During acid pickling of alloy steel pipes, hydrogen not only has a stripping effect on the oxide scale, but also diffuses into the interior of the billet matrix and accumulates at impurities or pores, leading to an increase in internal stress, reducing the plasticity of the steel, and increasing its brittleness. This effect is called hydrogen embrittlement. Hydrogen embrittlement is mainly influenced by the diffusion rate of hydrogen atoms. The diffusion rate of hydrogen is related to the following factors:
- The diffusion rate of hydrogen in sulfuric acid solution is higher than that in hydrochloric acid solution.
- With the increase of pickling temperature for 40cr alloy steel pipes, the diffusion rate of hydrogen rapidly increases, while increasing the concentration of acid solution does not significantly increase the diffusion rate of hydrogen. Therefore, it is not advisable to acid wash at low concentrations and high temperatures.
- Some inclusions such as arsenic (As), antimony (Sb), hydrogen sulfide (H2S), and sulfur dioxide (SO2) in 40cr alloy steel pipes during acid pickling increase the diffusion rate of hydrogen atoms, which increases the possibility of hydrogen embrittlement.
The chemical composition of 4.40cr alloy steel pipes also affects the diffusion of hydrogen atoms. The measures to prevent hydrogen embrittlement are to operate according to the specified acid concentration and temperature, shorten the pickling time as much as possible, and add corrosion inhibitors on time and in quantity.
- After under pickling and pickling, there is still iron oxide scale on the surface of the 40cr alloy steel pipe that has not been removed, which is called under pickling. Under pickling can be divided into full under pickling and partial under pickling based on the reasons for its occurrence. The reasons for all under pickling are short pickling time, low acid concentration, or low temperature; The reason for the local lack of pickling is due to the lack of padding iron or local oil stains on the surface of the steel billet during material stacking. Underpickling can be saved by continuing pickling.
- When subjected to acid pickling or 40cr alloy steel pipes are subjected to acid pickling, the surface is rough and the color is dark; When chromium stainless steel is pickled, the surface appears gray and extremely rough, with severe cases having circular holes of a certain depth – pinholes. The reasons for excessive pickling are high acid temperature, long pickling time, or high acid concentration. Once this defect occurs, it cannot be saved and has a harmful impact on the performance of the steel. Pickled 40cr alloy steel pipes are prone to sticking or crushing during the rolling process.