Process Research on Improving the Surface Coating Quality of 200L Steel Barrel

Process Research on Improving the Surface Coating Quality of 200L Steel Barrel

Zheng Yan (Jinzhou Petrochemical Packing Products Co., Ltd.)

Abstract: This paper analyzes the problems and causes of the surface coating quality of 200L steel drum, and proposes an effective solution. By improving the pre-spray treatment process, that is, increasing the phosphating process, a layer of phosphate film is formed on the surface of the steel drum. Significantly improve the adhesion and corrosion resistance of the paint coating and reduce the peeling of the paint film; change the artificial spray to mechanical automatic spraying to prevent the occurrence of sagging, ensure the thickness and uniformity of the paint film on the surface of the steel drum, and open up A new process for painting barrels.

Key words: steel drum; surface coating; phosphating

Since the establishment of the factory in 1982, our company has been producing a single product 200L galvanized drum. As the steel drum industry continues to develop, lowering costs and ensuring environmental protection have prompted the market to increase the demand for paint buckets. In order to adapt to this change and ensure the survival and development of the company in the future, our company has established a semi-automatic painting production line in 2011, which has increased the variety of paint buckets. After the mass production of the products, some sales markets were opened. However, the paint film peeling off, sag, uneven thickness of the paint film and other problems plagued the product quality, directly affecting the expansion of the market sales share. To this end, our company has set up a special technical research team to carefully analyze and study the causes of the above problems. At the same time, we draw on the coating technology of the automotive industry and the spraying experience of the same industry, and adopt the process improvement measures in a targeted manner: Increasing the phosphating process, forming a layer of phosphate film on the surface of the steel drum, significantly improving the adhesion and corrosion resistance of the paint coating, reducing the peeling of the paint film, and improving the qualified rate of painting from 87.1% to 99%; 2 The artificial spraying is changed to mechanical automatic spraying to prevent the occurrence of sagging phenomenon and ensure the thickness and uniformity of the paint film on the surface of the steel drum. After adopting automatic painting, the qualified rate of the film thickness reaches 100%, thus opening up a new process of painting barrels.

1 Problem analysis and solutions

1.1 Steel drum semi-automatic painting process

To be sprayed steel drum → alkaline washing → water washing → drying → painting → drying → lower parts → storage.

1.2 There are quality problems

Statistics on the quality of spray paint barrels in January 2013, the results are shown in Table 1:

Table 1 Statistics on the production of paint buckets in January 2013

It can be seen from the January 1 production statistics table that the primary pass rate is very low, only 87.1%.

Among the factors affecting the pass rate of painting, the factors such as peeling off of paint film, uneven thickness of paint film, and sag account for a large proportion. See Table 2:

Table 2 Proportion of non-conformities

It can be seen from Table 2 that the paint film shedding, sagging, and film thickness are not the main influencing factors of poor spray quality, so it is confirmed that the above three items are the main influencing factors of poor spray quality. In order to improve the quality of spraying, we must start from these three aspects.

1.3 Causes and solutions to problems

(1) The paint film is peeled off. It can be seen from the above process flow chart that only the alkali washing and water washing process in the pretreatment process, although the outer surface of the steel drum after the treatment is clean, the adhesion of the paint film on the surface of the steel drum is not strong, and the paint film is easy to fall off after drying. Poor defects such as peeling of the paint film make the appearance of the paint film poor.

Adjust the pre-spray treatment process, add the phosphating process after alkali washing and water washing to improve the adhesion of the paint film.

(2) Sag and uneven film thickness. The semi-automatic manual painting is uneven in stroke, and the distance from the workpiece and the injection angle are unstable.

Replace the semi-automatic manual painting with automatic mechanical painting to determine the optimum painting process parameters.

2 Research on improving the adhesion of paint film

In order to increase the adhesion of the paint film and reduce the peeling of the paint film, we changed the pre-treatment process, and the phosphating process was added in the new process. The new process is as follows:

To be sprayed steel drum → alkali washing → water washing → phosphating → water washing → drying → painting → drying → the next piece into the warehouse.

