Knowledge
The Differences Between Bluing and Blackening of Castings, Forgings and Machined parts
High-temperature bluing/blackening (alkaline oxidation method)
1. Principle: Steel parts are immersed in a concentrated solution of sodium hydroxide (NaOH) and sodium nitrite (NaNO₂) or sodium nitrate (NaNO₃) at high temperatures (approximately 135-150°C). A chemical reaction occurs on the surface of the steel, forming a dense oxide film mainly composed of Fe₃O₄.
(1) Color: Blue-black, aesthetically pleasing.
(2) Performance: Good adhesion of the film layer, relatively high corrosion resistance.
(3) Disadvantages: High energy consumption, high working temperature, presence of alkaline fumes, and high safety requirements for operation (risk of splashing of alkaline solution).
Blackening at room temperature (acidic oxidation method)
2. Principle: At room temperature (15-40°C), steel parts are immersed in an acidic solution mainly composed of copper sulfate (CuSO₄) and selenic acid (H₂SeO₃). Through the displacement reaction of selenium/copper, a black or blackish-gray compound film layer such as copper selenide (CuSe) is deposited on the surface of the steel.
(1) Color: Deep black or blackish-gray.
(2) Advantages: Energy-saving (no heating required), high efficiency (only a few minutes), simple and safe operation.
(3) Disadvantages: The film layer is relatively loose, and its adhesion, wear resistance, and corrosion resistance are far inferior to those of the high-temperature method. It must be subsequently immersed in anti-rust oil or wax for sealing protection to exert its anti-rust effect.
How to choose?
(1) Choose high-temperature bluing when the product requires good corrosion resistance, certain decorative properties (such as the aesthetic appeal of blue-black color), and cost control is not a major concern. For example: high-end tools, swords, firearms, and precision instrument parts.
(2) Choose blackening at room temperature when dealing with large quantities of parts with low performance requirements, and the primary goal is low-cost rust prevention and aesthetic appearance. For example: screws, nuts, washers, chains, machine tool casings, etc. It is an economical and efficient process.
