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Alumina coating was deposited on AZ31B magnesium alloy using atmospheric plasma spraying process by varying parameters such as power, stand-off distance and powder feedrate. The porosity level of alumina coatings were evaluated by the digital image analysis method. An attempt was also made to develop an empirical relationship to predict the porosity level of alumina coatings. Three factors five level central composite rotatable design matrix was used to minimise the number of experimental conditions. Response surface methodology was used to develop the relationship. The developed relationship can be effectively used to predict the porosity level of alumina coatings at 95% confidence level. The results indicate that the input power has the greatest influence on porosity level of the coatings, followed by stand-off distance and powder feedrate.

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Corrosion attack of aluminium and magnesium based alloys is a major issue worldwide. The corrosion degradation of an uncoated and atmospheric plasma sprayed alumina (APS) coatings on AZ31B magnesium alloy was investigated using immersion corrosion test in NaCl solutions of different chloride ion concentrations viz., 0.01 M, 0.2 M, 0.6 M and 1 M. The corroded surface was characterized by an optical microscope and X-ray diffraction. The results showed that the corrosion deterioration of uncoated and coated samples were significantly influenced by chloride ion concentration. The uncoated magnesium and alumina coatings were found to offer a superior corrosion resistance in lower chloride ion concentration NaCl solutions (0.01 M and 0.2 M NaCl). On the other hand the coatings and Mg alloy substrate were found to be highly susceptible to localized damage, and could not provide an effective corrosion protection in solutions containing higher chloride concentrations (0.6 M and 1 M). It was found that the corrosion resistance of the ceramic coatings and base metal gets deteriorated with the increase in the chloride concentrations.