Study on the thickness defect of phosphate crystallization before car coating.

Phosphate film is the basis of high-quality paint quality of the body, not only can reduce the roughness of the substrate surface, but also improve the corrosion resistance of steel and electrophoresis coating adhesion. In the pre-phosphate coating process, the table tone is the pre-phosphate process, which provides the active point of crystallization for the phosphate reaction. High-quality surface adjustment process can not only shorten the reaction time of phosphate film, but also help to form a fine and uniform phosphate crystallization

. The principle of the effect of the surface activity point distribution of the body substrate on the phosphorylation film is shown in Figure 1. When the active bits are densely distributed on the surface of the substrate, the growth space of a single phosphate crystal is small, it is easy to connect with each other to form a phosphate film, and then the growth of phosphorylation crystals is inhibited by each other, phosphorylation crystal fine, the skin film quality is light, is the ideal phosphorylation state; In this paper, through the investigation and analysis of the causes of the thickness of a phosphate crystal, the change process of surface modulation at the coating production site and its effect on phosphate crystallization are further studied.
1 Principle of the effect of the distribution of active bits on phosphorylation crystallization
1 Background introduction
A phosphate crystallization thickness defect appears in the pre-treatment of a car coating workshop (as shown in Figure 2). The results of the body phosphate film crystalline size and cort film quality analysis are shown in Table 1.
Figure 2 SEM photo of cold-rolled plate and galvanized plate phosphating film
Table 1 Crystallization size of phosphating film and leather film quality analysis results
The coating workshop uses liquid table tone matching phosphorying pre-treatment, pre-treatment process is: pre-wash (soaking) , pre-degreasing (spraying) , degreasing (soaking) , washing 1 (soaking) , washing 2 (spraying) , table tone (soaking) , phosphorying (flooding) , washing 3 (flooding) , washing 4 (spraying) , recycling pure water washing (washing)
2 Investigation and Analysis
In order to determine the factors affecting the crystallization anomalies of phosphate film, the process parameters of on-site surface modulation and phosphate fluid were investigated, and cross-matching tests were carried out on surface modulation and phosphate fluid.
2.1 Main raw materials
pre-treatment line table conditioning, phosphating liquid by Pakashi Chemical Co., Ltd. configuration, pre-treatment line with pure water from the production site homemade; 0.1mol/L EDTA (ethyl diamine tetacetic acid) solution, pH-5.5 buffer solution, 15% HCl solution, phenolic indicator, diphenol orange indicator, bromphenol blue indicator and amino sulfonate are provided by Pakatsin Essential Chemical Co., Ltd.
2.2 Main process parameter definition and measurement method
table modulation concentration: take 10mL table modulation to 200mL conical bottle, add 3 drops of 15% HCl mix evenly, add 5mLpH The buffer solution of 5.5 shakes well, then adds 4 drops of dimethol orange indicator mixed evenly, with 0.1mol/L EDTA solution for titration, the solution color from khaki to golden yellow, titration termination. For every 1mL of titration fluid consumed during the titration process, the corresponding surface modulation concentration is recorded as 1.
Phosphate total acid: take 10mL phosphate liquid to 200mL conical bottle, add 4 drops of phenolic indicator mixed evenly, with 0.1mol/L NaOH solution titration, color from colorless to light red, titration termination. For every 1mL of titration fluid consumed during the titration process, the total acid of the phosphate is recorded as 1.
Phosphate free acid: take 10mL phosphate to 200mL conical bottle, add 4 drops of bromophenol blue indicator mixed evenly, with 0.1mol/L NaOH solution titration, solution color from yellow-green to blue-purple, titration termination. For every 1mL of titration fluid consumed during the titration process, the free acid corresponding to the phosphate is recorded as 1.
Phosphate promoter concentration: take phosphate liquid into the fermentation tube, so that no bubbles in the fermentation tube produced, add 2 to 5 g amino sulfonate quickly flipped, placed 2min after reading the fermentation tube to produce gas scale value. For each 1mL gas produced in the fermentation tube, the concentration of the corresponding phosphate promoter is recorded as 1.
concentrations of phosphates are analyzed using ion chromatography (ICS-1500, Diane, USA) and inductively coupled plasma spectrometers (ICP, P-4010, Japan).
phosphate membrane shape using SEM (Hitachi SU70, Hitachi Corporation of Japan) analysis, phosphate film quality using chemical defilm analysis.
