Objective To study the effect of graphene (Gr) content on the corrosion resistance of nickel-based composite coatings. The optimal amount of Gr was determined by analyzing the influence of Gr on the corrosion resistance of the composite coatings. The corrosion behavior of nickel-based composite coatings with different Gr contents in three different pH solutions (acidic, neutral, and alkaline) was studied. Methods Five graphene/nickel-based (Gr/Ni60) composite coatings with different Gr contents (mass fractions of 0%, 0.3%, 0.5%, 0.8%, and 1%) were prepared by pre-powder laser method. The surface micromorphology of the composite coatings before corrosion, corrosion resistance test, X-ray photoelectron spectroscopy analysis, and surface morphology analysis after corrosion were performed. Results In the composite coatings with Gr added, C and Cr elements were mainly distributed between dendrites, and the intra-dendrite area was mainly composed of Fe and Ni. With the increase of Gr content in the composite coating, under acidic corrosion conditions, the self-corrosion potential increases with the increase of Gr content, from -0.466 V to -0.384 V, and the polarization resistance also increases from 87.71 Ω/cm2 of pure Ni60 coating to 153.53 Ω/cm2, but there is no obvious passivation interval in each coating, mainly hydrogen evolution corrosion occurs, and intradendritic corrosion is serious. Under neutral corrosion environment, each coating has an obvious passivation interval. When the mass fraction of Gr is 0.8%, the longest passivation interval reaches 0.285 V, at which time the phase angle value and impedance modulus value reach the maximum value, and the oxide generated on the surface improves the corrosion resistance of the coating. Under alkaline corrosion conditions, the five composite coatings with different Gr contents all showed longer passivation intervals compared with the composite coatings under neutral corrosion conditions. When the mass fraction of Gr was 0.8%, the length of the passivation interval reached 1.506 V, and the polarization resistance value also reached a maximum value of 3 030.32 Ω/cm2. When the mass fraction of Gr was 1%, the corrosion resistance under the three environments was reduced to a certain extent compared with the corrosion resistance when the mass fraction of Gr was 0.8%. Conclusion The addition of Gr has a significant positive effect on the corrosion resistance of Ni-based composite coatings. There is an optimal value for the addition amount of Gr. Excessive addition of Gr will reduce the corrosion resistance of the composite coating. The corrosion resistance of the composite coating under alkaline corrosion conditions is better than that under acidic and neutral corrosion conditions. When the mass fraction of Gr is 0.8%, the composite coating has the best corrosion resistance.

Steel materials have been widely used in many fields. However, factors such as high temperature, high humidity, and high pH in complex service environments can easily affect the thermodynamic stability of the material itself and cause corrosion damage to the surface of the steel material [1]. At present, the main method for protecting the steel surface is to coat nickel-based, iron-based, cobalt-based and other metal alloy powders on the steel surface through different methods such as plasma cladding [2], physical vapor deposition [3], and laser cladding [4] to form a layer of protective coating with good performance [5-7]. This not only maintains the original advantages of the material, but also enhances other properties of the material such as wear resistance and corrosion resistance [8-10]. Among the many self-fluxing alloy powders, nickel-based alloy powders have a wide range of application value due to their high hardness, good wear resistance and corrosion resistance. Laser cladding (LMD) technology can be used to manufacture 3D components, repair damaged parts surfaces, and prepare high-performance coatings [11]. Its principle is as follows: a molten pool is formed on the surface of the substrate using a laser, and metal powder is injected into the molten pool. Then, it is metallurgically bonded with the substrate by solidification and cooling to form a high-performance alloy coating [12]. By controlling the process environment and optimizing process parameters, LMD can produce high-performance, low-cost complex structural parts in different environments [13-15]. Metal matrix composites (MMC) are composite materials with metal as the main structure and ceramic or organic materials as the reinforcement phase. They combine the ductility and toughness of metals with the high strength and high elastic modulus of ceramics, and have become important engineering materials. Since Novoselov et al. [16] discovered graphene in 2004, people have conducted a lot of research on it. It was found that graphene has excellent electrical and mechanical properties due to its unique structure, and has a high thermal conductivity and specific surface area. Based on these excellent properties, it is considered to be an ideal reinforcement phase reinforced metal material. In recent years, people have begun to use graphene as the reinforcement phase of MMC and study its performance.

