Understanding the differences in film formation mechanisms of two comparable solvent based and water-borne coatings on bronze substrates by electrochemical impedance spectroscopy

Corrosion protection of bronze, a metal alloy which is commonly used in decorative architectural elements and for statuary, is a challenge particularly as new and previously untested water-borne coatings are supplanting traditionally used solvent based polymers as protective coatings. Studying film formation of coatings to better prevent corrosion of culturally significant works is an important problem in the field of conservation science. This work aims to further understanding of differences between films formed from water-borne lattices versus solvent based polymeric coatings on bronze substrates using electrochemical impedance spectroscopy and gravimetric analysis to investigate the changes of coatings as they formed solid films. Using equivalent electric circuit models, electrochemical snapshots of the developing films were produced. Those models established time points for the transitions between theoretical stages of the nascent films and allowed for a comparison between two major classes of coating types (using the water-borne Rhoplex (TM) WL-81 and the solvent based Paraloid (TM) B-44). Electrochemical analysis showed that diffusion occurs during early stages of film formation of both coating types. The prolonged coalescence of coatings produced from the water-borne latex was found to leave those films susceptible to water sorption. Annealing films as a means to drive out remaining hydrophilic additives and to facilitate film formation was investigated. (C) 2011 Elsevier Ltd. All rights reserved.