Actually, some of the 400 series stainless steels have little to no ferrite in them as they are martensitic stainless. The main problem with stainless steel rusting is contamination from free iron, which could come from tools, contaminated grinding wheels, polishing wheels, etc. All stainless steels, regardless of their microstructure (ie austenitic, ferritic, or martensitic) have enough chromium in them to form a passive oxide film if treated properly. Stainless steel will corrode in certain environments, but it is not in the form of iron oxide, or rust. But as John pointed out, some stainless steels will corrode in milder environments than others, and some of the less corrosion resistant stainless steels are in the 400 series. But they will all stand up in the right environment. The key is knowing what it will be exposed to, and which stainless steels are designed resist corrosion in that environment.