1. Introduction
As land resources are increasingly depleted, people have turned their attention to the development of marine resources. Human development of the ocean relies on various marine equipment. However, in the process of service, marine equipment is subject to severe corrosion due to the presence of moist, salt-laden air and water vapor in seawater. Marine engineering equipment such as ships and offshore platforms work under severe corrosive environments such as salt spray corrosion (caused by solid NaCl and water vapor) in high-temperature environments, which poses challenges to marine equipment materials [
1,
2,
3]. Although traditional metal materials have been studied to improve the corrosion resistance, their protective effect is limited in such harsh marine environments [
4,
5,
6]. In order to improve the durability of such equipment, spraying high-performance coatings has become a common and effective protective method, aiming to form a strong barrier to isolate the direct erosion of corrosive media and metal substrates [
7,
8,
9,
10].
Compared with metal alloys, amorphous alloys exhibit remarkable advantages in terms of the physical, chemical, and mechanical properties, such as the high strength and hardness, high elastic strain limit, and excellent wear resistance and corrosion resistance. In 2013, Ye and Shin [
11] synthesized Fe–Cr–Mo–W–Mn–C–Si–B metallic glass composite materials containing a large amount of amorphous phase using the laser direct deposition method. They found that the microhardness (HV
0.2 1591) of the amorphous phase was significantly higher than that of the crystalline phase (HV
0.2 947), and the wear resistance increased significantly with the increase in the amorphous phase ratio. However, due to the limitation of the glass-forming ability (GFA), it is very difficult to fabricate large-scale BMG workpieces or directly use them as structural materials, which limits the application of amorphous alloys. Fortunately, amorphous coatings prepared by thermal spraying have demonstrated their advantages in corrosion resistance [
12], showing great potential in industrial applications. Lin et al. [
13] prepared Fe
40Cr
19Mo
18C
15B
8 Fe-based amorphous coating on 316 stainless steel using the high-velocity oxy-fuel (HVOF) spraying method. The experimental results showed that the Fe-based amorphous alloy coating exhibited a transition behavior of activation, passivation, and overpassivation in seawater and 3.5% NaCl solution. After alternating cold and hot salt-spray corrosion and high-speed water erosion tests, although the coating surface showed rusting after 12 weeks, its weight loss was minimal and a stable passive film was formed. This indicates that Fe-based amorphous alloys have excellent stable passivation and corrosion resistance properties. Wang et al. [
14] prepared Fe-based amorphous coatings on an AISI 1020 steel tube surface using high-speed laser cladding technology, showing excellent corrosion resistance. The corrosion potential of this coating was as high as −0.471 V, the corrosion current density was as low as 2.7 × 10
−6 A/cm
2, and the polarization resistance value was as high as 22,149 Ω·cm
2. These excellent corrosion resistance properties are mainly due to the high content of amorphous phase (up to 95%) in the coating and the protection of the Cr oxide layer formed on the surface.
Adding a second phase to Fe-based amorphous alloys is often an effective means of improving their corrosion resistance. Chu et al. [
15,
16] prepared TiN/Fe-based amorphous composite coatings and AT13/Fe-based amorphous composite coatings through plasma-spraying technology. From the morphology point of view, the typical layered structure of the thermal spray coatings and the close combination of the two phases in these two composite coatings have no obvious structural defects, and they have good corrosion resistance and wear resistance.
In recent years, graphene, as a new emerging material, has attracted worldwide attention for its unique properties and wide application prospects. In the field of corrosion protection, graphene is a cutting-edge material that can be used as a nano-filler reinforcement material to enhance its anti-corrosion performance [
17,
18]. Various research results reveal that the effectiveness of graphene in preventing corrosion is mainly attributed to its unique “maze effect” [
19,
20,
21]. When corrosive media attempt to penetrate the graphene structure, its intricate layout makes the diffusion path of the corrosive media extremely tortuous and difficult. In addition, graphene can significantly fill the tiny pores in composite coatings, thereby reducing the porosity of the coating and enhancing its compactness [
22]. Moreover, graphene is also known for its excellent mechanical properties. Introducing graphene as an additive into composite coatings can greatly improve the wear resistance and other mechanical properties of the coating [
23,
24].
