Phosphating and descaling of steel drum surface
Text / Xin Qiaojuan
1. Phosphating treatment on the surface of steel drum
The steel drum chemically reacts with the solution in a phosphate solution containing zinc, iron and manganese to form a phosphate which is hardly soluble in water, so that a well-protected protective film is formed on the surface of the steel drum. This method is called phosphating of a steel drum, and this protective film is called a phosphating film.
The phosphating film has a microporous structure and is relatively stable under normal atmospheric conditions and has a certain rust preventing ability. As the bottom layer of the steel drum paint film, it can significantly improve the adhesion knife and corrosion resistance of the paint film coating.
The classification of phosphating treatment on the surface of steel drums can be divided into three categories: zinc system (including zinc phosphate, zinc zinc phosphate, zinc manganese phosphate), iron and manganese. . Phosphating membranes commonly used in steel drum enterprises are mainly zinc phosphate.
According to the processing temperature can be divided into: 1 high temperature phosphating. In general, the temperature of the phosphating solution is between 90 and 98 ° C; 2 is tempered at medium temperature. In general, the temperature of the phosphating solution is between 50 and 70 ° C; 3 is low temperature phosphating. In general, the temperature of the phosphating solution is between 20 and 30 °C. If the phosphating of the surface of the steel drum is divided according to the treatment temperature, it is preferable to select the medium temperature phosphating, and the phosphating effect is optimal at this time. However, at present, many steel drum companies have adopted low-temperature phosphating treatment in order to reduce production costs.
According to the treatment process, it can be divided into: impregnated phosphating, spray phosphating and brushing phosphating. However, the widely used process in the barrel industry is spray phosphating. It mainly uses steel drum cleaning production line to clean steel drums. The general process is: lye degreasing→one water washing→second water washing→phosphating treatment→water washing→passivation→drying. This steel drum washing production line has high production efficiency and low labor intensity for workers. The surface quality of the steel drum after cleaning can ensure the normal operation of the coating process.
In order to enhance the corrosion resistance of the phosphating film on the surface of the steel drum and the adhesion to the coating, the phosphating film on the surface of the steel drum can be passivated after phosphating. The passivation solution may be an aqueous solution containing 0.1% chromic acid, passivated and rinsed with water to remove excess chromic acid. For example, an aqueous solution composed of hexavalent chromium and trivalent chromium is used as a passivation solution, and the concentration thereof is 0.05-0.1%. After the passivation treatment, the water may be rinsed without water, and the purification film may be colorless or Light gray, corrosion resistance is better than pure chromic acid passivation film. Therefore, the surface passivation of the steel drum is mostly passivated by an aqueous solution composed of hexavalent and divalent chromium. The phosphating film is dried at a temperature above 120 ° C to improve its corrosion resistance.
At present, there are many obvious defects in the surface of steel drums after many barrel manufacturers have surface treated steel drums. For example, the surface of the steel drum is stuck with a white powdery precipitate, the phosphating film is uneven, the hair is blooming and the spots are present. One of the reasons for these defects is that the surface of the steel drum is stuck with a white powdery precipitate. The reason is that the free acidity of the phosphating solution is low and the amount of free phosphoric acid is small when the steel drum is phosphating; the iron ions contained in the phosphating solution are less when the steel drum is phosphating; the oxide on the surface of the steel drum is not removed; phosphating Excessive oxidant in the liquid, the total acidity is too high; too much sediment in the cleaning tank. The solution is to add zinc dihydrogen phosphate to the phosphating cleaning tank on the surface of the steel drum. In special cases, phosphoric acid can be added to adjust the free acidity; in the steel drum cleaning phosphating solution, a certain amount of sediment should be left, and the new solution is mixed with the old solution. Use; strengthen the phosphating cleaning (pickling) on ​​the surface of the steel drum, fully wash the water; add oxidant to the phosphating cleaning tank of the steel drum to adjust the ratio of acid; remove excessive sediment in the phosphating cleaning tank of the steel drum. Second, the phosphating film on the surface of the steel drum is uneven, hairy and speckled. The reason is that the oil on the surface of the steel drum is not cleaned; the temperature of the phosphating solution is too low in the phosphating cleaning of the steel drum; the surface of the steel drum is passivated; the ratio of the free acidity and the total acidity of the phosphating solution on the surface of the steel drum is out of balance. The solution is to strengthen the degreasing cleaning of the surface of the steel drum; to improve the temperature of the bath when the steel drum is cleaned; to strengthen the phosphating cleaning (pickling) on ​​the surface of the steel drum, and to fully wash the acidity in the phosphating bath when the surface of the steel drum is cleaned.
Second, the rust removal treatment of the steel drum surface
The surface of the steel drum is most easily oxidized and corroded before coating. The scale and rust on the surface are mainly ferrous oxide, ferric oxide, hydrous ferric oxide and triiron tetroxide. If the scale and rust on the surface are not removed before the steel drum is painted, it will affect the adhesion, decoration and service life of the paint film on the surface of the steel drum. Because the rust structure is loose, the surface of the steel drum is not firmly attached, and it is easy to fall off along with the coating. The scale will form triiron hydride under the action of water, and the volume will increase, causing the film to foam, crack and fall off. In the corrosive battery composed of the oxide film under the paint film and the metal iron of the steel drum, it is a cathode, which causes the metal on the surface of the steel drum to continue to corrode, so that the coating is quickly destroyed.
The rust removal method on the surface of the steel drum is divided into two major categories: chemical method and mechanical method.
Chemical derusting is the use of chemical reactions or electrochemical reactions to dissolve the general rust, oxide scale and various corrosion products from the surface of the steel drum without affecting the steel base metal. The most common chemical descaling method is pickling, that is, the use of organic or inorganic acids to precipitate oxides and yellow rust on the surface of the steel drum from the surface of the steel drum.
Mechanical derusting is the use of mechanical forces such as friction, hammering, scouring, etc., so that the abrasive particles remove rust, high temperature oxide scale and other dirt on the surface of the steel drum. The most common mechanical rust removal methods for steel drum companies are manual sanding and mechanical sanding. Manual descaling is the use of simple tools to remove rust by manpower. The method is simple and easy, but the labor intensity is high, the production efficiency is low, and the rust removal quality is poor. Generally, only loose or light rust can be removed. Mechanical descaling is generally performed by shot blasting or sand blasting, which has good treatment effect, low labor intensity and high production efficiency.
(China Packaging News)
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