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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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.

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

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

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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%.

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

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

Deionised. Cost about the same for 5L as a litre of the acid.

Neil