Corrosion and corrosion testing of magnesium alloys

The corrosion behaviour of magnesium alloys is not substantially comparable to other metals, such as iron, nickel and copper. It is always accompanied by hydrogen evolution. More hydrogen is evolved at a more positive potential or a higher anodic current density. The 'strange' hydrogen evolution behaviour is a common phenomenon for magnesium alloys and it is called negative difference effect (NDE). The NDE continues to receive considerable discussion. Furthermore, the corrosion behaviour of magnesium alloys depends mainly on the pH value of the surrounding electrolyte. Voluminous reaction products, formed in neutral electrolytes, lead to a diffusion-controlled dissolution on the surface of the underlying magnesium alloy. Therefore, influences from structure and alloying are suppressed very strongly. In alkaline environments, passivation occurs as a result of the formation of a hydroxide layer on the magnesium surface. Therefore, differences in the corrosion behaviour between the alloys are hardly detectable. Measurable effects can only be detected using very 'aggressive' corrosion conditions. Present methods do not adequately take into account the specific character of the corrosion of magnesium alloys. It can be better characterized using a rotating disc electrode for electrochemical measurements, which enables model defined flow conditions on the surface. Furthermore, the application of electrochemical noise offers the possibility of a simple and sensitive assessment of the corrosion susceptibility of magnesium alloys. Due to the high sensitivity of this measurement procedure, it is also possible to carry out examinations under more practical conditions.