Corrosion of Mg, AS31 and AZ91 alloys in nitrate solutions

The high strength to weight ratio of magnesium and its alloys makes them extremely attractive for transport and aerospace technology applications. However, their corrosion behaviour limits their general applicability compared to aluminium alloys. This study deals with the effect of nitrates on corrosion of Mg and two Mg-based alloys (AS31 and AZ91) in presence and absence of buffers (phosphate and borate) of pH 8. The results revealed that the nature of buffer has a critical effect on the protective layer formed on electrodes. When phosphate buffer used, a protective layer of magnesium phosphate formed. The rate of formation of such layer has been found to increase with increasing both nitrate concentration, in solution, and aluminium content, in the alloy. This increase continues up to a certain limit after which the protective layer may fall and a uniform corrosion is can be observed. In case of borate buffer, high rate of uniform corrosion is observed. In unbuffered nitrate solutions, localized corrosion takes place and its rate increases as the nitrate concentration or the aluminium content in the alloys increases.