Kumar et al. [17] synthesized graphene using graphite oxide as raw material and prepared Ni-Gr composite coatings on the surface of low-carbon steel by electrodeposition technology. They found that the incorporation of graphene in nickel electrodeposition changed the preferred orientation pattern, making the Ni-Gr composite material present a uniform, bright, smaller particle size and less hillock structure. They also found that graphene adsorbed on the surface of the coating during the deposition process hindered crystal growth and increased the nucleation sites for nickel ion reduction, resulting in the refinement of the nickel crystal grains in the composite coating and complete arrangement, thereby improving the hardness and corrosion resistance of the composite coating. Li et al. [18] modified graphene with nickel and synthesized Ni/Gr powder by chemical plating. Pure Ni60 coating, Ni60–Gr coating before modification, and Ni60–Ni/Gr coating after modification were prepared by laser cladding technology. The study found that the modified Ni/Gr powder improved the interface compatibility between Gr nanosheets and Ni60 coating, and Gr nanosheets showed better two-dimensional strip morphology, and its uniformity was better than that of the other two composite coatings. The wear resistance and corrosion resistance of Ni60–Ni/Gr composite coating were significantly improved. Kuang et al. [19] prepared Gr/Ni composite coating by graphene oxide (GO) sheets and electrodeposition. The study found that the thermal conductivity of the composite coating was about 15% higher than that of pure nickel coating, and the hardness value was 4 times that of pure nickel coating. This was mainly because the reduced graphene sheets significantly affected the preferred orientation of nickel growth in the composite film, thereby improving the performance of the composite material.

The above studies show that graphene has a significant effect on the performance of reinforced metal matrix composites. Laser cladding technology can be used to prepare composite coatings that are well bonded to the matrix and have excellent performance. The addition of Gr does help to improve the corrosion resistance of composite materials, but there are few reports on the optimal amount of Gr added when the corrosion resistance of composite coatings is the best. In order to explore the effect of Gr content on the corrosion resistance of Ni-based coatings, this paper adopts the pre-set powder method and prepares Gr/Ni60 composite coatings by laser cladding technology. The effects of different Gr contents on Gr/Ni60 composite coatings and the corrosion behavior of Gr/Ni60 in different environments are systematically studied.

1 Experiment

1.1 Materials

The nickel-based alloy powder system used in the experiment selects Ni60 self-fluxing alloy powder with a powder particle size of 180~300 mesh. The particle size of Gr powder used in the experiment is 5~15 μm, and Gr powder is a typical lamellar morphology. The substrate used in the experiment is Q235 carbon structural steel with a size of 160 mm×600 mm×8 mm. Before the experiment, the substrate was polished with an angle grinder to remove surface rust and oxides. After the metallic luster appeared, the substrate surface was polished with 200 and 400 grit sandpaper in sequence until the surface was smooth and flat, and the substrate surface was wiped clean with anhydrous ethanol. The chemical composition of Ni60 powder and Q235 substrate is shown in Table 1.

1.2 Methods

The laser cladding system uses IPG’s YLS-2000W fiber laser. The experiment uses the pre-set powder method. Deionized water is heated to 60 °C in a water bath, and then sodium carboxymethyl cellulose (CMC) powder is slowly poured in. The solution is slowly stirred with a glass rod until there are no pores in the aqueous solution. A CMC solution with a mass fraction of 0.65% is prepared as an adhesive. The mixed powder is evenly applied to the surface of the Q235 substrate using a homemade mold, and the thickness of the entire pre-set powder layer is ensured to be consistent. Through the optimization of process parameters, the optimal process parameters are determined as follows: laser power is 1 400 W, scanning speed is 6 mm/s, preset powder layer thickness is 0.8 mm, spot diameter is 2 mm, and overlap rate is 30%. Five composite coatings with different Gr contents were prepared on Q235 substrates, and the pure Ni60 coating was marked as a, and the Gr/Ni60 composite coatings with Gr mass fractions of 0.3%, 0.5%, 0.8%, and 1% were marked as b, c, d, and e, respectively. The corrosion resistance study adopted a three-electrode system: working electrode (coating), auxiliary electrode (platinum electrode), and reference electrode (silver chloride electrode). The potentiodynamic polarization curve test (LSV) and electrochemical impedance spectroscopy (EIS) test were carried out using Chenhua CHI660E electrochemical workstation to characterize the corrosion resistance of the coating. A 0.1 mol/L HCl solution was used to simulate the acidic environment; a 3.5% mass fraction NaCl solution was used to simulate the neutral environment; and a 1 mol/L NaOH solution was used to simulate the alkaline environment. The electrochemical workstation was used to test the potentiodynamic polarization curve (LSV) and the electrochemical impedance spectroscopy (EIS), and the experimental data were fitted using the Zview software. The microstructure of the coating surface before and after corrosion was observed using a SUPRA–55VP scanning electron microscope (SEM). The Thermo Fisher K–alpha+ X-ray photoelectron spectrometer was used to analyze the surface composition and element valence state of the coating after wear, and the measured data were analyzed and fitted using Xpeak and Origin software.

