Focused Review on Graphitic Carbon Nitride (g-C₃N₄) in Corrosion and Erosion Applications

Abstract

Inorganic, organic, and metallic coatings have received much attention due to their great protection applications in oil and gas industries and their development through finding eco-friendly materials for the coatings. Their unique structure, low cost, and facile synthesis are some of the few properties of graphitic carbon nitride (g-C₃N₄) materials that make them useful in a number of applications. Moreover, g-C₃N₄ offers exceptional chemical and thermal stability and a high specific surface area, and it is well known for its outstanding biocompatibility and biological activity. Numerous investigations have reported various types of C₃N₄-incorporated coatings that have enhanced corrosion, wear, and mechanical resistance properties. This review highlights the new applications of g-C₃N₄ (standalone, in an alloy, or composite) as a coating in the wear and corrosion protection fields. Furthermore, a strong focus on the structure, unique properties, and preparation technique of g-C₃N₄ are summarized, especially in metallic coatings, which is a highly novel trend. Lastly, various important issues based on current research are proposed for future prospective work that should be further studied in this attractive research topic.

1. Introduction

Corrosion of metallic-based materials is one of the oldest scientific problems on earth and is currently one of the industry’s significant threats. Its danger is attributed to its high life, economic, and environmental costs. World-leading industrial plants operate at around 95% or more asset utilization rates. Unfortunately, most continuous manufacturing facilities operate near 80% or less. This shortfall equates to an enormous loss in business earnings and opportunities, and a significant fraction of this is caused by corrosion-related equipment failures. An estimated 3 to 4% of industrialized nations’ GDP (gross domestic product) is lost yearly due to corrosion-related damage and failure [1]. NACE estimates the annual cost of corrosion worldwide to be 2.5 trillion USD [2]. A significant fraction of this cost comes from the oil and gas (O&G) sector, where the O&G field components (e.g., pipelines) are exposed to challenging environments, e.g., CO₂ and H₂S in the produced fluids. For example, in Qatar, the cost of corrosion was estimated to be 78 billion in 2011—which was double the amount in 2007 [3], and a large fraction of these costs are incurred by the oil and gas section. Furthermore, safety and environmental concerns make corrosion mitigation a key concern for industry and government. Therefore, several approaches have been employed to eliminate, mitigate, or control corrosion of metallic materials. Different corrosion mitigation strategies are adopted depending on the operational conditions and cost, such as corrosion inhibitors, coatings, cathodic protection, etc. Resistant coatings, in their various forms, are the most widely used technique across different sectors of the corrosion industry. Its market was 24.84 billion USD in 2017 and is estimated to increase to 31.73 billion USD by 2022, at a compound annual growth rate (CAGR) of 5.0% from 2017 to 2022 [4]. These coatings are mainly divided into metallic- and organic-based coatings, e.g., epoxy, polyurethane, acrylic, alkyd, Zinc, Ni and Ni-P, Ni-B, targeting many industries, e.g., marine, oil and gas, industrial, infrastructure, and power generation using different technologies to apply such coatings, e.g., solvent, water, powder. Metallic-based coatings are usually used when the coatings’ mechanical properties are required, other than the corrosion resistance ones. On the other hand, non-metallic-based coatings are usually used when insulation is required to act as a barrier between the undercoating metallic material and the corrosive environment. Recent trends in coatings technology included compositing the traditional coatings with new nanostructures to enhance the corrosion, mechanical, thermal, antimicrobial, and optical properties. Graphitic carbon nitride (g-C₃N₄) has recently started to be incorporated with metallic and nonmetallic coatings to enhance their anti-corrosion properties, or it can be used on top of other coatings. Carbon nitride is a novel category of carbon-based materials that is allegedly harder than diamond [5]. Recently, to improve the mechanical and tribological characteristics of alloys and metals, the utilization of CN_x coatings has been heavily researched [6,7,8,9,10]. The past literature has portrayed the standalone CN_x coatings as materials with a notable adhesion strength [11], increased hardness [12], decreased wear depth and friction coefficient [13,14], excellent biocompatibility, and exceptional corrosion resistance [15,16,17]. The preparation and deposition of the different structures of carbon nitride materials have been conducted using a range of techniques, including ion beam deposition, chemical vapor deposition, laser processing, spluttering techniques, and chemical processing [18].

As a result, in this review, the use of standalone CN_x coating and its different composites in various applications will be reviewed, with the main focus on enhancing its corrosion and wear resistance. In addition, the preparation and insertion of the carbon nitride in different traditional corrosion-resistant coatings, either metallic or nonmetallic, will also be discussed, shedding light on such g-C₃N₄ composite coatings’ achievements and future perspectives.

2. History, Structure, and Unique Properties of Carbon Nitride Materials

A relatively long history is known for carbon nitride materials since the 1830s, after the creation of the “melon” notation by Liebig and the release of the general formula (C₃N₃H)_n by Berzelius [19,20]. After that, Liu and Cohen were able to predict that an extremely hard material could be synthesized from the β-polymorph of hydrogen-free carbon nitride (C₃N₄) in 1989 [6]. Nowadays, graphitic carbon nitride (g-C₃N₄) is known to be the most stable allotrope, triggering significant attention as a result of its unique characteristics. Another similar structure to graphite is G-C₃N₄, which has strong C-N covalent bonds compared to C-C van der Waals bonds in graphite layers. Thus, g-C₃N₄ can be described as nitrogen-bridged poly (tri-s-triazine) defect-rich substances. The triazine rings or tri-s-triazine crosslinked by nitrogen atoms constructing extended networks are shown in Figure 1. This shows that graphitic carbon nitride, which is the most stable under ambient conditions, can be synthesized from two methods. The first method involves synthesizing graphitic carbon nitride from triazine (C₃N₃) units, while the second method synthesizes graphitic carbon nitride from tri-s-triazine (C₆N₇) units. Nevertheless, it is noteworthy to mention that the tri-s-triazine rings are preferred due to their better energetic properties and stability compared to triazine [21].

Figure 2 displays the large number of classes of carbon nitride materials based on the chemical or physical routes to synthesize them. In the most recent studies, a large number of produced carbon nitride materials have significant quantities of hydrogen (H) as a vital part of their structures, as well as carbon (C) and nitrogen (N). Thus, such materials are represented as “C_xN_yH_z” compounds. Nevertheless, as previously mentioned, graphitic carbon nitride (g-C₃N₄) compounds are structures that are built upon tri-s-triazine blocks (heptazine, C₆N₇) or triazine (C₃N₃) rings [22,23].

The high thermal and chemical stability of the polymer is founded upon the unique structure of the tri-s-triazine ring with a high-condensed degree. As a result, such compounds are chemically stable under organic, acidic, and basic solvents, as well as being thermally stable up to 600 °C in air. In addition, other features of the material include having a medium bandgap, being utilized as indirect semiconductors, and having a distinct electronic structure. Thus, due to their many significant features, these compounds can be used in many applications, such as corrosion-resistant coatings, biomedical applications, fuel cells, catalysts, etc. [24].

In general, graphitic carbon nitride (g-C₃N₄) has a number of benefits in comparison with other carbon nanostructures. Some of the most significant advantages are having a facile preparation method, a distinctive catalytic behavior, thermal stability, mechanical stability, chemical stability, a simple composition, and a lower cost [25]. The methods of preparation of graphitic carbon nitride include solvothermal methods, thermal condensation, single-step nitridation, physical vapor deposition (PVD), and chemical vapor deposition (CVD). Nevertheless, thermal condensation is the most utilized method due to its low cost and simplicity, where precursors like dicyanamide, urea, cyanamide, etc., are thermally condensed. After that, ultrasonic exfoliation, thermal oxidation, or chemical oxidation can be conducted to produce a single layer of g-C₃N₄ [26]. However, as a result of its relatively low cost and high efficiency, the chemical exfoliation method is considered the best compared to other methods [27,28,29,30].

The increasing demand for novel materials in a number of applications, including biomedical, sustainability, and energy fields, has caused a significant increase in research to advance new generations of g-C₃N₄ materials that have varied morphologies and shapes. This has resulted in new micro and nanostructures of g-C₃N₄, such as thin films, amorphous or bulky nanosheets, 0-dimensional quantum dots, mesoporous, and one-dimensional shapes like nanotubes, nanowires, and nanorods, through novel synthesis methods [21]. In its micron-sized graphitic form, it can be used for tribological coatings, biocompatible medical coatings, chemically inert coatings, insulators, and for energy storage solutions [29]. Figure 3 displays examples of the synthesized g-forms.

