Magnetically Induced Codeposition of Ni–Cd Alloy Coatings for Better

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Magnetically Induced Codeposition of Ni−Cd Alloy Coatings for Better Corrosion Protection Vaishaka R. Rao and Ampar Chitharanjan Hegde* Electrochemistry Research Laboratory Department of Chemistry, National Institute of Technology Karnataka, Surathkal Srinivasnagar 575025, India ABSTRACT: The effects of applied magnetic field, B (both parallel and perpendicular) during process of electrodeposition of Ni−Cd alloy coating on mild steel from a newly proposed electrolytic bath have been studied by using X-ray diffraction (XRD), energy-dispersive X-ray (EDX), and scanning electron microscopy (SEM) analysis. Both parallel and perpendicular B reduced the corrosion rates (CRs); however, the effect is more pronounced in case of perpendicular B. Progressive decrease of CR with increase in the intensity of B showed that corrosion protection efficacy bears close relation with changed composition and crystallographic orientation of the coatings. Under optimal condition, Ni−Cd coating deposited at 0.8 T (perpendicular) was found to be 35 times less corrosive than the conventional Ni−Cd coating (B = 0 T) deposited from the same bath for same time. The effect of B on thickness, microhardness, surface morphology, composition, and crystallographic orientation, and hence, the corrosion resistance of the coatings were analyzed in the light of magnetohydrodynamic (MHD) effect.

1. INTRODUCTION The nickel and its alloys were widely used for a variety of applications, the majority of which require corrosion and heat resistance, including aircraft gas turbines, steam turbine power plants, medical applications, nuclear power systems, and chemical and petrochemical industries.1−13 The Ni−Cd alloy coating finds its applications in decorative finishing of metals, protection of jet-engine parts from corrosion, manufacture of electrical contacts, and in batteries.14 The mechanism of Ni− Cd alloy deposition has been discussed from experimental and theoretical points of view, and effects of different parameters such as pH, polarization, and bath composition on the properties of the alloy has been widely investigated.16,17 The results of investigation demonstrated that the Ni−Cd alloy deposition follows anomalous type of codeposition. Kharlamov et al. studied the influence of Cu2+, Co2+, and Cd2+ on electrodeposition of nickel and reported that the presence of these metal ion impurities inhibited the nickel ion reduction process. Nickel with better texture was found to be deposited in presence of these impurities.18 A small change in double layer can bring large change in the properties of coatings. As corrosion resistance and other physical properties of any coatings majorly depends on phase composition, any modulation in the structure of electrical double layer (EDL) obviously brings modulation in their composition, and hence the properties.19−21 In this direction, the superimposition of the external magnetic field during process of deposition offers new route for developing such materials of enhanced properties.22−33 Number of metals and alloys of Ni, Ag, Cu, Co, and Zn have been developed by electrodeposition process by inducing magnetic field, and their composition and surface morphology have been studied using different characterization techniques. Zielinski et al. reported the mass transport controlled deposition of cobalt and cobalt alloys by magnetoelectrolysis. They demonstrated that numerous fractures that are formed on sample surface of Co−Mo, Co−W, Co−Mo−W alloys due to © 2014 American Chemical Society

residual stress were found to be disappeared when deposition was carried out under applied magnetic field. These changes were attributed to the combined effect of depletion of Nernst diffusion layer and hydrodynamic movement of ions at the interface.34 Tschulik et al. reported that magnetic field B could modify the surface morphology and preferred orientation of Cu and Bi by altering the diffusion of ions toward cathode.35 Li et al. studied the effect of high magnetic field up to 12 T on the morphology of nanocrystalline CoNi films. The grain size and the surface roughness of the films were found to be increased with field strength B, and reached its maximum value at 9 T. Further, it was reported that higher magnetic flux density could improve cobalt content in the film due to magnetohydrodynamic effect.36 The superimposing external magnetic field offers many possibilities to influence the orientation of the electrodeposit during codeposition process.37−39 When an electrochemical codeposition process is carried out under the influence of external magnetic field, additional convection in the electrolytic solution is induced. This effect is called magneto-hydrodynamic (MHD) effect. The effect of magnetic field on process of electrodeposition, in terms of the force acting on moving ions, is described by well-known Lorentz equation as FL = q(E + vB)

