Steel Pickling

Pickling is the process used to remove oxide scales and other impurities that can form during steelmaking, transportation, and storing. It is used to prepare steel for further processing. Hot-rolled steel is processed through a diluted acid bath to produce a steel surface free of scale, rust, inorganic contaminants, and stains. Pickling is used to remove a thin layer of metal from the surface of stainless steel using mixtures of nitric and hydroflouric acids.

During the process of slab reheating and hot-rolling, oxides are formed on the surface of the steel. Three types of scales are formed during the rolling process which are removed during pickling:

• Primary or Furnace Scale: This scale forms on the metal in the reheat furnace by reaction of combustion gasses including excess air, water vapour and carbon dioxide with the surface of the metal. Extended time in reheat furnaces due to delays contribute to abnormally thick scale layers that are not easily removed by the high pressure water sprays. Primary scale is not continuous but can be well dispersed along the strip surface. It is normally deep rooted and extremely difficult to completely remove by pickling.
• Secondary Scale: This defect classification is intended to cover scale remaining as a result of ineffective scale removal at the rougher and the high pressure water sprays prior to the Finishing Mill. Often called jet scale.
• Tertiary or Fleck Scale: This scale is a result of breakdown of the oxide layer on the roll in the finishing stands of the Hot Strip Mill. These are continuous scales formed on the strip surface. It may show up in bands or cover the whole surface depending on the condition of the work roll.

Pickling methods

The most commonly used pickling methods are as follows:

• Tank immersion pickling: In this process, the complete fabricated part is immersed in a tank. This usually involves off-site pickling at the fabricator or the pickling plant. This method has the optimum coating potential to treat all fabrication surfaces for corrosion resistance and uniformity of pickling finish.
• Spray pickling: Spray pickling is done on-site by spraying the pickling mixture onto the surface of the fabrication.
• Circulation pickling: This involves circulating the pickling mixture through the fabrication. It is best suited for pickling pipework meant to carry corrosive liquids.

Pickling Process

The pickling process involves two key steps: mechanical scale breaking and chemical pickling, followed by a final rinsing off of the pickling mixture.

• Scale Breaking: In this step, the surface of the strip is flexed and stretched so that the scales get cracked. This scale cracking allows the hydrochloric acid to penetrate effectively to the substructure dissolving the scale from the strip surface during pickling. When the steel strip is extended, the scales crack due to its brittleness and break away from the steel base. Scale dust is removed via a suction or vacuum system. Effective tension levelling is known to improve the pickling efficiency by around 20%.
• Pickling: The pickling process is carried out in three deep bath tanks with the highest acid strength tank 3 and the weakest in tank 1. The reaction involving the scale (iron oxide) and hydrochloric acid proceeds more quickly if pure iron (Fe) is in contact with the acid. Cracks made in the scale layer during the scale breaking stage allow the iron in the steel strip to come into contact with the acid and pickling becomes easier. Higher solution temperature and acid concentration makes the pickling process faster.
• Rinsing: Following the pickling of the strip, residue of acid solution remains on the surface. The residue needs to be removed and is carried out in the dip and rinse spray tanks. The strip then passes through a set of squeegee rolls and a hot air drier.

External Links

• Pickling Explained – Definition, Process & More | Fractory