The pretreated strip is sent to the acid tank for pickling. Pickling methods can be summarized into two categories; one is the acid leaching method, and the other is the acid electrolysis method. The former is a commonly used method, while the latter is a auxiliary method. In most cases, it is used in combination with acid leaching. Acid leaching method: Acids used alone or mixed include nitric acid, hydrofluoric acid, sulfuric acid, etc. Nitric acid is an oxidant that can passivate stainless steel, but it has no descaling effect when used alone. Hydrofluoric acid is highly corrosive, not only corroding the iron scale, but also corroding the base metal, making the surface rough. Therefore, neither of these two acids can work alone, and the use of their mixture has the best effect. A good surface can be obtained in a short time, and the risk of pitting corrosion is also small. Therefore, no matter what type of steel it is, this mixed acid is mostly used. The pickling effect of sulfuric acid is good at higher temperatures, but very poor at low temperatures, so it is generally only used in the following two situations: First, in order to improve the descaling properties of hot-rolled coils of chromium-based steel, a hot sulfuric acid tank is sometimes installed , used in combination with a mixed acid tank of nitric acid and hydrofluoric acid; second, in the cold rolled coil annealing and pickling line, in order to neutralize the alkali brought out from the strip after being treated in the alkali tank, it is set between the salt bath and the water washing tank Sulfuric acid tank to improve the efficiency of the subsequent pickling tank. When using sulfuric acid pickling, the key is to control the acid temperature. Stainless steel pickling generally does not use hydrochloric acid. Because hydrochloric acid not only corrodes iron scales, but also corrodes the metal matrix. Even if it is used, it should be mixed with nitric acid. However, this mixed acid is still corrosive to the metal matrix, resulting in rough metal surfaces, so it is rarely used in general stainless steel plants. Acid leaching electrolysis method (also called electrolytic pickling) Electrolytic pickling is a method of accelerating scale removal when it is difficult to completely remove scales by acid leaching alone. When used in combination with acid leaching, the pickling efficiency can be improved. The characteristics of this method are: due to the effect of electrochemistry, the surface is uniform and glossy. Two methods are usually used in electrolytic pickling: nitric acid electrolysis and sulfuric acid electrolysis, with nitric acid electrolysis being the most used. Nitric acid alone has no descaling effect when used as an acid leaching solution. However, when current flows through the electrode (silicon cast iron), the oxide film is ionized due to electrochemical action, and pickling can be performed. However, due to its low corrosiveness, descaling is not possible. The ability is small and generally not used alone. The descaling ability of sulfuric acid electrolysis is even worse. In addition, in addition to the electrolysis device, this method also requires a device to recover waste acid and sulfur oxides in waste gas, so the equipment cost is relatively high. Strip pickling is carried out continuously, and various pickling methods, including pretreatment, are combined in different ways. The combination method is determined by the steel type, strip surface condition and equipment conditions.    

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