(1)

where FL is the Lorentz force, q the charge of an ion, E the electric field strength, v the velocity of the ions, and B the magnetic flux density. Thus, the applied magnetic field brings significant change on composition and morphology, and hence the properties of electrodeposited coatings. It was observed that the effect of magnetic field is more prominent when it is Received: Revised: Accepted: Published: 5490

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as anode. The bath was maintained at a constant pH (4.0), using dilute HCl solution, and temperature was maintained at 303 K (30 °C). The optimized bath allowed the deposition of coatings in the range 1.0−7.0 A dm−2, having black-porous/ powdery, semibright to bright appearance. At very high current density, the deposit was observed to be very porous and peeledoff from the substrate. The current density (j), at which the Ni−Cd coating showed least corrosion rate (CR) was used to study the effect of magnetic field, B. All electrodeposition was carried for 600 s for comparison purpose using computer controlled power source (DC power Analyzer, N6705A, Agilent Technologies, U.S.A.). The magnetic field (B) was applied using an electromagnet (Polytronics, Model: EM 100, Flat pole pieces with 100 mm gap) to the direction of plane of cathode surface. The electrolytic cell was kept in the gap, keeping a constant distance between the cathode and anode. The magnetic field in the region of electrolysis was observed to be uniform and homogeneous. 2.1. Test Procedure for Characterization of Ni−Cd Coatings. The corrosion performance of the coatings developed under different conditions of current density, j, and magnetic field, B, were evaluated by potentiodynamic polarization and electrochemical impedance spectroscopy (EIS) techniques. All electrochemical measurements were performed using Potentiostat (ACM Instruments, Gill AC Series No. 1480), using standard three-electrode system. The test electrolyte was 5.0 wt % NaCl (analytical reagent) in deionized water maintained at 298 K (25 °C). The Ni−Cd alloy coating (on mild steel) was used as the working electrode with 2 cm2 surface area exposed to the test solution. The reference electrode was a saturated calomel electrode (SCE), which was connected to the working electrode via a Luggin capillary, and a platinum electrode was used as counter electrode. The potentiodynamic polarization and EIS tests were performed after 10 min immersion at open circuit. Potentiodynamic curves were measured by scanning the potential from −0.25 V below the open circuit potential to +0.25 V at a scan rate of 1 mV s−1. The corrosion potential, Ecorr, and the corrosion current density, icorr, were obtained through Tafel approximation, and corrosion rates (CR) were expressed in mm y−1. The EIS measurements were obtained using a polarization of ±10 mV in the frequency range from 100 kHz to 10 mHz, and corresponding Nyquist plots were analyzed. The surface morphology of the coatings was analyzed using Scanning Electron Microscopy (SEM, Model JSM-6380 LA from JEOL, Japan). The compositions of the coating were determined by energy dispersive X-ray (EDX) detector interfaced to the SEM machine. Each sample was analyzed at five locations, to average out the composition. The microhardness of coatings (15−20 μm thickness) was measured using a

superimposed parallel to the deposit plane of an electrode surface (perpendicular to the direction of flow of ions under the electric field).40 Further, it was observed that magnetoelectrolysis is found to enhance the surface smoothness of the electrodeposit.41,42 The morphology and phase structure of Zn−Fe alloys can be altered significantly when they are electroplated in the presence of external magnetic field, B. It was attributed to the change in the surface concentration of deposition inhibiting species (i.e., Fe2+) by MHD effect. Fahidy reported that the decrease in the surface roughness is also due to MHD effect on the surface of three-dimensional deposit film structure.43 Thus, sufficient information is available with regard to the effect of induced magnetic field on crystal orientation, surface morphology, and corrosion behaviors of many metals and alloys. Though a large volume of literature is available on the electrodeposition of Ni−Cd alloys using different baths, no previous study is available revealing the role of a magnetic field on the characteristics of Ni−Cd alloy deposited on mild steel from a bath containing gelatin and glycerol as additives. Hence, the present work is carried out to study the effect of magnetic field on electrodeposition of Ni−Cd alloy under superimposed magnetic field (both parallel and perpendicular) and to determine the effect of magnetic field on corrosion character of the coatings, in terms of composition, crystallographic orientation and surface morphology.

