Tin Plating Problems: black spots in random locations

A. Hello K. "Tin fretting" is caused by mechanical jostling of parts together and results in black spots. Does this happen before any such jostling or after it?

A. Dear Sir:

What you have is plating bath contamination.

A. How thick is your nickel undercoat? If it is less than 5 microns, it could be porous, thus exposing the brass substrate to any post-nickel treatments. Are you using an acid or alkali tin plating bath? This could make a difference. Do you melt the tin to brighten it or deliver it as plated? If you are heat melting it and you have porous nickel, you could well be forming a copper-tin intermetallic in the pores and this will be a dark colour. I presume you put the nickel down as a barrier layer to prevent migration of the tin into the brass and this is enhanced by elevated temperatures, so is there any connection here? Pretreatments to the nickel could cause attack on the brass. Does your process have any sulfides or sulphites that could attack either the copper or the nickel to produce dark sulfides? I hope these suggestions may stimulate some thought for you.

A. Heating tinplate is not a good idea once it has been flow melted. Tin melts at 232 °C; you are very close to it melting. Furthermore, tin diffuses into the base steel in tinplate to form a black alloy (FeSn2). Whilst there is some free tin available, the only alloy formed is FeSn2, but once all the tin reservoir has been used, you can get a range of alloys (FeSn, Fe2Sn3, Fe3Sn2). The rate of diffusion is relatively slow, but above its melting point, the tin goes in much faster. The rate of diffusion follows Fick's 3rd law of diffusion. I reckon you are using a very thin layer of tin on the base steel and allowing it to remain at this high temperature for too long. By the way, this alloying process is very important in generating the corrosion resistance that is normally associated with tinplate. The yellowing of the tin is probably due to oxidation of the tin.