2.3 tank liquid process parameters survey
production site table modulation and phosphate process parameters survey results as shown in Table 2, table modulation and phosphate process parameters are within the management reference range.Table
Table 2 Table Tuning and Phosphate Tank Liquid Process Parameters
2.4 Table Modulation and Phosphate Cross-Matching Test
Take table modulation and phosphate solution from the production site, and at the same time, in accordance with the production site process requirements in the laboratory to carry out the cross-matching test of surface modulation and phosphate liquid, through SEM analysis of phosphate crystallization, the results are shown in Table 3.
table 3 table modulation and phosphate cross-matching test results
As can be seen from Table 3, the on-site sample of the table conditioning liquid regardless of the field sampling of phosphate liquid or with the newly added phosphate liquid, the resulting phosphate film crystalline size is 1 5 to 20 m, beyond the management benchmark (10 m), and the laboratory new surface modulation, whether with on-site sampling of phosphate or with the newly matched phosphate liquid, the resulting phosphate crystallization size meets the management benchmark. Therefore, it can be confirmed that the surface modulation used in the production site is the main cause affecting the thickness of the phosphate film crystallization of the body.
then re-matched the table conditioning tank at the production site, with a new table conditioning concentration of 1.1 and pH of 9.8, and then with the vehicle to produce a test model to analyze the crystalline size and skin film quality of the phosphate film, cold-rolled plate phosphate crystalline size 3 to 5 m, skin film quality 2.6g/m2, galvanized plate phosphating crystallization size 4 to 8 m, skin film quality 3.2g/m2, all meet the management benchmark (base value see Table 1), phosphating crystallization quality returned to normal.
3 Principle Inquiry
After the above investigation, although it was found that the surface modulation is the cause of thick phosphorylation crystallization, but the process management parameters (pH and concentration) of the on-site surface modulation are normal, so it is necessary to further explore the principle of this phenomenon to improve the stability of the quality of products on the coating production site.
the coating production site using liquid table tone, the main component of its active particles is zinc phosphate particles, its structure is shown in Figure 3. The surface of zinc phosphate particles is covered with a layer of organic polymer with negative charge, and in normal condition zinc phosphate particles are dispersed from each other in the surface modulation, and when the body passes through the surface modulation, the zinc phosphate particles are evenly adsorbed at the surface of the steel plate to form the active point of phosphate crystallization. Therefore, in the liquid surface adjustment material system, the particle size control of zinc phosphate particles has a direct impact on the membrane state of phosphate. Figure
3 A diagram of the structure of liquid surface-adjusted active particles and a model of surface-toned reactions
combined with the phenomenon of thick phosphorylation crystallization, the principle of table-toneing and the structural characteristics of table-adjusted active particles, it is guessed that the dispersion of zinc phosphate particles after a period of time is used in the surface fluid changes to make the phosphorylation crystallization coarser, for example, zinc phosphate particles mutually aggregate to make the density of active particles in the surface fluid smaller. In order to verify this hypothesis, a qualitative test of effective zinc phosphate particles in table conditioning is designed by:
(1) starting from the new slot, synchronous detection table modulation (sampling directly from the field table transfer) and the concentration of table modulation filter fluid, detection frequency 1 time/week, the test results are shown in Figure 4. Table-conditioning filter fluid is filtered by intercepting filter paper with a particle size of 1 m (filter paper brand: ADVANTECFILTERPAPERQUANTITATIVEASHLESS quantitative filter paper, model: 5C).
Figure 4 Table modulation and table-conditioning filter fluid concentration varies with the time of use
From Figure 4, it can be known that the concentration of table-leveling fluid is the same as the concentration of table-conditioning filter fluid, but with the time of use, the concentration of table-conditioning filter fluid remains stable, to the 4th week when the concentration of table-conditioning filter fluid is only 0.5, below the baseline of table-toned concentration management .8.
(2) each time the detection table modulation concentration with the car to make a test model (cold-rolled plate) to phosphorylation crystallization shape and size analysis, the test results are shown in Figure 5.
