Exploring the surface treatment before the coating of steel drums (3)

(d) Derusting on the surface of steel drums

The surface of steel drums is most likely to be oxidized and corroded prior to painting. The scales and rust on the surface of the drums are mainly ferrous oxide, ferric oxide, hydrous ferric oxide, triiron tetroxide, and the like. Before the drum coating, if the scale and rust on the surface are not removed, the adhesion, decorative and service life of the paint film on the drum surface will be affected. Because the rust structure is loose, the surface of the steel drum is not firmly adhered and it easily falls off with the coating. The oxide skin will generate trihydrate iron hydroxide under the effect of water, and the volume will increase to cause blistering, cracking and shedding of the paint film. The oxide film under the paint film and the corrosion of the metal iron in the steel drum are the cathodes in the battery. It promotes the continuous corrosion of the metal on the surface of the steel drum and causes the coating to be destroyed quickly.

The method of removing rust on the surface of the steel drum is divided into two major categories: chemical method and mechanical method.

Chemical rust removal is the use of chemical reaction or electrochemical reaction to dissolve general rust, oxide scale, and various corrosion products from the surface of the steel drum without affecting the metal of the steel drum base. The most commonly used chemical rust removal method is pickling. That is, the use of organic acids or inorganic acids causes the oxide and yellow rust on the surface of the drum to precipitate from the surface of the drum.

The method of mechanical rust removal is the use of mechanical forces such as friction, hammering, and scouring to remove abrasive particles on the surface of the drum, such as rust, high temperature scales, and the like. The most commonly used mechanical rust removal methods for steel drums are hand sanding and mechanical sanding. Manual rust removal is the use of simple tools to remove rust with human labor, and its method is simple and easy. However, the labor intensity is large, the production efficiency is low, and the quality of the rust removal is poor. Generally, only loose or light rust can be removed, and the heavier rust layer and scale cannot be removed.

(E) Phosphating treatment of steel drum surface

The steel drum is in a phosphate solution containing zinc, iron and manganese and chemically reacts with the solution to generate a phosphate that is hardly soluble in water, so that a layer of a protective film is formed on the surface of the steel drum. This method is called phosphating of steel drums. This protective film is called a phosphating film.

Phosphate film has a microporous structure, is relatively stable under normal atmospheric conditions, has a certain degree of rust resistance, as the bottom layer of steel drum paint film, can significantly improve the adhesion and corrosion resistance of paint film coating.

1. Classification method of phosphating treatment on steel drum surface

According to the composition of the phosphating solution, the main cation can be divided into: zinc (including zinc phosphate, zinc calcium phosphate, zinc manganese phosphate), iron and manganese three categories. The commonly used phosphate film in steel barrels is zinc phosphate.

According to the processing temperature can be divided into: 1, high-temperature phosphating general conditions, the phosphating solution temperature between 90 ~ 98 °C; 2, the middle temperature phosphating general conditions, the phosphating solution temperature at 50 ~ 70 °C Between; 3, low temperature phosphating In general, the phosphating solution temperature between 20 ~ 30 °C. Phosphating the surface of steel drums, if divided according to the processing temperature, it is desirable to select medium-temperature phosphating, and the phosphating effect is best at this time. However, at present, many steel drum companies are adopting low-temperature phosphating in order to reduce production costs.

According to the treatment process can be divided into: impregnated phosphating; spray phosphating and brushing phosphating. However, spray-type phosphating is widely used in the barrel making industry. It is mainly used steel drum cleaning production line to clean steel drums, the general process used is: alkaline liquid degreasing → washing once → secondary washing → phosphating → washing → passivation → drying. This steel drum washing production line has high production efficiency and labor intensity of workers is low. The surface quality of the steel drum after cleaning can ensure the normal operation of the painting process.

2. Passivation treatment of phosphate film on steel drum surface

In order to enhance the corrosion resistance of the phosphating film on the surface of the drum and the adhesion to the coating, the steel drum can be treated for passivation after phosphating.

The passivation solution may be an aqueous solution containing 0.1% chromic acid, which is passivated and then rinsed with water to remove the chromic acid which is frequently used as a raw material. If the aqueous solution composed of hexavalent chromium and trivalent chromium is used as a passivation solution, the concentration thereof is (0.05-0.1)%, and the passivation treatment can be performed without water rinsing to reduce the pollution, and the passivation film is colorless or pale Gray, corrosion resistance is better than pure chromic acid passivation film. Therefore, the surface passivation of steel drums is mostly carried out using an aqueous solution composed of hexavalent and trivalent chromium for passivation. Phosphating film drying at temperatures above 120°C can improve its corrosion resistance.

