Pulsed Electrodeposition and Properties of Nickel-Based Composite Coatings Modified with Graphene Oxide

Abstract

Composite electrochemical coatings (CECs) on the basis of nickel modified with multilayer graphene oxide (GO) were deposited from a sulfate–chloride electrolyte in pulsed electrolysis mode. The microstructure of these CECs was studied by X-ray phase analysis and scanning electron microscopy. It was found that the microhardness of nickel–GO CECs increases by approximately 1.40 times compared to pure nickel. The corrosion–electrochemical behavior of nickel–GO composite coatings in 0.5 M H₂SO₄ was studied. Based on tests in 3.5% NaCl, it was found that the addition of graphene oxide particles into the matrix of nickel electrodeposits, increases their corrosion resistance by 1.40–1.50 times. This can be explained by the uniformity of the distribution of GO in the nickel matrix, which contributes to the reduction in grain size, as well as the impermeability and stability of graphene oxide.

1. Introduction

Composite electrochemical coatings (CECs) are formed during the co-deposition of metals with dispersed particles from suspension electrolytes [1,2,3]. Nickel composite coatings, which are characterized by hardness, stability in various corrosive environments, and good adhesion to base, are the most widespread among CECs [4,5,6,7,8,9,10,11,12,13,14,15,16,17,18,19].

The nature of the change in the structure and properties of composite coatings is significantly determined by the nature of the dispersed phase. At this juncture, considerable attention from scientists is focused on the study of various CECs modified with carbon materials, including carbon nanotubes [5,6,7], fullerenes [4,9], ultradisperse diamonds [10], and metal carbides [11,12]. Graphite and its derivatives stand out from carbon compounds. A characteristic feature of graphite is its layered structure, in which each C atom is bonded to three other carbon atoms. Layers of graphite (graphenes) are arranged in such a way that half of the atoms of one layer are under the centers of the hexagons of the other one, while the other half of the atoms are under each other. Due to this structure, graphite can act as a donor of electrons in interactions with various oxidizing agents. In particular, when graphite reacts with certain inorganic acids (for example H₂SO₄), graphene oxide (GO) is formed [20], in the structure of which graphenes are bonded chemically to various oxygen-containing functional groups (carbonyl, carboxyl, hydroxyl, etc.). Graphene and GO have outstanding operational properties (electrical conductivity, thermal conductivity, mechanical strength, etc.), which creates prospects for their use in various industries (machine building, chemical, petroleum, etc.). Therefore, it is of serious practical interest to study the process of GO codeposition with metals and the properties of the composite coatings that are formed.

The advantage of non-stationary electrolysis, in particular pulsed electrolysis, is the larger number of parameters that control the deposition process [21,22]. The application of a pulsed current contributes to an increase in the content of the dispersed particles in the metal matrix and the achievement of its distribution over the thickness of the electrochemical deposit. Additionally, it allows for control of the processes of crystal nucleation and growth. Consequently, composite coatings with high functional properties are formed.

The goal of this paper is to deposit nickel–GO composite coatings in pulsed electrolysis mode and to study their microstructure, physico-mechanical properties, and corrosion rate.

2. Materials and Methods

Nickel–GO CECs were deposited on a steel electrode (steel 45, Severstal, Cherepovets, Russia) from a sulfate–chloride electrolyte, the composition of which is presented in

Table 1. Electrolyte composition and deposition parameters used for nickel–GO CECs.

Electrolyte Composition	Concentration, g/L	Deposition Parameters
NiSO4·7H2O	220	Temperature t = 45 °C
NiCl2·6H2O	40	Constant stirring
CH3COONa	30	Cathode current density
Graphene oxide	10	ic = 7, 8, 9, 10 A/dm2

.

A GO dispersion was added into the electrolyte solution as a powder with a particle size not exceeding 10 μm. Electrochemical deposition of pure nickel was obtained from the above electrolyte without graphene oxide. The coatings were deposited on rectangular electrodes with a working surface area of 1 cm² (the non-working surface was isolated with lacquer). Pretreatment of the electrode surface included mechanical stripping with abrasive paper, anodic etching in 48% H₃PO₄ solution with a lead cathode and washing in distilled water.

The deposition of coatings based on nickel in pulsed electrolysis mode at cathode current densities of 7–10 A/dm² was studied. The ratio of the periods of the pulse (t_pulse) and pause (t_pause) was 1 s/0.1 s. The studied coatings thickness was 20 μm.

Electrochemical experiments were performed on a P-30J pulsed potentiostat (Elins, Moscow, Russia). The potentials were set relative to a saturated silver chloride reference electrode and recalculated using a standard hydrogen electrode (SHE).

There are various approaches to graphene oxide synthesis. In this work, multilayer GO was synthesized electrochemically in the galvanostatic mode by anodic oxidation of natural graphite powder GB/T 3518-95 (Sunshine Resources Holdings Ltd., Beijing, China) with a capacity of 700 Ah/kg. The electrolyte was 83% H₂SO₄ (h.p.). A detailed description of the procedure for the graphene oxide electrochemical synthesis was published earlier [20].

