Where the
world gathers for metal finishing
Q&As since 1989


-----

Uniformity in plating Co-P alloy

2004

Hi,

I am working on my thesis topic. I have to plate Co-P alloy from a bath consisting of Cobalt sulphate, cobalt chloride and Sodium Hypophosphite. The pH of the solution is about 4. I plate at room temperature and no agitation is required for my plating. The thickness I would like to about is in the order of 500nm and might go up to 2 microns. The current density ranges from 2.5mA/cm^2- 10mA/cm^2. The only problem with my plating is non-uniformity. I have read several queries about non- uniformity in this forum , some suggested to use levelers/ brighteners would these be the solution to my problem. If so what kind of levelers should I use , will they change the magnetic properties of my alloy? What will I have to do get a uniform plating , I use a potentiostat for constant current. I would appreciate if any one could help me in finding a solution to this problem.

Thanx and regards,

Fabbi
student - Philadelphia, Pennsylvania, USA

---

2004

I don't know if there are any levelers/brighteners for Co-P Plating, you can possibly try levelers that are used in Ni-P Plating, as Ni and Co are behaving very similar you may have success.

Another possibility to get uniformer coating is the use of pulse plating, but you would need an pulse amplifier for this and it will cost you quite much time for trials with the different pulses.

The easiest way to get a uniform thickness would be to set up a electroless cobalt bath, similar to a electroless nickel ( don't know if this actually is possible, but I see no reason why not), which will give you also an Co-P alloy, amount of P can be steered with pH of the solution.

Good luck,

Marcus Hahn
- Lucerne, Switzerland

---

I agree with Marcus Hahn, to get uniform Co-P coverage, your best bet is to use electroless cobalt and a bath formulation similar to those used for Ni-P. You will then be able to alter the amount of P in the deposit. Additives used for nickel plating do not necessarily work with cobalt and in fact, ones like saccharin definitely do not behave in the same way, so transposing nickel technology to cobalt is not a dead certainty. In general, electrolytic deposition is dependent on the local current density, the higher the local CD, the thicker the coating. The current density distribution can be modified by using screens, robbers or variable flow technologies. Pulse plating may be of some use in providing a better grain size and perhaps offer a modest improvement to thickness distribution if you get the forward and reverse pulse ratios correct, but I do not think that will totally solve your problems.

Trevor Crichton
R&D practical scientist
Chesham, Bucks, UK
2004