Mechanical and corrosion properties of electrochemically deposited Ni-Nb₂O₅ composite coatings on mild steel for marine applications

Abstract

The Nickel-Niobium Pentoxide Composite Coating is formed by electrochemically depositing nickel (Ni) and niobium pentoxide (Nb₂O₅). An investigation was conducted on corrosion resistance, microhardness, roughness, and deposit morphology. A comparison was made between composites and pure nickel coatings in scratch and contact angle tests. There is a homogeneous particle distribution with a maximum particle size of 99.27 µm in the coating. Adding Nb₂O₅ particles to the composite coating improved corrosion resistance with a maximum inhibitory efficiency of 87.9%. An increase of 44% in microhardness was observed for the Ni-Nb₂O₅ composite coating. The scratch test revealed no adhesion cracks and delamination at the surface of the cracks. The composite coating exhibited a hydrophobic nature in the contact angle measurement.

Keywords:

• Electrodeposition
• nickel
• niobium oxide
• corrosion resistance

1. Introduction

Scientists and engineers have long striven to develop new materials with improved properties. The development of new and non-equilibrium (metastable) processing methods has recently resulted in improved performance of existing materials. Researchers claimed that the lack of superior properties among conventional coatings opened up avenues for nanostructured coatings, and found extremely promising for a variety of applications (Gurrappa and Binder 2008). In the development of nanocomposite films of metal matrix materials, particles of pure metals, polymers, ceramics, and organic materials such as Cu, Ni, Co, Cr, and others have been added to submicron sizes to 100 mm sizes (Zhou et al. 2017). Nanocomposite materials exhibit high tailorability, making them an ideal material for a variety of applications including marine, aerospace, sensors, automotive, electronics, memory devices, construction, energy, and biomedicine (Torabinejad et al. 2017; Naveen Kumar et al. 2021; Naveen Kumar et al. 2022; Kumar et al. 2022; Naveen Kumar et al. 2023; Naveen Kumar et al. 2018). The composite coating offers substrate protection through the combination of two or more substances. Several reinforcing particles form the matrix of this coating, giving it its unique properties. To meet the varied needs of the industry, numerous techniques are used. Additionally, protecting against corrosion, wear, and mechanical qualities are the functions of these composite coatings (Chen 2011; Muller and Ferkel 1998; Chen et al. 2006; Lampke et al. 2006; Shrestha et al. 2001; Sasi et al. 2015; Wang and Wei 2003). A composite coating based on Ni has excellent corrosion resistance, tribological properties, hardness, and bonding strength, which makes them an excellent choice for a wide range of industrial applications (He et al. 2014; Flores et al. 2009; Manohar et al. 2021; Rodrı́guez et al. 2003). Various techniques have been employed by researchers to coat including gas tungsten arc cladding, plasmas, laser cladding, high-velocity oxygen fuel spraying (HVOF), thermal spraying, electrodeposition, electroplating, electroless plating, detonation spraying, electrophoretic deposition (EPD), pack cementing, cold spraying, and hydrothermal deposition (Karmakar, Maji, and Ghosh 2021). In addition to its wide range of configurable production parameters, electrodeposition is a preferred technique for producing coatings, as well as its low cost, control of the deposit thickness and structure even on complex shapes, high purity, rapidity, lack of heat treatment, and ease of production (Koch 2007; Ma, Wang, and Walsh 2015). The reduction of metallic ions from an electrolyte occurs when metal, alloy, or composite films are electrodeposited on conductive substrates by supplying a current or potential (Zhou et al. 2017). In electrodeposition, metal ions are reduced with undissolved oxide particles, such as (SiO₂) (Rathod et al. 2022), (TiO₂) (Huang, Hu, and Pan 2011; Lajevardi and Shahrabi 2010), (CeO₂) (Junli, Ruidong, and Zhang 2012), (Al₂O₃) (Sasi et al. 2015; Jeyaraj, Arulshri, and Sivasankaran 2016; Feng et al. 2008), (ZrO₂) (Kavirajwar, Basavanna, and Devendra 2022), (MoS₂) (Cardinal et al. 2009), (SrSO) (Liang et al. 2009), (Gr) (Varadaraj et al. 2022) carbides (SiC, B₄C) (Kılıç et al. 2013; Devaneyan and Senthilvelan 2014; Shrestha, Kawai, and Saji 2005), nitrides (Si₃N₄) (Krishnaveni, Narayanan, and Seshadri 2008; Eslami et al. 2014), polymers (PTFE) (Ger and Hwang 2002; Wang et al. 2010), and even metals (Cr, Co) (Saravanan and Mohan 2009; Srivastava, Grips, and Rajam 2007). These composite coatings have been reported to provide improved substrate protection against physical and corrosive damage.

