Tom's aluminum soldering demonstration failure

Personal anecdote — Tom's assistant-professor-era aluminum soldering demo using tin/zinc chloride flux fell apart in his hands a year later because retained flux on blind joints was hygroscopic. Illustrates why reaction fluxes for aluminum are limited in practice.

The problem is tin chloride and zinc chloride are extremely hygroscopic. If you leave this flux out for 4 hours in Cambridge exposed to the air, it's no longer any good. It will pick up enough moisture from the air that the tin reacts with the moisture to form tin oxide and hydrochloric acid. You no longer have any fluxing capability. You just destroyed your tin chloride or your zinc chloride. I once soldered up some aluminum for a demonstration back when I was an assistant professor — this is a few years ago, before you were born. I got some of this aluminum soldering flux and everything worked great. I did my little demonstration in class on slip planes, it had to do with deformation. The next year I took it off the shelf and it just fell apart in my hands, because I had some blind joints and I'd left some of the flux on the inside. Air got on the inside, attacked the solder flux residue, and just corroded the joint within a year. So I had to redo it and make it out of steel and weld it up. It was a mess. And now it weighs a ton when I'm trying to do the demonstration for the students. Actually, I haven't done it for 30 years because it does weigh a ton.