Soldering aluminum tubing to steel sheet metal

I've got a tank made of mild steel sheet metal, and I want to permanently attach some aluminum tubing to the bottom surface of the
tank. The tubing is for some hot fluid (200F-250F) that will be used to heat the contents of the tank. The aluminum tubing is only 3/8" diameter, with a wall thickness of about 0.060" (maybe 1/16"). I estimate that the sheet steel that I'm attaching it to is about 16 guage, maybe 18 guage (0.050" to 0.060").
What's the best way to do this and the best tools to use for the job for a DIY type person like myself. I'm not a professional, but I do have some experience welding and soldering. I have both an oxy-acetylene torch setup and a cheap Propane torch that mounts on a small hand held propane cylinder.
I've done a bit of reading and research, and I think I should probably use some Cadmium/Zinc solder, probably 30% or 40% Zinc. I'm not sure exactly what flux to use on the steel, or on the aluminum.
I'm kind of guessing that the propane torch would be a better choice for this job because I'll need more general heating of the materials and indirect heat. I've got some extra material of both types to practice on as well, so I won't start out ruining my main project.
I'm just asking for advice or confirmation that I'm on the right track here. Also, would a normal welding supply place have the appropriate solder? What kinds of flux would I need for this? I plan on calling them tomorrow and asking, so I guess I'll find out.
One resource I checked said not to use flux on the aluminum, but to apply the solder with steel wool to scrape away the surface oxidation on the aluminum. The procedure described was to heat the aluminum with the torch, drop a few drops of melted solder on the aluminum, then use steel wool to work the surface of the aluminum while continuing to apply heat to maintain the temperature slightly above the melting point of the solder. As the oxides are scraped clean, the solder will wet and tin the aluminum. I'm probably going to be soldering down 12 to 18 feet of this tubing, so that sounds like a lot of work with the steel wool. Would there be a better method using some kind of chemical treatment or flux to remove the surface oxides from the aluminum?
I plan to tin both surfaces to be joined with the solder, then sweat them together and apply additional solder to fill any gaps and build up a little under the tubing. I want as much heat transfer as possible from the aluminum to the tank in the finished product. I'm assuming that I'll need 30% to 40% zinc in order to have some decent paste range to work the extra solder into the gaps and build up around the bottom half of the tubing.
Also, this surface will be exposed to weather (after painting), so corrosion resitance is fairly important, as well as strength.
So, am I on the right track? Or am I about to make a huge mistake?
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On 9 May 2004 14:39:37 -0700, racer snipped-for-privacy@winning.com (Racer X) wrote:

I did a bit of research for one of my students that is restoring a Fuller-Hammond aircraft and found that (like this feat was apparently achieved 70 years ago) the best course is one of the alloys sold for joining dissimilar metals. There are quite a few welding rods available that meet the needs you describe and a Google will yield quite a few results....
Good luck on your endeavor....
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Racer X wrote:

Could you use copper tubing? In that case you can use regular lead/tin solder. Cadmium fumes are unhealthy to say the least, they can kill you. HTH
--
SATOR AREPO TENET OPERA ROTAS
Have 5 nice days! John
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This sounds like a great deal of work and you may be unhappy with the results. I would substitute copper or mild steel tubing instead for the heater coil, and solder with regular soft solder.
Jim
================================================= please reply to: JRR(zero) at yktvmv (dot) vnet (dot) ibm (dot) com =================================================
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I agree with Jim. It would probably cost less to buy copper or steel ( see your friendly scrap yard or auto supply ) tubing than to buy the solder and flux for aluminum. Soldering aluminum with ordinary solder is possible, but a PITA for anything but small items.
If you do try this, you might try scrubbing the oxide off the aluminum with steel wool or sandpaper under a film of oil. And leaving the oil in place while you solder. Cleaning under molten solder work well for small items.
Dan

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Off the wall but.. In this area, I have a plater that can plate copper and/or chrome onto aluminum. You might inquire in your area. Copper does solder to sheet metal. A thin plated finish might offer what you need to accomplish what you need. R. Wink
On 14 May 2004 11:11:22 -0700, snipped-for-privacy@krl.org (Dan Caster) wrote:

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Racer X typed:

The task you are trying to accomplish is very possible. You are dependent on your choice of solder and flux's. I would recommend the Harris brand. Harris Stay-Brite 8 silver solder would be my choice for the solder. It will bond both metals. http://www.jwharris.com/jwprod/solderalloys/#SBSolder
My choice for the flux on the steel would be Harris Stay-Clean flux. It is available in liquid and paste forms. http://www.jwharris.com/jwprod/brazesolderfluxes/#soldflux
Additionally, Harris makes an aluminum flux for soldering using the same silver solder mentioned above, Stay-Clean Aluminum. It is also on the flux page I posted above.
The solder has very high strength and is food safe. The web sites I posted tells it all.
I would "tin" the pieces with solder - using the appropriate flux - and then join them together as a unit by adding additional solder. A propane or Mapp torch should be more than adequate. I have tremendous success with this solder and flux combination. I do a bit of work in stainless steel, copper, brass and aluminum. Most heating/air suppliers that I'm familiar with carry this brand.
I hope this helps. PJ
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Thanks for the informative reply. I'll check into the Harris products.
Have you ever used the Harris Alsolder 500? It was on the same page with that silver solder. Would that be a better choice for me considering I'm working with aluminum?
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Racer X typed:

Alsolder is a possible choice for you, although, the Alsolder 500 does not produce joints as strong as the Stay-Brite 8 solder. I'm also not too thrilled with the zinc content. My preference is the Stay-Brite. It bonds very well with aluminum and has excellent strength. It also has a liquidus temperature range that allows it to be used as a filler metal.
Just my 2 cents. (BTW - I'm not affiliated with Harris. I just use the products.)
PJ
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I like the Stay Brite and a more active flux.
I'd lay lengths of Stay Bright aside the joint, adhered with a fillet of flux, and heat the whole job in the oven.
Preferably, you can block up the metal and support it flat, weight the tubing into contact with fire bricks, and just heat soak it at the reccommended liquidus using an oven thermometer to go maybe 15 degrees over so it won't take forever. You can shield the work from the infrared of an electric oven to prevent overheating.
The more active flux must be hand brushed with hot water until spotless, or it will corrode.
You can pull the lengths of solder to straighten them, then snip and handle carefully. You can also tack the ends of each to the tank before putting it all into the oven.
I have and recommend The Aluminum Association's Aluminum Soldering Handbook.
Yours,
Doug Goncz ( ftp://users.aol.com/DGoncz/ )
Read about my physics project at NVCC: http://groups.google.com/groups?q=dgoncz&scoring=d plus "bicycle", "fluorescent", "inverter", "flywheel", "ultracapacitor", etc. in the search box
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