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New EEC model separates water and ion transport in coatings
Researchers use odd random phase electrochemical impedance spectroscopy combined with a two-layer equivalent circuit model to independently characterise water and ion diffusion in organic coatings. The approach reveals how anion type, electrolyte concentration and pigment volume concentration jointly govern transport behaviour.
Understanding how water and ions penetrate organic coatings is central to evaluating their protective performance against corrosion. However, distinguishing the diffusion of water from that of ions has remained a challenge with conventional characterisation methods. A recent study addresses this gap by combining odd random phase electrochemical impedance spectroscopy (ORP-EIS) with a physically relevant two-layer electrochemical equivalent circuit (EEC) model to independently track both transport processes across a coating’s thickness.
The researchers investigated coatings with different pigment volume concentrations (PVC) and systematically varied the type and concentration of the electrolyte. The ion-ingress front position was resolved from the ORP-EIS data through the two-layer fit, and its validity was cross-checked using glow discharge optical emission spectroscopy (GDOES) depth profiling, which confirmed the modelled ion transport behaviour.
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Anion nature and electrolyte concentration shape transport
The analysis showed that higher PVC values promote both ion diffusion and water uptake, consistent with an increased number of pathways through the pigmented matrix. Increasing the electrolyte concentration had a differentiated effect: it enhanced ion transport but reduced water uptake, indicating that the two mechanisms respond independently to the surrounding chemical environment.
Comparative studies of different anions further revealed that ion mobility through the coating is strongly influenced by hydration-shell size. Anions with larger, more strongly bound hydration shells diffused more slowly, while less hydrated anions penetrated the coating more readily. According to the authors, the two-layer methodology provides a robust framework for separating water and ion diffusion contributions in coating systems, offering formulators a more precise tool to interpret barrier performance and to guide the development of protective coatings tailored to specific service environments.
Source: Jiryaeisharahi, Z. et al., On the effects of anion nature and electrolyte concentration on decoupled water and ion diffusion in organic coatings: Insights from two-layer EEC-modelled ORP-EIS data. Progress in Organic Coatings (2026). https://doi.org/10.1016/j.porgcoat.2026.110298
