THE ELECTROCHEMICAL AND CORROSION BEHAVIOR OF MAGNESIUM (3-PERCENT AND 5-PERCENT) ALUMINUM-ALLOYS IN ALKALINE CHROMATE SOLUTIONS

The open circuit potentials and corrosion rates of magnesium (3 and 5% )-aluminium alloys have been measured at 30 degrees C, in 0.05 to 0.5 M NaOH containing 10(-5) to 1 M K2CrO4. At low concentration of chroma te, the chromate ion promotes the corrosion of the alloys, catalysing the hydrogen evolution reaction. Above 10(-3) M, the chromate ion inhi bits corrosion satisfactorily, through the formation of protective fil m; the extent of inhibition increases with chromate concentration. Gen erally, the alloys are more resistant to corrosion than pure aluminium . Cathodic and anodic polarisation curves have also been measured unde r galvanostatic conditions, up to 10(-1) A/cm(2). The cathodic curves exhibited Tafel relation with slope of ca.0.14V, indicating that hydro gen evolution is governed by the slow discharge. At high currents the potentials deviate from linearity. Also Tafel relations were obtained on anodic polarisation; the slope amounted to 0.1V, indicating that th e first oxidation step of aluminium is the rate-determining step. At h igh currents the anode is passivated and passivation current density d ecreases as the chromate concentration increased.