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Progress in phosphating technology

Tan Hailin Pan Yiji Li Xinli (Wuhan Institute of Materials Protection, 430030)

[Abstract] The phosphating technology is discussed in the phosphating of composite parts, no phosphite phosphating, nickel-free phosphating, chromium-free passivation, phosphating of rare earth-containing additives, low-temperature phosphating and pre-treatment of phosphating surface. progress.

[Key words] phosphating pretreatment, progress

Development of Phosphating Technology

Abstract: Present status of phosphating in the aspect of simultaneous phosphating of Fe, Zn, Al, nitrite-free phosphating, nickel-free phosphating, Chromium-free passivation, rare-earth composite Additive in phosphating process, low temperature phosphating and pretreatment prior to Phosphating are reviewed

Keywords: phosphating, pretreatment development

 

Since the application of the first phosphating patent by Ross in 1869, phosphating technology has been developed for more than a century. In the 1930s, Tanner and Lodeesen added a small amount of copper and oxidant to the phosphating solution to reduce the phosphating time from 2 hours to a few minutes or even seconds, making breakthroughs in the application of phosphating [1] . With the development of anodic electrophoresis in the 1960s and cathodic electrophoresis in the 1970s, a new zinc-calcium phosphating system was discovered [2]. In order to prevent the corrosion of automobile body steel, since the 1970s, Germany first introduced the body of all-hot-dip galvanized steel sheet. Since then, the growth rate of galvanized sheet steel has been increasing, and the European automobile industry consumed galvanized steel sheet 1.30 in 1987. ×109kg, accounting for 26% of the total consumption of galvanized sheet, and about 46% in 1989 [3], galvanized sheet is widely used in the automotive industry. Previously, the traditional phosphating method was not suitable for phosphating of galvanized sheet, but in the 1980s, a ternary phosphating solution containing Ni and Mn was developed [4]. In addition, in terms of energy saving, in order to reduce the weight of the car, aluminum and aluminum alloy bodies and parts have emerged, and phosphating solutions of aluminum and aluminum alloys have been developed [5].

Due to environmental considerations, the progress of phosphating technology is mainly characterized by nitrite-free phosphating, no nickel phosphating, no chromium passivation, comprehensive utilization of phosphating slag, and others in steel, galvanized steel and Simultaneous phosphating of aluminum composite metal parts and biodegradable surfactants have been applied. In addition, phosphating further emphasizes safer and less toxic to operators; simple process, low temperature, and rapid film formation by phosphating.

1. Phosphating of composite metal parts

In order to improve the corrosion resistance of metal parts, galvanized steel sheets are increasingly used as automobile body materials. In addition, in order to reduce the weight of the vehicle body, the use of aluminum materials has also increased, such as aluminum automobile parts - steel rings, bumpers, heat dissipation. Tube and so on. Composite metal parts are more susceptible to corrosion due to their galvanic effect, so they have higher requirements for coating. Phosphating of composite metal parts, due to the insulation of the phosphating layer and the "anchor effect" - close combination with the paint film, slows the induction of corrosion and the expansion of corrosion, and improves the corrosion resistance [6, 7 ], phosphating is comparable to non-phosphorized metal parts, after coating, its corrosion resistance can be increased several times. Therefore, the development of automotive structural materials has promoted the research of simultaneous processing of composite metal phosphating products of steel, galvanized steel and aluminum.

At present, ternary zinc phosphating containing nickel and manganese is widely used in the automotive industry. Due to the use of galvanized sheet, it is difficult to form phosphorite [Zn2Fe(PO4)2•4H2O] on the surface, in order to obtain a high P ratio phosphorus. The film is used to improve the coatability and alkali resistance of the galvanized sheet. In the phosphating of the galvanized sheet, a ternary cationic phosphating system containing zinc, manganese and nickel is widely used [4]. The addition of nickel and manganese ions can form a phosphorite-like structure on the surface of the galvanized sheet, such as Zn2Ni(PO4)2•4H2O, Zn2Mn(PO4)2•4H2O, which is dense and finer. The resulting phosphating film has lower chemical activity and increases alkali resistance and secondary adhesion of the coating compared to nickel-free phosphating (Figure 1). In the phosphating of galvanized sheets, excessive dissolution and agglomeration of zinc occurs, which is improved by the addition of fluorides (such as fluorosilicic acid, fluoroboric acid, fluorotitanic acid) [4].

