Chrome stripping
 

I have moved this thread (hope this is OK mods) as it was originally part of a post regarding electroplaters in the 'wanted' section of this forum.

The first quote is a very helpful post by a member which is an excellent solution...IF... I can remove the remaining chrome plate.

I have to say, after compound polishing the alloy parts, they really do look very good. In fact, I would definitely settle for this if I could remove the remaining chrome.

Does anyone have any experience with chrome removal and is it a possible DIY job?

Thanks, Nick

Re: chrome stripping
 

I may be telling you something obvious but for the record I think you will have to remove the chrome chemically. Chrome is considerably harder than nails and seems almost impossible to remove abrasively. I spent half an hour with some emery trying once. I managed to scratch the surface but not much more :(.

Cheers,

GJ

Re: chrome stripping
 

I don't have any experience of removing it, but I do know a little about chrome plating.

Chromium itself is rather poor at being a metal - it's brittle, and it's porous. So it is only applied as a very thin layer (which is very hard, as GrimJosef says).

Because of it's thinness, the underlying metal shows through. So, to help keep things nice and shiny, the chromium is applied on top of an intermediate thicker nickel plate. This also helps with corrosion resistance.

Nickel, however, does not adhere very well to many base metals. So quite often, an initial layer of copper is often plated first (which adheres to most metals).

Thus, the plating is likely to be a 3-layer sandwich.

As for removal, rather than a chemical method, it might be worth trying electrolysis. Try water, with something added to make it conductive (washing soda is fairly benign), attach your item to the positive lead from a low-voltage DC power source, and take the negative lead to a copper plate immersed in the bath. You'll have to turn your item around every few minutes as metal removal will take place quicker nearer the cathode plate. And DEFINITELY try this out on a piece of scrap first!

Re: chrome stripping
 

You'll also have to watch out if your workpiece has sharp corners. The electric field enhancement there can cause the electrolysis to go several times faster than on the flat parts. So you may find that the plating on the corners etches away first at which point the base metal may start to be attacked. If this looks like it's a problem you could try painting something non-conductive (nail varnish ?) over the corners to stop the electrolysis. Of course the same effect would have occurred in reverse during the plating process. So the chrome/nickel/copper may be thicker on the corners to start with which will counteract the problem to some extent.

Cheers,

GJ

Re: chrome stripping
 

I've tried electrolytic rust removal using an alkaline electrolyte, with the work-piece as a cathode and an anode of scrap steel. A couple of amps gets things fizzing. I used washing soda - the more adventurous use caustic, I'm told.

I had thought of suggesting you try this to remove chrome - it certainly removes paint and rust - but I seem to remember caustic eats aluminium?

Re: chrome stripping
 

Please be careful, the solution that will remain after removing Chrome will be classed as hazardous waste - it is poisonous to marine life, and cannot be easily extracted from water at treatment plants.

Re: chrome stripping
 

Thanks for the advice all.
I have been on the phone to an electroplating supplier and he is sending me the necessary stripping solution along with health and safety data.
He tells me that the item to be stripped be made anodic and the process is fairly gentle, taking around 1 hour with a 12v supply.
I will post the results sometime next week, when I get the gloves and goggles off!

Nick.

Re: Chrome stripping
 

Here are a couple of pics of the Rockola alloy/mazak castings to be stripped and polished.

The far right casting in pic 1 was already stripped by a previous owner (I think he got fed up and gave up). I have done a first cut polish on this piece and, as you can see, it is already looking pretty good.

The piece far left is original chrome and in fair condition.

Re: Chrome stripping
 

Hydrochloric acid is very good at removing chrome chemically. I don't know whether it's easy to buy in the UK but here you can buy it in gallon containers at the local supermarket. The chrome layer will be very thin and it only takes a few seconds to dissolve it.

Re: Chrome stripping
 

Hi Stuart.

Somebody did mention this method to me recently. We can get hydrochoric acid from builder's merchants but I have no idea what % to use for stripping chrome, or indeed, whether to use via reverse electroplating.

Many thanks anyway and if you have more info on this please let me know.

Nick.

Re: Chrome stripping
 

The stuf sold at builders merchants for cleaning cement off brickwork is usually about 16%. The highest possible concentration in water is (I think) 37%.

Re: Chrome stripping
 

This is the Materials Safety Data Sheet published by Oxford University for hydrochloric acid http://www.arch.ox.ac.uk/files/rlaha...oric_acid.html.

I think that it can come in concentrations of up to 60-70%. Key thing is good eye protection and good ventilation and have lots of fresh tap water around to quickly douse down anything that is not quite 'as planned'.

