The corrosion behavior of Mg-9Al-xRE magnesium alloys modified by friction stir processing

Highlights:• The corrosion resistance of FSPed Mg-9Al-xRE alloys was improved with the increasing amount of rare earth addition.• The enhanced corrosion resistance is due to inhibition of micro-galvanic corrosion and improved corrosion product film.• The combination of rare earth alloying and FSP treatment is favorable in improving the corrosion resistance. -- Abstract: The combination of rare earth (RE) alloying and friction stir processing (FSP) was demonstrated to be favorable in improving the corrosion resistance of Mg-9Al alloys. The microstructure and corrosion behavior of the modified Mg-9Al-xRE alloys were documented in this paper. Electrochemical measurements revealed that the corrosion resistance of as-cast Mg-9Al-xRE alloys in 3.5 wt% NaCl solution was weaken with the addition of RE elements. It is found that the formation of a large amount of coarse strip-like Al-RE precipitates is responsible for the accelerated dissolution of anodic magnesium matrix. However, the corrosion resistance of FSPed Mg-9Al-xRE alloys was improved when the RE element was added and the improvement was further enhanced with the increasing amount of RE addition. This is mainly attributed to the alternation in the corrosion behavior of magnesium alloys owing to the refined and redistributed precipitates in α-Mg matrix. The refinement of cathodic precipitates enlarges the local area ratio of anode and cathode and retards the severe galvanic corrosion tendency in magnesium alloys. In addition, the presence of a large amount of fine Al-RE precipitates is favorable in accumulative growing of the corrosion product layer during the corrosion propagation, and thus improves the stability and compactness of this protective layer.