Chemical methods include: pickling (immersion, pickling paste or spray), auxiliary passivation (after pickling) and electrolytic polishing. Mechanical methods include sandblasting, shot blasting with glass or ceramic particles, immersion, brushing, and polishing, both of which are commonly used methods for cleaning square tube machines.
Although all methods can produce welded seams, no mechanical rear treatment provides corrosion performance suitable for demanding applications. The use of chemical methods to remove oxides and other pollutants from the surface, while mechanical methods may also remove contamination caused by previously removed, polished, or annihilated materials. All types of pollution, especially foreign iron particles, can become a source of corrosion, especially in humid environments. Therefore, it is best for mechanically cleaned surfaces to undergo formal cleaning under dry conditions.
After pickling, it is important to wash properly with water in order to remove all pollutants and pickling residues. The final flushing should be carried out by softening the water to prevent calcium stains and pollutants from embedding in the increasing oxide layer, which is essential for establishing a passivation layer. In addition, due to the use of chemical methods (acid washing and electrolytic polishing) to improve corrosion resistance, iron dissolves faster than other metals in acid washing and electrolyte, making the surface rich in chromium and more inert. Therefore, chemical methods such as acid pickling and electrolytic polishing are the only post-treatment methods that can restore the corrosion resistance of stainless steel at welds and other surface damages that occur before welding.
