Salt crystallization-assisted degradation of epoxy resin surface in simulated marine environments

Highlights

• Salt crystallization accelerates the degradation of epoxy in the surface layer.

• UVA irradiation further exacerbates the salt crystallization effect.

• There is a possible synergistic effect on epoxy degradation between the UVA irradiation and the NaCl solution wet-dry cycling.

• The salt-accelerated UVA degradation may be employed to rapidly evaluate the performance of an epoxy resin coating.

Abstract

Epoxy resin samples were cyclically exposed to ultraviolet A (UVA) and a 5 wt% salt solution in a specially designed marine environment simulation system. Surface analyses indicated that UVA irradiation could accelerate the degradation of the epoxy surface and the dissolution of the degraded products, while the salt crystallization in the drying process could further accelerate the chalking and peeling of the degraded products. A possible synergistic effect between the UVA irradiation and salt crystallization was proposed to interpret the degradation, which may provide a new insight into the damage of organic polymer materials in marine environments.

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.