Chromate leaching from inhibited primers: Part II: Modelling of leaching

Leaching profiles for two chromate-inhibited epoxy polyamide primers have been analysed and non-Fickian diffusion kinetics have been observed. The time dependence was found to be t0.25 instead of the standard Fickian t0.5. The t0.25 time dependence was consistent for different pH values (1, 3, 5 and 7) even though the overall release rates increased with decreasing pH. Abrading the primer surface increased the overall leaching rate but did not change the t0.25 dependence. The unusual release kinetics cannot be explained by a decreasing effective diffusion coefficient (via changes in porosity and tortuosity or a fractal structure) or pH-dependent chemical reactions. The leaching rates have been modelled using a simple power law, a polynomial, and the shrinking core model. The polynomial model produced the best fit to leaching data and provides some insight into the source of the non-Fickian leaching behaviour.