2 Results and Analysis

2.1 Microstructure

The SEM morphology of the Gr/Ni60 composite coating surface is shown in Figure 1. The microstructure of the Gr/Ni60 composite coating is composed of the intra-dendritic and inter-dendritic structures. It can be seen that there is a significant difference in element diffraction contrast between the inter-dendritic and intra-dendritic regions, which indicates that the distribution of different elements between the inter-dendritic and intra-dendritic regions is quite different. The Ni60 powder used in the experiment mainly contains Fe, Cr, Si, B and C elements, among which Ni and C are the forming elements of the austenite phase, and Cr and Si are the main forming elements of ferrite[20]. During the rapid condensation process of laser cladding, the introduction of Gr will cause element segregation. The unique structure of Gr and the high diffusion rate of C atoms will accelerate the homogenization process and cause organizational differences. As shown in Figure 1, the main elements in the inter-dendritic region are C and Cr. Cr precipitates between the dendrites in the form of carbides. The main elements in the intra-dendritic region are Fe and Ni. Since the atomic radius of Fe and Ni is not much different, Fe and Ni solid solutions will be formed in the dendrites.

2.2 Electrochemical corrosion performance of Gr/Ni60 composite coating in acidic (HCl) solution

2.2.1 Polarization test analysis

In the electrolyte, there is an electrode process on the surface of the laser cladding Gr/Ni60 composite coating. The potentiodynamic polarization curve of the Gr/Ni60 composite coating in HCl solution is shown in Figure 2, and its electrochemical performance parameters are shown in Table 2. The self-corrosion current density Jcorr can be obtained by Tafel extrapolation method, which represents the corrosion rate of the cladding layer. The smaller its value, the slower the corrosion rate. The self-corrosion potential Ecorr represents the possibility of corrosion of the coating. The closer its value is to the positive potential, the more difficult the coating is to be corroded. Both analyze the corrosion resistance of the coating from the perspective of thermodynamics of material corrosion.

As shown in Figure 2, the Gr/Ni60 composite coating does not show obvious anodic passivation in the HCl solution, that is, no stable passivation interval is produced. Among them, the self-corrosion current density in the cathode polarization curve far away from the potential area decreases slowly, indicating that this process is controlled by the hydrogen ion activation reaction. The corrosion products on the coating surface are easily dissolved in the acidic environment, so the passivation phenomenon is not significant. In addition, the coating surface is affected by the anion concentration in the corrosion environment, especially the Cl− concentration. In the HCl corrosion environment, it is difficult to form a stable passivation state on the surface in a short time. In addition, according to Table 2, compared with the self-corrosion potential of the pure Ni60 coating (−0.466 V), the self-corrosion potential values ​​of the composite coating with Gr added are all positively shifted. When the mass fraction of Gr is 0.8%, the self-corrosion potential value reaches −0.384 V, and the self-corrosion current density is 0.943 μA/cm2, indicating that Gr plays a positive role in improving the acid corrosion resistance of the Ni60 composite coating. Therefore, the order of the samples from good to bad in terms of acid corrosion resistance is: d>e>c>b>a.

2.2.2 AC impedance spectrum

In order to obtain more electrochemical corrosion performance parameters of Gr/Ni60 composite coatings, the Ni60 composite coatings with different Gr contents were subjected to AC impedance spectrum tests (EIS). The equivalent circuit diagram used in the experiment is shown in Figure 3. By changing the connection method of the resistor R and the capacitor C in the equivalent circuit, the electrochemical process of the coating can be simulated. In this experiment, all capacitors use constant phase angle elements (CPE) to weaken the influence of the unevenness of the coating surface. The measured data were analyzed and fitted by Zview software. The physical meaning of each component in the equivalent circuit is as follows: Rs is the solution resistance, Rpass is the passivation film resistance, Rct is the charge transfer resistance, Cpass1 (CPE1) is the passivation film capacitance, and Cpass2 (CPE2) is the capacitance between the coating and the corrosion solution interface. Cpass1 reflects the charge transfer process between the passivation film and the coating, which is determined by the constant coefficient Y1 and the diffusion coefficient n1; Cpass2 reflects the charge transfer process between the anode electrode surface and the corrosive solution, which is determined by the constant coefficient Y2 and the diffusion coefficient n2.

