Filiform Corrosion of a High Purity AZ31 Magnesium Alloy

It is generally recognized that impurity atoms affect the phenomena of corrosion. In the present study, we have prepared a low impurity magnesium alloy, and the corrosion behaviors of the alloy was compared with that of a commercial AZ31B magnesium alloy. The low impurity alloy was prepared with using 5N-Al, 4N-Zn and high purity magnesium, the last one was prepared by a vapor deposition technique. Chemical analyses showed that the composition of the alloy was about Mg-3% Al-1% Zn (in mass%). Slices with about 5 mm thickness were cut from an ingot of the alloy, and hot-rolled into 1 mm at 623 K, then solution heat treated at 673 K for 86.4 ks. Lowering the impurities resulted in improving a corrosion resistance. Corrosion test using a 1 mass% NaCl solution showed that the low impurity magnesium alloy had a superior corrosion resistance compared with those of the AZ31B and the AZ31PE alloys. In the AZ31B alloy, filiform corrosion occurred after a duration time of 0.6 ks. Heads of the filiform corrosion propagated with random walking, creating regular irtraces, while in the low impurity magnesium alloy, duration time for the filiform corrosion was prolonged to 7.2 ks. Moreover, morphology of tails were changed by lowering the impurity concentrations, into straightforward.