Why are there obvious clusters of corrosion pits on the surface of welded steel pipes after over-pickling?

After pickling, welded steel pipes, especially those welded by gas or furnace methods, are prone to developing corrosion pits along the longitudinal weld seams. The reason for the occurrence of these clusters of corrosion pits on the weld seams is mainly due to the temperature difference between the weld and the base metal during the welding process, which leads to internal stress. Areas with internal stress are susceptible to varying degrees of corrosion by the pickling solution. Additionally, oxides and other impurities pressed into the dissolved weld during the welding process have different corrosion rates in the pickling solution due to their different electrical potentials, causing uneven corrosion pits.