Some compression fitting spanners have a black oxide coating.
Black oxide coating
A black oxide coating is created when the iron in the metal reacts with a combination of sodium hydroxide, nitrates and nitrites to cause a chemical reaction.
The oxide coating does not affect the strength, weight or thickness of the object but does leave a decorative black finish which can give the tool a look of quality.
The advantage of using a black oxide coating on a compression fitting spanner is that it gives the tool a corrosion resistant surface which it does not gain from the drop forging process alone.
