Using Hull Cells for Silver Plating Problems

A. The Hull cell is basically a miniature plating tank where a test panel is used as the cathode and is placed at a diagonal to the anode so that it experiences very high current density at one end and very low current density at the other.

The Hull cell doesn't announce what to do, all it does is let you see what the plating looks like on a properly prepared substrate. The current density at the high current density end should be greater than the highest current density in your real plating tank and the current density at the low current density end should be lower than the lowest current density in your real plating tank so that you can see problems coming as they start to manifest themselves at one current extremity or the other.

From experience, you guess at what ingredients you need to add to get better plating in the Hull cell, and keep track of your additions, and after proving that some change worked, you simply scale up and add those ingredients to your plating tank in the same ratio.

AESF (www.nasf.org) has a very nice Illustrated Lecture on Hull Cell Testing, and references like The Canning Handbook will help you with the guesses about what to try based on your initial Hull cell result.

A. Perhaps I am wrong, but there are two other questions posted in the section from Taiwan in Thailand and they both relate to silver cyanide baths.

If I am wrong, I apologise ahead of time. I just wonder if Chris Owen was on the right track in his response in 26978, which asks about chloride contamination. It is possible that the chloride comes from the make-up water, but you should only use pure water, not tap water, when making up plating baths. 27119 talks about nitrate contamination. If you have nitrate contamination, you have a problem because virtually all nitrates are soluble so they cannot be precipitated out. The only other route may be to electrochemically reduce the nitrate, but this is very unpredictable. The reduction products can range between ammonia, nitrogen, nitrite, nitrosamine etc and the final product will depend on a wide range of operating parameters, including the type and structure of cathode being used.

There is now a suggestion the problem lies with carbonate. It is well known that cyanide will produce carbonates and these are not conducive to good cyanide plating, so this is quite possible. Your comments in this submission suggest there is an imbalance in the cyanide, so I would suggest you get a complete analysis of everything in the bath and tell us what it is. Also tell us whether the silver cyanide is a pure commercial grade or if someone has made it up on the cheap. Once we all know what you actually have, perhaps someone can advise you on how to get good silver plating.

You definitely win's this month's "read between the lines" award, Trevor, thanks!

I hadn't noticed it, but you are exactly correct that Tawan has posted 3 questions which seem to be related to the same issue, and his problem may be a need for a clearer understanding of his bath rather than any of the 3 problems he has consecutively thought he has. While it may be the need for removal of nitrates, it may be the need for removal of chlorides, or the need for removal of carbonates -- or none of the above :-)

A. You're probably right, Trevor. I could very well be on the wrong track. But, coincidentally, the 2 strangers that Tawan asked about, chloride and nitrate, just happen to to be the 2 that are involved in a AgNO3 to AgCl to AgCN makeup process. Tawan, what is the source of the suspected chloride and nitrate contamination?

Chris,

I think you could be on the right track with your suggestion that dodge-y silver salts are being used. I was only offering the water scenario as an alternative source. I am just wondering what Tawan is actually doing and what he has really got.

A. Hi Randy. You have to establish a workable process sequence before you order equipment. So if you can't get sample parts done by a supplier, you might try a Hull Cell setup. For example, sometimes you can't practically do silver plating without a silver strike. Sometimes you can't successfully do it without preceding it with copper plating. Lock down a workable process first!

Talk to a silver supplier about whether cast anodes or silver "popcorn" will prove most practical for you. You'll probably need anode bags and a small filter. Small molded 6-gallon polypropylene tanks are the least expensive part of the project.

For the benefit of other readers, we note that electroplating operations, even small ones, are regulated by the federal government under CFR433, with permitting authority usually passed down to your local POTW. You need to check if this is the category you will be in, or whether there is a special provision for jewelers, auto mechanics, etc., that covers your particular permitting situation. Remember that it's not only the ingestion of cyanide that can harm you; if you accidentally add acid to your cyanide tank or vice versa, you are building a hydrogen cyanide "gas chamber". Good luck.

Regards,

A. Hi Randy. That "make-up" info is in the Metal Finishing Guidebook .
but I think you've somewhat misunderstood what I was trying to say: I'm suggesting that you contact a supplier so they can help you figure out what you need, not so you can instruct them on what you need :-)

Regards,

A. I would not attempt to try this without a silver strike. Also, the solutions need to be well filtered. Any particles can result in silver treeing.

A. Hi Cody. Lowenheim says: "Free cyanide ... increases cathode polarization and thereby increases the throwing power".

Regards,