Pre-process test of traditional phosphating zirconium film pretreatment
Text / coating industry
With the continuous strengthening of environmental protection and the advancement of technology in various countries around the world, pre-treatment materials for automotive coatings are also emerging with new environmentally friendly products, such as low-temperature degreasing, liquid surface conditioning, and film pretreatment products. The application of new materials in the automotive coating industry helps to reduce energy consumption, improve environmental protection, and improve production efficiency and product performance in the automotive manufacturing process. Zirconium film pretreatment is one of the new technologies. The performance in energy conservation and environmental protection is particularly outstanding. The “Cleaning Industry Evaluation Index System for Coating Industry†classifies the film-type conversion film processing process as the highest benchmark value of Class I. Therefore, it is very meaningful to carry out the feasibility study on the traditional phosphating to film pretreatment design experiment, and to prepare for the old line conversion film pretreatment products.
1 Zirconium film pretreatment technology
The traditional coating phosphating pretreatment process has the disadvantages of serious pollution (including phosphorus and containing heavy metal ions such as nickel and manganese), sedimentation and energy consumption, and is a backward process with high pollution and high energy consumption that needs to be eliminated. Under the general trend of environmental protection and cleaner production, new film pretreatment products have emerged. The pretreatment of Zirconium film represented by PPG and Henkel and the pretreatment of silane film represented by Chemmit have been obtained globally. Promote the use. The coating of zirconium film is very dense, so it is often called ceramic coating, which can provide metal substrate with anti-corrosion, acid and alkali resistance equivalent to traditional phosphating process.
Zirconium film pretreatment film formation utilizes the principle of metal surface microbattery reaction, and the anode undergoes an oxidation reaction, such as formula (1); the cathode undergoes a reduction reaction, as shown in formula (2).
The local pH of the metal surface near the cathode is increased to promote the dissociation of the fluorozirconic acid and the precipitation of zirconia. At the same time, the fluoride ion scavenger in the solution forms a stable fluoro complex with the fluoride ion to reduce the fluoride ion decomposed by the fluorozirconic acid and promote the formation of the zirconium film.
The zirconium film pretreatment production line can eliminate the surface conditioning before phosphating and the water washing and passivation process after phosphating, and the phosphorus-free and nickel-free generation can reduce the wastewater discharge; the conversion tank can be operated at room temperature to reduce energy consumption. At the same time, the etching of the device can be reduced; the amount of sediment is reduced significantly, the use of the phosphating slag removal system can be omitted, and the cleaning frequency of the tank, the pipeline, the nozzle and the like can be reduced.
Zirconium film also has advantages in handling body-mixed panels, especially when aluminum body parts account for more than 20%. Traditional phosphating produces a large amount of cryolite/potassium cryolite crystal sediment due to the precipitation of aluminum ions, which affects the film formation of the whole vehicle parts. Zirconium film deposits zirconia on the aluminum or aluminum alloy parts, and the tank is compatible. The precipitated aluminum ions also have no large amount of crystal sediment. However, since the zirconium film is very thin and has a different color on the mixed substrate used in the vehicle body, it is difficult to cover the defects of the substrate, and it is easy to cause problems such as printing, so that the surface state of the substrate is required to be high.
The film materials studied in this paper were supplied by Henkel, and the electrophoretic coatings were supplied by BASF.
2 Applied research
2.1 Comparison of swimming permeability
The penetrating power is an important indicator of electrophoresis. After conversion from traditional phosphating to zirconium film, the electrophoretic penetrating power may also change. The cold-rolled sheet was used for the phosphating and zirconium film treatment. Then place it in the same factory in the target factory for the swimming penetration test. The result is shown in Figure 1.
Figure 1 Comparison of the throwing power after phosphating/zirconium film treatment
It can be seen from Fig. 1 that the bathing force strip using the zirconium film exhibits better throwing power, and the bathing force under the combination of the film pretreatment process and the high throwing power can meet the site requirements. The results of the two pass-throughs of the phosphating treatment were 235 mm and 265 mm, and the results of the throwing power using the zirconium film treatment were 267 mm and 278 mm. This may be because the zirconium film is thinner on the strip of the penetrating force, and the corresponding film resistance is smaller than that of the phosphate film. Therefore, the stripper is easier to electrophoresis, which is to improve the electrophoretic film thickness of the body cavity. helpful.
2.2 Power-on voltage comparison
It can be seen from the results of the bathing force test that the zirconium-based film-treated sheet has a large difference in electrophoresis power compared with the conventional phosphating sheet. Therefore, the experiment compares the power-on voltage of the two processes to treat the board, and the results are shown in Table 1.
Table 1 Comparison of power-on voltage of two kinds of process-treated plates
It can be seen from Table 1 that the zirconium film treated sheet can obtain a higher electrophoretic film thickness under the same electrophoresis time of the same voltage.
2.3 Film pretreatment stop line simulation experiment
The reaction mechanism of the zirconium film is different from the traditional phosphating reaction mechanism. After the line is stopped, the reaction does not automatically stop automatically. On galvanized sheets, the reaction rate was reduced after 20 min, as shown in Figure 2.
