What does soaking a knife in vinegar do?

Soaking a knife in vinegar removes some rust, loosens mineral deposits, and can force a dark patina on carbon steel. Vinegar contains acetic acid, so it reacts with the metal surface rather than sharpening the edge. A short soak can help clean light oxidation, but a long soak can etch the blade, dull the finish, darken the steel unevenly, and trigger fresh rust if the knife is not neutralized, rinsed, and dried well. Stainless steel usually resists the effect better than carbon steel, but it can still stain. Vinegar is a maintenance chemical, not an edge tool. Use it briefly and deliberately, not as routine knife care.