Bluing
A controlled finish that converts a steel surface to black oxide for modest corrosion resistance. The traditional treatment on blued barrels and actions.
Bluing chemically converts the surface of steel into black iron oxide, the stable form of rust, giving the familiar blue-black sheen on traditional barrels and actions. It offers light corrosion protection and is easy to refresh, but it is thin and relies on a film of oil to keep moisture off the metal.
It is the steel counterpart to anodizing on aluminum. Modern precision rifles increasingly wear tougher coatings such as nitride or Cerakote, which shrug off weather and fouling solvents better than bluing, but a well-kept blued rifle remains a classic.