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 Black Oxide


Black oxide (also known as bluing or blackening) is a chemical surface treatment. Steel fasteners are immersed in a high-temperature (typically >100 °C) alkaline solution containing oxidizing agents (e.g., nitrites).

Black oxide (also known as bluing or blackening) is a chemical surface treatment. Steel fasteners are immersed in a high-temperature (typically >100 °C) alkaline solution containing oxidizing agents (e.g., nitrites).


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Process Principle
Black oxide (also known as bluing or blackening) is a chemical surface treatment. Steel fasteners are immersed in a high-temperature (typically >100 °C) alkaline solution containing oxidizing agents (e.g., nitrites). This causes a chemical reaction on the iron surface, producing a dense, black, glossy magnetite (Fe₃O₄) film. The process does not involve electrochemistry, and the film thickness is extremely small, typically 0.6–1.5 μm.

 

Key Characteristics

Lowest cost – relatively simple process.

Negligible dimensional change – film is extremely thin.

Weak corrosion resistance – the oxide film alone provides little long-term rust protection; neutral salt spray test (NSS) duration is typically 3–5 hours.

Relies on oil sealing – must be immediately coated with anti-rust oil or sealer after treatment for even short-term protection.

 

Applications
Used where corrosion resistance is not critical, e.g., general tools, internal parts of machinery, precision instruments, and non-critical connections where a uniform black appearance is desired.

Process Principle
Black oxide (also known as bluing or blackening) is a chemical surface treatment. Steel fasteners are immersed in a high-temperature (typically >100 °C) alkaline solution containing oxidizing agents (e.g., nitrites). This causes a chemical reaction on the iron surface, producing a dense, black, glossy magnetite (Fe₃O₄) film. The process does not involve electrochemistry, and the film thickness is extremely small, typically 0.6–1.5 μm.

 

Key Characteristics

Lowest cost – relatively simple process.

Negligible dimensional change – film is extremely thin.

Weak corrosion resistance – the oxide film alone provides little long-term rust protection; neutral salt spray test (NSS) duration is typically 3–5 hours.

Relies on oil sealing – must be immediately coated with anti-rust oil or sealer after treatment for even short-term protection.

 

Applications
Used where corrosion resistance is not critical, e.g., general tools, internal parts of machinery, precision instruments, and non-critical connections where a uniform black appearance is desired.


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