Aluminum Workshop: What Is the Black Residue on Aluminum?

Q: I think I have a problem with the 5356 aluminum wire I am using for gas metal arc welding (GMAW). If I take a clean white cloth, wrap it around a length of the wire, and rub it vigorously, I get a pretty big black mark on the cloth. I think the wire is dirty. The company that sold it to me says it isn’t and that this is normal. What’s the truth, and what is in the black stuff on my cloth?

A: It is possible, but not likely, that the wire you have is contaminated and has a dirty surface. All aluminum wire is produced by a drawing process in which the wire must be lubricated. Otherwise, it would snap. It is up to the wire manufacturer to make sure that all of the lubricant is removed from the wire before it is put on the spool. Usually this is done in a solvent bath. However, if the manufacturer isn’t thorough, it’s possible to leave a residue from the lubricant on the wire. This could cause the black mark you see. This does happen, but only rarely.

There is another, much more likely explanation. If you remember, all aluminum forms a very thin oxide coating after just a few seconds’ exposure to the air. If you perform your “white cloth” test on any piece of aluminum, wire, plate, or extrusion, you will get a black residue (i.e., oxide) on your cloth. More than likely, this is what you are seeing. Any aluminum wire will show a black residue when rubbed on a white cloth.

So how do you know whether the black marks that you see are aluminum oxide or the drawing lubricant residue? It’s actually pretty easy. If you have a “black” lamp (an ultraviolet light lamp), take your white cloth with the black marks on it into a darkened room and shine the black light on it. If the black marks are residual lubricant, they will fluoresce (glow) under the black light because they are hydrocarbon compounds. If the black marks are aluminum oxide, they won’t glow.