Coating process of phosphating process

In order to establish a pretreatment production line for painting, the process design must be completed before the design, manufacture and installation of non-standard equipment can be carried out. Therefore, process design is the basis of establishing production line, and the correct and reasonable route will have a good impact on production operation and product quality. The content of process design mainly includes: treatment method, treatment time, process flow, etc.

1 processing mode

Workpiece treatment method refers to the way that the workpiece contacts the bath liquid to achieve the purpose of chemical pretreatment, including total immersion, total spraying, combination of spraying and immersion, brushing and so on. Mainly depends on the geometric size and shape of the workpiece, site area, investment scale, output and other factors. For example, workpieces with complex geometric dimensions are not suitable for spraying; Oil tanks and drums are not easy to sink in liquid and are not suitable for soaking.

1. 1 full immersion mode

The workpiece is completely immersed in the bath solution, treated for a period of time and then taken out, so as to achieve the purpose of oil removal or rust removal and phosphating. Different geometric shapes of the workpiece can achieve the treatment goal where the liquid can reach. This is the unique advantage of dip coating, which is incomparable to spraying and brushing. Its disadvantage is that there is no auxiliary use of mechanical scouring, so the processing speed is slow and the processing time is long. Especially when the workpiece is continuously suspended and transported, in addition to the running time of the workpiece in the groove, there are also uphill and downhill times of the workpiece, thus increasing the equipment, site area and investment. As far as phosphating is concerned, foreign countries tend to adopt full immersion method. It is said that full immersion phosphating is easy to form granular crystalline phosphating film with high iron content, which has a good match with cathodic electrophoresis.

1.2 full injection mode

Pressurize the liquid with a pump and spray it on the workpiece at a pressure of 0. 1 ~ 0.2 MPa to achieve the treatment effect. Because of mechanical washing and liquid renewal during spraying, the processing speed is accelerated and the time is shortened. The length of the production line is shortened, and the corresponding site, equipment and shortcomings are that the workpieces with complex geometries such as cavities and corners are difficult to reach, and the processing effect is not good, so it is only suitable for processing workpieces with simple geometries. Spray method is not suitable for pickling and rust removal, which will bring a series of problems such as equipment corrosion and rust between processes, so we must be very careful when choosing spray pickling. It is reported that full spray phosphating is easy to form a phosphating film with coarse dendrites and low iron content, and it is not recommended to be used as pretreatment before painting cathode electrophoretic paint primer abroad. Full spraying mode is mainly used for powder coating, electrostatic coating, anodic electrophoresis, etc. Household appliances and parts.

1.3 spray immersion combination

Spray-immersion combination, generally speaking, in a certain process, the workpiece is sprayed first, then immersed in the tank, and then sprayed after being discharged from the tank. All spraying and soaking are the same bath solution. This combination mode not only maintains the high efficiency of spraying and improves the processing speed, but also has a soaking process, so that all parts of the workpiece can be effectively processed. Therefore, the combined pretreatment of spraying and soaking can complete the treatment process in a short time, and the equipment occupies relatively small space, and at the same time, satisfactory treatment effect can be obtained. At home and abroad, for the automobile industry with high pretreatment requirements, the combination of spray and immersion is generally adopted.

1.4 Brush coating method

Brush the treatment liquid directly on the surface of the workpiece by hand to achieve the purpose of chemical treatment. This method is generally not easy to obtain good treatment effect and is rarely used in factories. This method can be considered for some large and simple workpieces.

2 treatment temperature

In order to save energy, improve working environment, reduce production cost, chemical reaction speed, treatment time and production speed, low or medium temperature pretreatment technology is widely used in production and application.

In addition to liquid oil stains, there is a small amount of solid grease on the workpiece, which is difficult to remove at low temperature, so the degreasing temperature should be in the middle temperature range regardless of soaking or spraying. If there is only liquid grease, low-temperature degreasing can fully meet the requirements.

