An in-situ corrosion sensor has been developed that allows early detection of coating degradation and substrate corrosion. The sensor is based on electrochemical impedance spectroscopy (EIS) and extends the use of this established laboratory technique to ambient environments and field applications. The sensor directly detects degradation of the structure or specimen of interest, as opposed to degradation of a sensor component. It has applicability both to field inspection and condition-based maintenance and to coating screening and development. A permanent, painted version is suited for inaccessible areas of structures or areas/specimens for which one wishes to take repeated measurements over time, such as a panel in an accelerated test chamber or a hidden area of an aircraft. A hand-held version is suited for spot inspection or if a permanent sensor is not desired for ascetics, aerodynamics, or other reasons. Both sensors utilize commercial potentiostats for data acquisition and analysis. The sensors are able to detect and quantify moisture absorption by a coating and the early stages of degradation well before any visual indication of deterioration. The sensor measurements have been shown to predict the amount of corrosion at the end of accelerated tests that have been correlated with service performance. They have also been used to evaluate coating systems and inspect structures in the field.
Electrochemical Impedance Spectroscopy (EIS) has long been used to evaluate coatings and study corrosion in the laboratory. 1-8 Correlations have been made with EIS parameters, such as low-frequency impedance and breakpoint frequency, and performance under different conditions. Initially, a coating exhibits capacitive behavior with very high impedance at low frequency. As the coating absorbs moisture, the impedance in the low-frequency region decreases and becomes independent of frequency (Figure 1). The low-frequency impedance is a sensitive measure of coating and substrate health; it will decrease by several orders of magnitude as moisture is absorbed and substrate corrosion occurs. This decrease in impedance occurs well before any visual indications of deterioration.
Figure 1. Left: Typical impedance spectra of a coating during degradation. Right: Low-frequency impedance as a function of exposure time showing three stages of degradation: moisture absorption, incubation, and substrate corrosion.
Conventional EIS requires immersion of a specimen into an electrolyte and the use of remote counter and reference electrodes. This procedure is suited for the laboratory as long as small specimens and immersion conditions are suitable for the study. Evaluation of larger specimens is possible with the use of flat cells (beakers without bottoms) that can be clamped to a specimen or structure and filled with electrolyte. Counter and reference electrodes are then inserted into the electrolyte and EIS spectra acquired. In some cases, gels or electrolyte-impregnated sponges can be used instead of a liquid electrolyte. This procedure allows measurements to be taken in the field in addition to the laboratory, 9-11 but requires an accessible, flat, smooth, and (preferably) horizontal surface. It provides a local indication of the coating and substrate health; data are acquired only for the area wetted by the electrolyte. The process generally is a time-consuming process involving the mounting of the cells, handling of the fragile electrodes, allowing the specimen to come into quasi-equilibrium with the electrolyte, acquiring data, removing and storing the cells and electrodes, and rinsing the surface. In some cases, the severalhour exposure to the electrolyte can cause artifactual