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Pickling and Passivation of Stainless Steel Tubes during Use

Date:2018-04-16    keywords:pickling and passivation, tainless steel tubes
Corrosion resistance is the primary property of stainless steel and stainless steel pipes that distinguish them from other steel grades and ordinary steel pipes. According to modern understanding, stainless steel in general can only maintain its “rust-free” or “corrosion-resistant” environment in a non-polluting atmosphere. In practical applications, especially in many industrial applications, stainless steel can still cause corrosion due to corrosion of corrosive media, and local corrosion such as intergranular corrosion, pitting corrosion, and stress corrosion cracking is often the main manifestation. Stainless steel corrosion resistance depends on the chemical composition of the steel, surface conditions and environmental conditions. In the specific application conditions, that is, after the steel type and environmental conditions are determined, a clean and smooth surface to produce a uniform and dense passivation film is a prerequisite to ensure its corrosion resistance, and the partial damage of the passivation film is the root cause of local corrosion. the reason. Stainless steel is not absolutely rust-free steel, and its good corrosion resistance is only relative and conditional. It is particularly important to keep its surface clean, ie, passivated. The pickling and passivation of stainless steel pipe during its manufacture and use should be done as follows:

(1) The chemically inactive and passive surface formed on the surface of stainless steel in the air or other oxygen-containing environment is the fundamental guarantee for corrosion resistance of stainless steel (tube). Stainless steel pipes and other stainless steel products must be removed by special procedures such as acid pickling, passivation or electrolytic polishing prior to delivery to remove foreign materials such as surface oxides and iron ions to obtain a passive surface.
 
(2) The passive surface of stainless steel is essentially an extremely thin passivation film with a thickness of 1 to 3 mm, and its partial contamination is the basic cause of corrosion of stainless steel.
 
(3) The protection of surface passivation film must be paid attention to in the process of storage, use, and especially reprocessing of stainless steel pipes. The rubbing, carbon steel banding or spreader, forklift carriage contact, place storage, sewage immersion, etc. strictly prevent.
 
(4) After the surface of the stainless steel pipe is stored and transported or re-processed, it is often necessary to passivate the surface after being contaminated. In addition to the rational selection of the chemical composition of the passivation solution, the soaking time must be strictly controlled to prevent the occurrence of excessive pickling and appearance of frost-like rough surfaces. Hole erosion. In general, 30 minutes should be sufficient, and immersing in a pickling or passivating solution for several tens of hours or even several days is not appropriate.
 
(5) Mixtures of nitric acid and hydrofluoric acid are only suitable for removing acidified austenite or annealed ferritic stainless steel pipes.
 
(6) 20% to 50% nitric acid or 4% to 10% citric acid solution can effectively clean the surface of the surface except the thermal processing scale and form an effective passivation film. Electropolishing is a special method for the high-gloss, clean, passive surface of super-clean surface stainless steel tubes that are required for biochemical engineering and semiconductor industries.

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