Pickling is a chemical process used to remove iron oxide scale from metal surfaces by treating them with acids of specific concentrations, temperatures, and exposure times.
Types of Pickling
- By Acid Type:
Sulfuric Acid Pickling
Hydrochloric Acid Pickling
Nitric Acid Pickling
Hydrofluoric Acid Pickling
The choice of acid depends on the material of the steel. For example:
Carbon Steel: Sulfuric acid and hydrochloric acid are commonly used.
Stainless Steel: A mixture of nitric acid and hydrofluoric acid is often employed.
- By Steel Shape:
Wire Pickling
Forging Pickling
Steel Pickling
Strip Pickling
- By Pickling Equipment Type:
Tank Pickling
Semi-Continuous Pickling
Full Continuous Pickling
Tower Pickling
Principle of Pickling for Seamless Steel Pipes
Pickling involves the chemical removal of iron oxide scale (Fe₂O₃, Fe₃O₄, FeO) from the metal surface, also known as chemical pickling. These iron oxides are insoluble in water but react with acids to dissolve.
The iron oxide scale on carbon structural steel or low-alloy steel is porous and cracked. During the pickling process, the scale undergoes further cracking due to mechanical actions like straightening and stretching. This allows the acid solution to penetrate and react with both the iron oxide scale and the base metal.
Key Reactions in Pickling:
Dissolution: Iron oxide scales react with acids to dissolve.
Mechanical Peeling: The reaction of metal iron with acid generates hydrogen, which helps peel off the iron oxide scale mechanically.
Reduction: Atomic hydrogen reduces iron oxides to ferrous oxides, which are then removed by further reaction with the acid.
Pickling Process for Seamless Steel Pipes
The pickling process includes immersion pickling, spray pickling, and acid paste rust removal. Immersion pickling is commonly used, while spray pickling is employed for mass production.
Typical Procedure:
Sulfuric Acid Pickling: Steel parts are immersed in a 10% to 20% sulfuric acid solution at around 40°C. The solution is replaced when the iron content exceeds 80g/L or ferrous sulfate exceeds 215g/L.
Hydrochloric Acid Pickling: At room temperature, a 20% to 80% hydrochloric acid solution is used to minimize over-corrosion and hydrogen embrittlement. Corrosion inhibitors are added to the acid to protect the metal.
Post-Pickling:
After pickling, the metal surface is cleaned to a silvery white and passivated to enhance corrosion resistance, especially for stainless steel.
Specialized Pickling
- Sulfuric Acid Pickling:
Uses a 5% to 20% sulfuric acid solution to remove scale and adhered salts. Industrial sulfuric acid typically contains 75% to 97% H₂SO₄.
- Chromatographic Carrier Preparation:
To prepare carriers for chromatographic analysis, they are acid-washed with 6M hydrochloric acid for 2 hours or concentrated hydrochloric acid for 30 minutes. This removes impurities like iron, aluminum, calcium, and magnesium but not silanol groups, making the carrier suitable for analyzing acidic samples.
By carefully selecting the pickling method and parameters, manufacturers can effectively remove scale and impurities from steel pipes, improving their quality and performance.
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