A review on graphene-based polymer composite coatings for the corrosion protection of metals

3.1.1 Using conductive polymer matrix systems

CPs are widely used as anticorrosion materials because of their superior environmental stability and exceptional redox properties linked with the chain of nitrogen (Yao et al., 2008). CPs can be readily deposited in the form of an electroactive film directly on the metal surface by electropolymerization technique. These CP conductive films when formed along with suitable dopants/anions create exceptional redox reactions with the base metal. Electroactive polymers when reacted with graphene nanoplatelets form nanocomposite coating that provides double layer protection against metal corrosion. In the first stage of the protection mechanism, the conductive polymer forms a passive oxide layer on the metal surface through redox reactions. In the second stage, the freely dispersed graphene nanosheets in the polymer matrix decreases O₂ and H₂O molecules penetration by increasing the tortuosity of the diffusion pathways (Jafari et al., 2016). The commonly used conductive polymers for coating applications are polyaniline (PANI), polypyrrole (ppy), poly-3,4-ethylenedioxythiphene (PEDOT) and polyimide (PI).

PANI composite coatings prevent gases/moisture particles from reaching the substrate surface and isolate it from the external environment by the following mechanisms (i) by controlling of the electrical conductivity of nanocomposite system (ii) by controlling redox activity of PANI (iii) by serving PANI as a sacrificial reservoir of inhibitor dopants/anions (Kraljić et al., 2003). Adherent graphene/PANI hybrid polymer coatings have been reported to be deposited on copper substrates by electropolymerization technique using cyclic voltammetry as shown in Figure 6. The resulted coatings showed outstanding barrier properties preventing water and oxygen molecules reaching coating-metal interface due to its high surface area and better dispersion of graphene in PANI matrix. Tafel curves indicated that in the presence of graphene nanosheets in PANI matrix causes corrosion potential shifting toward anodic regions with drastic improvement in the polarization resistance. The corrosion current density (I_corr) values derived from the Tafel curves were used to calculate protection efficiency with output of 98% (Jafari et al., 2016). Moreover, iterations of equivalent circuit model revealed that the coated samples possess high charge transfer resistance (R_ct) and low double layer capacitance (C_dl) at coating-metal interface indicating superior corrosion performance.

Figure 6:

Schematic diagram depicting the synthesis process of PANI/graphene composites coatings. Reproduced from Chang et al. (2012) with permission from Elsevier.

An identical synthesis technique was employed by Catt et al. (2017) to prepare a composite coating consisting of GO and an electroactive polymer (PEDOT) as matrix material and applied as a corrosion control layer on magnesium substrate. As-prepared coatings showed lower hydrogen evolution and corrosion rates of samples which were validated by hydrogen evolution test and Tafel extrapolation measurements. Moreover, the large radii of Nyquist curve obtained by impedance testing showed increased R_ct value indicating higher corrosion resistance of sample coated with GO/PEDOT. After the analysis of various test results, the author concluded that the increased corrosion protection and low hydrogen generation of coated samples were attributed to the following three factors; accumulation of negative ions in the coating layer, formation of passive layer hindering ions diffusion and formation of Mg-phosphate film by redox reactions. Besides being corrosion resistant, the coated samples also exhibited reduced toxicity when exposed to cultured neurons indicating strong resistance to in vitro corrosion.

A recent study on electrodeposited polypyrrole films proved that dopant mobility in the films can be minimized significantly by employing organic anions leading to escalated properties (Raudsepp et al., 2008). Li et al. (2017) fabricated successfully a novel rGO/polypyrrole composite by voltammetry reduction method as an effective protective coating for corrosion prevention of mild steel substrates. Analysis of potentiodynamic polarization, open circuit potential and EIS measurements revealed that the rGO/ppy coated samples have large polarization resistance (54.1 kΩ cm²) and exhibited approximately 7 times higher corrosion protection ability compared to bare samples. Moreover, the coating protection efficiency of this nanocomposite was turn out to be 95.9% with good hydrophobic property. The corrosion protection mechanism in this type of coating was explained as the combination of formation of passive layer of rGO/ppy hybrid and formation of oxide film that retards the electron transportation through the coating. The author also explained corrosion and rust formation phenomenon on mild steel substrate which consisted of the following reactions

