How does patina on knife steel prevent further rusting?

Discounting the “cleaning” action brought about by constant use, how does a thin layer of oxide prevent oxidation from penetrating deeper?

Oxygen molecules oxidize the steel by combining with the molecules on the surface of the knife. There is room for a large but finite number of molecules. Once all the places on the surface that an oxygen molecule could attach to are taken, it’s essentially sealed, until you scratch the surface.

Hehe. Nice and sweet. Thanks.

The problem with steel blades rusting is that while it slows down oxidation it does not stop it. Over time the rust will degrade the blade surface causing pitting which opens up more fresh iron surface for rusting. Eventually the entire bade will be consumed or damaged to point it’s unusable.