Graphene Oxide-Based Multi-Functionalization Coatings: Comparison
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Graphene oxide (GO), derived from the two-dimensional nanosheet graphene, has received unprecedented attention in the field of metal corrosion protection owing to its excellent barrier performance and various active functional groups. 

  • graphene oxide
  • two-dimensional fillers
  • surface modification

1. Introduction

Generally, the traditional composite coatings focus more on the long-term performance, durability as well as reliability for metal protection. However, when facing a complex service environment, the multi-functional anticorrosion coatings gradually attract more and more attention. These multi-functionalization anticorrosion coatings that are self-sensing, self-healing, wear-resisting, antibiosis and super-hydrophobic have intensively been developed to promote advanced applications [72][1]. To meet the demands of the multi-function in composite coatings, GO plays a significant role due to its excellent properties discussed above. Instead of focusing on its dispersion and compatibility, this part discusses several kinds of GO-based multi-functionalization coatings.

2. Self-Healing Composite Coatings

As discussed above, the presence of microcracks in polymer coatings degrades the structural stability and lifespan of the protective layers. In general, once the anticorrosive coating is applied and fully cured on the substrate, it is difficult to inspect the coating for damage and repair it in a timely manner. As a result, accomplishing coatings with self-healing properties has attracted considerable attention for polymer systems. The strategy of self-healing is mainly based on a responsive approach to repair material damage during its service life. External environment, such as vibration, pressure, pH value, heat, light and humidity can stimulate active response to change. Corrosion inhibitors can protect the metal by delaying or even inhibiting the corrosion. However, the direct addition of corrosion inhibitors to coating formulations can cause a deactivation of the inhibitor and/or fast degradation of adhesion and barrier properties of the coating [73,74][2][3]. To solve these issues, micro- or nanoparticles encapsulating/loading inhibitors were embedded in coatings [75,76,77,78][4][5][6][7].
GO, with outstanding barrier performance and rich surface groups, can also serve as a container of corrosion inhibitors in the field of intelligent self-repair. Microcapsules with reasonable mechanical stability, thin wall thickness and high loading capacity act as permeation barriers to prevent not only the diffusion of corrosive medium but also the leakage/solidification of healing agents. On this occasion, GO with outstanding impermeability, high specific surface area and abundant surface functional groups can be fabricated not only as platforms for stimuli-responsive nanocontainers but also as fillers to block corrosive components. Corrosion inhibitors can be entrapped in its capsules, and the existing pores of the container allow intelligent release by different triggering factors [79][8].
As is shown in Figure 1, the containers with inhibitors are well dispersed in the polymer matrix to enhance the “labyrinth effect” when the composite coatings are complete, which prolongs the path of corrosive media or even blocks their invasion to the substrate. At this stage, the containers serve as the physical barrier to improve the corrosion resistance of composite coating. On the other hand, once the damage of coatings occurs, corrosive substances from the environment penetrate the coating and reach the metal substrate. Thus, the microcurrent is easily generated in the microscopic areas and the micro-corrosion happens without any hesitation. Straightaway, a “stimulus signal” prompts inhibitors to be released, and the released inhibitors will be adsorbed on the metal surface, exerting their anticorrosion effect.
Coatings 13 01120 g009 550
Figure 1. Anti-corrosion and self-repair mechanism of coatings: (a) complete coating, (b) damaged coating, (c) local corrosion occurs and (d) coating self-healing [80][9].
Zhou et al. [81][10] reported the successful fabrication of a novel dual self-healing anticorrosion coating based on benzotriazole loaded TiO2 nanocapsule (BTC) modified GO sheets and the multibranched waterborne polyurethane (WPU). GO sheets were first modified with amino groups (GO-NH2) and then grafted with 3-isocyanatopropyltriethoxysilane (GNI) to further react with the BTCs (GNI-BTCs) as shown in Figure 2. After a series of reactions, BTCs were well dispersed on the surface of GNI sheets to improve the dispersion of GNI into the waterborne coatings.
Coatings 13 01120 g010 550
Figure 2. Fabrication of the dual self-healing anticorrosion coating [81][10].
Luo et al. [82][11] used isophorone diisocyanate, polytetramethylene ether glycol, dimethylglyoxime and glycerol to prepare polyurethane. GO was added to obtain a polyurethane/GO composite with self-healing and shape memory properties. Yu et al. [83][12] fabricated GO microcapsules containing light-curing epoxy resin based on the Pickering emulsions in a single step. The chemical stability of microcapsules was improved through chemical stitching of GO nanosheets with polyether amine. A self-assembly process was employed by Li et al. [84][13] to prepare the graphene oxide microcapsules (GOMCs) containing linseed oil as the healing agent. The nanometer-thick shells of GOMCs were built by the liquid crystalline assembling of GO sheets, forming at the liquid–liquid interface in Pickering emulsions. The as-prepared composite coatings not only possessed self-healing properties, but also showed excellent anticorrosion properties based on the physical barrier property of the GO shell. Chen et al. [85][14] developed a new strategy to improve anticorrosion performance of epoxy coatings, in which halloysite nanotubes (HNTs) were combined with GO. On one hand, the corrosion inhibitors loaded in nanocontainers would be released when the coating was damaged and subsequently prevented metal from further corrosion because of the pH-responsive ability of the nanocomposite. On the other hand, GO exerted the physical barrier property to protect the metal matrix against a corrosive medium.
In all, in order to avoid the direct addition of corrosion inhibitors that could be harmful to the polymer matrix, the GO-based containers provide a buffer for the release of an inhibitor and achieve long-term protection. When corrosion occurs, the GO-based containers can release corrosion inhibitor and repair the corroded area in time, realizing the self-healing effect.

