For the control of the hydrochloric acid washing tank, the most important thing is to control the pickling time and the life of the pickling tank, so as to ensure the maximum productivity and service life of the pickling tank.
To obtain the best pickling effect, the concentration of hydrochloric acid should be controlled first, and then the content of iron ions (iron salts) in the pickling solution should be controlled. Because not only the concentration of acid will affect the pickling effect of the workpiece, but also the content of iron ions will reduce the mass fraction of the pickling solution, which will also affect the pickling effect and speed of the workpiece. It is worth noting that in order to obtain the best pickling efficiency, the pickling solution must also contain a certain amount of iron ions.
(1) Pickling time
In fact, the pickling time basically depends on the concentration of hydrochloric acid/iron ions (iron salts) and the temperature of the pickling solution.
The relationship between pickling time and zinc content:
It is a well-known fact in hot dip galvanizing operations that the use of protective overpickling of galvanized workpieces results in more zinc loading, i.e. "overpickling" increases zinc consumption.
In general, immersion in the pickling tank for 1 hour is enough to completely remove the rust. Sometimes, under the working conditions of the factory, the plated workpiece can be placed in the pickling tank overnight, that is, immersion for 10-15 hours. Such galvanized workpieces are plated with more zinc than normal time pickling.
(2) Best Pickling
The best pickling effect of the workpiece should be when the concentration of hydrochloric acid and the concentration of precipitated iron ions (iron salts) reach a relative balance.
(3) Remedial method for acid effect decline
When the pickling solution decreases or loses the pickling effect due to the saturation of iron ions (iron salts), it can be diluted with water to restore the pickling function. Although the concentration of hydrochloric acid is reduced, the pickling function can still be exerted, but the rate is slower. If a new acid is added to the pickling solution with saturated iron content, the concentration of the new hydrochloric acid washing solution will fall above the saturation point, and the pickling of the workpiece will still not be possible.
(4) Treatment measures after the acid solubility decreases
When the pickling solution is used for a period of time, its concentration decreases and even becomes waste acid. However, the acid at this time cannot be recovered by the manufacturer, and still retains a certain value for use. In order to utilize the low acid with reduced concentration, at this time, the workpieces that have local leakage plating in hot-dip galvanizing and need to be re-dipped are generally placed in Among them, pickling and reprocessing is also an effective utilization of waste acid.
Method for replacing old acid with hydrochloric acid pickling solution:
When the iron salt in the old acid exceeds the specified content, it should be replaced with new acid. The method is that the new acid accounts for 50%, the old acid is added to the new acid after precipitation, and the amount of the old acid is ~ 50%. Iron salts with a content of less than 16% can increase the activity of the pickling solution, which is different from acid sparse, and also saves the amount of acid.
However, in this method, with the progress of hot-dip galvanizing technology, the amount of old acid added must be carried out on the basis of strictly controlling the content of iron salt of old acid, and the concentration of iron salt in the newly prepared hydrochloric acid solution should be controlled within the palm of your hand. Within the range, you must not blindly follow certain values.
Workpiece steel material and pickling speed
The pickling speed varies with the composition of the pickled steel workpiece and the resulting scale.
The carbon content in steel has a great influence on the dissolution rate of the steel matrix. The increase of the carbon content will increase the dissolution rate of the steel matrix rapidly.
The dissolution rate of the steel workpiece matrix after cold and hot processing is increased; while the dissolution rate of the steel workpiece after annealing will decrease. In the iron oxide scale on the surface of the steel workpiece, the dissolution rate of iron monoxide is larger than that of ferric oxide and ferric oxide. Rolled iron sheets contain more iron monoxide than annealed iron sheets. Therefore, its pickling speed is also faster. The thicker the iron oxide skin, the longer the pickling time. If the thickness of the iron oxide scale is not uniform, it is easy to produce local under-pickling or over-pickling defects.
Post time: Feb-27-2023