Electrochemical Studies of AC/DC Anodized Mg Alloy in NaCl Solution

Abstract

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 Mg(OH)2,Mg(OH)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. © 2004 The Electrochemical Society. All rights reserved.