Analysis of Dacromet Coating Technology and Inspection Points
The technological process of dacromet is described, and some detection methods of dacromet coating are briefly introduced.
Dacromet; Coating process; Common defects; test method
0. preface
Dacromet coating (Dacromet in Europe and America, Dacromet in Japan) is a kind of protective layer with brand-new structure and performance, which is composed of flaky zinc powder, aluminum powder, chromium-containing metal salt and adhesive, and is coated on the surface of parts by sintering. The coating has the characteristics of no pollution, no hydrogen embrittlement, excellent corrosion resistance, permeability and heat resistance. As the anticorrosion treatment of small and medium-sized parts in automobile, railway, highway traffic, electric power, construction, bridge and other industries, it has been widely used in the world.
Corrosion resistance mechanism of 1. coating
Dacromet treatment solution is mainly composed of aluminum particles, chromic anhydride and resin, which is solidified into a film. In the process of film formation, most of CrO3 is converted into Cr2O3, resulting in amorphous Cr2O3? CrO3 is used as a binder to bond the matrix with aluminum particles and aluminum particles with aluminum particles, and its structural branches represent Fe? (FeO？ Cr2O3？ CrO3？ Al2O3)？ Al-al. (Al2O3？ Cr2O3？ CrO3？ Al2O3)？ Al. In addition, the network structure formed by resin curing makes the coating structure more compact. Due to the unique structure of the coating, its corrosion resistance mechanism can be analyzed as follows:
1. 1 shielding effect
On the one hand, the coating generates complex compounds through passivation, which makes the coating combine with the substrate; On the other hand, it generates a three-dimensional network structure through resin curing reaction, which makes the combination of coating and substrate more firm. There are both chemical and physical effects between coating and substrate. This tightly bonded coating plays a better role in isolating corrosive media.
1.2 passivation
In dacromet treatment solution, chromic acid reacts with aluminum particles and matrix metal to form a dense chromic acid passivation film, which greatly improves the corrosion resistance of the matrix.
Electrochemical protection of 1.3
In essence, dacromet coating adopts cathodic protection to improve the corrosion resistance of metals. Fine aluminum particles in the coating are closely arranged on the steel surface. The electrode potential of aluminum is much smaller than that of iron. When the coating is partially damaged or immersed in corrosive medium, aluminum as the anode of the corroded microbattery loses electrons and is corroded, and the base metal as the cathode is completely protected.
2. Dacromet process
The process flow of dacromet treatment is as shown above. There are three important quality control points in the whole treatment process, namely pretreatment, coating and baking.
The purpose of pretreatment is to remove the oil, iron and scale on the surface of the part and improve the bonding force between the film and the substrate. There are many pretreatment methods, such as degreasing with organic solvent or alkaline cleaning agent, shot blasting, shot blasting and sand blasting. For more precise parts, organic solvents or alkaline cleaning agents are generally used for degreasing, while for parts with complex structures, the combination of spraying and polishing is generally used. Pretreatment should try to avoid using pickling for rust removal to prevent embrittlement.
Painting is to spray or impregnate parts with dacromet solution, and then remove more treatment solution by centrifugal drying, natural vertical flow and brush coating. Generally, the coating thickness is 3 ~ 6? M, 2 coating 2 baking is 6 ~ 9? M, 3 coating and 3 baking is 9 ~ 12? M, which can be selected according to product requirements (see appendix 1 for details). In the coating process, several process parameters need to be controlled: the immersion time is 200 ~ 300 r/min. In addition, the maintenance of dacromet solution should be especially careful. When not in use, its temperature should be kept below 14℃ and its normal working temperature should be kept at 20? 2℃ to ensure the stability of dacromet solution.
Baking is the process of converting dacromet treatment solution on metal substrate into film layer, which is usually carried out in a mesh belt circulating hot air oven. The whole process is divided into preheating zone, the temperature is 60 ~ 180℃ (treatment time is about 10min), and the parts should be preheated slowly to make the water in the coating escape gradually and prevent the coating from foaming and producing pores. Then it was cured at 280 ~ 330℃ for 25 ~ 30 minutes, and some physical and chemical reactions occurred in the curing zone. Baking process is very important for the formation of dacromet film properties.
3. Inspection of Dacromet coating
3. 1 coating amount
Due to the different shapes of parts, the coating amount of each part is also different, which refers to the average coating amount here.
China standard divides the coating amount into four grades: 70mg/dm2, 160mg/dm2, 200mg/dm2 and 300mg/dm2.
Detection method: Weigh 50 ~ 100 g of the painted part, then put it into 20% sodium hydroxide solution at 70 ~ 80℃, take it out after the coating is completely removed, clean it and blow dry it, and then weigh the difference between the two times and divide it by the surface area of the part to get the coating amount of the part.
3.2 coating thickness
The coating thickness can be measured by magnetic thickness gauge at multiple points, specifically according to the requirements of GB/T4956-2003 "Magnetic Measurement Method of Non-magnetic Coating Thickness on Magnetic Substrate".
3.3 adhesion
Stick the standard adhesive tape with the width of 18mm on the surface of the part, and rub it with your fingers to eliminate bubbles. The end of the adhesive tape and the surface of the part form a 45? Open the angle quickly, and judge the degree of attachment according to the amount of attachment attached to the tape. Generally, it is qualified if it exceeds 3 ~ 5.
3.4 Yellowing test
Yellowing test is a simple method to detect the baking degree of coating. Drop 1 ~ 2 drops of concentrated ammonia water on the surface of the cooled workpiece, and it will be qualified if it turns yellow within 30 ~ 60s, and it will not turn yellow if it is not baked adequately.
3.5 Rapid Corrosion Test of Ammonium Nitrate
Because of the long period of salt spray test, it is difficult to realize in production control, so the rapid corrosion of ammonium nitrate is often used to judge the corrosion resistance of coatings in China.
Test method: 20% ammonium nitrate solution is heated to 70? 2℃, immerse the parts in the solution, or partially immerse them in the solution.
Judgment method: Red rust is not allowed in the matrix.
3.6 salt spray test
The salt spray test of dacromet coating adopts neutral continuous spray test, and the specific operation method is implemented according to GB/T10/25-1997 "Artificial Atmospheric Corrosion Test-Salt Spray Test". It should be noted that the sample should be inclined by 60 ~ 75? Placed without touching each other, the solution leaked from it will not fall from one part to another.
4. Concluding remarks
With the progress of science and technology, there will be new technologies and methods for dacromet treatment technology and detection method. I hope to work with colleagues to develop and promote the wide application of Dacromet treatment technology in China. [partial]
refer to
Yi Shu, Bing Lin. Study on dacromet coating. Journal of Chongqing Normal University (Natural Science Edition), 1997.438+02.
[2] Xiao Hesen. Detection method of dacromet treatment. Electroplating and Finishing, February 2004.
Zou Zhiwu, Wang Hongzhou. Dacromet technology. Automotive Technology and Materials, 2003.
Wang Min, Xin Huang, Wang Jialu and He Zikai. Dacromet technology and its application. Electroplating and finishing, 200 1.4.
Click the next page for details >>& gt Paper surface coating technology.