I. Process Flow
1. Pretreatment
Degreasing and Cleaning: Soak in an alkaline degreasing solution (containing 15% sodium carbonate and 8% trisodium phosphate) at 70-80°C for 15 minutes to remove oil stains.
Acid Cleaning and Activation: Soak in a 10-15% hydrochloric acid solution (with a corrosion inhibitor) at room temperature for 3-8 minutes to achieve a uniform off-white surface.
2. Oxidation Treatment
High-Temperature Blackening: Use a mixed solution of sodium hydroxide (550g/L) and sodium nitrite (150g/L) at 138-142°C for 30-50 minutes.
Room-Temperature Blackening: Use a copper sulfate-based solution at pH 2.5 for 3-8 minutes. Circulate the solution to avoid color shifts.
3. Post-Treatment
Passivation and Sealing: Soak in an 80°C soap solution (3% sodium stearate) or potassium dichromate solution for 10 minutes to fill the pores of the oxide film. Dehydration and Rust Prevention: After centrifugation, apply an anti-rust oil (e.g., ISO VG32 base oil + 5% barium petroleum sulfonate) to the surface.
II. Key Control Parameters
Solution Ratio: The ratio of sodium nitrite to sodium hydroxide in the high-temperature blackening solution must be maintained at 1:3-3.5 to ensure a dense film.
Temperature Fluctuation: During high-temperature treatment, the deviation should not exceed ±2°C, requiring the assistance of a PID temperature control system.
Water Quality Requirements: Cleaning water conductivity ≤ 50 μS/cm, chloride ion content < 20 ppm.
III. Quality Inspection
Adhesion Test: Apply a 5% copper sulfate solution to the surface. A pass mark indicates no copper precipitation within 5 minutes.
Corrosion Resistance: Neutral salt spray testing requires 12 hours for standard parts to show no white rust, while reinforced sealed parts require 48 hours.
IV. Common Defect Treatments
Film Reddening: Add 5g/L of sodium nitrite and adjust the ratio to 1:3.
Partial film loss: Add a 5% hydrofluoric acid activation step for 1 minute.
V. Environmental Protection and Safety
Wastewater must be neutralized to a pH of 6-9, and chromium-containing wastewater must be treated separately for reduction.
Waste gas is treated in an acid mist purification tower (10% sodium hydroxide spray).
