Survey of literature.
Composite coatings can demonstrate improved property performance as compared to metals and alloy materials. One category of composite coatings is composed of metal or metal alloys with a dispersed phase of nonmetallic nanoparticles. The addition of these nanoparticles has been found to improve corrosion, wear resistance, and hardness. Producing metal composite coatings using electrochemical techniques can be advantageous due to reduced production cost, lower working temperatures, and precise control of experimental parameters. Metal coatings such as zinc have been successfully co-deposited with TiO2, SiO2, CeO2 and mica particles and nickel has been co-deposited with a number of materials including TiO2, SiC, Al2O3, PTFE and silicates. Zinc-nickel alloys have long been studied for a number of properties, most notably corrosion resistance and recently their tribological properties. This chapter reviews the literature on electrodeposition of ZnNi nanocomposite coatings. Although there has been much work done on composite coatings, there is much less literature available on composite coatings with zinc-nickel alloys. So in this review, we look at the general trends for nanoparticle incorporation, deposition mechanisms, system stability, microstructures of the coatings and general corrosion trends.
- Zn-Ni alloys
- metal matrix composites
Metal matrix composite (MMC) coatings are promising materials developed by inclusion of a dispersed reinforcing material into a metal matrix. MMC’s can replace traditional materials through their ability to offer improved mechanical and physical properties such as increased hardness, wear resistance, low thermal expansion coefficients, lubrication properties, antibacterial properties and improved corrosion resistance [1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11]. Nanosized particle incorporation in metal matrixes forms a nanocrystalline structure, leading to improved properties of the material due to modification of the growth of the deposit [7, 12]. The properties of the composite coating are dependent on concentration, size, distribution and type of nanoparticle incorporated, in addition to the method and parameters used during coating formation [13, 14]. Although there are a large number of successful metal/particle combinations, this review will focus on zinc-nickel nanoparticle coatings exclusively. Individually nickel has been successfully co-deposited with a number of materials including TiO2, SiC, Al2O3, PTFE and layered silicates such as montmorillonite (Mt) [2, 7, 15, 16, 17, 18, 19] and zinc has been successfully co-deposited with TiO2, CeO2, ZrO2, SiO2, mica particles and polymeric nano-aggregates (PNAs) [20, 21, 22, 23, 24, 25] but a review of current literature on ZnNi alloy nanocomposite coatings has not been compiled to our knowledge. An overview of the literature is shown in Table 1. The most commonly used reinforcement material for zinc-nickel coatings is Al2O3 constituting ~32% of the papers, followed by TiO2 and SiO2/SiC with ~20% each, carbon nanotubes and CeO2 with ~8% each, and Al2O3/SiC, CeO2/SiO2 and Mt with ~4% each [11, 12, 13, 26, 27, 28, 29, 30, 31, 32, 33, 34, 35, 36, 37, 38, 39, 40, 41, 42, 43, 44, 45, 46, 47, 48].
|Reference and application||Plating bath||Incorporated particle and deposition parameters||Dispersion method|
|Blejan et al. |
|106 g/L ZINCATE 75 (75 g/L Zn and 400 g/L NaOH), 12 mL PERFORMA 285 Ni-CPL, 100 mL PERFORMA Additive K. 82.6 g/L NaOH, pH = 13||Al2O3 60 nm powder, S.A. 74 m2/g|
5, 10, 15 g/L
2 A/dm2, 23 ± 2°C
|Ultrasonication and solution stirring during deposition.|
|Ghaziof et al. [33, 34]|
|35 g/L ZnSO4.7H2O, 35 g/L NiSO4.6H2O, 80 g/L Na2SO4, pH = 4||Alumina Sol|
iDC = iavg. = 80 mA/cm2
ipeak = 160 mA/cm2
Frequency (HZ) = 100 (ton = Toff = 5 ms), 500 (ton = toff = 1 ms)
|Bath agitated 10 min prior to deposition.|
|Ataie et al. |
|150 g/L ZnCl2, 250 g/L NiCl2.6H2O, 45 g/L H3BO3, 100 g/L KCl, 100 g/L NH4Cl, 0.1 g/L SDS, pH = 4||α-Al2O3|
18 a/dm2, 30°C
|Magnetic stirring 24 h prior to deposition, 500 rpm. Sonicated 2 h (500 W) (15 min on, 15 min off for 2 h). Magnetic stirring during deposition, 250 rpm simultaneously with sonication.|
|Shourgeshty et al. [36, 37]|
Corrosion, wear properties
|250 g/L ZnCl2, 150 g/L NiCl2.6H2O, 45 g/L H3BO3, 100 g/L KCl, 100 g/L NH4Cl, 0.5 g/L, pH = 4 ± 0.5||α-Al2O3|
~20 ± 5 nm
4 A/dm2, 30 ± 2°C
|Magnetic stirring 12 h prior to deposition, 300 rpm, followed by 1 h ultrasonication (250 W, 20 KHz). During deposition mechanical stirring, 150 rpm and ultrasonic waves (50 W, 20 KHz).|
|Zheng et al. [38, 39]||60 g/L ZnCl2, 120 g/L NiCl2.6H2O, 120 g/L KCl, 100 g/L NH4Cl, 30 g/L NaCH3COO, pH = 5.0||α-Al2O3, particle diameter ~100 nm|
4 A/dm2, 35 ± 1°C
|Magnetic stirring 24 h, 2000 rpm prior to deposition. Ultrasound generator and mechanical stirring (200 rpm) during deposition.|
|Momeni et al. |
Hardness, antibacterial properties
|57.5 g/L ZnSO4.7H2O, 52.5 g/L NiSO4.6H2O, 9.3 g/L H3BO3, 56.8 Na2SO4, 0.53 H2SO4, pH = 2.5||TiO2|
1 A/dm2, 35°C
|Stirring during deposition, 500 rpm.|
|Gomes et al. [40, 41]||0.10 M ZnSO4.7H2O, 0.30 M NiSO4.6H2O, 0.20 M MgSO4, 0.15 M H3BO3, pH = 4||TiO2, particle size ~25 nm|
−3.2 A/dm2, Room Temp
|Ultrasonic agitation 30 min prior to deposition. Stirring during deposition, 400 rpm.|
|Katamipour et al. |
|60 ZnCl2, 120 g/L NiCl2.6H2O, 120 KCl, 100 NH4Cl, 30 NaCH3COO, pH = 4.6||TiO2, ~25 nm|
3.5 A/dm2, 35 ± 1°C
|Magnetic stirring, 1500 rpm 24 h prior to deposition. Ultrasound generator and stirring during deposition, 600 rpm.|
|Praveen et al. |
|160 g/L ZnSo4.7H2O, 16 g/L NiSO4.6H2O, 12 g/L H3BO3, 40 g/L Na2SO4, 1.5 g/L cetyl trimethyl ammounium bromide, pH = 4||TiO2, ~100–200 nm|
2 A/dm2, 27°C
|Magnetic stirring 10 h prior to deposition.|
|Tuaweri et al. [30, 44]|
|57.5 g/L ZnSO4.7H2O, 131 g/L NiSO4.6H2O, 162 Na2SO4.10H2O, pH = 2.0–2.5||SiO2|
|Agitation through use of vibro-agitation with vibromixer prior to deposition.|
|Ullal et al. |
|100 g/L ZnSO4.7H2O, 100 g/L NiSO4.6H2O, 75 g/L NaCH3COO.3H2O, 2 g/L citric acid, 0.5 g/L thiamine hydrochloride, pH = 3.0 ± 0.05||SiO2 nanopowder|
Deposition current and temp—not specified
|Magnetic stirring 24 h prior to deposition. Agitation of solution with circulation pump during deposition.|
|Takahashi et al. ||1 M ZnSO4.7H2O, 0–0.7 M NiSO4.6H2O, pH = 2.0||SiO2 colloid (Cataloid SN)|
100 A/dm2, 50°C
|Poliak et al. |
|125 g/L ZnSO4.7H2O, 75 g/L NiSO4.6H2O, 25 g/L H3BO3, pH = 4||SiO2 powder, ~10 nm|
|Müller et al. ||0.16 M ZnO, 1.7 × 10−2 NiSO4.6H2O, 3.75 M NaOH, 3.4 × 10−2 M diethylenetriamine, pH = alkaline||α-SiC powder, ~7.0 μm|
|Stirring 24 h prior to deposition, substrate rotated during deposition.|
|Creus et al. [49, 50]|
|63 g/L ZnCl2, 100 g/L NiSO4.6H2O, 215 g/L KCl, 20 g/L H3BO3, pH = 5.3.||CeO2, ~80 nm|
Cathodic pulse, ip = 5.0 A/dm2 with ton = 4 ms, toff = 16 ms. Anodic pulse, Ja = 1.0 A/dm2 with ton = 4. Average current density ~0.67 A/dm2. ms, 25°C
|Stirred 24 h prior to deposition, continued stirring during deposition, 200 rpm.|
|Tseluikin et al. [18, 51]||10 g/L ZnO, 50 g/L NiCl2.6H2O, 220 g/L NH4Cl, 20 g/L NaCH3COO||Carbon nanotubes|
Reversing mode, ic = 6 A/dm2, ia = 1.5 A/dm2.
