1 Introduction

Electrodeposited nickel composite coatings containing ceramic particles like Al2O3, TiO2, ZrO2, SiC, Si3N4, etc. are the most widely studied and reported in the literature. Alumina (Al2O3) is the most widely used ceramic because of its superior properties like good chemical stability, high microhardness, and wear resistance at high temperature and lower cost. There have been a large number of publications on electrodeposition of Ni–Al2O3 composite coatings by varying the deposition parameters [1–12]. The corrosion resistance of composite nickel electrodeposits with alumina embedded in the nickel matrix prepared in sulfate and chloride bath has been reported [13]. Improved corrosion resistance of electrochemically deposited Ni–Al2O3 from a Watts bath has also been reported [14]. Electrochemical impedance spectroscopy (EIS) and corrosion behavior of Ni–Al2O3 nanocomposite coatings have been reported by Ciubotariu et al. [15]. Szczygiel and Kolodziej [16] have studied the corrosion resistance of Ni–Al2O3 coatings in NaCl solution. Improved microhardness and wear resistance for Ni–Al2O3 and Ni–SiC composite coatings have been reported by Wang and Wei [17]. Feng et al. [18] have observed an improved wear resistance for nanostructured Ni/Al2O3 composite coatings deposited under the influence of an external high magnetic field (8T). Most of the reported literature have discussed the synthesis and properties of Ni–Al2O3 composite coatings containing mostly α-alumina particles. In a recent article, we have reported the preparation of Ni–Al2O3 composite coatings containing different crystallographic forms of alumina [19]. To the best of our knowledge, there are no reports on the comparative study on the corrosion- and wear-resistant properties of Ni–Al2O3 composite coatings containing different phases of alumina. It is interesting to investigate the effect of different phases of alumina on the properties of Ni–alumina composite coatings. The aim of this study was to investigate the corrosion- and wear-resistant properties of electrodeposited Ni–Al2O3 composite coatings containing different forms of alumina.

2 Experimental

The Al2O3 particles prepared from (i) solution combustion process containing pure alpha alumina (crystallite size 40 nm), (ii) precipitation method containing pure gamma alumina (crystallite size 5 nm) and (iii) commercial alumina (Alcoa) powder with a crystallite size of 40 nm containing mixture of alpha, gamma, and delta crystalline phases were dispersed in a nickel sulfamate bath and stirred well overnight. Details regarding the preparation and characterization of alumina powders and Ni–alumina composite coatings have already been published [19]. The Ni–alumina composite coatings (i) Ni–Al2O3-1 (containing combustion synthesized alumina), (ii) Ni–Al2O3-2 (containing precipitation synthesized alumina), and (iii) Ni–Al2O3-3 (containing commercial alumina) were electrodeposited at 15.5 mA cm−2 for 36 min at 600 rpm (magnetic stirring). The coating thickness of the samples for corrosion studies was maintained at 11 ± 1 μm on mild steel substrate. To study the effect of current density on particle incorporation, the plating was carried out on brass coupons, and the thickness was not controlled precisely.

The microhardness and particle distribution of the composite coatings electrodeposited at 15.5 mA cm−2 at 600 rpm were measured at least on five different areas on the cross section of polished samples using a microhardness tester (Micromet 2103, Buehler, 50 gf load) and vertical metallurgical optical microscope.

Electrochemical studies on Ni–alumina-coated, circular-shaped mild steel samples were conducted using Autolab PGStat-30 (Potentiostat/galvanostat) system. The test was conducted in free air 3.5% NaCl solution (0.6 N). Platinum foil of 1 cm2 area was used as the counter electrode, and Ag/AgCl (3 M KCl) was used as the reference electrode. Details of corrosion testing had already been reported [20].

The tribological performance of Ni–alumina composite coatings was investigated by conducting wear tests on a pin-on-disk tribometer (DUCOM, India) under ambient conditions of temperature and humidity (30 °C, 50% RH) at an applied load of 9.8 N. Wear experiments were undertaken for a minimum of three specimens on semicircular brass pins of radius 6 mm coated with Ni–Al2O3. Brass pins were coated with Ni–Al2O3 coatings by electrodeposition at 15.5 mA cm−2 for 3 h (~40 μm thickness). All the wear tests were conducted at a wear track radius of 30 mm and 200 rpm (slide speed of 0.628 m s−1) to get a constant sliding distance of 4525 m. Hardened EN 31 steel disk with a Vickers hardness of 750 HV was used as the counter part. More details regarding the wear studies have already been reported [21]. The wear coefficient was calculated using the Holm–Archard relationship [22, 23]. The Raman spectra of the wear tracks on the disks and pins were recorded using a DILOR-JOBIN-YVON-SPEX (Paris, France) integrated Raman Spectrometer (Model Labram).