A novel three-fold protection mechanism was proposed by Chang et al. (2014a) in which PI formed two thin passive oxide layers i.e. Fe₂O₃ and Fe₃O₄ (due to the presence of active redox catalyst) while graphene nanosheets provided barrier effect. This graphene/PI composite coating was prepared by thermal imidization process on cold rolled steel samples. The coated samples were inspected for corrosion resistance by cyclic voltammetry studies and characterized by FTIR and TEM techniques. This drastic reduction in corrosion current densities and increase in polarization resistance (165.29 KΩ·cm²) with corresponding increment in R_ct values derived from Nyquist plot proved that the as-prepared composite coatings have superior corrosion resistance. Furthermore, gas permeability test results disclosed that gas barrier properties were also increased due to uniform distribution of graphene in PI matrix obstructing penetration of oxygen and water molecules.

Although many conductive polymer matrix based graphene composite coatings have been developed with superior corrosion protection efficiency, most of them have not fully emerged in practical industrial applications. The pre-eminent reason for this issue is that the conductive polymer coatings do not adhere firmly to the metal surface, hence cannot provide protection against large defects for a long period of time specifically formed during localized corrosion (Deshpande et al., 2014). In addition, most of the conductive polymers are overpriced which makes it impractical to employ it as main matrix materials for the synthesis of large-scale graphene coatings.

3.1.2 Passivating graphene sheets with polymeric resins

Graphene reinforced composite coatings (GRCCs) promotes galvanic corrosion similar to pristine graphene films when graphene particles are added above electrical percolation threshold in the composite due to abrupt increase in the magnitude of bulk conductivity (Potts et al., 2011). To phase out this corrosion furtherance effect in GRCCs, passivation is applied to graphene nanosheets. In graphene passivation process, each graphene nanosheet is enclosed by functionalizing with an insulated or conductive polymer to reduce graphene-graphene interactions in the matrix. Passivation by conductive polymer not only prevents restacking of graphene nanosheets but also their degradation by inhibiting corrosion by forming an oxide layer on the surface. Figure 7 shows the synthesis process of passivating polymer resin onto the surface of graphene nanosheets by using mechanical mixing method.

Figure 7:

Schematic diagram illustrating passivation of graphene nanosheets by conductive polymer and subsequent coating process.

To reduce the effect of galvanic corrosion, Sun et al. (2014) synthesized low electrical conductive pernigraniline (PG) capped rGO composite and embedded it in polyvinyl-butyral (PVB) polymer matrix to modify the coating properties. Potentiodynamic polarization and EIS analysis indicated that the rGO/PG adapted PVB coating served as an excellent corrosion resistant barrier compared to pristine rGO or PG based PVB with reduced corrosion rate of 6.99×10⁻⁷ mm/year and enhanced protection efficiency of 97.3%. This amended corrosion property can be correlated to a high surface resistance of the coating due to less interaction between graphene-graphene/metal interface and by prolonging diffusion channels for corrosive species in the coating matrix. Several reports have been published that shows PANI as a promising conductive polymer to passivate GO and rGO nanosheets to avoid direct contact between them. Similarly, Cai et al. (2016) explicitly demonstrated the preparation of conductive rGO/PANI composite as anticorrosive filler in waterborne PU matrix forming a novel coating with superior anticorrosive performance. Tafel plots and EIS spectra obtained for coated samples immersed in 3.5% NaCl solution revealed improved corrosion resistance in terms of lower I_corr values and lower corrosion rates. Moreover, there was a drastic improvement polarization resistance (3057.52 Ω·cm²) and protection efficiency of the coatings. The coatings also exhibited good adhesion ability and high impact resistance. With an intention to attain uniform dispersion of graphene particles and escalate adhesion capability, Zhao et al. (2017) used waterborne epoxy resin as matrix material instead of PU polymer keeping the same rGO/PANI composite as an additive. Moreover, the author employed a novel one-pot emulsion polymerization technique resulting in escalation in electrochemical properties from 1 to 3 orders of magnitude.

