A bulk and localised electrochemical assessment of epoxy-phenolic coating degradation

Water absorption is believed to be one of the main causes leading to the deterioration and degradation of protective organic coatings. Water uptake in coatings on metal may be measured, for example, by changes in electrical impedance (e.g. capacitance and resistance) as a function of time. Generally coating capacitance is expected to increase during initial stages of water uptake (due to the greater dielectric constant of water) while coating resistance is expected to decrease (due to the lower resistivity of water compared to most polymers). However, here we present evidence that water ingress is not the main determinant of damage in epoxy-phenolic can coatings. Thus, as the degree of cure is increased, the water saturated coating capacitance and resistance both increase while the time-to-failure also increases. We suggest that these observations are due, respectively, to an increase in the free polymer volume (permitting more water uptake) and to an increase in the charge transfer resistance at the metal-polymer interface (due to a higher density of polymer-to-substrate bonding). These results and interpretations are supported by local electrochemical impedance spectroscopy which has confirmed water absorption, coating failure and increased coating resistance for highly cured capacitive systems, at the microscale.