How to solve oxidation in Dalian black‑oxide surface treatment

Solution to Oxidation of Black‑Oxide Surface Treatment in Dalian

Black‑oxide (also known as blue‑ing) is a widely‑used anti‑rust & decorative process for steel parts. It forms compact Fe₃O₄ oxide film on metal surface, delivering matte black finish and basic corrosion resistance, applied in hardware, precision machinery, fasteners and other fields. However, oxidation and rust frequently occur on black‑oxide coating in production and service, shown as tan rust spots, film peeling or uneven gloss, spoiling appearance and shortening service life. Systematic solutions require identifying oxidation causes and full‑process control via process optimization and remedial measures.

I. Core Causes of Black‑Oxide Coating Oxidation

Oxidation essentially means failure of protective oxide film, mainly from four aspects:

Deficient pre‑treatment: Complete degreasing and derusting are required before black‑oxide treatment. Residual stamping oil, cutting fluid or rust hinders uniform film formation. Oil‑contaminated areas fail to form valid coating; residual rust weakens film adhesion and initiates corrosion. For small stamped hardware, cold‑water rinsing cannot remove mineral oil inside gaps, triggering rust spots upon moisture exposure.

Deviated black‑oxide process parameters: Standard parameters for high‑temperature strong‑alkali black‑oxide: temperature 135‑145 ℃, reaction time 15‑30 min, alkali concentration 400‑600 g/L. Insufficient temperature or short time produces thin porous Fe₃O₄ film; excessive alkali concentration causes brittle peeling; mismatched additives reduce film compactness and leave corrosive pores.

Failed post‑treatment sealing: Black‑oxide film has micro‑pores and needs saponification, oil‑immersion or passivation sealing. Skipping sealing leads to direct absorption of moisture and contaminants. Saponification below 85 ℃ gives incomplete hydrophobic saponified film. Un‑dried workpieces before oil immersion retain moisture and break oil film to accelerate oxidation.

Poor storage conditions: Environment humidity above 70 %, or exposure to acid‑alkali‑salt media (coastal salt mist, acidic fumes in chemical workshops) allows corrosive agents to penetrate film pores and corrode substrate.

II. Solutions for Oxidation Issues

Separate remedies for already‑oxidized parts and mass‑production prevention:

(A) Remedy for mild oxidation

For workpieces with slight rust spots without large‑area film peeling: remove rust by fine sanding or ultrasonic derusting. Conduct secondary pre‑treatment: thorough degreasing with alkaline cleaner (NaOH + Na₂CO₃) at 60‑80 ℃; add 0.5‑1 % urotropine inhibitor in pickling to avoid substrate erosion. Rinse and transfer immediately to black‑oxide bath. Strictly perform sealing after black‑oxide: soak 5‑10 min in hot saponification solution above 90 ℃, then immerse 10‑15 min in 105 anti‑rust oil to fully seal pores. Severe oxidation: apply electrolytic polishing to refine crystal structure before re‑black‑oxide.

(B) Systematic process optimization for prevention

Improve pre‑treatment: Adopt combined ultrasonic degreasing + shot‑blasting derusting. Ultrasonic cleaning removes residual oil in gaps; shot‑blasting homogenizes surface roughness. For precision parts, implement cyclic alkaline‑pickling to eliminate surface contaminants.

Control black‑oxide parameters: Establish parameter check sheet. Test bath temperature and alkali concentration every 2 hours, replenish additives periodically. Adjust reaction time by material: low‑carbon steel 25‑30 min, high‑carbon steel 10‑15 min, to obtain compact 2‑3 μm Fe₃O₄ film.

Enhance post‑sealing: Add chromium‑free weak‑alkaline passivation for 3‑5 min on basis of saponification (environmental compliance). Outdoor‑service parts adopt dual sealing: wax dipping plus anti‑rust oil for better moisture isolation. Pack precision components with vapor‑phase rust inhibitor.

Standardize storage: Store black‑oxide parts in dry warehouse with humidity ≤60 %. Keep away from acids and alkalis. Wrap with PE anti‑rust film or vapor‑phase anti‑rust paper; use buffered anti‑rust packing during transportation.

Conclusion

Black‑oxide oxidation stems from multi‑procedure defects. Full‑process standardized control covering pre‑treatment through storage combines emergency remedy and long‑term prevention, so as to improve appearance and anti‑corrosion performance for industrial applications.