Comparison of Ni-Co-WC composite coatings prepared by direct,pulse and pulse reverse current methods

Ranjith Bose *,G.Paruthimal Kalaignan ,K.N.Srinivasan

( 1.R & D Department,Reem Batteries & Power Appliances Co SAOC,Sultanate of Oman; 2.Advanced Nano-Composite Coatings Laboratory,Department of Industrial Chemistry,Alagappa University,Karaikudi-630 003,Tamilnadu,India; 3.Central Electrochemical Research Institute,Karaikudi-630 006,Tamilnadu,India )

Composite electrodeposition technique is one of the methods used to fabricate metal matrix composite coatings.The process involves embedding of insoluble metal micro-powders suspended in a solution by performing any one of the electrodeposition techniques such as direct current (DC),pulse current (PC) and pulse reverse current (PRC) electrodeposition techniques.Pulse electrodeposition technique is more effective in the fabrication of metal coatings which contain unique compositions and micro-structures when compared to DC electrodeposition since pulse electrodeposition offers more process controllable parameters which can be adjusted independently and can withstand much higher instantaneous current densities.Pulse electrodeposition is either done by pulse current (PC) or pulse reverse current (PRC) technique.

Tungsten carbide (WC) is one of the hard compounds widely used in tribological applications.Surender et al.analyzed wear characterization of Ni-WC composite coatings[1].WC-Co is a technologically important material and has been widely used as cutting tools,rock drills,punches and wear-resistant coating materials[2].For example,WC-Co coatings have been extensively used in industry to prevent wear,erosion,and corrosion of many metallic components.Recently,researchers in the field of surface engineering have been searching for the substitute for chromium plating techniques due to their environmental problems.A joint project is being executed by the US Department of Defense,the Canada Department of National Defense and Industry Canada to replace hard chrome plating on aircraft landing gear with HVOF (high velocity oxygen fuel) thermal-sprayed WC or WC-Co-based coatings[3].

In this paper,Ni-Co-WC composite coatings were prepared on mild steel substrate by direct current,pulse current and pulse reverse current methods.Their microstructure and properties were also investigated by hardness testing, AFM and SEM techniques.Electrochemical characterizations were carried out using potentiodynamic polarization and impedance measurements.The results showed that,Ni-Co-WC composite coatings deposited by pulse reverse current technique were found to be superior in their performance when compared to pulse current and direct current coatings.

1 Experimental

1.1 Electrode pretreatment

Mild steel (MS) plate has been used as a substrate for depositing Ni-Co alloy and Ni-Co-WC composite coatings.These substrates were subjected to pretreatment to remove organic and inorganic impurities.The MS plate cathode of size 2.5 cm × 2.5 cm × 0.2 cm was dipped in 10% HCl for 10 min and then cleaned with distilled water followed by drying in air and degreasing with trichloroethylene.Pure nickel (99.9%) bar of size 5 cm × 5 cm × 0.5 cm was used as anode.

1.2 Bath preparation and operation conditions

The plating bath was prepared by using AnalaR Grade chemicals and triple distilled water.The optimized bath composition and operation conditions are as follows:

Nickel acetate150 g/L

Nickel chloride45 g/L

Cobalt acetate15 g/L

Boric acid35 g/L

Tungsten carbide2-8 g/L

Temperature25 °C

pH4.5

Stirring rate150 r/min

Plating time90 min

The average size of WC particle is 100 µm.The bath was stirred well after the addition of WC particles and during the plating using a laboratory stirrer.

1.3 Electroplating parameters

The direct,pulse and pulse reverse current experiment was carried out using pulse rectifier (Komal Agencies,Mumbai).For DC electrodeposition,constant current density=6.4 A/dm2; for PC electrodeposition,ton=30 ms,toff=150 ms,peak current density=6 A/dm2,andf=100 Hz; for PRC electrodeposition,positiveton+toff=30 ms,negativeton+toff=30 ms,duty cycle=30%,positive average current density=6 A/dm2,negative average current density=1 A/dm2,andf=100 Hz.

1.4 Microhardness,SEM and XRD characterization

Microhardness was measured using MH6 Everyone Hardness Tester (Hong Kong).The hardness values were measured in three different locations for each sample.Surface morphologies of the composite coatings were determined by SEM (HITACHI S-570,Japan).Phase structures of the coatings was analyzed by XRD (X’ Pert PRO diffractometer with Cu Kα radiation).The samples were scanned between 20°-100° (2θ) at a scan rate 1°/min.

1.5 Electrochemical measurements

The corrosion behaviors of Ni-Co alloy and Ni-Co-WC composite coatings were characterized by potentio-dynamic polarization and AC impedance measurements.

