Tuning the oxygen reduction reaction activity of graphene through fluorination modification to inhibit its corrosion-promotion activity

Highlights

• Corrosion-promotion activity of graphene was inhibited by controling ORR activity.

• Partially fluorinated graphene with tuned-ORR activity was successfully prepared.

• PFG cannot accelerate the corrosion of Cu due to its lower ORR activity.

• The inhibition mechanism of the corrosion-promote activity of PFG was studied.

Abstract

Graphene is cathodic to most metals and can cause promoted corrosion to metals. Herein, we demonstrate that tuning oxygen reduction reaction (ORR) activity of graphene through fluorination modification is an effective strategy that can inhibit the corrosion-promotion activity of graphene. It is revealed that the cathodic ORR activity of partially fluorinated graphene (PFG) decreases with the increase of fluorine (F) content, which consequently increases the depolarization resistance of the cathodic ORR during corrosion. Especially, PFG containing 15.2 wt.% F exhibits a completely negligible corrosion-promotion activity. Furthermore, PFG can also improve the barrier property of polyvinyl butyral coating.

Introduction

Graphene has been proposed to be a superior material for corrosion protection of metals due to its outstanding physical and chemical properties, especially its impermeability to molecules [[1], [2], [3]]. Generally, graphene is used to prevent metals from corrosion in two forms. On the one hand, graphene film itself can act as a protective coating to provide protection for metals, such as graphene directly grown on metal substrates by chemical vapor deposition (CVD) [[4], [5], [6]]. On the other hand, graphene nanosheets can also be utilized as a filler to improve the barrier property of polymer coatings [7,8]. It has been pointed out, however, that graphene will accelerate rather than inhibit the corrosion of most metals at exposed graphene/metal connections whether it is used as coating or filler. And the essence of such an accelerated corrosion phenomenon is the micro-galvanic corrosion of graphene/metal couple [[9], [10], [11], [12]]. This phenomenon is called the corrosion-promotion activity of graphene. How to inhibit the corrosion-promotion activity of graphene has drown more and more attentions in the field of developing graphene-based corrosion protection coatings for practical applications.

In fact, earlier researches have already noted the problem of galvanic corrosion between graphite and steel, and it was revealed that graphite could accelerate the corrosion of steel when graphite and steel are in electrical contact and immersed in aggressive electrolyte [[13], [14], [15]]. In recent years, it was further pointed out that graphene and few-layers of graphite will also accelerate the electrochemical degradation of metals due to their cathodic nature [16]. In most recently, Cui et al. [17] demonstrated that, due to the fact that the potential of graphene is more positive than most metals, graphene can promote the localized corrosion of most metals at graphene/metal interfaces and seriously deteriorate the coated metals when being exposed to corrosive environment. It should be especially cautious that graphene actually accelerates the localized corrosion of metals, which is difficult to predict and prevent and is more harmful than general corrosion. Weatherup et al. [18] prepared single-layer graphene grown by CVD on various polycrystalline transition metals (Co, Fe, Ni, Pt and Cu). Their results showed that graphene film can only effectively prevent Ni and Co surface from oxidation over the long term. They found that a strong graphene-metal interaction is crucial to suppress the oxidation of substrate surface beneath the graphene film, achieving a long period protection. Subsequently, Xu et al. [19] demonstrated that H₂O can easily diffuse into the interface of graphene and Cu(100) and accelerate the corrosion of Cu(100). While, graphene films can protect Cu(111) from oxidation because H₂O will not diffuse into the interface due to the strong interfacial coupling of the graphene-Cu(111). Dutta et al. [20] proposed that graphene modified insulating-polymer composite coating is an alternative means to prevent the formation of galvanic coupling between graphene and metal. However, it should be noted that, when loading a high content of graphene, 3D electrical conducting network from graphene to metal and graphene to graphene would be fabricated randomly at the polymer/metal interfaces and in the polymer matrix so as to create condition for the graphene/metal galvanic couple [21].

Surface functionalization of graphene is a feasible method to enhance the corrosion protection performance of graphene/polymer nanocomposites [22]. Up till now, various surface modification materials have been investigated, such as silane [23,24], polyisocyanate resin [25], polyaniline-cerium oxide [26], polyaniline [27], tin-oxide [28], polyuria-formaldehyde [29], organic isocyanates sulfonated aniline trimer [30], vinyl polymer [31], layered double hydroxide [32], polypyrrole [33] and polymethylmethacrylate [34]. All the results showed that the functionalization of graphene improves the corrosion protection performance of graphene/polymer composite coatings. However, whether these functionalized graphene materials possess corrosion-promotion activity or not has not been studied. In theory, the galvanic corrosion couple will not form as long as the two materials, graphene and metal, are not electrical contacted each other. Therefore, our group firstly proposed the concept of “passivated graphene”, which is encapsulating graphene with insulating material to cut off electron pathways of the graphene/metal galvanic couple. We studied that different insulating materials, pernigraniline [35], silane coupling agent [36] and nanosized silicon oxides [37] can completely avoid electrical connection between graphene and metal, which consequently inhibits the corrosion-promotion activity of graphene.

Most researchers believe that the corrosion-promotion activity of graphene is related to its high electrical conductivity [10,36]. However, this knowledge alone is insufficient for the practical application of graphene-based coatings [38]. According to the mechanism of galvanic corrosion, the corrosion rate (I_corr) of anode is defined as:

$$I_{corr}=\frac{E_a-E_c}{R_a+R_c+\sum R_l}$$

where E_a and E_c are the potential of anode and cathode, respectively; R_a and R_c are the depolarization resistance of anodic reaction and cathodic reaction, respectively; $\sum R_l$ is the total resistance of conductors included in the corrosion cell. It can be seen that decreasing the conductivity of graphene, which belongs to the increase of $\sum R_l$, is only a small part of the inhibition of the corrosion-promotion activity of graphene. Furthermore, increasing the depolarization resistance of graphene would also be a possible way to inhibit the corrosion-promotion activity of graphene.

The most commonly cathodic process of metal corrosion is oxygen reduction reaction (ORR). To increase the cathodic depolarization resistance of ORR occurring graphene, we propose that fluorination is a possible way. As a graphene derivative, the electrochemical properties of fluorinated graphene is closely associated with the F content which can be controlled by manufacturing process [39,40]. In a previous work [41], we studied that nearly fully fluorinated graphene (F:C = 0.925) prepared by liquid-phase exfoliation method cannot accelerate metal corrosion due to its insulating nature. Lei et al. [42] fabricated hydrophobic epoxy coating by adding fully fluorinated graphite (F:C =∼1:1) in epoxy matrix, which also exhibits a prominent corrosion protection performance. However, it is difficult to prepare PFG with more fluorine, such as high costs, low yield and rigorous strictures imposed on its preparation environment. In addition, PFG with more fluorine have a poor compatibility and adhesion with polymer. Therefore, it is better to tune the appropriate fluoridation degree of graphene to achieve the best corrosion protection performance.

In this paper, PFG with low F content is prepared to study how the cathodic ORR activity affects the galvanic corrosion between PFG incorporated in the matrix of polymer and copper (Cu). The results show that, with the increases of F content in PFG, the ORR activity decreases, which finally weakens the corrosion-promotion activity of graphene. Besides, PFG which has a similar structure with graphene can greatly improve the barrier and corrosion protection performances of the polyvinyl butyral (PVB) coating because PFG extends the diffusion pathway of water and oxygen in the coating matrix. We believe that PFG may be a suitable and attractive substitute of graphene for corrosion protection applications.