Metal, alloy, or composite films are electrodeposited onto conductive surfaces using a current or potential to reduce metallic ions in the electrolyte. Since niobium pentoxide has surface valence cations, it is a semiconductor with a greater surface acidity. Furthermore, Nb₂O₅ exhibits polymorphism, resulting in different physicochemical properties at different temperatures (de Oliveira, de Souza, and Lopes-Moriyama 2020). The resistance of Niobium to corrosion in nearly all aqueous environments is greater than that of nickel (ASM Handbook 1993). Brazil has 80.2% of the world’s niobium reserves, accounting for more than 88% of global output (Smrithi et al. 2022).

R.Q. Fratari et al. (Fratari and Robin 2006) performed electrocoating of Ni-Nb on carbon steel. There was a study of microhardness and volume fraction of co-deposited Nb particles in relation to cathodic current density, electrolyte stirring rate, and Nb particle concentration in the bath. A Ni-Nb composite coating obtained at 20 or 40 mAcm⁻² exhibits a higher microhardness than a pure Ni coating. An improvement in corrosion resistance was achieved by incorporating Nb particles into Ni coatings in NaCl and H₂SO₄ solutions. Alain Robin et al. (Robin and Fratari 2007) developed Ni-Nb composite coatings for carbon steel through galvanostatic electrolysis of nickel baths containing suspended niobium powders. An evaluation of the effects of cathodic current density, electrolyte stirring rate, and Nb particle concentration in the bath was conducted for the purpose of determining the deposit morphology, texture, fractions of co-deposited Nb particles, and microhardness of the deposit. The morphology of composite layers of Ni-Nb is rough with Ni grains oriented randomly. The corrosion and the microhardness characteristics have improved with the addition of Nb. HCR Moreira et al. (Moreira et al. 2022) examined the tribology of electrodeposited Zn-Ni coatings using nanoparticles of niobium pentoxide (1 gl⁻¹) as a dispersant. It was found that Niobium pentoxide nanoparticles produced significantly denser coatings, which incorporated some Nb into the coating surface. A more durable and adherent coating was developed. T Oluyori et al. (Oluyori et al. 2017) developed composite coatings incorporating Zn-Nb₂O₅ for corrosion resistance and wear resistance using electrodeposition from a sulphate bath.

Nb₂O₅ exhibits excellent mechanical flexibility, durability, and optical properties (Rani et al. 2014). Despite its application as an electrodeposition process for composite strengthening, Nb₂O₅ has not been extensively tested for corrosion resistance and structural performance. Corrosion testing is carried out to improve the reliability and longevity of coated products and structures by evaluating the coating performance (Shashanka, Chaira, and Be 2018). This study aims to develop an improved coating that will offer enhanced service life, as well as mechanical and corrosion resistance. In this research, Ni-Nb₂O₅ composite coatings are electrodeposited on mild steel substrates using a sulphate electrolyte to achieve substantial corrosion resistance and structural integrity.

2. Experimental procedure

2.1. Materials preparation and electrodeposition

A nickel bath solution containing Nb₂O₅ particles generated a nickel-Nb₂O₅ composite using mild steel specimens in the size of 2 × 3 cm². In addition to distilled water, analytical-grade chemicals were used for the preparation of the electrolyte. A description of the constituents of the bath and deposition parameters is provided in Table 1. Magnetic stirrers are used to stir the electrolyte prior to electrodeposition at 300 RPM. The cathode of electrodeposition consisted of mild steel, while the anode consisted of nickel.

