Anodising

I've been having a first go at anodising. I've been intrigued by a few comments on the web about using Sodium Bisulphate as a substitute of Sulphuric Acid. My setup is a 30V 5A bench power supply and some lead cathodes in a plastic food container.

I used a commercial dye bought, like the Sodium Bisulphate, from ebay.

Results are as follows:

On the left are the first attempt in 25% Sodium Busulphate. A 720 rule calculator suggested 0.3A for the size of specimen. Very disappointing. I used some Ti wire to support the specimens and it seemed to be very difficult to maintain a current flow.

I have some legacy Sulphuric Acid which I have diluted down to the legal (in the UK) 15%. The middle specimens were anodised in this solution at 0.5A and I used Al suspension wire. Very happy with the results.

I then tried again with the Sodium Bisulpahte but with Al suspension wire. Again, it was tricky to get a really good contact with the specimen to achieve a current to flow. The maximum current I managed was 0.3A with the voltage turned up to the 30V max. The result was the specimen on the right, indistinguishable from the middle specimen.

I'm afraid the "experiment" was not very well designed - too many variables changed at once but it does seem that, given the right conditions, Sodium Bisulphate does work for anodising. 15% Sulphuric Acid is also perfectly good and would appear to be a little easier to use but not so nice to handle and store. I also don't know where one could buy even the legal 15% solution.

Anyway, I hope this helps anybody wanting to do some home anodising. Look out for model engines with purple heads and spinners.

Cheers,

Rod

Edited By Roderick Jenkins on 04/05/2019 16:08:48

I too have seen the use of Sodium Bisulphate on the web and have also bought some of it to experiment with but have not got round to trying it yet, so your efforts have encouraged me. I was wondering what strength of solution to use and see you opted for a 25% solution. Was that percentage based on anything other than a stab in the dark?

I have heard that the solution does not keep well which may be a disadvantage. The chemical is not that cheap so could be quite expensive if it is a case of use it once then have to throw it out. It might however be the only way to anodise if Sulphuric Acid is impossible to source.

Doug

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Link please.

The UK law on possession of explosive pre-cursors

Rod

Hi Rod,

I used diluted Sulphuric at about 12%. I found that I had to 'polish' the Ti wire and bend it into a 'springy shape' to ensure good contact. The work MUST be freshly 'stripped' in NaOH or you won't get any current.

You can tell if it isn't working as the current will be very low. The middle diameter tube here is 76mm and I only used a 4A 12V transformer so you can do pretty large pieces. By the way I had no trouble getting 3 to 4A at 12V:

Edited By Neil Wyatt on 04/05/2019 21:26:06

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Reading that UK law above makes me shudder... I can go to the nearest chemist's shop and get acids over the counter, no problem. 1 kg of H2SO4 they had to order from their supplier, and nitric acid they had only a small bottle (1/4 l) in stock.
Btw. when you leave some of that 15% acid in an open bowl, the water dries away and the concentration will go up. At some point - which I don't know where it is - there will be an equilibrum between evaporation and water absorption due to hygroscopicity (what a word...).

And in contrary to a widespread belief H2SO4 is not ccorrosive. What was the name of that famous model engineer which put his loco 'Como' in a glass box together with a bowl of concentrated H2SO4, to keep the humidity low and the rust at bay?

Regards,
Hans

Disposal: I don't know enough about chemistry to suggest how to deal with Sodium Bisulphate but you can neutralise Sulphuric Acid with limestone or chalk gravel (rock not blackboard, chalk). The reaction will produce a sludge of calcium sulphate, the mineral which occurs naturally in solid form as the rock, gypsum.

Anodising and plating tend to be more consistent if the electrolyte can be agitated in some way during the process. The nickel-plating firm for which I once worked used air blown through perforated pipes laid along the bottom of the tank. In a small scale you could probably use an aquarium pump and diffuser.

As well as e-Bay, if I recall correctly, Sodium Bisulphate is available from swimming-pool suppliers as "Dry Acid" - granular. Beware: dust!. It is used as a pH corrector in pools. The same suppliers also stock Hydrochloric Acid, or did only a few years ago, and Sodium Carbonate (washing soda, an alkali), for the same purpose.

Hydrochloric acid is the one that will corrode all your tools. It is actually a gas at room temperature, so is normally encountered dissolved in water. Left in the workshop, a little will come out of solution and waft around your workshop attacking everything. Even the killed spirits version, Zinc Chloride, seems to do this. Best to keep the soldering flux in the woodshed rather than the workshop!

Also the various forms of Chlorine sold for swimming pools will attack things too.

John

Possibly as an aside, when anodising, aluminium alloy components, at Rolls Royce, (Sentinel) the parts were hung on copper wire, in the bath of chromic acid. But the object was to form a protective coat, and not to add colour, so things would have been different.

Normally, in an electrolyte, copper causes Aluminium to corrode, but probably applying the voltage counteracts that.

Howard

Hans, the person you were asking the name of was Dr. J. Bradbury Winter, and the article referred to was in the first issue of "The Model Engineer and Amateur Electrician", January 1898.

Ian S C

Sodium Bisulphate is sometimes described as half-neutralised Sulphuric Acid. It's a white salt, becoming acid when added to water. It's considerably stronger than Citric Acid and Vinegar but not in the same league as its parent. Sodium Bisulphate does not absorb water, it cannot be used to make explosives, and throwing it in someone's face is moderately harmful rather than life-changing.

The main disadvantage of Sodium Bisulphate used as a pickle / metal cleanser is that the non-acid half of the salt leaves a lot of extra mess to be cleaned off the metal. Considerably more cleaning compared with Sulphuric Acid but not difficult - plenty of water and a little scrubbing.

Sulphuric Acid is used to unblock drains and Sodium Bisulphate was the main ingredient of Harpic Lavatory Cleaner. (The powder type.) Provided they are well diluted with water and kept away from metalwork domestic quantities of Sulphuric Acid and Sodium Bisulphate can both be poured down an ordinary British drain. I don't think there's any need to neutralise either.

Other chemicals, other acids, different rules! Read the Instructions and the Safety Data Sheets.

Dave

That process is not anodising, its a chromate conversion coating. One advantage is its conductive, disadvantage is the traditional process uses hexavalent chromium which is toxic and banned in most applications in europe and other counntries.

Robert G8RPI.

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The lifetime of sulphuric acid used for anodising is indefinite. It is not depleted by the process. Kept safely in a double container your 14.99% solution should last years.

Neil

Used to use hydrochloric to clean aluminium with, proper Jekyl and Hyde. Anodising was in the workshop with sulphurics and hydrochloric etc. Never had rust in 4 years of doing it until 2006 when the large tank broke, gallons every where at 21%.

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I think mine worked out at about 12-13% when diluted, and it worked very well,

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When diluting acid, just make sure that you add the acid to water, not the other way round, to avoid any chance of the solution erupting onto you and the surroundings!

Howard