Pickling process and quality control
Stainless steel pickling is affected by factors such as steel type, strip surface condition, annealing conditions, etc. The processes used are different and should be determined according to specific circumstances. Acid concentration control The pickling reaction in (HNO3+HF) mixed acid is relatively complex. According to the concentration balance law of the general reaction formula and the actual experience of many manufacturers, the acid concentration of the mixed acid tank, HNO3 is usually controlled at 80-160g/l, and HF is controlled at 8- Between 30g/l. It should be noted that the concentration of HF cannot be too high, otherwise metal loss will increase and the surface will be rough. Most manufacturers use a pickling process that combines a sulfuric acid tank, a nitric acid electrolytic tank and a (HNO3+HF) mixed acid tank, in which the concentration of the sulfuric acid tank is controlled between 150-200 g/l. Acid temperature control Increasing the temperature can increase the pickling effect just as increasing the concentration. When the metal ion concentration exceeds 45g/l, the pickling effect of increasing the temperature is more obvious than increasing the concentration, so the “low concentration, high temperature” pickling process is often used. The higher the temperature of the acid, the faster the reaction speed and the better the pickling effect. However, when the temperature is too high, the acid will evaporate seriously (especially HF acid). At the same time, the heat resistance of the pickling tank must be considered, so the temperature It cannot be too high, usually controlled at 50-60℃. The acid temperature is usually controlled automatically. To prevent instrument abnormalities, regular inspection is required. Fe ion concentration control During pickling, the acid liquid will react with Fe, Cr and other alloying elements in the base metal to form metal salts (mainly Fe), and the acid concentration calculated by wet analysis includes the acid ions in this metal salt. Even if the acid concentration analyzed in this way is the same, the free acid content will be different due to the different concentrations of metal ions (iron ions) in the acid solution. That is, as the iron ion concentration increases, the free acid content decreases and the pickling effect decreases. In addition, if the concentration of metal ions in the solution is too high, it will form insoluble fluoride and easily block the pump and piping. For this reason, the concentration of Fe ions must be managed to avoid over-pickling or poor pickling. The concentration of iron ions in the mixed acid tank should generally be controlled below 45g/l. In order to solve this problem, the AT-50 rapid analysis device is currently used to analyze the free acid content in the acid solution through electrochemical treatment. The basic working principle of the AT-50 rapid analysis device is to directly measure the amounts of H+ and F- in the solution through ion-selective electrodes, thereby calculating the concentration of the corresponding acid solution. To sum up, the core issue of the pickling process is good acid management. In addition, it is also important to conduct frequent inspections during the board passing process to prevent the occurrence of defects. The length of the pickling equipment is about more than half of the entire length of the entire deacidification unit. During the plate passing process, defects often occur on the surface of the strip and are covered. Defects are often discovered late, so spot inspections must be strengthened and a quality assurance system must be established. .
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