The specific surface area of graphene oxide was determined using the Brunauer–Emmett–Teller (BET) method using a NOVA 2000e analyzer (Quantachrome Instruments, Boynton Beach, FL, USA).

X-ray phase analysis (XPA) was performed using an ARL X’TRA X-ray diffractometer (Thermo Fisher Scientific (Ecublens) SARL, Ecublens, Switzerland). Structural studies of the coatings were carried out using an EXplorer scanning electron microscope with a built-in energy-dispersive analyzer (Aspex, Delmont, PA, USA).

Vickers microhardness (HV) values of electrolytic deposits were measured using a PMT-3 device (LOMO, Saint-Petersburg, Russia). A tetrahedral diamond pyramid was statically pressed into the studied nickel coatings under a load of 50 g. The time for load application on samples was 15 s. The distance between the indentations was at least two diagonals. The shape of the indentation was square. Taking into account the results of the conducted tests, the values of both diagonals of the indentation were determined. The calculation of the HV values was carried out according to the data from seven parallel experiments.

The surface roughness parameter Ra of nickel coatings was measured using a TR 220 profilometer (TIME GROUP Ink., Beijing, China) and the average value was calculated on the basis of five parallel experiments.

The Cochran’s C test confirmed that all experimental results were reproducible.

The corrosion–electrochemical behavior of nickel coatings was studied potentiodynamically in a 0.5 M H₂SO₄ (potential sweep rate V_p = 8 mV/s). To determine the corrosion rate, nickel-coated specimens were tested in 3.5% NaCl.

3. Results and Discussion

Particles of various origin in the dispersed phase can simply co-deposit with nickel electrochemically [2], but their application leads to various effects. The addition of GO powder into the nickel plating bath has a significant effect on the electrode processes and kinetics. In the chronopotentiograms (Figure 1), there is a shift of the potentials towards more positive values in the transition from nickel containing no graphene oxide to the nickel–GO coating. Along with this, there is a lessening in the potential jumps on the E–t curve obtained during the CEC electrodeposition. When the composite coating chronopotentiogram is moved in the positive side compared to the pure metal one depolarization appears, otherwise superpolarization takes place. Hence, in the presence of GO particles, the cathode process proceeds with depolarization.

The study of graphene oxide using scanning electron microscopy (SEM) allows one to establish that a layered structure has developed (Figure 2). The specific surface area of GO is 46.78 m²/g (determined by the BET method). The kinetics of the composite coating formation includes the stages of transporting dispersed phase particles to the cathode, holding them at the cathode surface and overgrowing the particles with electrocrystallized metal. Based on the data from previous publications [17,23], the following mechanism of the GO dispersed phase inclusion into the metal matrix can be proposed. The cations from the electrolyte solution are adsorbed on the GO surface and this causes the formation of a positive charge of the particles of the graphene oxide. Consequently, the transfer of the dispersed phase to the cathode occurs probably not only through stirring, but also under the influence of electrophoretic forces. Adsorbed ions, presumably, can participate in the “bridge” binding of the graphene oxide with the electrode. This binding weakens the disjoining pressure of the liquid layer between the graphene oxide phase and the cathode, thereby enhancing adhesion [23]. The dispersed phase held on the cathode surface stimulates nucleation at the points of contact of metal with graphene oxide particles, which in turn contributes to the ingrowth of these particles by electrochemical deposit.

In compliance with the results of the study of the nickel–GO deposit specimen by XPA (Figure 3), peaks belonging to the nickel and carbon phases were observed as a consequence of the addition of GO particles into the metal structure. This conclusion is confirmed by the SEM images (Figure 4), which show that the microstructure of the surface changes upon passing from a nickel containing no graphene oxide to a nickel–GO CEC. The microtopography of pure nickel is similar to the X-ray amorphous structure (Figure 4a), while composite coatings have an ordered “honeycomb” structure (Figure 4b). Apparently, GO particles act like centers of crystallization and stimulate the distribution of nickel metal over the cathode surface. As a result, dense and uniform deposits of nickel–GO CEC are formed. In this regard, it was of interest to investigate the surface roughness of nickel coatings.

There are several parameters of surface roughness (Ra, Rz, Rq, etc.), the choice of which depends on the functional purpose of metal products. For electrochemical coatings, the values of the parameter Ra are determined usually, which is the arithmetic mean of the absolute values of the profile deviations within the base length. The resulting values of the surface roughness parameter Ra for nickel–GO CECs decrease by approximately 1.40–1.50 times compared to pure nickel (

Table 2. Surface roughness parameter Ra values in μm of nickel coatings.

Cathode Current Density ic, A/dm2	Ni	Ni–GO CECs
7	0.465	0.338
8	0.661	0.472
9	0.945	0.673
10	1.477	0.986

), which confirms the formation of fine-crystalline and uniform deposits in the presence of graphene oxide (Figure 4). It should be noted that experimental conditions influence the roughness and can lead to non-homogeneous propagation of reinforcement with the ions and agglomeration in the coating. However, under pulse electrodeposition, for the composite coatings it is possible to achieve a roughness level of less than 0.40 μm (Table 2).