|Tulio et al. ||0.25 M ZnSO4.7H2O, 0.2 M NiSO4.6H2O, 0.4 M H3BO3, 0.1 M sodium citrate, pH = 4.9.||α-SiC ~9.5 μm, α-Al2O3 ~3.4 μm|
|Stirred 12 h prior to deposition. Substrate rotated during deposition.|
|Xiang et al. |
|Not specified||CeO2 modified SiO2, 400–500 nm|
Deposition current and temp—not specified
|Conrad et al. |
|0.2 M ZnSO4.H2O, 0.1 M Ni(NH4)2(SO4)2.6H2O|
0.1 M Na2B4O7.10H2O
pH = 9.5
1, 5 g/L
E1 = −1.45 V, T1 = 10 sec. E2 = −0.9 V, T2 = 2 sec, Room Temperature
|Sonicated 1 h prior to deposition, N2 gas bubbled through solution during deposition.|
Zinc-nickel coatings are well known in the field of corrosion resistance as a corrosion resistant material. Corrosion protective coatings are commonly used to extend the lifetime of materials such as stainless steel from corrosion onset as a substitute for more expensive, less available materials [49, 50, 51, 52, 53]. Coating zinc onto stainless steel, known as galvanization, is an industry standard to protect against corrosion. The zinc coating sacrificially corrodes, thereby protecting the stainless steel from corrosion [54, 55, 56]. Options are now being explored to withstand harsher conditions, longer lifetimes, reduced thickness and better overall strength of the protective coating layer. Although a large focus has been on the development of generalized corrosion resistant coatings, when considering cost, environmental impact and performance, zinc alloys have become an attractive option. An alloy modifies the composition of a material resulting in different corrosion properties then the original element which can significantly improve the stability of the protective coating [2, 3, 4, 5, 52, 57, 58], therefore, by picking the correct combination of alloys, one can greatly increase the corrosion resistance of the material [49, 52]. Alloy formation can result in various phases, dependent upon the experimental conditions at the time of formation. For zinc-nickel, there are 5 known alloy phases: α- and β- (30% Ni, nickel rich), γ- (Ni5Zn21), δ- (Ni3Zn22) and η- (1% Ni) (zinc rich), all dependent upon the Zn/Ni ratio and experimental parameters used to form the alloy [50, 55, 59, 60, 61]. The γ-phase and δ-phase are predominantly formed through electrochemical methods, with γ-phase showing the strongest protection against corrosion [57, 60, 62, 63, 64, 65]. Zinc nickel γ-phase alloys with approximately 8–18% have been found to be optimal for maximum corrosion protection [48, 57, 62, 65].
Although several methods are available for the development of nanocomposite coatings, electrodeposition remains a favorable choice due to relative ease of use, low cost, convenience, ability to work at low temperatures and overall control of experimental parameters [39, 48, 58, 65]. A general survey of the literature concerning zinc-nickel nanocomposite coatings found electrochemical deposition to be the main preparation method, so general trends and properties of the coatings formed through electrochemical methods will be the focus of this chapter.
2. Electrodeposition of zinc-nickel nanocomposite coatings
2.1. Dispersion of particles
A variety of particles, including Al2O3, TiO2, SiO2, SiC, ceria, carbon nanotubes and montmorillonite (Mt) have been successfully incorporated into zinc-nickel coatings. For optimal effect, the nanoparticles need to be dispersed throughout the metallic coating. To accomplish this, the particles first need to be suspended in the electrolytic solution and agglomeration of the particles needs to be kept to a minimum to prevent issues in coating formation. Particle agglomeration is an issue seemingly independent of particle concentration as it occurs under low to high concentrations, though smaller particle size does increase tendency to form agglomerations, leading to less incorporation in the final coating. To prevent agglomeration, various methods can be used such as organic additives, agitation of the solution, current density, etc. Treatment of the nanoparticles prior to deposition is varied throughout the field but the most common methods used for particle suspension are magnetic stirring, sonication or a combination of stirring and sonication prior to and during deposition [11, 12, 13, 26, 27, 28, 29, 30, 31, 32, 33, 34, 35, 36, 37, 38, 39, 40, 41, 42, 43, 44, 45, 46, 47, 48]. In addition to treatment of the nanoparticles, concentration in the bath also affects the quality of the coatings. As expected, as the concentration of nanoparticles in the bath increases, the concentration of nanoparticles in the resulting coating tends to increase. The small sized particles are easily incorporated into irregularities on the metal surface and positively charged particles are attracted to the cathode, so more easily incorporated into the coating . In the case of oxide nanoparticles, the oxides compete with the metallic ions for adsorption onto the active sites, creating more nucleation sites and perturbing metallic grain growth. Other particles are trapped during deposition, filing holes or gaps within the naturally forming coating [22, 40].
Concentration of nanoparticles in the bath varies from 0.05–300 g/L with most work using around 5–15 g/L. Müller et al., who relied on mechanical stirring to disperse the nanoparticles, found optimal concentration of SiC particles to be 60 g/L, beyond which the particles began to agglomerate. Beyond this concentration, stirring was not sufficient to keep the particles suspended in solution and a decreasing trend of SiC in the coatings was observed .
Katamipour et al. studied the effects of ultrasonic conditions to promote uniform dispersion of the coating particles, and to determine if improvement occurred in the corrosion and mechanical properties of the coatings. They found that increasing the ultrasonic power density lead to a decrease in particle size, an increase in nanoparticle incorporation in the coating, and initially, an improvement in corrosion and mechanical properties. The agglomeration often observed with nanoparticles also dissipated with the use of sonication . Nano-alumina particles were found to be uniformly imbedded in the ZnNi-Al2O3 coating after treatment of ultrasonic vibration [32, 33]. Without sonication, ceria nanoparticles were seen organized in long string-shape agglomerates. These agglomerates became trapped inside voids and pores during coating growth . Though sonication or mechanical disruption of the nanoparticles is needed to distribute them throughout the metal matrix, care must be taken as excessive agitation can lead to a lower quality of particles in the deposit .
Na-smectites, a type of clay mineral, specifically montmorillonite (Mt) were also examined, for incorporation into metal matrixes. Within aqueous solutions, Na-montmorillonite can be completely exfoliated and incorporated into other materials, forming continuous, crack free films, which is beneficial in corrosion resistant coatings [5, 8, 9, 52, 66]. Exfoliation causes the short range order of the clay particles to be disrupted, causing individual clay platelets to exist, unassociated from one another. The resulting clay platelets range 1–2 nm in width with 100–1000 nm in length . These platelets are easily incorporated into the coating during deposition, increasing the overall thermal stability and mechanical strength of the coating, which leads to increased corrosion resistance [2, 8]. As the alloy coating is forming, the exfoliated clay in solution is freely dispersed throughout the electrolytic bath. Mt is a cationic clay with a negatively charged surface which attracts metal ions, increasing incorporation of the platelets into the metal composite during deposition. The clay platelets settle onto the substrate surface as the coating is being formed, allowing them to be incorporated into the coating. Exfoliated Mt, which has a plate-like structure, increases the surface area of the material when imbedded in the coating and leads to a more tortuous mean free path of the corrosion cells upon onset . This technique has previously been successful with the incorporation of montmorillonite platelets into pure nickel, nickel-molybdenum and nickel-copper coatings [2, 5, 8, 9, 52, 58, 66]. However, many traditional particles used in composite coatings are spherical in shape. For example SiO2 nanoparticles coated with a layer of cerium oxide have been introduced into ZnNi coatings to improve corrosion resistant properties .
