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Aluminum Oxide Layer and Soot

Hello, metalworkers!

Haven't been by in a while. Been busy getting ready for a six month volunteer stint.

We know by sooting aluminum and torching it the soot will burn to indicate the annealed condition has been reached. I doubt this is more than a coincidence as I read the aluminum oxide layer become mobilized and can be removed by rub soldering at some temperature, so I speculate annealing happens to happen at near the same temperature. That is, buring off the soot doesn't anneal the aluminum, it's the heat that anneals the work.

Is this an effective method of preparing aluminum for soldering? Does the soot act as a powerful reducing agent, turning the Al2O3 into reduced Al at the surface? Can presooting aluminum, torching it to remove the soot, and applying flux with the tip of a solder wire, then rubbing with solder wire be a way of tinning aluminum for soldering?

Inquiring minds want to know. So do I. :)

I have the equipment, solder, and flux, but not a single scrap of aluminum other than a beverage can.

I'll be working with 0.080 inch thick aluminum soon.

Yours,

Doug Goncz ( ftp://users.aol.com/DGoncz/ ) Student member SAE for one year. Loves in my life: Dona, Jeff, Kim, Mom, Neelix, Tasha, and Teri, alphabetically. So that is who I spend my time with.

Reply to
Doug Goncz
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Hi Doug! (Snip)

The way that I misunderstand the process is as follows:

  • Use an uncontaminated (by other than aluminum) stainless steel brush to knock off the thickest part of the surface oxide of both pieces to be joined.

  • Clamp the pieces in place and cover them as much as possible with a thermal isolating blanket. (Kaowool)

  • Heat the bejeezus out of the pieces until tin solder flows.

  • As you add to the puddle, use the stainless brush in your third hand to scratch the aluminum oxide off the surface under the puddle.

  • Keep scratching for the entire length of the joint. The solder will act to isolate the new surface from oxygen. New oxides will not form.

  • Let the workpiece cool. Be careful that the pieces not move during the critical liquid - plastic - solid phases.

I have never used this process but it seems reasonable. A bright fella such as yourself will probably want to build an argon chamber to further limit oxide formation.

Let us know how it goes OK?

--Winston

Reply to
Winston

That's correct.

No. Aluminum's affinity for oxygen is much stronger than carbon's affinity.

The best expedient way of tinning aluminum I've found is to coat the aluminum with oil, scrap the surface with a sharp object to get below the oxide layer, then apply iron and solder to tin the surface *under* the oil. The oil will keep oxygen away from the cleaned aluminum so the solder will stick. Once you have it well tinned, you can clean off the oil and solder to it as you would any other tinned surface.

Gary

Reply to
Gary Coffman

Many years ago I saw a soldering iron advertised that incorporated a rotating brush driven by a small motor specifically for this technique. Tinning under a pool of oil is also supposed to work.

Leon

Reply to
Leon Heller

Thanks, Winston, Leon, and Gary!

Yours,

Doug Goncz ( ftp://users.aol.com/DGoncz/ ) Student member SAE for one year. Loves in my life: Dona, Jeff, Kim, Mom, Neelix, Tasha, and Teri, alphabetically. So that is who I spend my time with.

Reply to
Doug Goncz

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