The contribution of Zn(II) and phosphate anions to the inhibition of organic coating cathodic disbondment on galvanised steel by zinc phosphate pigment

In-situ scanning Kelvin probe (SKP) measurements are used to investigate the relative contributions of in-coating zinc (II) cations and phosphate anions to the inhibition of corrosion-driven cathodic disbondment by zinc phosphate (ZnPhos) pigments. Using hot-dip galvanized steel (HDG) substrates, delamination rates of model polyvinylbutyral (PVB) coatings comprising different pigment volume fractions (ϕ) of phosphate-loaded hydrotalcite and Zn(II)-exchanged bentonite are compared with those established in the presence of ZnPhos. The most powerful inhibitory effect is obtained using in-coating Zn2+, while ZnPhos pigments inhibit cathodic disbondment rather weakly and as such the principal function of phosphate is to control Zn2+ solubility.