I hope you chemists can help me here, so any advice is appreciated. I’ve done some reading but can’t find all the answers. I’ve got some anodised aluminium cam belt covers for a Ducati motorbike and would like to remove the anodised coating. They were machined from solid and there are machining marks that I wish to polish out.

I’ve read that a Sodium Hydroxide solution will remove the anodised layer although it will also dissolve the aluminium so solution strength and immersion time are both important. I’ve bought a 500g container of caustic soda and the label says it contains Sodium Hydroxide 98%w/w. This is the first bit I don’t understand as I’ve read that %w/w means “Percent of weight of solute in the total weight of the solution. Percent here is the number of grams of solute in 100 grams of solution.” My caustic soda is a solid so how can it be in solution? That’s not too important though as 98% means it’s nearly all Sodium Hydroxide, but I would like to understand the terminology.

I would like advice on solution strength and how to make that. I read that Sodium Hydroxide is often supplied as a 50% solution and that the recommended strength for removing the anodised layer is 10%. I’m not sure if that means 10% of the 50% solution or just a 10% solution.  I understand that %w/v  means “Percent of weight of solution in the total volume of solution. Percent here is the number of grams of solute in 100 mL of solution.”  Now if I wanted a 10% solution does that mean I would mix 10g of Sodium Hydroxide into 100ml of water? That would mean that the volume of the solution is the same as the volume of water used then? I’ve only read that a 10% solution is suitable so any advice on alternative strengths and the temperature of the water used is appreciated.
Finally, I do know that this material should be handled carefully with gloves and eye protection.

Thanks for reply Borek, and link to concentration lecture site. It looks like I'm not the only one confused by w/v solutions then. I think I understand how to make a 10% solution now; mix 10g of NaOH in water and then top-up to 100ml. That's if I need a 10% solution; I'm not sure.
Anodised aluminium is very hard and would need to be ground off to remove it; polishing will not touch it. The wall thickness of the component is only 2.5mm and the shape is quite intricate in places so I would prefer to remove it chemically.

Thanks for warning Mark but your suggestion about cold water contradicts what Dow says, which is 27°C-38°C? Have a look here http://www.dow.com/causticsoda/safety/dilution.htm
I'm still interested in a recommended concentration but perhaps a chemical forum is not the right place to ask?

I am curious what other forum members think about this.

I am slightly surprised too, although I have learnt eons ago that field practice differs from egghead thinking

One thing that occurred to me, although I am not sure why it would matter, is that a typical chemistry lab dilution is small scale, say around a liter or less, while the Dow information probably refers to much larger amounts, 'industrial scale'.

Everything else in that link, namely that you add your stuff (solid NaOH, concentrated solution) to water (not the other way around), slowly with stirring, is the standard issue rule-of-thumb, at least as far as I know.

mike: you really haven't heard about Dow Chemical, or are you just joking?

It occured to me - the plot presented by Dow is for dilution of 50% solution. Such solution has melting point of 12 deg C or something like that, so if you start adding it to cold water you risk solidyfying, which in turn will make stirring hard and may lead to local overheating. Perhaps that's the reason.

jethro, what do you think will happen if you use "cold" water?

Well although I'm a mechanical engineer I did do chemistry at school but it was many years ago and I've forgotten most of it. This is why the internet is great 'cos I can learn myself and when I get stuck I can ask you guys.
I've found a site here about the production of aluminium and to quote from it:-

With hot concentrated sodium hydroxide solution, aluminium oxide reacts to give a solution of sodium tetrahydroxoaluminate.
Al2O3+2NaOH=2NaOH+3H2O=2NaAl(OH)4

Note:  You may find all sorts of other formulae given for the product from this reaction. These range from NaAlO2 (which is a dehydrated form of the one in the equation) to Na3Al(OH)6 (which is a different product altogether).

What you actually get will depend on things like the temperature and the concentration of the sodium hydroxide solution. In any case, the truth is almost certainly a lot more complicated than any of these.

Apart from Borek's comment about the melting point, perhaps the above comment about the temperature is relevant to the reaction? I appreciate that the anodised coating is a very thin layer and this isn't a large industrial scale reaction but it's still aluminium oxide + sodium hydroxide.
On my container of sodium hydroxide it does say for clearing drains to add 100g to 1 litre of cold water stirring constantly so clearly it's safe to do that and nothing too vigorous is going to happen. Perhaps for clearing drains it's a good thing if the mixture thickens with cold water? Good point made by Mike about what is hot or cold though.
The caustic soda is not expensive  so perhaps I'm back at school with experimental chemistry? The anodised components were expensive though - £200 so I don't want to damage them!

Ok, thanks for warnings about splashing and that can't be said enough. I do have a workshop at home and wearing eye-protection is automatic for me. I've also got some long pvc gloves so I will be well protected.
Anyway, thanks to all for the comments and advice. I'm going to have a go and will report back with the results. The process may be useful if anyone does a search on here in the future so it's nice to have a conclusion even if it's a 'Don't do this'! I am confident of success here though.