Applied Surface Science 324 (2015) 482–489
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Effects of Co contents on the microstructures and properties of electrodeposited NiCo–Al composite coatings Fei Cai a , Chuanhai Jiang a,∗ , Peng Fu a , Vincent Ji b a b
School of Materials Science and Engineering, Shanghai Jiao Tong University, 800 Dongchuan Road, Shanghai 200240, PR China ICMMO/LEMHE, UMR 8182, Université Paris-Sud 11, Orsay Cedex, 91405 France
a r t i c l e
i n f o
Article history: Received 8 August 2014 Received in revised form 7 October 2014 Accepted 27 October 2014 Available online 4 November 2014 Keywords: NiCo–Al composite coating Electrodeposition Texture Residual stress Corrosion resistance
a b s t r a c t In this work, the NiCo–Al composite coatings with different Co contents were prepared by electrodeposition from the modified Watt baths containing different Co2+ concentrations. The effects of Co contents on the composition, crystal structure, texture, grain size, microstrain, surface morphology, microhardness, residual stress and corrosion resistance of the NiCo–Al composite coatings were investigated in detail. The composite coatings exhibited the solid solution NiCo crystal structure with the Co contents in the range of 18.3 wt% to 43 wt%. Further increasing the Co content to 60.5 wt% resulted in the formation of hexagonal (hcp) Co. As the Co contents increased, the NiCo–Al composite coatings exhibited the texture evolution from the (2 0 0) preferred orientations to the random orientations or slight (1 1 1) preferred orientations. The grain size decreased, while the microstrain and residual stress increased with increasing Co contents. The hardness of the composite coating attained the maximum at the Co content of 43 wt%. However, the increasing Co contents decreased the corrosion resistance of the NiCo–Al composite coatings. © 2014 Published by Elsevier B.V.
1. Introduction In recent years, there have been increasing interests focused on the electro-deposited NiCo and NiCo based metal matrix composite (MMCs) coatings due to their superior properties, such as higher hardness [1–5], improved anti-wear [2,4], better corrosion resistance [1,3,6–9] and oxidation resistance [5,10] as compared with the pure Ni and Ni based composite coatings. The properties of the NiCo or NiCo based composite coatings were mainly dependent on the incorporated particles and the microstructures of the NiCo matrix. The incorporated particles, such as carbon nanotubes (CNTs) [2], SiC [4,6,7], TiO2 [3], Al2 O3 [11–13], could significantly enhance the properties of the particles reinforced NiCo composite coatings including hardness and corrosion resistance as compared with the NiCo matrix. Shi et al. [4] fabricated the SiC nano-particles reinforced NiCo–SiC composite coatings and found that the SiC nano-particles increased the microharndess, wear resistance and corrosion resistance of the composite coatings. Similar particles enhancement effects were also found for the carbon nanotubes (CNTs) [2] and TiO2 [3]. Tian et al. [11] found that incorporation of Al2 O3 particles in the composite coating enhanced the
∗ Corresponding author. Tel.:+86 021 34203096; fax: +86 021 34203096. E-mail address:
[email protected] (C. Jiang). http://dx.doi.org/10.1016/j.apsusc.2014.10.159 0169-4332/© 2014 Published by Elsevier B.V.
co-deposition of Ni and Co and the increasing Al2 O3 particle contents in the composite coating increased the microhardness and wear resistance of the coating in the oil and sand slurry. The size of the incorporated particle also had an effect on the hardness and corrosion properties of the composite coatings. Bakhit et al. [6] found that the SiC nano-particle reinforced NiCo–SiC composite coating showed the higher hardness and better corrosion resistance than the SiC micro-particle reinforced composite coating. The properties of pure Ni coatings could also be enhanced by alloying with Co element [1,8,9]. Yang et al. [9] found that addition of about 20 wt%. Co in the pure Ni coating decreased the grain size of the NiCo alloy coating and made the surface smoother, which might be beneficial to the improvement of the corrosion resistance of the alloy coating. Srivastava et al. [1] found that the microstrucutures and properties of the NiCo alloy were closely dependent on the Co content and the coating attained the maximum hardness at Co content of 50 wt%. Moreover, the NiCo alloy exhibited better corrosion resistance at Co content of 20 wt% in comparison to the pure Ni coating and other NiCo alloy coatings. Kang et al. [8] pointed out that the presence of Co element increased the thickness of the space charge layer, which could enhance greatly the stability of the passive film for the NiCo alloy coating than that of the pure Ni coating. Similar to the pure Ni coatings, the properties of Ni composite coatings could also be enhanced by alloying with Co [5,10,14]. Srivastava et al. [5] found that addition of 25 wt% cobalt in the Ni–CeO2 composite coating increased the hardness and improved its wear resistance.