The EIS test results are fitted by the equivalent circuit diagram shown in Figure 3, and the results are shown in Figure 4. It can be seen that the fitted EIS curve is highly consistent with the curve obtained from the experimental data, indicating that the equivalent circuit selected for EIS fitting is more appropriate. Figure 4a is a Nyquist diagram, where Z″ represents the capacitive reactance in the system and Z’ represents the resistance in the entire system. It can be seen that under the action of double-layer capacitance and charge transfer resistance, all composite coatings with different Gr contents show a capacitive reactance arc. The relationship between frequency, phase angle and system impedance can be seen from the Bode diagram (see Figure 4b and Figure 4c). As shown in Figure 4a, with the increase of Gr content, the arc radius gradually increases, which means that the total resistance of the cladding layer gradually increases. Therefore, the corrosion resistance of the coating in HCl environment increases with the increase of Gr content in the composite coating. The Bode diagram is divided into three parts: low frequency, medium frequency and high frequency. In the high-frequency region where the phase angle of all coatings tends to 0°, the capacitance can be regarded as a conductor and the impedance is guided by the solution resistance. In the medium-frequency region, the phase angle of the sample gradually increases to 50°~65° but does not reach 90°, which is considered to be a “semi-capacitive” feature. The size of the phase angle is usually used to indicate the ability of the material to hinder the penetration of the electrolyte. This indicates that the coating surface also produces a passivation film similar to the capacitor layer and has good insulation properties, but it exists for a very short time. In the low-frequency region of 10’−1~10’2Hz, the impedance reflected by the corrosion is represented by the impedance modulus |Z|. Generally speaking, a larger impedance modulus and a larger phase angle indicate that the material has good corrosion resistance. As can be seen from Figure 4c, the impedance modulus |Z| of the composite coating with Gr added is greater than that of the pure Ni60 coating. When the mass fraction of Gr is 0.8%, the impedance modulus |Z| reaches the maximum value, indicating that charge transfer is difficult at this time and the corrosion resistance is the best. According to the author’s previous research [21], the addition of Gr will improve the uniformity of the structure, refine the grains, and increase the density of the grain boundaries, thereby blocking the corrosion channel to a certain extent and slowing down the corrosion. The diffusion rate of Cl− is increased, so the corrosion resistance is improved to a certain extent. In addition, although the generation rate of the surface passivation film in the acidic solution is lower than its dissolution rate, the generated passivation film also intercepts the corrosive ions that want to pass through to a certain extent, and has a certain protective effect on the surface.

In order to express the size of the resistance value of all film layers in the system, the concept of polarization resistance [22] is introduced, and the polarization resistance (Rp=Rpass+Rct) is used to represent the total resistance of the entire electrochemical system. Generally, the larger the polarization resistance, the better the corrosion resistance of the material. The electrochemical impedance fitting results of Gr/Ni60 composite coating in HCl solution are shown in Table 3. It can be seen that the order of the polarization resistance of each sample from large to small is d>e>c>b>a. This is consistent with the analysis results of the polarization curve. The corrosion resistance of the coating increases with the increase of Gr content, and the corrosion resistance decreases when it is added to a certain extent.

2.2.3 X-ray photoelectron spectroscopy

When the mass fraction of Gr is 0.8%, the composite coating is immersed in HCl solution for 3 The full XPS spectrum and high-resolution spectrum after d are shown in Figure 5. As shown in Figure 5a, the coating surface contains Fe 2p, Cr 2p, Ni 2p, O 1s, C 1s and other spectral lines. The Fe 2p3/2 high-resolution peak spectrum is composed of Fe2+ (709.93 eV) and Fe3+ (712.3 eV) peaks, and the Ni 2p fine spectrum peak is composed of Ni2+ (855.56 eV) and Ni2+ (873.21 eV) peaks. Under the strong corrosion of acid, Fe and Ni in the coating lose electrons at a fast rate, so only a few Fe and Ni peaks are detected. This may be related to the test position and X-ray sputtering. From the XPS results, it can be judged that the corrosion reaction formula in the acidic electrolyte is as shown in equations (1)-(3). As can be seen from Figure 5, Fe-O (530.42 eV), Cr-O (531.86 eV), and Ni-O (532.88 eV) peaks were detected in the fine spectrum of O. It can be judged that a passivation film mainly composed of oxides of elements such as Fe, Cr, and Ni was also produced in an acidic environment. The passivation film exists on the coating surface in a dynamic process, that is, a process of formation, destruction, and re-formation. However, in an acidic environment, the generation rate of the passivation film is lower than the dissolution rate of the passivation film, and a dense passivation film cannot be formed. Therefore, there is no obvious passivation interval at the anode in an acidic environment.

