Silver Flashing vs Tin Plating of Switchgear Bus Bar

• Thread 38265 "Why must Copper Bus Bar be Tin Plated?"

? Hi John. Khozem previously suggested a fix for this problem. Did you see it and have a comment on it? Do you do the silver plating or are you purchasing plated product? Thanks!

Regards,

Commenting: Generally, the silver is a far better conductor. Silver is the best single element conductor under normal conditions. There are others that are better, for example, superconductors that are cooled by liquid nitrogen, however, those are extreme exceptions.

In practice, silver is considered better, but tinned is more often used because of cost and durability. A special condition is to also consider the environment it will be used in. For example, in chemical and oil plants, silver must absolutely NOT be used because it can react with the sulfur [affil links] in hydrogen sulfide and can create whiskers that will then cause phase to ground or phase to phase faults.

A. Dear Mr. Steve,

The coating of Silver in this case appears to have been too thin, I presume its Copper that we are seeing below the busbar. Whatever the Silver thickness, you must use an anti-tarnish over Silver. The minimum should be 3 to 6 microns.

The prediction about Silver vs. tin corrosion resistance is like this: You can probably afford to put on 10 microns of tin for every micron of Silver at the same cost. 10 microns of Tin will outdo 1 micron of Silver hands down. Fixed Busbars like yours are traditionally only Tin plated + anti tarnished; only moving parts that are likely to arc are generally silver plated.

Q. It appears we are going to move back to using tin plated copper bus bars for our switchgear. The next question is what is the "standard" or recommended plating/flashing thickness of tin? What is it for silver?

A. I have plated busbars to a spec of 5 - 12 microns of Tin. You have to determine your plating thickness requirement based upon "Fitness of use" for your current / corrosion requirements. I do not believe that anyone else's spec may work for you!

A. Have you ensured that your Tin plating bath was always under 27 °C? You may have lost some of your stannous.

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Ed. note:
· Sujoy, We appended your questions to a thread where Khozem V suggested 5-12 µm is typical.
· Hussam, Khozem V also suggested that 3-6 µm of silver should be the minimum. But postings here are opinions not legal requirements, and they can get old & dated; they are not a substitute for a 'code check' on any project.

A. Hi Jim. Please see Khozem V's first response -- I found it very informative.

Regards,

A. I'm not in the plating business, but rather in the electrical design and installation side of the industry.
What I have noticed is that in a high H2S (Hydrogen sulfide) environment copper and silver have a high oxidation rate in a short period of time.

I have had generators fail because the silver solder holding the winding retaining bars oxidized completely away allowing the windings to flex under load and short.

This was a gas plant with 200,000 PPM H2S in the inlet stream.

Tin is not affected.

Just sharing.

A. Hi. The chemical to be applied is tin electroplating solution, but it's not a chemical that is wiped on or brushed on like a paint; rather, it's a manufacturing process where the bus bar is alkaline cleaned by immersion, rinsed, acid activated by immersion, rinsed, electroplated by application of DC electricity while the parts are immersed in a solution of stannous sulfate or potassium stannate (for example -- there are other choices) with proprietary addition agents, rinsed, and immersed in a bath of tarnish inhibitor.

The normal way this is done is by sending the parts to a plating jobshop. That is not to say that you can't do it in-house; but that it requires a knowledge base, a significant capital investment, and probably operating and waste generation permits. Good luck.

Regards,

Q. Dear sir,
What is the reason for "acid activated by immersion" for the bus bar? And does the bath consist of sulfuric acid and distilled water? If yes, can you provide me with the appropriate concentrations?
Regards.

---

"Electroplating Engineering Handbook"
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April 2022

A. Hi cousin Mahmoud. Chapter 3G of the Electroplating Engineering Handbook explains in detail the reasons for oxide removal and activation, and gives the suggested procedures for copper. But the short answer is that most metals including copper quickly oxidize or tarnish; so for proper plating it is necessary to remove the oxidation with acid in order to electroplate directly onto raw metal, not on top of tarnish. That reference suggests 5-10% sulfuric acid for copper.

Luck & Regards!

Readers: Please introduce your situation that motivates you to ask the question; that makes it informative & interesting, and other readers are usually enthusiastic problem solvers. Silence in response to an abstract question may be their polite way of saying your question is boring & highly repetitive, and they're not librarians so you should look it up yourself :-)

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

A. Hi Hesam. The theory they are explaining is that aluminum corrodes easily and doesn't comprise a good contact surface, so it should be copper plated and possibly silver plated before bolting. But a good answer unfortunately requires experience in the particular field you are inquiring about; for example ...

Decades ago the plating equipment supply company I was employed by made "workbars" ("flightbars") for plating & anodizing by simply bolting copper plates to extruded aluminum structural members after wire brushing and applying an anti-oxidation compound, and they functioned well enough. But that doesn't necessarily mean the method will function well enough in a different application. Good luck.

Regards,

A. Hi Ehab. Offhand, I don't see any reason to suspect that generic silver polish, used to remove tarnish from household flatware and silver would not work fine. Good luck.

Regards,

A. Hi Raveek. I have not tried it on busbar, but contact with aluminum foil in a warm or hot solution made conductive with TSP or similar chemicals is used on jewelry and silverware to cause a reduction process which converts the tarnished silver back to silver metal. Please see thread/topic 4785 for the theory and details.

I don't see any reason this couldn't work on busbar, although the details might take a bit of effort. Maybe get an aluminum bristle scrub brush and a bucket of strong hot TSP solution and see what the scrubbing does for you? Good luck.

Regards,

A. Hi Mohamed. As someone from the electroplating field, with some plating and corrosion knowledge, I don't personally foresee a particular reason to anticipate that such a joint would be problematical. But in most specialized fields we are not limited to theoretical predictions, but we have standards and design codes that have been developed from decades of experience around the world. I don't have any experience in switchgear application, so I would certainly suggest that you investigate your company and industry standards before deciding that it is okay. This sounds like it would be a pretty common question that switchgear manufacturers have encountered hundreds of times. Good luck.

Regards,

A. Irshad, what is the substrate metal (under the silver). If it is copper then tarnishing is expected. This is especially true in less than normal atmospheric conditions and higher temperatures. Please let us know the substrate or underplate.

? Hi Brandon. Sorry but I can't picture what you are describing. A photo would be a great help. And if they are copper plated before silver plating, can I assume they are made of aluminum? When you break a blister what do you see on the substrate and on the underside of the blister? Thanks.

Regards,

A. Following the rinse after plating, insert a 5% Sulfuric Acid dip to neutralize the cyanide, then continue.

A. Hi Irshad. In addition to the improvement suggested by Robert Probert, have you investigated your packaging. Silver is very vulnerable to tarnish in the presence of sulfides, and paper & cardboard can easily have lots of it if you don't pay attention.

Luck & Regards,