| The corrosion of heat-transfer walls is one of the main failure causes of heat transfer equipment, and has long been a much vexed issue in corrosion and protection field. The discovery of two-dimensional materials has provided new opportunities for the development of corrosion protective coating for heat-transfer equipment because two-dimensional materials have a unique crystallographic structure and many particular physical and chemical properties. Two-dimensional materials are now various, among which the most typical ones are graphene and graphene analogues. Graphene is believed to be the "thinnest" protective material due to its single-layer atomic structure and impermeability to molecules. However, as graphene is a conductive carbon material, protective graphene/polymer composite coatings, once damaged, may accelerate the corrosion of metal substrate through inducing a micro-galvanic corrosion between graphene and the metal substrate. This phenomenon indicates that graphene has "corrosion-promotion activity", and that greatly limits its application in the field of anti-corrosion. In addition, graphene analogues are two-dimensional materials with impermeability similar to graphene. However, research on the anti-corrosion performance and "corrosion promotion activity" of graphene analogues is still lacking. Starting from the problem that graphene can accelerate the corrosion of metal substrates, this paper proposes two strategy to promote the research and development of two-dimensional materials for the protection of heat-transfer equipment, including an insulation modification strategy of graphene to inhibit the "corrosion promotion activity" and a developing strategy of using insulating or semi-conductive graphene analogues as corrosion protective materials. The anti-corrosion performances of the as-prepared materials are investigated in this work. And the "corrosion promotion activity" and its inhibition mechanism of the materials is further revealed, based on which a selection criterion of semiconductor materials for anti-corrosion is also proposed. The main contents of this paper include the following aspects:(1) Investigation on the anti-corrosion performance of "passivated" graphene/polymer composite coatings and the inhibition mechanism of their "corrosion-promotion activity". Aniline, tetraethoxysilane and 3-aminopropyl triethoxysilane is used to carry out the insulating modification of graphene by means of in-situ modification technology, respectively, through which a series of lowly-conductive "passivated" graphene materials with a sandwich structure can be successfully prepared. This work reveals that the "corrosion-promotion activity" of graphene can be fundamentally inhibited through the insulating modification strategy because insulating material adsorbed on graphene surface can eliminate the electrical connection between graphene and metal substrate, and that hinders the formation of the graphene-metal microgalvanic corrosion, resulting in the fact that damaged "passivated" graphene/polymer composite coating cannot promote the corrosion of metal substrates. And the structure and surface properties of "passivated" graphene can be tuned by using different insulating materials to modify graphene. Furthermore, the as-prepared "passivated" graphene exhibits a flake-like structure, and "passivation" treatment can increase the stiffness of graphene materials. Compared with pristine graphene, "passivated" graphene is more likely to spread and disperse well in the coating matrix, resulting in a more effective "labyrinth effect" which prevents corrosive electrolyte from penetrating through coatings. The incorporation of "passivated" graphene into polyvinyl butyral coating matrix increases the coating resistance by 4-6 orders of magnitude and extends the coating lifetime during corrosion tests by at least 20 times. The strategy of graphene "passivation" via insulation modification can be generally applicable to molecules that can adsorb on the surface of graphene. It not only provides a new solution for the bottleneck problem in applying graphene for anti-corrosion, but also provides theoretical and technological supports for the development of graphene-based composite coating with a highly-effective coordination of thermal conductivity and anti-corrosion performance.(2) Investigation on the anti-corrosion performance of graphene analogues/polymer composite coatings and the mechanism of their "corrosion-promotion activity". Three graphene analogues with multilayer structure are prepared by a liquid-exfoliation technique, namely h-BN nanosheets (BNNSs), WS2 nanosheets (TDNSs) and MoS2 nanosheets (MDNSs). Experiments have shown that the coating resistance increases by 4-6 orders of magnitude at only 1.0 wt.% filler loading. Once damaged, all the composite coatings do not exhibit "corrosion-promotion activity" and cannot accelerate the corrosion of metal substrates, indicating that it is feasible to avoid the damage caused by "corrosion-promotion activity" via developing insulating and semi-conductive graphene analogues two-dimensional material as a substitute graphene for anti-corrosion. Considering factors that influence the resistance of electron transfer between the fillers and the metal substrates and the consumption rate of electron participating in cathodic reduction reaction that occurs on the surface of fillers, the nature of "corrosion-promotion activity" of graphene analogues is revealed after a further in-depth analysis. And a basis for selecting graphene analogues for anti-corrosion is proposed by considering their work function value, catalytic activity of cathodic reaction and conductivity, enriching the theoretical study of corrosion fillers. Using as anti-corrosion materials opens up new application areas of graphene analogues, and it will has a very broad application prospects in the field of preparing anti-corrosion coatings with multiple functions, e.g. insulating properties, good thermal-conductive properties, high abrasion resistance, etc. |
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