The addition of a dispersed phase into an electrochemical coating results not only in changes in its structure, but also in its functional properties. Of significant practical interest are the physical and mechanical properties of metal surfaces, especially their microhardness. With a growth in the cathode current density, the microhardness of the studied coatings increases (

Table 3. Microhardness HV_0.05 values in MPa of nickel coatings.

Cathode Current Density ic, A/dm2	Ni	Ni–GO CECs
7	1100	1528
8	1124	1573
9	1220	1695
10	1465	2028

). This is possibly owing to the addition of H₂ and hydroxides into their composition, which leads to deformation and compression of crystals. Earlier, it was noted that the addition of the GO dispersed phase into the metal nickel structure results in compaction and the formation of a fine-grained coating (Figure 4). In addition, the length of the grain boundaries increases in the composite coatings due to inclusions of the dispersed phase. Therefore, in the studied range of current densities, we can observe an increase in microhardness by approximately 1.40 times upon passing from pure nickel deposits to nickel–GO CECs (

Table 4. Corrosion rates in mm/y of nickel coatings.

Cathode Current Density ic, A/dm2	Ni	Ni–GO CECs
7	0.574	0.410
8	0.492	0.328
9	0.390	0.267
10	0.185	0.123

).

One more significant operational property of electrolytic coatings is corrosion resistance. The corrosion–electrochemical behavior of nickel deposits was studied potentiodynamically in a 0.5 M H₂SO₄ solution. E–lg I curves of nickel and nickel–GO coatings show that the graphene oxide phase increases the potential, and correspondingly decreases the current of the active anodic dissolution of this electrochemical deposit (Figure 5). The corrosion behavior of CECs is largely determined by the properties of the metal matrix; therefore, the potentials for the onset of passivation of nickel containing no dispersed phase and CEC nickel–GO are close. A distinctive feature of the E–lg I curve of the nickel–GO composite coating is the broadening of the passive area, while for pure nickel it is less obvious. In the far anode region of potential, graphene oxide in the nickel structure also affects the E–lg I curve (the overpassivation potentials of the nickel-based coatings differ). On the basis of potentiodynamic studies in a 0.5 M H₂SO₄ solution, it was to be expected that the resistance to corrosion of nickel–GO CEC would be higher than that of pure nickel.

The corrosion rates of the studied nickel and nickel–GO coatings were calculated based on the mass wastage when kept in 3.5% NaCl solution for 24 h (the specimens were weighed before and after immersion) with the help of the following relationship [24,25]:

$$\mathrm{Corrosion} \mathrm{rate}=\frac{\mathrm{KW}}{\mathrm{ATD}}$$

(1)

where K is a constant (8.76 × 10⁴), W is the mass wastage in g, A is the exposed area of the nickel-based coating specimen (1 cm²), T is the time of immersion in hours, and D is the nickel density (8.90 g/cm³).

Trials in a 3.5% NaCl solution revealed that the corrosion rates of nickel–GO CECs decrease by 1.40–1.50 times compared to pure nickel deposits (Table 4), correlating with the results of other studies [25]. The influence of graphene oxide on the corrosion resistance of nickel can be due to several factors. Matt porous coatings are formed from sulfate–chloride nickel-plating electrolytes [1,2]. The inclusion of GO particles into the metal structure leads to the overlapping of pores. Moreover, the studied CECs have an ordered fine-crystalline structure, in contrast to pure nickel (Figure 4), which contributes to a uniform surface distribution of the corrosion current. Additionally, this effect also stimulates a decrease in surface roughness of nickel–GO CECs compare to pure nickel (Table 2). The smaller the roughness of the metal surface, the more difficult it is to wet, which helps to slow down the corrosion rate. The impermeability and stability of graphene oxide contributes to the elongation of the corrosive medium diffusion path. Nickel’s tendency toward electrochemical corrosion depends on the crystallographic orientation, which determines the surface free energy per unit area of the material. The high impermeability of graphene oxide prevents the diffusion of nickel ions through the cross-section of the micro particles. It should also be noted that the influence of the dispersed phase in composite coatings can be seen only when particles form compounds that are more corrosion-resistant than the metal matrix at the phase boundaries or throughout the volume. Otherwise, the development of the corrosion process will not stop, but will bypass the particles (possibly with a decrease in speed). It is obvious that similar compounds are formed in the structure of nickel–GO CECs. All listed factors together provide an increase in the corrosion resistance of composite coatings as compared to nickel deposits that do not contain graphene oxide.

4. Conclusions

On the basis of the studies performed, we can draw a conclusion that composite coatings are formed from the sulfate–chloride nickel plating electrolyte containing a multilayer GO dispersion in pulsed electrolysis mode. The addition of GO to the matrix of nickel results in a change in the surface microstructure. The GO phase has a determining influence on the physico-mechanical and corrosion properties of the studied CECs. Modification of electrochemical nickel with a dispersed GO phase leads to an increase in the coatings’ microhardness by approximately 1.40 times and decreases in their corrosion rate by 1.40–1.50 times. Under the conditions studied, the nickel–GO CEC deposited at a current density of 10 A/dm² has optimal functional properties.