Phosphating treatment is the most widely used pretreatment process for coating at present, but its application is facing increasing environmental pressure due to the limitation of heavy metal ions such as zinc, nickel, manganese, phosphate and sodium nitrite in phosphating solution, and the high treatment temperature, the complex harmless transformation process of wastewater and waste residue. Silane treatment technology overcomes the above shortcomings and brings a revolutionary change in the field of coating pretreatment. The treatment effect of silane pretreatment technology has been comparable to that of zinc phosphating.
Since the first household appliances production line used silane technology 10 years ago, silane technology has moved from laboratory research stage to industrial production stage. By the end of 2011, hundreds of pretreatment production lines around the world have used silane technology, and dozens of production lines in China have used silane technology. The industry covers various fields, including household appliances, automotive parts, general industry, trucks, functional vehicles, cars and so on. The body painting has gradually entered the stage of vehicle production. At present, hundreds of thousands of cars have been off-line using silane as pretreatment technology in France, Spain, Brazil, Russia and other countries. China's first large-scale vehicle painting line will be put into operation in Wuhan by the end of 2012. Vehicle painting is the most anticorrosive and decorative coating. Silane technology can meet the requirements of car painting, and naturally can well meet the requirements of other industries.
Silane treatment technology and phosphating treatment have the same requirements, but also have some different requirements. In this paper, the specific requirements of process, equipment and management of silane technology in the practical application of painting are briefly introduced.
Advantages of 1-silane Technology
1) The film formed by silane technology can replace the traditional phosphating film. The quality of phosphating film is usually 2-3 g/m2. The quality of Oxsilan coating film is 0.1 g/m2, the difference is about 20 times, and the unit consumption is greatly reduced.
2) The bond between Si-O-Me covalent bond molecules is very strong, so the product is very stable, which can improve the anti-corrosion ability of the product. The corrosion resistance of silane treatment is better than that of zinc phosphating. Compared with zirconium series products, its performance is obviously superior.
3) Easy to use and easy to control. The bath liquid is composed of two-component liquid. It only needs to control the pH, activation point and conductivity. It does not need to control the content and temperature of free acid, total acid, accelerant, zinc, nickel and manganese as phosphating liquid does.
4) Saving energy, operating at room temperature or low temperature, reducing energy costs.
5) Save electricity, because of the reduction of pumps and power, can save 40% of the electricity.
6) The treatment time is short, only half of the phosphating treatment time, so the yield can be increased.
7) Excellent environmental protection performance, no harmful heavy metals, no slag, less waste water discharge, easy treatment, if installed filters and ion exchangers, can achieve closed recycling.
8) Polymetallic treatment process: cold-rolled plate, hot-dip galvanized plate, galvanized plate, coated plate, aluminium and other different sheets can be mixed line treatment.
9) The process is simple and the process is short. It can reduce the amount of maintenance, do not need to maintain meter adjustment, passivation equipment, and do not need regular cleaning of phosphating tank, pipelines and nozzles.
10) The comprehensive cost is low, the consumption of products is low, and the cost of three wastes treatment is low. The cost of using phosphating solution in the production line of Proposal plant in Europe is 730,000 Euro/a, and 390,000 Euro/a after switching Oxsilan products, which saves 340,000 Euro/a and 47% of the cost.
11) It is compatible with the original painting process and equipment, and can be put into production only by replacing phosphating solution without revamping the equipment.
12) System length reduction: Without surface adjustment and passivation process, the treatment time can be shortened. New production lines can reduce investment and occupation of land. The use of silane and phosphating in European automotive lines can save 2.2 million euros in equipment investment.
13) After silane treatment, electrophoresis can be performed directly without drying.  Silane has the same electrophoretic permeability as zinc phosphating.
2-silane technology treatment process
Silane treatment has been widely used in general industry instead of iron and zinc phosphating treatment. As long as the phosphating tank is cleaned, the old production line can be directly put into silane material. Phosphating tank and its pipeline usually contain phosphating slag, which needs to be thoroughly cleaned with a special slag cleaner, otherwise the quality of products will be affected.
2.1 General industrial silane pretreatment process
General industrial silane pretreatment process is degreasing washing silane treatment pure washing drying.
Silane pretreatment replaces the traditional surface adjustment, phosphating and passivation processes. The process is much simpler. After silane treatment, drying (removing moisture) is carried out directly by powder spraying or paint spraying. The curing process of silane coating is completed simultaneously with powder spraying or paint baking. The baking temperature should be above 140 C and the baking time should be more than 20 minutes. It can also be sprayed powder after silane treatment without washing and direct drying.
2.2 Main Process of Silane Pretreatment for Car Body
The main process of silane pretreatment for automobile body is shown in Table 1.
After silane treatment, cathodic electrophoresis was carried out directly without drying. The silane pretreatment process of automobile body is a complete process, and other processes can be reduced appropriately. For example, water washing after silane treatment can be reduced a little.
The difference between silane treatment and phosphating treatment
3.1 Differences in Phosphating and Silane Treatment Technologies
The difference between phosphating and silane treatment technology is shown in Table 2.


Some Technical Problems in the Application of 3.2 Silane Technology
3.2.1 Degreasing
German surface treatment experts pointed out that "although the silane process is very simple and stable, it still needs professional technical support in the application process to achieve the required anti-corrosion effect, paint bonding force and process adaptability".
