tips for oxy acetylene welding aluminized exhaust pipe

[Tmoose]

When patching together "custom" exhaust parts for various vehicles I often use sections harvested from cars with OEM heavily aluminized exhausts. Lately I've been having problems with achieving fusion with the aluminized parts. Removing the coating with abrasives works OK, but a convenient wire wheel seems to just smear the aluminum. Previously I could "break up" the aluminum coating by manipulating/dabbing the filler rod when the steel was molten, but recently that has not worked particularly well.

On a welding bulletin board someone suggested that the copper coated Oxy acetylene filler rod was part of the problem, and uncoated filler would work much better. I think someone claimed that coat hanger was a great filler for exhaust work, but I am reluctant to start using that unknown stuff.

Is there an "official" explanation for persistence of the aluminum coating, and a quick, easy, cheap way to overcome it?

thanks

Dan T

 

[berkshire]

Dan,
Have you tried dipping the ends of the tubes in a caustic soda solution? This will strip off the aluminum without harming the steel.
Don't put the tube in the solution any deeper than you want the coating removed.
B.E.

You are judged not by what you know, but by what you can do.

 

[tbuelna]

A coating of pure aluminum on steel provides a very durable and cost effective corrosion protection. The aluminum coating used on automotive exhaust systems is usually applied after fabrication. The mild steel exhaust parts you are attempting to weld have probably been aluminized on both the inside and outside surfaces. So you need to remove the aluminum coating from both surfaces well beyond the weld location. The aluminum coating has a melting point well below the temperature required to fuse the steel materials being welded.

 

[berkshire]

I have run into the same problem repairing Aircraft landing gear pieces that have been flame spray coated after fabrication.
Scrubbing with a wire brush after heating will remove some , but not all of the coating, and the oxide scum gets into the weld contaminating it. Heat the end, scrub it with a stainless brush to remove the worst of the coating, then dip it in caustic soda ( Sodium hydroxide.) until it stops fizzing.
B.E.

You are judged not by what you know, but by what you can do.

 

[Tmoose]

thanks all

 

[kingnero]

Surprised that noone mentioned this before, but I would strongly advise against using coat hanger wire...

If you know the type of steel that is underneath the aluminum coating, suitable filler can easily be chosen. Coating needs completely be removed. Won't it be a problem that after welding, the inside of the exhaust tubing won't have this protective coating?

 

[moltenmetal]

Regardless what you do, the welded areas will corrode first- not the weld itself, but the area adjacent to the weld where the coating has been stripped and the section is still thin. But that's not as much of a worry as not being able to weld the stuff in the first place.

Dipping the ends in strongly alkaline media (sodium hydroxide solution, a convenient source of which is a drain cleaner product, the solid versions of which sometimes also contain chunks of zinc or aluminum) will remove the aluminum coating, and will be more convenient than mechanical means especially on the interior.

 

[Tmoose]

" The aluminum coating used on automotive exhaust systems is usually applied after fabrication. "

Dunno. The way the OEM systems rust after a mere 15 years makes me think individual parts made of aluminized sheet or tubing/pipe are sometimes welded together.

https://volvo850wagon.files.wordpress.com/2012/06/dscn7940.jpg

And maybe gonna try MIG welding to blast thru the aluminization.

The low melting point of aluminum makes me surprised it is not easier to dislodge when it is floating like pond scum on top of the molten steel pool.

 

[LadaTrouble]

We struck this problem in the '70's when this stuff first appeared, it turned our nice gas welds into a mess. Grinding it off works, but not always possible in exhaust repair. Agitating and scraping the scum off the puddle with the rod helps - you're not going to get a good looking weld anyway, but it'll still be strong. The problem is still there with MIG, you have less time to work with, and no second hand to work the scum off the puddle. I have used a Dillon/Henrob torch too, and it doesn't help. I'm still frustrated 40 years later.