The performance of stainless steel is obtained by its special chemical composition, and the main element is chromium. The chromium and oxygen combine to form a dense chromium oxide film that protects the stainless steel pipe fittings from corrosion. When the stainless steel pipe fitting has chrome oxide film, it is in the passive state, and the stainless steel pipe fittings can be corrosion-resistant.
The stainless steel pipe fitting may be corroded when the protective film on the stainless steel pipe fittings is damaged or contaminated and the passivation film is formed naturally. In this case, most of the beneficial properties of stainless steel fitting may be destroyed during processing, such as heat treatment or welding, cutting, sawing, drilling and bending of such mechanical processing. Because such treatment will cause damage or contamination of the oxide film on the surface of the stainless steel pipe fitting, it will not achieve spontaneous and complete passivation. Therefore, local corrosion may occur, especially in relatively weak corrosion conditions, but also easy to rust. At the time of use, it may make the final product not satisfactory, or even cause the system to fail.
The weld seam and the inside and outside sides close to the weld will cause the acceleration of the oxidation of the stainless steel pipe fitting. The color is related to the thickness of the oxide layer due to the visible oxidation in the color change zone. Compared with the oxide layer on the stainless steel pipe fittings before welding, the oxide layer of the color change zone is relatively thick, and the chromium element is also reduced, and the local corrosion resistance is reduced. For stainless steel pipe fittings, oxidation and discoloration can be reduced to the minimum by using an appropriate blowing method. After welding, it is better to do the post weld treatment such as pickling and grinding to remove the oxide layer and restore the corrosion resistance.
In addition, mechanical treatment can also contaminate the surface of stainless steel fittings. Organic pollutants may be caused by lubricants. Inorganic contaminants, such as exotic iron particles, may be caused by tool contact. In general, all surface contamination may lead to plaque corrosion. In addition, the external iron particles may lead to galvanic corrosion. Spot corrosion and electrochemical corrosion are localized corrosion forms and need to be treated with water. Therefore, surface contamination usually reduces the corrosion resistance of stainless steel fittings.
In order to deal with the surface of stainless steel pipe fittings, remove discoloration and restore corrosion resistance, there are many rear treatment processes. It is important to distinguish between chemical and mechanical methods. Chemical methods are: pickling (immersion, pickling or spray), auxiliary passivation (after pickling) and electrolytic polishing. Mechanical methods are: sandblasting cleaning, glass or ceramic particles shot peening, annihilation, scrub and polishing.
Although all methods produce weld seams, no mechanical rear treatment will provide corrosion resistance for a strictly applicable purpose. The removal of oxides and other contaminants on the surface by chemical methods can also be used to wipe off the contamination of previously removed materials, polishing materials, or annihilation materials. All kinds of pollution, especially foreign iron particles, may be the source of corrosion, especially in humid environment. Therefore, the mechanical cleaning surface is best to be cleaned regularly under dry conditions.
After pickling, clean water is needed to remove contaminants and pickling residues. The final flushing uses softened water to prevent calcium stains and contaminants from being embedded in the growing oxide layer, which is essential for the passivation layer. Also because of the use of chemical methods (pickling and electrolytic polishing) and improve corrosion resistance, so the metal dissolution than iron in acid solution and the electrolyte quickly, according to the surface enrichment of chromium, with more inert. Therefore, the chemical methods of pickling and electropolishing are the only post treatment methods that can restore the corrosion resistance of stainless steel fittings at the weld and other surface damage.
Source: China Stainless Steel Pipe Fittings Manufacturer – Yaang Pipe Industry Co., Limited (www.yaang.com)