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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).
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.