Study on white-point defects of phosphate film for cold-rolled plate treatment before coating.

Automotive coating is one of the four major processes of vehicle manufacturing, which provides beautiful appearance and strong corrosion resistance for the body, and has an important role in the commerciality and durability of automobiles. Pre-coating treatment is the first portal of automobile coating, pre-treatment mainly includes: removing all kinds of dirt on the surface of the white body, such as oil, welding slag, dust, etc.; At present, the most widely used pre-treatment process in the industry is the traditional pre-phosphate treatment, which can improve the adhesion and corrosion resistance of the coating

. Therefore, the quality of phosphate film plays an important role in the performance of coating film. Pre-coating treatment has the characteristics of long process, many parameters control, strict reaction time, high degree of automation and non-reworkability. In order to achieve the goal of white body after pre-treatment surface clean and form high-quality phosphate film, not only requires strict management of the site to maintain stable process conditions, but also requires process personnel to pre-treatment of quality defects can be quickly resolved and measured. In this paper, a body cold-rolled plate phosphate film surface "white dot" defects, analysis of the phosphate film "white dot" defects occurred, after the successful elimination of this defect, summarized the solution to such problems.
1
background introduction
a car coating workshop pre-processing process occurred body phosphate film "white dot" defects, the coating workshop's pre-treatment process as shown in Figure 1, along the body into the direction of: pre-wash (immersion), pre-skimming (pre-fat (Spraying), degreasing (soaking), washing 1 (flooding), washing 2 (spraying), table tone (soaking), phosphate (flooding), washing 3 (flooding), washing 4 (spraying), recycling pure water washing (flooding), fresh pure water washing (spraying).
Figure 1 Pre-treatment process schematic
The white body plate processed by the pre-processing process consists of a mixture of cold-rolled plate and galvanized plate, the white body after pre-treatment, the surface of the phosphating film on the cold-rolled plate occurred "white dot" defects, affecting the quality of the paint film after the process. Generally speaking, the "white dot" on the pre-treatment plate occurs more commonly on the galvanized plate (the zinc layer rust on the galvanized plate produces white rust), but this time the phosphating film "white dot" defect only occurs on the cold-rolled plate.
2
Status Survey
In order to recognize the current situation of white spots occurring on the phosphate film on the surface of the body, the distribution and form of the body parts, the distribution and morphology of the white spots of the body, the process in which the white spots occurred, the parameters of the pre-processing process, the operating status of the equipment and so on.
2.1 Body phosphating film white dot distribution and morphological survey
through statistical analysis of body phosphating film white dot, the results show in Figure 2, the form of phosphating film white dot on the cold-rolled plate is "snowflake-like", distributed in the hood, roof, beam, tailgate, rear side circumference and other parts. There is no such phenomenon in galvanized plate parts such as four doors and wings.
Figure 2 body phosphate film white dot statistics
2.2 before the processing of white dot occurrence process investigation
in order to determine the white point occurred in the specific process of pre-processing, from the direction of the white body into the line, the body after each process to observe the state, the results are shown in Table 1.
Table 1 before the processing of white dot occurrence process investigation
from Table 1 can be found that the tailgate, rear side circumference and other cold-rolled plate area after the skimming process, before the table adjustment process, under the exposure of a strong photocliner, visually there is a certain degree of dot, phosphating process after the performance of white dot.
the survey of the fluid parameters of each tank before 2.3
the groove fluid parameters of the pre-treatment process were investigated, and the results were found in Table 2 compared with the differences between the measured values and the process management benchmark.
table 2 pre-treatment tank fluid parameters survey
from Table 2, it can be known that the table conditioning and phosphate process parameters are within the management range, the free alkalinity and temperature of the degreasing tank fluid is upper limit. The pH of the skimming fluid was then measured at 12.3, which is a risk of alkali corrosion on the plate and may cause the body phosphate film to produce white spots.
2.4 white body parts and pre-processing equipment status investigation
the white body parts investigation, no visual abnormalities. The equipment of the pre-treatment line is investigated, the circulation system of each process, the temperature control system, the drain/inlet spray, the humidification spray, the level control system, the dosing system are normal, the exhaust fan is also working normally, the air volume of the exhaust air is within the process management range.
Through the status quo investigation can be known: the pre-treatment line equipment and tank fluid parameters are normal (only the free alkali of the degreasing fluid and the upper limit of the temperature bias management benchmark), phosphate film white dots are concentrated in the body cold rolling plate, wherein the phosphated membrane white dot defects from the white body into the pre-treatment to the degreasing water wash before this process began to produce. Therefore, the degreasing fluid parameters are debugged to confirm whether they have an effect on the white spots of the phosphated membrane. Further, the white-point defects of phosphated membrane are reproduced on the test board, and their appearance and composition are analyzed to find out the causes and solutions of the defects.
