Carbon 15cm Utility Knife

Over time, Carbon-rich steel naturally forms a protective patina that is normally charcoal-colored from the dark iron oxide that forms on the surface.  However, if there is sufficient oxygen and moisture present, the patina forms too quickly and the iron oxide can be converted to hydrated ferric oxide which produces the red rust color.  Hydrated ferric oxide does not protect surfaces, it corrodes them, and needs to be removed and neutralized.

When caught early, removing red surface rust is easy to do. Products like Flitz and Barkeeper's Friend have light abrasives and a neutralizing agent in them, as well as a chemical which reverts the destructive ferric oxide back into the good iron oxide.  If you don’t have access to these chemicals, lightly scouring the surface with a nylon brush and Baking Soda can also do the trick.

For extra measure, remember to put a light coat of mineral oil on the metal surfaces which will disperse any moisture and protect your knife, giving that protective patina time to form and do what it is supposed to.

Acids like bleach, citric, vinegar or salt are common in the kitchen and with enough time in contact with a blade, these all can be catalysts for corrosion, especially if there is heat involved as well.  Avoid contact with these kinds of agents, especially when your knife is new.