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And their result also showed that the presence of 85 wt% cobalt in the Ni–CeO2 led to the formation of hcp structure, resulting in the composite coating better thermal stability. Also in Srivastava’s study [10], the NiCo–Al composite coatings were prepared and the coating with 30 wt% Co content exhibited the higher temperature oxidation and corrosion resistance. In reference [14], addition of 28 wt% Co content in the Ni–SiC matrix showed the higher microhardness than other coatings. Although considerably researches focused on the fabrication and characterization of the incorporated particle reinforced composite coatings, the effects of Co on the microstructures and properties, especially the texture, residual stress and corrosion resistance of the composite coatings need further investigation. In our early works, the fabrication and characterization of Ni-Al were reported [15,16]. In this study, the NiCo–Al composite coatings with different Co contents were prepared. Then, the effects of the Co contents on the microstructure, texture, grain size, micro-strain, surface morphology, hardness, residual stress and corrosion resistance of the NiCo–Al composite coatings were investigated in detail. 2. Experimental details 2.1. Coating processes NiCo–Al composite coatings prepared by the electrodeposition method from the modified Watt baths containing NiSO4 ·6H2 O (240 g/L), NiCl2 ·6H2 O (40 g/L), H3 BO3 (30 g/L) and C12 H25 NaSO4 (0.2 g/L). Co was added as the CoSO4 ·7H2 O and the additions ranged from 5 to 40 g/L of Co concentrations. The Al particles with a mean diameter of 1 m were added into the solutions at concentration of 50 g/L. In order to ensure a good dispersion of the Al particles, the electrolytes were ultrasonicated for 30 min followed by magnetically stirring for about 4 hr at a stirring rate of 300 rpm. The solution temperature and pH value were maintained at 50 ◦ C and 4.2, respectively. Pure nickel plate and stainless steel plate with an area of 1 × 1 cm2 were used as the anode and cathode, respectively. The stainless steel specimens were grounded with grade 600, 800 and 1200 emery papers step by step following a degreasing in acid (10% HCl) and a washing with distilled water before deposition. The applied current density was maintained at a constant of 4 A/dm2 for 1 h, which resulting in the composite coating with a thickness of about 30 m. 2.2. Coatings characterization Surface morphology of the coatings was observed with a field emission scanning electron microscope (FESEM, JSM-7600F), and the chemical composition was examined by using the Energy Dispersive X-ray Spectroscopy (EDX) method attached to the FESEM. Structure analysis of the coatings was performed by a Rigaku Ultima ˚ in IV X-ray diffractometer (XRD, Cu K␣ radiation, = 1.54056 A) standard 2 − mode and the voltage and current were 40 kV and 30 mA, respectively. The angular positions of the peaks were converted to the corresponding lattice plane spacing dhkl , which was then the lattice constant ahkl using the formula converted to ahkl = d
h2 + k2 + l2
[17]. The pole figures measurement of
the composite coating were performed on a Rigaku SmartLab Xray diffractometer with a four-circle goniometer and using Cu Ka radiation and the voltage and current were 40 kV and 30 mA, respectively. The Voigt method [18] was used to calculate the grain size and microstrain of the coatings using the integral breadth of (2 0 0) peak. Residual stress data on the as-deposited Ni–Zr composite coatings were collected on a Proto LXRD Residual Stress Analyser and analyzed using the classical sin2 method [19]. The voltage and current were 30 kV and 25 mA, respectively. The peaks
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Fig. 1. Element content in the deposits as a function of CoSO4 ·7H2 O concentrations in the bath.
