EIS studies of coated metals in accelerated exposure

One of the most popular uses of electrochemical impedance spectroscopy (EIS) is the characterization of the protective properties of coatings on corrodible metals. From early studies up to the present time, many EIS studies have been devoted to the study of the changes in the impedance of coated metals as they undergo either natural or artificial exposure to conditions that cause corrosive failure of such systems. With the current improvements in instrumentation and software for EIS studies of coated metals, one no longer needs to be an expert electrochemist to utilize EIS in one's studies of protection by coatings. In this paper, the use of EIS from the point of view of the coatings scientist will be presented, with an emphasis on its application simultaneous with accelerated exposure. EIS is used by coating scientists for several purposes, among them the detection of changes due to exposure, prediction of the lifetime of corrosion protection, identification of the corrosion processes that lead to failure, ranking of coatings systems, measurement of water uptake by coatings, and the development of models for coating/metal system performance. This paper will discuss several specific examples of the use of EIS in the study of coatings in accelerated exposure and the analysis of EIS data from such studies. The importance of cyclic vs. steady state exposure of samples will be shown by EIS results, and some of the problems in the use of standard continuous salt fog exposure as exemplified by ASTM B 117 for a coating specification will be discussed. Considering T-g effects on EIS data will show the importance of considering thermal effects in the testing of coatings. The extremely important role of water uptake in coatings during exposure will also be discussed using EIS results to analyze changes in both the coating resistance (low frequency 10 data) and capacitance (higher frequency Z data). During exposure to cyclic changes in temperature and electrolyte solution concentration, a coating over a metal substrate appears to undergo both "physical aging" and chemical degradation. The coating appears to have a memory of past exposure events such that each subsequent exposure to water and temperature creates and enlarges transport pathways within the coating for water and electrolyte. As cyclic exposure continues, damage to the bulk-coating layer above the coating/metal interface accumulates until there begins to be a permanent accumulation of electrolyte at this interface and local small-scale corrosion begins. This is the initiation of corrosion failure of the system, but it only occurs following the decrease of bulk-coating layer barrier properties caused by cyclic temperature and humidity processes characteristic of exterior exposure. This whole process can be accelerated by immersion in a flowing electrolyte, emphasizing the role of transport processes in coating degradation processes. If there is simultaneous UV exposure, as Skerry has so well described, one must also account for photodegradation of the outermost layer of the coating system. The role of the coating scientist is now to assimilate the data that EIS now provides us during the exposure process and develop meaningful models for the molecular level changes that occur in the coating film in order to enable use of the EIS results for true coating performance ranking and lifetime prediction. (C) 2003 Published by Elsevier Science B.V.