Study on coarse thickness defects of phosphating c

Research on coarse thickness defects of phosphating crystals in automobile coating pretreatment

Research on coarse thickness defects of phosphating crystals in automobile coating pretreatment

September 16, 2020

phosphating film is the basis of high-quality paint quality of car body, which can not only reduce the roughness of substrate surface, but also improve the corrosion resistance of steel and the adhesion of electrophoretic coating. In the pretreatment process of phosphating coating, surface modification is the pre process of phosphating, which provides crystallization active sites for phosphating reaction. High quality surface modification technology can not only shorten the reaction time of phosphating film, but also help to form a fine and uniform phosphating crystalline coating. The principle of the influence of the distribution of active sites on the surface of the body substrate on the phosphating film formation is shown in Figure 1. When the active sites are densely distributed on the surface of the substrate, the growth space of a single phosphating crystal is small, and it is easy to connect with each other to form a phosphating film. Then the growth of phosphating crystals is mutually inhibited. The phosphating crystal is fine and the film is light in weight, which is an ideal phosphating state; When the active sites are sparsely distributed on the surface of the substrate, the space for the growth of a single phosphating crystal is large, and the generated phosphating crystal is thick and sparse, which is not an ideal phosphating state. Through the investigation and analysis of the reasons for the coarseness of primary phosphating crystallization, this paper further studies the change process of surface regulating liquid in the coating production site and its influence on Phosphating crystallization

Figure 1 effect principle of surface active site distribution on Phosphating crystallization

1 background introduction

coarse thickness defects of phosphating crystallization occurred in the pretreatment of an automobile coating workshop (as shown in Figure 2). The analysis results of crystal size and film quality of body phosphating film are shown in Table 1

Fig. 2 SEM photos of phosphating film on cold-rolled and galvanized plates

Table 1 analysis results of phosphating film crystal size and film quality

it can be seen from table 1 that phosphating crystal size and film quality exceed the management benchmark, which needs to be investigated and analyzed. The coating workshop adopts liquid surface conditioning supporting phosphating pretreatment. The pretreatment process flow is: pre washing (immersion) → pre degreasing (spraying) → degreasing (immersion) → water washing 1 (immersion) → water washing 2 (spraying) → surface conditioning (immersion) → phosphating (immersion) → water washing 3 (immersion) → water washing 4 (spraying) → circulating pure water washing (immersion) → fresh pure water washing (spraying)

2 investigation and analysis

in order to determine the influencing factors of abnormal crystallization of phosphating film, the process parameters of on-site surface regulating solution and phosphating solution were investigated, and the cross matching tests of surface regulating solution and phosphating solution were carried out

2.1 main raw materials

the surface blending solution and phosphating solution of the pretreatment line are configured by pakase Fine Chemical Co., Ltd., and the pure water used in the pretreatment line is self-made by the production site; Cold rolled steel plate and galvanized steel plate are body plates, which are provided by Wuhan Iron and steel company; The 0.1mol/l NaOH solution, 0.1mol/l EDTA (ethylenediamine tetraacetic acid) solution, ph=5.5 buffer solution, 15% HCl solution, phenolphthalein indicator, xylenol orange indicator, bromophenol blue indicator and sulfamic acid used for analysis are all provided by pakase Fine Chemical Co., Ltd

2.2 definition of main process parameters and measurement methods

Table blending concentration: take 10ml table blending solution into a 200ml conical flask, add 3 drops of 15% HCl, mix evenly, add 5ml of buffer solution with ph=5.5, shake well, then add 4 drops of xylenol orange indicator, mix evenly, titrate with 0.1mol/l EDTA solution, the color of the solution changes from earthy yellow to golden yellow, and the titration ends. For every 1ml of titrant consumed in the titration process, the concentration of the solution in the corresponding table is recorded as 1

total acid of phosphating solution: take 10ml phosphating solution into 200ml conical flask, add 4 drops of phenolphthalein indicator, mix evenly, titrate with 0.1mol/l NaOH solution, the color changes from colorless to light red, and the titration is terminated. For every 1ml of titrant consumed in the titration process, the total acid of the corresponding phosphating solution is recorded as 1

