The Application of Electrochemical Impedance Spectroscopy to Immediately Diagnose the Protective Quality of Coatings on Artistic and Architectural Metalwork

Abstract

Corrosion is a spontaneous process that causes irreversible damage to nearly all metals, which has worldwide implications for architectural and artistic metalwork, such as bridges, buildings, airplanes and sculptures. Protective coatings such as wax, paint, or polymeric clear coatings are used to prolong the lifetime of metals such as steel and bronze. Unfortunately, these coatings fail over time due to oxidative damage by UV rays and by failure to exclude water that can carry salts and pollutants that cause corrosion of the underlying substrate. The current method of coating assessment is visual inspection but when coating failure is detected at this stage, irreversible damage has already occurred to the metal substrate. Diagnosing coating quality on artistic metalwork is a unique challenge as it requires a method that is non-destructive as to not alter the aesthetic of the piece. A non-destructive technique or device that can detect early signs of coating failure in the field (such as at a sculpture park) does not currently exist. The aim of this thesis is to develop a method that can be readily used in the field by a conservator to quickly diagnose the protective state of a coating on a sculpture in order to provide localized treatment. Electrochemical impedance spectroscopy (EIS) is a method currently used to study protective coatings in the lab. The technique itself is non-destructive but the most common electrochemical cell used with it must be used on a planar substrate and requires that a portion of coating be removed. Not only is the current method destructive, but the data produced by EIS is complicated and time consuming to analyze. These issues provide the foundation for this project.