SMLS pipe pickling process and principles

Pickling is an essential surface treatment process for seamless steel pipes (SMLS pipes), primarily used to remove oxide scale, rust, and other impurities that form during manufacturing. This process enhances the surface quality, improves corrosion resistance, and ensures better adhesion for subsequent coatings or treatments. By utilizing acidic solutions, pickling dissolves unwanted surface contaminants, leaving behind a clean and reactive metal surface suitable for further processing. The following sections outline the principles, process, and key considerations of seamless steel pipe pickling.

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1. Principles of Pickling

The pickling of seamless steel pipes is primarily based on chemical reactions. The acid reacts with oxides, rust, and other surface impurities on the steel pipe, dissolving or peeling them off to achieve surface cleaning.

(1) Chemical Reactions

The pickling solution (typically hydrochloric acid or sulfuric acid) reacts with iron oxides (Fe₂O₃, Fe₃O₄, etc.) on the steel pipe's surface to generate water-soluble iron salts and release hydrogen gas. The main reactions are as follows:

Hydrochloric Acid Pickling:
Fe₂O₃ + 6HCl → 2FeCl₃ + 3H₂O
Fe₃O₄ + 8HCl → 2FeCl₃ + FeCl₂ + 4H₂O
Fe + 2HCl → FeCl₂ + H₂↑

Sulfuric Acid Pickling:
Fe₂O₃ + 3H₂SO₄ → Fe₂(SO₄)₃ + 3H₂O
Fe₃O₄ + 4H₂SO₄ → Fe₂(SO₄)₃ + FeSO₄ + 4H₂O
Fe + H₂SO₄ → FeSO₄ + H₂↑

(2) Physical Action

In addition to chemical dissolution, the acid also facilitates the removal of oxide scale and impurities through physical means, such as bubble flushing. The hydrogen bubbles produced in the reaction create a mechanical impact on the oxide layer, accelerating its detachment.

(3) Surface Cleaning

After pickling, oxide scale, rust, and other impurities are dissolved or removed, revealing a clean metal surface. Residual acid and reaction products are then washed away with water and neutralized to ensure a contamination-free surface.

(4) Passivation (Optional)

In some cases, a passivation treatment is performed after pickling to form a dense oxide film on the steel pipe surface, further enhancing its corrosion resistance.

Key Factors Affecting Pickling:

Acid Concentration: Too high a concentration may cause excessive corrosion, while too low a concentration may result in insufficient cleaning.

Temperature: Increasing the temperature can accelerate the reaction, but excessively high temperatures may increase acid mist and energy consumption.

Time: A short pickling time may lead to incomplete cleaning, while excessive exposure may damage the substrate.

2. Pickling Process of Seamless Steel Pipes

The pickling process is mainly used to remove surface oxide scale, rust, and other impurities, improving the surface quality of seamless steel pipes. The main steps are as follows:

(1) Pretreatment

Degreasing: Use an alkaline solution or organic solvent to remove oil stains from the steel pipe surface.

Water Washing: Rinse with clean water to ensure no residue remains on the surface.

(2) Pickling

Pickling Solution Preparation: Commonly uses hydrochloric acid or sulfuric acid, with concentration and temperature adjusted according to the material and oxidation level.

Pickling Operation: Immerse the steel pipe in the pickling solution, with immersion time determined based on the thickness of the oxide scale and acid concentration.

Water Washing: Immediately rinse with clean water after pickling to remove residual acid.

(3) Neutralization

Neutralization Treatment: Use an alkaline solution (such as sodium carbonate) to neutralize residual acid and prevent corrosion.

Water Washing: Rinse thoroughly with clean water again to ensure no neutralizing solution remains.

(4) Post-Treatment

Passivation Treatment (Optional): Some steel pipes undergo passivation to form a protective film that enhances corrosion resistance.

Drying: Ensure the surface is free of water stains by drying with hot air or natural drying.

(5) Inspection and Packaging

Inspection: Check the surface quality to ensure no defects are present.

Anti-Rust Packaging: Apply anti-rust coatings or protective measures for easy storage and transportation.

3. Precautions

Safety Protection: Operators must wear protective equipment to avoid acid-related injuries.

Environmental Protection: Waste acid should be treated before disposal to meet environmental standards and prevent pollution.

By following this pickling process, the surface quality of seamless steel pipes is significantly improved, providing a solid foundation for subsequent processing and applications.