Why is it necessary to lift and lower steel pipes several times during bundled pickling?

During the pickling of bundled steel pipes, it is commonly observed that the pipes being pickled in the pickling tank are lifted and lowered several times during the process. This is the simplest form of the oscillating pickling method. By doing so, some of the advantages of oscillating pickling can be achieved, and even better results may be obtained. When the steel pipes, which have been pickled for a certain period, are lifted, the diluted pickling solution within the pipes, along with factors for pickling such as hydrogen, other impurities, and iron oxide scales adhering to the surface, basically flow out. When the pipes are lowered again, fresh pickling solution enters the inner bore of the pipes, bringing in a portion of oxygen from the air, which is beneficial for increasing the pickling rate. Since the conditions inside the bore of the steel pipes are much harsher than those on the outer wall during pickling, if pickling is carried out in a static pickling solution, it is likely that the outer wall will be fully pickled while the inner wall is still insufficiently pickled. If pickling is continued, the outer wall may become over-pickled when the inner wall is finally pickled. Therefore, stirring the pickling solution and oscillating the steel pipes can avoid this drawback. The practice of lifting and lowering bundled steel pipes several times serves this purpose.