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> Aluminium will have the opposite effect from what you wish to
> achieve, the steel will preferentially corrode and protect the
> aluminium from tarnishing.
>
> Iain, some folk, including me, would tend to disagree with
> this, but a lot depends on the particular circumstances.
I'm talking about the practical circumstances.
> Aluminium is really rather reactive, but normally protected by an
> oxide layer. Anything that disrupts that layer, including mild
> abrasion in water, can lead to it corroding, and so protecting any
> steel connected to it.
True, but all the time the oxide layer is intact, the steel will be
either unprotected or corroding to protect the aluminium - which
will be most of the time (the oxide layer re-forms in milliseconds).
According to all the chemistry text books aluminium should do the
job, but those texts are based on highly reactive "pure" aluminium
surfaces. In the real world of engineering, oxide bearing aluminium
(i.e. any aluminium not kept under vaccuum or in certain specific
chemical environments) will not give effective cathodic protection
to steel. There are some commercial corrosion protective coats that
contain aluminium as one component in a complex system - balanced to
have enough Al to be reactive but not enough to form a continuous
oxide coat - but these are rather expensive and I've never seen them
applied to anything other than aerospace parts. I believe sermetel
and sermetech were the trade names.
> Also right, but full-strength (or even quite well diluted)
> phosphoric acid is rather dangerous stuff. Its one of the more
> aggressive acids on skin (though by no means the worst). If I
> recall rightly, rather than causing the more usual "burn" (as if
> that wasn't nasty enough) a small amount will raise a large and
> exceptionally painful blister.
>
> All concentrated mineral acids should be treated with care,
> and that includes phosphoric. However, with sensible precautions it
> can be safely used.
Of course it can be safely used, but I think that requires adequate
care *and* adequate equipment - fume hood, adequate gloves, good
glassware (the anti-shatter coated kind), hand pipette pumps etc.
etc. An emergency shower bath would be nice if you have any
quantity... Not really the kind of hassle the home/small craft shop
operation would really want to be getting into, especially if there
is an easy alternative.
Phosphoric is I believe still used in school chemistry here in the
UK (it was used when I did school chemistry). However it was only
permitted for lessons involving older children (I think over 16,
certainly over 14) and it is probably the most dangerous substance
that a school can legally posess, certainly one of them.
Not *that* dangerous, but not something you want to keep lying
around if you don't have to, especially in a non-industrial, non-lab
environment.
> It is widely sold, even in the concentrated form, as rust remover
> (Navel Jelly, for one), in retail outlets. The acid that you might
> be thinking of, and which is _really_ dangerous, is hydrofluoric.
I said "by no means the worst". And that was what I meant.
Hydroflouric is the worst.
The survival rate for HF spill victims is low, and its doubly
insidious stuff because first thing HF does is kill off the nerves,
so you don't notice it burning you untill its too late.
If you survive the burn, and if you then survive the shock - it may
still poison you.
If you are not poisoned (i.e. you got the antidote jelly on quick),
then you have a future of brittle bones to look forward to.
Very horrible stuff.
My lab does posess some HF. I have never used it, nor do I ever
intend to. I do occasionally need to micro-etch aluminium, but I
went to the trouble of researching an HF free etchant rather than
use the HF based versions (even though they are the best). I cannot
think of anything important enough (to me) to warrant the use of HF.
I don't want a PhD badly enough to go messing with that stuff...
Phosphoric I have used and will use again if I need to. But only if
I truly need to. And if I don't need it I'd rather not have it
around - not even safely at the back of a locked acids cupboard.
Iain
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