free acid of phosphating solution: take 10ml phosphating solution into 200ml conical flask, add 4 drops of bromophenol blue indicator, mix evenly, titrate with 0.1mol/l NaOH solution, the color of the solution changes from yellow green to blue purple, and the titration ends. For every 1ml of titrant consumed in the titration process, the free acid of the corresponding phosphating solution is recorded as 1

concentration of phosphating accelerator: take the phosphating solution into the fermentation tube, so that there is no bubble in the fermentation tube. Add 2~5g sulfamic acid and turn it over quickly. After placing it for 2min, read the scale value of the gas generated in the fermentation tube by liquid drainage method. For every 1ml of gas generated in the fermentation tube, the concentration of the corresponding phosphating accelerator is recorded as 1

the ion concentration of phosphating solution was analyzed by ion chromatograph (ICS-1500, Diane, USA) and inductively coupled plasma spectrometer (ICP, P-4010, Hitachi, Japan)

the morphology of phosphating film was analyzed by SEM (Hitachi su70, Hitachi), and the quality of phosphating film was analyzed by chemical film removal method

2.3 investigation of process parameters of bath liquid

the investigation results of process parameters of surface mixing liquid and phosphating liquid at the production site are shown in Table 2. The process parameters of surface mixing liquid and phosphating liquid are within the scope of management benchmark

Table 2 process parameters of surface adjustment and phosphating bath solution

2.4 cross matching test of surface adjustment and phosphating solution

take surface adjustment and phosphating solution from the production site, and prepare new surface adjustment and phosphating solution in the laboratory at the same time. Carry out cross matching test of surface adjustment and phosphating solution in the laboratory according to the process requirements of the production site. Analyze the phosphating crystal morphology by SEM, and the results are shown in Table 3

Table 3. Results of cross matching test of mixing solution and phosphating solution

it can be seen from table 3 that the phosphating film crystal size formed by the surface mixing solution sampled on site is 15~20 whether it is used with the phosphating solution sampled on site or with the newly prepared phosphating solution μ m. Exceed Management Benchmark (10 μ m）; The phosphating crystal size formed by the newly prepared surface mixing solution in the laboratory meets the management benchmark whether it is used with the phosphating solution sampled on site or with the phosphating solution newly prepared for the plastic packaging market. Therefore, it can be confirmed that the surface mixing liquid used in the production site is the main factor affecting the crystallization thickness of the body phosphating film

then re prepare the surface mixing solution tank at the production site. The concentration of the newly prepared surface mixing solution is 1.1 and the pH is 9.8. Then make a test sample with the car to analyze the crystal size and film quality of the phosphating film. The phosphating crystal size of the cold-rolled plate is 3~5 μ m. The coating quality is 2.6g/m2, and the phosphating crystal size of galvanized sheet is 4~8 μ m. The film quality was 3.2g/m2, which met the management benchmark (see Table 1 for the benchmark value), and the phosphating crystal quality returned to normal

3 Principle Exploration

through the above investigation, although it is found that surface blending solution is the main cause of phosphating crystal coarseness, the process management parameters (pH and concentration) of surface blending solution on site are normal, so it is necessary to further explore the principle of this phenomenon, so as to improve the stability of product quality on the production site of coating

the coating production site uses liquid surface conditioning. The main component of its active particles is zinc phosphate particles, and its structure is shown in Figure 3. The surface of zinc phosphate particles is wrapped with a layer of negatively charged organic polymer. Under normal conditions, zinc phosphate particles are dispersed in the surface regulating solution. When the vehicle body passes the surface regulating solution, zinc phosphate particles are evenly adsorbed on the surface of the steel plate to form the active site of phosphating crystallization. Therefore, in the liquid surface modification material system, the particle size control of zinc phosphate particles has a direct impact on the phosphating film-forming state

Figure 3 Schematic diagram of liquid surface modified active particle structure and surface modified reaction model

combined with the phenomenon of coarse thickness of phosphating crystal, the action principle of surface modified active particles and the structural characteristics of surface modified active particles, it is speculated that the dispersion state of zinc phosphate particles changes after the surface modified solution is used for a period of time, which makes the phosphating crystal coarser. For example, the mutual aggregation of zinc phosphate particles makes the density of active particles in the surface modified solution smaller. In order to verify this conjecture, the qualitative detection test of effective zinc phosphate particles in surface regulating liquid is designed. The specific method is:

