(Keynote) Corrosion Protection of Galvanized Steel Using Smart-Release Inhibitive Pigments Containing Organic Anions
Authors: Williams Geraint , Dodds Patrick , Ansell Phil 
Published: February 2020

There is considerable current interest in developing credible, environmentally acceptable replacements for toxic anti-corrosion pigments based on sparingly soluble Cr(vi) containing salts such as zinc or strontium chromate. One of the most promising alternative technologies presently under investigation is the application of ion-exchange materials as so-called “smart-release” pigments incorporated within an organic polymer binder. These systems are “smart” because they release inhibitor species and sequester aggressive ions (e.g. chloride or H+ ions) only when a corrosive aqueous environment is encountered. A second advantage is that they can be used as generic inhibitor delivery system, within which it is theoretically possible to incorporate any inorganic (or organic) ionic species which can act as a corrosion inhibitor. Therefore, using this type of technology, the range of inhibitors which can be included within a coating is no longer constrained to ionic combinations which form sparingly soluble salts.

            In this work, a range of potential anti-corrosion pigments, based on an organic anion-exchange resin consisting of a poly(styrene-co-divinylbenzene) with quaternary amine functionality, are investigated as inhibitors of corrosion-driven organic coating delamination from a hot-dip galvanized steel (HDG) surface. A range of potential organic inhibitors were evaluated which included benzoate, benzotriazolate, phenyl phosphonate, 2-mercapto-benzothiazolate and phenyl borate anions. These were incorporated within the ion-exchange matrix by dispersing the as-received resin beads in 0.5 mol dm-3 aqueous solutions of the relevant sodium form of the organic anion followed by stirring for several hours. After settling and decanting, the beads were exhaustively washed by repeated cycles of centrifugation and re-dispersion in fresh distilled water, until no sodium ions could be detected in the supernatant. Finally the resin beads were dried in air and ground in a planetary mill to give a fine powder. A series of model coatings, prepared by dispersing various volume fractions of the ion-exchange resin pigments in a polyvinyl butyral (PVB) binder, were then 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. Preliminary screening of the in-coating inhibitors, at a fixed pigment volume fraction of 0.1, showed that all organic anions slowed the delamination rate compared to the uninhibited PVB coating, but that benzotriazole (BTA) was by far the most effective.

            Further study of BTA-containing anion-exchange resin (BTA-AER) pigments over a range of different volume fractions showed a marked and progressive decrease in delamination rate with increasing pigment loading as shown in Figure 1. In addition the plots of delamination distance (xdel) with time in this figure also demonstrated that increasing pigment volume fraction also produced a progressively longer time delay until underfilm corrosion became initiated. After delamination became established, the presence of in-coating BTA at a pigment volume fraction of 0.1 produced a 30 fold decrease in post-initiation disbondment rate. Figure 1 also demonstrates a transition from parabolic to linear kinetics when BTA-AER pigments were dispersed within the PVB coating, indicating 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. It was also demonstrated that BTA acted as a powerful inhibitor when added to the corrosive electrolyte applied to the defect region. In delamination experiments carried out using unpigmented PVB, delamination rates became progressively slower with increasing concentration of BTA, as shown in Figure 2, where an 8-fold decrease in the delamination rate constant (kdel) was observed at the highest inhibitor concentration. Mass loss experiments carried out on HDG specimens, immersed in chloride solutions containing various concentration of dissolved BTA, confirmed highly efficient inhibition of the exposed substrate surface at the penetrative coating defect. The contributions of these various modes of inhibition, comprising the action of BTA within both the defect and underfilm regions, to the overall mechanism will be discussed.

 Figure Legends

 Figure 1: Delamination distance (xdel) versus time plots observed for PVB-coated HDG substrates in the presence of a BTA-AER pigment at volume fractions of (i) 0, (ii) 0.02, (iii) 0.05 and (iv) 0.1. The initiating electrolyte applied to the defect region was 5% w/v NaCl (aq).

 Figure 2. Plots of xdel as a function of the square root of time (where ti is the time for delamination to become established) for PVB coated HDG substrates, where the corrosive electrolyte applied to the penetrative coating defect consisted of 5% w/w NaCl (aq) at pH7 in the presence of dissolved BTA at concentrations of (i) 0, (ii) 10-2 and (iii) 0.1 mol dm-3.

Figure 1