Electrochemical Behavior of a Magnesium Galvanic Anode Under ASTM Test Method G 97-89 Conditions

The electrochemical behavior of a magnesium galvanic anode in an aerated 5 g/L calcium sulfate (CaSO4) + 0.1 g/L magnesium hydroxide (Mg[OH]2) solution was investigated by measuring electrochemical impedance under the conditions of ASTM Test Method G 97-89. Impedance spectra showed the capacitance of a porous layer (Cpo) in the high-frequency (HF) region, the resistive component (Rpo) of the porous layer (Rpo) in the frequency range between 100 Hz and 1,000 Hz, and the resistance of charge transfer (Rt) in the low-frequency (LF) region. Rpo and Rt increased with time, whereas Cpo decreased with immersion time. Since the resistance of a film is proportional to its resistivity and thickness, an increase in Rpo was interpreted to mean passivation increased with immersion time. This increase in protection with time provided evidence for the existence of a protective layer over the magnesium anode surface. This layer was identified as Mg(OH)2 using x-ray diffraction (XRD) of corrosion products formed on the magnesium anode.