FYI, hard anodyzed aluminum has a surface that measures in the 80's on the Rockwell C scale, much harder than steel can be hardened to. It's basically the same Aluminum Oxide that is used for sandpaper and grinding wheels. Sig Sauer anodyzes all of their aluminum frames and I'd be shocked if S&W didn't do the same, because it's a very simple dip process in an acid bath. Don't remember the specific acid but expect a google search would reveal the details. After that dip the aluminum is then typically die to produce the desired color and finish.
The key to getting good service life with an aluminum frame is maintenance and lubrication. That means keep it clean and use enough grease that's formulated for good retention and wetting properties. On a Sig Sauer the recomended practice is to run them quite wet with grease, enough that when the slide is assembled to the frame the excess oozes out and has to be wiped off.
As for stainless steel and magnets, only the 300 series stainless steels are non magnetic and that's only if they haven't been cold worked. Want to freak out a car nut with a new stainless exhaust system? Run a magnet on the bends, every one will hold a magnet, however on a long straight section the magnet won't be attracted. Typically firearms use a 400 series stainless which is fully magnetic. Most common grade used is 416 because it's very similar to 4140 steel and can be heat treated using the same methods as 4140, in fact 416 can be heat treated in the same batch as 4140. Another common firearms stainless is 436, however it's use isn't as widespread.
