Effect of Ca addition on the corrosion resistance of gravity cast AZ31 magnesium alloy

The effect of Ca addition on the corrosion resistance of AZ31 magnesium alloys was evaluated by observation of microstructure and measurement of corrosion potential and average corrosion rate. The main mechanism of corrosion of AZ31 +xCa (x = 0 similar to 5 mass%) alloys was galvanic-corrosion between alpha-Mg and second phase. But the propagation behavior of corrosion was different with Ca content. In the alloy containing below 0.7 rnass%Ca, a micro-galvanic cell formed between matrix alpha-Mg and second phase formed semi-continuously at grain boundaries and the corrosion progressed in transgranular mode. The propagation of corrosion was retarded when the corrosion front met the second phase acted as corrosion barrier. But in the alloys containing above 1 mass%Ca, a micro-galvanic cell formed between eutectic alpha-Mg and discontinuous second phase in the eutectic region and the corrosion rapidly progressed along eutectic alpha-Mg formed continuously at grain boundaries because the discontinuous second phase could not prohibit the propagation of corrosion effectively.