Electrochemical corrosion of an Al---Mg---Cr---Mn alloy containing Fe and Si in inhibited alkaline solutions

The electrochemical corrosion of Al---2.7%Mg---0.19%Cr---0.04%Mn alloy containing 0.27% Fe and 0.14% Si, using open-circuit potential measurements, corrosion rate determinants, potentiodynamic and galvanostatic transients and anode efficiency experiments together with optical microscopy, scanning electron microscopy, transmission electron microscopy and energy dispersion X-ray has been studied to ascertain the effect of Cr and Mn. The experiments were performed at 25 and 50 °C in non-deaerated and stirred 4 M KOH containing K2MnO4 and combinations of NaVO3, NaBiO3, NaMoO4, ZnO and Al2O3. The open-circuit potentials were rapidly stabilized in values in the range between −1.4 and −1.6 V versus Hg/HgO/OH− (4 M KOH). The alloy dissolution at open circuit and also in the galvanostatic and potentiodynamic experiments were shown to be uniform, this being correlated with the homogeneous distribution of Mg in the material. The anodic oxidation took place with the formation and dissolution of a conducting aluminium oxide film. However, limiting values were found over which the anode became passivated. The limiting values depended on the temperature, being about 125 and 500 mA cm−2 at 25 and 50 °C, respectively. The anode efficiencies were sufficiently high, that is over 90%, only at current densities approaching the limiting value at the corresponding temperature. The anode passivation was related to an excess of Cr and the anode corrosion to an excess of Mg together with the presence of Fe and Si.