[ELI5] How does bluing work on metal? And why is it called bluing when it turns metal black?

Question

[ELI5] How does bluing work on metal? And why is it called bluing when it turns metal black?

In: Chemistry

Answer 1

Bluing is a process used on steel to convert red iron oxide,(Fe2O3, rust, prone to flaking when exposed to water) into black iron oxide(Fe304, magnetite, not prone to flaking, blue-black in color).

Based on the Wikipedia page the name comes from the fact that thin coatings will in fact be blue and that thicker coatings which appear black are termed black oxide. They are both the same chemical process though.