Knife Science / Can Stainless Steel Rust?
The common misconception is that stainless steel means "rustproof". The reality of it is that this is just not true. Stainless steel is an amazing material that makes a lot of things we take for granted in modern life possible—but it really is only rust-resistant.
There are a variety of conditions under which rust can form on stainless steel but the one that is of most concern to knife owners is extended exposure to low-oxygen environments. What does that mean?
In the simplest of terms stainless steel contains chromium. When chromium interacts with oxygen it creates a protective layer that does not allow rust to form. If this layer is removed by a scratch or abrasion it can repair itself on its own. It is in fact self-healing. However, if the chromium does not have access to the air (oxygen) to bond with this protective layer cannot form. The result is rusted stainless steel.
The most common culprit is when a dirty or recently cleaned, but still wet knife is returned to its sheath, or folded and stored away as in the case of a folding knife. Best practice is to dry your knife as much as possible and then allow it to continue to air dry. If this is not immediately possible then doing so at the next possible time should suffice.
If you're interested in learning more about conditions under which stainless steel can rust follow this link.