A number of factors can influence the final morphology and shape of synthesized g-C₃N₄ materials that are hastily improving, including materials composition, process conditions like condensation temperature, and preparation methods. Zhang et al. prepared nanosheets of graphitic carbon nitride from bulk material using a top-down liquid exfoliation method assisted with sonication [31,32,33]. As their method does not require applying temperature or oxidation process, the authors produced high-quality 2-dimensional nanosheets keeping the intrinsic properties of C₃N₄. On the other hand, Niu et al. developed a different method to obtain high-surface nanosheets from bulky g-C₃N₄ through a thermal oxidation process [34]. In this process, bulk graphitic carbon nitride material is etched in the air at an elevated temperature (500 °C), which results in the oxidation of bulk material away in the air until the desired nanoscale of 2D g-C₃N₄ is obtained. Other shapes, such as carbon nitride nanotubes, could be successfully achieved by Guo et al. through a facile non-catalytic benzene-thermal process, in which the reaction takes place at 220 °C. The authors reported that the synthesized nanotubes have a wall thickness of around 20–50 nm and 50–100 nm inside diameter. Additionally, other routes and conditions could achieve many other shapes of graphitic carbon nitride materials [21], which in turn expand their applications. In this paper, we shed light on the application of g-C₃N₄ in enhancing the anti-corrosion performance of various coatings.

3. Standalone g-CNs Coatings for Enhanced Wear and Corrosion Resistance, Their Fundamentals, Applications, and Mechanisms

For orthodontics, to enhance the treatment effect and lower the therapy time, the friction between the brackets and archwires should be minimized. Thus, due to the high friction of conventional alloys, the use of traditional alloys does not help in fulfilling the requirement of minimizing friction [35,36,37]. It was shown by M. Redlich et al. that the friction between the bracket and the archwire was successfully minimized using orthodontic stainless steel coated alongside inorganic fullerene-like WS₂ [38]. Lately, it has been shown that a decreased wear rate and friction coefficient were achieved when synthesizing CN_x films using the technique of ion beam–assisted deposition (IBAD) [39,40,41,42]. Comparing the wear rates of commercial DLC (diamond-like carbon) film and disks coated with CN_x film of the same thickness, it was seen that CN_x film reduced the wear rate by 10 times [43]. As a result, for orthodontics, as seen in Figure 4, Wei et al. [44] reviewed the corrosion resistance and friction characteristics of the CN_x films, which were conducted on 316L stainless steel by IBAD at a range of assisted N ion beam currents. The results showed decreased friction coefficients for the CN_x films for both artificial saliva and air. In addition, compared to the uncoated sample, decreased corrosion current density was exhibited by the CN_x film coated 316LSS by two orders of magnitude. Also, the CN_x film coated 316LSS had a higher breakdown potential by 0.2 V minimum. Therefore, the research results portrayed the possible use of CN_x film as a protective coating for orthodontics.

Yang et al. deposited the graphite carbon nitride (g-C₃N₄) film to enhance the AZ31B magnesium alloy’s corrosion resistance and mechanical characteristics [45]. As a result of its notable mechanical characteristics, including elastic modulus, proportional strength, and comparable fracture toughness to natural bone, magnesium is an optimized material for biodegradable metal implants [46,47,48,49]. Nevertheless, magnesium alloy undergoes corrosion easily in vivo, particularly in environments containing chloride. Thus, this leads to many unfavorable reactions, including premature mechanical failure and increased local pH. For this research, urea was utilized as a precursor for the one-step chemical vapor deposition method (OS-CVD). A ceramic crucible was utilized to place the magnesium alloy samples at its base, and 10 g of urea was added to cover the samples. After that, the crucible was positioned inside a tubular resistance furnace, up to 400 °C, 450 °C, or 500 °C, respectively. The rate of warming was 5 °C/min, and the aforementioned temperatures were kept for 2 h in argon. When the temperature cooled to room temperature, the sample was taken out of the furnace. The as-obtained g-C₃N₄ film was a hexagonal phase of g-C₃N₄, as evidenced by its well-documented XRD results. Thus, it can increase the corrosion resistance and successfully shield the utilized AZ31B magnesium alloy. The electrochemical and hydrogen evolution experiments confirmed that the carbon nitride film significantly improved the corrosion resistance of AZ31B magnesium alloys in phosphate-buffered saline (PBS) solution. Moreover, the g-C₃N₄ sample prepared at 500 °C had the lowest corrosion current, likely due to its more compact coating structure compared to those prepared at 400 °C and 450 °C. As illustrated in the schematic in Figure 5A,B, the uncoated magnesium alloy in PBS solution led to the formation of white sediments (Mg(OH)₂), which accelerated the corrosion of the AZ31B magnesium alloy. In contrast, the carbon nitride-coated alloy effectively resisted corrosion (Figure 5B). The formation of the coordination bonds between magnesium ions and g-C₃N₄ is attributed to the fact that magnesium ions can offer empty orbitals that can accommodate the lone electron pairs offered by N in g-C₃N₄. Therefore, as seen in Figure 5C, the g-C₃N₄ molecules were powerfully placed on the surface of AZ31B magnesium alloy. The g-C₃N₄-coated magnesium exhibited notable biocompatibility, with hemolysis ratios of 12.4 ± 2.3% (400-Mg), 3.6 ± 0.2% (450-Mg), 13.3 ± 1.6% (500-Mg), and 20.6 ± 1.0% (AZ31B). Among these, the 450-Mg sample demonstrated the best blood compatibility, highlighting the potential of g-C₃N₄ coatings to regulate biodegradation in Mg-based orthopedic implants. All details of this experiment can be found in Ref. [45].

Another new technique was presented by A. Markwitz [50] for the coating of stainless surfaces utilizing molecular carbon nitride beams with ion implantation technology utilizing electrically conductive sputter targets for surface engineering and conventional safe feeder gases. The technique is founded on the basis of molecular ion beam-based DLC coatings. The test for corrosion resistance of the CN⁺ implanted stainless steel coupons was conducted through the partial immersion of the coupons for 5 h at room temperature in a 10% HCl solution. Using mass spectra, the possibility of easily producing C, N, and CN ions is seen through Penning-type ion sources in multiple charge states. In addition, it was also seen that the ion source could produce other exotic ions like CNH⁺. Thus, this can be utilized in finding links to other advanced comparable characteristics of hydrogenated diamond-like carbon coatings. The results showed a significant increase in corrosion resistance of the stainless steel coupons implanted with CN⁺, up to 40%. This provides many possible applications where high corrosion resistance, a short processing time, and surface engineering are needed. Piedrahita [51] reviewed the effects of the deposition of hafnium carbonitride (HfCN) on AISI D3 steel at a range of bias voltages from 0 V to −150 V. The research utilized the multitarget magnetron reactive sputtering technique. Two graphite (C) and hafnium (Hf) targets with purity at 99.9% for both targets were utilized, and the r.f. source was set to 13.56 Hz. Compared to the uncoated samples, the corrosion rate was reduced by 98% when using the HfCN coatings. This shows the possibility of using HfCN in industrial applications. For industrial applications that need high performance, the merit index suggested that HfCN coating, which was deposited at a bias of −100 V, is the most promising because it shows the highest synergy for electrochemical and tribological characteristics.

