Monitoring degradation of single and multilayer organic coatings - I. Absorption and transport of water: Theoretical analysis and methods

This paper presents a mathematical model describing the change in the capacitance of both single and multilayer organic coatings exposed to a wet environment. The indication is that: the capacitance technique can successfully be employed to monitor, in situ, the absorption of water through single-layer organic coatings for values of equilibrium water uptake to a maximum of 5% by volume. In addition, this model indicates a relationship for d(ln C)/d root t, where C is the coating capacitance and t is the time of exposure to the environment, that can be used to rank different single-layer coatings. Conversely, no quantitative correlation between mass uptake and coating capacitance was found for a multilayer coating system, even for low values of water uptake. Although, at face value, these findings suggest that this technique may be inappropriate for in situ monitoring of water absorption in such complex systems, a comparison between gravimetric and capacitance data indicates that useful qualitative information may still be generated by the use of the latter technique.