Actually "odds are" was a manner of speech. It's the iron that rusts in lesser alloys of stainless and it's the iron that the magnet reacts to. No magnetism, no iron, no rust. It's actually a very good test, probably better even then trusting a chandlary that says it's 316. Got this trick from a Pardey book and confirmed it once with a friend who has a masters in metallurgy.
Med, I was trying to acknowledge that I understood what you meant by, odd are. Sorry, if I was confusing.
"No magnetism, no iron, no rust" may be helped with some clarification. All stainless steel has iron
. Its only a matter of how much chromium has been added or other minerals that will affect its grade. More chromium is generally more resistant to iron oxide. Ironically, the reason it avoids iron oxide is the chromium on the surface actually oxidizes (rusts) in the presence of air first. The layer is so small, you just don't see it. As a protective layer it prevents the iron from oxidizing. This is also why SS, including 316, that is not exposed to air will decay, as the iron does begin to rust without its chromium oxide protection.
While 316 is highly resistant to magnetism, lower grades may
become magnetic, but are not inherently magnetic. Your buddy with the degree in metallurgy would be able to explain better than I, but magnetism is the result of the alignment of the molecules, not just the presence of iron. When you mix other minerals, you change that alignment, just like you do when you heat a metal. When you mix chromium with iron, I understand you realign the molecules to a lesser, imperceptible magnetic state.
I understand it is possible, with lesser grade SS and lesser chromium to recondition it to be magnetic, depending on how it is processed into a screw or fitting. I also understand that it can also be heated and returned to non-magnetic.
In the end, this was always my rule of thumb. If a magnet does not stick, it is stainless, you just don't know the grade. If a magnet does stick, it is not a high grade stainless, if stainless at all.