Advances in Mg corrosion and research suggestions

Recent research is summarised with an emphasis on the use of Mg alloys for biodegradable medical applications. Mg melt purification using Zr has been shown to provide the opportunity to produce ultra-high-purity Mg alloys, which could lead to stainless Mg. Nor's solution may be a good starting model for the study of Mg for biodegradable medical implant applications. A systematic laboratory investigation is needed to elucidate the details of how the corrosion behaviour is controlled by the various constituents of the body fluids. In the evaluation of the Mg corrosion mechanism there is a critical lack of understanding of (i) the amount of hydrogen dissolved in the Mg metal during corrosion, and during anodic polarisation, and (ii) the size film-free area where corrosion occurs, and how to measure this area. In the evaluation of the apparent valence of Mg using an applied anodic current density, for reliable values, it is important to apply a sufficiently large applied current density. The available data are consistent with the slightly modified uni-positive Mg+ ion mechanism, which maintains that (i) the surface of Mg is covered by a partially protective film, and the film-free area increases as the potential becomes more positive (i.e. a catalytic activation process), (ii) corrosion occurs preferentially at breaks in the partial protective film, (iii) corrosion at the breaks in the partially protective film involves the uni-positive Mg ion, (iv) undermining of particles occurs when Mg is severely dissolved, and (v) there may be some self-corrosion not covered by these four processes, which may be associated with crevice-like features on a severely corroded surface or hydride dissolution at relatively negative potentials. Self-corrosion might also be possible under condition of essentially uniform corrosion. Mg+ has not been experimentally observed. Its existence is postulated as an extremely-short lifetime intermediate in the reaction sequence between metallic Mg and the equilibrium ion Mg++. There has been no direct experimental examination of this sequence, and a key challenge remains to devise an experimental approach to study the details of this reaction sequence and the intermediate steps. The apparent valence of Mg continues to be a critical question. If defendable values of effective valence for Mg less than 1.0 were measured, this would indicate that some phenomena contribute to these low values that are not currently accounted for in the uni-positive Mg+ corrosion mechanism. The most likely candidate would be self-corrosion.