Novel Corrosion Inhibitor Pigments Based on Benzotriazolium Cation Exchanged Resin

A novel corrosion-inhibitor pigment is described which is prepared by the reaction of benzotriazole (BTA) with a crosslinked polystyrene sulfonic acid resin (Amberlite 120H). This pigment is completely insoluble in water and may readily be ground to sub-micron particle size. The formation reaction relies on the resin sulfonic acid functions (pKa -2.8) being significantly more acidic than BTA (pKa 1.1) so that proton transfer produces the benzotriazolium cation ( BTAH+) in an exchangeable form. The kinetics of BTA uptake from aqueous solution at 60 o C are followed spectrophotometrically. It is thus shown that uptake is rapid (complete in minutes) and results in a BTAH+ exchange capacity of 143 milli-equivalents per100g. The BTAH+ pigment is evaluated as a corrosion inhibitor by dispersion in an ethanolic solution of polyvinylbutyral (PVB) which is then coated onto the cleaned surface of zinc-coated (hot dip galvanized) steel to produce a dry film thickness of ca. 30 microns. The BTAH+ pigmented PVB coatings thus prepared had BTAH+ pigment volume fractions of between zero and 0.2. A corrosive electrolyte solution (5% w/w aqueous NaCl) was introduced into a penetrative defect in the organic coating and the subsequent process corrosion-driven cathodic coating disbondment was followed using a scanning Kelvin probe (SKP) instrument in a “Stratmann type”experiment at 96% R.H. The SKP data show that increasing BTAH+ pigment volume fraction produces two well defined effects. First the potential of the intact (undisbonded) coated surface becomes depressed by up to 0.3 V versus the unpigmented case. Second the kinetics of cathodic disbondment are profoundly inhibited, becoming immeasurably slow for pigment volume fractions greater than 0.1. These results are interpreted in terms of two phenomena. First the “etching” of the zinc surface by the acidic resin with a consequent release of Zn 2+ cations and neutral BTA. Second the inhibition of underfilm cathodic O 2 reduction(COR) by surface adsorbed BTA or BTA-zinc complex. The inhibition of COR by BTA in solution is systematically investigated using a zinc rotating disk electrode (RDE) in conjunction with potentiodynamic polarization. The polarization data show that, although BTA profoundly inhibits COR on zinc, BTA is actually a mixed inhibitor and inhibits both anodic zinc dissolution and the COR.