In a separate investigation, Ramezanzadeh et al. (2017) reported the passivation strategy for the first time to improve the corrosion protection efficiency of ZREC. Covering GO nanosheets with PANI nanofibers not only prevented GO-GO contact but also resulted in the increase in electrical connections between zinc particles and base metal. Moreover, PANI nanofibers noticeably enhanced the sacrificial behaviour and barrier properties of zinc rich paint. Corrosion test results revealed improved corrosion performance with the addition of just 0.1 wt% GO/PANI composite to ZREC. In another report by the same author, Ramezanzadeh et al. (2015) successfully grafted polyisocyanate resin on graphene oxide nanosheets to enhance its compatibility in PU resin matrix. Polyisocyanate resin chains were adhered on GO surface because of the formation of amide and carbamate ester bonds. Salt spray tests results and EIS spectra analysis affirmed that the PU coatings with just 0.1 wt% GO/polyisocyanate hybrid nanosheets resulted in significant improvement in barrier and anticorrosion properties. Moreover, pull off adhesion test confirmed low adhesion loss in coatings even after a long period in corrosive conditions.

Deducing the advantage of a high rate of dispersibility and excellent compatibility of urea formaldehyde (UF) resin with epoxy polymer, Zheng et al. (2017) grafted UF resin on GO nanosheets via in-situ polycondensation process. These modified GO/UF hybrid nanosheets were dispersed into epoxy matrix and composed a novel GO/UF-epoxy composite coating. Sedimentation test confirmed the homogenous distribution of GO/UF nanosheets throughout the epoxy resin. Furthermore, EIS measurements revealed that corrosion protection efficiency and barrier performance of modified GO/UF-epoxy coatings were evidently distinctive compared with pristine epoxy coatings. The authors suggested that a plausible mechanism for enhanced corrosion performance is connected to freely dispersed GO hybrid nanosheets that blocked the penetration of electrolyte through diffusion pathways on the substrate, thus preventing the base metal from corrosion.

Several selective electroactive polymers have been exploited to refine anticorrosion properties of composite coatings. However, because of the intense intermolecular attraction between polymer chains, agglomeration problem persists, causing high porosity and weak solubility that cripple its anticorrosion performance (Mišković-Stanković et al., 2014). Nevertheless, this issue can be dealt with using conductive polymer pigments as fillers in neat organic resins such as acrylic resin or epoxy resin. Sheet templating method for conductive polymers is relatively a new technique in which conductive polymer pigments when mixed with graphene transforms into sheet-like structures because of the hybridization of graphene particles. To extend the free distribution of graphene nanosheets in polymer, Jiang et al. (2016) applied an innovative sheet templating approach to synthesize graphene/polypyrrole epoxy composite coatings. In this experiment, the author used inexpensive ppy pigments with graphene to form graphene/ppy hybrids. Composed hybrid graphene/ppy epoxy coatings revealed high electric conduction and improved corrosion protection efficiency by 50–100 order higher compared with neat epoxy coatings. Considerable enhancement of corrosion properties was ascribed to their hybridized graphene nanosheets, which causes sheet templating effect on ppy pigments. These hybrid sheets such as graphene/ppy pigments formed a large barrier in a parallel direction extending the pathways of corrosion agents to reach the coating/substrate interface. Moreover, the author pointed out that the mass percentage of graphene used in coatings was only about 0.05–0.15, which scales down the total cost of graphene/ppy epoxy coatings.

From the above literature reports, it can be concluded that passivation of graphene sheets by polymer resins can protect the metal against corrosion attack for a longer period as compared to pristine graphene films. However, the structural strength of these coatings is not sufficient to withstand external damage and becomes susceptible to the corrosion effect. Besides strength, another factor that limits the use of the conductive polymer to passivate graphene nanosheets is its high cost. Certain conductive polymers such as PANI, ppy and PEDOT are very expensive, eventually increasing the final price of the coatings when prepared in large scale for industrial applications.

3.1.3 Anchoring nanoparticles on graphene nanosheets

Numerous research articles have been reported on problems concerning to compatibility and dispersion of graphene sheets in the polymer matrix. As discussed in the earlier section, graphene nanosheets have a tendency to agglomerate when mixed with the polymer matrix, and to avert this difficulty, a new strategy of tethering graphene nanosheets with metal oxide nanoparticles is adopted by several researchers (Zhang et al., 2014; Archana et al., 2018). Recently, some investigators reported that the introduction of inorganic nanoparticles between graphene layers also prevents re-agglomeration. These anchored nanoparticles increase the surface area and interplanar spacing of graphene sheets and promote bonding with the polymer matrix on both sides (Wang et al., 2010). Apart from increasing the dispersibility of graphene sheets, in some cases, metallic nanoparticles also behave like corrosion inhibitors forming an oxide layer and provide extra protection against corrosion. Figure 8 represents the synthesis process of attaching nanoparticles onto the surface of graphene nanosheets by using simple mechanical mixing method. Furthermore, when these spherical shaped nanoparticles are grafted on the surface of graphene oxide, micropores generated in hybrid polymer coatings during the solvent evaporation process are covered inherently preventing electrolyte permeation (Ma et al., 2016). Among various available metal oxide nanoparticles, magnetic Fe₃O₄ nanoparticles are considered as ecofriendly materials with remarkable corrosion protection and adhesion properties when doped in a composite matrix (El-Mahdy et al., 2014).

