Inhibition of corrosion-driven organic coating disbondment on galvanised steel by smart release group II and Zn(II)-exchanged bentonite pigments

Bentonite pigments exchanged with either zinc or group II cations are characterised as inhibitors of corrosion-driven cathodic disbondment of model polyvinylbutyral (PVB) coatings adherent to the intact zinc surface of hot dip galvanised steel. An in situ scanning Kelvin probe (SKP) technique is used to quantify rates of coating delamination as a function of pigment volume fraction (ϕpt) and draw up a ranking order of inhibitor efficiency. Group II cation-exchanged bentonites show a moderate degree of inhibition, where rates of coating disbondment are reduced by up to 60–70% compared to the unpigmented case. In contrast, bentonite pigments containing exchangeable Zn2+ ions are markedly more effective, and no delamination is observed over periods of up to 24 h when ϕpt ≥ 0.1. The efficiency of in-coating Zn2+ is attributed to the ability to block underfilm cathodic oxygen reduction by reinforcing a pre-existing zinc (hydr)oxide layer.