1.What is the core principle?
In a high-temperature (approximately 135-155℃) strong alkaline (sodium hydroxide) solution, an oxidizing agent (sodium nitrite) is added to selectively oxidize the steel surface, generating a dense iron(II,III) oxide (Fe₃O₄) film.
2.How to perform surface pretreatment?
Chemical degreasing: Use alkaline cleaning agents or organic solvents to thoroughly remove rolling oil and grease. Heating to 60-80℃ can enhance the effect.
Acid pickling for rust removal: Immerse in a dilute hydrochloric acid or sulfuric acid solution (approximately 10-20% concentration) to remove loose rust and oxide scale, exposing a clean metal surface. The pickling time should be sufficient to remove all rust; excessive soaking should be avoided to prevent over-corrosion.
Water rinsing: Rinse repeatedly with running cold or hot water to thoroughly remove acid and residues from the workpiece surface. This is crucial to preventing contamination of the bluing bath.
3.What is the core process flow of alkaline high-temperature bluing?
Solution Preparation:
Add water to the bluing tank (stainless steel or carbon steel tank recommended).
Add sodium hydroxide (NaOH) and sodium nitrite (NaNO₂).
Typical ratio: NaOH 500-600 g/L, NaNO₂ 100-150 g/L. The specific ratio needs to be adjusted slightly according to the steel composition.
Heating and Immersion:
Heat the solution to boiling, typically at 135-155℃ (boiling temperature depends on solution concentration).
Completely immerse the thoroughly cleaned and dried cold-rolled coils (smaller pieces can be placed in a hanging basket) into the boiling bluing solution.
Processing Time: Typically between 20 and 90 minutes, depending on the steel grade, solution concentration, and temperature. Observe the surface color change: from yellow → red → purple → bluish-black.
Color Judgment: The ideal color is a uniform "steel blue" or deep black. A lighter color may indicate insufficient time or low concentration.
4.How does post-treatment determine the rust prevention effect?
Cleaning: After removing from the bluing tank, immediately rinse thoroughly with hot water (>80℃) to remove any residual strong alkaline solution.
Saponification or Passivation:
Saponification: Immerse in a 3-5% soap solution at 80-90℃ for 3-5 minutes. The soap (stearic acid) reacts with the oxide film to form a hydrophobic iron stearate film, significantly improving rust prevention.
Passivation: Alternatively, immerse in a 3-5% potassium dichromate solution for passivation filling.
Oil Immersion: This is the essential final step. Immerse the workpiece in hot (105-110℃) rust-preventive oil (such as transformer oil or spindle oil) for 3-5 minutes, then drain. The oil film fills the micropores of the oxide film, greatly enhancing its rust prevention and wear resistance. Rust-preventive grease or wax can also be applied.
5.What are the key control points and precautions?
Safety First: The operating environment must be well-ventilated. Wear acid and alkali resistant protective clothing, gloves, goggles, and face shields. Chemical disposal must follow safety procedures.
Material Limitations: Low- and medium-carbon steel and low-alloy steel are best suited for bluing. High-carbon steel has a darker color, and alloy steel (such as chromium steel) may have a reddish or uneven color. The composition of different batches of cold-rolled coils must be consistent.
Coating Characteristics: Bluing film has poor abrasion resistance and is mainly used in mild indoor environments or as short-term rust prevention. It cannot replace thick-layer protection such as electroplating zinc or chrome plating.
Mass Production: For continuous cold-rolled coils, a continuous through-type bluing production line can be used, including a pretreatment section, bluing tank, multi-stage rinsing tank, and drying and oiling section, to achieve automated production.
Quality Inspection:
Appearance: Uniform color, no mottling or red rust.
Corrosion Resistance: A 3% neutral copper sulfate drop test should show no red color after >30 seconds to be considered合格 (qualified).
Film adhesion: When vigorously wiped with a white cotton cloth, it should not peel off.