Figure 8:

Schematic diagram illustrating anchoring of nanoparticles on graphene sheets and subsequent coating process.

Di et al. (2016a,b,c) synthesized a novel hybrid GO/Fe₃O₄ epoxy coatings by a hydrolytic polycondensation process in which Fe₃O₄ nanoparticles were decorated on GO nanosheets. Fe₃O₄ nanoparticles were prepared by coprecipitation approach and tethered onto the GO nanosheets with the assistance of silane coupling additives. EIS spectra indicated an enhancement in anticorrosion efficacy with large impedance modulus due to excellent dispersion of GO/Fe₃O₄ hybrid sheets blocking the micropores to prevent the electrolytic diffusion into the epoxy matrix. Also, the long-term protection efficiency of the coatings was not up to the mark because of the generation of surface defects upon exposure to electrolyte for 20 days. This hybrid coating developed was compact in nature and cannot be deposited on elastic substrate surfaces such as flexible metal pipes and plates. For stretchable surfaces, Wang et al. (2017) pioneered a soft flexible GO/Fe₃O₄-natural rubber composite coatings using latex compounding technique. Interestingly, the composite coatings showed up exceptional corrosion resistance in terms of reduced corrosion current densities and reduced corrosion rate (5.2×10⁻⁴ mm/year). The proposed mechanism for improved anticorrosion performance was due to the aligned structure of Fe₃O₄-modified GO nanosheets that assisted to deter the diffusion pathways for corrosion agents. Furthermore, Fe₃O₄ nanoparticles attached on GO nanosheets also avoided the GO-metal interaction, therefore raising the polarization resistance of the coatings, which was validated by EIS calculations.

Similarly, silica nanoparticles can also be employed as GO modifier because of its good corrosion resistance, excellent chemical stability and high dispersion rate (Chen et al., 2012). Ramezanzadeh et al. (2016a,b) developed a novel two-step sol-gel method of preparing silica nanoparticles decorated GO nanohybrid and employed it in epoxy resin to make a hybrid coating with excellent corrosion resistance. On the contrary, Pourhashem et al. (2017a,b) used tetraethyl orthosilicate as organosilane to diffuse SiO₂ nanoparticles on the surface of GO nanosheets. Introduction of SiO₂ nanoparticles considerably improved dispersion and corrosion inhibition efficiency. Also, the cathodic delamination of coating notably decreased because of the existence of GO/SiO₂ hybrid nanosheets in the coatings. SiO₂ nanoparticles functioned as a barrier between the GO nanosheets and improved its degree of exfoliation and dispersion in the epoxy matrix, causing a decrease in moisture adsorption and penetration of corrosive media through the nanocomposite coatings.

Recently, lanthanide nanoparticles were used as a suitable substitute for SiO₂ and Fe₃O₄ nanoparticles because of their low toxicity and good corrosion inhibition effect. Lanthanide-based compounds can restrain corrosion reaction and preserve the metal surface against degradation even under strident circumstances. Lanthanide-based coatings have been proven as excellent corrosion inhibitors and protector against high-temperature oxidation (Bethencourt et al., 1998). Rahman et al. (2017) attempted to synthesize hybrid nanocomposite coating by incorporating graphene and ceria nanoparticles composite in water-borne PU matrix for corrosion protection. Polarization curves of coated mild steel samples after 15 day exposure to 3.5% NaCl showed higher corrosion potentials for G/CeO₂-PU coatings, indicating better corrosion performance compared with pristine PU coatings. The synthesized coatings exhibited remarkable anticorrosion properties by barrier and passivation effects, resisting cracks formation and maintained their integrity when exposed to extreme environment.