 

 

In recent years, the United States has proposed a mandatory manufacturer's fuel rating scheme (Corporale Average Fuel Economy), which urges auto manufacturers to actively develop lightweight technology. The most important step is to use aluminum alloy to make the car body. The best treatment for a single aluminum material is by chromic acid-chromate chemical treatment followed by paint application. There are also developed chromium-free processes for the treatment of aluminum, such as titanium fluorate/zirconium oxyfluoride, phthalate and organic film. In the automotive industry, steel, aluminum composite workpieces or steel, galvanized steel, aluminum composite workpieces, if the composite workpiece is poorly coated, it will produce more serious bimetal galvanic corrosion and filamentous corrosion, silk As a special form of corrosion, corrosion occurs on the metal substrate under the coating, and the corrosion products grow directly from the metal substrate in the form of filaments [8], in order to avoid this and subsequent consistency of the film. The composite part needs to be simultaneously phosphatized. For the treatment of aluminum alloy and its composite parts, the most important thing is that the phosphating is affected by the concentration of free fluoride. When phosphating, the free fluoride ion has an optimal concentration. When the free fluoride ion is lower than this concentration, The phosphating film without proper film weight can also be significantly reduced in resistance; when it is higher than this concentration, its secondary adhesion will be deteriorated, because Na3AlF6 and zinc phosphate are co-deposited on the aluminum surface, affecting the phosphating film. Formation. Figure 2 is a plot of film weight versus free fluorine content when H2SiF6 is added and H2SiF6 is not added [5]:

In the zinc phosphate treatment solution, the dissolved Al3+ reacts with F- to form a complex. When Al3+ is accumulated to several tens to hundreds of ppm, there is a certain influence on the formation reaction of the film. Therefore, in order to obtain a phosphating film of good quality, it is necessary to remove excess Al3+ in the treatment liquid. The method comprises the steps of: adding sodium fluoride and potassium fluoride to react with Al3+ to form a precipitate, and the reaction formula is as follows:

Al+3F-—→AlF3

AlF3+3F-+3Na+—→Na3AlF6↓ (cryolite)

The free fluorine in the phosphating solution should be controlled to 100-600ppm, free F<100ppm, insufficient etching, difficult to form a complete film, free fluorine>600ppm, affecting the formation of phosphating film. For the phosphating of the aluminum assembly, the spraying process is easier to operate than the dipping process, the treated area of the sprayed aluminum is 80%, and the dipping line can only reach 20% to 32%. In order to ensure the phosphating effect of aluminum immersion treatment, it is necessary to strictly control the amount of free fluorine in the working fluid. Under the process conditions of the workpiece liquid, the fluorine consumption and the time period of adding free fluorine or the acid sensation are determined through experiments. The electrode controls the fluorine.

2. No nitrite phosphating:

As the oxidation promoter in phosphating, nitrite is the most convenient, most extensive, most effective and economical phosphating oxidation promoter. Nitrite is toxic, nitrogen oxides are easily generated during phosphating, polluting the environment, and sediment is more likely to block the nozzle pipe. Since nitrite is easily decomposed, it needs to be formulated into a separate component, and it is continuously added during phosphating, which brings inconvenience to use and control. Based on the above reasons, people are working hard to develop a nitrite-free phosphating process. Among the many new accelerators currently developed, hydroxylamine sulfate (HAS) is more practical, and its reaction in the phosphating process is as follows: NH3OH+ + H2 → NH + H2O. It is reported that in 1996 there were several automotive production lines in Germany using hydroxylamine sulfate [9]. Hydroxylamine sulfate can be used as a promoter alone, and the optimum dosage range is 3.2~16g/L. The phosphating film promoted by HAS is more uniform and denser than that promoted by NaNO2, and the crystal structure is columnar or granular [10]. The alkali resistance is good, which is beneficial to cathodic electrophoresis. HAS can also be used in combination with other accelerators to reduce the amount of HAS [10], such as with sodium nitrobenzene sulfonate (SNBS). The final optimum amount of HAS is 3~ when SNBS is 1.0g/L. When 3.5g/L and SNBS is 1.5g/L, the optimal use amount of HAS is 2.5~3g/L. HAS is also prone to decomposition, but its rate of decomposition is lower than that of sodium nitrite.

In the nitrite-free phosphating process, the other type is promoted by hydrogen peroxide, and the reduced product is water, which does not cause any environmental pollution. The hydrogen peroxide promoting process is more advantageous on the spray line and is suitable for galvanized sheet. However, since hydrogen peroxide is unstable in an acidic solution, it is necessary to frequently detect the accelerator and separately add a neutralizing agent.

From the above, there is no nitrite phosphating, which can completely avoid the production of nitrogen oxides. However, the acidity of the HAS process is relatively unstable, and the hydrogen peroxide process control is more troublesome, which is their deficiency in phosphating.