For general corroded and scaled workpieces, medium temperature pickling should be selected to ensure that the corroded and scaled workpieces are completely removed within 10 minutes. Unless there are sufficient reasons, generally do not choose low-temperature or unheated pickling for rust removal. Low-temperature pickling is limited to, for example, small corrosion and no scale on the workpiece; Derusting time is not limited; Allow hydrochloric acid pickling.

The surface conditioning process usually does not require heating and is usually treated at room temperature.

There is no obvious difference in phosphating speed between low temperature and medium temperature, and phosphating film can be formed quickly in a short time. If the phosphatized workpiece needs a long storage time between processes, it should be phosphatized at medium temperature to achieve better antirust effect.

The whole pretreatment process can be washed at room temperature without heating. If the final washing is hot water blanching, the water temperature should be above 80℃.

3 processing time

Once the treatment method and temperature are selected, the treatment time should be determined according to the degree of oil pollution and corrosion of the workpiece. Generally, please refer to the treatment time requirements in the instruction manual of pretreatment agent.

4 process flow

According to the degree of oil pollution and corrosion of the workpiece and the requirements of primer, it is divided into different technological processes.

4. 1 The workpiece is completely rustless.

Pre-degreasing-degreasing-water washing-surface adjustment-phosphating-water washing-drying (electrophoresis primer can be put into electrophoresis tank without drying). This is a standard four-station process with a wide range of applications. Suitable for pretreatment of various cold-rolled plates and machined rust-proof workpieces. A surface conditioner can also be added to the degreasing tank, thus eliminating the surface conditioning process.

4.2 Common mixed workpieces with oil stain, corrosion and scale.

Degreasing and rust removal "two in one"-water washing-neutralization-surface adjustment-phosphating-water washing-drying (or direct electrophoresis). This process is the most widely used process in China, which is suitable for pretreatment of all kinds of workpieces (except heavy oil pollution); If moderate temperature phosphating is adopted, the surface adjustment and neutralization procedures can also be omitted for simple plate-shaped workpieces, which becomes the standard four-station process for rusty parts.

4.3 Workpieces with heavy oil stain, corrosion and scale

Pre-degreasing-water washing-degreasing and rust removal "two in one"-water washing-neutralization-surface conditioning-phosphating-water washing-drying (or directly entering the electrophoresis tank). For the workpiece with heavy oil pollution, pre-degreasing should be carried out first to remove most of the grease, so as to ensure that a completely clean metal surface can be obtained after the next "two-in-one" degreasing and rust removal treatment.

5 points for attention

We should pay great attention to some small places in process design, even if some of them are related to equipment design. If not carefully considered, it will have many adverse effects on the operation of production line and workers' operation, such as process interval, overflow flushing, phosphating and slag removal, process hole of workpiece, tank body, heating pipe material and so on.

5. 1 process interval time

If the interval between processes is too long, it will lead to the second rust of the workpiece during operation. Especially when there is pickling process, the workpiece is easy to oxidize, rust and turn green in the air after pickling. It is best to provide water film protection between processes to reduce rust. Rust, yellowing and greenness of the workpiece seriously affect the phosphating effect, resulting in the yellowing of the workpiece and the failure to form a complete phosphating film. The interval between processes should be shortened as much as possible. If the interval between processes is too short, the water in the water storage area of the workpiece can not be completely and effectively discharged, resulting in channeling phenomenon, especially in the spraying mode, which will cause mutual splashing and channeling, making it difficult to control the composition of the bath liquid and even destroying the bath liquid. Therefore, when considering the process interval, the appropriate process interval time should be selected according to the geometric size and shape of the workpiece.

5.2 overflow water cleaning

Advocate overflow flushing to ensure that the workpiece is fully cleaned and reduce the phenomenon of cross-slot. When overflowing, water should be fed from the bottom, and overflow holes should be opened at the upper diagonal to overflow.