2. EXPERIMENTAL SECTION All electrodeposition was carried out using analytical grade reagents, prepared in double distilled water. The bath ingredients and operating parameters optimized for binary Ni−Cd bath are given in Table 1. The Hull cell method was Table 1. Bath Composition and Operating Parameters of Optimized Ni−Cd Bath bath ingredients

composition (g L−1)

operating param.

NiCl2·6H2O CdCl2 NH4Cl H3BO3 glycerol gelatin

71.3 3.6 50.0 30.9 2.5 1.0

anode: pure Ni cathode: mild steel pH: 4.0 temp: 303 K (30 °C)

used to examine the effect of current density (j) and additives.44 Precleaned mild steel (MS) panel, having 7.5 cm2 active surface areas, were used as cathode. Surface was degreased with an alkali cleaner, activated by immersion into a solution of 1:1 HCl prior to coating. No nitrogen purging was done for electrolytic solution. The pure Ni with same exposed surface area was used

Table 2. Corrosion Data of Ni−Cd Alloy Coatings Deposited from Optimal Bath at Different Current Density without Applied Magnetic Field j (A dm−2)

wt % Ni

thickness (μm)

microhardness (GPa)

−Ecorr (V vs SCE)

icorr (μA cm−2)

CR (× 10−2 mm y−1)

nature of the deposit

1.0 2.0 3.0 4.0 5.0 6.0 7.0

13.24 35.34 55.47 70.77 84.33 92.05 89.33

6.6 10.5 14.9 19.1 22.8 24.3 26.0

1.30 1.61 2.02 2.40 2.66 3.07 2.82

556.9 463.6 478.2 444.4 380.3 327.8 281.8

8.10 6.25 4.82 3.42 3.97 5.51 6.89

12.38 9.55 7.36 5.22 6.06 8.42 10.53

porous semibright bright bright bright semibright dull

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Figure 1. X-ray diffraction peaks of Ni−Cd alloy coating deposited at different current densities from optimal bath at 303 K (30 °C).

Table 3. Corrosion Data of Ni−Cd Alloy Coatings Deposited from Optimal Bath at Different Current Density with Applied Magnetic Field coating configuration

wt % Ni

thickness (μm)

microhardness (G Pa)

−Ecorr (V vs SCE)

icorr (μA cm−2)

CR (× 10−2 mm y−1)

nature of the deposit

(Ni−Cd)B=0.05 T/Para. (Ni−Cd)B=0.1 T/Para. (Ni−Cd)B=0.2 T/Para. (Ni−Cd)B=0.3 T/Para. (Ni−Cd)B=0.4 T/Para. (Ni−Cd)B=0.6 T/Para. (Ni−Cd)B=0.8 T/Para. (Ni−Cd)B=1 T/Para. (Ni−Cd)B=0.05 T/perp. (Ni−Cd)B=0.1 T/perp. (Ni−Cd)B=0.2 T/perp. (Ni−Cd)B=0.3 T/perp. (Ni−Cd)B=0.4 T/perp. (Ni−Cd)B=0.6 T/perp. (Ni−Cd)B=0.8 T/perp. (Ni−Cd)B=1 T/perp. (Ni−Cd)B=0 T

70.79 70.81 70.83 70.85 70.87 70.90 70.93 70.96 71.69 74.02 76.55 78.89 80.93 84.10 88.93 92.56 70.77

19.0 18.9 18.7 18.5 18.3 17.9 17.5 17.1 18.9 18.3 16.7 16.2 15.8 14.7 13.6 12.5 19.1

2.422 2.432 2.452 2.471 2.491 2.540 2.599 2.658 2.452 2.491 2.540 2.589 2.648 2.766 2.913 3.089 2.403

0.564 0.539 0.550 0.588 0.598 0.611 0.625 0.612 0.323 0.327 0.378 0.428 0.445 0.470 0.520 0.492 0.444

3.37 3.29 3.15 3.08 2.89 2.54 2.36 2.48 3.34 3.10 2.49 1.72 1.13 0.81 0.10 0.62 3.42

5.18 5.02 4.81 4.70 4.41 3.88 3.60 3.79 5.10 4.73 3.80 2.62 1.72 1.23 0.15 0.94 5.22

dull semibright semibright semibright bright bright bright dull semibright semibright semibright semibright bright bright bright dull bright