3 Results and discussion

The microhardness of Ni–Al2O3 composite coatings electrodeposited at 15.5 mA cm−2 at 600 rpm was found to be 483 ± 19, 390 ± 9, and 350 ± 12 KHN, respectively for Ni–Al2O3-1, Ni–Al2O3-2, and Ni–Al2O3-3. The higher microhardness of Ni–Al2O3-1 may be attributed to the relatively higher microhardness of α-alumina compared to γ-alumina particles. The grain size of nickel was 25, 16, and 11 nm respectively for Ni–Al2O3-1, Ni–Al2O3-2, and Ni–Al2O3-3. The cross-sectional optical microscope images of the composites are shown in Fig. 1. The alumina particles in Ni–Al2O3-1 are in the form of various irregular-shaped particles, and in the case of Ni–Al2O3-2, more number of spherical particles are observed with a very few irregular-shaped particles. Ni–Al2O3-3 exhibits more number of particles with less agglomeration. From the optical microscope images, it is clearly seen that the area fraction of particles in the Ni–Al2O3-3 coating was higher. The area fractions of particles in the nickel matrix were 7.5, 13, and 17% respectively for Ni–Al2O3-1, Ni–Al2O3-2, and Ni–Al2O3-3.

Fig. 1
figure 1

Cross-sectional optical microscope images of electrodeposited a Ni–Al2O3-1, b Ni–Al2O3-2, and c Ni–Al2O3-3 coatings

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3.1 Corrosion resistance

The potentiodynamic polarization curves obtained for Ni and Ni–Al2O3 composite coatings are shown in Fig. 2. The corrosion potential, the corrosion current, and the polarization resistance obtained from the Tafel plots are listed in Table 1. When compared to the uncoated substrate, the corrosion potential of the coated samples shifted toward more positive side, indicating improved corrosion resistance of the coatings. Plain Ni showed corrosion potentials of −0.451 V and the corrosion current density of 0.6267 μA cm−2. In the case of Ni–Al2O3, corrosion potentials ranging from −0.267 to −0.184 V. The corrosion current density is an important parameter used for evaluating the kinetics of the corrosion reaction. Corrosion protection is inversely proportional to the corrosion current density. The corrosion current density for the nanocomposite coatings varied from 0.0119 to 0.0839 μA cm−2. The corrosion current density of the Ni–Al2O3-2 was very low indicating superior corrosion resistance of the coating. The R p value of Ni–Al2O3-2 was the highest (131.5 kΩ cm2), implying better corrosion resistance of the coating compared to other coatings.

Fig. 2
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Tafel plots of electrodeposited Ni (a) Ni–Al2O3-1, (b) Ni–Al2O3-2, and (c) Ni–Al2O3-3 coatings

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Table 1 Corrosion potential, corrosion rates and Tafel slopes calculated from potentiodynamic diagrams for Ni–Al2O3-1, Ni–Al2O3-2, and Ni–Al2O3-3 compared with Ni
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The Nyquist impedance plots and Bode plots for Ni and Ni–Al2O3 are, respectively, shown in Figs. 3, 4. From the Nyquist plot it is evident that the impedance offered by Ni–Al2O3-2 is higher than that of Ni–Al2O3-1, Ni–Al2O3-3, and pure Ni. In the Nyquist plot, the depressed semicircle with the center under the real axis is characteristic for the solid electrodes. At higher frequencies, interception with the real axis is ascribed to the electrolyte bulk resistance (R s), and at low frequencies, an interphase appears whose interception with the real axis is ascribed to the charge transfer resistance (R ct). In the Nyquist plot, the diameter of the semicircle corresponding to Ni and Ni–Al2O3 coatings increased compared to the substrate, thus indicating better corrosion resistance of the coatings. The shape of the impedance spectra describes the type of electrochemical reactions taking place on the electrode surface. The impedance plot of Ni shows single-time constant, and Ni–Al2O3 composites show the presence of two-time constant behavior. The higher-frequency time constant is related to the solution/coating interface, and lower-frequency time constant is related to the solution/substrate interface.

