Pickling and passivation of stainless steel

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1. Principles of pickling and passivation: The corrosion resistance of stainless steel is mainly due to the addition of a relatively high content of Cr element in the steel (for example, in 316L, W(Cr) = 16.00–18.00). The Cr element is prone to oxidation, which allows a dense Cr2O3 oxide film to form rapidly on the steel surface. This leads to a significant increase in the steel’s electrode potential and its resistance to corrosion in oxidizing environments. The corrosion resistance of stainless steel relies primarily on this extremely thin (approximately 1 nm) and dense passive film covering its surface; this film acts as a barrier between the steel and corrosive media. If this passive film becomes incomplete or is damaged, the stainless steel can still be corroded. During the lifting, assembly, welding, weld inspection (such as dye penetrant testing and pressure testing), and processing of the inner tank plates, equipment, and accessories of stainless steel chemical tankers, surface contaminants such as oil, scratches, rust, impurities, low-melting-point metal contaminants, paint, weld slag, and spatter are generated. These substances impair the surface quality of the stainless steel, damage its passive surface film, and reduce its corrosion resistance. Moreover, they can interact with corrosive agents present in subsequently transported chemicals, leading to pitting corrosion, intergranular corrosion, and even stress corrosion cracking. In operation, stainless steel chemical tankers typically undergo a process of tank washing with seawater. Seawater is rich in chloride ions, which have a significant corrosive effect on the passive film. Given such harsh operating conditions, acid pickling and passivation are indispensable. The passivation film on stainless steel has dynamic properties; it should not be considered as a complete cessation of corrosion, but rather as the formation of a protective layer that diffuses. This passivation film tends to be damaged in the presence of reducing agents such as chloride ions, while it can be protected and repaired in the presence of oxidizing agents such as air. When exposed to air, stainless steel forms an oxide film, but this film does not provide sufficient protection. Acid washing removes a layer from the surface of the stainless steel with an average thickness of 10μm. The chemical activity of the acid causes the dissolution rate in defective areas to be higher than in other parts of the surface; thus, acid washing helps to achieve a more uniform balance across the entire surface. More importantly, through acid washing, iron and its oxides dissolve preferentially over chromium and its oxides, eliminating the chromium-poor layers and resulting in a chromium-rich surface. Under the action of oxidizing agents, a complete and stable passive film is formed on the surface. The potential of this chromium-rich passive film can reach +1.0V (SCE), which is close to the potential of precious metals, thereby enhancing the corrosion resistance. 2. Methods of pickling and passivation: Depending on the operating method, there are six main methods for pickling and passivation of stainless steel: immersion, paste application, brushing, spraying, circulation, and electrochemical methods. Among these, the immersion, paste application, and spraying methods are particularly suitable for pickling and passivation of stainless steel chemical tankers and equipment. 1. Immersion method: This method is most suitable for stainless steel pipelines, elbows, small parts, etc., and yields the best treatment results. Because the parts to be treated can be fully immersed in the acid pickling passivation solution, the surface reactions occur completely, resulting in a dense and uniform passivation film. The East Sichuan Shipyard achieved good results by using its pickling workshop to carry out batch pickling of \"Ninghua 417\" stainless steel pipelines. This method is suitable for continuous batch operations, but requires constant replenishment of fresh solution as the reaction concentration decreases. Its drawback is that it is limited by the shape and capacity of the acid tank, making it unsuitable for large-capacity equipment as well as pipelines that are too long or too wide ； If not used for a long time, its effectiveness may decline due to reasons such as solvent evaporation; it requires a dedicated space, acid tanks, and heating equipment. II. Paste method: Stainless steel pickling and passivation paste is currently widely used in China, with a range of products available on the market. Its main components include nitric acid, hydrofluoric acid, corrosion inhibitors, thickeners, etc., combined in specific proportions. The ship standard \"Stainless Steel Pickling and Passivation Paste\" CB/T3595-94 specifies detailed inspection requirements. This method involves manual application and is suitable for use on-site; it can be used for passivating welds on stainless steel vessels, areas affected by welding discoloration, the top of decks, corners and hard-to-reach areas, the back of staircases, as well as large surfaces inside cargo tanks. The advantage of the paste method is that it requires no special equipment or space, no heating apparatus, offers flexibility in on-site operation, enables acid cleaning and passivation to be carried out in one step, and has a high degree of independence ； The passivation paste has a long shelf life; each application uses fresh passivation paste that is used once only. Once a layer of passivation is formed on the surface, the reaction stops, reducing the risk of over-corrosion. There is no limitation regarding the time required for subsequent rinsing, and areas such as welds that are more vulnerable can be given additional passivation treatment. The disadvantages include a poor working environment for workers, high labor intensity, high costs, and a somewhat poor treatment effect on the inner surface of stainless steel pipelines, requiring the use of other methods in combination. III. Spraying method: Suitable for pickling and passivation of single products