Table 1. Bath composition and deposition conditions.

Bath composition
NiSO4	300 gl^-1
NiCl2	60 gl^-1
H3BO3	45 gl^-1
NaC12H25SO4	4 gl^-1
Deposition conditions
Current density	5 Adm^-2
Temperature	50°c
PH	3–4

Pre-treatment of mild steel specimens prior to electrodeposition is fundamental to ensuring proper adhesion, corrosion resistance, and overall performance of the deposited coating. The following methods were deployed to clean the mild steel substrate

2.1.1. Cleaning

Mild steel surfaces are often contaminated with oils, greases, dust, and other impurities that can hinder proper coating adhesion. Cleaning methods such as solvent cleaning, alkaline cleaning, or ultrasonic cleaning are used to remove these contaminants.

2.1.2. Degreasing

Solvent or alkaline degreasing processes are employed to remove oils, grease, and other organic substances that can inhibit proper adhesion of the coating. This step is particularly important to ensure good bonding between the substrate and the electrodeposited coating.

2.1.3. Rinsing

After cleaning and activation steps, thorough rinsing is essential to remove any residual cleaning agents or acids. The residue left on the surface can negatively impact coating adhesion and performance.

2.1.4. Drying

Proper drying of the pre-treated surface is crucial to prevent the formation of water spots, residual chemicals, or rust. Thorough drying ensures that the substrate is ready to receive the electrodeposited coating (Yang et al. 2014; Hyie et al. 2022).

Several grades of Emery paper were used to polish the specimen, including 80, 800, 1000, and 2000. The mild steel plate was first immersed in HCl 10% solution and then in acetone solution to polish it. During the electrodeposition procedure, the temperature and current density were maintained at 50 °C and 5 amps/second, respectively. There were four Nb₂O₅ concentrations investigated in electro-deposition trials i.e. 0.25, 0.50, 0.75, and 1 gl⁻¹.

2.2. Characterisation of coating

The X-ray diffractogram was obtained using a Malvern Pananalytical Empyren 3rd Generation diffractometer (Cu = 1.5405998 A^o). Images of the surface structure were obtained using a Carl Zeiss Sigma FESEM. Tests of microhardness were conducted using the Vicker’s tester Matsuzowa MMT-X7A. Innova SPM atomic force microscope was used for the acquisition of the AFM images. An electrochemical workstation CHI 608D was used to perform the corrosion test in a conventional glass cell at a temperature of 27 °C. The coating profile was evaluated using a nano new ST 400 profilometer. For scratch testing, the DUCOM unitest 750 scratch tester was used. The contact angle was found using a Drop shape analyzer-100.

3. Results and discussion

3.1. The microstructure of Ni-Nb₂O₅ composite coating

A rough morphology with homogeneous particle distribution has been observed (Figure 1). The Nb₂O₅ particles are evenly distributed on the mild steel interface, resulting in a smooth, undistorted surface. The gaps and recessions are largely filled by the particles (Moreira et al. 2022). A majority of the particles are covered in metallic Ni, but only a few Nb₂O₅ particles are visible. These particles are visible in the form of solid flakes and nodular agglomerates.

Figure 1. (a) Represents the SEM image of bare mild steel. (b–f) Demonstrates the surface topography of electrodeposited samples by SEM at concentrations of 0, 0.25, 0.50, 0.75, and 1 gl⁻¹ of niobium oxide, respectively.

