Corrosion of magnesium and aluminium alloys in the atmosphere

Magnesium is the most chemically active of all industrial alloys, with a standard electrode potential of -2.3 V, about 2 V lower than the standard electrode potential of iron and about 0.7 V lower than the standard electrode potential of aluminium. in dry atmospheric conditions, the surface of magnesium is able to form an oxide film which is able to resist external corrosion of the magnesium substrate to a certain extent.

However, due to the low density of this oxide film and the large number of voids, its corrosion resistance is very limited and it also exhibits significant chemical and electrochemical activity and corrosion is further accelerated in air with high humidity and in the presence of Cl-. The electrochemical corrosion of magnesium is mainly manifested as a precipitation process of hydrogen ions, including pitting corrosion as well as full-scale corrosion, while the dissolution rate is very fast. In the dilute solution rich in Cl-, the corrosion potential of magnesium and its standard potential there is a large difference, its corrosion potential is about -1.7V, the analysis may be due to the magnesium surface oxide film protection about related. When the pH is high, the magnesium surface can form a thicker layer of Mg(OH)2 precipitation film, can play a better protective effect on the magnesium substrate.

In the atmospheric environment, the corrosion process of magnesium is mainly the depolarisation process of oxygen, the specific degree of corrosion is mainly affected by the humidity of the atmosphere and the content of various pollutants. Generally speaking, the size of the humidity of the air on the degree of corrosion of magnesium aluminium alloy is not obvious, only in the presence of other corrosive pollutants in the atmosphere, the size of the humidity of the air on the corrosion of magnesium aluminium alloy will be reflected. If there is basically no corrosive material in the atmosphere, then no matter how high the air humidity is, the corrosion behaviour of magnesium aluminium alloy will also show very weak. When the atmosphere contains a large amount of corrosive pollutants, the corrosion rate of the magnesium aluminium alloy surface will increase rapidly. The corrosion resistance of magnesium aluminium alloy as a material is poor when working in industrial and marine atmospheres. And in the dry and clean air environment, the conservation film on the surface of magnesium can form a strong protective effect, so that the corrosion of magnesium aluminium alloy is very weak.