Achieving a specified hardness in nickel plating

A. Yes, it is possible to control, to some extent, the hardness of electroplated nickel. Among the parameters which influence hardness are bath composition, pH, temperature, current density, agitation, contaminants, and the use of metallic and non-metallic addition agents. All of these things interact simultaneously, and it's a book-length subject.

I'd suggest you do a Google search and also contact suppliers of nickel plating processes.

A. Some nickel bath compositions are easier to control but are more expensive (sulfamate). Some are purposively hard as plated such as the ammonium sulphate-low nickel content bath. Generally, lower current densities and metal concentration, higher pH (above 5) and low bath temperatures favor higher hardness. Get a copy of a good plating book as these are not linear functions.

A. The hardness of a nickel deposit is controlled by the use of hardeners. The most common ones for sulphamate nickel are saccharin and NTS, although there are numerous other ones. Personally I prefer saccharin because it is easier to use and control. You can also use cobalt as an additive to harden nickel. The simplest way to control hardness is to control the amount of hardener you add to the bath. There are some splendid publications by NiDI on the control of nickel plating baths, so I suggest you look at their website for more details.

A. All of the above answers still hold true. You have to look at variables that you control; current density, pH, impurities, solution concentration, organics, temperature, addition of wetting agents, and hardeners like saccharin. May want to try a simple carbon treatment first and see how that does. Don't expect to pull out the hardeners though as they don't pull out easily. I would also get the literature that was suggested above by the Nickel Development Institute and keep it on file. Lots of great information in them. I know I review mine every six months or so.

A. If you are not using a hardener, then you very probably have a trash organic problem. Do some jar testing with a hull cell to see if it will take 2 or 3 carbon treatments to solve the problem. Also, if you are using SNAP A/M or the equivalent, it is easy to add too much which will affect stress and hardness.
How are you doing on your stress tests?

A. Dear Guy Boucher,

All reactions so far are right, there is still some more:

Probably your problem is the fact that, due to hydrolysis, the nickel sulphamate slowly decomposes into sulphate.
If the sulphate levels rise, inevitably the hardness of the deposited Ni rises too.
Hydrolysis is enhanced by lower pH, higher temperature and lower [Ni].
Probably the best thing to do for a constant results is renew some of the electrolyte on a regular base as well periodically and reflect this in the cost of the product.

Success,

Harry.

A. That's good advice, Guillermo -- no plating is more difficult than the plating on continuous caster molds. But based on some work he and I did together in that industry, I believe that finishing.com's supporting advertiser, Robert Probert, is one of those few who is capable of supervising such a development project.

Regards,