The Chemistry of Pickling Low-Carbon Hot-Rolled Steel Coils in Hydrochloric Acid

The pickling process plays a vital role in the production of low-carbon hot-rolled steel coils, ensuring the removal of surface oxides formed during high-temperature processing. Hydrochloric acid (HCl) is the most commonly used agent for this process due to its high efficiency and ability to preserve the integrity of the steel substrate.

Chemical Reactions in Pickling with HCl

The surface of hot-rolled steel typically develops a layer of scale comprising iron oxides such as hematite (Fe?O?), magnetite (Fe?O?), and wüstite (FeO). Immersion in an HCl bath initiates the following chemical reactions:

Iron Oxide Dissolution:

Fe?O? + Fe + 6HCl → 3FeCl? + 3H?O

Fe?O? + Fe + 8HCl → 4FeCl? + 4H?O

FeO + 2HCl → FeCl? + H?O

Reaction with Base Metal:

Fe + 2HCl → FeCl? + H?

These reactions remove the oxide layers effectively while producing ferrous chloride and hydrogen gas.

The latter requires careful handling to avoid issues like hydrogen embrittlement in the steel. To mitigate this, corrosion inhibitors are added to the pickling bath to form protective films on the steel surface, reducing acid attack and hydrogen production.

Comparison of Pickling Methods

Hydrochloric Acid Pickling (HCl)

Hydrochloric acid is the most widely used chemical in pickling for several reasons:

Fast Oxide Removal: HCl offers a higher reaction rate compared to sulfuric acid, reducing processing times.

Lower Operating Temperatures: Pickling in HCl can occur at ambient or slightly elevated temperatures, minimizing energy requirements.

Minimal Base Metal Loss: Its selectivity ensures that the steel substrate is preserved during oxide removal.

Surface Finish: HCl pickling delivers a clean and smooth surface, ideal for further processing.

Sulfuric Acid Pickling

Sulfuric acid (H?SO?) is another traditional agent for steel pickling but has distinct differences from HCl:

Slower Reaction Rates: H?SO? pickling requires higher temperatures (typically 70–80°C) for effective oxide removal, increasing energy consumption.

Increased Metal Dissolution: The steel substrate can suffer greater damage due to the less selective nature of sulfuric acid.

Economics and Disposal: While the acid is cheaper, the spent bath is more challenging to treat due to complex sulfates.

Mechanical Pickling

Mechanical pickling, such as shot blasting, offers a non-chemical alternative:

No Acid Use: Eliminates the need for acid handling, disposal, and associated environmental concerns.

Surface Texturing: Mechanical pickling can roughen the surface, which may be advantageous for certain coatings.

Limitations: However, it is less effective in removing fine or adherent oxide layers and is typically slower than chemical methods.

Waste Management and Sustainability

Spent pickling solutions present significant environmental challenges. Technologies such as acid regeneration have been developed to address this, particularly in HCl pickling. In acid regeneration, spent HCl is recovered, and by-products like ferric oxide are generated, reducing waste and promoting circularity in steel processing. Although implementation requires capital investment, it aligns with growing regulatory and sustainability demands in the steel industry.

Conclusion

Pickling with hydrochloric acid remains the industry standard for low-carbon hot-rolled steel coils, offering a balance between efficiency, surface quality, and material preservation. However, selecting the appropriate pickling method requires careful consideration of operational needs, environmental impact, and economic factors. Emerging advancements in waste treatment and inhibitors will continue to refine these processes, ensuring their relevance in modern steelmaking.

Luigi Villani is the owner of GTG Consulting and specializes in analyzing industrial trends in materials science. For more insights, visit www.gtgcons.com.