Stainless steel must contain at least 10.5% chromium. It is the element that reacts with the oxygen in the air to form a complex chrome-oxide surface layer that is invisible but strong enough to prevent further oxygen from staining or rusting the surface. Higher levels of chromium and the addition of other alloying elements such as nickel and molybdenum enhance the surface layer and improve the corrosion resistance of the stainless material.

Stainless does not “rust” as you think of regular steel rusting with a red oxide on the surface that flakes off. If you see red rust it is probably due to some iron particles that have contaminated the surface of the stainless steel and it is these iron particles that are rusting.

There are several types of stainless steel. Martensitic (400 series) stainless steels are strongly magnetic. Austenic (300 series) stainless steels which contain nickel have very low magnetic permeability when annealed. The situation is far less clear when these steels have been cold worked by wire drawing, rolling, heading or even heavy polishing,all of which (except the last) are common aspects of fastener manufacturing. After substantial cold working a Grade 304 stainless fastener may exhibit quite a strong response to a magnet. In general, higher the nickel to chromium ratio the more stable the austenic structure and the less magnetic response that will be induced by cold work. This means Grade 316 will be in most instances almost totally non-responsive because of its higher nickel content.