Black Auminum Oxide? 2

[metman]

One of the attributes of Aluminum alloys is that they quickly develop a tenacious surface coating of Aluminum Oxide upon exposure to air which tends to resist mild corrosive environment.

Aluminum Oxide, Al2O3 is white. Why then, after handling bare Aluminum alloy with bare hands does ones hands become coated with a black smut? But maybe it is not Al2O3 but some other compound of Aluminum Oxide that forms on the surface of Aluminum alloys.

 

[unclesyd]

This is one of those questions that is answered with "all of the above".
All of the analysis I've seen show oxides, hydroxides, and fine metal particles. The black is partially due to the extremely small particle size. If you sand Al with extremely fine grit paper the residue will be black. What you are probably referring to "weathering bloom". This is common on weathered aluminum and even on anodized aluminum if there is sealer failure.
One misconception is that a lot of the Al smut seen is carbon, which it is not true.


Aluminum is the worlds worst when it come to smut.

 

[CoryPad]

Great answer by Syd.

 

[tbuelna]

metman,

Sometimes oxidized aluminum appears white, sometimes it doesn't.

Anodized aluminum surfaces can be various colors, depending upon the chemicals involved. Anodizing is basically a controlled oxidation of the aluminum surface.

Fretting corrosion of an aluminum surface will also sometimes result in a black aluminum oxide residue on the surface.

I'm just guessing, but maybe the black residue left on your hands might possibly be the result of a reaction between the aluminum surface and the acids and oils present in your skin. Similar to how handling clean steel with your bare, sweaty hands will leave rusty finger prints on the steel.

Regards,
Terry

 

[IRstuff]

It's unlikely that any acid that one's body might excrete could attack aluminum oxide. It's basically resistant to most acids and bases.

We once accidentally grew some aluminum oxide on a metallization layer on an IC, and tried all sorts of dry and wet etches that we had available in our process, but they didn't do much at all. We actually had to sputter the oxide off, before we could make contact to the aluminum underneath.

TTFN

FAQ731-376

 

[EdStainless]

almost every material there is will look black as a very fine powder. When you get sub-micron the particle size becomes a factor in the scattering of the light.

= = = = = = = = = = = = = = = = = = = =
Plymouth Tube

 

[unclesyd]

Adding a little to the post by IRstuff.
When you look at an fresh weld on Al you sometimes see a parallel black streak on either side of weld bead due mainly to not having complete control of the purge gas. If you look closely there is normally a very narrow super shiny streak between the weld bead and the black streak and this is due to above mentioned sputtering, negative electrons from the welding arc. Super cleaning of Al by sputtering hasn't proven very practical so if you want super shinny AL you give the substrate a coat of very pure Al. You can mechanically polish Al for most reflective applications. All these polishes will turn the polishing media black. To remove the black you apply generous amounts of dish washing liquid, soak overnight and the wash, alone.

Coloring of Anodized Al is normally part of the sealing system used on the surface. There are many formulations of this coloring process with some being passive and some contributing to the sealing process.

 

[metman]

tbuelna,
Where I said, "Aluminum Oxide, Al2O3 is white." I mean pure Al2O3 as a separate compound as in a powder form OR sintered into an Al2O3 ceramic component rather than on the surface of Aluminum.

Sometimes oxidized aluminum appears white, sometimes it doesn't.

On the other hand (no pun intended)it seems like I have seen this white layer on weathered Aluminum which does not quite correlate with the weathering bloom that I think unclesyd is describing as being black. Not that we can't have either or both based upon Unclesyd's first response.

What I am referring to is relatively clean Aluminum alloy 2024, 6061, 7075, 7050 that has been protected from the weather during shipping. In this state prior to any surface treatment e.g. etch and alodine or anodizing these alloys exibit a shiny or semi-shiny surface luster. Sometimes they are provided with a thin film of plastic protection which certainly connot be weathered.

OK now I will have to peel off some plastic sheet from both unclad and clad and rub it with bare hands and I expect to find the black smut just as with the referenced clean alloys as described above all of which generate the smut when handled more than with just a few touches.

Could the white layer on weathered Al be Aluminum Hydroxide (water + Al + Al2O3)or would that be the black that uncle refers to? Are nano or near nano sized particles the only explanation for the black?