Joining copper to aluminized ( exhaust pipe) steel

pat32rf · Nov 8, 2016

Building a couple of sidearm heat exchangers for my outdoor furnace domestic water heater system.
A piece of 3/4" copper running in a piece of 2" steel exhaust pipe. The steel pipe is exposed to treated water while the copper pipe only has domestic water in it. A 3/4" hub welded to the side of the steel pipe at each end while the copper extends out the middle.
Max pressure is about 5psi at 190 deg F....
Made one last year and I used washers to reduce the steel pipe down to the copper od size but this time I want to swage/forge/ reduce the steel pipe and braze it right to the copper so it looks a little better.but I am having a hard time getting the steel clean enough to braze. No problem with the copper pipe, just the steel where it has been heated.
Any hints?

Texasmark · Nov 8, 2016

My guess is getting the right flux. Assume you are using a brass brazing rod. In the metal compatibility table I see that tin and steel are in the same group. I know that regular tin-lead solder adheres well to copper. Might try some 95/5 (tin/lead) "high temp" solder which I used in industry for high stress joints along with a good acid flux. I don't have the name handy, but my best acid flux is a white paste in a little white plastic jar, about 2" dia x 2" tall.

yomax4 · Nov 8, 2016

Couple things come to mind. If you want to use oxy-fuel just silver solder it. I would suggest the black flux for high heat but white will work if you don't over heat. I'd say 45% silver because the lower silvers struggle a little with vastly changing temps. I've even Tig welded steel to copper and brass with silicon bronze.

I put a coil of copper tube inside my stove water jacket and run a pump on the water heater to get unlimited hot water.
Good Luck !

muddstopper · Nov 8, 2016

I have never tried what you are trying to do so take this with a grain of salt.
My first thought is that you are trying to join three types of dissimilar metals in one process. Since one of the metals is aluminum, i dont think you will fine a flux and brass rod for brazing. Silicon/bronze maybe. I think you will most likely have to go to a silver solder product with a high silver content. Regular plumbing solder like at the hardware stores probably isnt going to have the silver content needed. At least Homedepot and lowes in my area doesnt carry anything I think would work. If you can remove all the aluminium coating from the steel pipe, then I think brazing or even lead solder should join the copper to steel without any problems. To remove the aluminum coating you would have to sand the pipe and to get it really clean maybe soak just the sanded part of the pipe in Muratic acid to dissolve any remaining aluminum coating you didnt get with the sanding, rinse with water and then try soldering. Like I said, I havent tried any of this, so it may or maynot work.

nikdfish · Nov 8, 2016

I used to use the white granular "cast iron" flux & bare bronze rod for brazing dissimilar metals, always worked for me ...

Nick

Gary Fowler · Nov 8, 2016

I have used the cheapest grade of silver solder (about 5% silver I think) on steel and copper with good results. I just use the white flux for it.

pat32rf · Nov 8, 2016

Actually my rod is left over silicon bronze from when I used to work with SB hot water tanks. I have a few different fluxes, including some for silver plus some muriatic acid. Guess it's just a case of cleaning and watching the temp. Been twenty five years since I was paid to do this kind of stuff....

yomax4 · Nov 8, 2016

pat32rf said:
Building a couple of sidearm heat exchangers for my outdoor furnace domestic water heater system.
A piece of 3/4" copper running in a piece of 2" steel exhaust pipe. The steel pipe is exposed to treated water while the copper pipe only has domestic water in it. A 3/4" hub welded to the side of the steel pipe at each end while the copper extends out the middle.
Max pressure is about 5psi at 190 deg F....
Made one last year and I used washers to reduce the steel pipe down to the copper od size but this time I want to swage/forge/ reduce the steel pipe and braze it right to the copper so it looks a little better.but I am having a hard time getting the steel clean enough to braze. No problem with the copper pipe, just the steel where it has been heated.
Any hints?

You are sure that it's Aluminized right? I know there is aluminized out there but a lot of it is 409 or 304. It would make a big difference in joining to copper.

Tractor Seabee · Nov 9, 2016

I can visualize that the fit up is probably not very precision. Silver solder is the way to go but use BCUP which is 5% silver. It will fill over gaps where the high silver content alloy works best with a tight fitting joint that relies on capillary attraction like soft solder. All solders rely on a mechanical connection joint for strength. Bronze rod will also work but requires a OA torch. Silver you can use a propane/air torch. Get the right flux for the application. I would just burn the aluminizing off as it is a thin coat; then polish the steel. I used tin (thin coating the steel with 45% silver then use the BCUP. Years ago I had some aluminum brazing rod that would join aluminum to copper and/or steel. A google search may find it yet as there are many aluminum brazing alloy and procedures. Brazing by definition does not melt the base alloy.

Ron