Black oxide coatings appear on a variety of products—most commonly on drill bits, folding knives, and other cutting accessories. Just what is black oxide coating? In a nutshell, it coats materials in a chemical conversion as opposed to an electroplating process. According to Electrochemical Products, Inc., you can apply this type of overlay to steel, stainless, aluminum, cast iron, copper, brass, bronze, zinc, and other materials.
Just the Facts
- Black iron oxide – Fe3O4
- Black oxide is a chemical conversion coating rather a steel blend
- Offers mild corrosion resistance
- Reduces friction
- Can scratch or wear off easily during use with some processes
Creating the Overlay
You create this type of coating directly on the material with a chemical conversion rather than applying it like an electroplating process. A black oxide solution includes salts that act as an oxidizer. That solution reacts with the iron in the steel to create a coating over the surface.
On products like knife blades, manufacturers coat the blade before putting on a final edge. This reveals that silver edge on brand new blades. Conversely, a drill bit gets its chemical bath last. You actually see the coating over the cutting edges when brand new. Use that bit or sharpen it with a Drill Doctor, and the natural uncoated edge shows through clearly.
Black Oxide Coating Benefits
The best drill bits use more expensive coatings or materials. Still, black oxide has its uses. Manufacturers choose it as an inexpensive way to improve steel compared to other coatings and blends. The two most popular benefits of this material include corrosion resistance and reduced friction. Both are relatively minor improvements, but it does give the product a bump up over bare steel.
Black oxide coatings also add very little thickness to the steel on which they are applied. For precision accessories like drill bits, adding only 5 to 10 millionths of an inch helps maintain their sharpness.
Black oxide coatings also give products a nice finish for a tactical look. It takes on the sheen of the original steel, so some appear glossy while others have a non-reflective matte finish.
- Better lubrication between parts
- Anti-galling surface
- Better adhesion for paint and other finishes
- Low effect on conductivity
- No increase in brittleness
- No additional fumes when welding
The Limitations of Coatings
The major limitation on any coating has to do with its characteristics not running through the entire steel blend. Any area of the steel which wears off loses the benefits of the black oxide.
Hot or Cold?
When people ask the ‘What is black oxide’ question, we also mention the process. Black oxide can form under hot (285° F) or cold (room temperature or a little higher) processes. Hot processes offer better corrosion and scratch resistance—the only type that automotive and military standards accept.
Cold processes don’t actually produce a true oxide, leaving a softer coating. You can take a coin and scratch off the coating on steels that went through a cold process. For a product like a drill bit, it doesn’t take much use before the coating comes off the areas that contact material if they undergo a cold oxide process.