or equipment with simple internal structures in fixed locations and enclosed environments. For example, it can be used in the spray pickling process on sheet metal production lines. For stainless steel chemical tankers, it can be applied to pickling the inner walls of cargo tanks. Chuandong Shipyard plans to use the spraying method for acid washing and passivation of the liquid cargo tanks on the CSIC 3,450t stainless steel chemical carrier \"Yingchun\", which is under construction. Its advantages are fast continuous operation speed, simple operation method, minimal corrosive impact on workers, and the possibility of re-acid-washing the pipeline during the liquid transfer process. The solution utilization rate is high. This method has many limitations. For example: 1. All scaffolding inside the tank must be removed, and the tank as well as the piping system need to be thoroughly cleaned; there should be no residues or impurities inside the pipes. 2. All deep well pumps, valves, bilge pumps, and other liquid cargo systems on the ship must be tuned up and kept in standby condition. 3. If the pickling solution remains in the tank for too long, reactions will occur that lead to excessive corrosion of the stainless steel; therefore, continuous operation is necessary. The shipyard must cooperate closely by having an ample supply of water for cleaning ready at all times. Power outages, water shortages, or interruptions in production can result in serious consequences. 4. Waste acid and wastewater must be contained in large containers or a temporary tank used as a substitute; however, they should be neutralized and discharged as soon as possible to prevent corrosion of the tank. 5. As the reaction time increases and the impurities in the solution rise, the effectiveness of the acid cleaning solution gradually declines; it is necessary to monitor the solution concentration regularly and replenish it with fresh solution as needed. 6. In areas such as the deck, walls, corners, and the back of escalators, the residence time of the spraying liquid is shorter, resulting in a slightly poorer pickling effect compared to the bottom plate. 3. Special precautions: Pretreatment for pickling and passivation: If there are contaminants such as grease on the surface of stainless steel, it will affect the quality of pickling and passivation; generally, the inner walls of the tank should first be cleaned using alkaline detergents or similar cleaning agents. Control of chloride ion concentration in pickling solutions/pastes and cleaning water: Excessive chloride content can damage the passivation film of stainless steel. Some stainless steel pickling solutions/pastes use chloride-containing corrosive agents such as hydrochloric acid or perchloric acid as main components or additives to remove the surface oxide layer, which is detrimental to preventing corrosion; therefore, this parameter must be controlled during testing. For example, the shipbuilding standard \"Stainless Steel Pickling and Passivation Paste\" CB/T3595-94 specifies that the chloride ion content in such pastes should be kept within the range of 25 ppm to 100 ppm. Additionally, the chloride content in the cleaning water should also be kept below 25PPM; this requirement can be met by adding sodium nitrate, a chloride corrosion inhibitor, to tap water during the processing. Neutralization of waste liquids and environmentally friendly discharge: When dealing with waste liquids resulting from pickling and passivation processes, it is necessary to meet **environmental discharge standards. For fluoride-containing waste liquids, lime milk or calcium chloride can be added for treatment; for chromium-containing waste liquids, ferrous sulfate can be used for reduction purposes. 4. Quality inspection of pickling and passivation: The quality inspection of the pickling and passivation effect on stainless steel can be carried out in accordance with the shipbuilding standard “Stainless Steel Pickling and Passivation Paste” CB/T3595-94, as well as the international standard “Inspection of the Surface Passivation Film Formed by Electrolytic Polishing, Grinding Polishing, and Pickling and Passivation” ISO15730. This inspection generally includes visual inspection and chemical tests; the chemical tests mainly involve artificial seawater salt spray corrosion testing, copper sulfate titration testing, and potassium ferricyanide titration testing (blue spot test). The pickling and passivation effect of the “Ninghua 417” vessel is inspected using the following three methods: Visual inspection: The surface after pickling and passivation should be uniformly silver-white in color, smooth and attractive in appearance, without any obvious signs of corrosion. Welds and heat-affected areas should not show any signs of oxidation, nor should there be any patches with uneven colors. Residual liquid inspection: Use phenolphthalein test paper to check the degree of cleaning of residual liquids on the stainless steel surface; a neutral pH value indicates compliance. Blue dot test: The basic principle of the blue dot test is that if the surface passivation layer is incomplete or contaminated with iron ions, free iron ions will be present. When a potassium ferrocyanide solution comes into contact with these iron ions, a blue precipitate is formed; the reaction equation is as follows: K+ + Fe2+ → KFe3+. In a 100 mL beaker, 10 g of potassium ferrocyanide is dissolved in 50 mL of distilled water. After dissolution, 30 mL of concentrated nitric acid is added, and then the mixture is transferred to a 1000 mL volumetric flask and diluted to the mark with distilled water. This resulting solution is used for the blue dot test, and it can be stored for up to one week. Test method: Drop the blue spot test solution on the test paper attached to the stainless steel surface; if blue spots appear on the test paper after 30 seconds, it is considered unqualified. It should be noted that chemical tests can damage the passivation film of stainless steel; typically, the test plates can be acid-washed and passivated simultaneously with the products, after which the aforementioned tests can be conducted on the test plates.