Why is seamless steel pipe subjected to acid washing process

Overview of Pickling 

The method of removing the oxide scale and rust from the surface of steel using an acid solution is called pickling. The oxides of iron (such as Fe3O4, Fe2O3, FeO, etc.) and rust react chemically with the acid solution, forming salts that dissolve in the acid solution and are thus removed. The acids used in pickling include sulfuric acid, hydrochloric acid, phosphoric acid, nitric acid, and mixed acids, etc. 

Common media for pickling: sulfuric acid, nitric acid, phosphoric acid, and hydrofluoric acid. During the pickling process, it is essential to add pickling inhibitors to prevent the corrosion of metals by the acid. 

Pickling process 

The main methods include immersion acid washing, jet acid washing and acid paste rust removal. The immersion acid washing method is generally used, while the jet method can be adopted in large-scale production. For steel parts, they are usually acid-washed in a 10% to 20% (by volume) sulfuric acid solution at a temperature of 40℃. When the iron content in the solution exceeds 80g/L and the ferrous sulfate exceeds 215g/L, the acid-washing solution should be replaced. At room temperature, steel parts are acid-washed with a 20% to 80% (by volume) hydrochloric acid solution, which is less likely to cause over-corrosion and hydrogen embrittlement. Due to the significant corrosion effect of the acid on the metal, corrosion inhibitors need to be added. After cleaning, the metal surface turns silvery white and simultaneously forms a passivation layer, enhancing the corrosion resistance of stainless steel. To eliminate the adsorption on the surface of the silica soil carrier and reduce the tailing of chromatographic peaks, the carrier needs to undergo acid washing or alkali washing before use. Acid washing involves soaking the carrier in 6mol/L hydrochloric acid for 2 hours or heating the concentrated hydrochloric acid for 30 minutes, filtering, washing with water until neutral, and drying. Acid washing can remove impurities such as iron, aluminum, calcium, and magnesium on the surface, but cannot remove the silanol groups. Acid-washed carriers are suitable for analyzing acidic samples. 

The function of pickling 

The surface of the precision seamless steel pipe is subjected to oil removal and rust removal to prepare for the subsequent processing. During the production process, the acid washing step is to remove the surface oxide scale. Then, it undergoes lubrication treatment (carbon steel - phosphating, stainless steel - lard and lime, copper and aluminum pipes - oil coating), using the old process - copper plating), and then undergoes drawing and deep processing. If the steel pipe is not acid washed, there may be oxides and oil stains on the surface, and the phosphating solution core cannot remove them, resulting in a decrease in phosphating quality.