AllanD - 28-11-2006 at 15:59
While stripping oxide corrosion from an aluminum casting
With HCl a purple-black colored oxide was formed.
Now I've seen a similar effect with phosphates on copper bearing aluminum alloys, but that wasn't even remotely purplish...
I'm thinking that this aluminum MIGHT be a Manganese
alloy, because this purple-black oxide is insoluble in water, for that matter I can't find anything handy that it is soluble
in, but I'd have thought MnO2 would be brown-black.
But MnO2 is the only thing I can think of that is "black-ish"
that could form from impure aluminum in HCl, of course I know that there are people here who are smarter than me.
So, can anyone think of any common alloying metals of aluminum or likely HCl contaminates that would create a purple-black oxide?
Because I'm drawing a blank...
AllanD
[Edited on 29-11-2006 by AllanD]
metaminct - 28-11-2006 at 16:14
It sounds like a mineral called purpurite - a manganese phosphate. It turns purple in acid. just a thought...
http://mineral.galleries.com/minerals/phosphat/purpurit/purp...
AllanD - 28-11-2006 at 17:20
Well what I'm seeing is a purple-black residue/oxide
on the aluminum.
Also as an insoluble sediment in the reacted acid solution
Present even after the acid was neutralized with NaOH
prior to disposal
It is persistant as an oxide coating on the metal
though can be brushed off with determination...
But even after an quick alkali wash it stayed purple.
AllanD
12AX7 - 28-11-2006 at 18:36
Cast aluminum is generally a silicon alloy with varying amounts of copper and magnesium (see alloy compositions for 320, 356 and in rare cases
(hypereutectic pistons, con rods, etc.), 390.
Tim