5.3 Machining hole of workpiece

For some tubular parts or workpieces that are easy to form dead corners and contain water, it is necessary to select appropriate positions to drill process holes to ensure that water can fully flow out in a short time. Otherwise, it will cause a series of grooves or drainage in the air for a long time, which will cause secondary rust and affect the phosphating effect.

5.4 Phosphating and slag removal

For any phosphating solution, there will be more or less deposits (there are few colored phosphating deposits in the light rail system), and a phosphating slag removal device should be indicated in the process design, especially when spraying phosphating, and the slag removal device is essential. Typical slag removal devices include: inclined plate precipitator, high-position precipitation tower, centrifugal slag removal device, paper bag filter slag, etc.

5.5 Materials of tank and heating pipe

Although the material selection of tank heating pipe is not the content of process design, if there is no reminder during process design, it may cause negligence of equipment designers and affect the operation of the whole production line. Pickling with sulfuric acid and hydrochloric acid, tank materials can only be made of FRP, granite and plastic, and heating pipes can only be made of lead-antimony alloy pipes and ceramic pipes, but not stainless steel materials. If phosphoric acid is used for pickling, the tank and heating pipe can be made of stainless steel, and of course, glass fiber reinforced plastic, plastic and granite can also be used.

6 Several typical pretreatment processes

6. 1 automobile body class

This kind of workpiece is formed by stamping and welding cold-rolled plates, and it is required that the workpiece cannot be corroded, even if there is a small amount of rust spots, it should be polished off and reloaded. Cathodic electrophoresis primer is adopted, which requires high pretreatment. The typical process is:

(1) Manual pre-wiping;

(2) Pre-degreasing spraying at 50 ~ 70℃ for 65438 0 minutes.

(3) degreasing spraying-soaking-spraying at 50-70 DEG C for 0.5-2.0-0.5 minutes.

(4) Water washing at room temperature and secondary spraying for 65438 0 minutes.

(5) Spray the surface at room temperature for 65438 0 minutes.

(6) phosphating spraying-soaking-spraying for 50-60℃1.0-3.0-0.5 min.

(7) Rinse and spray with water at room temperature for 65438 0 minutes.

(8) Circulating deionized water, washing, and spraying at room temperature for 65438 0 minutes.

(9) Wash with fresh deionized water and spray at room temperature for 65438 0 minutes.

6.2 Household appliances

Generally, it is also stamped from cold-rolled plates, and a very small amount of corrosion is polished off in advance. The main workpieces are refrigerators, washing machines, air conditioners and small household appliances. After pretreatment, it is powder coating, and some are electrostatic or air coating. The typical process is:

(1) pre-degreasing spraying at 50 ~ 70℃, 1 min;

(2) degreasing and spraying at 50 ~ 70℃ for 65438+/-0.5 minutes; ;

(3) Washing and spraying at room temperature for 65438±0min；;

(4) surface spraying for 0.5 minutes at room temperature; ;

(5) phosphating spraying at the temperature of 30-45 DEG C for 2.5 minutes; ;

(6) Water washing at room temperature and secondary spraying for 65438±0min；;

(7) washing with deionized water and spraying at room temperature for 65438+/-0 minutes;

(8) Drying at 10 ~ 140℃ for 10 min.

6.3 Auto Parts, Family Cars and Modified Cars

Generally, the batch of this kind of products is not very large, so the time between processes is long, and the workpiece has rust, scale, oil pollution and so on. Common processes are:

(1) degreasing and derusting "two in one" soaking, 15 ~ 40℃,10 min;

(2) cleaning and soaking in normal temperature water for 65438+/-0 min;

(3) Neutralizing and soaking at room temperature for 65438 0 minutes;

(4) Surface conditioning and soaking at room temperature for 65438±0min；;

(5) The phosphating soaking temperature is 65438 05 ~ 40℃ and the time is 65438 00 minutes;

(6) washing with water, and soaking twice at room temperature for 65438±0min；;

(7) Drying at 1 10 ~ 140℃ for 10 min.