In the experiment, the peaks of Cr3+ (576.58 eV), Cr6+ (577.88 eV) oxides and carbides and Cr element were also detected. Reference [23] shows that Cr can increase the stability of the amorphous structure in the alloy and enrich in the oxide film, making the oxide film more uniform, thereby slowing down the corrosion rate. Therefore, the acid corrosion resistance of the Gr coating is improved. In addition, the improvement of corrosion resistance is also closely related to the generation of Fe, Ni, and Gr oxides.

2.2.4 Corrosion morphology

The surface SEM morphology of the Gr/Ni60 composite coating after immersion in HCl solution for 3 days is shown in Figure 6. It can be seen that the corrosion surface of the pure Ni60 coating has a large number of corrosion pits of varying sizes. It is speculated that the coating has pitting corrosion in the acidic solution and local cracking due to stress corrosion. From the perspective of corrosion, the Cl− in the solution has a small radius and is easier to adhere to the passivation film, causing the passivation film to become an ion conductor locally. The current density at this point rises sharply, making the cations in the solution active [24]. Cl− will replace the O element in the passivation film and combine with the cations in the solution to produce soluble salts to corrode the passivation film, thereby forming pitting pits. At this time, the dissolution of this area is accelerated and has occlusion. The cation concentration continues to increase. In order to balance, Cl− continues to flow into the pitting pit, resulting in Cl− enrichment, aggravated corrosion, and enlarged pitting pits, which eventually achieves the transformation from passivation to activation. In addition, in acidic solution, H+ will also enter the pitting pit on the coating surface together with Cl−[25], and be released in the form of hydrogen, etc., and the generated hydrogen bubbles will produce a stirring effect to accelerate the diffusion of H+ in the solution, further expanding the defects. With the increase of Gr content, the number of pitting pits on the corrosion surface decreases, and the corrosion situation is also significantly improved. However, with the further increase of Gr content in the composite coating, the number of crystal nuclei increases accordingly, resulting in disordered organization of the cladding layer during cooling, increased impurities in the dendrite area, and carbon element agglomeration and uneven distribution. In form, it is manifested as an increase and uneven distribution of carbides, which reduces the barrier performance of the coating and may even open up new pathways for corrosive media. In addition, the carbides squeezed into the dendrites will form micro batteries with other elements, thereby accelerating intradendritic corrosion and reducing corrosion resistance.

2.3 Electrochemical corrosion performance of Gr/Ni60 composite coating in neutral (NaCl) solution

2.3.1 Potentiodynamic polarization curve

The potentiodynamic polarization curve of Gr/Ni60 composite coating in NaCl electrolyte is shown in Figure 7. The polarization curve in a neutral environment can be divided into four zones: active dissolution zone, transition passivation zone, stable passivation zone, and over-passivation zone. Its electrochemical parameters are shown in Table 4. It can be seen that with the increase of the applied voltage, the self-corrosion current density slowly increases, and the composite coatings with different Gr contents all show different degrees of passivation, that is, a passivation zone in which the current does not increase with the increase of potential. Eb is the pitting breakdown potential of the coating surface, which reflects the pitting resistance of the coating. ΔE=Eb−Ecorr reflects the size of the coating passivation zone and can also reflect the corrosion resistance of the coating. According to Table 4, the Eb and ΔE of the composite coating with Gr added are higher than those of the pure Ni60 coating, and the self-corrosion potential is also significantly positive than that of the pure Ni60 coating. In addition, when the mass fraction of Gr is 0.8%, the self-corrosion potential (−0.728 V), breakdown potential (−0.367 V) and passivation range (0.285 V) of the coating are higher than those of other composite coatings with Gr content. Therefore, through polarization analysis, it can be seen that the order of corrosion resistance of each sample from good to bad is d>e>c>b>a.

2.3.2 AC impedance spectrum analysis

The EIS test and fitting results of Gr/Ni60 composite coating in NaCl electrolyte are shown in Figure 8. As can be seen from Figure 8a, the impedance spectrum in NaCl electrolyte shows the characteristics of a capacitive arc, and the radius of the capacitive arc increases with the increase of Gr content in the coating. Similar to acid (HCl), the phase angle in the high-frequency region is close to 0°, indicating that the system impedance at this time is mainly dominated by the electrolyte resistance.

