SR-77 is a non stainless steel. A DC finish is bead blast, basically.

Steel and bead blast = small pitting on surface. Pitting attracts moisture. Moisture on non-stainless steel causes a redox reaction which means rust. The iron ( Fe(II)) in the steel reacts with the oxygen(O2 is the oxidizing agent) in water (not on the molecule but actual gaseous oxygen in water) and forms iron(III) oxide, a.k.a rust. Pitting creates extra moisture which also means a readier supply of O2. (little scientific side note, iron will not rust in water unless there is O2)

Hope that mini explanation helps.

The good thing is that SR-77 ain't like 5160. That stuff is a rust magnet. Cloak did a patina on his Ranger but even out in the rain for one day, there was a lot speckled rust spots throughout the blade.