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?