"Alochrome" / Iridite etc for Aluminium

In the past, when I needed to paint aluminium, was sometimes able to get it either "alochromed" or "iridited" (not sure of the spelling) first as I had access to the facility in the engineering industry via the "favour" route.

I did have a small "stock" of fluid, long since used up, which after cleaning the material with a "brillo pad" and distilled water, degreasing with a solvent, and then "painting" with the fluid gave the said finish.

Alochrome and Iridite are "chromate conversion" processes and, not only provide some corrosion resistance but also a better surface for painting.

Can anyone suggest a similar process I can do at home without the difficult to get fluids, please?

The parts are small(ish) so "dipping" isn't an issue- the particular parts for the current project are about 100mm largest dimension.


Reply to
Brian Reay.
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"the difficult to get fluids"

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expensive though ..

The main reaction is between chromic acid and aluminium. The chromic acid is sometimes introduced in the form of a chromate, but it's the chromate (VI) ions which count.

A very simple bath might contain sodium or potassium chromate - the latter is available on ebay - with a bit of nitric acid thrown in for good measure. Actually sometimes you can get chromic acid on ebay. but not right now.

However the commercial baths contain other nasties like hydrofluoric acid or silicofluoric acid, and cyanide or ferrocyanate. The fluoride-containing bits are, as I misremember, meant to give a less-spotty appearance, and the cyanide makes the coating more even - but don't quote me on either of those.

-- Peter Fairbrother

Reply to
Peter Fairbrother

What is colloquially referred to as "chromic acid" is usually just a solution of potassium dichromate in sulphuric acid. Can't remember the concentrations of each that are typically used; I used to use it by the gallon for cleaning laboratory glassware, but that was 40 years ago! Not sure why you specify nitric acid, not stuff you want lying around a workshop, the fumes will corrode any steel within reach. Dichromate is generally used in preference to chromate; both contain Cr(VI) but dichromate has more of it per kg.

Those are *really* nasty things; despite many years' laboratory experience, I wouldn't want them in my home - with the exception of hexacyanoferrate (if that's what you mean by "ferrocyanate", not a term I recognise) which is fairly innocuous. Cr(VI), despite the paranoia expressed about it in many websites, is fairly OK provided you know what to avoid (it's a very powerful oxidising agent).


Reply to
David Littlewood

Ah, the old chromate juice. Fond memories, not.

That would perhaps do, but it's not what is most usually used for chromate conversion coatings for aluminium, which is actual chromic acid, H2CrO4, or sodium/potassium (di)chromate and nitric acid, or both.

It was frequently used for that, and occasionally still is, but not often.

It was mostly replaced as the glassware cleaner of choice by "pirhana juice", a mixture of sulphuric acid and hydrogen peroxide, which is/was thought to be safer and which doesn't have the disposal restrictions which chromate juice has.

I'm not entirely sure why nitric acid works better than sulphuric acid in this application either. It may have something to do with the alloying elements, but apparently it's hard/impossible to get a uniform result with sulphuric acid.

Sometimes a small quantity of sulphuric acid is added to the bath, but there will usually be ten times as much nitric acid.

Also nitric acid is an oxidising agent (which may be why) and it's cheaper (not cheaper than sulphuric acid, but cheaper than chromic acid, which it partly replaces). But I doubt people would use nitric acid instead of sulphuric acid as exclusively as they do unless it had some other benefit.

In big commercial baths they use chromic acid, with hydrofluoric acid and cyanide additives. The bath is refreshed with nitric acid.

Sometimes they use sodium (di)chromate and nitric acid instead of chromic acid, or more often as a partial replacement. I don't think they ever use sulphuric acid as anything other than a low-concentration additive though.

I agree, nitric acid is not nice stuff to have around. The hinges on my acids cupboard need to be replaced every few years! Also, chronic low level exposure probably isn't very good for you health-wise.

Yes, they are nasty. However if you buy the commercial stuff which is available in smallish quantities without hassle you will, depending on brand, get (real) chromic acid with silicofluoric acid (or perhaps hydrofluoric acid) and potassium ferricyanide in it.

with the exception of

Duh! Sorry.

I was thinking "it's not ferrocyanide, it's the other one", and had a wet brain fart - came up with ferrocyanate rather than ferricyanide ! ;(

which is fairly innocuous.

Except when in contact with strong acids - if mixing a formula with acid and ferricyanide you want to make damn sure you do it in the right order, otherwise it will produce cyanide gas.

Also, many chromate conversion baths, and especially high concentration concentrates, release cyanide slowly ...

Yes. If you are also careful not to breathe it in any shape or form, and use simple general avoid-contact measures, the chances if it hurting you are small.

Disposal of small quantities isn't that hard either - reduce to Cr(III), and add/sneak the residue from that into a battery recycling scheme.

If you (the OP I mean, or anyone else) want an actual formula for that, let me know. Enough chemicals to treat a gallon or so of used solution will only cost about £2.50, it's cheap to do.

-- Peter Fairbrother

Reply to
Peter Fairbrother

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