2.2. Influence of nanoparticle addition on deposition mechanism
Though many researchers use electrochemical deposition as a tool to form a coating of interest, there is little published work on the electrochemical system used for the deposition of zinc-nickel nanocomposite coatings. A better understanding can lead to an improved deposition system, and an overall superior coating. Work continues to be done in acidic and alkaline conditions with a goal of further improving the materials, longer material lifetimes and a better understanding of the mechanisms involved in various alloy formations [49, 50, 55, 57, 59, 60, 65, 67, 68, 69, 70, 71] but little work has been done to examine systems with nanoparticle incorporation.
Zinc-nickel alloy formation follows an anomalous deposition mechanism which occurs when the electrochemically less noble metal deposits preferentially to the more noble metal. This is verified through examination of the voltammetry patterns of the zinc-nickel system as the individual zinc and nickel reduction peaks are shifted based on the presence of the other metal species in solution [48, 59, 62, 70, 72, 73, 74]. During deposition, a thin layer of nickel is initially deposited onto the substrate. As the deposition continues, zinc is intercalated into the nickel, leading to formation of the alloy [57, 61, 67]. In acidic systems under low current density, a transition from anomalous to normal codeposition has been noted. Normal codeposition is dominant when the applied potential is more similar to the free corrosion potential of zinc and zinc-nickel alloys. These systems also present with lower current efficiencies [57, 61, 73, 74, 75, 76, 77]. Normal deposition leads to alternate ZnNi phases, which are not preferred for maximized corrosion protection, so the goal is to remain under an anomalous deposition route, to further aid in the deposition of γ-phase ZnNi alloy. Within the research presented on the deposition mechanism with nanoparticle presence, the effect of SiO2, SiC, Al2O3, Mt and carbon nanotubes on the deposition mechanism has been explored and is discussed under their individual sections.
Hydrogen evolution at cathodic potentials is a concern in electrochemical deposition as it can lead to the formation of cracks and defects in the overall coating structure, both during deposition and later during use of the material. Hydrogen evolution competes with metal electrodeposition in this system and can play a major role in determining the composition of ZnNi coatings [35, 46]. In ZnNi deposition systems under alkaline conditions, boric acid was found to suppress hydrogen evolution. Hydrogen evolution is a larger concern for nickel deposition than zinc deposition as a larger overpotential is required for nickel deposition since the deposition is under kinetic control while zinc deposition is thermodynamically controlled . Our previous study examined the change in hydrogen evolution onset with varying borate concentrations in alkaline solutions, and found as the borate concentration is increased, hydrogen evolution is pushed to more cathodic values . A maximum borate concentration of 100 mM was used due to conductivity of borate in the system . In addition to borate, nanoparticles can have an overall effect on hydrogen evolution in the system as well. The hydrogen evolution onset was compared for solutions with and without the presence of Mt in Figure 1. For nickel, a large cathodic shift was observed when Mt was added to the system. For zinc and zinc-nickel, small cathodic shifts were observed with Mt. The Mt can help further shift the onset of hydrogen evolution within this system, in addition to borate [48, 57]. Alloy formation typically occurs at or near the onset of hydrogen evolution for this system. By shifting the onset in a cathodic direction, less hydrogen will be produced during alloy formation, leading to less entrapped hydrogen in the overall coating. Hydrogen evolution can hinder adsorption of nanoparticles on the surface of the coating material and lead to embrittlement .
The addition of Al2O3 and SiC was also found to cause a surface blockage preventing hydrogen evolution to occur. This effect is found to be dependent on the concentration of SiC in solution, but for addition of Al2O3 no dependency is observed. It is believed that SiC and Al2O3 are adsorbed onto the electrode surface, reducing the active surface area. At lower pH, H+ has a higher tendency to adsorb onto the SiC particles, leading to a reduction in hydrogen evolution .
2.3. Deposition methods
Electrodeposition techniques include potentiostatic and galvanostatic deposition, and further into both methods, applied vs. pulsed deposition parameters in the literature for zinc-nickel nanocomposite coatings . The particles co-deposit with the zinc-nickel coating which has advantages over other methods such as better control of coating thickness, deposition speed, working under controlled temperatures, and it is a single-step method. The nanoparticles are incorporated as the metal species are reduced onto the electrode surface, forming the nanoparticle coating. Applied methods include direct current or direct potential, where a constant current or potential is applied to the electrode. Pulsed methods include pulse current (PC), pulse reversed current (PRC), pulse potential (PP) and pulse reversed potential (PRP). PC and PP involve alternatively applying two or more cathodic direct current or potentials during the deposition, with off times, when no current or potential is being applied. PRC and PRP are similar to PC and PP as a cathodic pulse is applied but during the off times, an anodic pulse is applied to the electrode. Previous studies show an increase in incorporation of particles through a pulse deposition method with better overall coverage of the underlying material compared to a constant applied potential technique [7, 45, 48, 57, 62, 68, 78, 79, 80]. The nanoparticles are incorporated in a higher percentage because of the partial dissolution of the metal deposit during the anodic pulse. Pulse plating was found to improve overall quality of deposits and reduce grain size which inherently increases the corrosion protection of the coating [48, 57, 58, 79, 81]. Pulse deposition includes the following attributes: (1) better inclusion of nanoparticles in the metal matrix, (2) lower concentration of nanoparticles needed in the electrolytic solution, (3) selective entrapment based on size of nanoparticles, (4) release of trapped hydrogen prior to coating use which leads to longer coating lifetime and (5) a more opened grain structure which allows hydrogen to escape from the deposit without forming holes or pits in the coatings which could otherwise be used as corrosion cell development sites [7, 48, 57, 80]. Pulsed deposits help embed higher concentrations of nanoparticles because it helps eliminate a fraction of the electrodeposited metal during the off time . Pulse durations affect the shape and size of crystallite formation [21, 81, 82]. During off time, adsorbed metallic adatoms are able to reorganize and minimize surface energy. The grain growth continues during this time due to desorption of impurities leading to changes in grain morphology and size while chemical composition remains relatively constant .
2.4. pH studies
Coating composition and quality is dependent on the pH of the system at the time of formation. Although extensive work has been done on zinc-nickel coatings in both acidic and alkaline conditions, less work has been done on zinc-nickel coatings with nanoparticle incorporation. A review of the literature shows most studies being performed under acidic conditions [32, 33, 34, 35, 36, 37, 39, 40, 44, 46] with little work in alkaline conditions [12, 42, 48]. ZnNi-Al2O3 coatings were predominantly deposited under acidic conditions (pH = 4, 4.9 and 5.0) with one group examining deposition at pH = 13 [12, 27, 28, 29, 30, 31, 32, 33]. The literature for the deposition of ZnNi-TiO2 coatings was done under acidic conditions with pH = 2.5, 4 and 4.6 from a variety of groups [11, 34, 35, 36, 37]. The deposition of ZnNi-SiO2 particles was performed at pH = 2, 3 and 4 while the deposition of ZnNi-SiC was done under an unspecified alkaline pH [26, 38, 39, 40, 41, 42]. The deposition of ZnNi-ceria particles was undertaken with a pH = 5.3 [43, 44]. ZnNi-carbon nanotubes, though not specified are believed to have been deposited under alkaline conditions due to specified bath components [13, 45] and the deposition of ZnNi-Mt coatings was done at pH = 9.4 . Though the bulk of the work has been done under acidic conditions, focus of the research may benefit from pushing into the realm of alkaline deposition as throughout literature, optimal coating formation is realized under alkaline conditions. Although zinc-nickel coatings deposited under acidic conditions tend to have a higher current efficiency, alkaline processes tend to lead to better substrate coverage [12, 57, 62, 79]. A drawback of alkaline conditions is stabilizing agents are needed to keep the metal species from precipitating as metal hydroxides from the solution.
