Electrochemical Corrosion Behavior of AZ91D Magnesium Alloy under Thin Electrolyte Layer Containing SO₂

The self-built electrochemical measurement device for atmospheric corrosion, with a thin electrolyte layer thickness controlling set was utilized to investigate the electrochemical corrosion behavior of AZ91D magnesium alloy under thin electrolyte layer containing SO2 by means of polarization curve and electrochemical impedance spectrum. Furthermore the corrosion morphology was observed. Results show both the thickness of thin electrolyte layer and SO2 affected the anodic/cathodic polarization slope. SO2 addition reduced anodic/cathodic polarization slope, indicating the acceleration of corrosion for magnesium alloy. With the decrease of the thickness of thin electrolyte layer, the corrosion rate increased. The maximum value was found in 50 μm thickness. The formation of corrosion products under electrolyte layer with very thin thickness blocked the oxygen diffusion, leading to the slowing down of corrosion. The limited diffusion current density firstly increased and then decreased with the increase in the thickness of thin electrolyte layer. When the thickness of electrolyte layer was less than 50 μm, anodic process was inhibited because of pileup of corrosion products. The Nyquist curve consisted of high frequency capacity reactance arc and low frequency diffusion impedance, and showed two time constants. The order of corrosion tendency under electrolyte layer containing SO2 in different thickness was 50, 20, 200, 300 μm.