Fig. 3
figure 3

Nyquist plots of electrodeposited a Ni, b Ni–Al2O3-1, c Ni–Al2O3-2, and d Ni–Al2O3-3 coatings

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Fig. 4
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Bode plots of electrodeposited a Ni, b Ni–Al2O3-1, c Ni–Al2O3-2, and d Ni–Al2O3-3 coatings

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Bode plots of Ni and Ni–Al2O3 composite coatings are shown in Fig. 4. At frequencies below 103 Hz, a capacitive response was obtained for Ni–Al2O3-2 (Fig. 4c). However, in other coatings the capacitance response is observed at 104 Hz, and also narrow phase angle regions are observed. The Bode plot of Ni–Al2O3-2 shows a large negative shift in the Bode phase angle plot, and a straight line with a slope of about −1 in the Bode magnitude plot. These are the indications of higher corrosion resistance [24]. Coatings of Ni, Ni–Al2O3-1, and Ni–Al2O3-3 are less corrosion resistant than Ni–Al2O3-2, and they exhibit a low-frequency breakpoint just above 10 mHz. Coatings of Ni, Ni–Al2O3-1, and Ni–Al2O3-3 have much smaller capacitive regions across the frequency domain and distinct low frequency breakpoints which result in well-defined DC limits in the magnitude of the impedance. Ni–Al2O3-2 exhibits the highest DC limits. This type of EIS response shows improved corrosion resistance for Ni–Al2O3-2 compared with Ni, Ni–Al2O3-1 and Ni–Al2O3-3 [24].

The appropriate equivalent circuits used to fit the impedance spectra obtained for plain Ni and composite coatings are shown in the Fig. 5. The capacitance is replaced with constant phase element (CPE) for a better quality fit. CPE accounts for the deviation from ideal dielectric behavior and is related to the surface inhomogeneities. It must be noted that in the EQUIVCRT program, Q stands for constant phase element. The impedance data obtained from the spectra are tabulated in Table 2. The R ct value increases in the following order: Ni > Ni–Al2O3-3 > Ni–Al2O3-1 > Ni–Al2O3-2. Ni–Al2O3-2 which contains γ-Al2O3 shows higher R ct value of about 164.5 kΩ cm2, indicating better corrosion resistance compared to other coatings. The R coat value obtained for Ni–Al2O3-2 is the highest (155 kΩ cm2), which indicates that resistance offered by the coating is higher. Ni–Al2O3-1 containing α-Al2O3 shows very low Q coat and Q dl values of 35 and 4 μF cm−2. The decrease in Y 0 of Q coat and Q dl may be due to the decrease in the area involved in the electrochemical reactions, i.e., a decrease in the surface area of the barrier coating in contact with the solution. The n dl values for the nanocomposite coatings lie in between 0.82 and 0.87, indicating a deviation in the behavior of surface of the coatings from ideal capacitive behavior. From the potentiodynamic and impedance study, it can be concluded that the corrosion-resistant properties of the coatings decreased in the order: Ni–Al2O3-2 > Ni–Al2O3-1 > Ni–Al2O3-3 > Ni.

Fig. 5
figure 5

Equivalent circuit for a Ni and b Ni–Al2O3 coatings (R s solution resistance of test electrolyte between the working electrode and reference electrode, Q dl double layer capacitance, R ct charge transfer resistance, Q coat coating capacitance, R coat coating resistance, WE working electrode)

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Table 2 Electrochemical impedance analysis data
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The SEM images of the surface of the samples after corrosion tests are shown in Fig. 6a, c, e and their respective EDAX spectra are shown in Fig. 6b, d, f. Surprisingly, there was a difference in the morphology of the Ni-composite coatings with a change in the crystallographic forms of the alumina particles. The SEM images clearly show the superior corrosion resistance of Ni–Al2O3-2 surface which has not reacted much to the corrosive media. On the other hand, the surfaces of Ni–Al2O3-1 and Ni–Al2O3-3 have reacted with the corrosive media. The lowest corrosion resistance of Ni–Al2O3-3 is also evident from the appearance of Fe peak in the EDAX spectrum (Fig. 6f) which was absent in the EDAX spectra of other two samples. The lower corrosion resistance of Ni–Al2O3-3 may be due to the poor bonding between the particles and the matrix. There is a possibility of the dissolution of these loosely held alumina particles at high potentials; as a result, more nickel is exposed to the electrolyte for corrosion attack and hence poor corrosion resistance.