3. Characterization of the zinc-nickel nanocomposite coatings
3.1. Coating composition
Zinc and nickel content and nanoparticle incorporation were examined with various techniques including atomic absorption spectroscopy (AAS), inductively coupled plasma-mass spectrometry (ICP-MS), energy dispersive spectroscopy (EDS) energy dispersive x-ray (EDX) and EDX mapping. Uptake of the nanoparticles is of interest as varying concentrations of nanoparticles are found, dependent upon the character of the particle being added to the solution. ZnNi coatings with Al2O3 incorporation were found to contain anywhere from trace Al2O3 up to 8.9 wt % throughout the literature [12, 27, 28, 29, 30, 31, 32, 33]. Zinc-nickel coatings with TiO2 incorporation were found to contain on average 80–85% Zn, 12–17% Ni and 1.25–2.5% Ti [35, 36, 37]. ZnNi-SiC coatings contained 11% SiC . ZnNi coatings with ceria incorporation contained 10–11% Ni, with 2–3% ceria content [43, 44]. ZnNi-Mt coatings contained 86–90% Zn, 10–14% Ni with trace amounts of Mg and Al from Mt nanoparticles confirmed in ICP-MS analysis . Throughout the studies, the coatings maintain the Ni% needed (8–18%) for maximized corrosion protection [57, 59, 70, 83].
3.2. X-ray diffraction (XRD)
The phase of electrodeposited ZnNi alloy coatings is dependent upon the nickel content in the alloy and can be controlled by a number of factors including electrolytic bath conditions . ƔNi5Zn21 is known to be the most corrosion resistant ZnNi alloy phase and appears to be preferentially deposited under alkaline conditions in ZnNi systems without nanoparticle incorporation. The γ ZnNi has a preferred orientation with the (330) reflection as main peak in the XRD pattern [42, 59, 65, 66, 67, 71, 72]. This preferred orientation continues with the incorporation of nanoparticles although an overall decrease in peak intensity and broadening of peak suggest smaller crystallite size formation [12, 35, 48]. The peak width of the diffraction peak at half maximum height (FWHM) is dependent on crystallite size and lattice strains due to lattice imperfections such as dislocations or atom vacancies with the values dependent most heavily on crystallite size [84, 85]. If we assume there is little strain in the system, we can assume the broadening at FWHM is due to a decrease in crystallite size of the metallic particles . The average crystallite size of ZnNi coatings with TiO2, SiC, and Al2O3 nanoparticles are presented in Table 2. The trends show an overall decrease in particle size with the increase in nanoparticle incorporation as compared to pure ZnNi coatings.
Hardness (HV) values are a measurement of the microhardness or resistance to penetration of a sample and can be used to compare quality of the coatings. All composite coatings studied demonstrate improved microhardness values as compared to the base alloy as presented in Table 3. As expected, addition of nanoparticles to the coatings improve the overall hardness values, as demonstrated with an increase of 305 HV to 524 HV for ZnNi coatings with CeO2 treated SiO2 particles, an increase of 35 HV with the addition of TiO2 particles in Praveen’s work, a 300% increase in hardness with an incorporation of 11.2 wt % Al2O3 particles in Zheng’s work and noticeable increases in both Ataie’s and Ghaziof’s work with incorporation of Al2O3 particles as well [27, 29, 32, 37, 47]. The improved microhardness is believed to be due to dispersive strengthening as the ceramic like particles (TiO2) form a barrier to deformation commonly observed in metal matrix systems. As the incorporation of nanoparticles increases, the microhardness also increases . The higher hardness of the coating is due to the fine-grained structure. The dispersed particles in the matrix are able to obstruct easy movement of dislocations .
|Reference||Coating composition||Hardness values (HV)||Additional parameters|
|Xiang et al. ||Bare substrate||134||Direct deposition|
|ZnNi coating with incorporated SiO2 nanoparticles||535|
|ZnNi coating with incorporated CeO2 treated, SiO2 nanoparticles||524|
|Ataie et al. ||ZnNi Coating with incorporated Al2O3 nanoparticles||340||Direct deposition|
|ZnNi Coating with incorporated Al2O3 nanoparticles||640||30 W ultrasonic application during deposition|
|ZnNi Coating with incorporated Al2O3 nanoparticles||750||45 W ultrasonic application during deposition|
|Zheng et al. ||ZnNi coating||215||Ultrasound generation and magnetic stirring during deposition|
|ZnNi coating with incorporated 11.2 wt % Al2O3 nanoparticles||640|
|Praveen et al. ||Zinc-nickel coating||135||Direct deposition|
|ZnNi with incorporated TiO2 particles||170|
|Ghaziof et al. ||ZnNi coating with incorporated Al2O3 nanoparticles||235||Direct deposition|
|ZnNi coating with incorporated Al2O3 nanoparticles||310||Pulsed current deposition, 100 Hz|
|ZnNi coating with incorporated Al2O3 nanoparticles||323||Pulsed current deposition, 500 Hz|
3.4. Corrosion studies
An advantage to developing metal matrix composite coatings is for increased corrosion resistance as compared to pure metal coatings. Properties that may contribute to this added protection include a finer coating structure with refined grains, incorporation of electrochemically inert particles dispersed throughout the metallic coating, and filling of crevices, gaps, and micron sized holes on the coatings surface. These could otherwise lead to localized defects which are vulnerable to corrosion. Improvement of self-passivation of the coating is offered through improved barrier protection due to the incorporated particles in the naturally formed defects of the coatings. Common methods to examine the corrosion resistance of a material include open circuit potential (OCP) studies, linear polarization resistance (LPR), potentiodynamic polarization, and electrochemical impedance spectroscopy (EIS).
The open circuit potential (OCP) is the potential of the working electrode relative to the reference electrode when no external potential or current is being applied to the system. OCP is dependent on the composition of the working electrode, treatment of the electrode prior to the study, and the electrolytic bath in which the electrode resides. Linear polarization resistance is the measurement of current in relation to the electrode potential. This can be used to predict the corrosion rate of the coatings within a specific environment. The film is polarized by applying an external potential forcing the system away from equilibrium and monitoring the resulting potential and current. The deviation from an equilibrium potential is called polarization. The polarization resistance (Rp) is experimentally observed between the electrochemical current density and applied potential for the corroding electrode within a few millivolts of the polarization from the open circuit potential (Eocp). Potentiodynamic polarization pushes the potential even further from the equilibrium potential for the anodic and cathodic sweeps. From this data the anodic slope (βa) and cathodic slope (βc) are obtained from the curves. The Ecorr is determined from the intercepts of the curves. The icorr value is obtained by substituting the βa, βc and Rp values into a simplified rearranged Stern and Geary equation [86, 87].
The anticorrosive ability of ZnNi-nanocomposite coatings can be further investigated with EIS. Nyquist plots show a semicircle shape in the investigated frequency range with an increased axial radius, which is indicative of better corrosion resistance. Equivalent circuit models are used to simulate the metal-solution interface to better understand the system. A few studies have done corrosion work for these ZnNi nanocomposite coatings and shown improvement with addition of the nanoparticles. Table 4 lists some results which are discussed in sections below.