of (3 3 1)␣ of Ni were selected to calculate the residual stress. Microhardness tests were carried out using a Vickers microhardness tester with a load of 200 g and an indentation time of 15 s. The corresponding final values were determined as the average of 8 measurements. The potentiodynamic polarization measurements were carried out using a standard three-electrode method. A platinum sheet and a Ag/AgCl electrode [+207 mV(SHE)] were used as the auxiliary electrode (AE) and the reference electrode, respectively. The sample was used as working electrode (WE). The measurements were conducted in 3.5 wt% NaCl solution at the temperature of 25 ◦ C using an electrochemical apparatus (CHI660E) at a scan rate of 1 mV/s. Before each experiment, the sample was immersed for about 40 min and then the polarization potentiodynamic curves were recorded. The corrosion parameters such as corrosion potential (Ecorr ) and corrosion current density (Icorr ) were determined by using the Tafel curves [20]. The electrochemical impedance (EIS) measurements were conducted at the open circuit potential after immersion of a sample into solution for 30 min. The applied sinuous potential amplitude was 5 mV and the frequency ranged from 0.01 Hz to 100,000 Hz. 3. Results and discussion 3.1. Chemical component content analysis Fig. 1 shows the element contents of the NiCo–Al composite coatings as a function of the CoSO4 ·7H2 O concentrations in the bath. It was obvious that the Co contents linearly increased while Ni contents linearly decreased with the increasing CoSO4 ·7H2 O concentrations in the bath. According to the EDX results, the average Co contents of the composite coatings were 18.3 wt%, 29.9 wt%, 43 wt% and 60.5 wt% for the coatings deposited at CoSO4 ·7H2 O concentrations of 5 g/L, 10 g/L, 20 g/L and 40 g/L, respectively. Therefore, the composite coatings could be referred to as Ni–18.3Co/Al, Ni–29.9Co/Al, Ni–43Co/Al and Ni–60.5Co/Al. Fig. 2 shows the effect of Co/Ni ratio in baths on the Co/Ni ratio in the deposits. It could be found from Fig. 2 that the Co contents in the composite coatings were significantly higher than those in the baths exhibiting an anomalous co-deposition for the Ni–Co/Al composite coatings, which were also reported in references [7,21,22]. Several mechanisms have been proposed to explain this phenomenon, containing (1) a local increase in the pH [23], (2) two-step co-deposition of the cobalt cations and decreasing the nickel deposition in the second step [24], (3) the preferential deposition of the cobalt cations [25]. Previous researches have pointed out that the co-deposition of the incorporated particles, such as SiC and Si3 N4 , could be enhanced
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Fig. 2. Effect of electrolyte Co/Ni ratio on deposit Co/Ni ratio.
3.2. Coating microstructures Fig. 3 shows the XRD patterns of the NiCo–Al composite coatings with different Co contents. As shown in Fig. 3, the NiCo matrix peaks corresponding to the (1 1 1), (2 0 0), (2 2 0), (3 1 1) and (2 2 2) planes were detected. As the Co contents increased, the intensities of the NiCo matrix peaks decreased and the diffraction peaks broadened. The hcp Co was also detected with increasing the Co content to 60.5 wt%. Fig. 4 shows the lattice parameter a(2 0 0) calculated from the (2 0 0) planes and it clearly showed that the a(2 0 0) increased with increasing Co contents, which was due to that the smaller Ni atoms were partly substituted by the larger Co atoms to form the solution NiCo phases. It could also be found from the XRD patterns that the intensity of the NiCo (2 0 0) peak of the Ni–18.3Co/Al coating was higher
Fig. 3. XRD patterns of NiCo–Al composite coatings with different Co contents.
0.3540
Lattice parameter a, nm
by increasing the Co contents in the coatings [26,27], which was ascribed to that the adsorption of Co2+ cations on the particles surface is easier than the Ni2+ cations. However, reference [28] reported the opposite result that the Co contents increased accompanied by the decrease of SiC particle contents with reducing the current densities. Also in reference [29], the ZrC particle contents in the NiCo–Zr composite coatings decreased with increasing Co concentrations in the electrolyte. In this study, the Al particle contents in the composite coatings decreased with increasing Co contents as shown in Fig. 1, which might be due to the lower adsorption capacity of Al micro-particles with the Co2+ cations.