2.1 Principle of phosphating

The process of immersing the workpiece in a phosphating solution (some acid phosphate-based solution) and depositing a layer of water-insoluble crystalline phosphate conversion film on the surface is called phosphating.
The steel parts are immersed in a phosphating solution (a dilute aqueous solution consisting of Fe(H2PO4)2, Mn(H2PO4)2, Zn(H2PO4)2, pH 1-3, and the relative density of the solution is (1.05~1.10), phosphorus The formation reaction of the membrane is as follows:

3Zn(H2PO4)2=Zn3(PO4)2↓+4H3PO4

Or 3Mn(H2PO4)2=Mn3(PO4)2↓+4H3PO4

2.2 Phosphating before painting

(1) Enhance the bonding force between the coating film layer and the workpiece. That is to increase the adhesion of the paint film to the steel workpiece.

(2) Improve the corrosion resistance of the surface coating of the workpiece after painting. That is to increase the protective film and steel workpieces.

(3) Improve the decorativeness.

2.3 Selection of phosphating solution

(1) Phosphating film composition. The phosphating film is a crystal which is glittering and light, uniform and fine, gray and porous, and has strong adhesion. Most of the crystal is zinc phosphate, and a small part is iron hydrogen phosphate. Phosphating film types can be zinc or zinc calcium and iron. The mass per unit area of ​​the phosphating film is 0.2-1.0 g/m2 (for the bottom layer of the larger deformation steel piece); 1~5g/m2 (for the general steel piece paint bottom layer); 5~10g/m2 (for the non-occurrence) Deformed steel pieces paint the bottom layer). The proportion of zinc to iron depends on the composition of the solution, the phosphating time and the temperature.

(2) Nature:

Corrosion resistance. It has good corrosion resistance in the atmosphere, mineral oil, vegetable oil, benzene and toluene, but it has poor corrosion resistance in alkali, acid and water vapor. It has a certain corrosion resistance at 200 to 300 ° C. When the temperature reaches 450 ° C, the corrosion resistance of the film layer is significantly reduced.

Special nature. Such as increased adhesion, lubricity, anti-friction and wear resistance.

The choice of phosphating solution. In the coating industry, iron has its unique advantages, the pretreatment process is simple, cleaning and phosphating can be completed in the same process, no need to adjust the surface before phosphating, which can simplify the process; environmentally friendly, non-toxic, phosphating solution It does not contain sodium and chromium ions; it has good stability, less residue and less residue formation, easy to clean equipment; easy to control bath, single component addition, no need to add accelerator, etc., do not always check and adjust only need to consider pH The value and total acidity; the phosphating film is thin, has excellent elongation, and has good adhesion between the film layer and the organic coating; the composition of the iron phosphating solution is simple and the operating concentration is low.

The main components of the iron-based phosphating film are FePO4 and Fe2O3, and the film formation mechanism should be as follows:

4Fe+4NaH2PO4+3O2=2FePO2+Fe2O3+2Na2HPO4+3H2O

Iron phosphating mainly relies on the reaction of matrix metal with phosphating solution to form a film, which is a reaction in which metal atoms on the surface of the base metal are converted into an ionic state. Iron phosphating is also called conversion phosphating, and the phosphating film structure is amorphous. of.

Drawing on the experience of the same industry, the iron phosphating solution and the spray phosphating process were selected.

2.4 Factors affecting phosphating film formation

(1) Temperature. The higher the temperature, the thicker the phosphating layer and the coarser the crystallization. The lower the temperature, the thinner the phosphating layer and the finer the crystallization. However, the temperature should not be too high, otherwise Fe2+ is easily oxidized to Fe3+, the amount of precipitate is increased, and the solution is unstable.

(2) Free acidity. Free acidity refers to free phosphoric acid. Its function is to promote the dissolution of iron to form more crystal nuclei and make the film crystal dense. If the free acidity is too high, the action with iron will be accelerated, and a large amount of hydrogen will be precipitated, so that the interface layer phosphate is not easily saturated, resulting in difficulty in crystal nucleation formation, loose pore structure, porosity, and reduced corrosion resistance, which prolongs the phosphating time. The free acidity is too low, and the phosphate film becomes thinner and even has no film.