As can be seen from Figure 8c, in the low-frequency region, the impedance modulus |Z| of the pure Ni60 coating is one order of magnitude lower than the impedance modulus |Z| of the Gr/Ni60 composite coating, indicating that the corrosion resistance of the composite coating has been improved after the addition of Gr. In addition, in the medium-frequency region, the phase angle value of the composite coating with a Gr mass fraction of 0.8% is about 70°, and the phase angle peak can appear earlier. The impedance modulus |Z| of the coating impedance spectrum is almost linearly related to the frequency, which is a characteristic response of typical capacitance behavior. Combined with the polarization curve, it can be seen that there is obvious self-passivation behavior, indicating that a relatively uniform and stable passivation film is produced on the coating surface, mainly because the carbide distribution is most uniform when the Gr mass fraction in the composite coating is 0.8%. According to Figure 1, the distribution of Cr carbides is mainly between dendrites, and the literature [26] shows that chromium-rich carbides act as the nucleation core of chromium-rich oxides. Chromium-rich carbides distributed on the grain boundaries will preferentially form Cr2O3 at the interface of the oxide cladding layer, which plays a good protective role for the cladding layer and improves the corrosion resistance accordingly.

The electrochemical impedance fitting results under neutral environment (NaCl) are shown in Table 5. It can be seen that when the mass fraction of Gr is 0.8%, the diffusion coefficient of the constant phase angle element (CPE) in the equivalent circuit is close to 1, indicating that the generated passivation film is relatively uniform. At this time, the electrochemical system is close to a pure capacitance system. As shown in Table 5, the polarization resistance of the composite coating with Gr added is greater than that of the pure Ni60 coating. The order of the polarization resistance of each sample from large to small is d>e>c>b>a, that is, the corrosion resistance order of each sample under neutral environment is d>e>c>b>a. This is consistent with the results obtained from the polarization curve.

2.3.3 X-ray photoelectron spectroscopy

The XPS full spectrum and high-resolution spectrum of the composite coating after immersion in NaCl solution for 3 days when the mass fraction of Gr is 0.8% are shown in Figure 9. It can be seen that the coating surface contains Fe 2p, Cr 2p, Ni 2p, O 1s, C 1s, Ca 2p and other spectral lines. The high-resolution spectra of Fe 2p3/2 and Fe 2p1/2 are composed of the oxide peaks of Fe2+ (710.83 eV) and Fe3+ (712.68 eV), and the fine spectrum peak of Cr 2p3/2 is composed of the peaks of Cr element (573.65 eV), Cr3+ (576.24 eV), and Cr6+ (578.55 eV). In order to understand the process of formation and destruction of the passivation film of the Cr-Fe coating in a neutral (NaCl) electrolyte, the polarization test process was observed. It can be seen that the electrolyte gradually turned light yellow, and Fe-O bonds, Ni-O bonds, and Cr-O bonds were detected in the fine spectrum of the O element. It is speculated that Fe, Ni, and Cr oxides were generated on the coating surface. Since the solution is neutral, it cannot react directly with the anode material. The anode reaction is mainly controlled by Fe, Ni, and Cr. As the voltage increases, the current drops sharply, and the dissolved oxygen near the cathode undergoes a reduction reaction. The anode and cathode reaction equations of the coating are shown in equations (4) to (10).

Since there is an obvious passivation interval in the neutral polarization curve, it is speculated that stable passivation products Fe3O4, Cr2O3, and NiO2 are generated on the surface of the sample. Related studies have also shown that the improvement of the corrosion resistance of the coating is mainly due to the formation of difficult-to-dissolve oxides with double-layer structure characteristics on the surface. The inner layer is mainly Cr oxide, the outer layer is Fe or Ni oxide, and the Ni element under the passivation film will also prevent the passivation film from being reduced at this position, thereby reducing the active dissolution rate of the coating anode and improving its corrosion resistance. The formation and destruction of the passivation film are closely related to the corrosion resistance of the coating. In addition, a strong Cr3+ peak can be detected from the high-resolution spectrum peak of Cr, and Cr-O bonds are also detected in the high-resolution spectrum peak of the O element, indicating that there is a Cr2O3 oxide film on the surface of the cladding layer, which confirms the above analysis. As the voltage increases, the current density fluctuates greatly. The oxides that make up the passivation film will be adsorbed on the surface by Cl−, replacing oxygen atoms and combining with cations in the passivation film to form soluble salts, thereby destroying the dynamic balance of the passivation film and causing pitting corrosion, so the corrosion current density rises sharply.

