Evaluation of Protective Coatings by Electrochemical Impedance Spectroscopy (EIS) to Predict Coating Service Life
* Does EIS provide a better method of evaluating protective coatings compared to traditional visual standards and can EIS predict the coating service life for Reclamation infrastructure coatings?
Need and Benefit
Reclamation needs better evaluation methods to select new emerging coating technologies to meet performance criteria and to predict the coating service life.
EIS is a new technology for evaluating protective coatings to supplement conventional coating evaluation that provides more quantitative results for corrosion resistance of coatings. The corrosion protection of a coating increases as its impedance increases. Therefore, a newly applied, high performance coating will have a Log impedance in the range of 9 to 11. When the coating is exposed to simulated or real service conditions, its impedance gradually decreases over time. This rate of decrease is a function of the coating formulation and the severity of the service conditions. At some point in time, the impedance will drop to the point where coating permeability to water is very high and corrosion protection is lost.
The rate at which the impedance decreases over time can be used to predict the service life of the coating. This is very useful information to select, specify, and schedule for replacement of coatings before corrosion takes place. This will benefit the Reclamation by knowing which coating system will provide the longest protection in a specific environment and can be incorporated into Reclamation guide specifications. This method will also provide an estimated lifetime of a coating system, rather than waiting "x" number of years to determine a coatings lifespan. This is important because coatings technology has vastly changed in the past 20 years due to more stringent volatile organic compounds (VOC) regulations, especially in California, and the service life of these materials is vague.
EIS is a nondestructive, highly sensitive technique in which measurements can be obtained rapidly and is a more accurate evaluation technique than waiting for visible corrosion products to form. Primarily, this technique will be used for evaluating coating performance and service life prediction to determine the most suitable coatings for use on the Reclamation infrastructure. Using the highest performance coating systems will reduce the maintenance and down time of the structure, thus increasing water availability and reducing maintenance costs.
EIS can also be used in field studies to determine the remaining service life of existing coatings on Reclamation structures.
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