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

Abstract: 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 t 0.25 instead of the standard Fickian t 0.5. The t 0.25 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 t 0.25 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. [Copyright &y& Elsevier]