Letter 19028

Nickel/cobalt [Virginia] 

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What is the difference between nickel and cobalt in plating?

I mean:

1. When to use which?
2. Adhesion to metal substrate (carbon steel)?
3. Corrosion resistance?
4. Cost of application?

My regards,

Hamad Ababutain
- Blacksburg, VA, USA

Nickel plating is tried and tested. It is easy to do and most supply houses have a good range of nickel solutions to meet most peoples needs. The chemistry is well known. Cobalt is more expensive as a metal and its baths are not nearly as well documented. Cobalt salts are much more difficult to get and cost a lot more. Despite what is claimed in the literature, many of the additives used in nickel baths do NOT work in cobalt baths, so you may well have to do a fair bit of research to get the properties you desire. Generally cobalt deposits are much more tensile than nickel, so adhesion can be a problem. This is in part due to a change in crystal structure of cobalt metal. I do not really know the differences in corrosion resistance as that could well depend on the operating environment. Nickel is a sensitising metal and some of its salts are considered carcinogens. There are claims that cobalt is also a sensitising metal but there is little sustainable fundemental evidence for this. Almost all the sensitising tests (that I know of) have been done with cobalt salts that could contain nickel salts and still be within spec; the nickel levels are greater than that required to cause an allergic response in someone susceptable to nickel; funnily enough, where these tests have been done, the correlation betwen nickel and cobalt sensivity has been approaching 100%! Interestingly cobalt is also an essential trace element for a healthy body.

Trevor Crichton R&D practical scientist - UK

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