Any Chemists out there??

[spread-tee]

Make sure you only use it outside and don't inhale any fumes...

Absolutely, my asthma is bad enough already. Kickstart lever has been in the brick acid for about 4 hours now, the solution has gone very slightly green ish, no sign of any gas coming off which slightly surprises me, even from the bare bits of steel. I have another part in some saturated NaOH so I'll see what that does.

Desp

[spread-tee]

Went shopping for some spirit of salts on Friday after checking online who keeps it around here only to find what they call spirit of salts is not HCl anymore, apparently you need to sign the poisons register or some such paperwork to get it now. However I was able to get some One Shot drain cleaner which is 91% H2SO4, how bonkers is that??.

Anyway I have had various parts immersed in concentrated NaOH, and 15ish%HCl , for several days and the NaOH seems to have done a better job, I don't think it removed the Chrome plating but it may have removed the passivated surface that apparently Chromium metal develops in air, subsequently I was able to get a reasonable coat of Copper plated on the gear lever. Then a really nice coat of Nickel went on top of that
The HCl barely touched it.

I diluted the H2SO4 down to about 50% and tried a part in that which resulted in violent fizzing from the base steel but not much activity on the Chromium, so that's not going to work. I also have some conc HNO3, but that is really scary stuff and will probably dissolve the kickstart lever, my lungs, the garage door............so I'm not resorting to that just yet.

Maybe it will come down to the angle grinder and a bucket of patience, still it's and interesting project and I do have a growing pile of nuts, bolts spacers and what not ready to build up. Gonna have to get the spray gun out next

Desp

[Krill]

Stupid question, but how do you know the metal you are trying to remove is chromium?

[spread-tee]

Good point, honestly I can't be 100% sure, but it is generally accepted that old British bikes were Chromed on the decorative bits.

Desp

[Krill]

I just double checked on Wikipedia (I'm out of practice on this) but if it is pure chromium, the you need to remove the oxidated chromium layer. You could do tha mechanically...but it's really hard, so probably going to take a short while/corundum angle grinder, or a strong reducing agent.

Nitric acid is an oxidising acid and will actually make the protective layer thicker.

In a lab (and clutching at memories here) I'd use electrolysis, using the chromium item as the positive terminal in a solution of sulfuric acid, but then that would leave a toxic byproduct a lay person has no way to get rid of, and this also creates SO2 as a gas.

To be candid, it would be highly irresponsible and potentially lethal (to you and bystanders ie children) in both the short and long term if this went wrong. Angle grinder is safer.

[Countrypaul]

Conc. sulphuric acid will be unlikely to dissolve steel as it will result in passivating it rather than dissolving it. To store large amouts of c.H2SO4 (>98% iirc) they simply use mild steel tanks and if the concentration drops they add oleum (H2S2O7) until the concentratio of sulphuric acid becomes strong enough again.
c.HNO3 will lsso passivate steel rather than dissolve it but in dilute form will readily dissolve it.

If you are going dilute c.H2SO4 you should add acid to water and allow it to cool before adding more. Adding water to c.H2SO4 is dangerous and can lead to very unwanted events.

[spread-tee]

Sure thing CP, when I carefully diluted the 91% H2SO4 down to about 50% it liberated a tremendous amount of heat, the flask was almost too hot to touch despite adding the acid bit by bit. I always carry out these kinds of operations in a shallow water bath to contain any spillages.
Interesting about the passivation effects you mention on steel, I was not aware of that, not that I muck about with strong acids every day of course.

Thanks

Desp

[spread-tee]

I just double checked on Wikipedia (I'm out of practice on this) but if it is pure chromium, the you need to remove the oxidated chromium layer. You could do tha mechanically...but it's really hard, so probably going to take a short while/corundum angle grinder, or a strong reducing agent.

Nitric acid is an oxidising acid and will actually make the protective layer thicker.

In a lab (and clutching at memories here) I'd use electrolysis, using the chromium item as the positive terminal in a solution of sulfuric acid, but then that would leave a toxic byproduct a lay person has no way to get rid of, and this also creates SO2 as a gas.

To be candid, it would be highly irresponsible and potentially lethal (to you and bystanders ie children) in both the short and long term if this went wrong. Angle grinder is safer.

Thanks Krill, I didn't spot your post earlier..........

