Dark black oxide coatings look attractive. Aside from that smoky appeal, there’s another benefit to this technique. Designed to act as a corrosion-resistant finish, black oxide treatments stop ferrous parts from oxidizing. If there’s one factor that sets the process apart, it’s the amount of chemical processing that occurs during the procedure. As to how the coating performs its duties, read on for a more detailed answer.
Chemical Conversion Basics
Dipped in a hot oxide bath, the workpiece doesn’t accumulate a rustproof coating. In actuality, the ferrous surface of that part is transformed. It’s oxidized by a controlled substance. So there are no layers deposited here, no specially precipitated coatings. Instead of a separate film, the workpiece surface is altered by the hot oxidizing salts. Then, produced in the heat treatment shop, a quality assurance system checks for colour variance issues and non-uniform coating problems. That latter matter includes spotty finishes and other process shortcomings. After all, a hint of surface-corrupting residue will compromise the coating.
Does Black Oxide Treatment Stop Material Corrosion?
In the first place, this chemical conversion technique adds a mild corrosion resistance feature to the workpiece. It’s not intended as a full rustproofing solution. The newly converted ferrous layer is Magnetite (Fe3O4), a stable compound that won’t oxidize any further. Submerged in that hot pool full of sodium hydroxide, the freshly formed chemical bonds won’t react when another oxidizing substance comes calling. The corrosion-resistant surface rejects further oxidization, stays dark black, and adds a non-reflective aspect to the metal. Having said all that, is it possible for the black oxide treatment technique to go further?
Supplementary Post-Finishing Choices
The low-reflectance looks are appealing and the dark metal certainly exudes eye-catching allure, but the fact that it offers a moderate amount of corrosion stopping power is somewhat worrying. The process can stop here, and the final product will repel most oxidizing substances. Still, what happens when the metal is really exposed to the elements? If that black oxide coating is to realize its full potential, we need some post-processing polish. An oily veneer is one answer. Applied until the post-processing medium fills the slightly porous finish, the oil or wax impregnation stage reinforces the black oxide until the metal surface can really withstand a determined material oxidizing threat.
Simply put, this supplementary heat treatment stage uses a hot alkaline bath to control the part’s oxidization. Held in check in this manner, the chemically transformed metal surface stabilizes until it becomes mildly corrosion-resistant. It also turns black, forms a low reflectance finish, and adds an eye-catching aesthetic to the metal part.