Good Rust, Bad Rust

Rust is something every knife owner will eventually encounter. But did you know that not all rust is the same? There is “bad rust” and there is what many knife enthusiasts consider “good rust.”

Rust is the result of oxidation, a chemical reaction between metal and oxygen. The “bad rust” most people recognize is the reddish, rough rust known as red rust. While red rust itself can usually be removed by polishing, leaving it untreated allows corrosion to continue beneath the surface. Over time, a dark corrosion layer may develop underneath.

This hidden corrosion gradually eats away at the steel from within. You can think of it like a disease that slowly spreads through the metal. As it progresses, the edge becomes prone to chipping, sharpening becomes less effective, and the knife’s service life can be dramatically shortened.

On the other hand, the dark discoloration often called black patina is actually a protective oxide layer. This is the “good rust.” The patina forms naturally through oxidation and acts as a barrier that helps prevent destructive red rust from developing. Although it changes the appearance of the blade, there is usually no reason to remove it. It is also completely safe for food preparation.

This knife is an example of both good and bad oxidation on the same blade.
The thin, dark gray discoloration covering most of the blade is black patina—the protective oxide layer often considered “good rust.”
The dark spots highlighted by the red circles, however, are corrosion layers. These indicate corrosion that has progressed beneath the surface and represent the “bad rust” that can gradually damage the steel.

Interestingly, stainless steel relies on the same basic principle. Stainless steel contains chromium, which reacts with oxygen to form an extremely thin, transparent protective oxide layer called a passive layer. Although you cannot see it, this invisible film protects the steel from red rust and is the reason stainless steel is so corrosion-resistant.

So, while both may appear dark, they are very different. One is a destructive corrosion layer that weakens the steel, while the other is a protective oxide layer that helps preserve your knife. Understanding the difference is one of the keys to keeping a knife in good condition for many years.