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The removal of rust from the surface of steel is usually accomplished using hydrochloric acid. Due to the emphasis on production and the pursuit of high output, the concentration of the acid solution remains high, with little attention paid to maintaining an optimal concentration. Many manufacturers simply replace the acid solution once a week, or fail to change it at all; instead, they merely pour out some old acid solution and add some new one. This leads to excessive consumption of hydrochloric acid, increases production costs, and causes environmental pollution. Research shows that the speed of pickling depends not only on the concentration of the pickling solution, but more importantly on the saturation level of FeCl2 at that hydrochloric acid concentration. When the hydrochloric acid concentration reaches 10%, the saturation degree of FeCl2 is 48%; when the concentration reaches 31%, it is only 5.5%. Meanwhile, the saturation degree of FeCl2 increases as the temperature rises. To achieve the optimal clean surface on the acid-treated steel surface in the shortest possible time, the key lies in selecting the concentration of hydrochloric acid, the FeCl2 content, and its solubility at that concentration. Therefore, to increase the pickling speed, it is necessary to have an appropriate concentration of hydrochloric acid and a certain amount of FeCl2, as well as a high solubility level for FeCl2. Among these three parameters, the concentration of hydrochloric acid is the most important; reducing its concentration allows for a higher amount of FeCl2 to be used, and it also makes saturation less likely, thereby improving the quality of pickling steel products. Practice has shown that too narrow or too wide a control range for the hydrochloric acid concentration poses certain difficulties in both operation and production. Based on the realities of continuous production and the influence of various human factors, we believe that maintaining a hydrochloric acid concentration between 8% and 13%, an acid temperature within the range of 20°C to 40°C, and an acid specific gravity of 1.35 to 1.20 can effectively meet the requirements of production and maximize the service life of the acid. If the FeCl2 content is high, the hydrochloric acid concentration can be set at a lower value accordingly ； With a low FeCl2 content, a higher value for hydrochloric acid concentration can be used. In terms of practical operations, it is necessary to regularly prevent a decrease in the acid concentration and a drop in the acid level, by adding fresh acid. When adding acid, it is necessary to be frequent but use small amounts – that is, add acid frequently and in small quantities each time. If the pickling speed is slow in winter, the temperature can be raised to 20–25°C. If the concentration of FeCl2 is too high and the specific gravity of the acid solution exceeds 1.35, it can be diluted with water until the specific gravity is no more than 1.22. To address the issues of excessive acid mist emission from hydrochloric acid pickling tanks during storage and operation, which leads to environmental pollution by acid mist, as well as the dissolution of the iron matrix when pickling steel and resulting in over-corrosion and hydrogen embrittlement, high-efficiency acid mist inhibitors and corrosion inhibitors can be used in combination with hydrochloric acid solution to create a high-efficiency rust removal agent that works at room temperature. This agent enables the removal of oxide scales at room temperature, with fast rust removal rates, without causing over-corrosion; it improves both the surface quality and the internal quality of the workpieces. The pungent odor of hydrochloric acid is hardly noticeable near the pickling tank, and the rust removal efficiency is at least 98%. Continuous use of this agent helps extend the lifespan of both the rust removal solution and the equipment, while also saving fuel and reducing energy consumption. The preparation of its pickling solution and the process conditions are as follows: (by weight percentage) hydrochloric acid (33%) 55, rust removal additive 10, water 35, temperature 20–40°C. This rust-removing additive is composed of organic acids, sodium alkyl sulfate, hexamethylenetetramine, polyethylene glycol, phosphoric acid, and water. This post was last edited by JLH on 2007-12-19 21:17 ]