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Trivalent chromium high corrosion resistance passivation technology

Abstract: This paper summarizes the recent progress and mechanism of trivalent chromium passivation, and introduces a new sealing technology of trivalent chromium passivation. This sealing technology directly adds the nano sealing agent to the passivation solution, which greatly improves the corrosion resistance of the passivation layer, reaching or even exceeding the traditional hexavalent chromium passivation process, and meets the environmental protection and high corrosion resistance requirements of automotive materials and components electroplating. This new technology adjusts the color of the passive film by using different film-forming accelerators in different passivation solutions, forming blue white, multicolored and black passive films, which meet the decorative requirements. The paper also introduces the use and maintenance methods of these passivators in production

key words: galvanizing trivalent chromium passivation automotive parts high corrosion resistance sealing hole

1. Preface:

1.1 advantages and hazards of traditional hexavalent chromium passivation process

hexavalent chromium passivation process has many advantages, such as high corrosion resistance, self-healing ability of self-healing corrosion resistance, can easily passivate silver white, blue white, multicolored, military green, black and other colors, and raw materials come from a wide range of sources and are cheap. However, hexavalent chromium is a carcinogen, which is seriously harmful to the environment and human health

1.2 urgency of trivalent chromium passivation

the European Union signed a decree in Brussels in 2003, which stipulates that the use of hexavalent chromium in vehicle materials and components will be prohibited from January 1, 2003. According to this decree, the content of hexavalent chromium in each vehicle will not exceed 2G [1]. On August 14th, 2004, the EU's "e-waste treatment law" was officially issued. On August 13th, 2005, this law will officially implement the electronic universal testing machine, which is often used by researchers. The decree is based on two EU directives in 2002. These two directives are the directive on scrapped electronic and electrical equipment (WEEE brand awareness is further expanded) and the directive on the prohibition of the use of certain hazardous substances in electronic and electrical equipment (RoHS), requiring member states to ensure that new electronic and electrical equipment put on the market from July 1, 2006 does not contain six hazardous substances such as lead, mercury, cadmium, hexavalent chromium, polybrominated diphenyl ether and polybrominated biphenyls. The decree also stipulates that all laptop computers, desktop computers, printers, CPUs, mainboards, mice, keyboards, etc. produced and sold in the EU market must establish a complete classification, recycling, recovery, recycling system and bear the burden of product recycling before August 13, 2005. Chinese enterprises producing export products must stop using hexavalent chromium passivation process after August 13, 2004

2. Progress of trivalent chromium passivation technology

the research work on hexavalent chromium free passivation on zinc has been carried out for more than ten years, mainly in two directions: trivalent chromium passivation and chromium free passivation. Chromium free passivation systems include titanate, molybdate, tungstate, rare earth and silicate [2]. At present, although these chromium free passivation systems are non-toxic and environmental friendly, their corrosion resistance and appearance are not as good as those of hexavalent chromium passivation, which can not meet the electroplating requirements of ordinary hardware, let alone the high corrosion resistance requirements of automotive parts electroplating. Therefore, the chromium free passivation process has not been widely used in industry. Therefore, hexavalent chromium free passivation technology is mainly based on trivalent chromium passivation technology, which has been mature and applied to production. Just as the comprehensive performance of the modern alkaline cyanide free zinc plating new process has exceeded cyanide zinc plating and acid zinc plating, the latest third generation trivalent chromium passivation performance has reached or even exceeded the traditional hexavalent chromium passivation process

2.1 the first generation trivalent chromium passivation

complexing agent is mainly fluoride, and the complexion of fluoride with Cr (Ⅲ) is relatively stable, and the film layer is thin, so the color of the formed film is generally silver white and blue white, with poor corrosion resistance, and the neutral salt spray test does not exceed H. in order to achieve good corrosion resistance, only by sealing, and the color after sealing changes to silver white, with monotonous color. The Cr (Ⅲ) concentration of this system is high, and the operating temperature is also high