Degreasing is the first point to be paid attention to in the application of silane process. In the past, phosphating was carried out at higher temperature (50 C) and lower pH (about 3.0), so the dirt and grease on the body surface could be further cleaned and removed at this temperature and pH. However, the process conditions of silane are very mild (room temperature, pH is about 4.5), which basically does not have the possibility of further cleaning. This means that the silane process has a high requirement for degreasing. Selection of efficient and environmentally friendly degreasing can achieve excellent degreasing effect.
3.2.2 Washing
The effect of washing has a great influence on the quality of coating. Special chemicals can be used in the washing process, which is very effective in preventing flash rust.
3.2.3 Water quality
Pure water is needed in silane process. It is better to add a pure water wash after degreasing to reduce the pollution loss of silane tank liquid. Silane tank and subsequent washing all need pure water.
3.2.4 Swimming Permeability
The electrophoretic permeability of silane treated electrophoresis is different from that of phosphating. Because the thickness of silane film is obviously lower than that of phosphating film, the resistance of silane film is obviously lower than that of phosphating film, so the surface film thickness of cathodic electrophoresis will also be thicker. Due to Faraday effect, the effective voltage of the body chamber will decrease, so electrophoresis on the surface of the inner chamber may be more difficult to film. Dr. Schubacher, Germany, explained: "It is as important to realize that getting enough electrophoretic film thickness in the body chamber is as important as getting the same electrophoretic film thickness on the body surface."  Kermitl developed a solution to this problem by studying the chemical mechanism inside the silane tank. The success of this solution also gave electrophoresis suppliers new inspiration: to develop electrophoretic products specially for the new film pretreatment technology, the combination of the two achieved better penetration effect. Therefore, when selecting silane process, it is necessary to consider whether it is compatible with electrophoretic paint. At present, PPG, BASF, DuPont, Nippon and other companies have electrophoretic paint and silane technology to match.  The electrophoretic permeability of silane treated electrophoresis can be completely equivalent to that of phosphating.
4-silane process treatment equipment
There are still some differences between silane process and phosphating process in the design of new coating production line. Its characteristics are short process, less equipment, small trough and low cycle. This saves manufacturing and operating costs. Specific design of production line can refer to the following principles.
1) There is no need to design a table harmonization and passivation process.
2) Design of silane treatment tank.
The pH of silane tank solution is 4.2-4.8, which is acidic. The material of tank body is acid-resistant stainless steel, such as 316 or 304 stainless steel, or plastic material, such as polyvinylidene fluoride (PVDF) or polypropylene (PP). Cold-rolled ordinary steel plate is not feasible unless the inner cavity is coated with acid-proof materials, such as acid-proof plastics, stainless steel sheets, etc.
The reaction time is required to be 1 - 2 min, and the length of the tank can be calculated according to the passing time of 2 min. The number of cycles is required to be 1 to 2 times, and the number of design cycles is also calculated by 2 times/h. The designed heating system can be calculated according to the maximum tank liquid temperature of 35 C, and the actual operating temperature is 20 - 30 C. Silane tank liquid is clear and transparent, and no slag is produced during the reaction. However, when the workpiece encounters acid bath, iron ions will dissolve and particulate matter will also be carried on the workpiece. Therefore, it is suggested that bag filter be installed in the system, and filter bag with aperture of 25 micron or plate-frame filter press system be selected.
Because the chemical consumption is small and the silane tank feeding quantity is small, the chemical can be dripped by drip pump.
A small flow atomizing nozzle is installed at the outlet of the silane tank, and pure water is sprayed to wet the surface, so that the surface state is uniform and the best electrophoretic effect is obtained. Small flow rate will not increase the overflow of tank liquid.
3) Design of water-saving overflow pipeline.
Reasonable design and management of overflow pipeline can save water and ensure cleaning effect, as shown in Figure 1.
According to Kushener's approximation formula, the final dilution factor is equal to (X n+1-1)/(X-1), in which n is the number of washing times, and X is the amount of water supply/take-out. The same dilution factor can be calculated up to 500 times. The water consumption can be reduced by 55% compared with the production line without this method.
5. Process Management
5.1 Degreasing Process
It is necessary to use more effective degreasing methods, such as spray-immersion combination and demulsifying activator, to improve degreasing efficiency and effect and lay a good foundation for silane treatment. In daily management, we often pay attention to check the degreasing effect.
Daily management of 5.2 silane tank
The process management of silane technology is relatively simple, the control parameters are less than phosphating, and the control is easy.
1) Daily control parameters of tank liquid: tank liquid temperature is 20 ~ 30 (15 ~ 45);
The pH ranged from 3.8 to 4.8.
The activation point is 4.1 to 6.8.
The conductivity is less than 4 500 US/cm.
2) The portable instrument XRF can be used to detect the coating directly, and ICP can analyze the bath fluid, which can improve the accuracy of the detection.
3) The heating equipment is retained in the silane tank. Normally, heating is not necessary. The optimum temperature of silane treatment is 20 - 30 C. The temperature can be satisfied by degreasing carrier in daily work, but when production resumes on Monday or on holidays, the tank liquid needs to be heated to this temperature to obtain the best pretreatment effect.
4) Some production lines need to add fungicides to prevent bacterial growth in summer. Selection of high quality coating materials and good coating equipment, careful on-site management will be able to obtain excellent coating results.
After 10 years of development, silane technology has accumulated rich experience, and the process and technology have matured. This new energy-saving, environmental protection and cost-saving technology will quickly replace the phosphating process and be promoted on a large scale.