Tin Cobalt plating problems Q&A

A. Dear Mr Yiu,

I think you are referring to tin-cobalt plating done over Nickel plating to give a near chrome like look. It is not a replacement for Nickel. You should contact your nearest Enthone dealer for the chemistry. Regards

A. Nickel and cobalt are very nearly the same. Cobalt is slightly harder, slightly more magnetic, quite a bit whiter, same atomic weight and valence and etc. Any bath that you can find for bright nickel, you can substitute the same amount of a cobalt salt for as in cobalt sulphate for nickel sulphate.

Cobalt is quite a bit more expensive at the current time. Cobalt can be used in jewelry, avoiding the frequent prohibition of "no nickel". It is also unregulated, the last I knew, by EPA.

A. Well, I could not find cobalt-tin but I did find nickel-tin and they will be close. Since it is a stannous bath, you certainly do not want to use air. My guess is that you are forming some stannic tin which is precipitating out giving you shelf roughness. My first try would be to increase filtration which will also give you more fluid agitation.

A. Over this side of the world, SnCo alloy plating has been used as a replacement for Hex Chrome. It looks similar to chrome but not exactly like chrome. The deposit is very thin (about 0.3 micron). I may be missing something but why do you need such a thick deposit? Air agitation is a definite no-no as suggested by Mr.Watts.

A. Tin-cobalt alloy plating is quite unusual. I know the International Tin Research Institute in Uxbridge (UK) did some work some years ago with tin-nickel, but they used an alkaline bath and could only get very thin decorative deposits. If the tried to get thick ones, the deposit became very brittle and flakey.

However, none of this should deter you from trying to develop a tin-cobalt system. I would suggest you look at high tin and low cobalt compositions as one of the major problems with cobalt deposits is the co-deposition of the two phases. This leads to very high stress and poor adherence. It may be possible to use the ITRI basic system but replace the nickel salts with cobalt. There are numerous cobalt baths, based on the well known nickel formulations, that claim to deposit cobalt, but my experience is not as positive as those claimed! The baths tend to be inefficient and do not respond to additives as one would expect. Furthermore, there is the big problem with stress.

As far as dendritic growth is concerned, I would suggest you look at different levelers and brighteners. You may also find benefit from using low current densities and possibly temperatures about 40 °C. However, all this depends on your bath formulation, which you do not give.

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Ed. note: Mr. Chen and readers may wish to give a listen to "The Lost Art of Tin-Nickel Plating".

A. What bath are you using? Stannous is normally reserved for the Sn(II) cation whilst stannate is Sn (IV) as an anion - tin being amphoteric and all that.

However, the most common reason for tin being oxidised is because air is getting into the system -- so don't use air agitation.

A better answer can be given if we have your operating details.

A. Are you sure you don't mean that the tin is being oxidized to the stannic state? If this is what you are referring to then the most common answer is that some of this type of reaction is normal. If it is excessive it is generally due to air agitation or introducing air due to leaking seals in filter systems or finally too high an rpm for a barrel. If you are using a proprietary system (I presume) your vendor should advise you as to whether they have an additive which is compatible to the process you are utilizing which will reduce oxidation and/or a treatment additive to precipitate out the stannic tin.