39. Why does a distinct cluster of corrosion pits appear on the surface of welded steel pipes after acid pickling?

Welded steel pipes are prone to developing corrosion pits along longitudinal weld seams after acid pickling, particularly in gas-welded and furnace-welded pipes. This phenomenon primarily occurs due to temperature differences between the weld seam and base metal during welding, which generates internal stresses. These stress-induced zones become susceptible to varying degrees of corrosion from pickling solutions. Additionally, oxides and impurities embedded in the weld seam during welding exhibit differential corrosion rates in pickling solutions due to their distinct potential differences, resulting in uneven distribution of corrosion pits.