Additionally, in the computer hard drive industry, it has been seen that nitrogenated carbon (CN_x) materials have an excellent characteristic, which is their ability to produce ultrathin (about 2 nm or less) overcoats [52]. The investigation of synthesizing ultrathin (CN_x) films and their corrosion performance was conducted. Magnetron sputtering was utilized to grow the ultrathin (CN_x) films to produce protective, pinhole-free, and smooth CN overcoats on polished silicon (001). The application of a high-frequency pulsed bias on the substrate and target during deposition was conducted to aid in sustaining a stable plasma and enhance the ion bombardment of the developing film. It is known that such films have superior corrosion resistances at thicknesses as low as 1 nm. In addition, Li et al. [53] showed that the superior corrosion performance and tribological characteristics of the CN_x coating deposited on the Si (001) substrate utilizing the magnetron techniques were not altered under the condition that its N atomic percentages changed. For the synthesis of the CN_x coatings, two partial pressures were selected, which are 0 × 1 and 0 × 5 m Torr, with a range of N atomic percentages at unchanging d.c. bias of −100 V and a blend of N₂ and Ar gas. Peng [54] shows that the enhancement of unbalanced magnetron sputtering with microwave electron cyclotron resonance (MW-ECR) plasma can be utilized for synthesizing ultrathin amorphous Si–C–N films with thicknesses as low as 2 nm. The films are submerged for 12 h in an increasingly severe corrosion environment of 0.1 mol/L oxalic acid, compared with the normal conditions used in current computer industries, which are 0.05 mol/L for 4 min. The film’s dense, pore-free, and smooth structure are the reasons behind its notable characteristics. The unique characteristics are the main reason for the possible utilization of the Si–C–N film produced using the present techniques as a protective coating for magnetic storage devices and read/write heads.

Li et al. [55] investigated the synthesis of pinhole-free and smooth CN overcoats over extensive areas by regulating the substrate mounting geometry and the magnetron sputtering process parameters. During deposition, a tilt of 45 degrees is made to the substrate in regard to the direction between the sample and the target, as well as the rotation at a range of speeds. The exploration of how the film development was affected by rotation, substrate tilt, substrate bias, target power, and sputter gas configuration was conducted. The root mean square roughness was shown by atomic force microscopy scans over big sampling areas of the thin CN films produced at 100 V substrate bias with a rotation speed of 20 r/min rotation and substrate tilt of 45 to be around four times smaller compared to the CN films produced without substrate rotation and tilt. In addition, the surface roughness of the coatings can be enhanced with some percent of helium in Ar-5% N₂ sputter gas. As seen in

Table 1. Corrosion spot count of 1-nm CN overcoats deposited on hard disk substrates with 0 tilt and 0 r/min rotation (B), 45 tilt and 20 r/min rotation (C), 45 tilt and 25 r/min rotation (D), and 45 tilt and 25 r/min rotation under 0.06 m torr He partial pressure (E) at −100 V substrate bias and 200 W target power. (A) The corrosion spot count of bare supersmooth hard disks.

Sample	Corrosion Spot Count
A	about 38
B	about 15
C	about 8
D	about 6
E	about 4

, the 1 nm thick ultra-smooth coatings (sample E) reduce the corrosion damage with regards to the coatings without substrate rotation and tilt.

On the contrary, very thin C–N films with thicknesses of 3, 6, and 9 nm as an overcoat on CoCrTa magnetic discs using a complex treatment, which includes C–N reactive sputtering in an atmosphere of nitrogen and helium (N₂ and He) gaseous blend and nitrogen (N₂) and plasma irradiation, were reviewed by Miyake [56] in terms of their performance. The different thicknesses of films were produced through the regulation of the deposition time. The enhanced performance of C–N films is explained by the notable wear resistance characteristic, corrosion resistance resulting from He addition and complex treatment, and increased indentation hardness.

Over and above that, it is significant to put some emphasis on a range of different surface treatment methods that can be utilized to enhance the corrosion and wear resistance of engineering alloys and metals, such as plasma-assisted chemical vapor deposition [57], plasma ion implantation [58,59], physical vapor deposition [60,61,62,63,64], and gas nitriding [65,66]. Likewise, plasma-assisted microwave chemical vapor deposition [67] or electro-polishing and magneto-electro-polishing [68] can be utilized to enhance the biocompatibility of numerous materials. The utilization of plasma immersion ion implantation (PI³) for the duplex process for corrosion protection utilizing a CN_x coating on a nitrided martensitic stainless steel was reviewed by Short et al. [69]. The PI³ system was used to process two differing samples. The first sample received treatment by plasma nitriding, which is the nitrogen ion implantation, while the second sample received the duplex process treatment, which had the starting stage of plasma nitriding, and then the CN_x coating layer was deposited. The corrosion resistance of the sample undergoing the duplex process was enhanced compared to the other sample.

Ti6Al4V alloy is one of the most utilized biomedical materials compared to conventional cobalt-based alloys and normal stainless steel due to its notable corrosion resistance, superior biocompatibility, and smaller elastic modulus. Nevertheless, Ti6Al4V alloy suffers from bad tribological characteristics and a decreased work-hardening rate. Therefore, the duplex surface treatment process was used by Kao et al. [70] for the improvement of the characteristics of Ti6Al4V. As previously mentioned, the duplex process involves the high-temperature gas nitriding, and then the amorphous carbon nitride (CN) coating is deposited. The treatment can be conducted in single mode and duplex mode. Single mode involves either CN deposition or gas nitriding. Duplex mode involves gas nitriding and then a deposit of CN. Alternatively to PI³, a closed-field unbalanced magnetron sputtering (CFUBMS) system was utilized to deposit CN coatings on the original and nitride Ti6Al4V specimens. During the deposition process, the utilized substrate bias was −80 V, and the utilized sputtering gases were Ar and N₂ gas. A temperature of 900 °C for a time of 120 min was used during the gas nitriding treatment. The preparation of six different specimens was conducted, which are the original Ti6Al4V, nitrided Ti6Al4V (NT), nitrided and polished Ti6Al4V (NTs), Ti6Al4V coated with carbon nitride (CN-T), nitrided Ti6Al4V coated with carbon nitride (CN-NT), and nitrided and polished Ti6Al4V coated with carbon nitride (CN-NTs). The best tribological performance was achieved by depositing the CN coating on a polished nitride layer (CN-NTs). Specifically, as seen in

Table 2. Coefficients of friction of specimens when sliding against 316L and Ti6Al4V balls for 24 min.

Different Specimens	Coefficient of Friction
	316L	Ti6Al4V Ball
Ti6Al4V	1.84	1.90
NT	1.42	1.78
NTs	1.38	1.75
CN-T	0.44	1.71
CN-NT	0.54	0.95
CN-NTs	0.41	0.90

, when sliding the CN-NTs sample against 316L and Ti6Al6V spherical counterbodies, which have friction coefficients of 0.41 and 0.90, respectively, the friction coefficient of the sample was approximately 4.6–2.1 times smaller compared to the untreated sample. Additionally, it was seen that the CN-NTs sample had the maximum hardness compared to the other samples. The biocompatibility and corrosion resistance of the original Ti6Al4V sample and the nitrided Ti6Al4V sample is enhanced after the deposition of a CN coating (CN-T and CN-NT, respectively) by approximately 1.4 times and 1.7 times, respectively, in comparison to the original sample. Thus, there is high potential for improving the service life of Ti6Al4V biomedical implants using the duplex treatment, which consists of high-temperature gas nitriding and depositing the CN coating.

Similarly, it has been seen that the carbonitrides of transition metals (e.g., TiCN, ZrCN, etc.) overcome the problems of offering reliable protection. When comparing carbonitrides to their corresponding transition metal nitrides, the carbonitrides have enhanced wear performance, increased corrosion resistance, and hardness. For the further improvement of the mechanical properties of the CNx system and binary and ternary Ti- or Zr-based materials, different elements (B, Si, Y, W, Cr, Hf, Nb, and V) can be incorporated into the materials [71,72,73,74]. The addition of W and Ti was investigated by Yao et al., who showed an improvement in the wear resistance for ZrCN coatings. This was conducted by utilizing a reciprocating sliding wear tester alongside a cylinder on disk line contact geometric configuration [75]. Braic et al. [76] reviewed the effect of the addition of Ti on the basic ZrCN structure deposited on Ti6Al4V substrate utilizing the technique of DC magnetron sputtering in terms of its performance. Furthermore, the research also clarified the metal/non-metal ratios of the coatings. As a result, two CN coatings were produced (Zr, Ti). The first coating was a quasi-stoichiometric composition [(C+N)/(Zr+Ti) = 1], while the second coating was a highly over-stoichiometric composition [(C+N)/(Zr+Ti) > 2]. After the preparation of the coatings, their anticorrosive and mechanical characteristics were contrasted with the corresponding reference (ZrCN) coatings. Hank solution was utilized to evaluate the corrosion resistance. The results portrayed a significant increase in the polarization resistance (R_p) and reduction of the corrosion current (i_corr) after adding Ti to the basic ZrCN structure, which resulted in a protection efficiency of 99.89%. In addition, coatings with increased non-metal/metal ratios had further increases in the values of R_p and further decreases in the values of i_corr compared with the reference coatings. This is attributed to the increase in carbon content, which caused the production of a dense microstructure that acts as a protective layer against the corrosive solution.

