Improved Corrosion Resistance in AZ61 Magnesium Alloys Induced by Impurity Reduction

The effect of impurity element Fe on corrosion behavior of AZ61 magnesium alloys in various states has been investigated by immersion test and hydrogen evolution measurements in 3.5% sodium chloride solution. The corrosion rate is found to relay on the impurity Fe concentration in the alloys and decreases with decreasing Fe content. When Fe content drops from 150 ppm to 10 ppm, the corresponding corrosion rates under as-cast and solution treatment conditions are reduced from 8.54 mm/a and 8.61 mm/a to 2.54 mm/a and 0.21 mm/a, respectively. The corrosion pattern of the AZ61 alloys is the localized corrosion, and the galvanic couples are formed among the impurity particles, second-phase particles and the matrix. The Fe impurity particles tend to act as main cathodic to form micro-galvanic cell with the alpha-Mg matrix, which is harmful for corrosion resistance of AZ61 alloy.