It should be noted that the wt % values of Ni and Cd in the proposed bath (calculated based on atomic weight % of Ni and Cd in the bath) are 88.67% and 11.33%, respectively. From data reported in Table 2, it may be observed that wt % Cd in the deposit is much higher than that in the bath even up to 5.0 A dm−2. Hence, it may be inferred that at this current density limit, the bath followed anomalous type of codeposition with preferential deposition of less noble Cd. However, on increasing the current density (i.e. above 5.0 A dm−2), the codeposition process was changed from anomalous to normal type, as attested by the Ni content in the alloy (Table 2). At high current density, Ni deposited preferentially than Cd. Ni content was found to decrease once again at very high j (i.e., at 7.0 A dm−2). This decrease of Ni content can be explained through simple diffusion theory associated with regular normal plating system.45 According to which, the rate of deposition of metal has an upper limit, determined by the rate at which an ion can move through cathode diffusion layer. Hence, at high j, the rate of deposition of Ni (more noble metal) is much close

computer-controlled microhardness tester with Vicker’s diamond indenter (MMT-X7, Clemex). While the thicknesses of coatings were estimated by Faraday’s law they were verified through measurement using Digital Thickness Meter (CoatmeasureM&C, AA Industries/Yuyutsu Instruments). The phase structure of the coatings were analyzed using X-ray diffraction (XRD, JEOL JDX-8P) using Cu Kα radiation, λ = 1.5405 A0, 30 kV.

3. RESULT AND DISCUSSION 3.1. Electrodeposition of Ni−Cd Alloy without Magnetic Field. Using the optimal bath given in Table 1, electrodeposition of Ni−Cd coatings was accomplished on mild steel with no applied magnetic field, B. The optimized bath allowed the deposition of variety of coatings in the range 1.0− 7.0 A dm−2, having black-porous/powdery, semibright to bright appearance. The composition, corrosion rate, and physical appearance of the coatings, deposited at different j, are summarized in Table 2. 5492

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Figure 2. Comparison of relative intensities of XRD peaks showing the crystallographic orientations of Ni−Cd coatings deposited from optimized bath under magnetic field applied (a) parallel and (b) perpendicular to the direction of cathode surface.

Figure 3. Potentiodynamic polarization behaviors of Ni−Cd alloy coatings developed under different conditions of magnetic field B (perpendicular) from same bath (only representative).

to its limiting value than that of Cd. Consequently, wt % Cd in the deposit has increased.15 It may also be noted that the thickness of Ni−Cd coatings increased linearly with applied j, as shown in Table 2. This observation is attributed to the combined effect of increased hydroxide formation (due to local alkalinity caused by excessive hydrogen evolution) and Faraday’s law. Therefore, the optimized Ni−Cd bath follows both anomalous and normal type of codeposition depending on process of mass transfer imparted at the interface by altering the deposition current

Figure 4. EIS response of Ni−Cd coatings under different field intensity (perpendicular), deposited from same bath (optimal) in the frequency range from 100 kHz to 10 mHz using ±10 mV perturbing voltage.

density. The microhardness of Ni−Cd alloy coatings was found to increase with j up to 6.0 A dm−2 and decreased thereafter. This may partly be attributed to the increase in wt % Ni (the harder metal) in the deposit and partly to synergistic effect of gelatin and glycerol in bringing fine grain deposition at high 5493