3.2. XRD analysis

Figure 2 shows an XRD plot of the Nickel and Ni-Nb₂O₅ deposited coatings by the DC method. Nickel peaks with varying intensities are observed in deposits in crystallographic planes (111), (200), and (220). It is evident that the two deposits exhibit nickel peaks at the (111), (200), and (220) planes in varying intensities. The Fe peaks were visible in both the deposits at (020) and (121) planes. For determining the peak index, a nickel card (01-070-1849), Nb₂O₅ card (00-018-0910) and Fe card (96-900-6602) of ICDD were used. The Nb₂O₅ peak does not appear in this coating, because it has a lower percentage of Nb₂O₅ than nickel coatings. On XRD, there is no sign of the Nb₂O₅ peak, which is less intense than nickel (García et al. 2003). Figure 3 shows a composite coating that consists of nickel and Nb₂O₅ according to EDAX analysis. Table 2 represents the elemental composition of the EDAX analysis.

Figure 2. XRD plots of Ni and Ni-Nb₂O₅ coating.

Figure 3. EDAX analysis of Ni- Nb₂O₅ Coating.

Table 2. Elemental composition of Nickel- Nb₂O₅ coated specimen.

Elements (weight %)	O	Fe	Ni	Nb	C
Nickel- Nb2O5 coated sample	—	0.81	31.95	0.01	0.00

XRD data were used to estimate the texture coefficient (T_C) of a Ni-Nb₂O₅ composite coating. Based on Be’rube and L'Esperance’s methodology, the texture coefficient is calculated using the following relation, $$T_c\left(\mathit{hkl}\right)=\frac{I\left(\mathit{hkl}\right)}{\sum I\left(\mathit{hkl}\right)}\times \frac{\sum I_0\left(\mathit{hkl}\right)}{I_0\left(\mathit{hkl}\right)}$$

A peak intensity, I(hkl), is represented by Ni electrodeposits, and diffraction peaks are added together in this equation. In the ICDD card number (01-070-1849), index 0 refers to the intensities of peaks from the standard Ni sample. In Figure 4, the texture coefficient of Ni composite coatings and Ni-Nb₂O₅ composite coatings are shown graphically. There was a preferred orientation with nickel coating having the highest texture coefficient (T_C) value at (200) plane. An orientation change occurs at the (220) plane for Ni-Nb₂O₅ coatings. The results indicate that adding Nb₂O₅ to Ni electrodeposition changes the preferred orientation. When nickel was coated with Nb₂O₅, the coating morphology also changed. This is primarily due to the presence of Nb₂O₅ in the coating.

Figure 4. Texture coefficient plots of nickel and Ni- Nb₂O₅ coatings.

3.3. Corrosion test

In this study, corrosion resistance was evaluated for pure Ni and Ni-Nb₂O₅ steel substrates in corrosive solution (3.5% NaCl). The working electrodes were uncoated pure Ni and Ni- Nb₂O₅ steel substrates with 1 cm² surface area that were submerged in the corrosive liquid for five minutes. In this measurement, saturated calomel electrodes served as the primary electrode and platinum wire served as the secondary electrode. The technique of tafel extrapolation and impedance spectroscopy was used to evaluate corrosion resistance in detail.

Figure 5 shows a Tafel plot of Ni-Nb₂O₅ coating (1 gl⁻¹) at low I_corr values with high voltage. According to Table 3, the inclusion of Nb₂O₅ particles raised the Ni coating’s inhibitory efficacy from 44.3% to 87.9%.

Figure 5. Polarisation curves of Ni and Ni-Nb₂O₅ composite coated samples.

Table 3. Tafel extrapolation data.

Corrosion media	g/l	-Ecorr (V vs SCE)	Icorr (A cm^-2)	C.R. (mil/yr)	% IE (ɳp)
3.5% NaCl	Blank	0.638	8.958	3.875	—
	Nickel	0.573	4.986	3.584	44.3
	0.25	0.530	2.045	2.392	77.7
	0.50	0.525	1.756	2.113	80.4
	0.75	0.520	1.425	1.978	84.1
	1.0	0.461	1.078	1.683	87.9

An instant, non-destructive method to evaluate the corrosion resistance of a wide range of materials is Electrochemical Impedance Spectroscopy (EIS). The sinusoidal voltage signal included an amplitude of 0.01 V. Figure 6 shows Nyquist plots. Figure 7 shows an electrical equivalent circuit that is fitted to the impedance parameters. In electrochemical impedance (EI) spectra, a larger surface area semicircle was witnessed for a greater Nb₂O₅ content coating, whereas the smaller surface area semicircles appeared for Ni coating, without particles or with fewer particles as shown in Figure 6. Table 4 represents the impedance spectroscopy data. This indicates a better corrosion resistance characteristic of Ni- Nb₂O₅ (1 gl⁻¹) coating.

