Why do seamless black steel pipes need to be pickled?

Seamless black steel pipes are made from ingots or solid billets that are perforated into a burr tube and then hot-rolled, cold-rolled or cold-dipped. In its production process, pickling is an essential process, so why do seamless black steel pipes need to be pickled?

The method of using an acid solution to remove oxidation and rust from the surface of steel is called pickling. Oxide, rust and other iron oxides (Fe3O4, Fe2O3, FeO, etc.) react chemically with the acid solution, forming salts that dissolve in the acid solution and are removed. Acid used for pickling are sulfuric acid, hydrochloric acid, phosphoric acid, nitric acid and mixed acids.

Commonly used media for pickling of seamless black steel pipes.

Sulphuric acid, nitric acid, phosphoric acid, hydrofluoric acid. It is important to add a pickling corrosion inhibitor to the pickling process to prevent corrosion of the metal by the acid.

Methods of pickling seamless black steel pipes.

The main methods are dipping pickling, jet pickling and acid paste rust removal. Generally, the dip pickling method is used, and the jet method can be used in mass production. Seamless black steel pipes are generally pickled in 10%-20% (volume) sulphuric acid solution at a temperature of 40°C. When the iron content in the solution is more than 80%, the pickling process can be carried out at a temperature of 40°C. When the iron content in the solution exceeds 80g/L and ferrous sulphate exceeds 215g/L, the pickling solution should be replaced. At room temperature, pickling of seamless black steel pipes with 20% to 80% (by volume) hydrochloric acid solution is not easy to occur over corrosion and hydrogen embrittlement. As the acid has a great corrosive effect on the metal, corrosion inhibitors need to be added. The surface of the metal becomes silvery white after cleaning, while passivating the surface and improving the corrosion resistance of the seamless black steel pipe. In order to eliminate adsorption on the surface of the diatomaceous earth carrier and reduce peak dragging, the carrier needs to be acid washed or alkaline washed before use. Acid washing is done by boiling the carrier in 6mol/L hydrochloric acid for 2h or concentrated hydrochloric acid for 30min, filtering, washing with water to neutral and drying. Acid washing removes impurities such as iron, aluminium, calcium and magnesium from the surface, but not the silanol group. The pickling carrier is suitable for the analysis of acidic samples.

The role of pickling of seamless black steel pipes.

To remove oil and rust from the surface of seamless black steel pipes in order to prepare for the next process treatment. In the production process, the pickling process is to remove surface oxidation, followed by lubrication treatment (carbon steel – phosphorus saponification, stainless steel – butter lime, copper and aluminium tubes – oiling, with the old process – copper plating), and then deep processing by drawing. Steel pipe if not pickled, the surface may have oxides and oil, phosphate solution nuclear energy will not be able to remove them, phosphate quality will be reduced.

Tips: due to the small width-to-thickness ratio of the steel plate, the accuracy of the hot-rolled seamless pipe specification requirements are relatively low, not prone to plate shape problems to control the convexity of the main.

Chase Liu