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to conventional alloy coatings. To verify the above fact the electrodeposition of Ni−Cd coatings have been carried out under varying magnetic field intensity, applied both parallel and perpendicular to the plane of cathode surface i.e., at optimal conditions of the bath such as j = 4.0 A dm−2, pH = 4.0, and at 303 K (30 °C) (Table 1), and results are reported in the following sections. 3.2. Effect of Magnetic Field on Thickness and Microhardness. In conventional Ni−Cd coating (with B = 0 T), it was found that the thickness of coating increased with j whereas in B induced codeposition it decreased with intensity of the field, as reported in Tables 2 and 3. It is due to decrease of tangential velocity of the ions, causing the thickness of diffusion layer to reduce. It was observed that the thickness and microhardness of the coatings bears respectively inverse and direct dependency with strength of applied B, which more pronounced in case of perpendicular magnetic field as shown in Table 3. A drastic decrease in the thickness, and increase in the microhardness in case of perpendicular orientation of magnetic field, is due to MHD effect induced by Lorentz force. This makes the tangential velocity, induced by the applied B to decrease the diffusion layer thickness.45 Because of this reason only, it is impossible to obtain thick deposit at high magnetic field. Under this condition, dendritic growth of the coating takes place with no preferential direction. However, the effect of the Lorentz force is not substantial when electric and magnetic field lines are parallel, as there is no effect when electric and magnetic field lines are parallel.46 Similarly, an increase in the microhardness of the coatings with intensity of B is due to the grain refinement. Increase in the surface homogeneity of the coatings with increasing grain boundary and triple junctions acts as a barrier for the dislocation movement and causes the hardening of the material.38,39 Hence, the decrease in thickness and increase in the microhardness of Ni−Cd alloy coatings are due to the changed MHD convections. 3.3. Effect of Magnetic Field on Phase Structure. Ispas et al. have showed that the electrodeposition process (current efficiency, crystallographic orientation) of nickel−iron alloys greatly depends on both direction and strength of applied magnetic field.47 Hence, the type of electrodeposition changes greatly with changes in the direction of the magnetic field. As pointed out earlier, the corrosion resistance of Ni-based alloy coatings depends mainly on their stress, texture, phase structure, and chemical composition. In this study, change in composition, crystallographic phase structure, and, hence, surface morphology has been affected by applying varying B, configured both parallel and perpendicular to the plane of cathode surface. Comparison of relative intensities of XRD signals of Ni−Cd coatings deposited from optimal j under different intensities of B, applied parallel and perpendicular to the plane of cathode surface, are shown in comparison with the one under natural convection (B = 0 T) in Figure 2a and b, respectively. It may be observed that, in case of parallel B, as the field strength B increased the intensity of peak corresponding to Cd (200) increased progressively keeping other peaks corresponding to Ni (111), Ni (200), and Ni−Cd (862) almost unchanged. Since the deposition was carried out at constant current density, any change in phase structure of the deposit is due to orientation of crystals caused by MHD effect. However, a significant change in the crystal orientation was found as the intensity of applied B was increased in case of perpendicular B.

Figure 5. Surface morphology of Ni−Cd coatings under different superimposed magnetic field, deposited from same bath (a) B = 0.1 T, (b) B = 0.2 T, (c) B = 0.3 T, (d) B = 0.4 T, (e) B = 0.6 T, (f) B = 0.8 T, (g) B = 1 T, and (h) B = 0 T (by natural convection).

current density side, supported by the smooth surface morphology of the coatings. Corrosion data reported in Table 2, demonstrated that Ni− Cd alloy deposited at 4.0 A dm−2 is the most corrosion resistant, with CR = 5.22 × 10−2 mm y−1 compared to at other current densities, and same has been considered as the optimal j for the deposition. The XRD peaks of Ni−Cd alloy coating deposited at different j, deposited from optimal bath at 303 K, are shown in Figure 1. The peak corrosion resistance of Ni−Cd coating is supported further by its smooth surface, as seen under SEM, and specific phase structure. The Ni−Cd deposit referring to optimal j was found to have ∼70.77 wt % Ni, showing prominently Ni (111), Ni (200), Cd (200), and Ni− Cd (862) peaks, as shown in Figure 1. Due to changed composition and magnetically induced orientation of crystal lattice, the magnetoelectrolysed alloy coatings sometimes offer better corrosion resistance compared 5494

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compared to those at other B. It may be observed that considerable structural changes were found as soon as magnetic field was applied, these modifications could be due to the enhancement of the diffusion of H+ ions toward the cathode. An increase in the Ni content of the coating and field intensity could support the preferential orientation of Ni (111) and Ni (220) due to suppression of anomalous codeposition, in spite of common Cd (220) phase. Comparing the results referring to parallel B, the XRD peaks suggest that the applied perpendicular B favors the Ni rich crystallographic phases with preferential orientation (111) and (220). The topography of the coatings developed at optimal B (0.8 T) is found to be smooth and uniform, as shown in Figure 5f. However, the coating at high B (1 T) is observed to be more porous as shown in Figure 5g, and hence showed more CR. The roughness of the coating at high B may be attributed to excess hydrogen evolution.