3. Self-Warning Composite Coatings

Generally speaking, microcracks that are not easy to detect are the initial stage of various properties degradation of polymer materials. Lacking timely detection and repairment of these defects, corrosion deterioration may develop rapidly, resulting in rapid degradation of the protective coatings for substrate matrix. Therefore, the self-warning ability of anticorrosive coating materials is of great significance in practical applications. Timely sensing of the coating damage leads to proper healing and maintenance procedures to improve structural integrity and avoid unexpected failure [86][15]. It is of great significance to provide autonomous early warning by detecting the corrosion reaction at an early stage. Only in this way, the timely maintenance can be conducted. Although urgent as it is, this kind of research is still in the preliminary stage and needs more investigation.
Metal corrosion usually involves two reactions. Anodic reaction is metal electron loss oxidation to generate related metal ions, which may be accompanied by hydrolysis and acidification of metal ions according to environmental conditions. Cathode reaction includes electron reduction of oxygen or water in etched areas to form hydroxide ions [87][16]. Once corrosion occurs, a change in the concentration of metal ions is inevitably generated, while possible acidification due to the formation of hydroxide ions also results in a change in the pH of the etched area.
Ionic indicators are related to the detection of various metal ions. For example, sulfosalicylic acid and potassium thiocyanate are indicators of ferric ion; rhodaminohydrazine and phenanthroline are indicators of ferrous ions; 8-hydroxyquinoline and coumarin have a fluorescence response to aluminum ion; rhodamine ethylenediamine is an indicator of copper ions. Common pH indicators are sensitive to pH change within a given pH range. Here listed several pH-based indicators that can be used for corrosion monitoring in Table 1.
Table 1. Some pH-sensing molecules that have the potential for corrosion detection.
Sensing Molecules pKa Transition pH Range
Phenolphthalein OH− 8.2–10.0
Bromocresol green OH− 8.0–10.7
Cresol red OH− 7.2–8.8
Methyl red H+ 4.2–6.2
Bromothymol blue H+ 6.0–7.6
Similarly, it is not suitable to add the indicator directly to the coating, which may lead to their early leakage and properties reduction in the polymer matrix. In the study of Tiago et al. [88][17], the coating with direct addition of phenolphthalein performed even worse than that of pure coating samples. As their work presented, the uncoated phenolphthalein reduced the crosslinking degree of resin during curing. On the contrary, with the protection of the silica nanocapsules shell, detrimental interaction with the active compound in the coating could be minimized, which proved the importance of encapsulation of phenolphthalein by the container.
As a result, combined with GO, a novel design of corrosion alarming coating was proposed. Take for instance, 1,10-phenanthroline (Phen) was employed as the corrosion indicator, which was grafted on GO surfaces so that the Phen could distribute uniformly in the polymer matrix. In the work of Li et al. [89][18], they prepared a self-sensing polymer composite coating with functionalized GO. The GO was chemically modified with Phen which could form a red complex with Fe2+ to realize a corrosion alarm at an early stage. At the same time, the addition of laponite RD (laponite) improved the dispersion of Phen-modified GO in the polyurethanes (PU) matrix. Results illustrated that the as-prepared composite coatings showed higher corrosion resistance compared to the pure PU coatings.

