Salt crystallization-assisted degradation of epoxy resin surface in simulated marine environments
Graphical abstract
Introduction
Organic coating as an effective anti-corrosion barrier has been widely used for protecting reactive metals [1,2]. For example, in marine environments, organic coatings are generally favorably used for prolonging the lifetime of steel structures due to their high corrosion resistance [3,4]. They also have antifouling [5] and super-hydrophobic properties [6] after their surfaces are modified with some special organic function groups. However, marine environments are extremely aggressive because of the solar radiation, high-salinity, and wet-dry cycling, in which organic coatings usually break down much earlier than expected [7,8]. To extend the service life of the structures in these environments, organic coatings as thick as several hundred micrometers are usually applied [9] to prevent the ingress of corrosive media or the attachment of bio-fouling species [10,11].
The degradation of an organic coating normally starts from the outer layer, resulting in chalking, peeling and loss of its anti-fouling or super-hydrophobic function [12]. The failure of the surface layer is the rate-determining step in a coating degradation process [13], which normally results from photo-oxidation [14,15], hydrothermal degradation [16,17], dry-wet alternation [18] and cold-hot cycling [19]. In a natural environment, these degradation processes are often mixed together. It is difficult to identify which mechanism is dominating the degradation. For example, in the aggressive atmospheric environment, the photo-oxidation resulting from solar irradiation may trigger chain scission of polymer molecules [[20], [21], [22]]. Meanwhile, the wet-dry cycling can exert a swelling-shrinking effect on the coating, and consequently downgrade or even break the coating [23,24]. Moreover, hydrolysis of organic molecules may also occur, facilitating the chain scission in the organic coating in theory. Each the process can more or less exacerbate the coating degradation.
Although the influence of solar irradiation, high salinity, dry-wet alternation, and cold-hot cycling on coating degradation and substrate metal corrosion has been widely studied to date [[25], [26], [27], [28]], the coating degradation mechanism has not been comprehensively understood. In fact, epoxy coatings are porous [10,29]. In a salty solution, they may degrade in a way similar to the weathering of a porous material (e.g. rock or concrete), because the precipitated salt can grow in volume, just like an icing process in freezing weather, which can eventually destroy the micro-porous structure. Therefore, it is possible that for an organic coating in a typical marine atmosphere, salt precipitation occurs in the pores or defects of the coating during the drying period in each wet-dry cycling, similar to the crystallization in a porous material [[30], [31], [32], [33]]. This possible crystallization-assisted degradation of organic coatings (especially the highly protective epoxy resin) in marine environments has not been realized yet or reported so far. A study on such a possible mechanism will undoubtedly contribute to current understandings of the microstructure and performance of organic coatings.
The research presented in this paper comprehensively investigated the surface state of an epoxy resin in carefully controlled wet-dry cycling environments with various salt and ultraviolet (UV) influences, aiming to verify the possible crystallization-assisted degradation mechanism. It is expected that [15,16] the new knowledge gained from this study will eventually help enhance the quality and service life of organic coatings in marine environments.
Section snippets
Epoxy sample preparation
Two-component epoxy was purchased from Xiamen Dabang Electronic Materials Co., Ltd., China. The component A consists of epoxy resin (bisphenol A epoxy resin) and solvent, while the component B is an organic amine solidifying agent. Epoxy cylindrical disk blocks (3 mm thick and 2 cm in diameter) were prepared by encapsulating the mixture of 3 part A and 1 part B at a weight ratio in polyvinyl chloride disk molds, which were cured in an oven at 30 °C for 24 h. The top free surface of the disk
Surface morphologies
The surfaces of the epoxy samples after the different weathering tests in the marine environment simulation system were observed. Fig. 1 shows their typical morphologies after 28 days of the tests and water washing. The washing should have removed all the crystallized salt particles and some of the loosely degraded epoxy or chalked products. On the original epoxy surface, there were many uniformly distributed small cracks (see Fig. 1a). On the fully immersed sample, some small “particles” could
Water effect
Swelling is a common behavior for immersed polymers, which might narrow the cracks or pores (see Fig. 1(a)) on the epoxy resin surface after immersion (see Fig. 1(b)). The swelling process is schematically illustrated in Fig. 8(a) and (b). Obviously, the water uptake and polymer swelling depend on the porosity of the epoxy resin and the salt concentration in the water [[42], [43], [44]]. The water induced swelling has been experimentally verified in previous studies [45].
In this paper the
Conclusions
- (1)
Salt crystallization can generate a pressure on the inner walls of the micro-pores in epoxy resin, which may lead to collapse of the epoxy in the surface layer, eventually resulting in larger pores, cracks and chalked or peeled-off products on the surface.
- (2)
UVA irradiation can make an epoxy resin surface more porous, resulting in chalked products formed on the degraded surface, which further exacerbates the salt crystallization effect. UVA irradiation may also facilitate the dissolution of the
CRediT authorship contribution statement
Zhenliang Feng: Investigation, Methodology, Data curation, Investigation, Methodology, Writing - original draft. Guang-Ling Song: Funding acquisition, Project administration, Supervision, Conceptualization, Formal analysis, Investigation, Methodology, Writing - review & editing. Zi Ming Wang: Methodology, Validation. Yuqing Xu: Data curation, Validation, Resources. Dajiang Zheng: Methodology, Resources, Supervision, Software. Pengpeng Wu: Validation, Resources. Xiaodong Chen: Validation,
Declaration of Competing Interest
The authors report no declarations of interest.
Acknowledgments
The authors acknowledge the support by the National Key Research and Development Program of China (Grant No. 2017YFB0702100), Science and Technology Planning Project of Fujian Province (2018H6017) and the National Natural Science Foundation of China (key project Grant No. 51731008 and general project Grant No. 51671163).
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