Re: Chrome stripping
 

The stuff they sell in the supermarkets here claims to be 30%. You need good ventilation when using it. However it's not that dangerous, otherwise it wouldn't be so freely available. There is plenty of other stuff around such as bleach which should be treated with just as much respect.

Obviously you don't want to dissolve the castings. However chrome plating if done properly requires a layer of copper and a layer of nickel under the chrome which should protect the casting. The chrome layer is very thin and it only takes a few seconds to remove. If the back of the casting isn't plated then maybe you could just treat the front with a sponge and rubber gloves and then wash it off with water.

However chrome plating is really nickel plating. It's the nickel that protects the underlying metal, but nickel has a yellow tinge. The chrome is like a sort of varnish that gives the part the right colour. After removing the chrome, you'll need to remove the nickel. Then after that, the copper underneath. You can get nickel stripping solutions as well. I'm not sure about copper. Maybe you would need some combination of chemical and electrolytic stripping.

Having said all that, finishing metal surfaces is a complex, specialized subject and the experts generally advise "don't do it". If you really want to try, have a look e.g. here:
http://www.caswelleurope.co.uk/
It might give you some ideas.

Re: Chrome stripping
 

Thanks for the ideas and advice. I have to say I was mighty impressed with the compound polishing of the casting albeit very hard, long and filthy work. I would say I achieved a mirror finish of about 95% finish of genuine chrome but without the yellow tint. I am going to put a slight tint in the laquer and try to emulate this.
should I have no sucess with the reverse plating solution I will experiment with acid on scrap, starting weak and working up in strengh

Re: Chrome stripping
 

OK, the second line of the reverse plating guide reads "NOT TO BE USED WITH ALUMINIUM OR ALUMINIUM ALLOYS".
So I went with hydrochoric acid diluted to 20% and applied with a brush. This works very quickly and the casting was dowsed exposing just the nickel layer and a trace of copper.
I have been in touch with a motorbike restorer who assures me that 50% nitric acid will take care of the nickle without corroding the casting.
I am a bit concerned what effect this will have on the copper layer so back to the research.

Re: Chrome stripping
 

Nitric acid will definitely rip off the nickel and the copper. Are you hoping to find something that will just go for the chromium? Did the info with the reverse plating kit say what chemicals it uses?

Re: Chrome stripping
 

Take care, the reddish brown fumes of Nitrogen Dioxide which are given off from the reaction of Nitric acid and certain metals, of which copper is one, are very irritating to the lungs and in high enough concentrations are toxic.

John

Re: Chrome stripping
 

Thanks All
There is a lot of conflicting advice on the net and many 'home brew' methods of removing electro plate (none of which work).

The industrial chrome stripper contains safety data for sodium carbonate and sodium hydroxide (caustic soda?). The chap I got this from assured me this would be OK on mazak and would also remove nickel and copper if left in the bath long enough. I didn't use this because the written instructions supplied said not to use on aluminium or aluminium alloys. electroysis abandoned, at least for now.:wall:

I used the hydrocholic acid 20% (brushed on) which took off the thin, chrome layer almost immediately, leaving the heavier nickel layer untouched.

I am intending to stick with the chemicals and use nitric acid, well diluted initially, to dissolve the nickel (and the copper). The motorbike restorer tells me that nitric will not touch the copper and to use ammonia. 'O' level chemistry, from memory, tells me different and nitric loves to eat copper!

So far the castings (which are precious) show no sign of errosion where base metal is already exposed.

I share Johns concern regarding nitric on copper but it does seem the route to go (for the moment at least). As I understand, a weaker solution will dissolve nickel before it touches the copper?

Back to the research, Nick

Re: Chrome stripping
 

Sodium hydroxide is indeed caustic soda and it will eat aluminium, although not really quickly. I guess you probably don't want yours eaten at all though. Mazak is an alloy and sometimes alloys can behave surprisingly differently from pure metals (think steel and stainless steel). But unless you can find a really reliable reference saying Mazak will be OK I wouldn't risk it unless you've no alternative, especially since the instructions warn you not to.

Cheers,

GJ

Re: Chrome stripping
 

I'll second GJ about caustic soda (sodium hydroxide) attacking aluminium - it definitely does, liberating hydrogen (somewhat surprisingly).

Also, nitric acid does attack copper - and the brown fumes are very unpleasant.

Whatever you use, have a trial on a sample casting first. And have something to neutralise the chemical - if you use acid, have a weak alkali such as sodium bicarbonate (lots of it). And if you do use sodium hydroxide, wash it off with a weak acid (such as vinegar, afain, lots of it).