Fig. 2 Effect of quality change of zirconium film in different reaction time
The experiment simulates the situation that the vehicle body stops during the troughing process. The whole plate is fully immersed in the zirconium film bath. After the standard process time is reached, the plate is lifted and half exposed to the air. The reaction is continued in the bath. The effects of different zirconium film stoppage time on the final electrophoretic performance were observed. The results are shown in Table 2. Among them, the initial adhesion reference standard GMW14829, using Gardco or BYKGardner cross-cutting instrument and 3M company's 8981 tape for testing, according to the size of the failure area. The standard requirement is 0 or 1 grade, that is, the failure area is less than 5%; the test method of 240h damp heat aging adhesion is the same as the initial adhesion, but the electrophoresis plate should be at (38±2) °C according to the GMW14729 standard before testing the adhesion. Maintenance for 240 h in 100% environment.
Table 2 Effect of zirconium line stop time on the appearance and adhesion of electrophoretic plates
It can be seen from Table 2 that the semi-dip line will cause obvious chromatic aberration in the two parts of the plate, but the boundary is not touched and does not need to be polished. The extension of the stop time will lead to an increase in the electrophoretic roughness. The initial adhesion results of the electrophoresis of each plate were all acceptable. However, after 240 h of damp heat aging treatment on the electrophoretic sheet, the electrophoretic adhesion of the aluminum sheet to the zirconium film at 20 min and 30 min was unqualified (see Figure 3).
Figure 3 Failure template in the line stop experiment
At the same time, stone plate test and cyclic corrosion test were carried out on each electrophoresis plate. The test results are shown in Table 3. Among them, the stone strike test is carried out according to the standard GMW14700 using the gravel throwing test equipment indicated by SAEJ400 and the gravel of 8~16mm at room temperature and 2 temperatures of -18 °C. Generally, the evaluation is higher than 8 grade, that is, the electrophoresis damage width is not More than 1.5mm; cyclic corrosion test refers to GMW14282 for 26 salt spray cycles, observe the extension width, the standard requires cold plate expansion width is less than 6mm, galvanized plate, aluminum plate expansion width is less than 4mm. The stone hammer test results at room temperature and -18 °C were all qualified (≥8). In the cyclic corrosion, the test results of the cold-rolled plate stop line for 20min, 30min and the galvanized plate stop line for 20min are barely up to standard, and the cyclic corrosion test result of the galvanized plate stop line for 30min is not up to standard.
Table 3 Stone-resistant and corrosion-resistant performance of electrophoretic plates with different zirconium reaction times
It can be seen from the above test results that the zirconium film pretreatment stop line will not have a negative impact on the body panel for 10min, and the line stop car can be released directly; the performance of the line stop will be reduced in 20min, and it is necessary to comprehensively evaluate the release and determine the actual situation. No; stop line for 30min, due to the decline in corrosion resistance of the sheet, it is recommended to dispose of it.
2.4 Electrophoresis into the tank simulation experiment
In the traditional phosphating body, before entering the electrophoresis tank, there are all wet into the tank or semi-dry and semi-wet into the tank. After conversion to a zirconium film, it is necessary to study whether it has special requirements for the groove entry method. The experiment simulated five kinds of tanking methods, namely: semi-dry semi-pure water, semi-dry semi-filtrate, pure water spray wet, ultra-filtrate spray wet and full dry into the tank, the experimental results are shown in Figure 4. Shown. It can be seen from Fig. 4 that there are no obvious boundaries for the five kinds of electrophoretic plates in the slotting mode, and the surface is free from defects after the topcoat is sprayed.
Figure 4: Topcoat appearance of different electrophoresis into the tank
2.5 sanding cover cover simulation experiment
In view of the zirconium film pretreatment, the thickness of the conversion film formed on the sheet is only 10 to 200 nm, which is much thinner than the conventional 2 to 3 μm phosphating film. Therefore, its ability to cover the polishing of the sheet is also one of the properties that need attention.
Figure 5 Electrophoresis plate after 4 kinds of galvanized plates
The experiment selected the cold-rolled sheets, galvanized sheets and aluminum sheets used for the body panels, and polished the samples. The grinding methods are 100-mesh sandpaper hand-grinding, 100-mesh sandpaper sanding machine grinding, and oilstone grinding. Among them, the galvanized sheet additionally increases the coverage of the galvanized layer. After the sanding is completed, the sheets are subjected to film and electrophoresis according to the normal process, and the state of the sanding is observed, as shown in FIG. It can be seen from Fig. 5 that the electrophoretic appearance of the cold-rolled plate and the aluminum plate after polishing is good, and only the galvanized plate has a color difference with other regions after being punctured. In terms of roughness, appearance and adhesion, the roughness of the sanding area is substantially flat or slightly higher than that of the unpolished area, and the adhesion is acceptable. The results are shown in Table 4.
The above data shows that the current body grinding state can be well covered by zirconium film and electrophoresis, and conventional polishing will not adversely affect the appearance. After the topcoat was sprayed on the above panels, there was no obvious defect in appearance.
Table 4 Performance evaluation of different grinding states
3 Conclusion
The film pretreatment process is a green coating process, and the advantages of energy saving and emission reduction are outstanding. However, many OEMs are slow to apply domestically due to concerns about the stability of new processes. Through the study of the above aspects of performance, we believe that the film pretreatment process can meet the current quality requirements in all aspects. Through further familiarity with its process characteristics, it is entirely possible to completely replace the traditional phosphating process in the future.
Author: Zhang Xiaochun 1, 1 Yezi Ping, Cao 1, 1 song Chen, Chen Jun 2 can (1. General Motors Co., SAIC vehicle manufacturing, Shanghai 201201; 2 SAIC General Motors Co. plant in Jinqiao, Shanghai 201201)
Source: Coatings Industry, No. 1 of 2018
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