Figure 5 Phosphate crystallization profile and size with the change in the use time of the table modulation
From Figure 5, it can be known that with the use time of the surface modulation, the phosphate crystallization size gradually increased, to the 4th week phosphation crystallization size is 15 m, higher than the phosphate crystallization size management benchmark upper limit (10 m), indicating that the distribution of surface active bits of the pre-phosphate steel plate substrate from dense to sparse, and then indicating that the content of effective zinc phosphate particles in the surface modulation is gradually decreasing.
combined with the analysis of the test results of Figures 4 and 5, it can be found that with the time of use of table modulation, the increase of phosphate crystallization size and the decrease of the concentration of surface modulation filter fluid show a more consistent correspondence, while the stable concentration of table modulation is not There is a correspondence, so the effective content of zinc phosphate particles in the corresponding surface fluid can be qualitatively measured qualitatively by the concentration of the corresponding surface filtration fluid, and the lower the concentration of the phosphate filter fluid, the lower the effective content of zinc phosphate particles in the corresponding surface fluid.
the state of the meter conditioning during use and the principle of reducing the concentration of filter fluid after filtration is shown in Figure 6.
Figure 6 Table conditioning state changes and filtration principle
Combined with Figure 6 analysis, the table modulation of the new slot zinc phosphate particles evenly dispersed, with the use of time, the table conditioning gradually aging, such as the body carrying the pre-process of the work water into the table conditioning tank so that Ca2, Mg2 plus ions accumulated in the table conditioning fluid, Ca2, Mg2 plus ions. The negative charge on the surface of some zinc phosphate particles reduces the electrostitive rebuke force between them, the zinc phosphate particles are converted from a uniformly dispersed state into a partial particle mutual cluster, when the surface fluid is filtered, these clusters of zinc phosphate particles are intercepted on the filter paper, the effective zinc phosphate particle content in the surface filtration liquid decreased, so the concentration of the synchronously detected phosphate filter liquid is lower than the table conditioning fluid concentration.
Based on the above analysis, the series of changes caused by the use of table conditioning process in the production site can be summarized as follows: table conditioning from the new distribution to the use of aging, due to the accumulation of zinc phosphate particles so that the concentration of effective zinc phosphate particles in the surface of the surface of the modulation decreased, when the body through the surface of the surface of the steel plate substrate attachment of zinc phosphate active sites also from dense to thin, and thus make the surface of the body from phosphate crystallization to thickness. The process parameters of on-site surface modulation are mainly tested daily pH and concentration, where the table-toned concentration can measure the total content of active ingredients in table conditioning, but it does not reflect the dispersion of active particles, so there is the above mentioned "on-site surface modulation process management parameters are normal, but there is a phenomenon of thick phosphate crystallization
4 Production site process parameters testing recommendations
In order to monitor the crystallization status of phosphate, coating production site usually in accordance with a certain period (such as 1 time / month) with the car production test model for SEM analysis of phosphate film, in the case of stable production site this way can meet the process personnel on the quality of phosphate film monitoring needs. However, when the production site sudden anomalies rely solely on SEM photo analysis there is a certain degree of hysteresis, because from the field plate making, sample delivery to test results often take a few days, which can not meet the production site needs to quickly pre-judge the quality of high-time requirements, at this time in the production site if you can directly through the tank liquid state to assess the quality of phosphate film is more time-efficient. As far as the liquid surface tone described in this paper is concerned, the effective qualitative test of zinc phosphate particles designed in this paper can be added on the basis of the daily detection table modulation pH and concentration, the relevant data of the concentration of the surface modulation filter fluid and the phosphate crystallization size can be accumulated, the concentration range to be satisfied with the production site table conditioning filter fluid, and the state of phosphate crystallization can be qualitatively predicted by the concentration of the table conditioning filter fluid, and then the need to update the surface modulation fluid.
5 Conclusion
Automotive coating pre-treatment for automated assembly line, with high processing efficiency and non-reworkable characteristics, which is the production site when the cause of abnormal quality analysis and countermeasures to develop a high timeliness requirements, to minimize the sudden quality anomalies caused by the production stop line and subsequent quality risk. Therefore, on the basis of solving the field problems, the coating process technicians also need to understand the principle of the problem, and constantly accumulate the process data on the site, improve the analysis efficiency of the field problems. Coating production site on the process parameters management can be combined with material suppliers to improve testing methods, from state monitoring to quality prevention direction, reduce the frequency of quality defects and defective product outflow, for the production site quality efficiency.
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