Smart-Release Inhibition of Corrosion-Driven Organic Coating Delamination on Galvanized Steel Surfaces

In-coating cation and anion-exchange pigments are studied with respect to their ability to inhibit corrosion-driven organic coating disbondment on the zinc surface of hot-dip galvanized (HDG) steel. The efficiency of inorganic smart-release cation-exchangers, including bentonite and zeolite based pigments containing stored Zn(ii) inhibitor are compared with an organic cation-exchange cross-linked polystyrene based system. In-situ scanning Kelvin probe experiments carried out on HDG samples coated pigment-containing model polymer organic films showed that only the bentonite system was capable of halting delamination over a 24 h holding period, despite having a cation exchange capacity of only one third of the other exchangers. The reasons for this are discussed in terms of the kinetics of underfilm ion-exchange process. A preliminary investigation of anion-exchange hydrotalcite-based pigments is also described. The inhibition efficiency is shown to be highly anion specific, with chromate and phosphate exchanged pigments providing the best protection against organic coating cathodic disbondment.