The characteristics of the oxide film formed on a Mg-based WE43 alloy using ac/dc anodization techniques in an alkaline silicate solution have been investigated using scanning electron microscopy and Rutherford back-scattering spectroscopy, and the corrosion resistance of these films has been determined using the ac impedance technique. The oxide film was found to be composed of MgO, Mg(OH) 2 , MgF 2 , and SiO 2 , and consisted of an inner barrier and an outer porous oxide film, both of which increased linearly in thickness with the applied anodization voltage. High applied voltages and current densities also led to the formation of large pores, having diameters greater than 6 m, and the partial sealing of small pores in the outer layer. Allowing the current to decay for a period of time in the later stages of anodization led to an increase in the thickness of both the barrier and porous films and further sealing of the film pores. A correlation of the oxide corrosion resistance with the barrier film thickness is also demonstrated.

The corrosion behavior of a Mg-based alloy, WE43, ac/dc anodized using a waveform involving a final period of current decay in an alkaline silicate solution, has been investigated using electrochemical impedance spectroscopy during exposure to a 0.86 M NaCl solution. The change of the oxide film morphology with immersion time was also examined using scanning electron microscopy. Anodization of the WE43 alloy significantly improves its corrosion resistance and greatly increases the time to pitting in the NaCl solution, which for air-formed films is in the range 2-3 h. By fitting the impedance data to a two-time-constant equivalent circuit and by tracking the open-circuit potential, it is demonstrated that film hydration initially decreases the corrosion resistance, followed by an increase in the resistance due to the gradual conversion of MgO to Mg͑OH) 2 , which leads to partial blocking of the film pores. During this time period, the underlying barrier film is slowly thinned and/or penetrated by chloride ions, consistent with its increasing capacitance and decreasing resistance, ultimately leading to a loss in corrosion resistance. Overall, both the porous and barrier oxide layers contribute to the corrosion protection of WE43, and the higher the voltage and the longer the time of current decay in the latter stages of anodization, the lower the alloy corrosion susceptibility.

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