6.4 Bicycles, motorcycles and tractors

Because most workpieces have rust, scale and oil stain, the output is generally large and the processing speed is required to be fast. The general process is:

(1) Degreasing and derusting "two in one" soaking at 60 ~ 70℃, 50 ~ 70℃,10 min;

(2) cleaning and soaking in normal temperature water for 65438+/-0 min;

(3) Neutralizing and soaking at room temperature for 65438 0 minutes;

(4) Surface conditioning and soaking at room temperature for 65438±0min；;

(5) phosphating and soaking at 30-70 DEG C for 6 minutes;

(6) washing with water, and soaking twice at room temperature for 65438±0min；;

(7) Drying at 1 10 ~ 140℃ for 10 min.

Phosphating (ⅵ) —— Quality Control and Detection Methods

After phosphating, the quality index of the workpiece should be tested according to its purpose. The main quality control indicators include the appearance of phosphating film, the thickness or weight of phosphating film, and the corrosion resistance of phosphating film or after treatment. According to the purpose of phosphating, it is sometimes necessary to test the compatibility between phosphating and paint film, hardness, friction coefficient and scratch resistance of phosphating film. Please refer to the following standards and methods for the three indexes of phosphating.

Appearance of phosphating film: Visual inspection method is adopted, and relevant standards GB 1 1376-89 "Metal phosphating conversion film" and GB 6807-86 "Technical conditions for phosphating treatment of steel workpieces before painting" are adopted.

Thickness or film weight of phosphating film: The film thickness can be measured by the microscope method of GB 6462, or by the thickness gauge according to GB 4956 Method for Measuring the Thickness of Nonmagnetic Coating on Magnetic Metal Substrate or GB 4957 Method for Measuring the Eddy Current of Nonconductive Coating on Nonmagnetic Metal Substrate. The weight of paint film is determined by gravimetric method, which can be in accordance with GB 6807 "Technical Conditions for Phosphating Treatment of Iron and Steel Workpieces before Painting" or GB 9792 "Determination of paint film quality per unit area of conversion film on metal materials".

Corrosion resistance: the corrosion resistance of phosphating film itself can be detected by copper sulfate drop method, sodium chloride salt water immersion method and salt spray test method. Drip method and salt water immersion method can meet GB 6807-86 "Technical Conditions for Phosphating Treatment of Steel Parts before Painting". After the phosphating film is treated by oiling, waxing and painting, it is generally inspected by salt spray test. Salt spray test can be conducted according to GB 177 1-79 "Determination of Salt Spray Resistance of Paint Film" or GB 6458 "Neutral Salt Spray Resistance Test of Metal Coatings".

1 Phosphating primer before painting

The main purpose of phosphating the primer before painting is to improve the adhesion of the paint film and the corrosion resistance of the coating system, so the focus is on the compatibility with the paint film. The inspection indexes of phosphating quality generally include the appearance of paint film, film thickness and performance after matching with paint film. The appearance of paint film should be uniform, fine and complete phosphating film. For light-colored iron-based phosphating, its appearance should be uniform, fine and complete phosphating film. For light-colored iron-based phosphating, its appearance should be complete red-blue film. Phosphating film should not be too thick. Generally, the membrane importance is less than 7.5g/m2, preferably 1.5 ~ 3.0g/m2. For light-colored iron-based phosphating film, the weight should be 0.5 ~ 1.0g/m2. The phosphating film is too thick and rough, which is not conducive to painting. Corrosion resistance indicators include the corrosion resistance of phosphating film itself and yellowing and rust should not occur before painting. The corrosion resistance of phosphating film and paint is the most important, which reflects the overall corrosion resistance of phosphating film and paint. After the phosphating film is matched with the paint, besides the corrosion resistance, the mechanical and physical properties of the phosphating film, such as adhesion, impact strength and bending resistance (flexibility), should also be measured.

