Corrosion Protection of Galvanized Steel Using Smart-Release Hydrotalcite Pigments Containing Benzotriazolate Derivatives
Authors: Williams Geraint , Dodds Patrick Charles , Ansell Phil , McMurray Hamilton Neil 
Published: February 2020

The development of credible, environmentally-friendly replacements for toxic, inhibitive pigments based on sparingly soluble Cr(vi) containing salts remains one of the major challenges in the field of corrosion science. One of the most promising alternative technologies presently under investigation is the use of ion-exchange materials as smart-release pigments incorporated within an organic polymer binder. Anion-exchange layered double hydroxides, such as hydrotalcite (HT, Mg6Al2[OH]16CO3.4H2O) have received significant recent attention and have been previously shown to act as highly effective inhibitors of chloride-induced filiform corrosion on organic coated AA2024 alloy surfaces 1,2. The efficiency is principally attributed to their ability to sequester aggressive chloride ions and moderate underfilm pH, although specific effects due to the nature of the stored inhibitor anion type are also observed. Of the numerous potential inhibitors evaluated within HT pigments, benzotriazolate (BTA) anions were previously shown to be among the most effective in forestalling FFC 3. In this work, BTA-containing HT pigments are investigated as inhibitors of corrosion-driven organic coating delamination from a hot-dip galvanized steel (HDG) surface. A series of model coatings, prepared by dispersing various volume fractions of the BTA-HT pigments in a polyvinyl butyral (PVB) binder, were applied to HDG coupons and an in-situ scanning Kelvin probe technique was employed to evaluate their efficiency in inhibiting corrosion driven coating failure via cathodic disbondment. A marked and progressive decrease in delamination rate with increasing pigment loading, as shown in Figure 1, was obtained. In addition the plots of delamination distance (xdel) with time in this figure also demonstrate that increasing pigment volume fraction also produce a progressively longer time delay to the initiation of underfilm corrosion. Figure 1 also demonstrates a transition from parabolic to linear kinetics, suggesting that the inhibition mechanism may in part be due to an underfilm interaction of BTA- with the underlying zinc, leading to a significant blocking of cathodic oxygen reduction. Identical experiments were carried out using stored BTA derivatives, including 5-methyl benzotriazole and benzotriazole-5-carboxylic acid to assess whether further improvements in performance could be obtained. Pigments based on 5-methyl benzotriazole produced a marginally more profound inhibition of cathodic disbondment when compared with BTA-HT, while in-coating 5-carboxy-benzotriazolate inhibitor acted far less effectively. It will also be demonstrated that BTA acts as a powerful inhibitor when released from the coating into a corrosive electrolyte contacting the bare zinc surface at a penetrative coating defect. A combination of in-situ-scanning vibrating electrode technique (SVET) and potentiodynamic studies, along with mass loss experiments were carried out on galvanized specimens immersed in chloride solutions. A comparison of results obtained in corrosive electrolyte containing various concentration of dissolved BTA confirmed highly efficient inhibition of the exposed substrate at concentrations of greater than 0.01 mol dm-3.

References

1. G. Williams and H.N. McMurray, Electrochemical and Solid state letters, 6, (2003), B9.
2. H.N. McMurray and G. Williams, Corrosion, 60, (2004), 219.
3. G. Williams and H.N. McMurray, Electrochim. Acta, 69, (2012), 287.

Figure Legend

Figure 1: Delamination distance (xdel) versus time plots observed for PVB-coated HDG substrates in the presence of a BTA-HT pigment at volume fractions of (i) 0, (ii) 0.005, (iii) 0.02 and (iv) 0.1 and (v) 0.2. The initiating electrolyte applied to the defect region was 5% w/v NaCl (aq).

Figure 1