The effect of pH on the selective dissolution of Zn and Al from Zn-Al coatings on steel

Selective dissolution of Al and Zn from 5 wt.% Al–Zn (Galfan) and 55 wt.% Al–Zn (Galvalume) galvanized steel coatings was investigated by comparing Zn and Al dissolution rates in 30 mM NaCl at pH from 2 to 12 using in situ kinetic analysis (atomic emission spectroelectrochemistry, AESEC) and in a 5 day immersion test. The selective dissolution of Zn occurred at low pH and selective dissolution of Al at high pH. Results from AESEC and from the immersion test were compared and interpreted in terms of the inhibiting and passivating effect of corrosion product films.