Corrosion of magnesium alloys

Corrosion products on the surface of magnesiumThe mechanism of corrosion processes in magnesium and its alloys is very complex and not completely understood, in spite of the large number of studies. While it is clear that the corrosion of magnesium is highly localized (often it occurs on the level of structure grains and intergranular boundaries) and the key role in the corrosion process belongs to impurities, it is much less clear why the same phases in a magnesium alloy may sometimes accelerate the corrosion and sometimes inhibit it. It is known that the corrosion behavior of magnesium alloys strongly depends on the technology of casting and subsequent cold and hot working, but the details of this dependance are known only for some cases. Even the measurement of the corrosion rates of magnesium alloys is a problem, because different techniques of measurement present very different results, a minor error in the preparation of a specimen completely changes the rate and even the character of the corrosion process; and even when specimens have been prepared ideally perfect, significant deviation are observed. Particular interest present MRI series magnesium alloys patented by the Dead Sea Magnesium and Volkswagen. Being relatively new, these alloys have been only poorly studied, which makes such a research "hot" and actual. The research involves a systematic comparison of the measurements of the corrosion rates by different techniques, both electrochemical and purely chemical. The comparison will hopefully reveal the details of the corrosion mechanisms and will also allow the choice of the most appropriate test technique. The corrosion behavior of Mg alloys is measured by the open circuit potential (OCP) measurements, electrochemical impedance spectroscopy (EIS), linear polarization tests (LPR), linear sweep voltammetry (by Tafel extrapolation) and cyclic voltammetry, gas evolution and mass loss in neutral 3.5 wt% NaCl solution. The validity of the above techniques for the evaluation of corrosion characteristics of magnesium alloys is compared.