Comparison of corrosion resistance of microarc oxidation coatings prepared with different electrolyte concentrations on AM60 magnesium alloy

The microarc oxidation (MAO) of AM60 magnesium alloy in two kinds of electrolytes is studied in this paper to compare the influence of electrolytes on the properties of the MAO coatings. One concentrated electrolyte is composed of 100 g L-1 NaOH+20 g L-1 Na2B4O7 center dot 10H(2)O+50 g L-1 C6H5Na3O7 center dot 2H(2)O+60 g L-1 Na2SiO3 center dot 9H(2)O and the other is a dilute solution, which is composed of 5 g L-1 NaOH + 15 g L-1 Na2SiO3 center dot 9H(2)O. It was found that the sparking behaviour in dilute electrolyte is more intensive and the breakdown voltage U-B is higher (the U-B in dilute electrolyte is 282 V, while it is 82 V in the concentrated electrolyte). The thermal effect during the MAO in the two electrolytes is also measured, and the temperature increment in dilute electrolyte is more than twice of that in the concentrated electrolyte. The corrosion resistance of the MAO films is studied by electrochemical impedance spectroscopy and potentiodynamic polarisation techniques and the microstructure of the films is examined by SEM. The electrochemical tests show that the MAO film formed in the concentrated electrolyte is more corrosion resistant. Scanning electron microscope shows that the microstructure of the MAO film in dilute electrolyte is porous and thin, which accounts for its poor corrosion resistance. The experiment results show that high U-B does not mean good corrosion resistance.