Additionally, CN_x is the main reason for improving the oxidation resistance of tribomaterials and the friction behavior in the water environment to increase water lubrication. This is linked to the water lubrication field, which has gained popularity over conventional lubrication methods due to saving energy sources and solving the pollution problem instigated by oil lubrication in hydraulic systems, drainable pumps, and food and medicine factories [77,78,79]. The tribological and mechanical characteristics of CrCN coatings in different carbon content (0–79.0 at. %) were reviewed by Wang et al. [80]. When the carbon content was increased over 15.4 atomic %, the hardness of the coatings significantly decreased from 22.5 to 14 GPa. The wear rates (2.27 × 10⁻⁶ to 5.15 × 10⁻⁵ mm³/N m) and the coefficient of friction (COF) (0.197 to 0.496) experienced increases due to the sliding against Si₃N₄ balls in water lubrication. Likewise, Ye et al. [81] stated that the increase in carbon content worsens the wear resistance of coatings, and the CrCN coating had the maximum tribological and mechanical characteristics at a low carbon content (12.8 at. %) as sliding against WC balls in seawater. However, a low carbon content does not reduce the friction coefficient [82]. As a result of the solid solution effect and grain refinement, the addition of Si into the CrCN coatings causes further enhancements in their mechanical characteristics [83]. The doping of Si (CrSiCN) causes the COF of the coating to decrease from 0.63 to 0.32 and the hardness of the coating to increase from 22 to 43 GPa [84]. This is attributed to the tribochemical reaction of amorphous silicon nitride sliding against steel balls in the air. Furthermore, the addition of Si can also be utilized to enhance the wear resistance of CrN-based coatings [85]. Wu. et al. [86] attributed the improvement of the friction behavior for the CrSiCN coating to the friction-induced hydration of a-Si₃N₄ and a-SiC. The coating was produced on 316L stainless steel substrates and Si (100) wafers utilizing the magnetron technique for a closed-field unbalanced magnetron sputtering system. As a result of its decreased wear rate and COF, CrCN coating was more appropriate for mating Al₂O₃ balls.

In marine environments, vanadium carbon nitride (VCN) and vanadium nitride (VN) are utilized as reliable marine lubrication materials. The occurrence of C was a pivotal factor in enhancing the characteristics of the VN coatings. Utilizing magnetron sputtering technology, Yu et al. [87] produced the VCN and VN coatings. The results showed that the mechanical characteristics of the VCN coating drastically improved compared to the VN coating. The comparative discussion of the tribology characteristics of VCN and VN coatings in environments with high temperatures was conducted by Mu et al. [88]. It was stated that the occurrence of C decreased the COF of VN coating. It was verified by Chen et al. [89] that the occurrence of C reduces the wear loss of VN coating when testing the anti-wear abilities of VCN and VN coating in seawater. Cai et al. [90] stated that the VCN coating comprising 19.14% C has the highest anti-wear ability when researching the tribology characteristics of VCN coatings with a range of C contents. A study by Ye et al. in 2020 reached the successful preparation of the vanadium carbon nitride (VCN) coatings utilizing the multi-arc ion plating technology at a range of bias voltages (−25, −50, −100, −150 V) on Si wafer and 304 stainless steel [91]. For the source material in the CH₄ atmosphere, two V targets were utilized. The authors tested the tribological performance, mechanical characteristics, corrosion resistance, and microstructure of the VCN coating in seawater. The increase in the bias voltage causes the reduction of the average coefficient of friction (COF) (Figure 6a) and the corrosion current (i_corr) (Figure 6b). In addition, the smallest i_corr and average COF values occurred at a voltage bias of −100 V. This indicates that an appropriate bias voltage can enhance the self-lubrication performance of VCN coating in artificial seawater environments.

In addition, the titanium boron carbon nitride (TiBCN) coatings deposition on high-speed steel substrates and Si (100) is conducted by using the reactively pulsed DC magnetron sputtering technique. The TiBCN coatings are utilized as a protective coating for cutting tools. Tillmann et al. reviewed the mechanical characteristics of the coatings with a range of N₂ content [92]. The increase in the nitrogen content in the gas blend caused a decrease in the hardness of the coated samples and a decrease in the wear rate. The decreasing wear rate with increasing nitrogen content can be attributed to the reduction of the amorphous phases of carbon nitride and carbon. Increased wear resistance and hardness are impossible due to the films’ short C and B content. This causes a very small TiB₂-fraction, which is accountable for the increased hardness, and a small amount of free carbon in the phase formation, which is accountable for the low friction coefficient. Therefore, there is an assumption that reaching a distinct carbon and TiB₂ content enhances the wear resistance and the hardness adequately. As a result, the DC magnetron sputtering deposition technique proposed in this research utilizing Ti and B₄C- targets and for the selected process factors is decreasingly reliable compared to those utilized by the other authors. As a result, more research is required in this area to reach the appropriate technique and process factors that will cause the TiBCN coating to have the wanted tribological and mechanical characteristics. Therefore, much of the past literature has portrayed that the coating produced by the arc ion plating technique has inherent advantages of notable adhesion and increased ionization rates compared to magnetron sputtering [93].

Wang et al. reviewed the tribocorrosion characteristics and structure of TiSiCN coatings deposited on Ti6Al4V alloy utilizing arc ion plating technique at a range of bias voltages [94] and a range of carbon contents [95]. TiSi targets (90 at. % Ti, 10 at. % Si; purity 99.99 at. %) in a gas mixture of Ar (~470 sccm), N2 (420 sccm), and C2H2 (60 sccm) using bias voltages of −20 V, −40 V, −60 V, −80 V, and −100 V were utilized to produce the TiSiCN coatings. Cathode current of 65A, 0.004 Pa background pressure, 3 Pa work pressure, and 90 min deposition time were applied. Moreover, tribocorrosion tests of the TiSiCN coatings were conducted using SiC balls with a 6 mm diameter in artificial seawater. The experiments were carried out at room temperature (18 ± 3 °C) and a relative humidity of 52 ± 5% using a reciprocating ball-on-plate tribometer and a potentiostat. The test parameters included a normal load of 5 N, a sliding speed of 20 mm/s, and a wear track length of 5 mm. It is worth mentioning that the potentiodynamic polarization curves were presented under both non-contact and sliding conditions during ball motion. As proved in literature [96], the ion plating technique implanted lots of droplets in the coatings. However, the increase of the bias voltage to −100 V causes the structure to become dense and compact, as illustrated in Figure 7. This can be attributed to the increased kinetic energy of the positive ions from the targets that seal the gaps among peens and grains on the depositing coating when under an increased bias voltage. Therefore, the coating deposited at −100 V has the highest tribocorrosion resistance under ball motion with non-contact the coating and under sliding, as seen in Figure 8a and Figure 8b, respectively, thanks to its increasing dense and refined grain structure caused by ion bombardment. It is important to highlight that the combined effects of wear and corrosion can significantly accelerate the material’s degradation. Moreover, as seen in Figure 8c, the Ti6Al4V substrate shows a higher friction coefficient under both OCP and CP conditions than the TiSiCN coatings, indicating the coatings’ friction-reducing ability. Under OCP, the TiSiCN coatings’ friction coefficient initially rises and then falls with increasing bias voltage, peaking at 0.37 for the coating deposited at −60 V.