Titanium oxide (TiO₂) nanopowder has been reported to be successfully integrated into the neat epoxy coating to improve its barrier and corrosion performance (Radoman et al., 2014). Similar to graphene nanosheets, TiO₂ nanopowder also tends to segregate when mixed with other materials because of its high polarity and large specific surface area. To remove this limitation and enhance the anticorrosion performance of neat epoxy coatings, Yu et al. (2015a,b) anchored TiO₂ nanoparticles on GO nanosheets, and this hybrid was reinforced in epoxy coatings via solvent evaporation method. As prepared GO/TiO₂-epoxy coatings exhibited remarkable corrosion protection behavior with maximum corrosion protection efficiency surging up to 99.96%. Scanning electron microscopy (SEM) micrographs revealed that GO/TiO₂ hybrid sheets distributed homogenously in the epoxy matrix and formed sheet-like structures blocking the micropores in coatings to limit the penetration of electrolytic molecules. Experimental results also demonstrated that the anticorrosion performance of GO/TiO₂-epoxy nanocomposite coatings largely depends on the extent of dispersion and exfoliation of graphene particles.

Similar to titania nanoparticles, aluminum oxide (Al₂O₃) and zirconium oxide (ZrO₂) nanoparticles were also anchored on GO nanosheets by Yu et al. (2015a,b) and Di et al. (2016a,b,c) and dispersed in epoxy resin matrix via solvent evaporation and ultrasonic mixing process using silane coupling agents to form highly corrosion resistant GO/Al₂O₃-epoxy and GO/ZrO₂-epoxy coatings, respectively. Introduction of GO/Al₂O₃ and GO/ZrO₂ nanohybrids in epoxy resin exhibited excellent dispersion and exfoliation property, thereby enhancing the corrosion performance of the coatings. However, the low weight percentage of nanoparticles in epoxy was not able to enhance notably the anticorrosion properties to a higher extent. Moreover, both the composite coatings showed similar corrosion protection mechanism. Exceptional anticorrosion behavior of both the hybrid nanocoatings was associated to its active double barrier property and free dispersion of GO hybrid sheets in epoxy coatings.

Considering the above literature reports, it can be noticed that the nanoparticles selected for anchoring on GO nanosheets are metal oxide-based nanoparticles that are expensive to synthesize. Hence, to reduce the overall cost of graphene-epoxy coatings, low-cost calcium carbonate (CaCO₃) nanoparticles were exploited for the first time to modify the surface of graphene nanosheets. Di et al. (2016a,b,c) fabricated GO nanosheets anchored with CaCO₃ nanoparticles by means of 3-aminopropyltrimethoxysilane (APTES) additive. These modified GO/CaCO₃ hybrid nanosheets were diffused in the epoxy matrix and cured to form highly anticorrosive composite coatings. SEM and TEM micrographs revealed homogeneous dispersion of CaCO₃-anchored GO nanosheets in the epoxy matrix, which in turn enhanced the anticorrosion performance of the coatings by providing tortuous passage to the electrolyte.

The foremost limitation of using inorganic nanoparticles in polymer hybrid coatings is its compatibility with graphene nanosheets and the polymer matrix (Kim et al., 2010). Moreover, expensive coupling agents such as APTES are needed to covalently attach nanoparticles onto the surface of graphene sheets. Also, the thickness of the final coated layer on the metal surface increases with the type of polymer matrix used ranging from 40 nm to 100 μm, which can strongly influence the structural properties of the metal (Watcharotone et al., 2007). Thus, the implementation of this strategy of anchoring expensive inorganic nanoparticles on graphene oxide nanosheets makes GPMCCs inappropriate for large surface area substrates.

3.1.4 Chemical functionalization of graphene nanosheets

Functionalization of graphene sheets by chemical modification is one of the convenient approaches to counter flocculation and stabilize graphene suspension within the polymer matrix. Chemical functionalization is an effective way to amend the defects in graphene by altering the electronic and crystal structure to induce specific electronic and magnetic properties (Boukhvalov & Katsnelson, 2008). The promising chemical methods to modify a graphene surface are covalent functionalization and noncovalent functionalization, which increase the interlayer distance between graphene layers and promote steady interfacial interactions between graphene nanosheets and the polymer matrix (Boukhvalov & Katsnelson, 2009). In covalent functionalization, covalent bonds are formed between the epoxide groups and carboxylic groups of GO and the chemical coupling agent. However, covalent grafting of GO sheets involves lengthy chemical processes that can induce irregularities changing the inherent corrosion properties, whereas in noncovalent functionalization, strong π-π bonds are established between GO nanosheets and stabilizer conserving its properties (Kuila et al., 2012).