4. Other Multi-Functional Coatings

In addition to excellent barrier properties, its large surface area, rich surface functional groups and good mechanical properties also promote the GO application in coating fields of super hydrophobic, antibacterial, wear resistance and so on. GO with abundant oxygen-containing functional groups and nanoscale size was prepared and incorporated into waterborne polyurethanes (WPU) by chemical grafting to improve the dispersion in WPU [26][19], resulting in excellent mechanical properties and solvent resistance of the coating. Besides the good compatibility with WPU, the tensile strength of that coating film increased by 64.89%, and the abrasion resistance and pendulum hardness increased by 28.19% and 15.87%, respectively. This chemical grafting strategy provides a feasible way to improve the dispersion of GO in coatings and of reference value in the modification of waterborne coatings.
Superhydrophobic silane/GO composite coating, with excellent anticorrosion performance and durability, was also successfully synthesized on copper surface using simple dipping and subsequent curing procedure [90][20]. The prepared superhydrophobic silane/GO composite coating possessed the largest impedance modulus and the protection efficiency was over 99% after exposed to 1 M NaCl solution for 120 h. Apart from these applications listed above, studies on the nanotribological properties of GO indicated that excellent lubrication performance and wear resistance of GO made it a potential high-performance nano-lubricating material [91,92][21][22]. Zhang et al. [93][23] mixed GO with WC-17Co alloy powder. GO was embedded in the coating as transparent thin sheets. The friction coefficient of the GO coating was reduced by approximately 22% compared to that of the original coating. The formation of lubrication films in the micro-area improved the self-lubrication and anti-wear effects of the coatings. Similarly, GO was incorporated into WC-12Co powder via wet ball milling and spray granulation, and GO was embedded in the structure in a transparent and thin-layer state [94][24]. Compared to the friction coefficient (0.6) of the WC-12Co coating obtained at room temperature, the friction coefficient of the GO/WC-12Co coating was decreased by approximately 50%. In another work [95][25], the NiCr-WC-Al2O3 composites with the addition of GO were fabricated by powder metallurgy technique. Results showed that in high temperature ranges, the composites with the addition of 3 wt% GO exhibited the lowest friction coefficient (0.41) and wear rate (1.0 × 10−5 mm3/Nm) at 700 °C. In the above studies, the improvement of GO nanosheets to the tribological performance of metal is mainly due to excellent mechanical and tribological properties. In addition, GO is thought to be a promising antibacterial material. The oxygen-containing functional groups endowed GO with good hydrophilicity, dispersity and biocompatibility, making GO a promising biomedical applications candidate [96,97,98][26][27][28]. A number of studies have proved the strong antibacterial activities of GO, and its antibacterial activity is considered to be mediated by the physicochemical interaction between GO and microbes [99,100][29][30].
In all, the multi-functional composite coatings refer to the materials with practical functions including self-healing, self-warning, superhydrophobic, wear-resistant, antibacterial, etc. Apart from enhancing the barrier property of coating as reinforcements/fillers, more efforts should be put into the study of multi-functional coatings based on the excellent properties of GO.

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