic corrosion,and can be used as a barrier to prevent contact between electrolyte and the magnesium,thus improved the corrosion resistance of magnesium alloy.For the hot-extruede AZ31 magnesium alloy,the corrosion rate decreased from15.98×10-4 to 11.19×10-4 g·cm-2·h-1 in the transverse section and from 8.57×10-4 to6.20×10-4 g·cm-2·h-1 in the longitudinal section.Compared to the unmodifed alloy,the corrosion potential of the Sm modified alloy in the transverse and longitudinal sections increased by 98 and 62mV,respectively,and the Rct value?charge transfer resistance?in the transverse and longitudinal sections of the modified alloy increased from 1764 and 1756?·cm2 to 2928 and 2408?·cm2,respectively.The results showed that the corrosion resistance of hot-extruded AZ31 magnesium alloy was significantly improved by Sm addition due to the grain refinement caused by the formation of Al2Sm particles,the decreased dislocation density,and the suppression of micro-galvanic corrosion caused by Al-Sm-?Mn?intermetallic compounds.For the as-cast AZ91 magnesium alloy,when the added amount of rare earth Sm was 1wt.%,the corrosion rate was the lowest.The weight loss of the alloy was 0.35mg/cm2day and 0.029 ml/cm2day respectively,about 73%and 88%of the matrix alloy.At the same time,the electrochemical test results shown that the corrosion current density of AZ91-1.0 Sm alloy was about 1/3 of the matrix alloy,and the polarization resistance was about 175.8%of the matrix alloy.It found that the content of Sm element has a significant impact on the distribution of Mg17Al12 phase:a small amount?0.5wt.%?and excessive?1.5wt.%?of Sm made Mg17Al12 phase distribution more uniform,but the continuity of the mesh structure was poor,so the improvement of corrosion resistance was weak.However,a moderate amount of Sm?1wt.%?transformed the originally large and concentrated dendritic Mg17Al12 into a uniform,refine and continuous network structure,which could effectively improve the barrier effect of the Mg17Al12 phase.For the ultrasound treated AZ91 magnesium alloy,the results of weight loss and hydrogen collection shown that the corrosion rate of alloys dropped significantly with the increase of Sm content,when Sm element content is 1.5 wt.%,the corrosion rate of the alloy was the lowest,which was 0.167 mg/cm2day and 0.247 ml/cm2day,respectively.The electrochemical results shown that alloy has the minimum corrosion current density?16.582 mA/cm2?,only 57.5%of the matrix alloy,and the corrosion potential increased of 34mV.With the amount of Sm was increasing,Mg17Al12 phase dendritic structure gradually disappear and a more uniform granular Mg17Al12 phase formed,which inhibited the degree of micro-galvanic corrosion.At the same time,the ultrasonic treatment also made more small Al2Sm particles uniformly distributed in the grain and grain boundary,thus increasing its protective ability.Serious local corrosion corrosion morphology changed into a relatively mild general corrosion,and a wider and better corrosion product layer formed,thus significantly improve the corrosion resistance of alloy. |
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