0.3535 0.3530 0.3525 0.3520 0.3515
Ni-4 Ni-6 Ni-2 Ni-0 N 3 0.5C 9 Co/A i-18.3C o/Al o/Al .9Co/Al Co/Al l
Fig. 4. Effect of Co content on the lattice parameter of the NiCo–Al composite coatings.
than those of other peaks, exhibiting the (2 0 0) preferred orientation. However, the intensity of the (2 0 0) peak decreased while the intensities of the (1 1 1) and (2 2 0) peaks increased with increasing Co contents, indicating the random orientation or slight (1 1 1) preferred orientation. The pole figure measurements were conducted to further investigate the texture of the NiCo/Al composite coatings and the measured pole figures of Ni (1 1 1), (2 0 0) and (2 2 0) planes for the Ni–18.3Co/Al and Ni–60.5Co/Al coatings are
Fig. 5. Pole figures of the NiCo–Al composite coatings with different Co contents (a) Ni–18.3Co/Al and (b) Ni–60.5Co/Al.
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to the distortion of the NiCo–Al composite coating. Therefore, the microstrain of the composite coatings increased with increasing Co contents. 3.3. Surface morphology
Fig. 6. Grain size and microstrain of the NiCo–Al composite coatings.
presented in Fig. 5(a) and (b), respectively. It could be found from Fig. 5(a) that the Ni–18.3Co/Al composite coating exhibited typical (2 0 0) fiber texture. However, the (2 0 0) texture disappeared and the random orientation appeared for the Ni–60.5Co/Al coating as shown in Fig. 5(b). The results of XRD and pole figure showed that the increasing Co contents could change the texture from the (2 0 0) preferred orientation to the random orientation or slight (1 1 1) preferred orientation of the NiCo–Al composite coatings. Fig. 6 demonstrates the calculated grain size and microstrain of the NiCo–Al composite coatings with different Co contents. As seen in Fig. 6, the grain size of 112 nm was obtained for the Ni-Al composite coating. Increasing the Co contents led to the decrease in grain size of the NiCo–Al composite coatings and the grain size obtained the minimum of 22 nm at Co content of 60.5 wt%. As mentioned above, the Al particle contents in the composite coatings decreased with increasing Co contents. Thus, the decrease in grain size could be ascribed to the addition of Co [30,6]. During the electrodeposition, Co2+ and Ni2+ in the bath could react with OH− to form the metal mono-hydroxides Co(OH)+ and Ni(OH)+ , which were usually adsorbed on the surface of electrodeposits [31]. The greater adsorption of Co(OH)+ than Ni(OH)+ could block the growth centers, which were beneficial to the formation of the smaller grain size [32]. Thus, the grain size of the NiCo–Al composite coating decreased with increasing Co contents. From Fig. 6, it could also be found that the microstrain of the composite coating increased with increasing Co contents and obtained the maximum at Co content of 60.5 wt%. This was because that the Ni atoms could be partly substituted by Co atoms to form the solid solution NiCo phases, which led
Surface morphology images of the NiCo–Al composite coatings with different Co contents are displayed in Fig. 7. Compared with the Ni–Al composite coating (presented in Fig. 1b from our previous paper [15]), the surface of the NiCo–Al composite coating became more compact and smoother with the addition of Co. As the Co content increased to 43 wt%, the smaller globular structure appeared. With further increasing the Co content to 60.5 wt%, some sharp corner structures appeared. The surface morphology observation showed that addition of Co could make the surface of the NiCo–Al composite coating more compact and smoother as compared to the Ni–Al composite coating. Fig. 8 shows the surface morphologies and Al dot-map patterns of the NiCo–Al composite coatings with different Co contents. The Al dot-map patterns confirmed that the Al particles showed fairly uniform distribution over the surface of the composite coating. In addition, it could be found from the Al dot-map patterns and the EDX patterns that the Al contents in the coatings decreased with increasing Co contents. In order to study the chemical composition and Al particle distribution uniformity across the composite coating, the elemental line scans of Ni, Co, Al and Fe (from the substrate) on the cross sectional FESEM image of the NiCo–Al composite coating with Co content of 60.5 wt% is shown in Fig. 9. It could be found that there was a fairly uniform distribution of the elements, which confirmed the uniform distribution of Al micro-particles through the thickness of the NiCo–Al composite coating. 3.4. Residual stress analysis Fig. 10 presents the residual stresses of NiCo–Al composite coatings with different Co contents. It could be found that all the coatings exhibited the tensile residual stress. As seen in Fig. 10, a smaller residual stress of 26 MPa for Ni-Al composite coating was obtained and the addition of Co significantly increased the residual stresses of the NiCo–Al composite coatings. As the Co contents increased from 18.3 wt% to 60.5 wt%, the residual stresses increased from 188 MPa to 782 MPa. The increase in residual stresses was closely related to the formation of sold solution NiCo and texture
Fig. 7. Surface morphology images of NiCo–Al composite coatings with different Co contents of (a and e) 18.3 wt%, (b and f) 29.9 wt% (c and g) 43 wt% and (d and h) 60.5 wt% at two different magnifications: 1000× and 3000×.