2.3.4 Corrosion morphology

The surface SEM morphology of the Gr/Ni60 composite coating after immersion in NaCl solution for 3 days is shown in Figure 10. It can be seen that there are more corrosion pits on the surface of the pure Ni60 coating and the composite coating with a Gr mass fraction of 0.3%, and the corrosion within the dendrite is more serious from the local enlarged image. Overall, the surface morphology in a neutral environment is mainly pitting pits and cracks caused by corrosion stress. The Cl−-rich environment of the neutral NaCl solution is similar to the acidic environment. Cl− can destroy the passive film on the surface of the coating, thereby inducing pitting corrosion and promoting stress corrosion cracking. The accumulation of Cl− in the pits hinders the repair of the passive film and thus expands the defects [27]. With the increase of the Gr content in the composite coating, the number of corrosion pits decreases, which is consistent with the results of the polarization test and EIS test. The corrosion resistance first increases and then decreases, which is mainly due to the formation of the passive film. Through microscopic morphology analysis, it can be seen that the carbon atoms in Gr have relatively high activity and fast diffusion speed during laser cladding. Carbon atoms are very easy to dissolve and thus form carbide phases with carbide-forming elements (Fe, Cr, etc.) and are distributed at the grain boundaries. According to the above analysis, when the mass fraction of Gr is 0.8%, the carbide distribution is the most uniform, and a passivation film that protects the cladding layer is also formed at the grain boundaries, so the corrosion resistance is also the best. However, as the Gr content increases further [21], the highly active carbon elements will be segregated, resulting in uneven distribution of the generated carbides, which is easy to form microscopic corrosion galvanic cells, generate corrosion stress during the corrosion process, and thus accelerate the corrosion of the coating. As shown in Figure 10e, cracks appear on the corrosion surface and the corrosion resistance is reduced.

2.4 Electrochemical corrosion performance of Gr/Ni60 composite coating in alkaline (NaOH) solution

2.4.1 Potentiodynamic polarization curve

The potentiodynamic polarization curve of Gr/Ni60 composite coating in NaOH solution is shown in Figure 11, and its electrochemical performance parameters are shown in Table 6. It can be seen that all composite coatings have an anodic current peak at point A (about −1.102 V) and an anodic current valley at point B (about −1.043 V), which is speculated to be caused by the change in OH− concentration. Different from the shape of acidic and neutral polarization curves, the alkaline passivation interval is longer. It is worth noting that when the mass fraction of Gr is 0.8%, the current peak at point A is lower than that of other coatings, which indicates that the anodic reaction is slow at this time.

As shown in Table 6, the self-corrosion potential Ecorr of all samples in alkaline (NaOH) solution is generally low and not much different. With the increase of Gr content, the self-corrosion potential slightly shifts positively. When the mass fraction of Gr is 0.8%, the stable passivation interval of the coating is the longest, reaching 1.506 V; however, there is no obvious change pattern of the self-corrosion current density, which shows that the self-corrosion current density measured by the Tafel extrapolation method is not completely accurate. The evaluation of the corrosion resistance of the coating should be combined with a variety of test methods and a variety of test data, which also explains the necessity of AC impedance spectrum analysis and a series of subsequent analyses. According to the self-corrosion potential, it is preliminarily believed that the coating performs best in the polarization test when the mass fraction of Gr is 0.8%. The order of the corrosion resistance of each sample from good to bad is d>e>c>b>a.

2.4.2 AC impedance spectrum

The EIS test and fitting results of Gr/Ni60 composite coating in NaOH solution are shown in Figure 12. As shown in Figure 12a, all coatings show a capacitive reactance arc in the mid-frequency region, and the radius of the capacitive reactance arc also increases with the increase of Gr content. When the Gr mass fraction is 0.8%, the corresponding capacitive reactance arc radius is the largest, indicating that the total resistance of the entire system reaches the maximum value at this time. In the mid-frequency region, the phase angle of the coating with a Gr mass fraction of 0.8% is about 75°, indicating that the coating has shown insulation characteristics dominated by capacitive reactance behavior. In the low-frequency region, the total impedance value of the system is also higher than that of other coatings. The electrochemical impedance fitting results of Gr/Ni60 composite coating in NaOH solution are shown in Table 7. It can be seen that the polarization resistance of the composite coating with Gr added is greater than that of the pure Ni60 coating, and the polarization resistance value is 2 orders of magnitude higher than that in acidic and neutral environments. The order of polarization resistance of each sample from large to small is d>e>c>b>a, which is consistent with the results measured by the polarization curve. Therefore, when the mass fraction of Gr is 0.8%, the coating has the best alkaline corrosion resistance. There is a relatively wide passivation range in the polarization curve. It is speculated that the corrosion resistance of the coating under alkaline conditions is the best compared with neutral and acidic conditions.