In fact, the corrosion damage of high-temperature seawater to materials is more severe. For marine equipment that must operate in high-temperature environments, they need to be exposed to high-temperature seawater for long periods, such as tens or even hundreds of degrees Celsius. For example, the service environment of the armor layer of submarine oil and gas pipelines is generally between 20 °C and 130 °C [
25]. Such extreme conditions can easily lead to equipment failure due to corrosion, so there are extremely stringent requirements for its corrosion resistance.
There have been a lot of studies on the corrosion resistance of Fe-based amorphous coatings in ambient seawater [
13,
14,
15,
16], but there are few research articles on the high-temperature seawater corrosion resistance of Fe-based amorphous coatings. For the high-temperature corrosion resistance of Fe-based amorphous coatings, some scholars believe that high temperatures affect the formation of passive films [
26], while other scholars’ research has proved the negative impact of high temperatures on metal passive films [
27,
28]. The working conditions of marine equipment are complex, and a large number of corrosion studies focused on ambient seawater cannot meet the service needs of certain special equipment, so it is very important to study the corrosion behavior of Fe-based amorphous coatings in high-temperature seawater.
In this study, Fe-based amorphous composite coatings of reduced graphene oxide (RGO)/copper (Cu) were prepared using plasma-spraying technology. The Fe-based amorphous composite coating containing 15% RGO/Cu was immersed in simulated seawater at 90 degrees Celsius for up to 18 days. Its resistance to high-temperature seawater corrosion was comprehensively evaluated and the protective mechanism of the coating was reviewed.
2. Experimental Materials and Methods
2.1. Preparation of the Coating
GO/Cu composite powder was prepared by the gas-atomizing drying method, in which the mass ratio of GO to Cu was 1:9. Then, the GO in the GO/Cu composite powder was reduced by the thermal reduction method to obtain RGO/Cu composite powder. The Cu powder and Fe-based amorphous powder required for this experiment were purchased from Shanghai Naio Nanotechnology Co., Ltd. (Shanghai, China). The purity of the Cu powder was 99.9%, and the particle size was 1 μm. The chemical formula of the Fe-based amorphous composite powder was Fe
45Cr
16Mo
16C
18B
5, with a particle size ranging from 15 μm to 45 μm. The GO was purchased from Suzhou Carbon Feng Technology Co., Ltd. (Suzhou, China), with a layer count of 1–2 and a purity of over 98%. The sheet diameter ranged from 0.2 μm to 10 μm. The mechanical mixing of the Fe
45Cr
16Mo
16C
18B
5 amorphous powder with m (RGO/Cu mass accounting for 5%, 10%, 15%, and 20% of the total) resulted in RGO/Cu/Fe-based amorphous composite powder. Finally, an RGO/Cu/Fe-based amorphous composite coating was prepared on the surface of 45# steel (0.45 wt.% C) with a size of 10 mm × 10 mm × 12 mm by plasma spraying. The powder used for spraying is shown in
Figure 1, and the plasma spraying parameters are shown in
Table 1 [
29].
In order to confirm the introduction of graphene, Raman spectroscopy (as shown in
Figure 2a), a conventional method for characterizing graphene, and infrared spectroscopy (as shown in
Figure 2b) were used for the composite powder. It can be calculated from the Raman spectrum that the ID/IG value of RGO/Cu was 1.14, and the ID/IG value of GO/Cu was 1.10. After the thermal reduction of the powder, the ID/IG value increased slightly, indicating a slight increase in disorder. The content of various oxygen-containing functional groups is shown in
Figure 2b. It can be seen that at 3440 cm
−1, 1630 cm
−1, 1400 cm
−1, and 1060 cm
−1, oxygen-containing functional groups corresponding to -OH, C=O, C–OH, and C–O–C appeared, respectively. Moreover, the intensity of the oxygen-containing functional groups of the reduced RGO/Cu composite powder decreased, indicating the presence of GO, and GO forms RGO through reduction.