I did in fact try reverse electrolysis with brick acid <10% HCl but it didn't touch the chrome, instead it fairly rapidly removed the iron surface from the base metal where the chrome had flaked off due to rust, it did leave the part super clean though.

So far the most successful approach has been to soak the part in strong NaOH for a few days which I think removes that oxidised Chrome, it certainly changed its appearance a bit anyway, and didn't attack the steel below. Then I was able to get a light Copper plate on it Followed by Nickel. I now have a very smart gear lever. All other methods just wont plate the chrome at all or it just wipes.

Regarding the disposal of waste chemicals, I am trying to use the smallest quantities of all solutions, if they are clean and useable for further plating store them, or if unsure of the composition then I plan to neutralise them and evaporate off the water and pay for specialist disposal'

I promise I won't flush them down the bog

Thanks for your help

Desp

[Moxi]

I just double checked on Wikipedia (I'm out of practice on this) but if it is pure chromium, the you need to remove the oxidated chromium layer. You could do tha mechanically...but it's really hard, so probably going to take a short while/corundum angle grinder, or a strong reducing agent.

Nitric acid is an oxidising acid and will actually make the protective layer thicker.

In a lab (and clutching at memories here) I'd use electrolysis, using the chromium item as the positive terminal in a solution of sulfuric acid, but then that would leave a toxic byproduct a lay person has no way to get rid of, and this also creates SO2 as a gas.

To be candid, it would be highly irresponsible and potentially lethal (to you and bystanders ie children) in both the short and long term if this went wrong. Angle grinder is safer.

Thanks Krill, I didn't spot your post earlier..........

I did in fact try reverse electrolysis with brick acid <10% HCl but it didn't touch the chrome, instead it fairly rapidly removed the iron surface from the base metal where the chrome had flaked off due to rust, it did leave the part super clean though.

So far the most successful approach has been to soak the part in strong NaOH for a few days which I think removes that oxidised Chrome, it certainly changed its appearance a bit anyway, and didn't attack the steel below. Then I was able to get a light Copper plate on it Followed by Nickel. I now have a very smart gear lever. All other methods just wont plate the chrome at all or it just wipes.

Regarding the disposal of waste chemicals, I am trying to use the smallest quantities of all solutions, if they are clean and useable for further plating store them, or if unsure of the composition then I plan to neutralise them and evaporate off the water and pay for specialist disposal'

I promise I won't flush them down the bog

Thanks for your help

Desp

To be honest Desp by the time you have done all that its probably way cheaper and safer for you to have sent the part to a platers to be de-plated and cleaned. However, I appreciate that price may not be the driving factor here and that you may simply want to do it yourself in which case the systems you have tried so far should have had some success unless the chrome plating is hard chrome plating and if it is then as Krill and some others suggested you will need to mechanically degrade the finish to allow the chemicals to get an effective contact and do their work which with hard chrome plate is not an easy task. Most plating and automotive shops use wet or dry blasting with garnet or other media to achieve this task and there are suitable blasting kits for the enthusiast to us to get similar results (you might already have something?) The only other possible option I can think of is something along the lines of heat treatment to degrade the hard chrome plate finish before abrading. IF the item to be de-coated is not a complex shape with varying cross section then you could make up a small soaking furnace from some firebrick and or kaol wool insulation (wear a mask as this stuff is a potential source of silicosis when heated to high temperatures) then using a propane burner bring up the temperature to ideally 400 to 500degrees C to remove the hardness of the chrome plate finish, it will need to sit in that heat long enough to hold a uniform temperature and therefore you need to be sure that the metal the item is made of will not begin to creep at those temperatures over a prolonged period if in doubt you can soak at just 200 degrees but thats only going to reduce the chrome hardness by around 10% as opposed to the 40% at 400 degrees, magic things happen above 600 degrees but thats probably going to be too much for motorcycle components that could be made from light metal alloys etc. Once well soaked allow the component to cool naturally before using abrasives to strip away the now softened chrome plating and then a dip and clean to remove the rest.

Just be really sure of the metal composition of the items you are going to heat treat so that you dont damage them

Moxi

[Krill]

I've had to double check this, but I think the entire electrolysis angle might fail because the basic metal which has been chrome plated will act as a sacrificial anode, and is going to get depleted quickly because of the amount of current that needs to get pushed through the Chromium (which is usually double or more other metals for some reason). Essentially the component might be too degraded for use at the end.

I would need to second Moxi on this, at a certain point one has to consider professionals for a job.