Figure 6. Nyquist graphs for Ni and Ni-Nb₂O₅ composite coating.

Figure 7. Equivalent circuit for the Nyquist plots. R: Resistance; CR: Current Rectifier; QR: Quick Response.

Table 4. Impedance spectroscopy data.

Corrosion media	gl^-1	Rp (Ωcm^2)	Cdl (µF/cm^2)	%IE(ɳz)
3.5% NaCl	Blank	10.92	386	—
	Nickel	20.44	313	46.6
	0.25	49.97	275	78.1
	0.50	58.99	205	81.5
	0.75	72.42	176	84.9
	1.0	98.29	159	88.8

Ni-Nb₂O₅ composites exhibit a lower corrosion rate than pure Ni, owing to a number of factors. Further, deposits may contain Nb particles, which are intrinsically more corrosion-resistant than Ni in most aqueous environments. In addition, the microstructure of a coating can change from columnar to non-columnar. In columnar microstructures, the coatings have a high aspect ratio, with their growth direction being perpendicular to the substrate surface. This type of microstructure can create pathways for the corrosive medium to penetrate and reach the substrate, leading to higher corrosion rates. On the other hand, non-columnar microstructures are characterised by a lower aspect ratio, with their growth direction being parallel to the substrate surface (R and Chaira 2014; Chaira 2015; Shashanka 2019; Shashanka and Chaira 2017). This type of microstructure can create a more uniform coating with fewer pathways for the corrosive medium to penetrate and reach the substrate, leading to lower corrosion rates (R and Chaira 2014).

The incorporation of Nb₂O₅ particles acts as a nucleation site for crystal growth with smaller grain size composite coating and further adds to the corrosion resistance of the substrates (Mohan Reddy, Praveen, and Kumar 2017). As a result, Ni-Nb₂O₅ composites strongly decrease the corrosion rate of pure Ni compared to Ni-Nb₂O₅ composites because they have Nb particles in the deposits and a non-columnar coating microstructure.

3.4. Microhardness of Ni-Nb₂O₅ composite coating

An indenter equipped with diamonds from Vicker was used to test the microhardness of two load segments of 50 and 100 g over a dwell time of five seconds. Figure 8 shows the microhardness of nickel and nickel-Nb₂O₅ coatings (1 gl⁻¹). Ni-Nb₂O₅ composite coatings exhibit higher microhardness than pure nickel coatings. Ni-Nb₂O₅ (1 gl⁻¹) coatings are reported to have 321.3 HV and 270 HV hardness, respectively, compared to the nickel counterpart’s 213.6 HV and 195.78 HV. The Nb₂O₅ has a positive impact on the hardness in both the 50 and 100 g load segments. The inclusion of Nb₂O₅ particles in the composites disturbs the growth mechanism of nickel and reduces the grain boundary size and therefore requires more energy required for deformation (García et al. 2003).

Figure 8. Microhardness of Ni and Ni- Nb₂O₅ coating.

3.5. Tribological characteristics of Ni-Nb₂O₅ composite coating

3.5.1. Atomic force microscopy

Figures 9 and 10 show AFM images of nickel and nickel-Nb₂O₅(1 gl⁻¹) coatings. Granular grains are present on the surface of the composite thin films that resemble flakes. A 3D image of the surface coatings shows a bulgy-like spherical structure with microscopic grain boundaries. The boundaries of larger grains appear to contain a good number of grains. Nickel-Nb₂O₅ coatings had a smaller particle size, seemed uniform, and were tougher than nickel coatings alone, indicating that nickel and Nb₂O₅ were dispersed evenly in the coating. As viewed in Table 5, Nb₂O₅ particles did not have much impact on the roughness characteristics.

