Chromium is the element that make`s stainless steel stainless or corotion resistant. Stainless steels are iron-chromium alloys (most have a low carbon content) that contain at least 11% chromium. The level is the content at which effective resistance to atmospheric corrosion begins. Other compositions , Nickle, Manganese, Molybdenum, Silicone, Sulfer.
Remember Stainless has many different types 200,300,400 each series has several different types say 301,302,302b,304,304l,305,308, ext. ext. all have a diferent makeup for many different applications. Stainless, brass,aluminium all are non ferrous or non magnetic family even thought they do have a little iron content. As Riffer said stainless can become magntic once worked at once cold worked and would need to be annealed to be brought back to a non magnetic state. David