|Coating [ref]||Ecorr (V)/SCE||icorr (A)||Rp (Ω cm2)|
|Zn ||−1.17||2.09 × 10−4||1333|
|Ni||−0.45||2.75 × 10−5||6790|
|ZnNi γ phase||−0.74||1.06 × 10−5||30,485|
|ZnNi-Mt γ phase||−0.73||3.72 × 10−6||34,900|
|ZnNi ||−0.92||6.20 × 10−5||—|
|ZnNi-CeO2||−0.77||3.30 × 10−5||—|
|ZnNi-CeO2 (sonicated)||−0.78||2.80 × 10−5||—|
|ZnNi-TiO2 ||−1.09*||9.90 × 10−5||122.2|
|ZnNi||−1.05*||4.30 × 10−5||352.0|
|ZnNi-TiO2 (24 h immersion)||−1.11*||1.25 × 10−5||97.3|
|ZnNi (24 h immersion)||−1.03*||—||94.1|
|ZnNi ||−0.62||2.51 × 10−6||1167.6|
|ZnNi-Al2O3 5 g/L||−0.52||1.23 × 10−6||4024.9|
|ZnNi-Al2O3 10 g/L||−0.63||2.37 × 10−6||2038.3|
|ZnNi-Al2O3 15 g/L||−0.70||2.57 × 10−6||1190.0|
4. Zinc-nickel nanocomposites
4.1. ZnNi-Al2O3 and Al2O3/SiC
Most work to date has examined ZnNi-Al2O3 coatings (~32% of papers) with 4% examining the effects of Al2O2/SiC combined in the nanocomposite. Though the deposition mechanism of ZnNi-Al2O3 coatings was not explicitly discussed, Tulio et al. examined the effects of SiC and Al2O3 in slightly acidic pH with rotating disc. They first examined the effect of SiC and Al2O3 on nickel and zinc, without the other metal ion present in solution and found the addition of SiC and Al2O3 encouraged deposition of both Ni and Zn individually. For nickel, a marked increase in current densities was observed. In the Zn system when the solution was scanned cathodically without the presence of nanoparticles, the deposit exhibited many discontinuities, or areas without a deposit present. When the SiC and Al2O3 particles were added to the solution, there was a noticeable increase of coating coverage so much that the discontinuities almost disappeared entirely, suggesting encouragement of Zn deposition. SiC and Al2O3 do not affect the initial nucleation and growth in the ZnNi system when the metal species are combined, though at higher concentrations of nanoparticles, surface blockage has been observed. Larger current densities are observed for systems with SiC and Al2O3 as compared to systems free of nanoparticle presence and a positive shift in potential was noted at the onset of secondary nucleation. This is due to an increase in the mass-transport of the particles to the electrode surface during the rotation. During the scans the quantity of particles reaching the electrode increased, leading to an increase in current density. The ZnNi deposition did remain anomalous under all conditions examined . Blejan and Muresan examined the XRD patterns of deposited ZnNi-Al2O3 films (using a Cr x-ray tube), which only exhibited γ-phase ZnNi alloys, showing small growth of the (330) plane with addition of Al2O3 particles with deposition giving a preferred (600) orientation . Improvement of nanoparticle incorporation was noted through the use of ultrasonication [29, 32, 33].
Zhang and An found an increase of hardness with the addition of Al2O3 . Ataie et al. examined the effect of sonication during the deposition. Without sonication, the hardness was 340 HV, with 30 W sonication it was 640 HV and with 45 W sonication it was 750, a 220% increase over the coating with no sonication . The hardness of ZnNi and ZnNi-Al2O3 coatings under direct current and pulse current deposition conditions was examined. ZnNi under applied current was 235 HV while pulsed ZnNi was 310–323 HV, a 40% increase and ZnNi-Al2O3 was 338 HV, a slight increase over pulsed ZnNi coatings . Shourgeshty et al. examined multilayer coatings of ZnNi and ZnNi-Al2O3 deposits. As expected, an increase in the number of layers improved the hardness values of the coatings but addition of Al2O3 also had a positive effect [30, 31].
ZnNi-Al2O3 coatings were studied in Na2SO4 solution. ZnNi-Al2O3 coatings (Table 4) present corrosion potentials of the composite coatings at more positive potential with initial Al2O3 incorporation as compared to ZnNi alloys which is attributed to the chemical inertia of the incorporated particles . The corrosion current decreases from 1.83 × 10−5 to 0.92 × 10−5 as the Al2O3 content is doubled from 4.5 to 8.9 wt% . EIS of ZnNi-Al2O3 with varying incorporation of Al2O3 and varying immersion (0, 24, 48 and 120 h) is presented in Figure 2. The coatings were studied in 0.2 g/L Na2SO4 (pH 5) using a potentiostat PARStat 2273 (Princeton Applied Research). The frequency domain was 10 kHz to 100 mHz and temperature was maintained at 23 ± 2°C. The plots were fit with ZSimpWin 3.21 software. The impedance modulus of the nanocomposite is higher than pure ZnNi films. The charge transfer resistance for the composite coating is higher than ZnNi films, yet the double layer capacitance is smaller. Initially the measurement decreases at a systematic rate, suggesting a rapid degradation of the coating due to corrosion but after 50 h the rate of degradation decreases, likely due to the formation of corrosion products on the surface of the coating . Incorporation of Al2O3 particles results in γ-phase zinc-nickel alloys with nanoparticle incorporation. ZnNi-Al2O3/SiC coatings still follow an anomalous deposition route. Improved hardness and corrosion properties are observed with incorporation of Al2O3 [12, 27, 29, 32, 46].
ZnNi-TiO2 coatings comprise ~20% of the papers on ZnNi-nanocomposite coatings. As demonstrated with the Al2O3 composite coatings, improved corrosion and mechanical properties of the ZnNi coatings occur with the incorporation of TiO2 particles into the metal matrix. In a study by Praveen et al. they varied the TiO2 concentration in the bath from 0.5 to 5.0 g/L. Lower current densities were observed at 3 g/L and above this concentration the corrosion current increased so it was chosen as the optimal concentration .
The deposition with TiO2 gave coatings with preferential γ-phase alloy, though small amounts of a pure zinc phase are seen in ZnNi coatings without TiO2 incorporation. Textural modifications due to the presence of TiO2 nanoparticles are suggested due to slight changes in peak intensity in the XRD patterns as compared to ZnNi coatings without TiO2 incorporation. The metallic grain size also decreases with the incorporation of TiO2, due to changes to nucleation and growth due to disruption of the metallic growth by incorporation of semiconducting particles during coating formation . TiO2 incorporation can also cause a considerable decrease in grain size for the metallic phase, with rough and irregular deposits as demonstrated by SEM and AFM (Figure 3) [34, 35]. The ZnNi coating without TiO2 exhibited multiple defects, cracks, gaps, crevices and microholes. The TiO2 nanoparticles fill these gaps, leading to an overall decrease in the corrosion rate. The crystal size of the composite coating also appears smaller as compared to the ZnNi coating . The compact size is preferred as it also better protects from corrosion onset. The effects of soniciaton on morphology were also examined. Ultrasonic vibration during deposition was found to result in increased nanoaparticle incorporation and a more homogeneous coating, suggesting the vibration promotes uniform distribution of the particles and decreased agglomeration of the particles. Improvement of nanoparticle incorporation due to ultrasonicaiton was also noted .
TiO2 particles restrained the growth of the ZnNi alloy grains leading to a significantly higher microhardness in the presence of TiO2 . As expected, with increasing nanoparticle incorporation, the hardness increases, which is believed to be due to the dispersion of the ceramic like TiO2 particles throughout the metal matrix . As observed with Al2O3 addition, sonication of the electrolytic bath lead to increased microparticle incorporation, with hardness increasing from 253 HV for ZnNi coatings, and 464 for ZnNi-TiO2 coatings without sonication, to 754 HV for ZnNi TiO2 with sonication during deposition, a 60% increase in hardness with sonication and a 200% increase in hardness as compared to coating with the addition of Al2O3 particles incorporated under sonication. TiO2 particles hinder the dislocation of movement, leading to an increased hardness of the material though a reverse trend which was observed at higher concentrations of TiO2 in the deposition bath and believed to be due to agglomeration of the nanoparticles in solution .
The OCP was monitored over time for ZnNi and ZnNi-TiO2 coatings in 3.5% NaCl and near-neutral 0.05 M Na2SO4 solutions, respectively . The initial OCP values show that OCP of ZnNi and ZnNi-TiO2 coatings appear at more noble values due to the presence of nickel (a more noble metal as compared to zinc) in the coating. Initially, the OCP values were − 1.49, −1.51, −1.43 and − 1.23 V (vs. Hg/Hg2SO4) for Zn, Zn-TiO2, ZnNi and ZnNi-TiO2, respectfully. After 24 h submersion, these values changed to −1.47, −1.49, −1.18 and −1.10 V for Zn, Zn-TiO2, ZnNi and ZnNi-TiO2, respectfully. The ZnNi coating undergoes the most drastic change in OCP in the 24 h time frame. The ZnNi-TiO2 appears to reach a steady state at a faster rate than ZnNi, possibly due to the smaller grain size of the particles due to nanoparticle incorporation . There is a small positive shift in all coatings, due to dissolution of zinc on the surface of the coating, as zinc undergoes a sacrificial protection method.
