Effectiveness of non-Fickian diffusion model on the water uptake determination of organic coatings

Water uptake and permeation rates are critical parameters for the assessment of anti-corrosive organic coatings. Theses parameters can be estimated conveniently using electrochemical impedance spectroscopy measurements (EIS). Most of the defects and the origin of catastrophic failures of organic coatings are associated with water and concomitant pollutants transfer through the protective film to the substrate.

Although, the diffusion of water in organic coating is well understood, there is a lack of experimental comparison between in situ electrochemical techniques and standardised methods for water uptake determination in free-standing films.

Continuous immersion

In a recent study, the anti-corrosive performance and water uptake rates in 4 commercial anti-corrosive organic coatings applied on galvanised steel were determined under continuous immersion in 0.6 mol dm⁻³ NaCl and 0.6 mol dm⁻³ Na₂SO₄ by in situ EIS measurements. Parameters associated with water uptake were compared against water vapor permeation rates in free-standing films measured by wet-cup method (ASTM-D1653).

Consistent and suitable methods to study the diffusion of water

It was found that EIS estimations of water permeation rates only agree with the wet-cup method when non-Fickian diffusion and coating swelling effects were considered. Remarkably, the presence and nature of two contaminants (Cl⁻ and SO4²⁻) did not affect considerably the permeation rates but had a significant impact on water saturation times and swelling. EIS and wet-cup methods demonstrated to be consistent and suitable methods to study the diffusion of water. However, EIS provided a more detailed survey of water uptake and associated degradation processes.

The study has been published in Progress in Organic Coatings, Volume 136, November 2019, 105206.

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