pH-dependency of the corrosion inhibiting resp stimulating effect of polyacrylic acids on aluminium pigments

The isoelectric point (IEP: pH approximate to 9) of aluminium oxide is of significant influence for the corrosion inhibiting effect of polyacrylic acids with respect to aluminium pigments in alkaline aqueous medium: 1. Above the IEP (pH 10): High- and low-molecular weight polyacrylic acids inhibit the corrosion whereby their corrosion inhibiting effect decreases with increasing molecular weight. 2. Below the IEP (pH 8): Low-molecular weight polyacrylic acids inhibit the corrosion but less effective asat PH 10; the corrosion inhibiting effect decreases with increasing molecular weight. High-molecular weight polyacrylic acids stimulate the corrosion reaction; i.e., at the IEP a change of their effect takes place.