Inhibition of Corrosion-driven Organic Coating Disbondment on Galvanized Steel by Smart Release Zn(II)–Exchanged Bentonite Pigments

In this work, an in-situ scanning Kelvin probe (SKP) technique is used to follow the delamination kinetics of model organic coatings adherent to hot dip galvanized steel (HDG) substrates. These coatings comprise films of polyvinyl-butyral (PVB) in which have been dispersed varying concentrations of cation-exchange bentonite pigments. The principal aim has been to study the influence of the nature of the exchangeable cation on the efficiency of bentonite pigments containing either Zn²⁺ or alkali earth cations as inhibitors of corrosion-driven cathodic delamination on HDG steel.

When Zn²⁺ exchanged bentonite pigment was dispersed in the coating, delamination rates were found to be significantly decreased compared to the unpigmented case. The extent of inhibition increased with pigment volume fraction and delamination kinetics became linear for all ø ≥ 0.02 and tended to zero for all ø ≥ 0.1. These findings are consistent with cation exchange occurring between the underfilm electrolyte and the in-coating Zn²⁺ bentonite, resulting in Zn²⁺ aquo-cation hydrolysis to precipitate solid zinc hydroxide. The observed linear delamination kinetics is consistent with Zn²⁺ cations acting to block cathodic electron transfer processes at the coating delamination front. This blockade results from Zn²⁺ hydrolysis producing a buffering of pH, so preventing the dissolution of amphoteric zinc (hydr)oxide at the delamination cell cathode. Additionally zinc (hydroxide) formed by cation hydrolysis is also thought to reinforce the same (hydr)oxide layer and obstruct interfacial electron transfer. Similar studies carried out using alkaline earth cation-exchanged bentonites showed a moderate degree of inhibition, where rates of coating disbondment were reduced by up to 60-70% compared to the unpigmented case. In contrast to the Zn²⁺ case, delamination kinetics remained parabolic throughout. The retention of parabolic kinetics for alkaline earth cations is inconsistent with cathodic blocking, but is consistent with a reduction of under-film electrolyte conductivity resulting from aquo-cation hydrolysis.