UCTEA Chamber of Metallurgical & Materials Engineers Proceedings Book

740 IMMC 2016 | 18th International Metallurgy & Materials Congress

Eff ects of pH and Cobalt Concentration on the Properties of Nickel – Cobalt Alloy Plating

Mertcan Başkan¹, Metehan Erdoğan², İshak Karakaya¹

¹Middle East Technical University, ²Yıldırım Beyazıt University - Türkiye

Abstract

Nickel-Cobalt alloys are among the most prominent materials due to their magnetic properties, wear resistance, thermal conductivity and electrocatalytic activities. Although nickel and/or nickel-cobalt alloys have been plated from Watts solution for a long time, today, especially for the engineering applications, nickel sulfamate based solutions are commonly preferred due to the advantages of low internal stress and high deposition rate. The properties of nickel-cobalt alloy platings are directly related to the pH and the cobalt amount of the solution. In the literature, the optimum pH range was determined as 3.0-5.0, but the effects of the changes within this range on the properties of the coatings have not been investigated deeply. Three different solutions were prepared to perform coatings from solutions containing different pH and amounts of cobalt in this study. The coatings were characterized in terms of grain size and shape, composition, and hardness properties.

1. Introduction

Nickel-cobalt alloys are important coating materials due to their magnetic properties [1-2], excellent wear resistance [3-4] and electrocatalytic properties [5]. Their deposition from sulfamate electrolytes has many advantages including low internal stress even deposition at high current densities, high current efficiencies and higher hardness as compared to the conventional Watts electrolyte [6].

The properties of the coating strongly depends on both the plating characteristics and bath conditions. The former subject was studied by many authors [referanslar]. For instance, Chung et.al [7] studied the effects of current density and temperature on the anomalous behavior of nickel-cobalt plating and concluded that both of them decreased the impact of anamolous codeposition. The increase in the current

density and temperature lead to the reduction in the amount of cobalt in the coating. Wang et al. [4] studied the tribological properties of Ni-Co alloy platings. They revealed that as the Co content of the coating increased, the wear resistance of the plating also increased due to the improved hardness values. Also, the friction coefficient of the coating layer exhibited a sudden drop around 70% Co, which was attributed to the fcc to hcp transition within the matrix phase.

Although many studies focussed on the deposition parameters existed in the literature, the effects of bath constituents and pH have not been thoroughly investigated yet. The chemicals like cobalt sulfamate and cobalt chloride in Ni-Co alloy deposition have been studied deeply in the literature, however, the use of cobalt sulfate has not been examined in detail. The effects of solution pH and cobalt ion concentration in the electrolyte on the properties of Ni-Co deposits were investigated in this study.

2. Experimental Studies

Ni-Co alloys were electrodeposited in typical sulfamate baths containing nickel sulfamate (350g/l), nickel chloride (15g/l) and boric acid (30g/l) under galvanostatic conditions by DC current at 50 C. Additives were not used throughout the experiments to follow the effects of each parameter independently. The pH of the baths was adjusted by the addition of sulfamic acid. Nickel source was high purity nickel anodes, whereas the cobalt source was only the Co+2 ions from the cobalt sulfate added to the electrolyte. Stirring was kept constant at 600 rpm in all depositions. The cathode materials were polished to a roughness of . Cu plates, whose plating area were about 5 cm2, were cleaned by hot water and soap to remove the dirt from the surface dipped into 1M NaOH solution at 40 C to remove oil and activated by 20 % HNO3-balance water solution. Table 1 showed

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the design of experiments followed in this study. Bath 1 and 2 were used to study the effects of Co amount in the electrolyte, whereas Bath 1 and 3 were used to see the effect of pH of the solution. Four different galvanostatic depositions were applied in each bath at the given current densities.

Table 1. The properties of the electrolytes

Bath 1 Bath 2 Bath 3

pH 3.5 3.5 4

[Co+2]/[Ni+2] 0.05 0.10 0.05

i(A/dm2) 1,2,4,8 1,2,4,8 1,2,4,8

3. Results and Discussions

3.1. The Effects of Co Content

To see the effects of Co amount in the bath, bath 1 and 2 were prepared with the same composition except their Co content. Figure 2 showed the microstructures of the coatings. Figure 2 (a) and (b) were deposited at current densities of 1 and 8 A/dm2, respectively, from bath 1. Figure (c) and (d) were also deposited at the same current densities, but they were from bath 2. At low current densities like 1A/dm2, the addition of cobalt yielded larger grain sized coatings. However, at higher current densities, the addition of Co did not change the grain size of the coatings.

