The corrosion behaviour of magnesium alloys is not substantially comparable to other light metal alloys, it is more similar to that of steels. Voluminous reaction products, formed in neutral electrolytes, leads 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. Presently used methods do not adequately take into account the specific character of the corrosion of magnesium alloys. 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.
Magnesium is very reactive and its alloys are not corrosion resistant under normal conditions [1-3]. The low corrosion resistance of the magnesium alloys is currently limiting their applications. For example, because of the poor corrosion behaviour, the use of magnesium alloys in the automotive industries is currently limited to some components [4]. The corrosion problems are a major issue that often prevents the application of magnesium alloys. Therefore, understanding and improving the corrosion behaviour of magnesium alloys is important for their future applications [3, 5]. For the assessment of the corrosion resistance of magnesium alloys no suitable methods are readily available at the present time. Special mechanisms, which do not occur in other metal alloys to that extent, have to be taken into account. Methods like the salt spray test, which originally were developed for testing of coated materials, do not adequately take into account the specific character of the corrosion of magnesium and magnesium alloys. Salt spray test procedures are frequently used in the industry for testing the quality of semi-finished products and end products with regard to their corrosion resistance. For this reason, salt spray testing is one of the best-established corrosion test procedures. Furthermore, the test conditions for salt spray tests are intensified in order to shorten the testing periods, which can lead to a change in the corrosion mechanism and, consequently, may provide misleading information about the corrosion behaviour.
The suitability of electrochemical noise measurements to determine the influence of passivation events and the quality of protective layers is shown in this work. The electrochemical noise measurements reflect the sub-microscopic processes of the corrosion, although diffusion controlled processes also take place [6-8]. Therefore, different magnesium alloys are investigated under open circuit conditions to detect very small changes in the corrosion system (alloy, medium, design) with high precision. The use of the electrochemical noise measurement in the corrosion examination makes it possible to see the time dependence of corrosion events. In order to do that, defined protective layers, like Alodine® layers, are produced on magnesium alloy working electrodes.