The design of the range of carbon contents (6.9, 11.9, 16.7, and 22.2 at. %) in the TiSiCN coatings, as previously mentioned [95], is conducted through the variation of the proportion of N₂ (300, 280, 260, and 240 sccm) and C₂H₂ (20, 40, 60, and 80 sccm) that are utilized in depositing the coating with the Ar gas and the occurrence of TiSi targets, with the remaining amount of N₂ and C₂H₂ constant. The Ar is also utilized for the stabilization of the chamber pressure. These parameters were maintained at a constant temperature of 450 °C, a pressure of 3 Pa, and a negative bias of 20 V. The conclusion reached by Wang was that the increase in the C content in the TiSiCN coatings causes a gradual decrease in the elastic modulus, and the hardness increases at the start and then decreases [95]. Furthermore, as seen in Figure 9, for all coating specimens, the corrosion current is noticeably higher under sliding conditions compared to non-contact conditions, likely due to the rapid dissolution of the coating caused by the wear-induced disruption of the passive film. The highest tribocorrosion resistance was achieved using the C content of 11.9 at. %. The unique nanocomposite structure and the decreased friction coefficient (Figure 9c,d) of the TiSiCN coating connected to the graphitization effect in sliding in the atmosphere are the reasons behind the highest tribocorrosion resistance. It has been stated by much of the literature that increases in the C contents cause a gradual decrease in the crystallite size [97,98]. In addition, The dislocation movement is restricted by the barriers, which are the amorphous phase (Si₃N₄, SiC, and a-C) surrounding the TiN/TiC/Ti(C, N) crystallites. The grain boundary amount increases as the fineness of the grains increases and the distinct surface area increases, which obstructs the sliding or dislocations. The high-resolution electron microscopy and the selected-area electron diffusion (SAED) pattern verified that the amorphous phase surrounding the nanosized TiN, TiC, and Ti(C, N) crystallites of the TiSiCN coating had a C content of 11.9 at. %.

Lately, reactive plasma spraying (RPS) has been utilized to produce different coatings by lots of researchers [99,100]. RPS is a thermal spraying technique that offers a simplistic method of producing thick coatings through the combination of high-temperature synthesis and the atmosphere spray process. The preparation of the TiCN coating conducted by Mi et al. [101] utilized the Ti-graphite aggregates. It was revealed that approximately 5 min is needed to spray a coating with a thickness of 300 μm. The coating constituents were TbO, graphite, amorphous CN_x, and TiC_0.7N_0.3. The results showed that the coating had a hardness of 1674 ± 197 HV_0.1. Moreover, the coating was seen to have a small number of cracks and pores. Many studies on the structure of RPS TiCN coatings have revealed the production of voids, cracks, and pores on the coatings. Recently, the TiCN ceramic coatings were synthesized by Zheng et al. [102] utilizing RPS on the carbon steel, and an ion nitriding furnace was utilized for the nitriding of the coated samples for alteration. Titanium and graphite powders were used as starting materials in a mass ratio of 6:1. During the plasma spraying process, argon (Ar) served as the carrier gas, nitrogen (N₂) as the reaction gas, and hydrogen (H₂) as the secondary plasma gas. The RPS spray parameters were as follows: 500 A current, 35 kW power, 40 L/min Ar, 4 L/min H₂, 3 L/min N₂, and a spraying distance of 100 mm. It was seen that, following nitrification, there are still voids and pores present in the nitrided TiCN coatings; however, as seen in Figure 10a–d, they have an increasingly compact structure compared to the sprayed TiCN coating. Thus, the nitrided TiCN coatings had higher corrosion resistances in comparison with the sprayed TiCN coatings, as seen in Figure 10e. Furthermore, the improvement of the corrosion resistance of different substrates is successfully confirmed using ion nitriding through the production of nitrides, as in [103,104].

The recently highly attractive laser cladding technique was utilized by Li et al. to produce the Ti(TiBCN) cladding coatings, considering a range of TiBCN contents (0, 5, 10, and 15 wt%), on the surface of the 7075 aluminum alloy to achieve an enhancement in its strength [105]. The features of the laser cladding technique are fast cooling and heating speed, little distortion, increased efficiency, small dilution rate, and small heat effect in comparison to the conventional surface treatment technique [106]. The clad material utilized was TiBCN and Ti powder. A 4000 W continuous-wave semiconductor laser, set to a wavelength of 980–1020 nm, was utilized to produce the cladding coatings. A 6-axis robot ABB was connected with the laser cladding system. A coaxial nozzle DMS-3D was utilized to project the powder coaxially alongside the laser beam trough. The nozzle tip was 15 mm above the focus of the powder. At the time of the laser cladding, the samples’ surfaces received protection from the continuous high-purity argon. The cladding coatings were analyzed in terms of wear behavior analysis, corrosion characteristics, and microhardness. The increase in the TiBCN content in the coating caused the reduction in the average friction coefficient and the enhancement of the average hardness values. As seen in Figure 11a, the lowest wear rate and highest hardness were achieved at 15 wt% TiBCN. Comparing the cladding coating at 0 wt% TiBCN to the 7075-aluminum substrate, the corrosion resistance of the coating is significantly smaller. This is attributed to the Al substrate’s destruction of the Al₂O₃ protective film and the production of the Al₃Ti. Next, as seen in Figure 11b, the increase in the content of the corrosion-resistant TiBCN causes an increase in the corrosion resistance of the cladding coating. The cladding coating achieves the highest corrosion resistance after the content of the TiBCN reaches 15 wt%.

4. G-C₃N₄ Metallic Nanocomposite Coatings: Preparation, Importance, and Mechanism

Metallic coating holds many great advantages, such as a long life that allows it to be suitable for corrosion. The primary metals used in this type of coating are aluminum, zinc, gold, tin, nickel, and cadmium [107]. Since nickel has good corrosion resistance, it is considered one of the essential metallic coatings [107]. Nickel coating can be classified into three categories: nickel (Ni) alone, nickel–boron (Ni-B), and nickel–phosphorus (Ni-P). The most common methods for depositing metallic coatings on the substrate are hot dipping, galvanizing, anodizing, spraying, electroplating, and electroless coating. Nevertheless, they are considered the most marketable method due to the ease of the handling procedure and the unique characteristics of the electroless-plated coatings. More than 95% of industrial electroless coatings are electroless Ni-P alloy compared to Ni and Ni-B alloy coatings. Ni-P is autocatalytically deposited on the substrate through the use of a reducing agent (source of electrons), instead of the application of external power, in the electroless plating technique [108]. Past literature has been successful in the incorporation of nanoparticles to enhance the characteristics of electroless Ni-P coating such as ZrO₂, TiNi, B₄C, Ti, TiO₂, graphite (C_g), diamond, etc. [109,110,111,112,113,114,115]. Despite the good properties of g-C₃N₄ previously mentioned, there are no references on metallic coating with C₃N₄ except for a few, which utilized it with Ni-P coating using the electroless technique. Knowingly, the electroless Ni-P coating holds exciting properties such as good wear, high hardness, and uniformity of coating thickness [116]. Hence, the composite of Ni-P-C₃N₄ is expected to have excellent properties. Bearing these beneficial properties in mind, this section synthesized Ni-P coating with C₃N₄ (nanocomposite) for corrosion and erosion resistances of different substrates.

Lee et al. [117] demonstrated that adding diamond-like carbon nitride (DLC nitride) using an ion beam–assisted deposition technique over a layer of electroless Ni-P, which was deposited on commercial grade 5088Al-Mg alloy substrate, would increase the corrosion resistance of NiP coating. Furthermore, it was found that the corrosion resistance increases as the thickness of the DLC nitride increases. The utilized thicknesses of DLC nitride were 1.5, 2.0, 2.5, and 3.0 nm. The corrosion potential and corrosion current density of the examined coatings were evaluated using potentiostat equipment in a corrosive solution of 1 M NaCl+1 M H₂SO₄. The potential ranges from 250 mVSCE to 1600 mVSCE with a scan rate of 1 mVs⁻¹ was used during the potentiodynamic polarization process. Scanning electron microscopy (SEM) was used to demonstrate the surface morphologies of the specimens after wear–corrosion tests. Lee found that the corrosion current densities of all DLC nitride films were lower than that of Ni-P coated substrate, as explained in

Table 3. Electrochemical characteristic data obtained from potentiodynamic polarization curves.

