Temperature Effects on Two-Cell Field Impedance Testing of Epoxy and Polysulfide-Modified Epoxy Coatings

Abstract

Two-cell field impedance spectroscopy provides coating condition assessment in situ, i.e., on coated structures. This study evaluated the dependence of impedance data on environmental conditions encountered during testing, especially temperature. Field impedance spectroscopy measurements were applied to two coated steel substrates over the course of a diurnal heating-cooling cycle. One coated substrate provided a high-barrier epoxy and the second a low-barrier polysulfide-modified epoxy (polysulfide). The temperature range examined was approximately 80 degrees Fahrenheit (ºF) to 110ºF for air, surface, and solution temperature, with baseline measurements made at 70ºF. The results showed coating impedance decreases as the coating surface temperature increases. The coatings showed time-dependent recovery of their initial properties during the cooling cycle, with the greater hysteresis observed by the high-barrier epoxy. Linear regression of temperature and impedance provided a strong correlation, with R2 exceeding 0.80. Therefore, field impedance data with varied temperatures can be corrected via a linear regression formula derived by measuring one sample at two or more temperatures within the encountered range. Coating surface temperature is preferred based on a stronger correlation with impedance than atmospheric or test cell solution temperatures. An Arrhenius relationship is also acceptable for temperature correction and may be preferred where the highest accuracy is needed. This correlation had an R2 of 0.96 or higher, demonstrating a logarithmic dependence of impedance on the inverse of the absolute temperature, i.e., the temperature in Kelvin. The effect of humidity on the impedance was low and could not be resolved from the strongly correlated effect of temperature. The repeated impedance testing, about 40 repetitions at a single location, applied in this experiment showed no discernable effect on the coating impedance, suggesting the voltage amplitudes utilized of 100 mV for low-barrier polysulfide and 500 mV for high-barrier epoxy were nondestructive.