2.4.3 X-ray photoelectron spectroscopy analysis

The XPS full spectrum and high-resolution spectrum of the composite coating with a Gr mass fraction of 0.8% after immersion in NaOH solution for 3 days are shown in Figure 13. It can be seen that the coating surface contains Fe 2p, Ni 2p, Cr 2p, O 1s, C 1s, Si 2s, and Si 2p spectral lines. The high-resolution spectrum of Fe 2p3/2 is composed of Fe2+, Fe3+, and Fe single peaks. The high-resolution spectrum of Ni 2p is composed of Ni2+ and Ni single peaks. The high-resolution spectrum of Cr 2p3/2 is composed of Cr6+, Cr3+, and Cr single peaks. In an alkaline environment, single peaks of three elements, Fe, Ni, and Cr, were detected, indicating that the anode reaction in an alkaline environment is weaker than that in an acidic environment. As described in Section 2.4.1, the polarization curve of the Gr/Ni60 composite coating in alkaline solution has an anodic current peak and a slightly decreasing current valley. Combined with the high-resolution spectrum of O 1s in XPS, which contains Fe-O bonds, Ni-O bonds, and Cr-O bonds, it can be judged that in an alkaline environment rich in OH− ions, the dynamic conversion of passivation film destruction and repair is significant, and the alkaline environment rich in OH− is conducive to the formation of highly stable oxides.

In an alkaline environment, the Gr/Ni60 composite coating mainly undergoes oxygen absorption corrosion. The spectrum corresponding to the figure is the oxidation reaction of the anode Fe, Ni, and Cr. The reaction equations are shown in equations (11)-(20).

In NaOH solution, the OH− content is high, and the cations in the solution will first react with the OH− in the solution to form hydroxides, and then continue to react with the OH− in the solution to form relatively stable high-valent oxides, i.e., corrosion products. This can be confirmed from the oxygen-containing bonds in the high-resolution spectrum of O 1s. During the entire dynamic conversion process of the passive film, as the corrosion proceeds, the generated corrosion products cover the surface of the passive film in the form of protective oxides, and continue to increase to compensate for the dissolution process of the passive film, preventing corrosion from alkaline solutions, and the generation rate of the passive film is higher than its dissolution rate, which improves the overall wear resistance. In addition, in the over-passivation zone, due to the instability of Cr(OH)6, the possible reaction is shown in formula (21), which will cause the corrosion current density to increase, thereby destroying the equilibrium process of the dynamic conversion of the passive film and generating corrosion pits.

2.4.4 Corrosion morphology

The surface SEM morphology of the Gr/Ni60 composite coating after immersion in NaOH solution for 3 days is shown in Figure 14. It can be seen that, unlike the acidic and neutral environments, the pitting pits under alkaline corrosion conditions are smaller and more dense, and no cracks or cracks are found, especially when the Gr mass fraction is 0.8%, almost no corrosion pits can be seen on the coating surface. According to the EIS test results, the polarization resistance value of the whole system reached 3030 Ω/cm2 at this time, and the corrosion resistance was the best. This is also attributed to the fact that the carbides distributed at the grain boundaries can protect the grain boundaries, thereby improving the corrosion resistance. With the further increase of Gr content, the number of corrosion pits increased again. Therefore, in alkaline solution, the corrosion morphology of Gr/Ni60 composite coating is good, it is not easy to produce more serious corrosion pits and cracks, and the overall corrosion resistance is good.

3 Conclusion

The effect of graphene (Gr) on the corrosion resistance of nickel-based cladding was studied by electrochemical workstation, and the specific conclusions are as follows.

1) Gr/Ni60 composite coating was successfully prepared on the surface of Q235 steel by laser cladding technology. The addition of Gr caused the segregation of elements. Cr and C elements were distributed between dendrites, while Fe and Ni elements were mainly distributed in dendrites. The interdendrites were mainly composed of Cr carbides, and the dendrites were mainly composed of Fe and Ni solid solutions.

2) Compared with the single active dissolution in 0.1 mol/L HCl solution (acidic), Gr/Ni composite coating produced obvious passivation interval in 3.5% NaCl and 1 mol/L NaOH solutions, and the composite coating had a higher self-corrosion potential than Ni60 coating. When the mass fraction of Gr was 0.8%, the coating showed excellent corrosion resistance.

3) The Nyquist curves in the three solutions all showed the characteristics of a capacitive arc. X-ray photoelectron spectroscopy analysis confirmed that a passivation film was formed on the surface of the composite coating. In 1 mol/L NaOH solution, the polarization resistance value of the composite coating with a Gr mass fraction of 0.8% reached 3 030.32 Ω/cm’2, indicating that the corrosion resistance of the composite coating was much higher than that of the Ni60 coating, and the corrosion resistance in NaOH solution was better than that in HCl and NaCl solutions.