A Smart-Release Corrosion Inhibitor Pigment for Galvanised Steel Based on Salicylaldoxime-Loaded Hydrotalcite

Abstract

Smart-release anticorrosion pigments are a promising alternative technology to traditional anticorrosive pigments such as sparingly soluble salts. Due to legislation prohibiting the use of chromium (VI) containing protective coatings, new environmentally friendly alternatives are required. Smart release pigments have numerous advantages over traditional pigment technologies. Sparingly soluble salts slowly leach out their constituent ions over time, which both reduces the lifetime of the coating and leads to environmental effects. In contrast, smart release pigments utilise ion-exchange properties to release inhibitor species on demand, whilst simultaneously sequestering corrosive species such as chloride ions. In this work intelligent release pigments based on anion-exchange hydrotalcite (HT) like materials were studied with respect to their efficiency in the inhibition of cathodic disbondment of a model coating. The ability of the various organic-anion loaded HT pigments to inhibit cathodic delamination of hot dip galvanised steel was evaluated using polyvinyl butyral (PVB) coatings containing a range of volume fractions of the inhibitive pigments. An in-situ Scanning Kelvin Probe (SKP) technique was used to measure the rate of delamination over time, by measuring time-dependent changes in free corrosion potential of the zinc surface under the organic coatings. Of the selection of organic anions evaluated, the most effective corrosion inhibitor was found to be salicylaldoxime, a well-known chelating reagent for various divalent metal cations. The slowing of cathodic delamination rates with increasing pigment volume fraction compared well with the performance of a hexavalent chromate-containing benchmark. To determine the importance of the release of salicylaldoxime from the coating into the electrolyte in contact with bare metal at a penetrative coating defect, experiments were also conducted using unpigmented PVB coatings in a situation where inhibitor additions were made to the initiating electrolyte. The conclusion of this investigation is that for the corrosion protection of galvanised steel surfaces, salicylaldoxime-loaded HT intelligent-release pigments represent effective, yet environmentally friendly alternatives to the current chromate containing coating technologies.