Specimens	Ecorr (mV)	Icorr (nA/cm2)	Eb (mV)
Ni–P	−66	2369	–
IBD/N2 1.5 nm	204	23.33	–
IBD/N2 2.0 nm	197	10.23	–
IBD/N2 2.5 nm	273	3.98	525
IBD/N2 3.0 nm	295	3.07	572

, which clarified the different electrochemical values. This concept was verified when the SEM pictures revealed that as the thickness of the coating increases, the corrosion pits decrease. Also, the author extended his research to cover the wear loss of the specimens by subjecting them to the corrosive solution, and it was found that as the DLC nitride thickness increased, the wear loss declined, as shown in Figure 12. Lee suggested that incorporating DLC nitride reduces the friction coefficient, lowering the wear loss, as shown in Figure 12.

In the other contribution, Fayyad et al. [118] reported the direct insertion of the 2D C₃N₄ nanosheets in the electroless Ni-P plating bath by ultrasonication of the solution for 2 h to ensure the excellent distribution of C₃N₄ in the Ni-P matrix, as shown in Figure 13c. The TEM micrograph of the 2D C₃N₄ nanosheets is represented in Figure 13a. The novel Ni-P-C₃N₄ nanocomposite coating was deposited on the carbon steel and underwent various tests. The study revealed that from the SEM figures, the surface of the Ni-P-C₃N₄ coating looks like the cauliflower-like morphology feature of the Ni-P coatings, as shown in Figure 13b,c. However, the presence of C₃N₄ makes the surface finer and more compact and leads to higher microhardness, surface roughness, and a more hydrophobic coating. To understand the structure clearly, XRD analysis was performed on the samples, and the generated graphs suggested that Ni-P is amorphous and Ni-P-C₃N₄ is a mixed amorphous-crystalline structure.

Another parameter worth looking at is the effect of heat treatment on the samples. After heating the samples to 400 °C for 1 h, the Ni-P coating becomes smooth, and the granules in the structure start to fade. In contrast, in the Ni-P-C₃N₄ coating, the particles became bigger and more agglomerated. Further examination of the structure after heating evinces the presence of nickel phosphide (Ni₃P) particles that cause changes in different properties. This was clear when the author conducted the surface roughness and contact angle tests to find that the coatings were hydrophilic and smooth after heating. However, the microhardness test contrasted this phenomenon as the precipitation of Ni₃P leads to a strengthening effect that increases the microhardness for both coatings, Ni-P and Ni-P-C₃N₄ nanocomposite, as illustrated in Figure 14.

Furthermore, the author presented the corrosion results using electrochemical impedance spectroscopy (EIS) and Tafel analysis to verify that the composite exhibits better corrosion resistance. As seen in Figure 15, as the |Z| value increases, the corrosion resistance is raised. The incorporation of C₃N₄ increased the corrosion resistance by about 95%. This can also be seen in the Tafel analysis as the corrosion current density drops as C₃N₄ is introduced.

Moreover, Fayyad et al. [119] extended the study on the Ni-P-C₃N₄ nanocomposite in a different paper to highlight the effect of changing the bath parameters on the efficiency of the nanocomposite coating. The impact of plating time showed that 2 or 3 h would be optimum for the C₃N₄ nanocomposite coating as the microhardness value has its highest number at this specific plating time. The effect of pH showed that at pH 8, the thickness and microhardness of the coating would be at the foremost. Additionally, adding a surfactant helped to have refined and defectless structures. The comparison between several surfactants demonstrated that PVP (polymeric) surfactant has the highest microhardness value. In contrast to Lee, the study showed that increasing the C₃N₄ concentration leads to an increase in corrosion resistance up to a certain limit (0.5 g/L). As the concentration influences the size of the nodules and their uniformity, as shown in Figure 16, a very high concentration of C₃N₄ changes the structure to a fibrous-like look, losing its uniformity due to C₃N₄ blocking the active sites for deposition of Ni-P. These findings might be because Lee added C₃N₄ on top of the Ni-P coating, while the former author mixed the C₃N₄ with the Ni-P coating in the same solution.

To emphasize the point about the limit of C₃N₄ to be added, Fayyad et al. performed Tafel analysis and EIS on different concentrations from 0 to 2 g/L. The results showed that the lowest current density is at 0.5 g/L, which means this concentration has the highest corrosion resistance, confirming the theory suggested by the author. Moreover, the authors concluded that as the immersing time of the composite coating in the corrosive solution increases, its protection efficiency diminishes. However, after 30 days in the corrosive solution, the C₃N₄ nanocomposite coating still has a satisfactory protection efficiency (75%) compared to the substrate, as shown in Figure 17.

Recently, the hardness and corrosion protection performance of metallic coatings incorporated with C₃N₄ (e.g., Ni/g-C₃N₄) have been evaluated using a combination of electrochemical analysis and advanced machine learning techniques. Two distinct machine learning approaches were employed to complement traditional methods, providing deeper insights into the coating’s properties and performance. This integrated approach represents a promising direction for optimizing material properties and expanding the applications of C₃N₄ in protective coatings. Alireza Zarezadeh et al. [120] studied the influence of electrolysis bath parameters (varying current density and g-C₃N₄ concentration) on the corrosion, microhardness, and wear properties of Ni/g-C₃N₄ coatings. Electrochemical impedance spectroscopy (EIS) and polarization methods revealed that a bath containing 0.3 g/L g-C₃N₄ at 0.1 A/cm² yielded Ni coatings with optimal corrosion resistance, wear performance, and hardness. Machine learning models (ANN and ANFIS) were trained using corrosion current density, g-C₃N₄ concentration, and plating time as inputs. ANFIS outperformed ANN in prediction accuracy (R² = 0.99 vs. 0.91), allowing for deeper insights into parameter effects. ANFIS provided more precise predictions, highlighting its robustness in modeling corrosion behavior.

5. G-C₃N₄ Organic Nanocomposite Coatings: Preparation, Importance, and Mechanism

Due to their excellent anticorrosive and barrier properties, organic coatings, such as epoxy, are considered among the cheapest coatings that are commonly utilized in industry and various applications. Nevertheless, a number of these coatings have microporous defects, which show up at the time of the curing process and permit the diffusion of electrolytes into the substrate. Thus, this causes a reduction in the barrier characteristics of the coating due to the occurrence of adhesion loss in a short time [121,122,123]. Nowadays, the addition of the g-C₃N₄ nanoparticles into the epoxy matrix for the preparation of nanocomposite coatings has been shown to provide substantial enhancement in the anticorrosive characteristics of a number of organic coatings. In this section, two main categories of epoxy–carbon nitride composites are demonstrated to verify their properties for corrosion and erosion, where both the corrosion and mechanical properties of such coatings are presented.

The first category of the epoxy–carbon nitride composite is synthesized by adding the carbon nitride alone or with organic compounds to the epoxy. An excellent example of such a composite is made by adding ethylenediamine with carbon nitride as nanofiller (AF carbon nitride) in epoxy coating to increase the corrosion resistance of mild steel, as illustrated by Pourhashem et al. [124]. The procedure of obtaining such a composite contains two steps: first, oxidizing carbon nitride with HNO_3, followed by adding ethylenediamine to the oxidized carbon nitride. The corrosion resistance of the composite shows a great performance compared to the epoxy alone. The author reveals that adding 0.5 g of AF carbon nitride would give the highest resistance value of the composite coating. Another form of such composites uses a polymer with carbon nitride in epoxy. Zuo et al. [125] used the chemical oxidative polymerization method to synthesize polyaniline/g-C₃N₄ in the epoxy composite coating, which resulted in a superior anti-corrosive coating with enhanced barrier behavior compared to pure counterparts. Understanding the shape of the composite is crucial before conducting corrosion analysis, which was performed using TEM analysis. The TEM images of the various composites confirm the presence of uniformly encapsulated polyaniline films on the surface of epoxy-g-C₃N₄, significantly enhancing its anticorrosion performance. Furthermore, electrochemical measurements using the Tafel potentiodynamic technique indicate that the best corrosion protection was achieved with the polyaniline/g-C₃N₄ composite coating at a mass ratio of 3:1. This composition exhibited the lowest current density, resulting in the highest protection efficiency of 98.64%. The authors attribute this superior performance to the synergistic anticorrosion effect between polyaniline and g-C₃N₄.