(3) Total acidity. Total acidity refers to the sum of phosphate, nitrate and acid. The total acidity is generally controlled at the upper limit of the specified range, which is favorable for accelerating the phosphating reaction and making the film grain fine. During the phosphating process, the total acidity is continuously decreased and the reaction is slow. The total acidity is too high, the film layer becomes thinner and can be diluted with water. The total acidity is too low and the film layer is loose and rough.

(4) PH value. The manganese phosphating solution is generally controlled between 2 and 3. When the pH is >3, the surface of the common part is easy to form a powder. It is difficult to form a film when pH < 1.5. The iron system is generally controlled between 3 and 5.5.

(5) The ion concentration in the solution. 1 Fe2+ is easily oxidized to Fe3+ in the solution, resulting in difficulty in film formation. However, the concentration of Fe2+ in the solution should not be too high. Otherwise, the formed film grains are coarse, and the surface of the film has white floating ash, and the corrosion resistance and heat resistance are lowered. 2Zn2+ effect, when the concentration of Zn2+ is too high, the phosphating film grains are coarse, the brittleness is increased, and the surface is white floating ash; when the Zn2+ concentration is too low, the film layer becomes loose and dark.

2.5 Selection of preferred process conditions for phosphating process

Spray phosphating:

Features: Suitable for medium and low temperature phosphating processes, can handle large area workpieces. The treatment time is short, the film formation reaction speed is fast, the production efficiency is high, and the phosphating film obtained by the method has the advantages of compact, uniform crystal, thin film and good corrosion resistance.

Selection of spray phosphating process parameters: The process conditions such as pH value, total acidity, phosphating time and phosphating temperature were investigated.

(1) Determination of PH value

The experimental conditions were set: the total acidity of phosphating was 30 points, the phosphating time was 3 min, and the phosphating temperature was 30 °C. The effect of pH on the adhesion of the paint film was examined. The data is shown in Table 3.

Table 3 Effect of PH value on adhesion of paint film

It can be seen from Table 3 that the adhesion of the paint film is good when the pH is controlled at 2.8 to 3.0.

(2) Determination of total acidity value

The experimental conditions were set: pH was 3.0, phosphating time was 3 minutes, and phosphating temperature was 30 ° C. The effect of total acidity on the adhesion of the paint film was examined. The data is shown in Table 4.

Table 4 Effect of total acidity on adhesion of paint film

It can be seen from Table 4 that when the total acidity is controlled at 28 to 30 points, the adhesion of the paint film is good.

(3) Determination of phosphating time

The experimental conditions were set: pH value was 4.5, total acidity was 30 points, and phosphating temperature was 30 °C. The effect of phosphating time on the adhesion of the paint film was examined. The data is shown in Table 5.

Table 5 Effect of phosphating time on adhesion of paint film

It can be seen from Table 5 that the phosphating time is controlled at 3 to 3.5 minutes, and the adhesion of the paint film is good.

(4) Determination of phosphating temperature

The experimental conditions were set: pH value was 4.5, total acidity was 30 points, and phosphating time was 3 min. The effect of phosphating temperature on the adhesion of the paint film was examined. The data is shown in Table 6.

Table 6 Effect of Phosphating Temperature on Adhesion of Paint Film

It can be seen from Table 6 that the phosphating temperature is controlled at 25~30 °C and the adhesion of the paint film is better.

