Corrosion inhibition of aluminium and zinc pigments by copolymers

Aluminium and zinc pigments corrode in aqueous alkaline paint media with the evolution of hydrogen. Maleic acid-styrene-acrylic ester copolymers were synthesized by copolymerisation of maleic acid anhydride, styrene and different (meth)acrylic esters. Three acrylic esters (ethyl, n-butyl, n-hexyl) and two methacrylic esters (n-dodecyl, n-octadecyl) were used; the copolymers with long-chain acrylic esters are amphiphilic. Additionally, a commercial (non-amphiphilic) styrene-maleic acid copolymer (SMA) with similar molecular mass and acid number was tested. The corrosion reaction of aluminium and zinc pigments in aqueous alkaline media can be inhibited by addition of these copolymers. But aluminium and zinc pigments react completely differently with the examined copolymers. With addition of the amphiphilic maleic acid-styrene-acrylic ester copolymers to aluminium pigment dispersions the evolved hydrogen volumes decrease with increasing chain-length of the acrylate monomer in the copolymers, whilst with zinc pigment the hydrogen volumes increase, which is just the opposite compared with aluminium. Furthermore, there exist mathematical correlations between the number of carbon atoms of the ester alcohol of the acrylate monomer in the copolymers and the hydrogen volumes evolved.