In-Coating Phenyl Phosphonic Acid as an Etch-Primer Corrosion Inhibitor System for Hot Dip Galvanized Steel

In-coating phenyl phosphonic acid (H 2 PP) additions are investigated as inhibitors of corrosion-driven organic coating disbondment on hot-dip galvanized (HDG) steel surfaces. In-situ scanning Kelvin probe (SKP) experiments under atmospheric corrosion conditions are used to study the influence of the quantity of dissolved H 2 PP in the organic coating on the kinetics of delamination. It is demonstrated that increasing levels of in-coating H 2 PP progressively lengthen the time for organic coating delamination to become established. Once underfilm corrosion becomes initiated, rates of cathodic disbondment are significantly slowed by H 2 PP additions in comparison with the uninhibited case. From the observed delamination kinetics, an inhibition mechanism is proposed whereby H 2 PP additions interact with the underlying zinc to form an interfacial salt layer which blocks underfilm oxygen reduction. The contribution of other possible modes of inhibition, such as H 2 PP release from the coating into the external defect electrolyte and a buffering of underfilm pH, are also discussed.