The polarization of ZnNi and ZnNi-TiO2 coatings were found to have a larger corrosion current after 24 h of submersion in 0.05 M Na2SO4 solution than the as deposited coatings but the ZnNi-TiO2 coating still maintained a smaller corrosion current value than the ZnNi coating even after immersion (Table 4). The microstructure of as deposited and submerged coatings was examined to determine any structural design which could affect the corrosion current of each coating. The incorporation of TiO2 nanoparticles decreased the grain size of the metallic phase and the coatings appear more rough and irregular in surface morphology . The initial increase in corrosion current observed by ZnNi-TiO2 coatings prior to submersion are attributed to the smaller grain size and more porous structure observed in the coatings. The higher porosity of the coatings could be the cause of the increased corrosion resistance [34, 35]. Polarization curves and kinetic data show ZnNi-TiO2 and ZnNi deposits initially have a high corrosion density (0.4–0.6 mA/cm2) and low polarization resistance. At 24 h immersion in the Na2SO4 solution, the icorr of the ZnNi-TiO2 coating has decreased by a factor of 5 and the Rp had increased by a factor of 3. The ZnNi-TiO2 coating presented the highest corrosion protection after 24 h immersion . ZnNi-TiO2 coatings were examined in 3.5% NaCl solution and a decrease in corrosion current density was observed as TiO2 was incorporated into the coating, with a decreasing trend following increased sonication of the particles prior to deposition . Coatings throughout literature demonstrate a wide array of corrosion potentials, varying from E = −0.5 to −1.2 V, which follow values found for ZnNi coatings [12, 28, 30, 31, 32, 34, 35, 36, 37, 39, 43, 44]. The value of the corrosion potential, which can show corrosion tendencies, is indicative of the components of the coatings. The optimal corrosion potential will lie between that of a pure zinc coating and a pure nickel coating, as it will have character of each metal and with that, corrosion behavior of each metal. The corrosion current, which is proportional to the corrosion rate, does decrease with the incorporation of nanoparticles as demonstrated in Table 4. The addition of nanoparticles, even in small amounts shows an overall improvement on the corrosion potential, corrosion current and resistivity of the systems.
Momeni et al. studied ZnNi-TiO2 coatings on copper substrates as a possible coating for antibacterial inhibition, specifically the antibacterial resistance toward Gram positive (Staphylococcus aureus PTCC1431) and Gram negative (E. coli PTCC1394) bacteria through an inhibition zone method (Figure 4). The bacterial strains were transferred into flasks containing nutrient broth and bacteria which had been cultured at 37°C under aerated conditions. An agar diffusion test was used to study antibacterial activity. Inoculums of E. coli and S. aureus were spread over the surface of the nutrient agar, and the ZnNi-TiO2 sample was placed onto this sample and incubated for 24 h at 37°C. The best coating was found to be ZnNi-TiO2 prepared with 3 g/L TiO2 in solution, which had an inhibition zone of 23 mm for E. coli and 28 mm for S. aureus .
4.3. ZnNi-SiO2 and ZnNi-SiC
SiO2 and SiC comprise ~20% of the literature on ZnNi-nanocomposite coatings. SiO2 was examined by Tuaweri and Wilcox. They studied the change in current density as a function of % Ni in the deposit with and without bath agitation, and with varying SiO2 particle size. SiO2 is believed to deposit with the ZnNi coating under a codeposition mechanism. As expected, without SiO2 presence in the bath, nickel appears to follow an anomalous deposition mechanism as the current density of the system is increased from 3 to 6 A/dm2. When 26 g/L of 20 nm SiO2 particles was added to the system, a transition from anomalous to normal deposition is noted at 4 A/dm2 . The SiO2 colloids have been previously noted to increase deposition rate of Fe group metals . SiO2 colloids shift this deposition from an anomalous mechanism to a normal mechanism. A possible explanation is due to adsorption of the Fe group metals onto the SiO2 nanoparticles in the electrolytic bath. During the deposition process, the pH of the electrolyte at the working electrode surface increases, or becomes more alkaline due to removal of hydrogen by the generation of hydrogen gas, also known as hydrogen evolution. The SiO2 particles tend to agglomerate once a neutral pH is reached, so the agglomerated colloid can suppress Zn(OH)2 formation causing a slowing in the diffusion of zinc ions from the solution, through the inner layer and to the cathode for reduction. As the SiO2 particle size was increased from 20 nm to 2 μm, a slightly higher nickel wt. % was observed in the coatings. Addition of the SiO2 nanoparticles resulted in increased Ni wt. % at all current densities, as compared to coatings without SiO2. This suggests that the SiO2 in the bath encourages the deposition of nickel in the coating. Throughout the studies SiO2 appears to have an overall effect on the deposition mechanism of ZnNi coatings through emergence of a normal deposition route, while SiC continues to follow an anomalous deposition pattern. Further studies need to be completed in this area to determine if increased particle presence will encourage a transition from anomalous to normal deposition for other systems or if this is unique to the behavior of SiO2 nanoparticles in the ZnNi electrolytic system.
Tuaweri et al. found the corrosion potentials of ZnNi and ZnNi-SiO2 coatings were more anodic as compared to zinc. Under open circuit potential conditions, ZnNi and ZnNi-SiO2 coatings behave in a similar manner, but once the applied potential is increased, the ZnNi-SiO2 coatings shift toward more anodic potentials as compared to ZnNi coatings. This suggests the presence of SiO2 promoted shifting of the dissolution potential to more anodic values as compared to ZnNi due to the inert nature of SiO2 particles and possible changes in the deposition mechanism in the presence of SiO2. SiO2 appears to have an overall effect on properties such as deposit texture, morphology, microstructure due to the ability of the SiO2 particles to provide barrier protection to the coating through packing of microholes, gaps and crevices in the coating . The incorporation of SiC and SiO2 nanoparticles shows no changes on phase composition, with γ-phase being the predominant phase in the XRD patterns. Some Zn101, Zn102 and δ-phase XRD peaks were observed, but this was expected as these coatings were deposited under acidic conditions. Low intensity peaks corresponding to SiO2 confirms incorporation of the nanoparticles into the coatings without leading to any structural phase changes [39, 42]. In other nanoparticle coatings, we find similar trends such as finely grained, uniform, clearly pronounced crystal structures with the incorporation of SiO2 and SiC [26, 38, 40, 41]. Finer grains were common with even distribution of the nanoparticle in the coatings . SiO2 coatings were examined for hardness changes, and showed an increase in hardness with an increase in current density during deposition of the coatings. Coatings were deposited from 2.0–5.0 A/dm2 in increments of 1, and hardness values increased correspondingly from 155 to 210 HV. The improved hardness of the coating was attributed to the incorporation of SiO2 particles which add mechanical strength due to embedded SiO2 particles .
4.4. ZnNi-CeO2 and SiO2/CeO2
ZnNi-CeO2 coatings comprise ~8% of the literature, while ZnNi-SiO2/CeO2 comprises ~4%. Nanoparticle incorporation was found to be improved through pulsed deposition methods [43, 44]. Exbrayat et al. examined ZnNi coatings with ceria incorporation and confirmed the presence of single phase γ Ni5Zn21 with preference to the (330) plane as previously observed in other deposition systems. The intensity of the (600) reflection increases with the addition of ceria particles, which could be attributed to the preferential incorporation of ceria nanoparticles at the grain boundaries which affects the overall growth of the crystals . The incorporation of CeO2 nanoparticles is shown in Figure 5 for the SEM micrographs. Ceria nanoparticles were first added to the electrolytic bath without prior sonication (Figure 5a), and the nanoparticles agglomerated into long string-shape structures. Due to the agglomeration tendancies of the nanoparticles, sonication of the nanoparticles prior to depositon was examined. The coatings obtained from the sample post sonication (Figure 5b) take on a pyramidal growth pattern and appear more coarse. EDX was used to determine placement of the nanoparticles in the coating and the CeO2 particles appear to be primarily adsorbed onto the electrode surface. The agglomerated nanoparticles appear uniformly trapped inside the metal matrix. Ultrasonic agitation was done at 20°C with an amplitude value of 35 (power of 41 W/cm2, output frequency of 20 kHz) for 20 min prior to deposition. As the samples were sonicated prior to deposition, the agglomerated particles dispersed and were able to better fill the voids and pores naturally formed in the matrix, leading to better overall corrosion protection [43, 44]. Improvement of nanoparticle incorporation through the use of ultrasonication, previously noted for other systems including TiO2 and Al2O3 was also noted for CeO2 [32, 33, 36, 43, 44].
