The pretreated strip is sent to the acid bath for pickling. The methods of pickling can be grouped into two categories; one is the acid leaching method and the other is the acid electrolysis method. The former is the commonly used method, while the latter is an auxiliary method, and in most cases a combination of acid leaching methods is used. Acid leaching method. The acids used alone or in combination are nitric acid, hydrofluoric acid, sulfuric acid, etc. Nitric acid is an oxidizing agent, can promote stainless steel passivation, but used alone is useless to remove the scale effect. Hydrofluoric acid corrosion, not only erosion of iron scales, but also erosion of the base metal, so that the surface rough. Therefore, these two acids can not be used alone, and the use of their mixture works best, a short time to get a good surface, the risk of pitting is also small, so no matter what the steel, most of this mixed acid. Sulfuric acid at higher temperatures pickling effect is good, but the temperature is very poor, so generally only in the following two cases: First, to improve the chrome steel hot-rolled coil scale-removal, sometimes set up hot sulfuric acid tank, with nitric acid, hydrofluoric acid mixed acid tank combination; Second, in the cold-rolled coil annealing pickling line, in order to neutralize the alkali brought out on the strip after alkali tank treatment, set up sulfuric acid tank between the salt bath and water washing tank to improve The efficiency of the latter pickling tank. When using sulfuric acid pickling, the key is to master the acid temperature. Stainless steel pickling generally does not use hydrochloric acid. Because hydrochloric acid not only erodes the iron scale, but also erodes the metal matrix, even if used, should be mixed with nitric acid, but this mixed acid is still aggressive to the metal matrix, resulting in rough metal surface, so the general stainless steel plant is rarely used. Acid immersion electrolysis method (also known as electrolytic pickling) Electrolytic pickling is a method used to accelerate the removal of scale in cases where it is difficult to completely remove the scale using only the acid dip method, and the combination of acid dip method can improve the efficiency of pickling. This method is characterized by a good uniform glossy surface due to electrochemistry. The two methods usually used in electrolytic pickling are nitric acid electrolysis and sulfuric acid electrolysis, with nitric acid electrolysis being used most often. When nitric acid is used as pickling solution alone, there is no scale-removal effect, but when current flow is generated through the electrode (silicon cast iron), the ionization of the oxide film is possible due to electrochemical effects, and pickling can be performed, although the scale-removal ability is small due to low corrosiveness, so it is not used alone. The scale removal capacity of sulfuric acid electrolysis is even worse. In addition to the electrolysis device, this method also requires the recovery of waste acid and sulfur oxides in the exhaust gas, so the equipment cost is higher. Strip steel pickling is continuous and is a combination of various pickling methods, including pretreatment, used in different ways. The combination is determined by the steel grade, strip surface condition and equipment conditions.

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