Pickling and passivation of seamless steel pipes is a chemical reaction. First, the pickling solution can remove the scale and rust on the surface of steel, and the passivation process is completed at the same time. It can effectively prevent the oxidation of seamless steel pipes and achieve the purpose of corrosion protection. Before pickling and passivation of seamless steel pipes, oil, degreasing, and impurities such as surface drawing compounds must be removed. The surface of the seamless steel pipe after pickling and passivation becomes uniform silver-white, greatly improving the corrosion resistance of stainless steel.
Phosphating of seamless steel pipes is a process in which seamless steel pipes are immersed in a phosphating solution (a solution mainly composed of certain acid phosphates) to deposit a layer of water-insoluble crystalline phosphate conversion film on the surface. Phosphating is a process in which a chemical and electrochemical reaction forms a phosphate chemical conversion film. The phosphate conversion film formed is called a phosphate film, which prevents metal from being corroded to a certain extent; it is used for primer before painting to improve the adhesion and corrosion resistance of the paint film layer; it is used for friction reduction and lubrication in metal cold processing.
“Phosphating and saponification” of seamless steel pipes refers to a process of saponification after phosphating. It is generally used in machining processes such as drawing or stretching. The phosphating film is generally 8-12 microns, and then enters the saponification liquid, and then draws or stretches. The purpose of saponification of seamless steel pipes is to further increase the lubrication performance.
The pickling, phosphating, and saponification of seamless steel pipes are simple process procedures: pickling → high-pressure washing → phosphating → high-pressure water washing → saponification → drying → transportation and stacking.
First, pickling
1. Pickling definition: Acids are used to remove iron oxide scale by chemical methods according to a certain concentration, temperature, and speed, which is called pickling.
2. Pickling classification: According to the type of acid: sulfuric acid pickling, hydrochloric acid pickling, nitric acid pickling, hydrofluoric acid pickling. Pickling must use different media according to the material of the steel, such as pickling carbon steel with sulfuric acid and hydrochloric acid and pickling stainless steel with a mixed acid of nitric acid and hydrofluoric acid. According to the shape of the steel: wire pickling, forging pickling, steel plate pickling, strip pickling, etc. According to the type of pickling equipment, it can be divided into tank pickling, semi-continuous pickling, full-continuous pickling, and tower pickling.
3. Principle of pickling: Pickling is a process of removing iron oxide from the metal surface by chemical methods, so it is also called chemical pickling. The iron oxide scale (Fe203, Fe304, Fe0) formed on the surface of the steel pipe is an alkaline oxide that is insoluble in water. When they are immersed in acid or sprayed with acid on their surface, these alkaline oxides can undergo a series of chemical changes with the acid.
Second, passivation
1. Passivation principle: The passivation mechanism can be explained by the thin film theory, that is, it is believed that passivation is due to the action of metal and oxidizing substances. When the action occurs, a very thin, dense, well-covered, and firmly adsorbed passivation film is generated on the metal surface. This film exists as an independent phase, usually a compound of oxidized metals. It plays the role of completely separating the metal from the corrosive medium, preventing the metal from contacting the corrosive medium so that the metal stops dissolving and forms a passive state to achieve the effect of corrosion protection.
2. Advantages of passivation:
1) Compared with the traditional physical sealing method, the passivation treatment has the characteristics of absolutely not increasing the thickness of the workpiece and changing the color, improving the precision and added value of the product, and making the operation more convenient;
2) Since the passivation process is carried out in a non-reactive state, the passivator can be added and used repeatedly, so it has a longer life and more economical cost.
3) Passivation promotes the formation of an oxygen molecule structure passivation film on the metal surface, the film layer is dense, the performance is stable, and it has a self-repairing effect in the air. Therefore, compared with the traditional method of applying anti-rust oil, the passivation film formed by passivation is more stable and more corrosion-resistant.
Third, phosphating
Phosphating treatment is a surface treatment process in which a layer of film (phosphating film) is formed on the surface by a chemical reaction. The phosphating treatment process is mainly used on the metal surface, and its purpose is also to provide a protective film for the metal surface to isolate the metal from the air and prevent it from being corroded; it is also used for primer before painting some products. With this phosphating film, the adhesion and corrosion resistance of the paint layer can be improved, the decorativeness can be improved, and the metal surface looks more beautiful, it can also play a lubricating role in the cold processing of some metals. After phosphating treatment, the workpiece will not oxidize and rust for a long time, so phosphating treatment is widely used and is also a commonly used metal surface treatment process. It is increasingly used in the automotive, shipbuilding, machinery manufacturing, and other industries.
Third, the process flow of pickling and passivating seamless steel pipes
The second cold drawing of seamless steel pipes is the raw material (rough pipe or finished pipe) → heading → pickling → phosphating (i.e. lubrication) → cold drawing. If there is a second cold drawing, whether annealing is required depends on the type of steel.
Generally, low-carbon steel does not need annealing. After the second cold drawing → lubrication → cold drawing → annealing → straightening → cutting → finished product inspection → packaging and storage. After that, pickling and passivation treatment is performed for high-quality requirements.
Fourth, pickling and passivating seamless steel pipes should pay attention to the following points:
(1) Seamless steel pipes cannot be pickled with a highly corrosive formula, otherwise local corrosion will occur. However, austenitic stainless steel can be pickled with the process of pickling stainless steel.
(2) Formulas containing chloride ions have a faster pickling speed and are only used for the initial pickling of thick oxide films. After the initial pickling, other pickling processes are used for the second pickling. Chloride ions can easily cause pitting corrosion of stainless steel. When preparing the pickling solution, try to use distilled water to prevent the mixing of chloride ions.
(3) When scrubbing, rolling, or pickling with pickling paste, the pickling surface should always be kept moist and not dry, otherwise the local acid concentration will be too high, which will produce a cloud-like pattern.
(4) In various pickling processes, it is necessary to pay attention to strictly control the pickling temperature, time, and other process conditions, otherwise the pickling will be insufficient, over-pickling, or even corrosion and scrapping of the workpiece.
Post time: Oct-11-2024