Which metal cannot be soldered?

Aluminium is the metal hobbyists run into most often that standard electronics solder won't join. The moment bare aluminium is exposed to air it forms an oxide layer that reforms almost instantly if you scrape it away, and ordinary rosin-core solder and flux aren't aggressive enough to break through it. The same problem applies, to a lesser extent, to stainless steel and cast iron — both need specialist high-temperature solders and much stronger acid-based fluxes designed for the job, not the rosin flux used for circuit boards and wiring.

Metals that solder well with a standard electronics iron and rosin-core solder are copper, brass, tin-plated steel, and pre-tinned component leads — anything the solder can "wet" and bond to at 300–370°C. If you're working with an unfamiliar metal and the solder is beading up and rolling off instead of flowing flat, that's usually the oxide layer or the base metal itself refusing to accept it, not a fault with your iron or technique.

This is a different issue from why electricians avoid solder for fixed household wiring — that's a wiring-regulations decision, not a metallurgy one. For getting a joint right on metals that do solder, see our beginner's soldering guide.