Principles, methods, and quality inspection of stainless steel pickling and passivation

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1 Principle of pickling and passivation: The corrosion resistance of stainless steel is primarily due to the high content of Cr element added to the steel (e.g., in 316L, W(Cr) = 16.00–18.00). Cr is prone to oxidation, and it can rapidly form a dense Cr2O3 oxide film on the surface of the steel, thereby significantly improving the steel’s electrode potential and its corrosion resistance in oxidative environments. The corrosion resistance of stainless steel relies mainly on this extremely thin (about 1 nm) dense passivation film covering its surface; this film acts as a barrier that separates the steel from corrosive agents and serves as the fundamental protection for stainless steel. If this passivation film is incomplete or damaged, the steel will still be susceptible to corrosion. Stainless steel chemical tankers suffer from issues such as surface oil stains, scratches, rust, impurities, low-melting-point metal contaminants, paint, slag, and splashes that arise during the handling, assembly, welding, and weld inspection (such as dye penetrant testing and pressure testing) of the inner liner materials, equipment, and accessories. These substances affect the surface quality of stainless steel, damage its passive film, reduce its corrosion resistance, and can also interact with corrosive agents in the chemicals carried later on, leading to pitting, intergranular corrosion, and even stress corrosion cracking. GB150-1998 \"Steel Pressure Vessels\" stipulates that: \"For vessels made of stainless steel that require corrosion protection, their surfaces shall be pickled and passivated.\" Stainless steel chemical tankers, due to the transport of various different chemicals, are subject to strict requirements regarding the prevention of product contamination. The surface quality of domestically produced stainless steel sheets is relatively poor; therefore, it is necessary to carry out mechanical, chemical, or electrolytic polishing treatments on the stainless steel sheets, equipment, and accessories before cleaning, pickling, and passivation, in order to enhance the corrosion resistance of the stainless steel. Stainless steel chemical carriers typically undergo the process of cleaning their tanks with seawater during operation. Seawater is rich in chloride ions, which have a strong corrosive effect on the passivation film; given these harsh operating conditions, acid cleaning and passivation are absolutely essential. 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, whereas 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. By pickling, a layer on the surface of the stainless steel with an average thickness of 10μm is corroded away. The chemical activity of the acid causes the dissolution rate in defective areas to be higher than in other parts of the surface; thus, pickling helps to achieve a more uniform balance across the entire surface. More importantly, pickling promotes passivation, allowing iron and its oxides to dissolve preferentially over chromium and its oxides. This removes the chromium-poor layers, resulting in a chromium-rich surface. Under the action of oxidizing agents, a complete and stable passivation film is formed on the surface, and the potential of this chromium-rich passivation film can reach +1.0V (SCE), which is close to the potential of precious metals, thereby enhancing the corrosion resistance. 2 Acid cleaning and passivation methods: Depending on the method of operation, there are mainly 6 ways to carry out acid cleaning and passivation of stainless steel, including immersion method, paste method, brushing method, spraying method, circulation method, and electrochemical method. Among these, the immersion method, paste method, and spraying method are more suitable for the acid cleaning and passivation of stainless steel chemicals ships and equipment. I. Impregnation method: This method is most suitable for stainless steel pipes, 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 processing, but requires the addition of fresh solution as the reaction concentration in the solution 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 an extended period, its effectiveness will decline due to factors such as solvent evaporation; it requires a dedicated facility, an acid tank, 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 rinsing, and areas such as welds, which are more vulnerable, can also be given enhanced passivation. The disadvantages are 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 combination with other methods. III. Spraying method: Suitable for the pickling and passivation of single products in a fixed, enclosed environment, or of equipment with a simple internal structure; for example, it is used in the spraying pickling process on sheet metal production lines. For chemical tankers made of stainless steel, it can be employed for pickling the inner walls of the liquid cargo holds. Chuandong Shipyard plans to use the spraying method for acid washing and passivation of the liquid cargo tanks on the CNPC 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 (WeChat official account: Pump Manager). For example: 1. All scaffolding inside the liquid 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 work can result in serious consequences. 4. Waste acid and wastewater must be stored in large containers or a temporary tank used as a substitute, but 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 concentrations in pickling solutions/pastes and cleaning water: Excessive chloride levels can damage the passivation layer of stainless steel. Some stainless steel pickling solutions/pastes use corrosive agents containing chloride ions 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 aspect needs to 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 25ppm to 100ppm. 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 acid washing and passivation, 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 treatment, etc. 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 inspection of the pickling and passivation effect on the \"Ninghua 417\" vessel is conducted using the following three methods: Visual inspection: The surface after pickling and passivation of stainless steel should be uniformly silver-white in color, smooth and attractive in appearance, without any obvious signs of corrosion. There should be no oxidized colors in the welds and heat-affected areas, nor any patches with uneven coloring. 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 strip attached to the stainless steel surface; if blue spots appear on the strip 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 plate can be acid-washed and passivated simultaneously with the product, after which the aforementioned process can be carried out on the test plate