**1. Introduction**

The sea is a potentially huge source of energy, but the development of Marine Renewable Energy (MRE) technologies able to harness that energy is at a relatively early stage [1]. Part of the problem is that seawater is an aggressive medium, and that metallic structures are submitted to corrosion. An effective solution to protect against corrosion is organic coatings [2,3] which can be applied with high thicknesses and/or can be associated

**Citation:** Renaud, A.; Pommier, V.; Garnier, J.; Frappart, S.; Florimond, L.; Koch, M.; Grolleau, A.-M.; Puente-Lelièvre, C.; Sebastien, T. Aggressiveness of Different Ageing Conditions for Three Thick Marine Epoxy Systems. *Corros. Mater. Degrad.* **2021**, *2*, 721–742. https://doi.org/ 10.3390/cmd2040039

Academic Editor: Markus Valtiner

Received: 30 September 2021 Accepted: 30 November 2021 Published: 3 December 2021

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**Copyright:** © 2021 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

with cathodic protection [4]. Among them, epoxy-based marine coatings have been widely applied for the anticorrosion of metallic materials due to their low cost and their efficiency in aggressive mediums such as seawater [5,6]. However, all organic coating formulations have to be tested before being applied in order to offer a guarantee in terms of lifetime and effectiveness [7] because they suffer from degradation from the environment. In the past, much research has been devoted to evaluating the performance of organic coatings using natural or artificial tests [2,8–16].

Because anticorrosive coatings for marine environment are usually very robust, accelerated ageing tests are needed in order to promote the coating degradation. The physicochemical properties of the coatings are monitored during these tests, and many fields in material chemistry and physics are involved [17]. The coatings are often tested using standardized methods, such as the salt fog, the climatic chamber, the Q-UV chamber, thermal cycling, etc., and, in some cases, a combination of these methods is proposed. Many accelerated ageing cycles are used in practice to try to find a balance between various tests and obtain the most natural possible exposition [18]; however, very often, there is not a good correlation between accelerated ageing tests and degradation in a natural environment [19]. Research is still in progress in order to find reliable protocols and/or techniques to better understand organic coating degradation [20–29]. However, it is necessary to keep in mind that the degradation mechanisms have to be the same as the real life conditions [30], and that only the intensity of the degradation factors (temperature, humidity, UV radiation, mechanical stresses, etc.) can be increased, respecting the limit properties of the organic material. It is known that the continuous salt fog test is unrealistic [30–32], and a recent work showed that it did not correlate with other tests [33]. On the other hand, a simple UV-accelerated test is not sufficient to simulate the natural exposure and it is important to study the synergic effect of different accelerated factors (for example, UV radiation plus thermal cycling) on many different types of organic coatings [10,32]. In fact, fast temperature changes with high humidity and low humidity periods appear as the most harmful accelerated ageing conditions [9,12,34]. Recently, Qu et al. [35] used different wetting and drying cycles for the ageing of a coating system constituted by a thick epoxy primer and an acrylic polyurethane topcoat. They found that, the longer the wetting time in one cycle, the more serious the degradation of the coating samples. For offshore applications, organic coatings can be tested according to ISO 20340 standard, which alternates UV/condensation (ISO 11507), neutral salt spray (ISO9227), and a freezing phase ( −20 ◦C), with a total duration of 4200 h. However, Le Bozec et al. [33] showed that there was no impact from the freezing phase and that the ISO 20340 cycle showed a satisfying correlation with test variants where UV/condensation was replaced by a wet and dry transition at a similar temperature. Davalos-Monteiro et al. [36] also compared ISO 20340 conditions to natural exposure testing, but attempts to correlate natural exposure results with cyclic test results proved unsuccessful. The authors highlighted the requirement of performing further research to provide an indication of whether a correlation with natural exposure might or might not be possible.

As one can see, the perfect accelerated ageing test that fully represents service conditions may not exist because of the very numerous factors that have to be considered, including the chemical nature of organic coatings. However, attempts to find realistic ageing conditions that one allow to accelerate the coating degradation and to rank different systems must be undertaken [37]. This work aimed to study the ageing of an epoxy coating system applied onto three different metallic substrates, using different ageing tests in natural or artificial seawater, neutral salt spray, and an alternate immersion test, in order to evaluate the aggressiveness of the different ageing conditions. Coated systems with and without initial artificial defect were considered and monotonous ageing conditions were compared to cyclic ageing conditions in order to define the most aggressive ageing condition for thick marine epoxy coatings. The evaluation of the coating degradation was performed by visual inspection (corrosion degree ISO 4628-8) for panels with defect and by electrochemical characterization (barrier properties, ISO 16773) for defect-free panels.
