What black oxide is
Black oxide forms a very thin black oxide layer on steel by a chemical reaction, with almost no dimensional change.
Protection is baseline and needs an oil/wax top-coat; it is chosen for looks and for adding no thickness.
What black phosphate is
Phosphate—especially manganese phosphate—builds a rough dark-grey/black phosphate crystal layer that holds oil/paint very well.
It out-protects black oxide when oiled and reduces run-in wear; popular in automotive/firearms.
Which to choose
Precise dimensions, thin black skin: black oxide.
Corrosion / oil retention / paint primer: phosphate.
Both should be oiled or painted for maximum corrosion resistance.
FAQ
Which protects better?+
Phosphate (especially manganese) holds oil better, so it out-protects black oxide once oiled.
Which changes dimensions less?+
Black oxide—its layer is extremely thin, ideal for tight fits/fine threads.
Do both need oil?+
They should be oiled to seal porosity and boost protection—especially phosphate, which is designed to hold oil.
