Pickling Process of Seamless Steel Tube

What is pickling?
Acids remove iron oxide scale by chemical method according to a certain concentration, temperature and speed, which is called pickling.

Pickling classification: According to the type of acid: sulfuric acid pickling, hydrochloric acid pickling, nitric acid pickling, hydrofluoric acid pickling. Pickling must choose different media according to the material of the steel, such as pickling carbon steel with sulfuric acid and hydrochloric acid, pickling stainless steel with mixed pickling of nitric acid and hydrofluoric acid, etc.
According to the shape of steel: wire pickling, forging pickling, steel pickling, strip pickling, etc.

According to the type of pickling equipment: tank pickling, semi-continuous pickling, full continuous pickling, tower pickling.

The principle of pickling:

Pickling is a process of chemically removing iron oxide scale on the metal surface, so it is also called chemical pickling. The iron oxide scales (Fe203, Fe304, Fe0) formed on the surface of the seamless steel tube are all alkaline oxides that are insoluble in water. A series of chemical changes can occur with acids.

Due to the loose, porous and cracked properties of the iron oxide scale on the surface of carbon structural steel or low-alloy steel, and the repeated bending of the iron oxide scale along with the strip on the pickling unit through straightening, stretching and conveying, these The pores and cracks further increase and expand, so the acid solution reacts chemically with the iron oxide scale, and also reacts with the matrix iron of the steel through the cracks and pores. That is to say, the chemical reaction between the three kinds of iron oxide scales and metal iron and the acid solution is carried out at the same time at the beginning of the pickling.

 (1) The iron oxide scale is dissolved by chemical reaction with acid (dissolution).
 (2) The metal iron reacts with acid to generate hydrogen, which mechanically peels off the iron oxide scale (mechanical peeling effect).

 (3) The generated atomic hydrogen reduces iron oxides to ferrous oxides which are susceptible to acid action, and then reacts with acid to be removed (reduction action).

Seamless steel tube pickling process:

The pickling process mainly includes immersion pickling, spray pickling and acid paste rust removal. Generally, the dipping pickling method is used, and the spraying method can be used in mass production. Steel parts are generally pickled in 10% to 20% (volume) sulfuric acid solution at a temperature of 40°C. When the iron content in the solution exceeds 80g/L and the ferrous sulfate exceeds 215g/L, the pickling solution should be replaced. At room temperature, pickling steel with 20% to 80% (volume) hydrochloric acid solution is less prone to over-corrosion and hydrogen embrittlement. Because acid has a strong corrosion effect on metals, it is necessary to add corrosion inhibitors. After cleaning, the metal surface becomes silvery white, and the surface is passivated at the same time, improving the corrosion resistance of stainless steel. The process of using sulfuric acid aqueous solution with a concentration of 5% to 20% to remove scale and adhered salts on the surface of the workpiece is called sulfuric acid pickling. The industrial concentrated sulfuric acid sold on the market usually contains H2SO475%~97% (mass fraction), and brown industrial sulfuric acid solution can also be used, which contains H2SO475%~78% (mass fraction).

In order to eliminate surface adsorption of diatomaceous earth carrier and reduce chromatographic peak tailing, the carrier needs to be acid-washed or alkali-washed before use. Pickling is to soak the carrier with 6mol/L hydrochloric acid for 2h or concentrated hydrochloric acid for 30min, filter, wash with water until neutral, and dry. Pickling can remove iron, aluminum, calcium, magnesium and other impurities on the surface, but cannot remove silanol groups. Acid-washed supports are suitable for the analysis of acidic samples.