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Distilled water for anodising

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Steviegtr21/05/2021 01:25:08
2275 forum posts
313 photos

Hi all. I am making some parts in Aluminium. I think they would look a bit Bling if they were anodised a nice colour. The problem with buying Sulphuric acid in the UK is a no no.

My family owned motorcycle shop cannot even buy it. The Motorcycle batteries now come ready charged & sealed. Before it came with the acid in a pack for you to fill yourself. Unfortunately due to tossers throwing the stuff at people it is now banned unless you have a licence. So i am going down a different route by using Sodium Bisulphate.

So on to the next question. Using tap water for washing down is another no no. Distilled water or de ionised water has to be used.

My question is can i use the water that my dehumidifier collects. I am thinking i can , but not 100% sure.


Graham Stoppani21/05/2021 05:14:28
105 forum posts
21 photos

I just bought 5 litres of sulphuric acid from Gateros for anodising (which I haven't done yet). It will be less concentrated than battery acid for sure. I used to store my sulphuric acid at full 97% concentration and dilute it as needed until the law changed.

I use the water distilled from my garage dehumidifier for making up my electroplating baths with no ill effect.

Edited By Graham Stoppani on 21/05/2021 05:14:58

Andy_G21/05/2021 08:56:29
137 forum posts

I use the battery water from Halfords - it’s only a few £ for 5l.

There can be a lot of dirt in dehumidifier water, but it might still work OK.


Note that you don’t need to use sulphuric acid - sodiumbisulphate works just as well, is easier to get hold of and less dangerous to handle and have around.


(I’ve posted details on this forum previously, but they’re also here:

**LINK** )

Edited By Andy_G on 21/05/2021 09:00:02

Tony Pratt 121/05/2021 08:59:50
1767 forum posts
10 photos

Yes dehumidifier water will be same as distilled but as Andy G says can be dirty.


Graham Stoppani21/05/2021 09:18:24
105 forum posts
21 photos

" There can be a lot of dirt in dehumidifier water, but it might still work OK. "

Simple test if you're not sure. Pour a very small amount into a glass container and let it evaporate to see what's left behind.

Probably the first science experiment you did at primary school! smiley

Brian Wood21/05/2021 09:18:30
2475 forum posts
39 photos

Be-friend to your window washing people, or bribe them with tea, the ones that use long poles and lances with a pumped supply of water in the hosing.

They use demineralised water by the gallon so that the windows dry without leaving streaks. We have been using ours as a supply for years


Hopper21/05/2021 09:37:51
5505 forum posts
137 photos

My local grocery store sells 5 litre bottles of demineralised water for use in steam irons etc. Cheap as chips.

Dehumidifier water is going to have minerals/pollutants from airborne dust etc. Probably not much, but the demin water is so cheap to buy you might as well use it if available.

Oldiron21/05/2021 09:47:08
865 forum posts
23 photos

I buy Demin' water from my local truck parts supplier in 25ltr containers for about £12 a go. We use it in the iron & for window washing.


SillyOldDuffer21/05/2021 10:31:32
7714 forum posts
1705 photos
Posted by Hopper on 21/05/2021 09:37:51:

My local grocery store sells 5 litre bottles of demineralised water for use in steam irons etc. Cheap as chips.


Check what's in steam-iron water before buying, and - usually - cheapest is best. The problem is Ladies like their Ironing water to be perfumed, and the effect of perfume on Anodizing is unknown.

Car Battery Water is also suitable, but more expensive than Ironing water. (Same stuff, but battery water is sold to men!)

Freezer ice and dehumidifier water are both demineralised, but likely to be slightly grubby. Rainwater is probably dirtier again: it's slightly acid due to pollution plus muck collected by running over roofs and guttering. My guess is all these waters would be OK, but de-ionised water would eliminate the possibility of contamination.


Neil Wyatt21/05/2021 12:32:05
18809 forum posts
733 photos
80 articles

Sulphuric at just under the legal strength limit for unlicenced home use is fine.

I've used deionised water from a motor factors, available in 5L containers, got excellent results.

My tip is use professional dyes.


Bazyle21/05/2021 12:54:03
6087 forum posts
221 photos

If you are worried about dirtin the dehumidifier water just put it through a new paper coffee filter.

Since the purpose of the acid is just to make the water conduct better probably anything that does that would work if it doesn't have a side reaction, perhaps citric or white vinegar would do, or phosphoric too.

Steviegtr21/05/2021 14:39:47
2275 forum posts
313 photos

Thanks for all the replies. The dehumidifier is not hat old & catches the water in a plastic container. When i empty it the contents are always crystal clear. I have ordered the proper dye's. I have also ordered 1/2 kg os Sodium Bisulphate. I already have Caustic soda to hand.


Grindstone Cowboy21/05/2021 15:29:47
761 forum posts
60 photos

+1 on Bazyle's suggestion of a filter, I find the odd speck of rust in the water from mine.


SillyOldDuffer21/05/2021 15:35:06
7714 forum posts
1705 photos
Posted by Bazyle on 21/05/2021 12:54:03:


Since the purpose of the acid is just to make the water conduct better probably anything that does that would work if it doesn't have a side reaction, perhaps citric or white vinegar would do, or phosphoric too.

There's more purpose to the Acid than making the water conduct! Anodising grows a layer of oxide, and, if the layer is to be colour stained, it needs to be microscopically porous. A porous layer is created by anodising with an acid that partially dissolves the Aluminium Oxide as it forms, giving the layer a spongy structure to take the dye. Also, the acid should dissolve Aluminium Oxide but not the metal.

Not all acids dissolve Aluminium Oxide and others are too aggressive. 10% Sulphuric Acid is suitable and cheap, but my book says Phosphoric Acid and Oxalic Acid (Rhubarb!) also work.

May be worth trying alternatives like Citric Acid or Vinegar, but my guess is they either remove the sponge entirely, or leave it too thin to take a dye. Sponge size is also affected by the amount of current flowing and by the particular Alloy used. I suspect this is why results vary between excellent and abject failure! Using iffy materials and equipment at home it may be necessary to experiment.

Acid isn't used at all when a hard protective oxide layer is wanted; Ammonium Tartrate solution makes a thick solid layer that can't be dyed a pretty colour. No sponge!


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