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 aerat ed 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 sh owed the capacitance of a porous layer (C-po) in the high-frequency (H F) region, the resistive component (R(po)) of the porous layer (R(po)) in the frequency range between 100 Hz and 1,000 Hz, and the resistanc e of charge transfer (R(t)) in the low-frequency (LF) region. R(po) an d R(t) increased with time, whereas C-po decreased with immersion time . Since the resistance of a film is proportional to its resistivity an d thickness, an increase in R(po) was interpreted to mean passivation increased with Immersion time. This increase in protection with time p rovided evidence for the existence of a protective layer over the magn esium anode surface. This layer was identified as Mg(OH)(2) using x-ra y diffraction (XRD) of corrosion products formed on the magnesium anod e.