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Tin Plating Problems: black spots in random locations

Q. We are tin plating brass sheet with a nickel undercoat. These parts go through a DI water wash prior to shipment. We have seen black spots appearing in random locations on the part. Any ideas as to what this may be? Thanks for any responses.

K Link
- Little Rock, Arkansas

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?

Ted Mooney,
Ted Mooney, P.E.
Striving to live Aloha - Pine Beach, New Jersey

These spots seem to form in the presence of water with no motion.

K Link [returning]
- Arkansas

A. Dear Sir:

What you have is plating bath contamination.

Joel Garcia
- McAllen Texas.

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.

trevor crichton
Trevor Crichton
R&D practical scientist
Chesham, Bucks, UK

Tin plating turns yellow with time, and blackens when heated

Q. Dear Sir,

We have had problem at the surface of a tin plate. When heat at about 200 °C is introduced to the surface, it will turn black and not shiny. Besides, it also turns yellow by just leaving it for a few months. I would like to know what are the possibilities that cause this to happen? Thanks in advance.

Best regards,

Lau Lai Kuan
- Ipoh, Perak, Malaysia

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.

trevor crichton
Trevor Crichton
R&D practical scientist
Chesham, Bucks, UK

Q. We are exporter of Engine Components manufactured from Steel with one side coating of copper alloy i.e. Cu 80%, Sn 10% , Pb 10%. After plating the component our product gets tarnished from inside i.e. alloy side. We need a very very thin coating which is drying, colourless, and retain the tin plating finish for about 1 year. We don't want to apply any oil which gives wet finish. We need a product which has the properties of repelling the water.

Neeraj Maheshwari
Manufacturer of Auto Parts - Agra, Uttar Pradesh, India

A. Hi Neeraj. There are benzotriazole [on eBay or Amazon] based tarnish inhibitors for copper, and chromate based tarnish inhibitors for tin, which would be almost immeasurably thin.

One well known brand name is Tarniban from Technic.

Unfortunately, your problem is not quite clear to me. This object is first plated on one side with a Cu-Sn-Pb alloy? Then both sides are tin plated and the the Cu-Sn-Pb alloy side starts tarnishing? Or is just one side (the inside?) plated at all and you are unconcerned with the bare steel side?

Ted Mooney,
Ted Mooney, P.E.
Striving to live Aloha - Pine Beach, New Jersey

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