In the case of pickling, the
seamless steel pipe has a stripping effect on the iron oxide scale, and also diffuses into the interior of the steel billet matrix and accumulates at the impurities or pores, resulting in an increase in internal stress, a decrease in the plasticity of the steel, and an increase in the brittleness of the steel. The effect is called hydrogen embrittlement. Hydrogen embrittlement is mainly affected by the diffusion rate of hydrogen atoms. The rate of hydrogen diffusion is related to the following factors:
1) The diffusion rate of hydrogen in a sulfuric acid solution is higher than that in a hydrochloric acid solution.
2) As the pickling temperature increases, the diffusion rate of hydrogen increases sharply, and the concentration of acid increases, and the diffusion rate of hydrogen does not increase much. Therefore, it is best not to pickle at low concentrations and high temperatures. This is a must for both seamless and
oil casings.
3) Some inclusions in pickling such as arsenic (As), antimony (Sb), hydrogen sulfide (H2S) and sulfur dioxide (SO2) increase the diffusion rate of hydrogen atoms, ie increase the possibility of hydrogen embrittlement.
4) The chemical composition of steel also has an effect on the diffusion of hydrogen atoms. The measures to prevent hydrogen embrittlement are to operate according to the specified acid concentration and temperature, to minimize the pickling time, and to add corrosion inhibitors in accordance with the amount.
There is still a scale of iron oxide on the surface of the slab after pickling, which is called under-picking. Under-picking can be divided into all under-picking and partial under-washing depending on the cause. The reason for causing all the acid pickling is that the pickling time is short, the acid concentration is low, or the temperature is low; the reason for causing partial under pickling is that there is no horn on the material or oil on the surface of the steel billet. Under-pickling can be saved by continuing to pickle.
When it is pickled and carbon steel is pickled, the surface is rough and the color is dark. When the chrome stainless steel is pickled, the surface is gray and extremely rough. In severe cases, there is a circular hole with a certain depth - pinhole. The reason for the pickling is that the acid temperature is too high, the pickling time is too long, or the acid concentration is too high. Once this defect is unrecoverable, it has a detrimental effect on the performance of the steel. The acid-washed steel is easily stuck or broken during the rolling process.
After pickling, in addition to the iron salt remaining on the surface of the slab, there is a residue consisting of oxides or salts of other elements in the steel. These residues are not good for cleaning the surface defects of the slab. If it is not washed out in time, the surface of the slab will continue to rust and appear yellow.
