ANODIC BEHAVIOR OF MG IN HC03-/CO23- BUFFER SOLUTIONS, QUASI-STEADY MEASUREMENTS

Anodic behaviour of Mg in HCO3-/CO32- buffer solutions was studied by quasi-steady measurements on high purity magnesium electrodes in solutions of pH between 9 and 11, and with total concentration of carbonates between 10(-2) and 1 M. Two domains of passivity were observed, each with essentially potential independent corrosion rates. Passivity breakdown occurred both at cathodic and high anodic polarization, however. The corrosion rate could be correlated to the concentration of HCO3- in the solution. No pH dependence could be seen, but some dependence on solution stirring was observed. The corrosion rate (determined from metal mass loss) at open circuit under natural convection increased from about 5 x 10(-6) A cm-2 at 10(-3) M HCO3- to 3 x 10(-2) A cm-2 at 0.9 M, accompanied by a change in the corrosion potential from -1.55 to -2.05 V (vs saturated calomel electrode, sce). Comparison of the cds obtained electrochemically and by mass loss measurements revealed that the anodic efficiency of the electrodes increased from about 0.6 at -1.8 V (sce) to 1.0 at -0.3 V (sce). Formation of MgH2 may be the main reason for this. MgH2 was detected by ex situ X-ray diffraction at the surface of electrodes polarized cathodically.