Similarly, Karimi et al. [126] presented the usage of polystyrene instead of polyaniline, and the results were also promising compared to polystyrene alone. The author used copper as a substrate instead of iron to test the performance of the coating and found that at 3.5 g/L of carbon nitride, the highest protection efficiency (98.6%) would occur, agreeing with the former author. Moreover, Yan et al. [127] synthesized g-C₃N₄ nanosheets to increase the protection efficiency and used them as nanofillers during the synthesis process of g-C₃N₄/epoxy nanocomposite coatings. A number of epoxy-based nanocomposites were synthesized using a range of concentrations of g-C₃N₄ nanosheets (1.00, 1.50, 2.00, 2.50, and 3.00 wt%) and utilized as a coating for the Q235 steel substrate. It was seen that the corrosion resistance in 0.1 M H₂SO₄ and 3.5% NaCl electrolyte solutions provided substantial improvement in the corrosion resistance properties of g-C₃N₄/epoxy nanocomposite coatings in comparison with pure epoxy, as conveyed in Figure 18. In addition, it is shown that 2 wt% of g-C₃N₄ nanosheets that are consistently distributed over the epoxy matrix provide the maximum corrosion resistance of the epoxy coating, having a protection efficiency of 99.6%. However, increasing the carbon nitride amount to more than 2 wt% causes the nanoparticles to agglomerate, reducing the corrosion resistance. Furthermore, the authors extended their research to demonstrate the coating’s adhesion by performing an adhesion test. The samples with carbon nitride showed excellent adhesion, whereas the pure epoxy peeled off and deteriorated, as shown in Figure 19. The results indicate that the g-C₃N₄ nanosheets positively interacted with the epoxy matrix, which enhanced the coating’s adhesion to the steel substrate.

In addition, adding organic additives to epoxy composites is also considered in this section. Xia et al. [128] demonstrated that adding polydopamine (PDA) and silane coupling agent (KH560) to carbon nitride enhances the dispersion of g-C₃N₄ (GCN) nanosheets in waterborne epoxy, as shown in Figure 20, hence increases the corrosion resistance of P110 steel. Polydopamine exhibits a strong adhesive ability, allowing it to attach nanoparticles to substrates by forming covalent bonds on the surface. Moreover, incorporating a saline coupling agent would also enhance the adhesion and dispersion of nanofillers. SEM analysis shown in Figure 21 illustrates that adding carbon nitride alone would result in undesirable dispersion. In contrast, adding PDA and KH560 enhances the dispersion of g-C₃N₄, and no aggregation is seen due to enhanced adhesion and interaction between the epoxy and composite.

Furthermore, electrochemical corrosion and salt spray tests reveal that the modified GCN nanosheets are promising nanofillers that significantly improve the anticorrosion performance of waterborne coatings. The polarization measurements indicate that adding dopamine and saline coupling agents to the composite would increase the corrosion resistance of P110 steel with a protection efficiency of almost 100% (99.99%), as shown in Figure 22. The visual inspection of the composite coatings in the corrosive solution shows that the epoxy coating with modified GCN nanofiller outperforms the other coatings. It provides corrosion resistance for up to 15 days before it starts to have defects due to delamination and a decrease in the barrier effect of the composite.

The last point to mention in this type of composites is about examining their mechanical properties. This could be retrieved from the work conducted by Song et al. [129] in testing the carbon fibers (CFs)/carbon nitride in epoxy composite for different mechanical properties. The main mechanical tests investigated were tensile, bending, and hydrothermal aging resistance. As seen below in Figure 23, the tensile, bending strength, and modulus increase after introducing carbon nitride. The main reason for this increase could be attributed to the synergistic effects of increased wettability, mechanical interlocking, and chemical bonding between CFs and epoxy resin that resulted from the incorporation of g-C₃N₄. Another crucial mechanical factor in the composite is the aging resistance. The test was conducted by immersing the samples in boiling water for 48 h and measuring the interlaminar shear strength (ILSS). Figure 24 shows that the decrease of the ILSS in the composite is smaller than that of the carbon fiber alone, and this is due to the strong and powerful adhesion created by the carbon nitride.

The second category of epoxy/carbon nitride composite is adding different metal oxides with carbon nitride to form a nanocomposite and then using it as a nanofiller inside the epoxy. There are various ways to classify the research work in this form. However, all the works here are categorized based on the shape of the nanofiller composite. Two different shapes of carbon nitride/metal oxide nanocomposite were utilized in epoxy as a nanofiller: rod/tube and sheet. The first shape (rod) was demonstrated by Kumar et al. [130] as the author synthesized carbon nitride with MoO_x inside epoxy to improve the corrosion resistance of AA2024 alloy. It can be observed from Figure 25 that the presence of carbon nitride/MoO_x nanofiller significantly increases the adhesion between the composite and substrate. The adhesion enhancement is attributed to the uniformly disturbed nanofiller inside the matrix. However, increasing the concentration of the nanoparticles (to more than 2 wt%) leads to agglomeration and, hence, destruction of the matrix. Considering the corrosion test, which agrees with the adhesion test, the nanocomposite with 2% of carbon nitride gives the highest value of corrosion resistance. Similarly, Pourhashem et al. [131] reported the synthesis of TiO₂ nanotubes with carbon nitride instead of MoO_x to enhance the corrosion resistance of carbon steel.

In addition to nanotubes, nanosheets are considered the other form of this type of epoxy composite. As illustrated by Kumar et al. [132], adding ZnO to carbon nitride nanosheets would improve the corrosion resistance of carbon steel. The composite has the shape of sheets with small spherical particles on it, emphasizing the presence of ZnO on the carbon nitride. Figure 26 represents the SEM images showing the nanofiller’s shape carrying out the corrosion analysis on different composite components; the results reveal that ZnO with carbon nitride in epoxy gives the highest corrosion resistance. This can be deduced from the following Bode plots (Figure 27), where all components are added, and the highest impedance is achieved. Another way the author measured the corrosion resistance was by the salt spray test, where most miniature cracked coatings had ZnO/carbon nitride, agreeing with the previous experiments. Also, incorporating SiO₂ instead of ZnO would enhance the corrosion resistance of mild steel, as revealed by Xia et al. [133]. Several concentrations of carbon nitride at SiO₂ were investigated. According to the results, the author recommends using 0.3 wt% of carbon nitride, as this would give the highest corrosion resistance.

Finally, for simplicity,

Table 4. Corrosion protection performance of various g-C₃N₄ coating, including standalone, alloy-based, or composite-based coatings, over the past five years.

