What is Black Oxide Coating? Ask the PTR Pros
Black oxide is something you’ll hear tossed about on a variety of products. In our neck of the internet woods, it’s most common on drill bits and knife blades, though there are plenty of other examples to choose from. So what is black oxide?
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
Black oxide is a coating on steel. You create it directly on the steel 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 black iron oxide coating over it.
On products like knife blades, you coat the blade first before putting a final edge on it. You’ll see the silver edge on a brand new knife. A drill bit gets its chemical bath last, so you see the coating over the cutting edges when it’s new.
Black Oxide Benefits
Black oxide is so popular as a finish because it is a very inexpensive way to improve steel compared to other coatings and blends. Corrosion resistance and reduced friction are the two most popular benefits of a black oxide coating. Both are relatively minor improvements, but it does give the product a bump up over bare steel.
One of the minor benefits is how thin the coating is. That might sound like a negative, but when you’re trying not to affect the precision of a drill bit, the addition of 5 – 10 millionths of an inch isn’t noticeable.
For products going for a tactical look, black oxide adds a coating that gives you a black finish. It takes on the sheen of the original steel, so some will be glossy while others with 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
Black Oxide Limitations
The major limitation on black oxide is that fact that it’s a coating. Since the characteristics don’t run through the entire steel blend, any area of the steel that loses its coating also loses the benefits.
Hot or Cold?
A black oxide can form can form under hot (285° F) or cold (room temperature or a little higher) processes. Hot processes offer better corrossion and scratch resistance. It’s the only process the automotive and military standards accept.
Cold processes don’t actually produce a true oxide and leave 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.