Copper-coated chassis -- 'spotting' remedy?

[Al (astral highway)]

This post was inspired by one yesterday on the removal of rust/staining from metal.

I have a chassis here that is copper coated, in some places with beryllium copper and in others with a different grade containing a small amount of zinc.

I don't know what the substrate is, but it is likely to be steel. What's happened is that the steel has slowly been attacked by moisture over the years and spotting and staining of the copper surface has resulted.

Overall, the chassis is in very good condition, but in some places, I need to 'contain' and seal the rust to prevent mositure attacking the metal in future. This applies especially where the front panel meets the chassis, a moisture trap.

What would you do? I was thinking I could just seal this exposed surface with shellac or something.

[Skywave]

What immediately springs to mind is to carefully coat the iron-rusted surface areas with something like "Ku-Rust" * - obtainable from car accessory / parts dealers. This is a chemical compound that reacts with the rust (ferric oxide & ferric hydroxide, brown in colour) and converts it to a stable compound of iron, (mainly ferrous oxide and ferrous hydroxide, black in colour). [Works wonders on old cars ]

* Or something similar.

As for then restoring the copper coating / plating - well, that's not so easy! I suppose small areas of corrosion could be masked with carefully applied copper paint. Theoretically, it should be possible to get the plate entirely re-plated (electro-plating; copper sulphate solution, etc.) - but the entire plate would have to be stripped down to the iron substrate to do this - and it hardly seems worth the effort / cost.

For overall protection in future, a spraying with a clear acetate varnish should do the job: something like "Letracote" springs to mind (if this is still available).

Let's see what other readers come up with - this is an interesting Q., since it has applications over a wide range of engineering / maintenance issues, not limited to radio & electronics.

Al / Skywave.

[Al (astral highway)]

Originally Posted by Skywave

What immediately springs to mind is to carefully coat the iron-rusted surface areas with something like "Ku-Rust" * - obtainable from car accessory / parts dealers. This is a chemical compound that reacts with the rust (ferric oxide & ferric hydroxide, brown in colour) and converts it to a stable compound of iron, (mainly ferrous oxide and ferrous hydroxide, black in colour). [Works wonders on old cars ]

As for then restoring the copper coating / plating - well, that's not so easy! I suppose small areas of corrosion could be masked with carefully applied copper paint. Theoretically, it should be possible to get the plate entirely re-plated (electro-plating; copper sulphate solution, etc.) - but the entire plate would have to be stripped down to the iron substrate to do this - and it hardly seems worth the effort / cost.



Al / Skywave.

Interesting idea, Al. Do you know what the ferric oxide/ferric hydroxide will do to the copper. I don't know any chemistry and was just wondering if I'd end up with an unwanted (equally unaesthetic) compound of copper as a by-product?

[Skywave]

That's a good Q. - and, of course, highly relevant!

Off-hand: no. But I'm sure that I have some of that stuff (or "Jenolite" - much the same) somewhere. If I can dig it out, I'll try the experiment - and report back here.

Al / Skywave

[Mike Phelan]

Kurust is based on phosphoric acid, as is Jenolite. There are other rust-removing componds based on acetic acid which are kinder to the metal.
Another method that does not leave a black coating is electrolysis using a sodium carbonate solution.

[Al (astral highway)]

Originally Posted by Skywave

That's a good Q. - and, of course, highly relevant!

Off-hand: no. But I'm sure that I have some of that stuff (or "Jenolite" - much the same) somewhere. If I can dig it out, I'll try the experiment - and report back here.

Al / Skywave

Thanks Al, look forward to hearing the results!

[Al (astral highway)]

Originally Posted by Mike Phelan

Another method that does not leave a black coating is electrolysis using a sodium carbonate solution.

This sounds like the way to go. Thanks Mike, I'll try it out.

[Skywave]

Astral -
Report on 'KuRust and copper experiment, as promised.

I used a piece of copper-laminate board. I made two test samples. In one place, I left the board with its slightly off-coloured oxide coating intact. In another place I removed this oxide with metal polish - and then carefully removed all traces of this polish. I then applied 'KuRust' to both samples. (I also applied some to a piece of surface-rusted iron - just as a control sample).

Left these for three hours - then inspected the result.

In neither case were there any signs of any vigourous chemical reaction against the copper: it was not attacked. All that remained was a slight discolouring of the copper - in effect, another (thin) layer of oxidisation. The reaction on the iron was as expected.
In your application, I suspect that this would not be objectionable, but for something that required a 'visibly-seen' decorative finish, it probably wouldn't be.

'KuRust'; phosphoric acid. Of course. It slipped my memory when I sent in my last post, which was early in the morning [] (Thanks Mike).

Al / Skywave.

[Skywave]

Originally Posted by Mike Phelan

Another method that does not leave a black coating is electrolysis using a sodium carbonate solution.

That is an interesting article.

One thought: the author refers to using a battery-charger, but he does not provide any indication of the current that one would expect to flow. Is it the case that the magnitude of this current (for the given voltage) will be automatically determined by the size of the electrodes and the concentration of the electrolyte? Or should some external current-limiting device be employed?

Your thoughts would be appreciated.

Al / Skywave.

[Al (astral highway)]

Originally Posted by Skywave

That is an interesting article.

One thought: the author refers to using a battery-charger, but he does not provide any indication of the current that one would expect to flow. Is it the case that the magnitude of this current (for the given voltage) will be automatically determined by the size of the electrodes and the concentration of the electrolyte?



Al / Skywave.

Hi Al, I ran some tests earlier.

In my rig, the electrolyte concentration and electrode area enabled a current of close to 2 Amps to flow @ just 12V, and with the electrolyte at about 50 degrees centigrade.

I learnt that it is possible to get quite a surprising shock from the live electrolyte even at 12V. I won't be using a higher voltage than this.

I've reduced the spotting on the worst-affected parts, but the results aren't attractive and am now thinking of electroplating some of the assemblies on the chassis, if not the whole thing.

[Mike Phelan]

I forgot about this stuff which I have used with great success - it does not leave any residue at all. They also do another product, Exit Rust, that is used prior to painting, and is based on tannic acid.

[Al (astral highway)]

That's very interesting, Mike, thanks. I will be invesigating 'exit rust' which seems to be a canny composition with, as a bonus, no environmental fall out.

[Ian B]

If I remember correctly from my school chemistry lessons (long ago) iron or steel can be given a thin coating of copper by immersing it in copper sulphate solution. I can't remember the time required, but I suspect not very long. I believe copper coated mild steel welding rods were produced by effectively this method.

The copper film can only be a molecule or so thick, so if you need a polished surface, you have to polish the steel before "plating" with copper.

Ian Blackbourn

[Sean Williams]

If you se a copper sulphate, and a copper electrode the result is a much thicker deposit - something i have done with the kids here - turning a 10p piece into a 2 p piece....

Professional plating isnt that expensive though, and you do get excellent results

[Al (astral highway)]

Originally Posted by Sean Williams

If you se a copper sulphate, and a copper electrode the result is a much thicker deposit - something i have done with the kids here - turning a 10p piece into a 2 p piece....

Professional plating isnt that expensive though, and you do get excellent results

Sean, I do have a superb copper 'anode' ready here, made from a chunk of busbar, as well as access to CUSO4.

However, if you can recommend any platers who can electroplate CU onto steel, I'd ben interested, as the finish is likely to be far superior!

Thanks,

Al