The OCP of ZnNi-CeO2 coatings was measured in 3.5% NaCl and near-neutral 0.05 M Na2SO4 solutions, respectively and monitored over time . Exbrayat et al. studied two differing ZnNi-CeO2 samples, ZnNi-CeO2 which was determined to contain 84% Zn, 14% Ni and 2% CeO2 and ZnNi-Ce2 (sonicated) which was found to contain 85% Zn, 12.8% Ni and 2.2% CeO2. When first submerged in the Na2SO4 solution, the OCP values changed drastically for ZnNi, ZnNi-CeO2 and ZnNi-CeO2 (sonicated). For ZnNi, a significant ennoblement was observed moving from an OCP value more cathodic than E = −0.95 V to E = −0.55 V after ~20 h of submersion. The OCP then begins to decrease steadily before stabilizing at ~E = −0.65 V after 96 h of immersion. ZnNi-CeO2 (sonicated) follows a similar pattern to ZnNi, with a shift in OCP from ~E = −0.85 V initially to ~−0.57 V after 30 h of submersion, while ZnNi-CeO2 which was not sonicated prior to deposition, stayed relatively stable throughout the 4 day submersion test, decreasing in OCP from ~E = −0.82 V to ~E = −0.75 V. Zinc coatings often settle over the initial submersion due to the formation of corrosion products, which then begin to protect the coating [43, 44]. EIS of ZnNi-CeO2 coatings were examined using a PGP 301 Autolab potentiostat after 24, 48, 96 h immersion in 35 g/L saline solution at 25°C with a frequency range of 64 kHz to 1 mHz, AC voltage amplitude of ±10 mV. Analysis was completed with Zview software. The Nyquist diagrams exhibit two capacitive loops at middle and low frequencies, with similar time constants. The loop diameter of the ZnNi coating remains relatively constant, suggesting stability in the corrosion rate. In the nanocomposite coatings, the loop diameter increased with immersion time. The incorporation of ceria enhances the corrosion resistance by ennoblement of the surface through reduction of galvanic corrosion of the steel .
4.5. ZnNi-carbon nanotubes
ZnNi-carbon nanotubes comprise ~8% of the literature to date. The dispersion, linear sweep voltammetry, surface morphology and friction properties of ZnNi coatings with nanotube incorporation was discussed. When carbon nanotubes were introduced into a ZnNi electrolytic solution, a positive shift (~0.1 V) in the polarization curves were observed and the deposition current of the system increased. The transport of the carbon nanotubes to the cathode surface and their incorporation into the coating is believed to be due to adsorption of Zn2+ and Ni2+ ions onto the nanotubes which are then reduced onto the coating, thereby entrapping the nanotubes in the coating. Initially the nanoparticles are weakly adsorbed onto the cathode, but once the particles lose their ionic and solvation shells, they become securely attached to the surface of the deposit. The adsorbed metal ions on the surface of the dispersed phase discharge at this point permanently attaching the nanotube to the coating [13, 45]. The actual deposition mechanism is not discussed in this work, so it is unclear if the nanotubes have an overall effect on the deposition mechanism or if anomalous deposition is still followed for this system.
In the case of carbon nanotubes, they are believed to act as nuclei for crystallization, further promoting even distribution of the nanotubes throughout the cathode surface. Microcracks are often observed in ZnNi coatings, but once carbon nanotubes have been added to the electrolytic mixture, the surface appears uniform and dense.
Another property examined for ZnNi-carbon nanotube nanocomposites was the sliding friction coefficient of the coatings as compared to ZnNi coatings. The ZnNi-carbon nanotube coatings were found to have a sliding friction coefficient 1.3–1.5 times smaller than ZnNi coatings without nanotube disbursement both in direct current and reverse current deposition modes. ZnNi coatings showed a decrease in friction coefficient values from 0.30 to 0.24 for current densities changing from 1.0 to 2.5 (A/dm2) while the corresponding ZnNi-carbon nanotube coatings decreased from 0.23 to 0.17 for the same current density values. Under a reverse current mode, ZnNi coatings had friction coefficients starting at 0.31 and decreasing to 0.23 as the ratio between cathodic and anodic periods was increased from 10:1 to 16:1 while for ZnNi-carbon nanotube coatings under the same conditions, the friction coefficients decreased from 0.24 to 0.15 [13, 45].
The effect of montmorillonite (Mt) addition to the ZnNi bath was examined through anodic linear sweep voltammetry (ALSV) as presented in Figure 6. Montmorillonite is a smectite mineral and has a 2:1 layered structure, with two layers of silicon tetrahedral sandwiching one layer of aluminum octahedral. The layers can be stacked together, but when the van der Waals forces holding the individual clay layers together are overwhelmed, the individual layers become exfoliated (also known as delaminated). For this work, mechanical agitation and/or sonication was used to exfoliate the layered silicate and produce individual nanoplatelets. Individual montmorillonite nanoplatelets exist as coordinated layers, measuring 1–2 nm thick. Mt is a hydrous aluminum silicate with approximate formula (Na,Ca)(Al, Mg)6(Si4O10)3-(OH)6.nH2O. The Al3+ and Si4+ locations can be replaced by lower valent cations, causing the montmorillonite structure to have an excess of electrons. The negative charge is compensated through loosely held cations from the associated water. Sodium montmorillonite, the clay mineral in which the loosely held cation is the Na+ ion, was the clay source used throughout the work. ALSV was used to obtain initial dissolution data of Zn2+ and Ni2+ ions in solution, as well as any electrochemical effect of the Mt nanoparticles on the metal dissolution peaks and the electrochemical behavior of Mt. The potential was scanned from OCP to E = −1.5 V (vs. SCE) at a sweep rate of 50 mV/s, held briefly and scanned back to OCP. During the anodic scan, the metals of interest were stripped back into the electrolytic solution. As previously observed for zinc-nickel systems under anomalous deposition control [57, 62, 69, 79], the anodic stripping peaks of the metals in solution are shifted based on other metal species in solution. According to the linear sweep voltammetry (LSV) data, zinc in the electrolytic solution had two anodic dissolution peaks present at potentials of E = −1.12 V and E = −1.08 V. During the cathodic scan, a small Zn(OH)2 layer deposits on the steel surface, slowing down dissolution kinetics. The dissolution of this species caused the second peak in the LSV [48, 57, 88]. Nickel had an anodic dissolution peak present at a potential of E = −0.48 V. When combined in solution, the zinc anodic dissolution peaks were shifted to potentials of E = −0.91 V, E = −0.83 V and the nickel anodic dissolution peak was shifted to a potential of E = −0.55 V. As previously stated, the zinc-nickel dissolution peaks of zinc and nickel are shifted in potential with respect to the individual metals in solution and this is indicative of an anomalous deposition system [51, 70, 76]. With the presence of Ni2+ in the system, Zn2+ is able to deposit at a more positive potential, and the nickel potential is shifted cathodically as previously observed in ZnNi systems [57, 62, 79, 82]. As ZnNi is known to undergo anomalous deposition, the dependence of the metal dissolution peaks relative to one another was expected [48, 50, 65, 68]. The added Mt appeared electrochemically inactive itself and has no overall effect on the anomalous deposition previously observed for ZnNi coating deposition .
