The Electrochemical Impedance Study of Corrosion Protection of Bronze by Waterborne Organic Coating/Phosphonic Acid Protective System

Abstract

The use of organic coatings is the most common method for corrosion protection of metals. However, the majority of the organic coatings are solvent borne. Nowadays, there is a great interest for their replacement by more environmentally friendly waterborne coatings. Unfortunately, their application poses practical problems due to their poor surface wetting, lower level of protection as well as the corrosion of metal substrate. This problem is especially pronounced in protection of bronze cultural heritage due to the requirement that coating does not change the visual appearance of the substrate, which limits the use of protective pigments in the coating. In addition, coating must successfully protect, not only the bronze substrate, but the adhering patina as well.This work examines the use of waterborne acrylic coating in combination with pretreatment by a long-chain phosphonic acid as a solution for protection of bare and patinated bronze surface. Electrochemical impedance spectroscopy studies are conducted on bare and chemically patinated bronze protected by waterborne coating or by combined protection system phosphonic acid/waterborne coating. In this way it is possible to evaluate the influence of phosphonic acid pretreatment on overall coating protection. Studies are conducted under various aging conditions, including the continuous immersion into artificial acid rainwater, climatic chamber with NO2 introduction and outdoor exposure. The obtained results show that phosphonic acid pretreatment improves the overall corrosion protection level of acrylic coating as the phosphonic acid acts as a corrosion inhibitor. However, some differences in the EIS spectra evolution in time are observed when comparing the results from different aging conditions. This differences are related to the coating swelling and underlying patina transformation under studied corrosion conditions. Acknowledgment: This work was fully supported funded by the Croatian Science Foundation under the project IP-2019-04-5030