Research Article
A new insight into promotion action of Co2+ in Ni-diamond composite electrodeposition

https://doi.org/10.1016/j.jmst.2019.03.019Get rights and content

Highlights

  • Addition of Co2+ in Watt’s bath remarkably increased the content of particles in deposits for Ni-diamond system.

  • The cathodic zero potential has a significant effect on the entry of particles into coatings.

  • Addition of Co2+ can not only increase Faraday resistance but also reduce reduction rate of metallic cations.

  • Physical model of diamond particles deposition state before and after the addition of Co2+ was established.

Abstract

A new insight into the promotion action of Co2+ on both particle and metal deposition in Ni-diamond composite electrodeposition system was analyzed according to electrochemical measurements. The results showed that the addition of Co2+ made particles content in deposits increased remarkably. The change of particles content in deposits was related inversely to the change of cathodic zero potential with the increase of the concentration of cobalt sulfate. Zero charge potential of cathode was shifted to much more negative region. The negative shift of the zero potential, combining with positive shift of the zeta potential, increased the electrostatic force between the particle-adsorbed metallic cations and the cathode. It not only benefits to the transportation of particles in solution towards cathode, but also shortens their residence time on cathodic surface. Meanwhile, entry of particles is also promoted. For metals deposition, reduction resistance of metallic cations rises greatly and deposition current at cathodic potentiodynamic polarization decreases after cobalt sulfate has been added into electrolyte. These factors are favorable for increasing particles content in deposits. In addition, physical model of diamond particles deposition state before and after the addition of Co2+ has been discussed.

Introduction

Composite electrodeposition has been fascinated in the last decades because the properties could be adapted by controlling the fraction of incorporated particles in coating according to design [[1], [2], [3]]. Composite coatings are also endowed with new functions in order to satisfy different demands. For example, many researches have focused on the excellent tribological properties [4] and corrosion resistance [5] of metal-diamond composite coatings. Recent research has proved that thermal conductivity of nickel-diamond coating can be much higher than that of pure nickel coating [6].

It is well known that the fraction of incorporate particle in composite coating plays an essential role in improving properties of coating [[1], [2], [3],[7], [8], [9], [10], [11], [12]]. Many methods, such as surfactant, magnetic field and double-pulse technique and so on, were studied in order to favor particle entries into deposits [10,[8], [9], [10], [11], [12]]. Addition of Co2+ cations in Watt’s bath has been suggested to enhance particle incorporation in composite coatings several decades ago [4,11,[13], [14], [15], [16], [17]]. It was explained that stronger adsorption of Co2+ on particle surface than that of Ni2+ would change zeta potential of particles, which increased positive charge of the surface and enhanced the electrostatic attractive forces between the charged particles and cathode surface [4,11,17]. However, Wu et al. [14,16] indicated that the zeta potentials kept more positive with increasing concentration ratio of [Co2+] to ([Ni2+]+[Co2+]), i.e., [Co2+]/([Ni2+]+[Co2+]), in Ni-Co-Al2O3 composite system. Consequently, the content of particles in deposits increased with the increasing concentration of Co2+ and held up to 0.4 ratio of [Co2+]/([Ni2+]+[Co2+]) with further marginal increment at higher Co2+ concentrations, indicating that the change of particles content in deposits was not just determined by the change of zeta potential.

It is obvious that cathodic potential also has a significant effect on the electrostatic attraction between the particles and the cathode surface. A sharp positive shift in the cathode potential can not only result in a decrease of electrostatic attraction between positively charged particles and the cathode surface, but also affect the motions of particles in solution and their immobilization on cathode surface. However, there has been no report about the influences from the addition of cobalt sulfate on the change of cathodic potential.

In this work, the promotion action of cobalt sulfate in Ni-diamond composite electrodeposition system was verified by various technical experiments. Deposition state of diamond particles was discussed based on the changes of the cathodic zero potential and the zeta potential data that are caused by the addition of cobalt sulfate. The effect of the concentration of cobalt sulfate on the metal deposition was discussed according to Faraday resistance and cathodic potentiodynamic polarization. Physical model of diamond particles deposition state before and after the addition of Co2+ was established. And so the promotion actions of Co2+ in Ni-diamond composite electrodeposition were studied from a new perspective.

Section snippets

Deposition of Ni-diamond coatings on carbon steel sheet

Ni-diamond coatings were electrodeposited on carbon steel sheets (10 × 10 × 0.2 mm) with experimental conditions as present in Table 1. Bath composition for composite coatings was similar to our previous work [18]. Prior to coating, the steel sheets were polished with emery paper up to #2000 grade and then washed in a degreasing bath containing 30 g/L Na2CO3, 30 g/L Na3PO4 and 40 g/L NaOH at 70 °C for 3 min. Finally, the sheets were activated in dilute sulfuric acid at room temperature for

Cobalt sulfate vs. Weight percentage of diamond particles in composite

Fig. 1 displays the relationship between the content of diamond particles in the deposits and the concentration of cobalt sulfate in the electrolyte. It is observed that the weight percentage of diamond particles in the deposits first increased from 4.6 wt.% to 8.2 wt.% and then reduced to 7.8 wt.% as the concentration of CoSO4 increased from 0 g/L to 1.25 g/L and further to 2.5 g/L. Finally it tended to remain a steady value of 6.4 wt.% when the concentration of CoSO4 was increased to 5 g/L.

Conclusions

In summary, the addition of cobalt sulfate in Watt’s bath remarkably increased the content of diamond particles in deposits for Ni-diamond system. It also shifts cathodic zero potential to more negative regions. It is observed that the change of the content of particle in deposits is inversely related to the change of the cathodic zero potential. The negative shift of the zero potential, combining with the positive shift of the zeta potential, greatly raises the electrostatic attractive force

Acknowledgement

This study was supported by the State Grid Scientific and Technological Research Program of China (Grant No. 5211DS17001X).

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