(1) starting from the new tank, the concentration of surface regulating liquid (directly sampled from the on-site surface regulating tank) and surface regulating filter liquid are synchronously detected, and the detection frequency is once a week. The detection results are shown in Figure 4. The surface adjusted filtrate passes through the intercepted particle size of 1 μ M filter paper (filter paper brand: advantecfilterpaperquantitativeathless quantitative filter paper, model: 5C)

Fig. 4 Variation of the concentration of surface conditioning solution and surface conditioning filter solution with the use time

it can be seen from Fig. 4 that the concentration of surface conditioning solution in the newly prepared state is the same as that of surface conditioning filter solution, but with the passage of use time, when the concentration of surface conditioning solution remains stable, the concentration of surface conditioning filter solution continues to decline, and by the fourth week, the concentration of surface conditioning filter solution is only 0.5, lower than the lower limit of surface conditioning concentration management benchmark (≥ 0.8)

(2) each time the concentration of liquid in the meter is detected, a test sample (cold rolled plate) is made with the car to analyze the morphology and size of phosphating crystal. The test results are shown in Figure 5

Fig. 5 changes of phosphating crystal morphology and size with the use time of surface mixing solution

it can be seen from Fig. 5 that with the passage of surface mixing solution use time, the phosphating crystal size gradually increases. It is necessary to select the injection molding machine length equivalent to the main product capacity of injection molding, and the phosphating crystal size is 15 by the fourth week μ m. Higher than the upper limit of phosphating crystal size management benchmark (10 μ m) , indicating that the distribution of active sites on the surface of steel substrate before phosphating is from dense to sparse, which further indicates that the content of effective zinc phosphate particles in surface conditioning solution is gradually decreasing

combined with the analysis of the test results in Fig. 4 and Fig. 5, it can be found that with the passage of the service time of the surface conditioning solution, the growth of phosphating crystal size shows a relatively consistent corresponding relationship with the decrease of the concentration of the surface conditioning filter solution, but not with the stable concentration of the surface conditioning solution. Therefore, the concentration of the surface conditioning filter solution can be used to qualitatively measure the effective content of zinc phosphate particles in the corresponding surface conditioning solution. The lower the concentration of the surface conditioning filter solution, It indicates that the lower the effective content of zinc phosphate particles in the corresponding surface conditioning solution

the principle of the change of the state of the surface blending solution during use and the reduction of the filtrate concentration after filtration is shown in Figure 6

Figure 6. The state change of surface regulating liquid and the principle of filtration function

according to the analysis of Figure 6, the zinc phosphate particles are evenly dispersed when the surface regulating liquid is newly prepared in the tank. With the passage of time, the surface regulating liquid gradually ages. For example, the working water of the previous process carried by the car body enters the surface regulating tank, so that ca2+ and mg2+ ions accumulate in the surface regulating liquid, and ca2+ and mg2+ ions neutralize the negative charge on the surface of some zinc phosphate particles, so that the electrostatic repulsion between them decreases, The zinc phosphate particles are transformed from a uniformly dispersed state into some particles agglomerating with each other. When the surface conditioning solution is filtered, the zinc phosphate particles of these agglomerates are intercepted on the filter paper, and the content of effective zinc phosphate particles in the surface conditioning filtrate decreases. Therefore, the concentration of the surface conditioning filtrate detected synchronously is lower than that of the surface conditioning solution

based on the above analysis, the series of changes caused by the use of surface regulating liquid in the production site can be summarized as follows: surface regulating liquid from new preparation to aging, due to the accumulation of zinc phosphate particles, the concentration of effective zinc phosphate particles in surface regulating liquid decreases, and when the body passes through the surface regulating liquid, the zinc phosphate active sites attached to the surface of steel plate base material also change from dense to sparse, thus making the phosphating crystals on the body surface from fine to thick. The process parameters of on-site surface regulating liquid are mainly tested for pH and concentration. The surface regulating concentration can measure the total content of active ingredients in the surface regulating liquid, but it can not reflect the dispersion state of active particles, Therefore, the above-mentioned "process management parameters of on-site surface mixing solution are produced. When it is easy to process, bond and has a larger net surface area, the phenomenon of phosphating crystal coarseness appears"

4 suggestions for the detection of process parameters on the production site

in order to monitor the phosphating crystal state, paint the production site