Process and Applications of Black Oxide

14 May 2020

The process of forming black iron oxide to ferrous metals and other types of materials is often called black oxide. The black oxide process is a chemical conversion coating for ferrous materials, stainless steel, copper and copper-based alloys, zinc, powdered metals, and silver solder. This process is done to add corrosion resistance, enhance appearance, and minimise light reflection. 

Unlike nickel or zinc electroplating, the black oxide is not deposited on the surface of the substrate. Instead, a chemical reaction is generated between the iron on the ferrous metal surface and the oxidizing salts that are present in the black oxide solution. The generated chemical reaction results in the formation of magnetite, a black iron oxide, on the surface of the coated material.

The Black Oxide Processes

The black oxide chemical conversion can be done through the hot process or cold process. 

  • Hot Process

The hot process starts by fixturing, which is the hanging of individual parts from a wire, placing them in baskets, or putting them in a tumbler. After fixturing, the parts will be cleaned by submerging them in an alkaline bath and then rinsing them in clean water. For blackening ferrous materials, they must be placed in a bath of propriety blackening solution that contains sodium hydroxide, nitrates, and nitrites. This solution converts the material surface into magnetite. The proprietary blackening bath is not applied when applying hot black oxide for stainless steel parts. 

The cleaning and rinsing of materials must be done in warm baths, up to 160°F. The blackening, on the other hand, must have temperatures ranging from 265°F to 285°F. The final step in the hot process is the application of oil to heated parts. To avoid corrosion, the oil seals the black finish by sinking into the porous layer of the black oxide.

  • Cold Process 

Cold black oxide is only introduced since it reduces hazards and saves energy. With the cold process, a compound called copper selenium is applied to the materials at room temperature. This process is contrary to the hot process wherein the parts are subjected to numerous chemicals at high temperatures. The final step in the cold oxide process also deals with the application of oil to parts for added corrosion resistance. 

While cold black oxide can offer better productivity and convenience for in-hose blackening, its coating property tends to rub off easier and offers less abrasion resistance. Hot blackening, on the other hand, is known to be cost-effective, provides superior coverage, and offers enhanced corrosion resistance. 

Applications of Black Oxide

The blackening process of materials is typically used in industries that revolve around retail, automotive, electrical, home, garden, gearing, and firearms. Some products and tools that are processed with black oxide include store displays, fixtures, oil filter cans, hood fasteners, nuts, washers, wire strippers, cutters, clipping tools, small gears, electrical switches, gun components, and shotgun shell magazines. With the black oxide process, you can easily get products and tools that have a remarkable surface look and have enhanced corrosion resistance property. To know more about black oxide, feel free to contact us now at Alpha Detroit Heat Treatment.

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