Black Oxide Coating

Black Oxide Coating MIL-C-13924 & MIL-DTL-13924 CLASS 1 (STEEL) CLASS 4 (S/S)

The Black Oxide process that Seminole Metal Finishing uses on Steel and Stainless Steel is a hot alkaline penetrating finish.

The Black Oxide for steel (Class 1) is a boiling proprietary alkaline solution that operates at 290 °F. This system forms a black iron magnetite finish and is a true conversion coating for steel. The dwell time in the tank depends on the surface hardness, alloy and the prior heat treating of the part being plated. The Black Oxide itself does add some corrosion protection; however, the supplementary oil treatment is the sealant that provides for the best corrosion resistance.

The stainless steel Black Oxide (Class 4) is also formed using a boiling proprietary alkaline solution that operates at 250°F. Besides plating stainless steel, it is also used to plate cast and malleable iron. 300 and 400 series stainless steels are a more typical example of types of stainless steel plated. However, many other series stainless steel can be blackened to some degree. To activate stainless steel before oxidizing requires a 50% sulfuric or hydrochloric acid solution. This activates the surface of the part and prepares the material surface to allow the black oxide to penetrate the stainless steel surface. The material itself does not show any sign of etching from this process when done properly.

Black Oxide on Stainless Steel


The mil-specs associated with the Black Oxide process are Mil-C-13924 and Mil-DTL-13924D. They are basically the same spec with the Mil-DTL-13924D being the latest revision. The specification covers the Black Oxide coatings applied to ferrous metals (wrought iron, carbon, low alloys and corrosion resistant steels) as well as any testing procedures that are required. Black Oxide coatings; with or without supplementary oil treatment may be used where a black surface is required. Only very limited corrosion protection under mildly corrosive conditions is obtained as a result of Black Oxide coatings.

The classifications of Black Oxide are as follows:
Class 1: Alkaline oxidizing process for steel and steel alloys
Class 2: Alkaline chromate oxidizing process for certain corrosion resistant steel alloys which are tempered at less than 900 °F. (482 °C.)
Class 3: Fused salt oxidizing process for corrosion resistant steel alloys, which are tempered at 900 °F. (482 °C.) or higher.
Class 4: Alkaline oxidizing process for all other corrosion resistant steels.

***Seminole Metal Finishing currently only offers Black Oxide, Class 1 and Class 4.***


A typical example of an application for Black Oxide coatings are nuts, bolts and screws, tool handles, crimping dies and other associated tools that require an attractive appearance to the end consumer. Black Oxide Coating is also used in printing press parts, moving parts and other ferrous materials that cannot tolerate dimension change or have very tight tolerance levels that are in need of some kind of corrosion protection prior to going to the end consumer. Black Oxide can be used for decorative applications and can be used to reduce light reflectivity on sandblasted or glass beaded parts. Black oxide is often used in all types of weapons systems such as gun sights & gun barrels and other parts associated with weaponry. Call or email Seminole Metal Finishing today to get your quote.