2.2. Experimental Route
In this paper, firstly, potentiodynamic polarization curve scanning was conducted on the coatings with different RGO/Cu addition ratios after being immersed in simulated seawater at room temperature for 30 days. The long-term corrosion performance of the four coatings in simulated seawater at room temperature was compared, and the optimal RGO/Cu addition ratio was obtained. Based on this optimal ratio coating, it was immersed in simulated seawater at 90 degrees Celsius for 18 days, and its high-temperature corrosion performance was tested. A series of characterizations were performed on its microstructure, phase, etc. The simulated seawater composition is shown in
Table 2 [
15].
2.3. Coating Performance Testing Method
The electrochemical workstation interface 1010E produced by Gamry was used. During the test, a three-electrode system was used, with the coating sample as the working electrode, the saturated calomel electrode as the reference electrode, and the platinum sheet electrode as the counter electrode. Before performing the potentiodynamic polarization curve scan and electrochemical impedance spectroscopy (EIS) experiments, the coating was polished and buffed until it was in a mirror state, and it was then encapsulated with epoxy resin, leaving only a 1 cm2 surface to be tested. The measurement of the potentiodynamic polarization curve was performed after the open circuit potential (EOCP) stabilized, with a scan rate of 1 mV/s and a scan range set at EOCP ± 500 mV. The test frequency of the electrochemical impedance spectroscopy ranged from 100,000 Hz to 0.1 Hz, with an additional sine wave AC perturbation frequency of 5 mV. The coating was subjected to polarization curve and EIS tests after being immersed in high-temperature simulated seawater for 0 day, 1 day, 4 days, 7 days, and 18 days. Each set of samples was tested three times and averaged to avoid accidental errors. After the testing was completed, data analysis was performed using Gamry Echem Analyst software (version 7.0.0.7). For each sample, the potentiodynamic polarization curve test was conducted after the EIS test.
For the composite coatings with different soaking times, scanning electron microscopy (SEM, S4800, Hitachi, Tokyo, Japan) was used to observe the surface morphology of the coatings. For the composite coatings soaked for 18 days, energy dispersive spectrometer (EDS) was used to analyze the elemental distribution of the coating surface, and X-ray diffraction (XRD, Bruker D8 Focus, Billerica, MA, USA) and X-ray photoelectron spectroscopy (XPS, Thermo Scientific K-Alpha, Thermo Fisher Scientific, Waltham, MA, USA) were used to analyze the phase composition of the coating surface. In the XRD test, the specific scanning angle was 20°–80°, and the scanning speed was 2°/min. In the XPS test, the fine spectra of the Cu and Fe elements were tested.
4. Discussion
In order to further reveal the corrosion mechanism of RGO/Cu/Fe-based amorphous composite coatings, a schematic diagram is shown in
Figure 16. During the spraying process, it is inevitable that some pores will appear in the single Fe-based amorphous coating, even if the spraying quality of the coating is excellent. Pitting defects are prone to occur and expand at the pores. In high-temperature seawater, the difference in thermal expansion coefficients between different phases can easily lead to the expansion and cracking of the coating, resulting in the corrosion medium reaching the coating–substrate interface through defects [
34]. Copper in the composite coating is beneficial for sealing the pores generated by thermal spraying, reducing the stress concentration at the pores, and reducing the occurrence of pitting corrosion. This may be why there are few pitting corrosion defects on the surface of the coating after removing the corrosion products in the SEM images. Graphene can increase the toughness of the coating and hinder the generation of cracks during exposure to high-temperature environments [
35]. At the same time, due to its “maze effect”, the corrosion medium is difficult to penetrate through graphene to reach the substrate, which is beneficial for forming a physical barrier, extending the corrosion channel, and prolonging the service life of the coating.
In high-temperature seawater, the following chemical reactions mainly occur on the surface of the coating:
In high-temperature seawater, the high temperature accelerates the dissolution of the corrosion product films Fe(OH)2 and Cu(OH)2, resulting in the formation of more Fe3O4 and CuO on the coating surface, which is also consistent with the phase analysis results.