Figure 9. AFM image of Nickel coating.

Figure 10. AFM image of Ni- Nb₂O₅ Coating.

Table 5. Roughness data.

Coating	Ra (nm)	Rq(nm)
Nickel	78.8	102
Ni-Nb2O5	102	135

3.6. Profilometry

The Ni-Nb₂O₅ (1 gl⁻¹) electrodeposited coating surface profile is shown in Figures 11 and 12. The coating profiles typically range from 20 to 70 µm.

Figure 11. 3D profile of Ni- Nb₂O₅.

Figure 12. 2D profile of Ni- Nb₂O₅.

The coating texture appears to have fewer dense spikes, as can be observed in the surface profile of the Ni-Nb₂O₅ coated sample. A maximum height of 99.27 µm was measured for the particles.

3.7. Scratch test

The scratch test measures the coating’s adhesion to the substrate and its micromechanical properties. Rockwell diamond indent was used to test the scratch resistance of Ni-Nb₂O₅ coating (1 gl⁻¹) at an incremental load of (0–20 N) at 0.1 mm/sec and a scratch length of 5 mm.

During scratch testing, as the indenter load increases, friction between the indentor and coating will increase plastic and elastic deformation. The scratch path is illustrated in Figure 14 by a SEM image. In Figure 13, load vs. stroke is represented by a graph. There is no noticeable material piling at the scratch’s edges. The plastic deformation has occurred for the coating material during the scratch test and a small portion is found at the edges of the scratch. Neither the tested coating nor the substrate material has visible adhesion cracks or delamination.

Figure 13. Normal load vs stroke.

Figure 14. SEM Image of the scratch path.

3.8. Contact angle measurement

The measured contact angle of the coatings is given in Figures 15 and 16. The wetting behaviour of Ni-Niobium oxide (1gl⁻¹) coatings can be determined by measuring contact angles following the incorporation of Niobium Pentoxide into the Ni matrix. Wettability is improved by the less porous nature of the composite coating, its low or no stress and its lack of agglomeration of particles on the outer coating. The improvement in contact angle can be found from 86.4° to 107.6°.

Figure 15. Contact angle of nickel coating.

Figure 16. The contact angle of Nickel- Nb₂O₅ (1 g/l) coating.

4. Conclusion

In this study, the effects of niobium pentoxide on the morphology, mechanical characteristics, and corrosion resistance of Ni electrodeposited coatings were researched. The following conclusions were drawn based on the results obtained in the study:

• Smooth, compact and undistorted surfaces were obtained for the coatings with the flake-like visibility of the Nb₂O₅ particles.

• XRD studies indicate that Nb₂O₅ modulates crystal size and orientation. The maximum T_C was observed for 200 for Ni element.

• As Nb₂O₅ concentration increased, corrosion resistance increased, with a maximum corrosion resistance observed for Nb₂O₅ (1 gl⁻¹)

• With the incorporation of Nb₂O₅ particles in a sulphate solution, the microhardness value has increased by about 44% when compared to pure nickel.

• The inhibitor efficiency almost doubled with the inclusion of Nb₂O₅ particles in the coating (1 gl⁻¹)

• Nb₂O₅ particles produced a more steady and tougher coating with reduced particle sizes, with a maximum particle size of 99.27 µm.

• The Ni-Nb₂O₅ (1 gl⁻¹) coating material noticed a plastic deformity during the scratch test with the least heaping at the edges of the crack.

• The hydrophobic nature of the protective coating was revealed by the inclusion of Nb₂O₅ (1 gl⁻¹). As a result of these enhanced corrosion and mechanical properties, Ni-Nb₂O₅ electrodeposited coating could be utilised as an alternative protective coating to cater for the needs of the marine industry.

Disclosure statement

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this article.