One of the most widely used chemicals for functionalization of graphene is APTES, which is a type of silane coupling agent and promotes strong interfacial bonding between graphene sheets and the polymer matrix. Pourhashem et al. (2017a,b) evaluated the influence of amino silane on GO nanosheets as nanofillers in epoxy matrix coatings to increase dispersion quality. Functionalization of GO nanosheets with APTES showed a good interfacial interaction of GO with polymer and superior anticorrosion performance as confirmed by FESEM and EIS analysis. However, the addition of modified GO of more than 0.1 wt% resulted in the agglomeration of nanosheets in the matrix reducing the barrier properties. Another study (Mo et al., 2015) was carried out to investigate the impact of modified graphene oxide by APTES in PU composite coatings. The team fabricated PU composite coatings by reinforcing functionalized graphene oxide (FGO) and functionalized graphene with exceptional corrosion and tribological properties. The graphene nanosheets were chemically treated with APTES to enhance the dispersion and compatibility with PU matrix. However, corrosion testing results of FGO/PU coatings exhibited lower corrosion protection when compared with functionalized graphene/PU coatings vowing to the presence of plentiful oxygenated groups. These oxygenated groups of FGO promoted stronger interfacial interaction but induce imperfections in graphene lattice structure. Although salinization is a popular method to functionalize graphene, it suffers from the limitation of hydrolytic degradation. When a salinized composite is subjected to the aqueous atmosphere, the salinized interfaces absorbs moisture and gets hydrolyzed, thereby opening additional passages for water molecules to diffuse (Matinlinna et al., 2004).

Apart from silane, there are other types of coupling agents that can be employed to functionalize graphene such as titanate and zirconate coupling agents. Table 1 shows various chemicals used to modify graphene nanosheets to increase dispersion and bonding with the polymer matrix. Li et al. (2014) employed titanate coupling agent for the first time in rGO/PU composite coatings and found that the titanate not only improves the homogeneous distribution of rGO nanosheets in PU matrix but also aligns itself parallel to the substrate surface. This auto-alignment property of graphene nanosheets was directed by the decrease in total excluded volume. Nevertheless, this unique in-plane alignment property guides in utilizing the full surface area of rGO layers to confront the electrolyte and provide tortuous path to limit its permeation through the coatings. High anticorrosion performance of the composite coatings was confirmed by Nyquist plots even after 96 h of exposure to electrolyte. However, the titanate-treated graphene did not display a self-alignment behavior when added up to 0.2 wt% in the matrix and was scattered in random three-dimensional directions. In another research, Diouf and Asmatulu (2014) carried out a study to determine the effect of organofunctional alkoxysilane modified graphene on corrosion and weathering resistance of fiber-reinforced composite substrates. The graphene nanoplatelets were salinized and mixed in different proportions in PU primer matrix via mechanical mixing sonication process. Experimental results indicated that with the addition of 2 wt% salinized graphene sheets produced drastic improvement against corrosion and UV degradation.

Ramezanzadeh et al. (2016a,b, 2018) and his team conducted two separate experiments with p-phenylenediamine (PPDA) as a functionalizing agent. In the first experiment, they covalently grafted PPDA aromatic diamine on GO nanosheets to enhance dispersion and interfacial bonding of GO nanosheets with epoxy matrix. In the second experiment, they synthesized PPDA functionalized GO nanosheets with three different lateral sizes, i.e. small area GO nanosheets with 0.85 μm size, medium area GO nanosheets with 8.2 μm size and large area GO nanosheets with 38 μm size. The covalent functionalization of GO surface was achieved through wet transfer method and resulted in successful intercalation leading to increased corrosion inhibition efficiency. Addition of FGO into the epoxy enhanced ionic resistance and hydrophobicity of the coating while hindered the penetration of Cl⁻ ions due to creation of covalent bonds between amine groups of PPDA and epoxide groups of GO. Furthermore, the GO nanosheets with medium lateral size delivered highest barrier performance and anticorrosion properties. A similar investigation was reported by Gu et al. (2015) in which carboxylated aniline trimer stabilizer was used as dispersing agent with graphene nanosheets and synthesized water-soluble graphene/epoxy hybrid coatings. After functionalization, stable high concentration of graphene nanosheets (>1 mg/mL) was able to disperse in epoxy system by forming strong π-π bonds between graphene and carboxylated aniline trimer stabilizer. A series of electrochemical measurements revealed that the increased dispersion of graphene nanosheets into waterborne epoxy drastically enhanced corrosion resistance properties and minimized the coating delamination.

