What kind of pipelines require pickling and passivation?

The function of acid washing and passivation: It removes various types of oils, rust, oxide scales, weld spatter, and other contaminants; after treatment, the surface becomes a uniform silver-white color, **which enhances the corrosion resistance of stainless steel. Stainless steel generally requires pickling and passivation treatment, as its corrosion resistance relies mainly on the passive film on its surface; if this film is incomplete or defective, the stainless steel will still be susceptible to corrosion. During the processes of forming, assembling, welding, weld inspection (such as flaw detection and pressure testing), and marking stainless steel equipment and components, surface contaminants such as oil, rust, non-metallic dirt, low-melting-point metal pollutants, paint, slag, and spatter can accumulate. These substances affect the surface quality of the stainless steel equipment and components, damage their oxide layer, reduce the steel’s resistance to general corrosion as well as to local corrosion (including pitting and crevice corrosion), and may even lead to stress corrosion cracking. Pickling and passivation in the manufacturing process of stainless steel equipment 1: Cleaning, pickling, and passivation after machining. After machining, stainless steel parts often have residues such as iron shavings, steel particles, and cooling emulsions on their surfaces, which can cause stains and rust on the surface of the stainless steel. Therefore, it is necessary to remove fats and oils first, followed by cleaning with nitric acid, which not only eliminates the iron shavings and steel particles but also performs passivation. 2: Cleaning, pickling, and passivation before and after welding. Since grease is a source of hydrogen, gas bubbles can form in welds where grease has not been removed. Moreover, contamination by low-melting-point metals (such as zinc-rich paint) can cause cracking after welding; therefore, it is necessary to clean the groove area as well as the surface within 20 mm on each side before welding stainless steel. Grease can be removed by wiping with acetone, while paint and rust should first be removed using sandpaper or a stainless steel brush, followed by cleaning with acetone. Regardless of the welding technique used in the manufacturing of stainless steel equipment, it is necessary to clean the material after welding; all slag, spatter, stains, and oxidation residues must be removed. The methods for removal include mechanical cleaning and chemical cleaning. Mechanical cleaning includes grinding, polishing, and sandblasting or shot blasting. The use of carbon steel brushes should be avoided to prevent rusting on the surface. To achieve the best corrosion resistance, it can be immersed in a mixture of HNO3 and HF, or acid pickling passivation paste can be used. In practice, mechanical cleaning and chemical cleaning are often used in combination for Cerenium-4. 3: Cleaning of forged and cast parts. Stainless steel components that have undergone heat treatment such as forging and casting often have an oxide layer, lubricants, or other contaminants on their surface; these contaminants include graphite, molybdenum disulfide, carbon dioxide, and others. It should be treated by shot blasting, salt bath treatment, and multiple acid washing processes. The treatment process for stainless steel turbine blades in the United States is as follows: salt bath (10 min) → water quenching (2.5 min) → sulfuric acid washing (2 min) → cold water washing (2 min) → alkaline permanganate bath (10 min) → cold water washing (2 min) → sulfuric acid washing (1 min) → cold water washing (1 min) → nitric acid washing (1.5 min) → cold water washing (1 min) → hot water washing (1 min) → air drying. 4: Acid washing and passivation treatment before the commissioning of new installations. Stainless steel equipment and pipelines in many large-scale chemical, textile, fertilizer, and other types of installations require acid washing and passivation before they are put into operation. Although the equipment has been pickled in the manufacturing plant to remove slag and scale, contamination by grease, sediment, rust, etc. is inevitable during storage, transportation, and installation. To ensure that the quality of the plants and equipment as well as the products produced during testing (especially chemical intermediates and refined products) meets the required standards and to guarantee a successful first trial run, pickling and passivation are necessary. For stainless steel equipment and pipelines in H2O2 production plants, cleaning must be carried out before operation; otherwise, contaminants and heavy metal ions can poison the catalyst. Furthermore, substances such as grease and free iron ions on metal surfaces can cause the decomposition of H2O2, releasing large amounts of heat, which can lead to fires or even explosions. Similarly, in oxygen pipelines, trace amounts of oil and metal particles can also generate sparks, leading to serious consequences. 5: Acid washing and passivation during on-site maintenance. In the equipment and materials used in production facilities for purified terephthalic acid (PTA), polyvinyl alcohol (PVA), acrylic fibers, acetic acid, etc., austenitic stainless steels such as 316L, 317, and 304L are widely used. Due to the presence of harmful ions such as Cl-, Br-, SCN-, and formic acid in these materials, or as a result of dirt and material aggregation, pitting, crevice corrosion, and weld corrosion can occur in the equipment. During shutdown for maintenance, the equipment or components can undergo comprehensive or partial pickling and passivation to repair their passivation layer and prevent the spread of local corrosion. Acid washing and passivation have been carried out for tasks such as the renewal and maintenance of stainless steel pipes in the PTA unit at Shanghai Petrochemical, as well as the maintenance of stainless steel heat exchangers in the acrylic fiber unit. 6: Descaling and cleaning of operational equipment. In petrochemical plants, stainless steel equipment, especially heat exchangers, accumulate various types of deposits on their inner surfaces over time as a result of operation. These deposits include carbonate scales, sulfate scales, silicate scales, iron oxide scales, organic deposits, and catalyst residues. Such deposits impair the heat exchange efficiency and can also lead to corrosion beneath them. An appropriate cleaning agent must be selected for descaling; options include nitric acid, nitric acid + hydrofluoric acid, sulfuric acid, citric acid, EDTA, water-based cleaners, etc., with an appropriate amount of corrosion inhibitor added. After descaling and cleaning, passivation can be performed if necessary. Chemical treatment. Stainless steel heat exchangers in facilities such as Shanghai Petrochemical’s PTA, acetic acid, and acrylic fiber plants have all been subjected to descaling cleaning. This post was last edited by zjiris on 2008-1-22 09:50]

Are there any strict regulations for this, or are there national standards?

This post was last edited by zhhwang63 on 2013-7-3 at 12:57. It is an essential process after processing stainless steel equipment. The naturally formed passivation film on stainless steel contains contaminants attached to its surface (such as those resulting from rolling or finishing processes); therefore, pickling is essential. Hydrofluoric acid is usually used to break down this passivation film, after which a passivation solution is applied to the clean and fresh surface to form a uniform, dense, and pure chromium oxide film.