Generally, the quality index and detection method of primer phosphating before painting should refer to the national standard GB 6807-86 "Technical Conditions for Phosphating Treatment of Steel Workpieces before Painting", which has detailed provisions on the quality index and detection and evaluation method of phosphating film, and its main contents are as follows:

(1) The appearance of phosphating film should be dense, continuous and uniform light gray to dark gray, and it should be a continuous light-colored iron-based phosphating film. The following defects are allowed: slight water mark, chromate mark, slight ash hanging, phosphating film defects caused by different surface conditions such as heat treatment, welding and processing. The following defects are not allowed: the phosphating film is yellow and rusty, the phosphating film is loose, the phosphating bottom layer has no film, and the ash hanging is serious.

(2) The weight of phosphating film for painting should be less than 7.5g /m2.

(3) The corrosion resistance of phosphating film is soaked in salt water, and the phosphating workpiece should not rust in 15 ~ 25℃ 3%NaCl aqueous solution1h.. The corrosion resistance test after phosphating and paint film matching is that the phosphating work piece is coated with 25 ~ 35μ m A04-9 white amino paint, and the paint film should be free of foaming, rust and peeling after scratching (according to GB 177 1-79) and 24h salt spray test (8h salt spray test for iron phosphating).

GB 6807-86 doesn't take the dropping method of copper sulfate as a necessary inspection item, but thinks it can be used as a quick inspection method of phosphating quality between processes, and the corrosion resistance after phosphating coating is a necessary inspection item.

Please refer to GB 1 1376-89 "Phosphating Conversion Film of Metals" for inspection indexes and methods of phosphating before painting.

Therefore, judging from the inspection items specified in the standard, the phosphating of primer before painting should be a dense, uniform and thin phosphating film, and the corrosion resistance and mechanical and physical properties of phosphating film and paint should be emphatically inspected.

2 phosphating rust prevention and corrosion prevention

For this kind of phosphating, its main purpose is to prevent corrosion, and its corrosion resistance is the most important index. General quality inspection indicators include copper sulfate drop 1min or more, salt tolerance for 2 hours or more, and salt spray test1.5 hours or more. Salt spray test is the best method to detect corrosion resistance after oil coating or wax coating, and the specific salt spray resistance time can be agreed by both parties.

3 Lubrication, wear resistance, friction reduction and phosphating

Phosphating for lubrication is mainly used for cold working, generally zinc series. Wear-resistant and antifriction phosphating is a workpiece used for load friction movement, and conventional phosphating is manganese phosphating.

For phosphating with lubricating function, the appearance, film weight, corrosion resistance and lubricity after saponification are mainly tested, and sometimes the friction coefficient is determined. The appearance of phosphating film is required to be uniform and complete. Generally, the film is more than 5g/m2 to ensure a certain film thickness. After saponification, the friction force is obviously reduced, the die damage is reduced, and the cracking of the workpiece during cold working is reduced.

Wear-resistant and antifriction phosphating generally adopts manganese phosphating, and the appearance of phosphating film should be uniform and complete dark gray or black film. For parts with small fit clearance, the film weight should be 1 ~ 3g/m2, and for workpieces with large dynamic fit clearance, the film weight should be above 5g/m2. This phosphating requires high hardness and scratch resistance, and the specific indicators can be agreed by both parties. At the same time, wear resistance, antifriction and phosphating should have good corrosion resistance, and the salt spray resistance should be above1.5h. Please also refer to GB 1 1976-89 for lubrication, wear resistance, antifriction and phosphating.

4 Phosphating for other purposes

Phosphating can be used not only in the above three fields, but also in electrical insulation and decoration. The conventional quality inspection indexes are appearance, film weight and corrosion resistance. For phosphating of electrical insulation, it is necessary to detect the surface resistance per unit area. Decorative phosphating should be dyed according to different requirements, requiring different colors, chromaticity and corrosion resistance. The detection method and control range of these indicators are generally agreed by the supply and demand sides.

The detection and control of phosphating quality index have different requirements according to its different uses. In addition to the conventional appearance, film weight and partial phosphating corrosion resistance standards, most indicators and detection methods are agreed by both the supplier and the buyer.