Study On The Corrosion Resistance Of As-cast GWNK And ZWK Series Rare-Earth Magnesium Alloys

Developing high strength and good corrosion resistance Mg-RE (rare earth) alloys has become a hot research area in recent years because the disadvantages of low strength and poor corrosion resistance impede the wide application of magnesium alloys. Our research group carried out a study on the effect of RE content on the microstructure and mechanical properties in Mg-xZn-yY-0.6Zr (GWNK) and Mg-xGd-3Y-yNd-0.6Zr (ZWK) and acquired good results. Therefore, this work conducts a study on the effect of Zn and Y content on the corrosion resistance of Mg-xZn-yY-0.6Zr alloys and the effect of Gd and Nd content on the corrosion resistance of Mg-xGd-3Y-yNd-0.6Zr alloys. The main results obtained are as following:(1) The effect of Y contend on the corrosion resistance of Mg-xZn-yY-0.6Zr alloys depends on the Zn content. When the Zn content is 3%, the microstructure of the alloy is equiaxed grain and the corresponding corrosion rate increases slowly with increased content of Y element. Mg-3Zn-5Y-O.6Zr alloy has the best corrosion resistance in Mg-xZn-yY-0.6Zr series alloys. When the Zn content is 5%, the 5%Y alloy has the best corrosion resistance, the second one is 11%Y alloy and the 8%Y alloy has the lowet corrosion resistance. When the Zn content is 7%, the 5%Y alloy has the wrost corrosion resistance, the second one is 11%Y alloy and the 8%Y alloy has the best corrosion resistance.(2) The effect of Zn contend on the corrosion resistance of Mg-xZn-yY-0.6Zr alloys depends on the Y content. When the Y content is 5% and 11%, the corrosion resistance of the alloy decreases with increased Zn content. When the Y content is 8%, the 3%Zn alloy has the lowest corrosion rate, the second one is 7%Zn alloy,5%Zn alloy has the wrost corrosion resistance.(3) The effect of Mg-xGd-3Y-yNd-0.6Zr constitute on the corrosion resistance is closely related to the microstructure. The microstructures of the Mg-xGd-3Y-yNd-0.6Zr series alloys present three forms:dendrite shape, equiaxed crystal and lamellar shape. The corrosion resistance of the alloy is closely related to its microstructure. The precipitated phase of the dendrite shape alloys discontinuously distributes on the grain boundary, which cannot prevent the transverse and longitudinal corrosion. As a result, the dendrite shape alloys has the wrost corrosion resistance with the corrosion products of Mg(OH)2 and Y2O3. The grain size of equiaxed crystal alloy is small and the precipitated phases are continuously distributes on the grain boundary, which can prevent the longitudinal dissolve of Mg matrix. As a result, equiaxed crystal alloy has the best corrosion rate with the corrosion product of Mg(OH)2. The lamellar shaped alloy has the dispersal distribution of matrix and the lamellar precipitated phase served as an internal channel of external anion, promoting the dissolve of alloy. Consequently, the corrosion resistance of lamellar shaped alloy is between the two other alloys, and the corrosion products are Mg(OH)2 and Y2O3.(4) For the as-casted Mg-xGd-3Y-yNd-0.6Zr alloy, the corrosion resistance of the alloy decreases with the increased Nd content. For the different contents of Nd,8%Gd alloy has the best corrosion resistance, the second one is 6%Gd alloy and the 10% Gd alloy has the wrost corrosion resistance.(5) The microstructure of as-cast Mg-xGd-3Y-yNd-0.6Zr alloys appears as equiaxed crystal and the precipitated phase were typical eutectic organization distributing in the form of continuous net on the grain boundary of the alloy. The corrosion type of this alloy is intercrystalline corrosion. The alpha and the second phase alternately distributes on the grain boundary which accelerate the dissolution of eutectic phase. The matrix of Mg-xGd-3Y-yNd-0.6Zr has better corrosion resistance due to the solution of RE element and the corresponding corrosion products of Mg-xGd-3Y-yNd-0.6Zr alloys is Mg(OH)2.