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Fig. 8. The Al particle dot-map and spectra patterns of the NiCo–Al composite coatings with different Co contents of (a, e and i) 18.3 wt%, (b, f and j) 29.9 wt%, (c, g and k) 43 wt% and (d, h and i) 60.5 wt%.
Fig. 9. Cross sectional image (a) and line scan element curves (b) of NiCo–Al composite coating with Co content of 60.5 wt%.
evolution from the (2 0 0) preferred orientation to random orientation or slight (1 1 1) orientation. As mention above, the substitution of Ni atoms by Co atoms resulted in the distortion of the NiCo–Al composite coatings, which could increase the residual stress of the coatings. Therefore, the residual stresses increased with increasing microstrain by increasing the Co contents in the composite coatings. Moreover, the Young’s moduli in different directions are different and the average Young’s moduli are 303 GPa and 137 GPa for (1 1 1) and (2 0 0), respectively [33]. The higher strain energy density in (1 1 1) textured coatings likely led to the higher tensile internal stress in the (1 1 1) textured coatings compared with that in (2 0 0) textured coatings, as also reported in references [34–36]. 3.5. Hardness measurements
Fig. 10. Residual stress of composite coatings with different Co contents.
The measured microhardness for NiCo–Al composite coatings with different Co contents is shown in Fig. 11. It was obvious that
F. Cai et al. / Applied Surface Science 324 (2015) 482–489
Fig. 11. Hardness of composite coatings with different Co contents.
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Fig. 12. Potentiodynamic polarization curves for NiCo–Al composite coatings in 3.5% NaCl solution.
Table 1 Corrosion potential Ecorr and corrosion current Icorr of coatings with various Co contents. Composite coating with different Co contents
Ecorr (mV)
Icorr (A/cm2 )
Ni–18.3 Co/Al Ni–29.9 Co/Al Ni–43 Co/Al Ni–60.5 Co/Al
−145 −144.7 −163.2 −224.9
0.537 0.604 0.635 1.197
the hardness of the composite coatings increased with increasing Co contents and obtained the maximum of 444 Hv at Co content of 43 wt%. As mentioned above, the incorporated Al particle contents decreased with increasing the Co contents. Thus, the hardness enhancement of the composite coatings was related to the NiCo matrix material. First, the grain size of the composite coatings decreased with increasing Co contents (as shown in Fig. 6), resulting in an increase in hardness, which was consistent with the well-known Hall–Petch law [37]. Second, the solid solution strengthening caused by substitution of Ni atoms by Co atoms also contributed to the increase of hardness [6], and the more Co contents the composite coatings contained, the higher the hardness were. However, the hardness value of the coatings showed a sharply decrease with further increasing the Co content to 60.5 wt%, which might be due to the formation of hcp Co [26]. 3.6. Potentiodynamic polarization The potentiodynamic polarization curves of the NiCo–Al composite coatings are presented in Fig. 12 and the corrosion parameters, such as corrosion potential (Ecorr ) and corrosion current (Icorr ) obtained by using the polarization curves, are summarized in Table 1. It could be found from Table 1 that the corrosion current (Icorr ) of the NiCo–Al composite coatings increased with
increasing Co contents. Comparing with the smaller corrosion current (Icorr ) of 0.501 A/cm2 for Ni–Al coating (seen in our previous paper [15]), it could be speculated that addition of Co deteriorated the corrosion resistance of the NiCo–Al composite coatings and the corrosion resistance decreased with increasing Co contents. The decrease of the corrosion resistance of the NiCo–Al composite coatings could be due to several factors. First, alloying with Co decreased the corrosion resistance of the Ni by changing the nobility of materials [14,38] because Co is more active than Ni and the electrochemical activity of the NiCo alloy is expected to be greater than that of pure Ni [14,39]. Second, the formation of hcp Co also deteriorated the corrosion resistance of the coating. In general, two-phase structures are less corrosion resistant than single-phase structures because galvanic cells can be easily formed between phases with different nobilities [6]. In addition, the decrease in grain size also decreased the corrosion resistance of the coatings. It is acknowledged that the grain boundaries are prone to corrosion attacks especial for the multi-phase structure materials [40]. For the NiCo–Al composite coatings, the grain size decreased with increasing Co contents, resulting in the decrease of the corrosion resistance. Fig. 13 shows the corroded surface morphology images of Ni–18.3Co/Al and Ni–43Co/Al composite coatings. It could be observed from Fig. 13(a) and (b) that the corrosion pit in circular shape appeared, showing typical localized corrosion. Comparing with Ni–18.3Co/Al composite coating, the amounts of corrosion pits increased for Ni–43Co/Al composite coating, confirming that increasing Co contents decreased the corrosion resistance of the NiCo–Al composite coatings.
