I am NOT a scientist. Some of you who may be....please verify or correct me here. Iron oxide is NOT magnetic. Steel, of course, is magnetic at room temp. So if you have a layer of iron oxide on steel, depending on how thick the oxide layer is, the magnet may not stick. But in application.......
Take a carbon steel knife and let it lay outside for a month to collect rust. My guess is the magnet will still stick to the blade. Because the oxide layer is very thin relatively, and there is plenty of mass of steel underneath to attract the magnet.
I realize now that is kinda opposite of what you are asking. Magnetising a blade will not cause it to resist corrosion.