Diffusion of mineral acids into an epoxy coating. Phosphoric acid diffusion model

• V.A. Golovin, A.B. Il’in and A.D. Aliev

A.N. Frumkin Institute of Physical Chemistry and Electrochemistry, Russian Academy of Sciences, Leninskii pr. 31, 119071 Moscow, Russian Federation

Abstract: A series of studies on the diffusion and sorption of phosphoric acid solutions with concentrations of 54–87 wt% into epoxy coating materials was carried out. Local X-ray analysis and luminescence analysis were used to determine the penetration depths and to plot concentration profiles of diffusion penetration of several mineral acids. It was shown that in the case of phosphoric acid, in contrast to hydrochloric and sulfuric acids, a significant gradient of diffusant concentration was observed in the diffusion zone, and the distribution profile in the diffusion zone had indications of a 2-step structure. The diffusion data obtained and the results of thermodynamic calculations were used to determine the parameters of the diffusion penetration model with partial chemical binding of the penetrating acid. The diffusion model with hydrates of constant composition (H₃PO₄·nH₂O, where n = 10–11) in the inner solution was shown to describe correctly (R² = 0.9986) the kinetics of the penetration of concentrated solutions of phosphoric acid. The use of the diffusion model combined with a calculation of the chemical potential of non-volatile electrolytes based on the Gibbs–Duhem equation makes it possible to significantly reduce the amount of diffusion studies and simplify the prediction of the impermeability time of coating materials in highly corrosive media, including nonvolatile ones.

Keywords: epoxy coatings, phosphoric acid, concentration profile, diffusion model, Gibbs–Duhem equation, hydrate number, fluorescence spectrum, phosphorescence time

Int. J. Corros. Scale Inhib., 2021, 10, no. 4, 1587-1605
doi: 10.17675/2305-6894-2020-10-4-13

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