2.2 second generation trivalent chromium passivation technology

the early second generation trivalent chromium passivation process contains oxidants, and its corrosion resistance is close to hexavalent chromium with the film color, but it is eliminated because the film contains hexavalent chromium. The second generation process in the later stage does not contain oxidants, and the multicolored color is light. The common feature of the second generation trivalent chromium passivators is that they use organic complexing agents and add other metals to greatly improve the corrosion resistance, and can get different colors of passivation films, such as blue white, multicolored, black. The requirements for operating conditions are relatively low. Typical products include rodip248 from Atotech, chromitirng680 from surec and tr-173 from JASCO, which have been successfully applied in production

921 zinc acid passivation 261 trivalent chromium blue white

2.3 the latest third generation trivalent chromium passivation technology

is to directly add sealing agent into the second generation passivation solution, which overcomes the defect of trivalent chromium passivation without self-healing ability, and greatly improves the corrosion resistance of the film. The corrosion resistance of the film has reached or exceeded the hexavalent chromium passivation process, so it meets the environmental protection and high corrosion resistance requirements of automotive parts electroplating. Using this process, the R & D team led by dipsol, researcher tianxingyou of the advanced materials center of the Institute of utilization technology, Hefei academy of materials science, Chinese Academy of Sciences, and associate researcher Zhang Xian won the new development division and ekem (Hongzheng) Company in the research of the preparation and anti electromagnetic interference performance of light fluoroplastic matrix nanocomposites. The typical products are 251 and 252 trivalent chromium five color passivation, 261 trivalent chromium blue white passivation 271 trivalent chromium black passivation has been successfully applied in production at present

221 alkaline cyanide free passivation 252 trivalent chromium multicolor

3.trivalent chromium passivation mechanism and composition

the traditional hexavalent chromium passivation film is formed through the dissolution of zinc, the reduction of chromate and the precipitation of trivalent chromium gel. The film contains hexavalent chromium. Therefore, the passivation film has self repairing ability, also known as self-healing ability. The trivalent chromium film layer forms zinc ions through the dissolution of zinc. At the same time, the dissolution of zinc ions causes the pH value of the zinc surface solution to rise. Trivalent chromium reacts directly with zinc ions, hydroxyl, etc., forming insoluble compounds to precipitate on the zinc surface, forming a passive film. The specific reaction formula is as follows:

trivalent chromium passivator generally contains the following components:

trivalent chromium Cr (Ⅲ): the main component source of the passivation film. Trivalent chromium can be chromium sulfate, chromium nitrate, chromium chloride, chromium acetate, etc

oxidant: oxidant: produces zinc ions to promote membrane formation. Oxidants can be hydrogen peroxide, nitrate, halite, persulfate, cerium, etc. In the passivation process, the passivator using oxidant will oxidize trivalent chromium into hexavalent chromium due to the automatic increase of pH, which will be mixed in the coating, so that the coating contains hexavalent chromium. Therefore, the film formed by the passivator containing oxidation has a darker color, but it does not meet the requirements of environmental protection

other metals: other metals: mainly adjust the appearance color and corrosion resistance. Mn, Sb, Mo, Ti, Fe, Co, Ni, CE and other lanthanide rare earth elements can be used

complexing agent: complexing agent: control the speed of film formation and the stability of passivation solution. It is through changing the rotation speed of the electromechanical, the adaptability is too strong, the film forming speed is slow, the film layer is thin, and even the film layer cannot be formed; The complexing property is too weak, the stability of passivation solution is poor, and the film is Matt. Complexing agents include fluoride, organic carboxylic acid and their mixtures

film forming accelerator film forming accelerator: adjust the color of the film. Different colors can be formed by choosing different film-forming accelerators. Organic and inorganic anions can be used, such as no3-, so42-, -po43-, f-, cl-, sio32-, sif62-, bf4-, rcooh

sealing agent: in order to overcome the corrosion resistance and other problems existing in the second generation passivator process and meet the environmental protection and high corrosion resistance requirements of automotive parts electroplating, ekem (Hongzheng) company has made a breakthrough in technology through years of research. Its research shows that the film of trivalent chromium passivation process has certain stripes and has no self-healing ability, which affects its corrosion resistance, so the sealing agent is directly added to the passivation solution, The pore sealing agent can fill the pores of the passivation layer by adding particles with a diameter of up to nanometer, making the film more fine, so the corrosion resistance of the film is greatly improved

