Exploring the surface treatment before steel drum coating (3)

Exploring the surface treatment before steel drum coating (3)

(4) Derusting the surface of the steel drum

The surface of the steel drum is most susceptible to oxidation and corrosion before coating. The scale and rust on the surface are mainly ferrous oxide, ferric oxide, hydrous iron 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 with the coating, and the scale will generate 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 promotes the corrosion of the metal on the surface of the steel drum and causes the coating to be destroyed quickly.

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 organic acid or inorganic acid is used to precipitate oxides and yellow rust on the surface of the steel drum from the surface of the steel drum.

The mechanical descaling method utilizes mechanical forces such as friction, hammering, and scouring to remove dirt such as rust and high-temperature scale 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 hand, and the method is simple and easy. However, 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, and the heavy rust layer and scale can not be removed.

(5) 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, is relatively stable under normal atmospheric conditions, and has a certain rust-preventing ability. As a bottom layer of the steel drum paint film, the adhesion and corrosion resistance of the paint film coating can be remarkably improved.

1. Classification method of phosphating treatment on steel drum surface

According to the composition of the phosphating solution, the cations can be mainly divided into three categories: zinc (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 treatment temperature can be divided into: 1, high temperature phosphating in general, the temperature of the phosphating solution is between 90 ~ 98 ° C; 2, medium temperature phosphating in general, the temperature of the phosphating solution is 50 ~ 70 ° C 3, low temperature phosphating in general, the temperature of the phosphating solution is between 20 ~ 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 are adopting 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.

2. Passivation treatment of phosphate film on the surface of steel drum

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 steel drum can be treated by passivation after phosphating.

The passivation solution may be an aqueous solution containing 0.1% chromic acid, passivated and rinsed with water to remove the more 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 to 0.1)%. After passivation treatment, the water can be rinsed without water, and the pollution can be reduced. The passivation film is 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 trivalent chromium. The phosphating film is dried at a temperature above 120 ° C to improve its corrosion resistance.

3. Analysis of defects after phosphating on the surface of steel drum

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 flowering and speckle are present, and the like. The reasons for these defects are analyzed as follows:

(1), the surface of the steel drum is stuck with white powdery precipitate

Causes:

1. When the steel drum is phosphating, the free acidity in the phosphating solution is low, and the amount of free phosphoric acid is small.

2. The phosphating solution contains less iron ions during phosphating of the steel drum.

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

4. The oxidizing agent has an excessive amount of oxidizing agent and the total acidity is too high.

5. Excessive sediment in the cleaning tank.

Solution:

1. 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.

2. A certain amount of sediment should be left in the steel tank 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 wash it thoroughly.

4. Stop adding oxidant to the phosphating cleaning tank of the steel drum to adjust the ratio of acid.

5. Remove excessive deposits from the phosphating tank of the steel drum.

(2), the phosphating film on the surface of the steel drum is uneven, hair and spotted

Causes:

1. The oil on the surface of the steel drum is not cleaned.

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

3. The surface of the steel drum is passivated.

4. The ratio of free acidity and total acidity of the phosphating solution on the surface of the steel drum is out of balance.

Solution:

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

2. Increase the temperature of the bath when the steel drum is cleaned.

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

4. Adjust the acidity in the phosphating bath when cleaning the surface of the steel drum.

4. Environmental protection during surface treatment of steel drums before painting

As we have already said before, the surface treatment of steel drums should go through the processes of degreasing → one-time washing, two-time washing, pickling, washing, passivation, drying, etc., and in the process of cleaning and manufacturing steel drums, many Harmful substances such as benzene, aldehydes, amines and heavy metals, which cause serious pollution to the environment and endanger people's health, and this problem has not yet attracted the attention of most barrel companies. I used to Many barrel manufacturers have been visited. In addition to the few sewage companies that have established sewage treatment facilities, most enterprises directly discharge the sewage after the surface treatment of steel drums. Therefore, it is imperative to comprehensively control the environmental pollution caused by the surface treatment of steel drums.

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

In order to treat the wastewater from the surface treatment of steel drums, we must first analyze the harmful substances contained in the wastewater to determine the method used for comprehensive treatment.

The waste water discharged from the surface treatment process of the steel drum is of course a pre-treatment process of the steel drum, and the type of the waste water is waste water such as degreasing, pickling, phosphating, and circulating water for rinsing. Its main components are acid, lye and heavy metal salts; these sewages vary with the type and concentration of the pretreatment agent used, so the degree of environmental pollution is not the same.

(2) Treatment method for surface treatment wastewater of steel drum

The waste water brought by the surface treatment of steel drums is rarely contaminated with separate harmful substances, and a large number of pollution sources containing both inorganic substances and organic substances, which brings great difficulties to its treatment, so steel drums The sewage brought by surface treatment should be treated comprehensively.

Under normal circumstances, the treatment methods for surface treatment of steel drums fall into three categories. That is, physical law, chemical law, and biological law.

1. Physical processing methods include:

1. The precipitation method separates the precipitated material according to gravity.

2. Filtration The suspension is separated from the filtrate by a porous material.

3. Heat treatment is concentrated, dried and burned.

4. Aeration gas aeration to oxidize the reducing substance.

5. Adsorption method uses activated carbon or other adsorbing substances to adsorb harmful substances and pigments.

6. The floating method utilizes the adhesion of bubbles and solid particles.

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

2. Chemical treatment methods include:

1. Neutralization method is neutralized with acid and alkali.

2. Redox method uses sodium sulfite, air, chlorine, etc. to redox.

3. The drug treatment method uses a flocculant to separate the solid and liquid.

4. Ion exchange method uses ion exchange resin for ion exchange to treat heavy metal-containing wastewater.

3. Biological treatment methods include:

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

2. Anaerobic biological treatment Anaerobic bacteria decompose organic matter.

3, aerobic biological treatment of aerobic bacteria to decompose organic matter.

As an industrial enterprise, under normal circumstances, it will not put too much energy into it. Therefore, most steel drum companies use chemical methods for comprehensive treatment when dealing with their own industrial wastewater.