1.5.1Potentiodynamic polarization studies

The electrochemical measurements were carried out in a conventional three-electrode cell at 25 °C.The MS specimen was masked with lacquer to expose 1 cm2area and served as the working electrode.A Pt foil (1 cm2) was used as the counter electrode and a saturated calomel electrode (SCE) as the reference.The EG&G -Autolab Analyzer (Model:6310) was employed for the polarization studies and the potential of the working electrode was varied with respect to SCE.The specimens were immersed in the test solution of 3.5% NaCl and allowed to attain a steady potential value.The potentio-dynamic polarization was carried out from -0.75 V to -1.25 V with respect to the OCP at a scan rate of 2 mV/s.The potentialE(Vνs.SCE) was plotted against lg[j/(A/cm2)]to obtain polarization curve.From this polarization curves,the corrosion potential (Ecorr) and corrosion current (icorr) of the specimens were obtained using the Tafel extrapolation method.

1.5.2Electrochemical impedance spectroscopy (EIS)

The electrochemical impedance spectra for electrolytes with various amounts of WC particles were measured using an EG&G -Autolab Analyzer (Model:6310).The same three-electrode cell setup was used for this experiment.The EIS were carried out between 10 kHz and 0.01 Hz frequency range (r.m.s.amplitude 5 mV).

2 Results and discussion

2.1 Effect of particle concentration on co-deposition

The relationship between the content of codeposited WC and the concentration of WC particles varying from 2 g/L to 8 g/L in the bath is shown in Figure 1.The particle concentration in the bath as well as the particle content in the composite also influenced the rate of codeposition[4].The highest content of codeposited WC particles was achieved at the WC concentration of 4 g/L.

Figure 1 Effect of WC concentration in the bath on the content WC in the composite coatings prepared by DC,PC and PRC图1 镀液中WC 浓度对DC、PC 和PRC 法所制复合镀层中WC 含量的影响

For further increase of WC particles concentration above 4 g/L,there was no change in the content of WC in composite coating.There was no significant change in the incorporation of WC particle in the Ni-Co matrix during the deposition of DC,PC and PRC.The curves in Figure 1 are quite similar to the well-known Langmuir adsorption isotherms,supporting a mechanism based on an adsorption effect.The codeposition of WC particles on the cathode surface was suggested by Guglielmi’s two-step adsorption model[5].Once the particles are adsorbed,metal begins building around the cathode slowly,encapsulating and incorporating the particles.The highest concentration of WC particles on the codeposit is due to the saturation reached by adsorption on the cathode surface.

2.2 Microhardness

Vickers microhardness measurements were performed for the mechanically polished electrodeposited Ni-Co alloy and Ni-Co-WC composite coatings prepared by DC,PC and PRC respectively (see Figure 2 ).The average coating thickness is 38 µm.The microhardness of the composite coatings increased with WC particles content in the Ni-Co matrix.All the Ni-Co-WC composite coatings have higher microhardness,compared to Ni-Co alloy coating.With the addition of WC in Ni-Co matrix,the improvement of hardness of the deposits may be expected as the carbide particle is intrinsically hard and may help to impede dislocations motion.

Figure 2 Effect of WC content in bath on microhardness of different coatings图2 镀液中WC 含量对不同镀层的显微硬度的影响

The hardness of the PC composite coating is higher than that of DC composite coating.The pulse current affects the structural uniformity and controls the particle distribution.Such hardness enhancement is attributed to the reduction of grain size,according to the Hall Petch’s effect,and it is induced by WC particles which segregate to the grain boundaries of Ni,stabilize the grain structure and suppress grain growth[6-7].

The hardness of the PRC composite coating is superior to that of the PC and DC composite coatings.There are two reasons contributing to the increase of microhardness in the PRC coating.On the one hand,the method of PRC prevents the edges and corners of the coatings from becoming thicker,so that the composite coatings deposited is uniform.On the other hand,the pulse intervals and the reversal current suppress the absorption of hydrogen atoms by the coatings.PRC reduces the tendency to buildup thick deposits in traditional high current density areas and improves step coverage without pores reaching down to the substrate.

2.3 Electrochemical measurements

2.3.1Electrochemical impedance spectroscopy

As there is no improvement in the overall properties of the coating deposited under DC condition,only PC and PRC methods were considered for the corrosion measurement studies.Figures 3 represent the Nyquist plots observed for the Ni-Co alloy and Ni-Co-WC composite samples for PC and PRC methods respectively.All the Ni-Co-WC composite coatings showed the higherRctand lowerCdlvalues than Ni-Co alloy coating for both PC and PRC methods.Hence,the WC incorporation into Ni-Co alloy coating offered higher polarization resistance for PC and PRC methods (see Table 1 ).

