Portland State University. Department of Chemistry
Tami Lasseter Clare
Date of Publication
Master of Science (M.S.) in Chemistry
Protective coatings -- Testing, Corrosion and anti-corrosives -- Measurement, Impedance spectroscopy
1 online resource (vii, 58 pages)
Corrosion is a spontaneous process that causes irreversible damage to nearly all metals, which has world-wide 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.
This thesis first proposes multiple methods to quickly analyze the complicated data traditionally collected through EIS. Three quick analysis methods, including the estimation of the derivative at one single frequency in EIS spectra, was successfully used to categorize coating quality of five different coating types. Using this quick analysis can aid conservators in assessing coating condition without the need for extensive training in EIS data interpretation.
This thesis also proposes a method to measure early warning signs of coating degradation through a co-planar hydrogel electrochemical cell paired with EIS. The configuration of this co-planar hydrogel cell negates the need for the removal of the coating and can be used on multiple types of surfaces because of its flexibility, therefore overcoming the drawbacks of the traditional EIS electrochemical cell. Data provided demonstrates that this co-planar hydrogel provides similar information, when compared to the standard electrochemical cell, about the bulk of the protective coating. Unique to the co-planar hydrogel cell, information about surface degradation is provided during EIS measurements. This provides a warning sign before bulk degradation of the coating and therefore before any damage to the underlying substrate has occurred.
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Hosbein, Kathryn Nicole, "The Application of Electrochemical Impedance Spectroscopy to Immediately Diagnose the Protective Quality of Coatings on Artistic and Architectural Metalwork" (2016). Dissertations and Theses. Paper 3324.