The patina on a frequently used knife will change with use. You wear off old patina as new patina builds. I'm not sure how well patina protects against rust in the long run. Frequent use of the knife may actually reduce rust better by simply wearing away the rust that tries to build up. That might be why a blade with heavy patina resists.
Recently I read an interview with the owner of a company that builds knives for use under extreme conditions. (I forget both the company and the owner's name. Sorry) His knives are all made with carbon steel. He said he observed natives in the Amazon used only carbon steel machetes and knives and they never had to worry about rust on their blades despite contact with corrosive fluids and substances and the hot, humid weather. The reason being the knives might form rust but, since they were in constant use, the rust was worn off immediately. So, on a knife in frequent to constant use, being able to maintain a sharp edge was more important than rust prevention.
Recently I read an interview with the owner of a company that builds knives for use under extreme conditions. (I forget both the company and the owner's name. Sorry) His knives are all made with carbon steel. He said he observed natives in the Amazon used only carbon steel machetes and knives and they never had to worry about rust on their blades despite contact with corrosive fluids and substances and the hot, humid weather. The reason being the knives might form rust but, since they were in constant use, the rust was worn off immediately. So, on a knife in frequent to constant use, being able to maintain a sharp edge was more important than rust prevention.
) that the different acids or other caustics used in oxidizing metals will create different colors of oxide, based at least in part on the specific pH of the solution used, and the specific chemical reactions taking place. I'd assume the protective qualities of the oxide will also vary widely by the same reasoning.