Figure 3 Nyquist plots for Ni-Co alloy and Ni-Co-WC composite coatings prepared by PC and PRC methods图3 单向脉冲法制备的Ni-Co 合金及Ni-Co-WC 复合镀层的Nyquist 谱图

Table 1 Parameters derived from the electrochemical impedance spectra for Ni-Co alloy and Ni-Co-WC composite coatings prepared by PC and PRC methods表1 从PC 和PRC 法制备的Ni-Co 合金及NI-Co-WC 复合镀层的电化学阻抗谱中求得的参数

There was a significant change ofRctandCdlvalues for the coating deposited from the electrolyte containing 4 g/L WC compared to other compositions like 2,6 and 8 g/L WC.With the increase in WC particle concentration from 2 g/L to 4 g/L in the electrolyte,more WC will be adsorbed and incorporated in the coating.The WC particles not only have higher corrosion resistance but also contribute to reduce the porosity of the coatings[4].When the WC concentration in the electrolyte increased to above 4 g/L,there was almost no change inRctandCdlvalues.It is indicated that,the adsorption of WC particles in the coating has reached the saturated state.Thus,it is reasonable to conclude that the optimal WC concentration in the present bath is 4 g/L.

2.3.2Tafel polarization behaνior of Ni-Co-WC composite

Parameters derived from potentiodynamic polarization curves (see Figure 4 ) of Ni-Co alloy and Ni-Co-WC composite coatings prepared by PC and PRC methods were summarized in Table 2.Corrosion currents were calculated using the Stern-Geary equation[8].

The corrosion potential is shifted to low negative values for Ni-Co-WC (4 g/L) composite coatings prepared by both PC (-0.478 V) and PRC (-0.457 V) methods.This positive shift leads to improve the corrosion resistance of the Ni-Co-WC composite coatings prepared by PRC method.

Table 2 Parameters derived from potentiodynamic polarization curves of Ni-Co alloy and Ni-Co-WC composite coatings prepared by PC and PRC methods表2 从PC 和PRC 法制备的Ni-Co 合金及NI-Co-WC 复合镀层的动电位极化曲线中求得的参数

Figure 4 Tafel polarization curves for Ni-Co alloy and Ni-Co-WC composite coatings prepared by PC and PRC methods图4 PC 和PRC 法制备的Ni-Co 合金及Ni-Co-WC 复合镀层的塔菲尔极化曲线

From the experimental data,it was observed that,corrosion current density is 3.521 μA/cm2for Ni-Co-WC (4 g/L) composite coating prepared by PRC method,but is 3.587 μA/cm2for the composite coating prepared with the same WC concentration by PC method.Furthermore,the passivity of the coated steel is enhanced by incorporating more WC particles in the coating as shown in Figure 4.

2.4 Surface morphology by SEM studies

Scanning electron microscopic (SEM) pictures of the electrodeposited Ni-Co alloy and Ni-Co-WC (4 g/L) composite coatings prepared by PC and PRC methods are shown in Figure 5.

The PC-electrodeposited Ni-Co alloy showed regular surface and fine crystallites as observed in Figure 5 a and the Ni-Co-WC composite coating prepared with 4 g/L WC by PC was found to be more compact than the Ni-Co alloy and consisted of small and spherical sized WC grains as observed in Figure 5 b.The Ni-Co-WC composite coating obtained with 4 g/L WC by PRC showed more uniform distribution of WC particles as compared to the Ni-Co-WC composite coating obtained by PC method (see Figure 5 c).Hence,the PRC-electrodeposited Ni-Co-WC (4 g/L) composite coating possessed excellent hardness and corrosion resistance.

2.5 AFM investigation of the surface morphology

Figure 6 illustrates the AFM morphologies of the Ni-Co-WC (4 g/L) composite coatings prepared by PC and PRC methods.It is seen that the Ni-Co-WC composite coating has small particle size,uniform and compact surface,which indicates that the codeposited WC were uniformly distributed in the Ni-Co matrix of the composite coating and the presence of WC results in the decreasing of particle size.The WC uniformly distributed in the Ni-Co matrix has contributed significantly to increase the hardness of the Ni-Co alloy coating.

Figure 6 AFM analysis results of Ni-Co-WC (4 g/L WC) composite coatings prepared by PC and PRC methods图6 以PC 和PRC 法在含4 g/L WC 镀液中所得Ni-Co-WC复合镀层的原子力显微镜分析结果

3 Conclusion

The preparation and optimization of WC concentration for Ni-Co-WC composite coatings were carried out using an environment-friendly acetate-based electrolytic bath.Based on the experimental results,it is conformed that the Ni-Co-WC composite coating prepared with 4 g/L WC in the plating bath has a better performance than all the others.

(1) PRC technique could obtain Ni-Co-WC (4 g/L) composite coatings with finer crystal grain,smoother surface,and more homogeneous microstructure.Under the experimental conditions,WC particles uniformly distributed throughout the PRC composite coatings and combined very well with the Ni-Co matrix.

(2) The coatings obtained by PRC have superior hardness and corrosion resistance to DC and PC coatings,so the PRC technique is a promising method for surface modification and improvement.

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