The Hot-Temperature vs. Mid-Temperature Black Oxide Process: What Are the Differences?

28 February 2020

Black oxide is a conversion coating that is suitable for ferrous metals along with copper-based alloys, copper, zinc, stainless steel, silver solder and powdered metals. It accomplishes this through a chemical reaction since it is made of caustic oxidisers and additives. Also, it gives items a lustrous, black appearance and provides some corrosion protection. On top of all this, it offers other advantages such as anti-galling properties, smut-free finishes and dimensional stability.

Examples of Items That Benefit From a Black Oxide Coating

A wide assortment of items can benefit from being coated with black oxide, including such ones as:

• Assorted components ranging from parts to nuts and bolts
• Architectural pieces
• Tools
• Surgical instruments

Types of Processes to Apply Black Oxide

While you can achieve a black oxide coating through the hot-temperature, mid-temperature and cold-temperature processes, the first two are the most popular since they bond better to the various metals and offer improved abrasion-resistance. For the rest of this information, we will be discussing only the first two processes.

How the Hot-Temperature and Mid-Temperature Black Oxide Processes Differ

The hot-temperature method uses a temperature of 140.556-degrees C to produce a finish with black oxide iron magnetite or Fe3O4, and it will blacken a wide range of metals. The mid-temperature method accomplishes its task at temperatures ranging from 107.222-degrees C to 123.889-degrees C to coat various items with Fe3O4. The following are some other differences between these two processes:

• Hot process takes about 10 to 20 minutes while the mid-temperature one requires 30 to 45 minutes to blacken metals
• A glossy blue-black colour is achieved with the hot process, and the mid-temperature method produces a glossy black finish
• The hot method produces less sludge than the mid-temperature method does
• Mid-temperature process produces fewer fumes than the hot-temperature method
• The hot method produces a finish that is longer lasting than that of a mid-temperature method, even when the components are exposed to flexing

It is important to note that both methods create finishes that resist corrosion and abrasion. Matte finishes are possible upon request.

Which Process Is Right for Your Purposes?

Select the process that provide your workpieces the ideal finish for the conditions that they will face. Also, consider the alloy, steel or other metal in your components or other elements that need the black oxide coating.

Benefits of the Black Oxide Coating

To finish, read about the advantages of this coating in the following:

• More economical than other types of protective coatings
• All methods for applying black oxide can be performed in large batches of small components or items
• No significant dimensional changes
• Black oxide can conform to military standards

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