In order to treat the surface of the machine, remove discoloration and restore corrosion resistance, there are many post-treatment and means. Here again we should distinguish between chemical and mechanical methods. Chemical methods include: pickling (immersion, pickling paste or spray), auxiliary passivation (after pickling) and electrolytic polishing. Mechanical methods include: sand blasting, shot blasting with glass or ceramic particles, immersion, brushing and polishing. These two methods are commonly used to clean the square tube machine.
Although all methods can produce weld seams, no mechanical posttreatment will provide corrosion properties suitable for demanding applications. Chemical methods are used to remove oxides and other contaminants on the surface, while mechanical methods may be used to remove the contamination caused by previously removed materials, polishing materials or annihilating materials. All kinds of pollution, especially foreign iron particles, may be the source of corrosion, especially in humid environment. Therefore, the mechanically cleaned surface should preferably be formally cleaned under dry conditions.
After pickling, in order to remove all contaminants and pickling residues, it is important to wash properly with water. The final flushing shall be carried out with softened water to avoid calcium stain and pollutants embedded in the increasing oxide layer, which is necessary for establishing the passivation layer. In addition, due to the use of chemical methods (pickling and electrolytic polishing), the corrosion resistance is improved, so iron must be dissolved more quickly in pickling solution and electrolyte than other metals. According to this, the surface is enriched with chromium and becomes more inert. Therefore, chemical methods such as pickling and electrolytic polishing are the only post-treatment methods that can restore the corrosion resistance of stainless steel at weld seams and other surface damages, which occur before welding.
