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Topic: Remove anodised coating  (Read 11828 times)

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jethro

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Remove anodised coating
« on: March 08, 2006, 01:17:39 PM »
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.

Offline Borek

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Re:Remove anodised coating
« Reply #1 on: March 08, 2006, 01:27:35 PM »
2% of water is not anough to dissolve NaOH, just like a teaspoon of water won't dissolve a pound of sugar :) Besides, it is likely that your NaOH ontains 2% impurities, not water. But you may probably safely ignore them.

As for concentration calculations - take a look at these concentration lectures.

Why do you want to remove coating using chemical methods? I think it will be mechanically removed when you start polishing.
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jethro

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Re:Remove anodised coating
« Reply #2 on: March 08, 2006, 02:43:11 PM »
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.

Offline pantone159

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Re:Remove anodised coating
« Reply #3 on: March 08, 2006, 04:59:29 PM »
Be careful when diluting NaOH, the solution gets hot when you do this, and can boil and splatter caustic stuff on yourself.
Dissolving it into cold water is a good idea.

jethro

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Re:Remove anodised coating
« Reply #4 on: March 09, 2006, 05:42:27 AM »
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?

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Re:Remove anodised coating
« Reply #5 on: March 09, 2006, 12:01:16 PM »
I hadn't heard of the suggestion to avoid cold water.  Thanks for the link.  I am curious what other forum members think about this.

I am looking at Shakhashiri, "Chemical Demonstrations", Vol 3, p. 31 where there is a preparation for NaOH solution, it says to keep your recieving beaker in an ice-water bath.  This is based on diluting solid NaOH, not 50% solution, not sure what would be different.



Offline Borek

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Re:Remove anodised coating
« Reply #6 on: March 09, 2006, 12:42:18 PM »
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 :)
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jethro

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Re:Remove anodised coating
« Reply #7 on: March 09, 2006, 07:39:13 PM »
Ok, I've done a bit more reading but keep finding articles about making soap! However there does appear to be more support for cool water. I've found this comment:-
Quote
The water for the caustic solution must be cool (<20ºC / 68ºF). Never use warm water to dissolve the alkali!
When you add the alkali to the water, a chemical reaction occurs, which generates heat... and quite a lot of heat, for that matter! The temperature of a standard strenght caustic solution for bar soap (27% sodium hydroxide, 73% water) will easily reach 80ºC / 176ºF or more.
That recommendation was also based on adding solid NaOH to water so perhaps the majority view is to use cool water. I still don't know about the concentration though. I'm thinking I should try a 10% solution and watch what happens to the component? I suppose I should try to find a piece of scrap anodised aluminium to practise on.
Thank you both for your interest though.

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Re:Remove anodised coating
« Reply #8 on: March 10, 2006, 12:28:54 AM »
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.

Offline mike

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Re:Remove anodised coating
« Reply #9 on: March 10, 2006, 12:42:26 AM »
"never start with hot or cold water"  well hot and cold are relative terms anyway so this isn't very helpful from DOW (who is dow anyways?)

And according to their graph the colder the water is initially the colder the final solution is, generally, so starting with cold water doesn't seem to pose any risk stated on their webpage.

jethro, what do you think will happen if you use "cold" water?
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Re:Remove anodised coating
« Reply #10 on: March 10, 2006, 03:25:14 AM »
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.
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Offline mike

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Re:Remove anodised coating
« Reply #11 on: March 10, 2006, 05:22:07 AM »
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mike: you really haven't heard about Dow Chemical, or are you just joking?

Sorry Borek, I think I was in a silly mood when I wrote this. I sometimes forget that sarcasm is hard to read over email/forums/internet. Yes I have heard of Dow chemicals, although there is no dow company anywhere near where I live, in fact I couldn't tell you where the nearest chemical factory is to me. The big ones would be at least 1500km away. :)

I think your thought sounds correct about the graphs.

I still also think that as a general rule there is nothing wrong with adding to cold water.
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jethro

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Re:Remove anodised coating
« Reply #12 on: March 10, 2006, 06:48:08 AM »
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:-
Quote
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!




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Re:Remove anodised coating
« Reply #13 on: March 10, 2006, 12:43:31 PM »
The reaction between aluminum and hydroxide will certainly happen faster when warm or hot.

My curiosity is about the dilution of NaOH.  In this case, I'm not worried about the rate, rather avoiding dangerous splashing/misting/etc.  I have always heard that diluting into cold water minimizes the chances of these bad things happening.


jethro

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Re:Remove anodised coating
« Reply #14 on: March 11, 2006, 07:20:40 AM »
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. :)

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