Coating’s Type	Deposition Method/Substrate	Dopant/Concentration/Form of Coating	Corrosive Solution	Electrochemical Technique Used	Remarks and Main Results	Ref.
g-C3N4/Graphene oxide in epoxy coating. GO@CN nanohybrids are chemically functionalized with silane (F-GO@CN)	Solution mixing method through probe-sonication and mechanical mixing/carbon steel	0.1 wt% F-GO@CN nanohybrids	5 wt% NaCl	EIS and salt spray test	The epoxy coating loaded with 0.1 wt% silane-functionalized GO@CN nanohybrid has the highest chemical stability and corrosion resistance.	[15]
g-C3N4 prepared at different temperatures (400, 450, and 500 °C)	one-step chemical vapor deposition method (OS-CVD)/AZ31B Mg	Coating is prepared at different temperatures (400, 450, and 500 °C)	phosphate-buffered saline (PBS) solution	EIS, PDP, and immersion test	The corrosion resistance and biocompatibility of AZ31 B Mg alloy were significantly improved by g-C3N4 coating. The highest protection was at 500 °C.	[45]
VCN coatings	Multi-arc ion plating technology/Si and stainless steel	Coating was prepared under various bias voltages, −25 to −100 V)	Artificial seawater	PDP	The V-50, V-100, and V-150 coatings significantly reduced corrosion current density compared to the V-25 coating, demonstrating that increasing the bias voltage enhances the corrosion resistance of VCN coatings.	[91]
TiCN ceramic coatings.	Reactive plasma spraying (RPS)/carbon steel	Sprayed and nitride TiCN coating	Seawater	PDP	The corrosion resistance of nitrided TiCN coatings was better than that of the sprayed TiCN coating, which was mainly due to the compact structure and the phase change resulted from nitridation.	[102]
NiP-C3N4	Electroless deposition/API X100 carbon steel	0.5 g of C3N4/NiP, with and without heat treatment	3.5 wt% NaCl	EIS and PDP	The microhardness and corrosion resistance of the as-plated nanocomposite and the heat-treated nanocomposite coating were significantly enhanced compared to the Ni-P.	[118]
NiP-C3N4	Electroless deposition/API X100 carbon steel	0.25, 0.50, 1.0, and 2.0 g L−1 C3N4	3.5 wt% NaCl	EIS and PDP	An electroless bath of 0.5 g L−1 C3N4 offered a nanocomposite coating with the highest microhardness and superior corrosion resistance (96%), which decreased gradually, losing about only 2% after one week and 20% after one month of immersion time.	[119]
Ni/g-C3N4	Electrodeposition/carbon steel	0.02, 0.05, 0.1, and 0.2 C3N4 are loaded in Ni-coating	3.5 wt% NaCl	Electrochemical impedance spectroscopy (EIS) and potentiodynamic polarization (PDP) techniques	0.3 g/L g-C3N4 at 0.1 A/cm2 yielded Ni coatings with optimal corrosion resistance, wear performance, and hardness	[120]
Amine functionalized g-C3N4 (AF g-C3N4)/Epoxy coating	Spraying technique/mild steel	0.05, 0.1, 0.3, 0.5, and 0.7 wt% of AF g-C3N4	3.5 wt% NaCl	EIS	The superior corrosion protection of nanocomposite coatings due to the barrier performance of AF g-C3N4. Meanwhile, an epoxy coating containing 0.5 wt% AF g-C3N4 shows the highest corrosion protection (10,650 Ω·cm2) compared to a pure epoxy sample (913 Ω cm2).	[124]
polystyrene/g-C3N4 (PS/g-C3N4)	One-step immersion method/Copper	2.5, 3.0, 3.5, 4.0 and 4.5 mg mL−1 C3N4	3.5 wt% NaCl	EIS and PDP	The optimal concentration of g-C3N4 for achieving favorable corrosion resistance was determined to be 3.5 mg/mL. Additionally, a preparation time of 15 min was ideal for forming a uniform PS/g-C3N4 coating on copper. This coating demonstrated excellent anticorrosion performance attributed to the synergistic effect between PS and g-C3N4. Even after 216 h of immersion, the coating showed minimal corrosion.	[126]
g-C3N4/epoxy nanocomposite coatings	Spraying/Q235 steel	1.00, 1.50, 2.00, 2.50, and 3.00 wt% C3N4	0.1 M H2SO4 and 3.5% NaCl solutions	EIS and PDP	Incorporating 2 wt% of g-C3N4 nanosheets into the epoxy coating results in optimal corrosion resistance. This is demonstrated by an increase in the electrochemical impedance of epoxy by two orders of magnitude in a 3.5 wt% NaCl solution and by one order of magnitude in a 0.1 M H2SO4 solution.	[127]
polydopamine (DA) and silane coupling agent (KH560)/g-C3N4 /Epoxy [g C3N4@(PDA+KH560)] on Epoxy coating	Mechanical spreading/P110 steel substrates	Epoxy g-C3N4 g-C3N4@PDA g-C3N4@PDA+KH560	NaCl	EIS, PDP, and salt spray	Polarization measurements reveal that incorporating dopamine and a silane coupling agent into the composite significantly enhances the corrosion resistance of P110 steel, achieving an almost complete protection efficiency of 99.99%.	[128]
g-C3N4/MoOx inside epoxy. GN-MoOx nanocomposite	Using drawdown bar coater at a constant rate/AA2024 Al alloy	1, 3, and 5 wt% of GNMoOx	3.5% NaCl solution	Different electrochemical techniques	Maximum protection is achieved with epoxy coatings reinforced by 3 wt% GN/MoOx.	[130]
TiO2 nanotube/g-C3N4)/Epoxy	Brush painting/carbon steel	0.1, 0.3, and 0.5 wt% TiO2/g-C3N4 hybrid in epoxy	3.5% NaCl solution	EIS	0.3 wt% hybrids have higher corrosion resistance than that of the neat coating, and silane-functionalized nanofillers more effectively enhance the corrosion resistance.	[131]
g-C3N4-ZnO2 nanocomposites/Epoxy (EP) CN-ZnO/EP	Drawdown bar coater/carbon steel	5 wt% of each GCN, ZnO, and GCN/ZnO in epoxy	3.5% NaCl	EIS, Salt spray	Water uptake studies demonstrated that incorporating GCN/ZnO into the PE matrix enhanced its barrier properties by effectively filling micro-defects and pores.	[132]
g-C3N4@SiO2 nanocomposites. g-C3N4@SiO2/EP	Deposition using compressed air spray gun/carbon steel	0.1, 0.3, and 0.5 wt% g-C3N4@SiO2	3.5% NaCl	EIS, PDP, and salt spray	The impedance value of the g-C3N4@SiO2 coating increased by 969% compared to the neat epoxy coating. 0.3 wt% g-C3N4@SiO2 provided the most effective shielding and achieved the highest corrosion resistance.	[133]

summarizes the corrosion protection performance of graphitic carbon nitride coatings deposited on different substrates, either as standalone layers, alloys, or composites, over the past five years.

6. Conclusions, Recommendations, and Future Perspectives

Generally, it can be said that the graphitic carbon nitride (g-C₃N₄) material and its composites can be utilized for corrosion mitigation and wear resistance applications in a wide range, namely because of their significant effect on enhancing the mechanical and corrosion resistance properties of various coatings, metallic or organic, compared to pure counterparts. The anti-wear and anti-corrosion properties of various types of C₃N₄ coatings—standalone, alloy-based, or composite—are influenced by numerous factors such as temperature, doping elements, deposition time, and stirring power, as well as the preparation methods, which often rely on traditional techniques. Integrating C₃N₄ as a doping or filler element in coatings contributes to grain structure refinement, improved density, and effective resistance to the infiltration of corrosive agents, thereby enhancing overall coating performance. Research on the incorporation of carbon nitride in metallic coatings is relatively limited and underexplored.

The g-C₃N₄ materials remain a focus of research for wear- and corrosion-resistant coatings, especially in oil and gas applications and sustainable practices. The diverse shapes of carbon nitride material that can be produced, e.g., nanosheets, nanorods, nanoflowers, nanospheres, etc., open the door for a significant area of future research for corrosion protection, where a new generation of composite coatings can be addressed. Accordingly, these composite coatings could be improved, and further research on C₃N₄ composite coatings, either metallic or organic, could result in novel outcomes.

Despite advancements in their preparation and modification, challenges persist, such as environmentally unfriendly or time-intensive synthesis methods. Future efforts should prioritize seeking to produce environmentally friendly coatings and efficient production techniques. Utilizing renewable and natural resources like plant leaves or other organic materials for synthesizing g-C₃N₄-based materials could offer greener alternatives while enhancing their functionality to use as second phases in different types of composite coatings.

Currently, most studies on carbon nitride-based coatings focus on artificial seawater environments. However, the complexity of these experimental conditions does not fully replicate the actual marine environment, highlighting a gap in the research. Therefore, improving the anti-corrosion performance of the coatings, especially in aggressive environments like sour conditions, as well as under elevated temperatures and pressures, is a key focus for future research.

Additionally, the influence of altering the morphology of C₃N₄ nanoparticles, such as using microporous and mesoporous forms, in wear and corrosion applications has not been thoroughly explored. Moreover, the antibacterial and antifouling properties of carbon nitride-based protective coatings remain underexplored. In addition, the application of g-C₃N₄ in tribological coatings is still in its early stages, with the potential for improved mechanical strength and wear resistance under harsh conditions.

Finally, with the ongoing advancements in computer technology, there is an increasing demand for the development of specialized software designed to simulate marine environments. Furthermore, integrating artificial intelligence (AI) with interdisciplinary methods can revolutionize the design of g-C₃N₄-based coatings. AI models enable predictive correlations between experimental conditions and corrosion performance, reducing trial-and-error reliance and streamlining development. This approach supports the creation of advanced composite structures tailored for specific applications. It is worth mentioning that despite current challenges, C₃N₄ materials are poised to significantly advance protective and anti-corrosion coatings. Emerging synthetic methods for functionalizing these materials are improving their compatibility and dispersion in coatings, paving the way for more effective and durable applications in the future.