Zhu et al. (2017) very recently developed a novel 3-(1-(2-aminopropoxy)propan-2-ylamino)propane-1-sulfonate sodium functionalized water-dispersible graphene via nucleophilic ring opening reaction. These chemically modified graphene nanosheets were dispersed in acrylic modified alkyd resin matrix to enhance the corrosion protection performance. SEM images showed the homogeneous distribution of graphene sheets in alkyd resin nano-emulsion forming compact and uniform coatings that can improve barrier properties against moisture and oxygen. Corrosion tests indicated that 3-(1-(2-aminopropoxy)propan-2-ylamino)propane-1-sulfonate sodium functionalized coatings has superior corrosion-resistant properties compared with pristine alkyd resin coatings. EIS measurements further revealed that the formed coatings exhibit dual corrosion protection mechanisms, i.e. barrier effect and anodic protection mechanism. Yu et al. (2014) demonstrated successfully for the first time the application of vinyl-polystyrene/GO (V-PS/GO) hybrid coatings in anticorrosion system. The nanocomposite composite coating was prepared by in situ miniemulsion polymerization process and tested for anticorrosion properties. The as-prepared hybrid coatings exhibited high corrosion resistance properties with inhibition efficiency of 99.53% by introduction of 2 wt% of functionalized GO in the polystyrene matrix. Apart from corrosion properties, the GO-modified coatings showed an improvement in mechanical properties, thermal stability, storage modulus and thermal decomposition temperature. The author explained the reason for this enhanced performance by V-PS/GO nanocomposite coatings as the increased dispersion of GO nanosheets in nonconjugated polystyrene matrix due to strong π-π interactions between polystyrene and modified GO.

Undoubtedly, chemical modification of graphene sheets is a beneficial approach to counter the agglomeration, but there are certain demerits associated with it. First, the use of chemicals could alter graphene properties that can subsequently affect the corrosion properties of the coatings. Second, chemical functionalization of graphene is a time-consuming and tedious process. Third, most of the chemicals used for functionalization are expensive and toxic in nature (Hirsch et al., 2012).

Graphene-embedded composite coatings containing well-dispersed graphene sheets in the matrix material not only improves barrier properties but also improves structural durability and thermal and electrical properties (Hu et al., 2014). However, the graphene sheets used in these coatings are synthesized employing presently available methods generating various types of structural defects that can affect the protection efficiency. The direct impact of these local defects in graphene can cause accumulation of oxygen molecules that can degrade its performance (Rohini et al., 2015). All the demerits described above restrict the free usage of graphene-based coatings for anticorrosion application in a large scale. More advanced processes and methods with regard to the enhancements in exfoliation, dispersion and adhesion property of graphene sheets are needed so that it can be utilized effectively in the preparation of anticorrosion coatings. Apart from requiring costly equipment and tedious preparation methods, most of the strategies described in the above section to improve corrosion performance of GPMCCs need expensive chemicals and materials that can harm the surrounding environment.

Recent research reports on corrosion performance of graphene-based composite coatings and related electrochemical parameters are summarized in Table 2. From the table, it can be observed that the corrosion inhibition efficiency of prepared coatings depends upon several factors such as synthesis method, type of deposition method and substrate material. Furthermore, the corrosive environment to which the coatings are subjected and the adhesion ability between the polymer and the substrate surface could also affect the performance of the coatings. Over the past 5 years, there is a drastic increase in research conducted on graphene coatings with polymer as matrix material compared with pristine graphene coatings (Bonavolontà et al., 2017). Future research could be focused on experimenting with coatings containing low-cost inorganic fillers combined with graphene with superior properties. Only a few innovative synthesis techniques are reported in the recent years to improve the performance of the coatings. Even though magnesium and titanium are being used widely nowadays for a variety of applications, most of the research is directed toward the deposition of coatings on mild steel and copper substrates only.