Why do we need pickling?
The temperature of hot-rolled products is very high, and the surface is covered with iron oxide scale after rolling. For some products with high surface requirements, it is necessary to remove the surface iron oxide scale in a certain way to improve the surface quality. At this time, pickling is necessary.
Pickling is a method of cleaning the surface. The surface iron oxide scale and some rust and dirt can be removed by pickling.
Principle of pickling
Iron oxide scale is composed of Fe2O3/ Fe3O4/ FeO, etc. It will react chemically with acid to form salt, which dissolves in acid to remove the iron oxide scale.
The most commonly used acid is hydrochloric acid.
The concentration of hydrochloric acid is generally 10%-15%. The temperature of the solution is basically slightly higher than room temperature, 30-40℃.
After pickling, the surface quality is improved, and it can replace cold-rolled products to make some components.
Surface grade
Generally, the surface of hot-rolled products is FA, and the surface of pickled products is FB. Cold-rolled products are FB/FC/FD.
Ordinary products have low surface requirements, such as the inner panels of automobiles, which use FB surfaces. For some general outer panels of automobiles that do not have particularly high requirements, FC is used. For high-end cars, FD surfaces are used.
Usually pickled products have FB surfaces, so they can replace cold-rolled products to make some structural parts.
There are fewer processes later, which can save costs for the company. Moreover, the surface oxide scale is removed, which makes welding easier.
Relatively speaking, pickled products are easier to weld than hot-rolled products. If there is subsequent oiling, painting, etc., it will be easier and the quality will be better.
In addition, the dimensional accuracy is much better than that of hot-rolled products, and the dimensional accuracy is relatively high compared to hot-rolled products. In addition, pickled products are flat, the plate shape is also better, and the unevenness is better. The surface finish is also much better than that of hot-rolled products, and the appearance is higher.
Pickling defects
While pickling improves the surface quality, it will also produce some defects. So what are the defects caused by the pickling process? Let's take a look together.
1 Hydrogen embrittlement
Hydrogen embrittlement defects do not occur much during the pickling process.
During pickling, the acid reacts with the iron oxide, and hydrogen is produced while producing salt. Salt dissolves in the acid, which is a very slow process. The hydrogen formed in this process expands and peels off the iron oxide and the matrix, playing a peeling role. While playing the peeling role, the hydrogen diffuses into the matrix, and diffuses into the steel through the pores and loose structure on the surface. In the steel matrix, it gathers in the places where the steel has pores and impurities. Hydrogen easily increases the brittleness of the steel, reduces the plasticity, increases the internal stress, and increases the brittleness, resulting in hydrogen embrittlement.
2 Under-pickling
Under-pickling means that the acid concentration is insufficient/the temperature is not high/the belt speed is too fast, and the local iron oxide scale is not removed cleanly and fully.
This is related to the process of the acid solution and the surface condition of the steel plate. For example, the thickness of the iron oxide scale is uneven, and the steel coil is wavy, and some cannot be immersed in the acid solution, etc., which are prone to under-pickling.
So, in general, where are the parts that are not cleaned when pickling is insufficient?
That's right, it's the head and tail and the edge. From the perspective of the entire coil length, the iron oxide scale at the head and tail is thicker and is prone to under-pickling. In the width direction, it's the edge.
3 Over-pickling
In another case, if the acid concentration is high/the temperature is high/the tape speed is slow, it is easy to produce over-pickling, that is, over-pickling.
So the head and tail and the edge we just talked about are pickled clean, where should the over-pickling part be? It is the area between the middle and the width direction.
Over-pickling is prone to pitting, roughness, and spots. It is equivalent to the acid corroding the surface of the steel. Once over-pickling occurs, there is basically no way to save it.
4 Rust
Rust refers to rust that reappears after pickling.
How does this rust occur?
There are several stages in pickling, pickling, cleaning, rinsing, and drying in the later stage. If there is a certain factor, such as a blockage, the acid will remain, which is equivalent to direct contact between the acid and the metal, further corroding, so rust will occur.
Another reason is that the acid cannot be completely and 100% rinsed clean, and there will be some residue. If it is not blown dry later, and it is packaged after pickling, if the temperature is relatively high, some heat inside will not be completely dissipated, and there will be acid residue, and the heat will be stuffy inside, causing rust, which is the so-called yellow spot. Or it is packaged before it is completely dried, and the moisture is sealed inside, which also causes rust.
There are also some external factors, such as seawater erosion and moisture entering, which are easy to cause new rust.
5 Oxygen bite/oxygen pressure
When the pickling process is just right, the surface is washed better, but there is iron oxide bite or pressure in some places. At this time, the pickling is not clean, which is called oxygen bite or oxygen pressure.
If the pressure is not deep or the iron oxide piece pressed in is not large, it may be cleaned later, but a small pit will remain. There was iron oxide inside, which was pickled. At this time, we call it oxygen residue.
Oxygen bite and oxygen pressure are the pressure of iron oxide scale, which cannot be washed off and remain in the steel.
GNEE is a professional supplier of cold-rolled and hot-rolled pickling products with 16 years of export experience, providing one-stop service for steel products. The main production units are pickling mill combined unit, continuous annealing unit and continuous hot-dip galvanizing unit. Our products are widely used in high-end home appliances, light industry, construction, automobile and other industries.
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