Current Density Through a Polymer Coating

The model must deal with the situation encountered when the electrodes are coated with a polymer and ionic leakage eventually causes failure. Ionic species and molecules diffuse to the electrodes as migration takes place under the effect of the electric field, the latter governed by the electrolytic process. Before steady state is attained, there is a transient period during which the incoming diffused species and the migration component establish concentration profiles and an equilibrium ionic concentration at the electrodes. 

At equilibrium, the rate of ionic transpat due to the diffusion component Iq from the sample surface equals the transport component i between the electrodes. 

The mass flux density, Q, of water and other species that diffuse from the outside through the polymeric film, destined to be electrolized on arrival at the electrodes, can be expressed as  

The following differential equation describes the process to a first approximation  

The time-to-fallure, t (which is implicit in Eq. 20) occurs when the current reaches afaii criterion i(, ForZFDE/(RTd)t 1, which requires t 1000 hrs at test temperature, Eq. 20 can be rewritten as  

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