Diffusion cell investigations into the acidic degradation of organic coatings

Protective organic coatings work by preventing contact between an aggressive environment and a vulnerable substrate. However, the long required lifetime of a barrier coating provides a challenge when attempting to evaluate coating performance. Diffusion cells can be used as a tool to estimate coating barrier properties and lifetime. In this work, a diffusion cell array was designed, constructed, and compared to previous designs, with simplicity being the most important design parameter. Sulfuric acid diffusion through five different coatings was monitored using a battery of cells, and a mathematical model was developed to simulate the experimental data and to study diffusion mechanisms. The diffusion cells allowed an objective and fast analysis of coating barrier properties. It was found that sulfuric acid deteriorated these properties as it diffused through the films. This was also expressed in the modeling results, where a three-step time dependency of the diffusion coefficient was required to simulate both acid breakthrough time and the subsequent steady-state flux. A vinyl ester-based coating proved to be the most effective barrier to sulfuric acid diffusion, followed by a polyurethane coating. Amine-cured novolac epoxies provided the least effective protection.