Influence of bicarbonate concentration on the conversion layer formation onto AZ31 magnesium alloy and its electrochemical corrosion behaviour in simulated body fluid

Abstract

The electrochemical corrosion behaviour of a magnesium carbonate conversion layer-coated AZ31 magnesium alloy was evaluated in simulated body fluid (SBF) solution. Bicarbonate solution was used to produce conversion layers and its concentration significantly influenced the conversion layer formation and electrochemical corrosion behaviour. Formation of a MgCO₃ conversion layer with Mg(OH)₂ onto AZ31 Mg alloy was identified by X-ray diffraction (XRD) and attenuated total reflectance infrared (ATR-IR) spectroscopy studies. The surface morphology and chemical composition of the conversion layer-coated AZ31 Mg alloys were analysed using scanning electron microscopy (SEM) attached to energy dispersive X-ray analysis (EDAX). Formation of a mud-crack pattern was ensured from the SEM morphologies. Potentiodynamic polarization, linear polarization, potentiostatic polarization and electrochemical impedance spectroscopy (EIS) techniques were used to study the electrochemical corrosion behaviour. An increase in the bicarbonate concentration up to 5 wt% resulted in about a 9 times reduction in the corrosion current density (i_corr) value. However, further increase of the bicarbonate concentration increased the i_corr values and thus the corrosion rate. EIS studies further supported the polarization results. The increase in the conversion layer and charge transfer resistance was evidenced when the concentration of bicarbonate is increased up to 5 wt%. A further increase in the bicarbonate concentration resulted in a decreasing resistance up to 9 wt%. These observations were confirmed from the equivalent circuit curve-fitting analysis. The surface morphology and compositional changes of the conversion layers altered the corrosion resistance of the AZ31 Mg alloy in SBF solution.