Carbon steel blades passivation homemade: the "bluing" method

The result of the reactions of a metal with environmental factors (air/water) is what we usually call "rust":  rust is an iron oxide, usually red oxide, formed by the redox action of iron and oxygen, in the presence of water or air moisture.

"Passivation", in physical chemistry, refers to a material becoming "passive", being less affected by environmental factors (air, water). Passivation involves a thin outer-layer of insoluble metal oxide at the surface, so the underneath metal cannot react with the acid anymore.

Since centuries, to protect metals by rusting, mankind treats swords and knives blades making what is called "bluing": bluing is a passivation process in which steel is partially protect against rust, and is named after the blue-black appearance of the resulting protective layer. This thin layer is made by magnetite (Fe3O4).

You may bluing a knife blade, just using what you have at home. 

First of all, clean carefully the blade from grease and fingerprints, using alcohol

Now, the blade is ready to be treated. To "passivate" the blade I use vinegar, but lemon or onion juice is effective as well

To accelerate and improve the chemical reaction, just warm the vinegar at the boiling point. Put the blade on the hot vinegar for 10 minuets or so

Final result: the knife blade is "blued", a thin magnetite layer will protect (partially) the metal from rust

   

Enjoy your bushcraft! :-)

Cheers,

Alfredo