Electroless Co-B Deposition Using Dimethylamine Borane as Reducing Agent in the Presence of Different Amines

The therm “electroless plating” was originally adopted by A. Brenner and G. Riddell [1, 2] to describe a method of plating metal substrates with cobalt or nickel alloys without the benefit of an external source of electric current [3].

Electroless metal deposition processes are recognized to be electrochemical by nature and to result from the coupling of two partial electrochemical reactions – the cathodic reduction of metal ions and the anodic oxidation of the reducing agent [4]. Electroless cobalt deposition process is consisting of anodic oxidation of proper reducing agent and cathodic reduction of Co(II) ions occurring simultaneously on the same cobalt surface; the electrons appeared in anodic oxidation of reducing agent are transferred  through the metallic surface to adsorbed Co(II) ions, and the autocatalytic deposition of Co takes place. It is noteworthy the fact, that pure cobalt coatings are obtained seldom. In most cases cobalt alloys containing boron or/and phosphorus are formed – phosphorus and boron are originated from reducing agents.

When comparing the action of amines on duration of induction period it is seen that in the case of amines with the shorter chain (propylenediamine (prop), diethylenetriamine (dien), triethylenetetraamine (trien)) the induction period is very short and the mixed potential of electrode becomes most negative values also very rapidly. The situation changes dramatically, when using ammines with longer chains and containing more amine-groups (pentaethylenehexaamine (pnten), tetraethylenepentaamine (ttren) ) – the induction period becomes longer: ca. 3 min in the case of pentaethylenehexaamine and more than 6 minutes using tetraethylenepentaamine. The prolongation of induction period is connected with changes in electrode mixed potential values: electroless cobalt deposition begins only when negative enough value of mixed potential is established. It can be noted, that after leveling of quasy-stationary plating conditions, the electroless cobalt deposition rate is practically the same for all amines investigated.

Therefore, on the basis of EQCM experiments, we can conclude that the optimum operating conditions (rational low induction period, high enough plating rate and moderate temperature) are as follows: 0.05 M CoSO₄, 0.2 M DMAB, amine (preferably diethylenetriamine), pH ca. 6 and temperature of 50 °C.

 Acknowledgement

The work was carried out within the project VP1-3.1-ŠMM-08-K-01-009 that is partly supported by the National Programme “An Improvement of the Skills of Researchers” launched by the Lithuanian Ministry of Education and Science.

References

 1. A. Brenner and G. Riddell, J. Res. Nat. Bur. Stand., 37, 31 (1946).

2. A. Brenner and G. Riddell, J. Res. Nat. Bur. Stand., 39, 385 (1947).

3. Electroless Plating: Fundamentals and Applications. Eds. G. O. Mallory and J. B.  Hajdu. American Electroplaters and Surface Finishers Society, Orlando (1990).

4. A. Vaskelis, A. Jagminiene, I. Stankevicienė, E. Norkus. “Electroless deposition of cobalt alloys”. Unites States Patent 7,794,530 B2 (2010).