Splotchy aluminum

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jcfx
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Splotchy aluminum

Post by jcfx »

Any alumnium finishing guru's here ?

I've been trying for a better part of the day to get rid of what looks like fingerprints on some sheet
6061 alu, they were degreased with 99% IPA then random orbit sanded with a new green abrasive pad to a satin/matte surface
the weird part is the greasy fingerprints wiped off cleanly before sanding, but they appear as the surface is sanded.
I've attached a picture, it's the darker ovals in the lower left.
The marks seem shinier than the sanded surface, what's going on ?
IMG_7314.jpeg
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Bill Shields
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Re: Splotchy aluminum

Post by Bill Shields »

May not be on the surface...may be contamination of the alloy during rolling
Too many things going on to bother listing them.
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Harold_V
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Re: Splotchy aluminum

Post by Harold_V »

I'm inclined to agree with Bill, although any etching of the material could be the culprit. That the spots are shinier is a little troubling, though.

Aluminum that is unprotected has a tendency to pit (aluminum converts to aluminum oxide easily), and that may be what's causing the discoloration. Like rust, the etching may be deep enough to be difficult to remove.

Love to hear what you learn if you happen to gain some ground on this issue.

H
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jcfx
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Re: Splotchy aluminum

Post by jcfx »

I concur with what Bill said but without some sort of hardness tester to see of those spots are harder/softer than the rest
it'll probably remain a puzzlement.
The marks in the picture were finger marks, whether they were mine or the person handling the sheet at McMaster,
it must be very acidic or alkaline sweat to etch aluminum.
In the meantime a deadline has to be met, I'll investigate this further....
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Bill Shields
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Re: Splotchy aluminum

Post by Bill Shields »

If acetone and Scotch Brite do not take it off ..then it probably is not coming off.

Blast with silicon carbide grit?

Phospho might help
Too many things going on to bother listing them.
Mr Ron
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Re: Splotchy aluminum

Post by Mr Ron »

I'm inclined to believe it's acidic fingerprints that have etched into the surface deep enough that only removing metal will the spots disappear. Microscopic view of the spot may reveal if the spot is porous from the acid.
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RSG
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Re: Splotchy aluminum

Post by RSG »

I agree with Mr Ron!

It's known as "finger print corrosion" in the anodising industry. It's a phenomenon common to 6000 series alloys. I experience this preping for anodizing if I don't wear protective gloves.

Skins oils are acidic and contain chloride ions which removes the magnesia film freshly exposed from sanding it. Once it's had time (even minutes) after it was sanded for it to form a new oxide layer this problem won't happen.

Surface blemishes as a result of the extrusion usually reveal a very specific pattern once exposed. Some patterns are wavy looking when looking at flat extruded bars and annular rings like patterns for round bar extrusion. If you were to wipe it with a dilute solution of Phosphoric acid and it goes away then it's finger prints, if it doesn't then it's a tempering issue during extrusion. In the 20 years of finishing aluminium I've only ever experienced the later once and that was with an alloy from an extruder in India. Since then I've learned to buy only NA extruded alloys.

If you sand the sheet again try a bit of WD-40 immediately after and leave it for a bit before touching it. That should mitigate the problem. Once it\s had time to form a new oxide layer you should be able to touch it again without issue.
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