Blacking aluminium

[Rik ShawParticipant @rikshaw]

I need to machine a small part in aluminium 150mm long by 18mm diameter. After I've machined it I would like to blacken / blue it. As this will be a one off job I would prefer to do it on the cheap rather than spend my loot on a commercially available product. Can it be done a' la cheapskate?

Rik

[Rik ShawParticipant @rikshaw]

[Thor 🇳🇴Participant @thor]

There is "aluminium black" available, I don't know how good it is but wouldn't anodising be better if blue is OK?

Thor

[Jim NicParticipant @jimnic]

At £85.00 for just the 5l of the blacking, never mind the preparation copmpounds, that stuff from Blackfast would seem to be ruled out by Rik's proviso that the process needs to be done "a la cheapskate"

Jim

[mike TParticipant @miket56243]

A 'rattle can' may be the cheapest option, but cans are no longer that cheap.

Mike

[Dave Smith 14Participant @davesmith14]

Birchwood Casey Aluminium Black if you do not want to spend very much

Aluminium Black – Ebay

[duncan webster 1Participant @duncanwebster1]

Anodising without suphuric acid. anodise

you still have to buy chemicals, so it's not free

[SteviegtrParticipant @steviegtr]

I did a video of anodising without battery acid. But it still cost quite a bit to get set up if just starting out. Shame you did not know someone who would do it for you.

Steve.

[GeorgineerParticipant @georgineer]

Just throwing an idea out, without any idea at all if it could be useful – a lot of alkalis (such as washing soda (sodium carbonate) and caustic soda (sodium hydroxide) will blacken aluminium, or at least turn it dark grey.

George

[Howard LewisParticipant @howardlewis46836]

Caustic Soda (Sodium Hydroxide ) will definitely dissolve Aluminium.

Sodium Carbonate, when it dissolves in water becomes a fairly weak acid, so, again, will dissolve Aluminium.

Anodising, with a suitable dye in the solution would do the trick. Probably a 12 volt battery, or maybe even a simple battery charger would provide the power.

The solution has be an electrolyte, possibly sulphuric acid (So I can imagine all sorts of problems getting that, nowadays ) and a small volume of the dye.

It may be that the cheapest solution would be a visit to a motor accessory shop, such as Halfords for an aerosol of black pint?

Howard

[GeorgineerParticipant @georgineer]

Posted by Howard Lewis on 18/06/2022 09:52:29:

Sodium Carbonate, when it dissolves in water becomes a fairly weak acid, so, again, will dissolve Aluminium.

Are you sure about that? I think you may be thinking of carbon dioxide, which when dissolved in water becomes carbonic acid.

Sodium carbonate dissolved in water gives an alkali (either that, or I taught my science classes wrong for all those years, and my chemistry master before me).

I presume that the black finish alkalis give is caused by the alloying elements which aren't dissolved by the alkalis, but I stand open to correction on that.

George

[Howard LewisParticipant @howardlewis46836]

Ah! Bi Carbonate

Either way, the Aluminium will dissolve slightly.

Maybe if a dye can be added to either solution, as an electrolyte; that might produce the desired effect.

The problem is likely to be obtaining the small quantities needed for a one off job.

Howard

[Tim StevensParticipant @timstevens64731]

Whether you get black from acid (etc) will depend on what else is in the aluminium you use. And – perhaps just as important – whether the black sticks or rubs off too easily. As Georgineer hints, above. (But Howard is not correct with his Washing soda >> acid suggestion).

And it also depends on the usage, when made. A handrail will not stay black very long, if you rely on the silicon, etc in the alloy.

So, as usual, it all depends – sorry

Cheers, Tim

[bernard towersParticipant @bernardtowers37738]

coloured anodising is achieved after the anodising process is completed. As I understand it anodising that is going to be coloured is done at a higher amperage to make the pores larger to more readily accept the colour,

[Graham MeekParticipant @grahammeek88282]

Pore size is dependant upon the applied voltage. In my experience I have never known it affect the colouring or dyeing of the anodised surface afterwards.

Regards

Gray,

[Clive HartlandParticipant @clivehartland94829]

The level of anodising is time based, ranging from AA% up thro. AA25.

Colour is a hot dip again on a time basis. I did read that pen ink can be used.

[Michael GilliganParticipant @michaelgilligan61133]

Be wary of ‘unsophisticated’ black anodising … it tends to fade to purple with prolonged exposure to light, and also on contact with soluble cutting oil.

Obviously, these characteristics will vary with the specific materials.

MichaelG.

.