Fig. 13. Typical corroded surface morphology images of (a) Ni–18.3Co/Al and (b) Ni–43Co/Al composite coatings.
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hcp Co was detected with increasing the Co content to 60.5 wt%. Increasing Co contents in the NiCo–Al composite coatings resulted in the texture evolution from the (2 0 0) preferred orientation to the random orientations or the slight (1 1 1) preferred orientation. The grain size decreased and microstrain increased with increasing Co contents. The residual stress also increased with increasing Co contents. The hardness of the composite coatings increased with increasing Co contents and attained the maximum at Co content of 43 wt%, which was ascribed to the solid solution strengthening and grain refining strengthening. However, the addition of Co deteriorated the corrosion resistance of the composite coatings. References Fig. 14. Nyquist impedance diagrams for NiCo–Al composite coatings in 3.5% NaCl solution.
Fig. 15. Equivalent circuits used for fitting of impedance plots for NiCo–Al composite coatings in 3.5% NaCl solution. Table 2 Equivalent circuit parameters obtained by fitting the experimental EIS of NiCo–Al composite coatings. Composite coating with different Co contents
Rs ( cm−2 )
Rct (k cm−2 )
CPE (F cm−2 )
n
Ni–18.3 Co/Al Ni–29.9 Co/Al Ni–43 Co/Al Ni–60.5 Co/Al
7.2 6.2 8.1 8.7
162.24 130.43 108.11 36.1
46.1 45.6 35.1 60.8
0.89 0.88 0.92 0.9
3.7. Electrochemical impedance spectroscopy (EIS) Fig. 14 shows the Nyquist impedance plots of the NiCo–Al composite coatings in 3.5 wt% NaCl soulution. It could be found from Fig. 14 that all the coatings exhibited a single semicircle and the diameters of the semicircle decreased with increasing Co contents, indicating the highest corrosion resistance of the Ni–18.3Co/Al composite coating. In order to account for the corrosion resistance of the composite coating, the equivalent circuit model (Fig. 15) was applied to fit the impedance spectra and a good fit with this model was obtained. The Rs represents the solution resistance. The constant phase element, C.P.E, is introduced in the circuit instead of a pure double layer capacitor to give a more accurate fit. The impedance of a CPE can be expressed by ZCPE = [A(jω)n ]−1 , where ω is frequency; A is the CPE magnitude and the exponent n is between 0 and 1. The Rp represents the charge transfer resistance whose value can be used to measure the corrosion resistance. Table 2 lists results extracted from the EIS by simulation. It could be found that the Rp value decreased with increasing Co contents, indicating that corrosion resistance of the NiCo/Al composite coatings decreased with increasing Co contents. 4. Conclusions NiCo–Al composite coatings with different Co contents were prepared by the conventional direct current electrodeposition from the modified Watt baths containing different Co2+ concentrations. The composite coatings showed an anomalous co-deposition and the Co contents in the deposits increased from 18.3 wt% to 60.5 wt% with increasing the Co2+ concentration from 5 g/L to 40 g/L. As the Co contents increased from 18.3 wt% to 43 wt%, the composite coatings exhibited solid solution NiCo crystal structure. However, the
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