Figure 2 shows the variation of cobalt amount in different baths. It was predictable that Bath 2, which contained more Co ions, yielded Co-enriched deposits. Secondly, it was observed that as the current density increased, the deposited Co got reduced, despite 2 and 4 A/dm2 yielded similar Co contents in bath 2,. This behavior was also verified by many studies [2,3,6].

Figure 1. Microstructures of Ni-Co alloys deposited at (a) 1 A/dm2 and (b) 8 A/dm2 from bath 1 and (c) 1

A/dm2 and (d) 8 A/dm2 from bath 2.

Figure 2. The dependence of Co (at.%) in the coating on current density and bath composition.

The amount of Co in the deposit played a key role for the mechanical properties of the deposits. Figure 3 shows the dependence of hardness on the current density. When they were compared with Figure 2, it can be inferred that in bath 1, hardness and Co amount had a positive correlation, whereas in bath 2, they changed just in an opposite manner. The reason for this behavior can be seen in Figure 4 where the relationship between the Co content in the deposit and the hardness is shown. According to the figure, the hardness reached a maximum near 40 % Co, then started to decreased as the Co content increased. This could be explained by the physical metallurgical phenomenon called solid-solution strengthening. In this model, as the density of solute atoms increased, hardness and strength also

UCTEA Chamber of Metallurgical & Materials Engineers Proceedings Book

742 IMMC 2016 | 18th International Metallurgy & Materials Congress

increased until a certain level. After that level, the solute atoms acts as matrix material and the efficiency of strengthening diminishes and the properties start to decrease.

Figure 3. Effects of (a) applied current density and (b) Co content of the deposit on hardness.

Figure 4. The change of deposit hardness with cobalt concentration of the deposit

3.2. The Effects of pH

Close control of solution pH is very crucial for the quality of the deposits. Very low pH values can cause excessive hydrogen evolution reaction, which leads to the pitting problem. On the other hand, if the pH of the solution is too high, formation of nickel hydroxide layer can take place, which deterioarate both the optical quality and mechanical properties of the deposit [7].

Figure 5 shows the effect of electrolyte pH on the microstructure of the deposits. The samples deposited at the same current density are compared in this figure. It was observed that there was a reduction and better

homogenity in the grain size of the coatings from bath 3 as compared to the microstructures from bath 1.

Figure 5. Microstructures of coatings deposited at (a) 1 A/dm2 and (b) 8 A/dm2 from bath 1 and (c) 1 A/dm2

and (d) 8 A/dm2 from bath 3.

The pH of solution also changed the amount of Co in the coatings. Figure 2 showed that the amount of Co in the deposit increased when pH of the solution increased from 3.5 to 4.0. This behavior was probably due to the decrease in the rate of hydrogen evolution reaction when the pH was increased to 4.0. The reduction of hydrogen and cobalt ions occur simultaneously just at the surface of the cathode; therefore, the less hydrogen evolution reaction means more reduction of cobalt from the solution to the cathode. Figure 6 revealed that both bath 1 and 3 gained less Co as the current density increased.

Figure 3 showed the results of hardness measurements changing with current density. Bath 3 did not show a proper relationship between the hardness and current density as the previous case. However, it can be stated that the increase in the Co content and the reduction in the grain size yielded deposits with higher mechanical properties at all current densities. Also, it should be noted that the effect of increase in hardness was more dominant when the difference between the grain sizes was large between deposits at the same current density. For example, at 8 A/dm2, the compositions of deposits were very close to each other. Yet, their grain sizes were so different that the small grain sized deposit exhibited higher hardness.

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4. Conclusion

The effects of Co ion concentration and solution pH were studied with a proper design of experiment. It was revealed that the enrichment of baths with Co+2 ions yielded deposits of higher Co. The amount of Co in the deposit enhanced hardness up to a certain point, then decreased again, obeying the solid-solution strengthening phenomenon. The increase in the pH of the solution caused a reduction in the grain size and slightly increased Co content. Both effects helped the increase in hardness as compared to deposits from bath 1 at lower pH.

References

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[2] G. Chatzipirpiridis, E. Avilla, O. Ergeneman, 50 (2014) 10 12.

[3] L. Xuewu, X. Yunhua, Q. Yi, and S. Lilong, Integr. Ferroelectr., 152 (2014), 144 151.

[4] L. Wang, Y. Gao, Q. Xue, H. Liu, and T. Xu, Appl. Surf. Sci., 242 (2005) 326 332.

[5] C. Hu, C. Tsay, and A. Bai, Electrochim. Acta, 48 (2003) 565 572.

[6] M. Saitou, S. Oshiro, and S. M. Asadul Hossain, J. Appl. Electrochem., 38 (2008) 309313.

[7] J. a. A. C. E. Inc., C. Orange, U. Stanko R. Brankovic Cullen College of Engineering, Modern Electroplating. 2010, Houston, Houston, Virginia,