Aluminum Reaction, Not Steel
Date: Winter 2009-2010
Is there a chemical, single or a compound, that will react
with aluminum but leave steel untouched?
USE APPROPRIATE SAFETY PRACTICES
Aluminum is an amphoteric element, so it reacts, not only with acid but also
base. You could try moderately dilute acid (HCl).
Or you could try concentrated NaOH. The reaction with aluminum is very, very
exothermic, so it should be done in a hood. One of the products is hydrogen
gas, so avoid ignition sources. Depending upon which grade of stainless
steel (there are dozens of grades) there should be little reaction with
Any alkaline (basic) water solution, is your answer, in principle.
Aluminum dissolves in acid or base, only neutral pH is OK with it.
But Iron dissolves in acid, rusts in neutral, and precipitates strongly in base.
It tries to corrode, but then makes a dark-colored highly adherent oxide coating,
which then stops corrosion before a noticeable amount of metal has been removed.
It is called a passivation coating.
Gun bluing is related to this. I think it uses sodium basic phosphates instead of
So if you put your aluminum/steel assembly in a bath of sodium- or potassium-
hydroxide (such as lye), the aluminum will hopefully corrode fast,
perhaps with flammable hydrogen bubbles (be careful),
and the steel will be very little touched,
and might stay pretty light and shiny, or get slightly darkened.
When done, rinse in water, dip in sodium bicarbonate, (baking soda),
dip in water again, blow dry, and convection-bake at maybe 80- 120 degrees C.
Steel needs all the help it can get to not rust later.
It is possible that the effect on steel can be improved by
adding a pinches of sodium carbonate or sodium phosphate,
so the steel does get darkened by the bath.
Wherever there is a thin hard dark coating,
you can be pretty sure the steel is not corroding,
at least not fast enough to change dimensions perceptibly.
And if slightly oiled it will be less likely to rust, later, too.
I think too much phosphate in solution might slow down the aluminum etching.
Sodium and potassium may have subtly different effects on the steel too,
mostly because traces of metal ions left behind may
encourage rust breakouts later, or not, depending on the ion.
Both erode aluminum and do not erode steel while in the bath,
but one may leave the steel subtly better long after drying.
( Chloride, bromide, iodide ions are much worse rust-starters;
keep them out of your alkaline bath.)
I imagine there should be commercial solutions,
of similar chemistry, to do this kind of thing.
They would also include surfactants to make sure the whole surface
of each metal gets attacked or protected uniformly.
One caveat is that aluminum alloys may well include metals
that are not very soluble in alkaline solutions.
Then your aluminum could have a tendency to stop etching after a bit,
or refuse to ever quite finish etching away.
This problem could be quite puzzling and aggravating.
There are lots of things that might help it some,
such as vigorous stirring or bubbling or ultrasonic or surfactants,
and fairly frequent replacements of the solution.
And these are the kind of improvements that
commercial products or services do best.
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Update: June 2012