Surface treatment of magnesium alloys by artificial corrosion-oxidization method

A new technique has been proposed for improving the poor corrosion resistance in magnesium and its alloys. The specimens were immersed into solutions with high pH values, such as 10% NaOH, (1% NaCl + 10% NaOH) and (10% NaCl + 10% NaOH) solutions, at R. T. for 3.6 ks, and then heat treated in air at 673-773 K for 3.6 ks. Corrosion resistance of the specimens were evaluated by the time for occurring filiform corrosion, t(f), in salt immersion test using 1% NaCl solution. Hydrogen bubble evolution at the early stage of the salt immersion test was suppressed by the corrosion-oxidization treatment, and also t(f) was prolonged by this treatment. On the non-treated specimen of AZ31 alloy, tf was about 1.7 ks, and on the specimen only heat-treated in air at 673 K without the primary immersion, tf was about 1.9 ks. On the other hand, when the (10% NaCl + 10% NaOH) solution was used for the primary immersion and then heated in air at 673 K for 3.6 ks, t(f) was prolonged up to about 35 ks, about 20 times longer than that in the non-treated specimen. It is considered that magnesium hydroxide formed on the surface of the specimen by the primary immersion treatment changed into magnesium oxide which protected the specimen from corrosion. Formation of magnesium oxide on both the surfaces of the specimens treated by the corrosion-oxidization method and heated without the primary immersion was confirmed by TEM observations. Microstructures in both the oxide layers were different one another, cohesion of the oxide formed directly from metallic magnesium seemed to be weak compared with that formed from magnesium hydroxide.