4. Production and application of the third generation trivalent chromium passivation

4.1 application methods and characteristics of the third generation trivalent chromium passivation:

it can be seen from the table that the corrosion resistance of trivalent chromium passivation reaches or even exceeds that of hexavalent chromium process

4.2 production condition control and maintenance

4.2.1 pH value control:

ph value is controlled at 1 5. Different passivation processes have different optimal values. The pH value of trivalent chromium passivation solution is generally higher than that of hexavalent chromium. The control of pH value of trivalent chromium passivation solution is stricter than that of hexavalent chromium. If the pH value is too low, the film is thin and prone to blooming. If the pH value is too high, the film formation speed is slow and prone to fog. Generally, the pH value will increase automatically in the production process. Different passivation solutions are adjusted with different acids. This method is troublesome to adjust the pH value in actual operation, because the passivation solution is dark in color, and it is difficult to measure the pH value with pH test paper. The supplement of passivation solution already contains acidity. As long as the concentrated solution is supplemented normally, the acidity can automatically restore balance, and the pH value changes little, so it does not need to be adjusted frequently

4.2.2 temperature control:

blue and white passivation is carried out at room temperature, and colorful and black passivation can be carried out at ℃. The higher the temperature, the faster the film formation rate and the thicker the film; The lower the temperature, the slower the film formation speed and the thinner the film

4.2.3 passivation time:

passivation time is seconds. The longer the passivation time, the thicker the film; The shorter the passivation time, the thinner the film. The passivation time is too long or too short, and the film is thin, so it is necessary to control the time. Too long passivation time of blue and white will make the film blue and white with yellow, uneven, and affect the appearance

4.3 trivalent chromium passivation and sealing technology

the requirements for high corrosion resistance of automotive parts are that h does not appear white rust and H does not appear red rust. If the second or third generation passivator is used together with a special sealing agent, it can fully meet or even exceed the high corrosion resistance requirements of the automotive industry, such as the 1131 and 1133 sealing agents of ekem (Hongzheng) company. 1131 sealing agent film is transparent and uniform. 1133 sealant, the film is transparent and has strong covering power. After the black passivation layer is sealed, the film is bright and smooth, revealing attractive black. Traditional sealing agents such as gold oil have no significant effect on the improvement of corrosion resistance

5. Trivalent chromium passivation and selection of zinc plating process

cyanide free zinc plating is mainly chloride zinc plating and alkaline cyanide free zinc plating. The advantages of acid zinc plating are excellent decoration and high current efficiency. It is precisely because of these advantages that the uniformity ability of acid zinc is poor, and the coating has a layered structure, which makes its corrosion resistance inferior to alkaline cyanide free zinc plating. The advantages of the new generation of alkaline cyanide free zinc plating are good deep plating ability and dispersion ability, and the coating shows columnar shape, which has excellent corrosion resistance, From the perspective of functionality, the corrosion resistance of zincate zinc plating is greater than that of acid chloride zinc plating. And from the perspective of coating softness, the softness of the new generation zincate zinc plating is better than that of acid zinc plating

the adhesion and appearance of trivalent chromium passivation film are closely related to the galvanizing process. Whether hexavalent chromium passivation or trivalent chromium passivation, the adhesion of alkaline cyanide free zinc plating passivation film is far better than that of acid zinc plating. The trivalent blue white passivation of zinc acid is bright sky blue, and the alkalinity can also reach the color of zinc acid, but the operation requirements are relatively high, for example, the water washing requirements are higher, and the passivation time cannot be too long, otherwise it is easy to turn yellow at the edge. The colorful passivation color of zinc acid is light, only