Edited By Michael Gilligan on 19/06/2022 13:15:09

[BazyleParticipant @bazyle]

Anyone tried printer ink as the dye since there is so much left when they decide they need you to buy more?

[Tim StevensParticipant @timstevens64731]

Bazyle asks if 'printer ink' will work here – as you might expect, it all depends.

[A] If you mean ink as used by proper printers in the days following Gutenberg, the black was basically soot – amorphous carbon, in particle form, held together by some sort of usually-water-based glue. So – no.

[B] If you mean 'ink' as used in laser-jet printers it is just a clever powder version of [A], often with magnetic black oxide as well as carbon, and a synthetic resin which becomes sticky when heated. Again – no.

[C] If you mean the liquid ink used in ink-jet printers, then you may have a chance – as the black is a dye (or a mix of dyes) in a water base (with other solvents). Anodising may produce a porous surface which is friendly to the dyes – but there is a risk that the dyes may not all be affected the same way, so you could get a brown or a purple etc. And the result is not going to be waterproof.

[D] modern printer ink used for magazines etc can vary a lot depending on the type of paper, the printing method and speed etc. The results tend to be fairly waterproof. So while this might work, I promise you will spend a long time and get your fingers thoroughly stained as you try to find out.

My suggestion would be to look at the alcohol-based dyes sold to renovate the canvas hoods on elderly motorcars (etc). They are intended to be waterproof and rub-proof, and are dye-based not carbon particles. And – a big advantage – they are sold in small bottles and you won't have a lot of wasted time explaining 'what do you want it for, sonny?'

Cheers, Tim

[Andy_GParticipant @andy_g]

"Can it be done a' la cheapskate?"

If you already have a suitalble power supply, sulphuric acid free anodising can be quite cheap and easy.

(My setup: https://misterg.org.uk/anodising-html/ )

If the part mentioned (150 dia x 18) was substantially a solid cylinder, I would use ~0.6 amps for ~2 hrs for a dyed finish.

If you look around the internet, it'll become obvious that people have tried pretty much everything under the sun as a possible dye. Some work, some dont.

Gettin a good black can be tricky. I use commercial anodising dye in a heated bath (55°C). The dye is available in small quantities on eBay for £10 to £15. It produces a good, deep black that has proved colour-fast.

If the dye is leaching out (as per the example above) it would imply that the sealing stage hasn't been successful (the process is Clean -> De-oxidise -> Anodise -> Dye -> Seal ) – sealing by immersing the part in boiling tap water for 20 minutes has worked for me.

Make sure that the process works on a test piece of the same alloy before committing to the part!

(I could do it for you if you covered the postage and could provide a witness sample of the same alloy to check the process first.)

[Neil WyattModerator @neilwyatt]

I'd agree with Tim, except the finish with inkjet inks should be waterproof, assuming you seal the anodising by boiling or with proprietary sealer.

I anodised this laser collimator with gold and black dies from Gateros Plating.

Neil

Edited By Neil Wyatt on 22/06/2022 15:15:33

[GeorgineerParticipant @georgineer]

It's probably too late for Rik, but I've just come across this useful page in Engineering Workshop Data by Arthur W. Judge (Caxton 1950). I'll add it in case it's of use to anybody :

[duncan webster 1Participant @duncanwebster1]

Dissolving caustic soda in water is exothermic, so I'd start with cold, then warm it up if need be. Caution, hot caustic soda solution is nasty stuff, be very careful and use goggles and thick rubber gloves

[Tim StevensParticipant @timstevens64731]

A couple of comments on the latest from Georgineer and Duncan W:

1. There are likely to be two difficulties with the Anodising recepies – the difficulty, now, of poppping down to Boots for a few ounces of chromic Acid, and then, not knowing what the current density should be. It is easy to measure the current in Amps, but no clue is offered regarding the relevant area. Is (or was) current density measured in Amps per square inch, or per square foot, or some other unit? Sorry, I can't help, here.

2. And yes, a useful reminder that dissolving caustic soda (or potash) in water generates lots of heat, so do it slowly. It is interesting, though, that washing soda shows the opposite effect. Chuck some in your bath (Like we did in the old days) and the water gets colder. Most noticeable if you take a handfull of soda and dip it into the warm bath water. The things we used to do before the internet was invented!

Cheers, Tim

[SamsarandaParticipant @samsaranda]

Tim, I remember bathtime when I was youngster back in the 50’s, my abiding memory is of having to sit on undissolved washing soda crystals when getting into the bath, funny what memories we have stored away in our brains. Dave W

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