Fig. 6
figure 6

SEM images of electrodeposited composite coatings after corrosion studies: a Ni–Al2O3-1, c Ni–Al2O3-2, and e Ni–Al2O3-3, and their corresponding EDAX spectra: b Ni–Al2O3-1, d Ni–Al2O3-2, and f Ni–Al2O3-3

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3.2 Wear studies

Wear results of Ni–Al2O3 composite coatings are tabulated in Table 3. It is evident from Table 3 that Ni–Al2O3-1 has lower wear volume and hence better wear resistance compared with Ni–Al2O3-2 and Ni–Al2O3-3 coatings according to Holm–Archard relationship [23, 24]. For Ni–Al2O3-1, the coefficient of friction was higher, which is probably due to the higher hardness of α-alumina phase present in the sample [23, 24]. The Ni–Al2O3-2 and Ni–Al2O3-3 coatings exhibited lower coefficient of friction values compared with Ni–Al2O3-1 probably because of the presence of gamma phase which is relatively softer compared with corundum α-phase. The optical micrographs obtained on the wear track and wear pin for Ni–Al2O3-1, Ni–Al2O3-2, and Ni–Al2O3-3 after wear tests are shown in Fig. 7.

Table 3 Wear result data of Ni–Al2O3 composite coatings
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Fig. 7
figure 7

Optical micrographs of the wear tracks of the pins coated with a Ni–Al2O3-1, c Ni–Al2O3-2, and e Ni–Al2O3-3 after wear tests; and b, d, and f are their corresponding wear tracks on discs

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The pin and disk corresponding to Ni–Al2O3-1 (Fig. 7a, b) show very less scouring marks, and the pin surface shows colored passive film. The pin surfaces corresponding to Ni–Al2O3-2 (Fig. 7c) and Ni–Al2O3-3 (Fig. 7e) do not show the passive film formation; instead, the disk and pins show more scouring marks compared with Ni–Al2O3-1. It is also evident that the Ni–Al2O3-1- and Ni–Al2O3-3-coated pins have undergone uniform wear, and the Ni–Al2O3-2-coated pin has undergone non-uniform wear. More material transfer from the pin to the disk is seen in Ni–Al2O3-2 (Fig. 7d) and Ni–Al2O3-3 (Fig. 7f).

The plots of sliding distance versus the wear loss for Ni–Al2O3 coatings are shown in Fig. 8. From the plots, it is evident that Ni–Al2O3-1 undergoes lower wear loss compared with Ni–Al2O3-2 and Ni–Al2O3-3. The plots of coefficient of friction versus sliding distance are shown in Fig. 9. From the plots it is evident that Ni–Al2O3-2 and Ni–Al2O3-3 exhibit similar coefficients of friction, which is lower than that of Ni–Al2O3-1.

Fig. 8
figure 8

Plots of wear loss versus the sliding distance measured using a pin-on-disk tribometer

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Fig. 9
figure 9

Plots of coefficient of friction versus sliding distance measured using a pin-on-disk tribometer

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The transfer of material from the pin to the disk was confirmed by the Raman spectra recorded on the wear track of disks (Fig. 10). The Raman spectrum of wear track of Ni–Al2O3-1 shows a peak around 650 cm−1 which corresponds to Raman active phonon modes for corundum (α-Al2O3) as reported by Porto and Krishnan [25]. The Raman spectrum of wear track of Ni–Al2O3-2 shows peaks at 481, 558, and 680 cm−1. The peaks at 481 and 680 cm−1 correspond to gamma phase of alumina, clearly indicating the transfer of material from the pin to the disk.

Fig. 10
figure 10

Raman spectrum of wear track of (a) Ni–Al2O3-1 and (b) Ni–Al2O3-2 transferred disk

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4 Conclusions

Different crystallographic forms of alumina particles like pure α-Al2O3 were prepared by solution combustion method, pure γ-Al2O3 was prepared by precipitation route, and commercial Alcoa alumina containing mixture of α and γ particles were codeposited with Ni at a current density 15.5 mA cm−2 and 600 rpm magnetic stirrer speed. Ni–Al2O3-2 composite coating containing gamma-alumina phase showed an improved corrosion resistance relative to Ni–Al2O3-1 (containing alpha alumina) and Ni–Al2O3-3 (containing mixture of alumina phases). The wear resistance of Ni–Al2O3-1 containing alpha alumina was higher than the Ni-composites containing gamma-alumina and mixture of alumina phases. Thus, it has been shown that the corrosion- and wear-resistant properties of nickel–alumina matrix vary with the phase of the incorporated alumina particles. Thus, by proper selection of pure phases of alpha- and gamma-alumina phases, it may be possible to get the preferred properties.