I have pretty freshly machined aluminum but with white silicone grease (heat transfer compound) on it, and using toluene or mineral oil to wipe the aluminum, my rag keeps coming up black, which never stops. What am I getting on the rag? Chemical compatibility charts say aluminum has excellent compatibility with both. Seems to me I remember other metals never seeming to come clean, too. What is this stuff?
Thank you!
I suspect you’re getting aluminum oxide coming off. which will form on the surface of aluminum pretty rapidly. Nominally white, it probably looks black mixed with other substances on the surface.
It’s a good thing that oxide layer is there. It prevents worse things like mercury from interacting with and corrupting your aluminum
What @CalMeacham said.
Mineral oils and Toluene undergo slight oxidation from atmosphere. I suspect the small amount of acidity from this oxidation is aiding in picking up the oxide layer.
Try a fresh bottle of solvent and see if it helps.
Is it pure Aluminium or an alloy? Some of the metals commonly alloyed with Aluminium have dark coloured oxides
Great question! Should have thought to say, sorry. It’s 6061:
| Al | Mg | Si | Fe | Cu | Cr | Zn | Ti | Mn | Remainder |
|---|---|---|---|---|---|---|---|---|---|
| 95.85 - 98.56 | 0.8 - 1.2 | 0.40 - 0.8 | 0.0 - 0.7 | 0.15 - 0.40 | 0.04 - 0.35 | 0.0 - 0.25 | 0.0 - 0.25 | 0.0 - 0.15 | 0.05 each, 0.15 total |
Not sure what to expect from those constituents. The magnesium ought to act pretty much like aluminum, right? Silicon, chromium, and titanium would I guess not contribute dark oxides. The iron and the copper would, I suppose. But they’re all pretty minor components.
As does powdered aluminum.
Of course what we’re looking at is undoubtedly aluminum oxide – I couldn’t find any pix of Al in a nitrogen chamber.
I guess what I’m befuddled by is this – why is the toluene or mineral oil so much better at getting black stuff than just rubbing with the dry cloth? It certainly gives the impression that it’s a reaction with the liquid. I’ve tried isopropanol too, and it has the same effect but much less strong. That makes am77494’s suggestion more attractive (but alas I have no fresh bottles here).
I might try some WD40. I have that in spray cans that would of course prevent any oxidation of the contents from the outside.
I don’t know anything about the chemistry, but what color is your rag? IME if it’s a white rag anything that gets on it will look black and if you use a black rag anything you get on it will look white. I wonder what the actual color of the substance is.
Aluminium oxide is white in colour. Powdered Aluminium is grey because the metal is grey/silver
Some aluminum oxide is white, but definitely not all of it. When’s the last time you saw a white sapphire or a white ruby? Or white sandpaper? (Black aluminum oxide is often used as the abrasive medium in sandpaper).
@am77494 and @CalMeacham got this in two. Aluminum oxide has many phases and many different colors.
I third their suggestion that the black stuff is just the standard Al oxide layer that forms almost instantly on the aluminum.
I know that whenever I’ve been handling a lot of aluminum conduit (especially the flexible stuff), my hands have always ended up blackened, without any oil or other solvent beyond what my skin produces.
It may be that the oil is helping you lift the oxide off of the metal – otherwise you’re just rubbing it in, which stops cutting when you get a polished surface.
I only mention it because it’s one of the things that happens with a fine whetstone: it gets filled up and smoothed off, and stops cutting. A drop of oil or water prevents that. I don’t actually know anything about rubbing cotton on aluminum.
Those colours typically come from the presence of other elements in the mix (small amounts in gemstones, disrupting the crystal structure, larger amounts in industrial materials). Remember I said:
The color of aluminum powder depends on the shape of the particles. If they have a lot of flat or curved surfaces they’ll reflect a lot of light. Erosely shaped particles will appear dark grey and almost black against the right background. Also, if you want to see aluminum it would have to be kept in a container of inert gas because it oxidizes on exposure to oxygen faster than they eye can detect. Luckily a passivating layer of aluminum oxide forms on the surface of it or it wouldn’t be worth refining in the first place.
For what it’s worth, I’m using white cotton cloth, and white paper towels, and the blackness is moderate to profound. I might try to find a bit of black cloth to see how its appearance changes.
Tell you what, though – I have tried a few times to get very glossy surfaces on aluminum or copper or stainless, to give them a low emissivity, and also having tried to do this and also deliver flatness, to give good thermal contact between parts. I don’t have my own machine shop but I have also made some efforts to get this through professional metal finishers. Glass optical surfaces seem comparatively easy to make glossy and also flat or accurately spherical. Metals, especially soft sticky ones like aluminum and copper, are surprisingly difficult!
Hey Napier, that’s a cool thing you do. Would love to see some videos if you have any.
I read somewhere long back that professional astronomers spin a tub of mercury to get concave lenses of high focal lengths.
Also wanted to add that flat aluminum shiny surfaces for heat transfer did not work for the microchips. They had to resort to thermal paste for heat sinks. A lot of good research has gone into that.
Meant concave mirrors not lenses. (Past edit window)
Robert W. Wood did this about a hundred years ago
Raymond Z. Gallun used the idea in his 1934 short story Old Faithful, which has a Martian making the mirror for his telescope this way.
The idea is attractive because the surface of the spinning mercury is very nearly a perfect parabola (rather than the spherical section that standard hand-grinding gives you, assuming that you have very uniform rotation) And, of course, a paraboloid is the shape you want for an astronomical telescope.
The drawback is that it only has this shape if it’s pointing straight up. If you tilt it, it’s no longer a perfect paraboloid, because gravity distorts it.
Astronomers are using liquid mirrors, though. If you google it, you’ll find lots of articles.
There are other uses, too. I knew someone who suggested using spinning liquid metal mirrors for focusing laser fusion beams, because the mirrors wouldn’t be damaged by flying debris. Also, I know of a hydrogel contact lens manufacturer who used spinning cups to form one surface of the contact lenses.
The metal doesn’t have to be mercury anymore, either. There are lots of low-temperature melting eutectics that don’t need much heating (the classic example is Wood’s metal, named after the same R.W. Wood). In fact, at least one such eutectic – galinor – is liquid at room temperature, without having the toxicity of mercury.
Absolutely! I’m working on two different problems – studying heat transfer from shiny metal surfaces in air, and also the construction of equipment for that work, said equipment requiring thermal connections between metal parts. So in some cases I am using thermal compounds, and dealing with their stiffness in large joints. There’s lots of information out there about IC sized thermal compound connections to heat sinks, but not so much about e.g. 300 mm square connections. Some people use diaper cream…
I wonder if these metals would become less sticky (and easier to precisely grind) if they were chilled (and somehow kept chilled during the grinding)