3, the defect analysis of the surface of the drum after phosphating

At present, after many barrel-making enterprises have surface-treated the steel barrels, there are often obvious defects on the surfaces of the steel barrels. For example, white powdery deposits adhere to the surface of steel drums, the phosphating film is not uniform, and hair and spots are present. The reasons for these defects are as follows:

(1) A white powdery deposit adheres to the surface of the drum

Causes:

1. The phosphating solution in the steel drum is low in free acidity and free phosphoric acid.

2. When the drum is phosphating, the phosphating solution contains less iron ions.

3. The oxide on the surface of the drum is not removed.

4, excessive oxidizer in the phosphating solution, the total acidity is too high.

5. Too much sediment in the cleaning tank.

Solution:

1. Add zinc dihydrogen phosphate to the phosphating cleaning pool on the surface of the steel drum. In special cases, add free phosphoric acid to adjust the free acidity.

2. A certain amount of sediment should be left in the steel drum cleaning phosphating solution. The new solution is mixed with the old solution.

3. Strengthen the phosphating cleaning (pickling) on ​​the surface of the steel drum and fully wash it.

4. Stop adding oxidants to the phosphating bath of the steel drum and adjust the acid ratio.

5. Remove excessive sediment in the phosphating bath of the steel drum.

(2) Difficult phosphating film on the surface of steel drum, hair and spots

Causes:

1. The oil on the surface of the drum has not been cleaned.

2. The temperature of the phosphating solution during the phosphating of the steel drum is too low.

3, drum surface passivation.

4. The proportion of the free acidity and the total acidity of the phosphating solution on the surface of the steel drum is disordered.

Solution:

1. Strengthen the degreasing and cleaning of the steel drum surface.

2. Increase the bath temperature during drum cleaning.

3. Strengthen the phosphating cleaning (pickling) on ​​the surface of the steel drum and fully wash it.

4. Adjust the acidity in the phosphating bath when the drum surface is cleaned.

Fourth, environmental protection before surface treatment of steel drum coating

As we have discussed before, the surface treatment of steel drums must go through the following processes: degreasing → one-time water washing → secondary water washing → pickling → water washing → passivation → drying and so on. During the washing process of steel drums, many processes will occur. Harmful substances such as benzene, aldehydes, amine organics, and salts of heavy metals, all of which can cause serious environmental pollution and endanger people's health. This issue has not yet attracted the attention of most barrel makers. I used to Visited a lot of barrel manufacturers, in addition to a few few barrel companies set up a sewage treatment plant, most companies still directly discharged the surface treatment of steel drums. Therefore, it is imperative to comprehensively treat the environmental pollution caused by wastewater treatment on the surface of steel drums.

(I) Sources and types of surface treatment wastewater from steel drums

In order to process the wastewater from the surface treatment of steel drums, we must first analyze the hazardous substances contained in the test wastewater in order to determine which method to use for comprehensive treatment.

The wastewater discharged from the steel drum surface treatment process is naturally the source of waste water before the steel drum is coated. The types of waste water are degreasing, pickling, phosphating, and other waste water and circulating water for rinsing. Its main components are acid, alkali, and heavy metal salts; these wastewaters vary according to the type and concentration of the pre-treatment agent used, so their pollution to the environment is not exactly the same.

(II) Treatment of surface treatment wastewater from steel drums

Wastewater from the surface treatment of steel drums contains few harmful substances, and a large number of pollutants contain both inorganic and organic pollutants. This poses great difficulties for its treatment. Therefore, steel drums Wastewater from surface treatment should be treated comprehensively when it is treated.

Under normal circumstances, there are three major types of treatment methods for steel drum surface treatment wastewater. That is, physical, chemical, and biological methods.

1, physical processing methods include:

1. Precipitation Separation of precipitated material by gravity.

2. Filtration method The porous material was used to separate the suspension from the filtrate.

3, heat treatment concentrated, dried and burned.

4, aeration method of ventilation to reduce the oxidation of the material.

5. Adsorption method Adsorption of harmful substances, pigments, etc. with activated carbon or other adsorptive substances.

6. The float method uses bubbles and solid particles to adhere.

7. The membrane separation method uses ultrafiltration and reverse osmosis to reduce the concentration of pollutants.

2. Chemical treatment methods include:

1. Neutralization and neutralization treatment with acid and alkali.

2. Redox method uses sodium sulfite, air, chlorine and other oxidation reduction.

3, drug treatment method using flocculants to solid-liquid separation.

4. Ion exchange method Ion exchange resin is used for ion exchange to treat wastewater containing heavy metals.

3. Biological treatment methods include:

1. The long-term exposure and decomposition of the biological filter bed method.

2. Anaerobic biological treatment Anaerobic bacteria decompose organic matter.

3, aerobic biological treatment of aerobic bacteria decomposition of organic matter.

As an industrial enterprise, under normal circumstances, it will not invest too much effort here. Therefore, most of the steel drum companies use chemical methods to comprehensively treat their own industrial wastewater.