3. Nickel-free phosphating

In the 1970s, in order to improve the corrosion resistance of automobiles, zinc sheets began to be used as automotive body materials, and the amount of use was increasing. From the point of view of coating, its binding force to organic film is much lower than that of steel plate, so phosphating before coating is more important [11], and it is usually carried out in phosphating solution containing higher Mn2+, Ni2+, etc. In order to obtain a phosphating film which is more resistant to alkali. Due to the strict restriction on heavy metal ions in wastewater, China's current emission standard is 1mg/L, while German wastewater requires no more than 0.5ppm. Therefore, a new nickel-free phosphating process needs to be developed. Nickel can be replaced by other ions without sacrificing the comprehensive performance of phosphating [4]. The subsidiary of Bayer Automotive has established the world's first nickel-free, nitrite-free phosphating line, nickel in wastewater. The concentration is reduced to below 0.2 ppm [9]. The phosphating process uses Zn, Mn, and Cu, wherein Cu is a trace amount of HAS as a promoter to form a phosphate film. In order to prevent the formation of "white spots" during the phosphating process of the galvanized sheet, a fluorine complex is also added to the phosphating solution. VERTAK also treats aluminum with nickel-free phosphating using process parameters similar to galvanized sheets. At present, the process can meet the requirements of corrosion protection and adhesion on steel, galvanized steel and aluminum. The following are the process parameters for nickel-free phosphating:

Phosphating time 200s

Hydroxylamine Promoter 0.9~2.0g/L

Copper content 5±2ppm

Zn content 1.9±0.1g/L

Temperature 53 ° C

Total acidity 26 points

Free acidity 2.5~3.5 points

Free fluorine 150~200ppm

The promotion of copper as one of the promoters in phosphating is that the copper potential is more positive than iron and zinc, and is deposited on the substrate to form a microcathode, and the enlarged ratio of anode to anode area promotes phosphating to form a film. In this phosphating process, traces of copper replace nickel, and the innovation lies in the precise monitoring and measurement of copper. Although the principle of action of copper in this process is not known at present, it can be confirmed that copper is embedded as an oxide layer in the phosphating film [9]. The copper concentration in the phosphating solution is determined by photometric determination of 2,9-dimethyl-4,7-diphenyl 1,10-phenyl porphyrin disulfonic acid complex, and copper is added to the phosphating solution by adding copper. The concentration remains the same.

4, no chromium passivation

The passivation treatment effectively increases the performance of the phosphating film by filling the pores of the phosphating film, especially the secondary adhesion. The conventional method uses a dilute solution of chromic acid or chromate to seal, and most automobile factories in Europe and the United States use a passivator. Order, while Japan and China do not have this process [12]. Due to the carcinogenicity and toxicity of Cr6+, wastewater is difficult to handle and the cost is high. At the same time, the discharge standard of wastewater is strictly Cr6+ not more than 0.1mg/m3. Therefore, foreign countries are vigorously developing and applying chromium-free passivation.

At present, two types of passivation have been developed which have been successfully applied and have been practically applied. One of them is a zirconia inorganic passivation solution containing a promoter [13, 14, 15]. The salt exposure test and the paint adhesion test show that Performance is comparable to chromium passivation. This process has been used in ten production lines in Germany [12]. The second is the use of chromium-free organic polymer passivation solution in the mid-1980s. The Florida exposure test and the scratch test of General Motors show that the corrosion resistance is equivalent to or even higher than that of chromium-containing treatment [4]. It is also reported that 0.01 to 10 g / L (optimally 0.01 ~ 1 g / L), Ti (iv), Mn (II) or Ni (II) or 0.03 ~ 0.1 g / L Cu (II) metal 2, 4 - The pentanedione salt is blocked by titanium dioxide. The same effect can be obtained by chemical treatment of dimerized or trimerized titanium oxyacetylacetonate or copper acetate anhydride [14].

From the environmental point of view, no chromium passivation, or no passivation is the direction and trend of post-phosphorization treatment.

5. Phosphating with rare earth additives

China's rare earth reserves account for 80% of the world's total, which provides favorable conditions for the promotion of rare earth applications in China. Rare earth elements are known for their unique physical and chemical properties and their small amount of use, and are widely used in the field of materials science. The application of rare earth in the surface treatment of materials started late, but the development speed is very fast. It has been used in a series of surface treatment technologies such as electroplating, chemical heat treatment, thermal spraying, coating conversion film, metal anticorrosion, etc. [16] , has obtained huge economic benefits. The application of rare earth to phosphating treatment is still in the development stage. Foreign countries have not conducted research in this area, and there are a few reports on the application of rare earth in phosphating [17, 18, 19].

After adding the rare earth additive to the phosphating solution, the corrosion resistance of the phosphate film is improved, and the film thickness is slightly reduced [14]. The phosphating of the rare earth added during the phosphating process is a negative shift of the φ-t curve than that of the phosphating system without the addition of rare earth, which indicates that the phosphating solution added with the rare earth compound is more likely to form a phosphate film [19]. After adding a trace amount of rare earth compound to the phosphating solution,


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