The preferential orientation of Cd (200) phase is always favored to exist in both parallel and perpendicular B. However, increase in peak intensity with field strength corresponding to Ni (111) and Ni (220) phase was observed only in case of perpendicular B, shown in Figure 2b. This is due to limited diffusion current for hydrogen evolution, which suppresses the anomalous codeposition (due to decreased local alkalinity). The comparison of XRD peaks corresponding to parallel and perpendicular B of varying field strength reveals that the magnetic convection is more effective in case of perpendicular field compared to parallel field. 3.4. Effect of Magnetic Field on Corrosion Character. Different electrochemical methods can be used to assess the corrosion protection ability of the coatings of metals and alloys. In this study, electrochemical polarization and electrochemical impedance spectroscopy (EIS) methods are used. 3.4.1. Potentiodynamic Polarization Study. The corrosion behaviors of Ni−Cd coatings deposited under different conditions of B were evaluated by Tafel’s extrapolation method, and corresponding data are reported in Table 3. The potentiodynamic polarization behavior of Ni−Cd alloy coatings developed under natural and varying B (perpendicular) is shown in Figure 3 (only representative). It may be noted that corrosion current, icorr, decreased progressively with intensity of B until B = 0.8 T and increased thereafter. From the data, it may be noticed that a drastic decrease of CR with increase of field intensity was found only in case of perpendicular B. This decrease of CR is due to characteristic Ni (111) and Ni (220) peaks observed in case perpendicular B as shown in Figure 2b. Further, the extent of decrease of CR was found to bear close relation with changed composition and intensity of applied B. The CR decreased with increase in intensity of Ni (111) and Ni (220) peaks, as seen in Figure 2b. Hence, it may be concluded that characteristic Ni (111) and Ni (220) peaks are responsible for decreased CR of Ni−Cd coatings, deposited under perpendicular B. However, at very high B (i.e., at 1 T), the CR once again started decreasing due to excessive hydrogen evolution. At the same time, it is worth to note that though CR decreased with increase of field intensity under parallel field, it is not significant as in perpendicular field. It is due to presence of common Cd (200) phase and absence of Ni (111) and Ni (220) phases, as shown in Figure 2a. Hence, from corrosion data summarized in Table 3, it may be concluded that Ni−Cd coating at 0.8 T (perpendicular) is found to exhibit the least CR (= 0.15 × 10−2 mm y−1) compared to conventional Ni−Cd coating (showing CR = 5.22 × 10−2 mm y−1), deposited from same bath under natural convections. The magnetically induced Ni−Cd alloy plating offers 35 times better corrosion resistant coatings compared to conventional Ni−Cd coating. 3.4.2. Electrochemical Impedance Study. EIS study was made to understand the factors responsible for the enhanced corrosion protection of Ni−Cd coatings. The Nyquist plots corresponding to the coatings, developed at perpendicularly induced B is shown in Figure 4. Increase of polarization resistance, RP with field intensity clearly indicates that the capacitive behavior of the coatings increases. The corrosion protection efficacy of the coating increases with field intensity only up to 0.8 T and then decreases, as depicted in Figure 4. 3.5. Effect of Magnetic Field on Surface Morphology. The SEM image of Ni−Cd alloy coatings developed at different B (perpendicular) is shown in Figure 5, in comparison with one without applied B. It may be noted that the grain size of the coatings at optimum B is more fine and homogeneous,

4. CONCLUSIONS The Ni−Cd alloy coating has been electrodeposited on mild steel in presence and absence of applied magnetic field (B), using gelatin and glycerol as additives. The corrosion resistances of the coatings were found to be improved drastically when deposition was carried out under superimposed B, applied both parallel and perpendicular to the plane of the cathode. The perpendicular B was found have better effect on deposit character than parallel B. The improved corrosion resistances of the coatings were attributed to the changed composition, crystallographic orientation, and surface morphology, supported by EDXA, XRD, and SEM analysis, respectively. A drastic decrease of thickness and increase of microhardness, observed in case of perpendicular B was attributed to increased magneto-hydrodynamic effect. Under optimal condition of B (perpendicular) Ni−Cd coatings show about 35 times better corrosion performance compared to the same coating under natural convection (with B = 0 T), deposited at j = 4.0 A dm−2, from same bath deposited for same time.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: hegdeac@rediffmail.com. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Mr. Vaishaka R. Rao acknowledges National Institute of Technology Karnataka (NITK), Surathkal, for financial support in the form of Institute Fellowship.

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REFERENCES

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