Elsevier

Carbon

Volume 79, November 2014, Pages 605-614
Carbon

Synthesis of low-electrical-conductivity graphene/pernigraniline composites and their application in corrosion protection

https://doi.org/10.1016/j.carbon.2014.08.021Get rights and content

Abstract

Graphene can be a corrosion-promotion material because of its high electrical conductivity. This paper aims at eliminating the undesired corrosion-promotion effect of graphene, and reports a promising application of graphene/pernigraniline composites (GPCs) for the corrosion protection of copper. The composites were synthesized by an in situ polymerization-reduction/dedoping process. The synthesized composites have a flake-like structure, and their conductivity is as low as 2.3 × 10−7 S/cm. The GPCs are then embedded into polyvinylbutyral coating (PVBc) to modify the coating. Potentiodynamic polarization and electrochemical impedance spectroscopy measurements reveal that the GPC-modified PVBc is an outstanding barrier against corrosive media compared with pernigraniline or reduced graphene oxide (rGO) modified PVBc. Scratch tests also show that the corrosion-promotion effect of rGO in GPCs is inhibited. The enhanced corrosion protection performance is observed because on the one hand the pernigraniline growing on rGO surface avoids graphene–graphene/metal connections increasing the electrical resistance of coating; on the other hand the as-prepared GPCs are less flexible than polymer-free rGO and they are more likely to unfold during the coating process, which can greatly prolong the diffusion pathway of corrosive media in the coating matrix.

Introduction

The development of material science provides new opportunities for developing “smart” and effective methods for anticorrosion [1], [2]. Recently, graphene, one of the most compelling materials, has been reported to be an excellent anticorrosion material because it possesses many unique characteristics that are ideal for anticorrosion, such as chemical inertness, excellent thermal and chemical stability, remarkable flexibility, and impermeable to molecules [3], [4], [5], [6], [7], [8], [9], [10].

Chen et al. [11], for the first time, demonstrated that graphene films grown by CVD (CVD-graphene) can prevent the surface of the metallic substrates (Cu and Cu/Ni alloys) from oxidation. Subsequent studies also showed that metals (such as Cu, Fe, Cu/Ni alloy, Ag) coated by CVD-graphene were significantly more resistant to thermal oxidation (150–1100 °C) and wet corrosion (corrosive electrolyte such as NaCl, Na2SO4 and H2O2) than bare metal in the short term (2 min–5 h) [9], [12], [13], [14], [15], [16]. Krishnamurthy et al. [17] successfully prevented the microbial corrosion of a foam nickel anode in glucose-based electrolyte for more than 2700 h through using a CVD-graphene coating. However, Schriver et al. [18] found that the graphene-coated copper succumbed to thermal oxidation (185, 250 °C) because of the migration of oxygen through graphene defects over long time scales (>17 h). Furthermore, they suggested that graphene was able to promote copper corrosion at room temperature over time scales of 1 month to 2 years. They stated that a graphene coating on Cu promotes long-term Cu corrosion in several ways, including maintaining a conductive pathway between Cu and graphene, causing nonuniform oxidation at graphene defects and providing a driving force for the anodic polarization of Cu. Zhou et al. [19] also observed the corrosion-promotion effect (CPE) of graphene film. And they believed that the CPE was associated with the defects and high electrical conductivity of graphene films. Therefore, the anticorrosion application of graphene films should be carefully considered.

Besides graphene anticorrosion films, another graphene-based anticorrosion method is the use of graphene-reinforced composite coatings (GRCCs). Chang et al. [20] presented the first application of polymer/graphene composites for the anticorrosion of steel. The coating was able to effectively protect steel because of its good impermeability to O2 and H2O. Yu et al. [21] successfully prepared well-dispersed polystyrene/modified-GO composites by in situ miniemulsion polymerization and used it for corrosion protection. The as-prepared composites exhibited superior anticorrosion properties compared with pure polystyrene. Singh et al. [22] reported a robust graphene reinforced composite coating with excellent anticorrosion performance by cathodic electrophoretic deposition. They believed that cathodic electrophoretic deposition was more advantageous than CVD on producing coating of controlled microstructure on a wide range of substrates. Chang et al. [23] directly duplicated the surface features of fresh Xanthosoma sagittifolium leaves and developed an excellent epoxy/graphene composites anticorrosive coating with hydrophobic surfaces. GRCCs are promising anticorrosion methods because they possess not only passive properties inherited from coating matrix (e.g., barrier, color, adhesion), but also improved physical properties (e.g., good mechanical properties, high thermal and dimensional stability, superior barrier properties) [24], [25], [26], [27], [28], [29]. However, similar to CVD-graphene films, the GRCCs are likely to promote the corrosion of metal substrate because the bulk conductivity of an insulating coating may be increased by several orders of magnitude when graphene material is imbedded above the percolation threshold [24]. Therefore, for evaluating the anticorrosion performance of GRCCs, it is vital to experimentally determine whether or not GRCCs possess the CPE property. However, to our best knowledge, there is no investigation on the CPE of GRCCs. In this work, we focus on investigating the anticorrosion performance of GRCCs from the perspective of CPE. We aim to eliminate the corrosion-promotion effect of graphene by graphene passivation, through which each graphene sheet is surrounded by insulated materials with minimal graphene–graphene connections. In order to achieve such a graphene passivation, herein, low-conductivity pernigraniline is synthesized on the surface of reduced graphene oxide (rGO). Our experiments reveal that, when coating defects emerge, rGO in the polyvinylbutyral coating (PVBc) will promote the corrosion of metal substrate, while pernigraniline-passivated rGO will not. The anticorrosion mechanism of passivated rGO is found to be the same as for other two-dimensional materials [30], [31], [32], [33]. We believe that the passivated rGO is a new class of two-dimensional material for corrosion prevention and has broad application prospects.

Section snippets

Synthesis of graphene/pernigraniline composites (GPCs)

Graphene oxide (GO) was prepared from natural graphite (∼30 μm) by Hummers’ method [34]. And the GPCs were synthesized by an in situ polymerization-reduction/dedoping method [35]. Here, dedoping refers to the removal of protonic acid from emeraldine salt through alkali treatment, which can result in the decrease of emeraldine salt conductivity. Typically, 2 mg mL−1 GO in 180 mL ethylene glycol was exfoliated by ultrasonication for 1 h. Then, 1 mL aniline was slowly added into the yellow–brown

Characterization of materials

The Raman spectroscopy was implemented to characterize the structure of the synthesized materials. Fig. 1 shows the Raman spectra of the rGO, emeraldine salt, pernigraniline and GPC. It reveals that the rGO displays a D band at 1350 cm−1 corresponding to the vibration of graphene defects or edges and a G band at 1598 cm−1 which is assigned to the in-phase vibration of the graphite lattice (Fig. 1(a)). However, the Raman spectra of emeraldine salt, pernigraniline and GPC show more peaks than that

Conclusion

In conclusion, low conductive, inflexible, and flake-like GPCs are synthesized by the in situ polymerization-reduction/dedoping method in this study. The composites possess not only impenetrable property inherited from rGO, but also insulating property inherited from pernigraniline. Potentiodynamic polarization and electrochemical impedance spectroscopy measurements show that PVBc can maintain high anticorrosion performance for a long time when using the GPCs as fillers. The anticorrosion

Acknowledgments

The authors thank the financial support of National Natural Science Foundation of China (No. 21403030) and Fundamental Research Funds for the Central Universities (No. 2100-852014).

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