Description
Organic coatings that come into contact with aqueous solutions are prone to coating degradation. The first steps in this process are commonly identified as to be water uptake and uptake of corrodents, in the form of ions [1, 2]. These steps happen in conjunction, although water uptake precedes the ion uptake. To identify the water uptake in organic coatings, Electrochemical Impedance Spectroscopy (EIS) is widely used because it is an in situ and non-destructive technique that is also capable of measuring other states of coating degradation, including ion diffusion and the onset of corrosion. However, with most EIS techniques, the system needs to be stationary before meaningful impedance spectra can be obtained.In this work, the initial behaviour of model organic coatings shortly after coming into contact with an aqueous solution is studied using Odd Random Phase Electrochemical Impedance Spectroscopy (ORP-EIS). The coatings under study display rapid water and ion uptake, and therefore non-stationary behaviour in the first two hours after immersion. This shows a need to resolve the impedance spectra over time, which can be done using the instantaneous impedance calculation from ORP-EIS results [3]. The calculated spectra show a non-ideal capacitive behaviour. The cause of this non-ideal behaviour is identified by fitting with Voigt elements and looking at the resistivity distribution over the thickness of the coating [4]. Interestingly, two distinct layers can be identified using this method.
The two different layers are most likely related to the ion diffusion inside the coating, as is described by the two-layer model identified by Nguyen et al. [5]. Ions are penetrating inside the outer layer, whereas no ions are present in the inner layer yet. Using this model to fit the EIS results, many physical parameters can be obtained, such as the water uptake, the resistivity of the coatings and the penetration depth of ions in the coatings. It is found that the coatings under study reach saturation of water uptake within two hours after immersion, identifying the need for the instantaneous impedance approach for accurate results. The physical interpretation of the fitting results is verified using DSC measurements, identifying the physicochemical structure of the coatings, and gravimetrical analysis for the water uptake.
[1] H. Leidheiser Jr., Progress in Organic Coatings 7 (1979) 79-104
[2] M. Kendig and J. Scully, Corrosion 46 (1990) 22-29
[3] T. Breugelmans et al., Electrochimica Acta 76 (2012) 375-382
[4] Y.-M. Chen et al., Electrochimica Acta 219 (2016) 312-320
[5] A. S. Nguyen et al., Corrosion Science 109 (2016) 174-181
| Period | 30 Aug 2021 |
|---|---|
| Event title | 72nd Annual Meeting of the International Society of Electrochemistry |
| Event type | Conference |
| Degree of Recognition | International |