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Actual examples and perspectives of corrosion protection of magnesium alloys

  A growing number of new and innovative applications of Mg alloys is observed recently due to unique weightstrength ratio. Unfortunately, magnesium has also a number of undesirable properties including poor corrosion resistance. Therefore a development of more durable protective systems is concerned as the challenge. The main problem results from the fact that mechanism of corrosion of Mg alloys varies with a kind of alloy, impurities from mechanical processing, environment and from potential applications. Frequently, corrosion process contains complex steps of phenomena that overlap each other (general, pitting, bimetallic, fi liform types of corrosion). There are two main kinds of surface engineering techniques that modify surface properties: physical or chemical treatment. Based on the own results a review of selected technologies was made taking into consideration improvement of corrosion resistance as the priority. The example of the fi rst group of methods, high-energy beam surface melting of magnesium alloys, produces surface layers with a fi ne microstructure of signifi cantly reduced dimensions of intermetallic precipitates. A resistance to general corrosion was improved as a result of this surface treatment for AZ91, AZ31 tested alloys. The susceptibility to pitting corrosion in terms of Epit value was not improved. Indications to improve pitting resistance are discussed. Better results in terms of resistance to local corrosion were obtained after chemical surface treatment based on phosphate/permanganate process. Taking into account conversion coatings as a sub-layer for an organic coating, some examples of the phosphating process in comparison with sol-gel applied coating are shown. In both cases an optimization of technological parameters is required, however, even for the time being corrosion resistance obtained is satisfactory. Since Mg alloys are considering as a material of increasing use in automotive industry, electroplat[...]

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