The effect of surface modification and stannate concentration on the corrosion protection performance of magnesium alloys

This paper reports the effect of surface modification and tin oxide concentration on the corrosion protection performance of AZ91D in chloride containing solution. Environmentally friendly chemical conversion coatings based on stannate solutions of different concentrations (25, 50 and 75 g/l) were used to improve the corrosion resistance of AZ91D. Different surface modification such as alkaline and/or acidic etching prior to applying stannate were investigated, Potentiodynamic polarization tests and Electrochemical Impedance Spectroscopy (EIS) were used to analyze the corrosion of the alloys in 3.5% NaCl solutions and to determine the optimum conditions for corrosion protection. The polarization resistance (Rp) for the uncoated AZ91D alloy was measured by EIS to be approximately 1.2 × 103 Ω cm2. The Rp increased by a factor of 4-5 for alloys coated with the stannate conversion coating. The corrosion rate decreased for the coated specimens, indicating that the stannate coating behaves as barrier to prevent the chloride attack and hence, decreases the susceptibility of the alloys to corrosion. Increasing stannate concentration was found to have an adverse effect on the corrosion resistance. Results showed that a simple immersion in a dilute stannate solution is very effective for improving the corrosion resistance of Mg alloys.