Though the effect of pH was not discussed in most works, it was studied with the addition of Mt nanoparticles by monitoring the pH of the baths with and without Mt addition over a period of days to determine overall stability of the system. More acidic plating conditions lead to nonuniform coatings, specifically areas of low to no corrosion protection on the underlying substrate . In Figure 7, line A represents the system with zinc, nickel and ammonium hydroxide (starting pH = 9.40), line B represents the system with zinc, nickel, ammonium hydroxide and borate (starting pH = 9.40) and line C represents the line with zinc, nickel, ammonium hydroxide, borate and Mt (starting pH = 9.40). The horizontal dot-dash line represents pH 9.21, where the zinc equilibrium species exists (Zn2+ and HZnO2−) . Since this work is based at a pH range near this equilibrium, careful control of the pH is needed throughout all studies. The systems were closed to air for 7 days, then opened to atmosphere and monitored for an additional 32 h. Upon exposure to atmosphere, there was a definite decrease in pH as compared to closed systems for the previously stable baths. Line C (containing Mt in the system) decreased in pH at a slower rate than line B (not containing Mt) suggesting the Mt has an additional effect on the stabilization of metal species in solution. The system without borate or Mt addition passed through pH 9.21 (zinc equilibrium) even as a closed system (Line A). After 7 days the pH of the system with Zn, Ni and NH4OH had decreased from pH = 9.40 to 9.17, the system with Zn, Ni, borate and NH4OH had decreased from pH = 9.40 to 9.37 and the system with Zn, Ni, borate, NH4OH and Mt had decreased from pH = 9.40 to 9.38. Once opened to atmosphere, it took the borate system 4.5 h to reach a pH of 9.21 and a pH of 8.73 after 32 h, a total decrease of 0.64 pH units. The borate/Mt system reached pH 9.21 after 10 h of exposure to atmosphere and a pH of 9.05 after 32 h, a decrease of 0.33 pH units. The nonstabilized system (no borate or Mt) reached pH 7.94 after 32 h of exposure to atmosphere, a total decrease of 1.46 pH units. The decrease in pH in the closed system is due to formation of metal hydroxide species forming and precipitating out of solution. The large decrease in pH upon exposure to the atmosphere is due to absorption of carbon dioxide from the air [48, 57]. The system with borate demonstrates a clear stabilization of the system when in a closed system, and absorbs CO2 at a slower rate as compared to the system without borate. The addition of exfoliated Mt nanoparticles further stabilized the system, as shown in the relatively slow pH decrease in this system when closed to air and when opened to atmosphere. The nanoparticles stabilize the pH of the bath improving the deposition of the nanocomposite coating .
In the case of ZnNi-Mt nanocomposites, Figure 8, XRD pattern had a strong (330) reflection present at 2θ = 42.9°, indicative of ZnNi γ-phase alloy formation with a (330) preferred orientation as previously observed in coatings without nanoparticle incorporation [57, 62, 79]. The coating of pattern B was formed under the same conditions as pattern A, but Mt nanoparticles were dispersed into the electrolyte solution and incorporated into the resulting coating. Since Mt nanoparticles do not give diffraction peaks upon exfoliation, no additional peaks were observed due to its presence [2, 8, 52, 58, 66, 89]. The coating with Mt incorporation was thin as compared to the alloy coating without Mt but the γ-phase alloy was still formed even in the presence of Mt, confirming the Mt did not affect the deposition of the alloy phase of interest. The incorporation of Mt into the coating is shown in Figure 9. Films with Mt incorporation have strong adherence, small grain size and overall good coverage of the stainless steel substrate. Small spherical particles covered the surface and no voids appeared present in the coating. The structure was not affected by the incorporation of Mt under these conditions. A strong overall coverage of the substrate material was observed, and particles of exfoliated clay were observed, confirming clay presence within the coating.
The potentiodynamic polarization curves of the electrodeposited zinc-nickel and zinc-nickel-Mt nanocomposite alloys in 3.5% NaCl solution are illustrated in Figure 10 and the corrosion current and potential are given in Table 4.
In previous studies ZnNi coatings with optimal corrosion resistance was found to have a corrosion potential (Ecorr) more anodic as compared to pure zinc but more cathodic as compared to pure nickel. The optimal coatings had a corrosion potential around E = −0.74 V in that study . In this study, the ZnNi-Mt nanocomposite coating had a corrosion potential of E = −0.73 V, which is in agreement with previous findings (Table 4). This value is slightly more cathodic (10 mV) than the coatings without Mt incorporation. The high zinc content (~90%) of the coating (confirmed with AAS and ICP-MS) but more cathodic corrosion potential are in the optimal range for improved protection. Corrosion current density is the primary parameter used for evaluating the kinetics of the corrosion reaction. The lower corrosion current density, the better corrosion protection. The corrosion current density for the zinc-nickel γ-phase alloy was 1.06 × 10−5 A/cm2 and the corrosion current density for the ZnNi-Mt γ-phase nanocomposite alloy was 3.72 × 10−6 A/cm2. The corrosion current density of the nanocomposite alloy was lower as compared to the alloy without Mt denoting an improved corrosion resistance. Rp of the coating with Mt was 34,900 Ω cm2 as compared to 30,485 Ω cm2 for the coating without Mt further confirming the results of improved protection with incorporation of Mt .
ZnNi nanocomposites can be formed by incorporating nanoparticles into the coating during an electrochemical deposition. The nanoparticles under study do not appear to affect the electrochemical behavior or electrochemical deposition mechanism of zinc-nickel γ-phase alloy formation. Anomalous deposition of the zinc-nickel alloy was observed which is consistent with formation of the γ-phase alloy, but small anodic shifts were observed in the ALSV scans of the metal species in the ZnNi-Mt bath as compared to baths without Mt nanoparticles present. Al2O3 also noted no overall effect on the electrochemical behavior of the system. The addition of nanoparticles, including Mt, SiC and Al2O3 also affected the onset of hydrogen evolution, pushing the onset to more cathodic potentials, which can be an advantage in an aqueous plating system as it broadens the working window for the deposition. Zinc-nickel γ-phase deposition requires a high overpotential to overcome the kinetic limitations of nickel deposition, this added benefit of shifting the reduction of the metals anodically with the onset of hydrogen appearing more cathodically, leads to alloy formation with less entrapped hydrogen.
Particle dispersion in the electrolytic bath is an important factor when considering deposition. Optimal corrosion protection is acquired from systems with better dispersion of nanoparticles in the system. When nano-Al2O3 particles were dispersed uniformly throughout the coating, and incorporated in the matrix, they were able to protect the coating from corrosive medium, increasing the corrosion potential and retarding corrosion onset. Agglomerated nano-Al2O3particles on the other hand, would combine loosely with the metal matrix, or fall off the coating entirely, leading to gaps and pores in the coating which were easily attacked by corrosion cells. All studies treated the nanoparticles prior to deposition to cause disruption and dispersion of the particles in the plating bath through stirring or sonication, but no studies have been found discussing the overall effect on the particles and any benefits or drawbacks of one method compared to another. It was noted that sonication during deposition leads to better incorporation and higher concentrations of nanoparticles in the final coatings. In addition to sonication, pulse current and pulse potential deposition tends to lead to better incorporation of the nanoparticles in the resulting coating. Up to 11% nanoparticle incorporation was noted throughout the studies, with confirmation of ZnNi-TiO2, Al2O3, SiO2, SiC, CeO2, Al2O3/SiC, CeO2/SiO2, Mt and carbon nanotubes.
XRD results confirm formation of γ-phase ZnNi-nanocomposite coatings throughout the studies. Interestingly, in acidic conditions without nanoparticle incorporation, acidic electrolytic baths tended to give impure ZnNi coatings, with a mixture of γ and δ-phase ZnNi coatings. The studies with ZnNi nanoparticles show almost exclusive γ-phase ZnNi alloys, which previously was primarily observed under alkaline conditions. The crystallite size decreased with the increase of nanoparticle incorporation in the coatings. Mt nanoparticles have been successfully incorporated into the alloy coatings with no disruption in the crystal structure of the zinc-nickel γ-phase alloys, deposited with a preferred (330) orientation. The incorporation of TiO2, Al2O3 and CeO2 was also confirmed with XRD. The morphology of the coatings shows incorporation of the nanoparticles, with small, compact like structures and few cracks or holes. The hardness of the coatings increased as nanoparticle concentration in the coating was increased.
Corrosion studies all show nanoparticle incorporation into ZnNi coatings leads to lower corrosion currents, suggesting a lower corrosion rate for the coatings. The nanoparticles are believed to fill the crevices, gaps and holes within and on the surface of the coatings, leading to improved corrosion resistance. Overall the corrosion protection offered by the ZnNi-nanocomposite coatings was improved as compared to pure ZnNi coatings.