Characterisation of rapid water uptake in model coatings using instantaneous impedance

Organic coatings are a key component in corrosion protection for a wide array of metal constructions. Breakdown of these coatings can occur due to exposure to water during prolonged periods of time. The water uptake of organic coatings has been studied extensively in literature [1]. It is understood to proceed in three different steps, the first of which is identified as homogeneous diffusion, after which saturation occurs, and later heterogeneous water accumulation and coating breakdown. The water uptake in these coatings can be studied with electrochemical impedance spectroscopy (EIS) [2]; the coating capacitance is followed as a function of time, and through the extraction of the dielectric constant of the coating the water content in the coating is estimated through the Brasher-Kingsbury formula. Recently, a study was published that challenged the validity of this equation [3], since it does not take into account the swelling of the coating.

In this work, the initial water uptake for model coatings is studied through odd random phase electrochemical impedance spectroscopy (ORP-EIS) [4]. The model coatings are based on acrylic and/or methacrylic backbones. Several different monomers are used to form the polymer matrix of different model coatings, phosphonic acid or carboxylic acid are used as adhesion promoters. Another batch of model coatings feature no adhesion promotor as a comparison. From gravimetrical measurements it is found that the first step in the water uptake from these coatings, Fickian diffusion, already occurs within the first 20 to 30 minutes. Therefore, it is crucial that the water uptake is measured immediately after immersion of the coating in solution. As the coating properties are changing rapidly, non-stationary behaviour is observed. This non-stationary behaviour can be quantified using the time-resolved instantaneous impedance calculation [5] from ORP-EIS measurements, and the instantaneous coating capacitance can be found through electrical equivalent circuit fitting of the results. The results of the different studied coatings are then compared. As a secondary objective, the most fitting equation between the observed coating capacitance and water uptake in this case study is searched for.