After the above tests, the preferred process parameters are determined as follows:

PH value: 2.8 ~ 3.0

Total acidity: 28~30 points

Phosphating time: 3 ~ 3.5min

Phosphating temperature: 25 ~ 30 ° C

3 Improve spraying technology

3.1 Process requirements for surface coating of steel drums

The spraying process is a simple physical process that refers to the process of uniformly coating the coating on the surface of the steel drum to the required thickness. Although the spraying process is simple, it must meet the two requirements that it requires: a certain thickness and uniform coating, and no defects such as leakage coating, sagging and coarse grain defects. The thickness of the coating is a very important process parameter during the spraying process. The cured coating formed by any coating has a certain gas permeability, which is related to the structural pores of the coating. The pores of the structure of the coating have an average diameter of 10-6 to 10-7 mm, which is larger than the molecular diameter of the gas and the liquid, so that not only the gas can pass, but also the general ion and the low molecular liquid can pass. In order to maximize the water resistance and gas permeability of the coating, the coating must be of a certain thickness to overlap the structural pores with each other. The minimum thickness at which the coating is normally protected is referred to as the critical thickness. For example, the critical thickness of the steel drum coating process is 0.02 mm, that is, the thickness of the paint film formed by spraying the steel drum is not less than this critical thickness. Only such a film thickness can protect the steel drum from external corrosion. In addition, the uniformity of spraying is also very important. If it is not uniform, the excessively thin places will reduce the corrosion resistance, and if it is too thick, the defects such as sagging and wrinkling will easily occur.

Semi-automatic painting has the following weaknesses:

Due to improper spraying pressure adjustment, improper spraying distance, improper spraying angle, improper spraying range, etc., the corresponding quality problems are caused:

The spraying distance is too close, the spraying angle is not appropriate, and the spray width is too large to overlap too much to cause sagging;

When the air pressure is too high during spraying, the solvent balance is destroyed, and pinholes are easily generated;

Insufficient spray pressure, poor atomization; spray distance is too far, spray gun running speed is too fast, spray thickness is not enough, easy to produce orange peel.

Fully automatic spraying can avoid the above problems.

3.2 Automatic spraying process parameter selection

Under the existing conditions of workpiece rotation speed of 60r/min, the injection pressure, station spray time, drying temperature and other parameters were investigated, and the preferred process parameters were selected, as shown in Table 7.

Table 7 Effect of injection pressure, spraying time and drying temperature parameters on painting quality

Note: The above data applies to amino paints.

3.3 Nitrocellulose and alkyd paints have the same method and different parameters.

The process parameters of automatic spraying can be seen from Table 7 as follows:

Workpiece rotation speed: 60 r/min

Injection pressure: 0.45Mpa

Station spraying time: 10s

Drying temperature: 150 ° C

4 Quality tracking statistics after adopting new technology

All measures for the steel drum spraying process were completed before February 28, 2014. And after the implementation, the pass rate of the paint bucket is significantly improved. The qualification rate from March to May is shown in Table 8:

Table 8 Product improvement rate statistics after process improvement

As can be seen from Table 8, the average pass rate from March to May after the implementation was 99% on average.

Randomly sampling the currently produced spray paint buckets, the measured values ​​of the paint thickness are in line with the quality requirements.

5 Conclusion

After adding the phosphating treatment before the steel drum is sprayed, the adhesion of the paint film is increased, and the pass rate is 99%; the pass rate of the paint film after automatic painting is 100%.

Before the improvement, our company's paint bucket did not have large-volume production orders. After the improvement of the process, the quality of the spray was greatly improved, and it was recognized by the users. It has signed long-term supply contracts with many users inside and outside the province. Increasing year by year.

references

[1] Technical conditions for phosphating treatment of steel workpieces before painting. GB6801-86. National Bureau of Standards. 1986, Beijing.

[2] HP-2 normal temperature phosphating solution research report. Institute of Corrosion and Protection, Hainan University. 1992.

[3] Li Wensheng, Ding Yuzhong. Application of SAMES automatic spraying system in automobile spraying. Electroplating and finishing. 2000, 26:12~14.

[4] China Petroleum Jinzhou Petrochemical Company Enterprise Standard QJ/JSH BZ 03.01- 2006 "Barrel Device Operation Stacking".

[5] "Steel drum standardization" GB/T 325- 2008.

[6] "Paint Painting Technology 1000 Questions" Zhejiang Science and Technology Press.

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