Wiping aluminum with toluene or mineral oil - why do I keep getting black on the cloth?

Factual Questions
21

I am not exactly sure of your application, but I’ve seen shiny metal surfaces (aluminum foil actually) being used as one of the layers for insulating cryogenic equipment/piping. The shiny layer faces outside and the primary purpose is to reflect radiative heat transfer. Is this your application ?

In the chemical industry, aluminum is used a lot in Cryogenic services. The primary method of joining them is to weld (using Metal Inert Gas welding) or Brazing (as in Brazed Aluminum heat exchangers).

It is also the general practice to have aluminum fins attached to surfaces (like the car radiator) for better heat transfer.

22

I’ve worked with a lot of uncoated copper mirrors in my time (copper is an excellent reflector in the infrared – we used it for CO2 laser beams all the time). It can be made very shiny. So can aluminum. In most visible applications the oxide coating doesn’t have an effect, and front-surface aluminum optics are very common.

If your issue is obtaining a very shiny aluminum or copper surface, you can purchase such items from optics suppliers. If the oxide coating is a problem, you can get the surfaces overcoated with something like magnesium fluoride, which gives you a very shiny surface that won’t oxidize. In fact, for a project where I needed precisely this I had parts constructed of aluminum first polished, then vacuum coated with aluminum, then overcoated with mag fluoride to give me extremely highly reflecting, non-tarnishing surfaces.

If you are looking for flat, extremely shiny reflecting surfaces that won’t tarnish there are chemically coated metals and alloys like Coilzak that are wonderfully shiny and resistant to degradation.

23

Hi CalMeacham, I always look to you first for answers that rely on working with optics. And I know about getting things coated with metals for reflectivity, and in fact we are doing that with one of the items we work with, relying on somebody with a small shop that specializes in custom telescope mirrors, which warms the heart of this one-time astronomy student and ATM.

But the components I’m working on currently need to be large thick plates of highly conductive metal, around 2 or 3 inches thick and of a foot or more diameter or square size. We bit the bullet on one copper slab that is so heavy it requires equipment to move it, but currently have six other components of aluminum, each light enough that one person can lift it. You did say one thing I didn’t think of, which is getting aluminum polished and then vacuum coated. I’m intrigued by the thought of doing this with an aluminum coating or a gold coating, and maybe an overcoat to block tarnish.

The whole goal is to get low emissivity. I have applications that operate in air at temps up to 60 C, and other applications in air up to 260 C, and some in air up to 400 C. This has come up in several different ways over the past 20 years. In fact for a while I was working pretty hard to get gold electroplating to work over copper at up to 400 C, and we were plagued with copper diffusing through the gold and tarnishing at the surface, talking about nickel intermediate layers, considering going with massive nickel parts electroplated with gold to fix the copper problem, etc etc.

I guess my best question to ask you is, how do I search for vendors that can create glossy polished surfaces in aluminum or copper? “Metal finishing” has only been a little successful, and I think they don’t produce surfaces that would be appreciated in optics. Is there a business niche I don’t know to search for?

In particular one challenge has been getting an accurately flat glossy surface in a heavy conductive slab. If I only needed flat and glossy, a first surface mirror on Pyrex or silica would be perfect, but how to get it in metal? Some shops will quote on quite stringent flatness and also surface roughness specs, but then have some guy with a right angle grinder and buffing pads supposedly do it. And I’d much rather believe I can get what I spec from the right kind of vendor, rather than start trying to figure out how to verify performance.

I have thermal laboratory facilities, but not optics, other than a few boxes of bits and pieces I ordered from Edmund and Thorlabs.

24

I could put some numbers on this. Our simulations and modeling have maxed out at 10 nm Ra surface finish and 100 nm flatness, which I think metal coated glass optics achieve. But we would have appreciated realizing 100 nm Ra and 1 um flatness for hand-sized metal parts. If we were spending $10 per square cm to achieve this we would have been buying, though maybe only a little bit if $100 per square cm.

25

Good Lord, there’s a lot to unpack in those two posts.

I’m not even ngoing to try to answer right now. I’ll send you a message with my e-mail, and you can get in touch with me. I think I;'d like to know a bit more about your project before I reply.

For a quick reply, though, you can find optical parts and services through the buyer’s guides at Photonics Mrketplace – Photonics Buyers' Guide & Marketplace for the Industry and at Optics dot org – optics.org - Photonics Buyers Guide and the Laser Focus Buyer’s Guide at https://www.laserfocusworld.com/directory

26

I don’t know exactly what @Napier does for a living, but it sure sounds like fun. Difficult, brain-bending fun, but fun nonetheless. A Mad Scientist with lotsa corporate money behind him! The guidance counselor forgot to mention that job when I was asking for advice.

27

Absolutely. And doing CFD at the same time.

But don’t get too jealous. I have to give PowerPoint talks about it, too…

28

Aside: One of my professors (a solar physicist) once went to a Halloween party as a CFD junkie. He wore a signboard that said on the front “Addicted to CFD. Please help. God bless.”, and on the back, “Will work for supercomputer time”, plus some MHD equations.