Black Oxide has unique properties. It's a process that produces a highly desirable finish resulting in a set of features that are perfect for use in certain types of applications.
What is Black Oxide?
(black oxide finishing, black oxide treatment, also known as Black Penetrate, Black Magic, Black N, and Gun Bluing)
Black Oxide is a coating that's created by dipping metal parts into a specially formulated solution at specific temperatures. The chemical reaction that occurs during this process creates magnetite (Fe3O4) on the surface of the parts. The solution used is generally a hot bath of sodium hydroxide, nitrates, and nitrites at 286 degrees F (131 degrees C), with water added periodically by a controlled method. It is possible to create similar effects at different temperatures, however the results and their effectiveness will vary. For example, a “hot” oxide process will yield highly effective results that are consistent. Alternatively, the cold oxide process will tend to show variations in color and can potentially cause the black finish to rub off. Given this, it's the “hot” black oxide process that's most desirable and meets military, automotive and other specifications. For example, when applying Hot Black Oxide for stainless steel, a solution of caustic, oxidizing, and sulfur salts is used. And if applied to 300 and 400 series, and the precipitation hardened 17-4 PH stainless steel alloys, it can be used on cast iron and mild low-carbon steel. It's this process that complies with military specification MIL-DTL-13924D Class 4 and provides abrasion resistance.
What are the steps of the process?
The black oxide conversion finishing process occurs in multiple steps. These steps are clean, wash, create the finish, wash, apply an after-finish or coating. Depending on the parts, neutralizing and/or additional rinses may be necessary to prevent bleed out or “flowering”.
If the parts have any build-ups like scale or rust, these may have to be resolved before creating the black oxide coating. Black oxide cannot be applied to parts that are plated. The plating would have to removed before the black oxide process.
There's a wide variety of options for the after-finish. The after-finish is critical for adding corrosion protection to your pieces, therefore a good after-finish is always recommended. Which after-finish you use should be based on the parts' configuration and intended use. For example, if you need your parts to be dry to the touch, you should specify it. As another example, if you're looking for a matte finish, you might want to choose a wax after-finish since the oil tends to be glossy. If your specifications don't indicate a specific after-finish, “Black Oxide” generally will be understood as Black Oxide with an Oil after-finish. Popular after-finishes like oil, wax, and lacquer attribute effective corrosion protection, especially for indoor use. Brass alloys and stainless steel parts will have a high level of corrosion protection with Black Oxide. Supplementary coatings may be required for outdoor applications of steel parts.
Here's a quick list of factors to consider when choosing an after-finish for your next black oxide project:
- Appearance (glossy or matte)
- Level of corrosion protection needed
- Environmental conditions (temperature, humidity, vapor)
- Intended use and function
Features and Benefits of black oxide:
- No dimensional change: This means your parts before and after the black oxide finishing process will have the same exact dimensions. The process only colors the base metal and does not add or remove from it.
- Does not affect conductivity: Because the black oxide process is only coloring the base metal, it does not interfere with the conductivity of the original part.
- Increased corrosion resistance: Especially with the proper after-finish, you can amplify the corrosion resistance of your parts. Popular after-finishes include wax, lacquer, chromic seals, and of course oil.
- No hydrogen embrittlement: Since the black oxide process doesn't require acid activation and it's not an electro-process, your parts are immune to hydrogen embrittlement. Even if the parts have to be treated to remove scale or rust buildups, most hydrogen used for treatment will quickly dissolve away first during the black oxide process, and completely dissolve away within a couple of days after processing.
- Reduced light glare: One of the benefits of black oxide is that the finish reduces reflection which makes it great for applications where this matters.
- Decorative Finish: Yet another benefit of black oxide finish is that the coating looks pleasant and can be given a nice touch with the various after-finish options available.
- Provides anti-galling properties: Black oxidized parts will cause less friction as they tend to impart better lubrication. Not only is this good for applications where the parts will be running together but it also adds to the improved feel.
- Base for organic coatings: The combination of all the characteristics imparted by the black oxide process creates a good foundation for being able to apply other organic coatings.