Corrosion Protection of Galvanised Steel Using Organic Coatings Containing Inhibitive Additives Based on Benzothiazoyl Succinic Acid

Abstract

It is increasingly apparent that the replacement of highly effective, but environmentally unacceptable chromium (VI)- based inhibitive pigment technologies within protective coatings requires the use of combinations of anti-corrosion additives. There is currently significant interest in using organic inhibitors containing hetero-atoms of nitrogen and/or sulphur in conjunction with conventional Cr(vi)-free inorganic pigments as a means of providing equivalent corrosion protection. One such organic inhibitor is benzothiazoyl succinic acid (BTSA), which is presently commercially available as an in-coating anti-corrosion additive, which can improve the protection capability in existing chromate free formulations. In this work we investigate the ability of BTSA to mitigate corrosion-driven organic coating failure within model coatings applied to hot-dip galvanised steel. In addition, the ability of BTSA to inhibit localised corrosion on bare zinc surfaces when released from a protective coating into corrosive electrolyte within a penetrative defect is also evaluated. A combination of time-lapse imaging and in-situ scanning Kelvin probe (SKP) potentiometry is employed to quantify cathodic delamination rates of model poly-vinyl butyral (PVB) coatings containing varying levels of BTSA additions. It is demonstrated that in-coating BTSA, both added directly to PVB or stored as an exchangeable anion in a layered double hydroxide “smart-release” pigment reduce rates of cathodic disbondment by up to a factor of 20. For both methods of incorporating in-coating BTSA, an increasing loading produces a progressive decrease in delamination rates and a transition from linear to parabolic coating failure kinetics. In addition, it is also demonstrated that BTSA dissolved in the aqueous NaCl (aq) electrolyte applied to a defect can also significantly reduce the rate of cathodic disbondment observed for an unpigmented PVB coating. Highly effective inhibition of localised corrosion of bare HDG surfaces by BTSA additions made to NaCl (aq) electrolyte is demonstrated by a combination on SVET and potentiodynamic studies. Linear polarisation resistance experiments demonstrate an inhibition efficiency of 95% for a 10 mM BTSA addition made to 1% w/v NaCl (aq) at pH 7, while SVET-derived, area averaged current density value are shown to decrease by over an order of magnitude compared with an un-inhibited case over a 6 h immersion period. Using polarisation curves obtained for HDG surfaces in the presence and absence of varying concentrations of dissolved BTSA, a mixed inhibition mechanism is proposed. At intermediate concentrations (1 – 10 mM), BTSA strongly adsorbs on the oxide covered Zn surface via its two carboxylate functional groups, thus hindering electron transfer reactions and providing net cathodic inhibition. At higher concentrations (> 10 mM) an additional reinforcement of any weak points in the adsorbed layer is achieved by virtue of the formation of an insoluble salt formed by reaction with Zn(ii) cations, producing a positive shift in free corrosion potential. In-coating BTSA appears to stifle cathodic disbondment by adsorbing strongly on the intact organic-coated metal, which in turn hinders underfilm oxygen reduction and the dissolution of the amphoteric zinc(hydr)oxide layer at the metal-polymer interface.