electrochemical and corrosion behaviour of aluminium-magnesium [3 and 5%] alloys in alkaline chromate solutions

ABSTRACT

Corrosion rates of aluminium-magnesium [3 and 5%] alloys have been measured at 30C, in 0.05 M NaOH containing 10-5 to 1 M potassium chromate. At low concentrations /< 10-3 M, the chromate ion promotes the corrosion through catalyzing the hydrogen evolution reaction. At higher concentrations, corrosion inhibition was noticed through the formation of a protective film. Generally, the alloys are more resistant to corrosion than pure aluminium. Galvanostatic cathodic and anodic polarization was carried out at different current densities up to 10-2 A/cm-2. The cathodic curves exhibited Tafel relation with a slope of ca 0.14, indicating that hydrogen evolution was governed by slow discharge. Tafel relations were also obtained on anodic polarization, the slope of the V/c.d. curves amounted to 0.1, indicating that the first oxidation step of aluminium is the rate- determining step. At relatively high current densities anodic passivity occurred at c.d. decreasing with increase of the chromate concentration