Electrochemical Corrosion Behavior of Magnesium Alloy AZ31D in NaCl Solutions
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The high specific strengths and biodegradability of Mg and its alloys, combined with their other useful properties, such as high damping capacity and excellent castability, makes magnesium and its alloys widely used as light-weight structural materials and biomaterials in the automobile, aerospace, and biomedical industries. However, the low corrosion resistance of most Mg-based alloys restricts their further applications compared to other more noble metallic materials, such as aluminum alloys and stainless steels. In this dissertation, the electrochemical corrosion behavior of the as-cast Mg alloy AZ31D in NaCl solutions is investigated using electrochemical techniques and the weight loss method. The surface characterization methods of optical and scanning electron microscopy and energy dispersive spectroscopy are also used. The alloy corrosion morphology, surface oxide film growth kinetics, evolution of the surface film properties with immersion time, and the special anodic dissolution phenomenon known as the “negative difference effect” are investigated in multiple concentrations of NaCl solutions. It is found that the galvanic corrosion initiates at the α-Mg matrix around the Mg2Si particles and stops at the massive Mg17Al12 phase located at the α-Mg matrix grain boundary. The electrode open-circuit potential curves display four distinctive segments referred to as t1-t2, t2-t3, t3-t4, and t4-t5. The oxide film growth occurs primarily in the t2-t3 segment, which consists of two consecutive linear growth stages. The oxide film growth in both stages follows a direct logarithmic law, and the growth rates are unaffected by Cl- ion concentrations. The surface film properties at different conditions are analyzed using electrochemical impedance spectroscopy with equivalent electrical circuit simulation. The alloy corrosion resistance increases with immersion time due to film growth at immersion time t<t4 and decreases with prolonged immersion periods due to the increasing cathode/anode area ratio. Potentiodynamic polarization results show that cathodic polarization yields better film corrosion resistance. A Cl- ion-facilitated, current-governed surface film breakdown model is proposed to explain the negative difference effect of the alloy during anodic dissolution.