3
cause analysis and verification
3.1 main raw materials
sodium nitrite phosphating promoter, degreasing agent A: Pakashi Chemical Co., Ltd.; cold-rolled steel plate: body plate, Wuhan Iron and Steel Company; pre-treatment line degreasing liquid, table conditioning fluid, phosphid liquid: Pakatsu Fine Chemical Co., Ltd.; pure water for pre-treatment line: homemade;
2
SO
4
solution, 0.1mol/L NaOH solution, 0.1mol/L EDTA solution, pH-5.5 buffer solution, 15% HCl solution, phenolic indicator, diphenol orange indicator, bromphenol blue indicator and aminocarbons: Pakaminated Chemical Co., Ltd.
3.2 Verification Method
Pre-treatment white dot defect plate reproduction test method: before the body enters the coating pre-processing line, the same cold-rolled plate will be hung on the body with wire, and the body synchronously through the pre-treatment. Subsequently, the cold-rolled plate with phosphate film attached to the pre-treatment line is removed as a test model, and the white-point defect of the phosphated film on the test plate is then SEMHitachi SU-70, Hitachi Corporation of Japan and the scanning electroscope energy spectrum analysis oxford Max50, United Kingdom.
the free alkalinity of the degreasing fluid is adjusted by adding the amount of the skimming agent A, and the temperature of the skimmed fluid is adjusted through the temperature control system of the tank fluid.
Coating pre-treatment main parameter definition and measurement method:
free alkalinity: take 10mL test sample liquid to 200mL conical bottle, add 4 drops of phenolic indicator mixed evenly, with 0.05mol/L H2SO4 titration, solution color from light red to colorless, titration termination. For every 1mL of titration fluid consumed during the titration process, the free alkalinity of the test sample fluid is recorded as 1.
table tone concentration: take 10mL table conditioning to 200mL conical bottle, add 3 drops of 15% HCl mix evenly, then add 5mLpH to 5.5 buffer solution shake well, and then add 4 drops of diphenol orange indicator mixed evenly, with 0.1mol/L EDTA solution for titration, solution color from earthy yellow to golden yellow, titration termination. For every 1mL of titration fluid consumed during the titration process, the corresponding table modulation concentration is recorded as 1.
total acid of phosphate: take 10mL phosphate to 200mL conical bottle, add 4 drops of phenolic indicator mixed evenly, with 0.1mol/L NaOH solution for 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 liquid 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 phosphate free acid 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 after rapid flip, placed 2min after observation read the gas scale in the fermentation tube. A reading of the gas scale in the fermentation tube indicates the concentration of phosphate promoter.
3.3 Degreasing tank fluid parameter adjustment
to the degreasing fluid free alkalinity and degreasing temperature will have an impact on the body phosphating white dot, in the process management range to gradually reduce the free alkalinity and degreasing temperature of the degreasing tank, the results are shown in Table 3.
Table 3 test results on the effect of the adjustment of the lipid parameters on white spots
As can be seen from Table 3, when the free alkalinity is reduced from 15.3 to 13.2 (corresponding to pH 11.6), or the temperature of the degreasing fluid is reduced from 50 degrees C to 46 degrees C, there is no improvement in the white dot defects of the phosphated membrane of the body.
3.4 pre-treatment white dot defect hanging plate reproduction and analysis
for the front treatment of white dot defects, in the body roof trailer with the car into the line test, cold-rolled plate surface reproduction and body the same snowflake white dot defects. SEM and scanning electroscopic energy spectrometer analysis of cold-rolled plates that reproduce white-dot defects in phosphated membranes, the results are shown in Figure 3.
figure 3 cold-rolled plate phosphate film white dot site SEM map and white dot area of the local amplification map
from Figure 3, it can be seen that (b) point C is the white dot defect position, did not see phosphate film. The scanning electroscopic energy spectrum analysis was made on point A, point B and point C with phosphate film crystallization in Figure 3 (b), and the results were shown in Figure 4.
Figure 4 phosphate film white dot defect model A, B, C point scanning electroscope energy spectrum analysis
From Figure 4, it can be seen that, relative to A point and B point, the main components of C point are iron and carbon elements, for the cold-rolled plate substrate itself components, no phosphorus, zinc, manganese and other components of phosphate film, indicating that the cold-rolled plate surface of the phosphate film white dot defect area almost no phosphorus film cover.
The C-point that does not cover the phosphate film may be considered white because: the color of the normal phosphate film is silver gray, if the cold-rolled plate after phosphation there is no phosphate film attached to the blank area, then this area shows the metallic luster of the steel plate itself, relative to the silver-gray phosphate film is brighter, in the light as a white dot. Further analysis, the test board C point does not have phosphate film, indicating that C point C in the phosphate tank did not carry out a normal phosphation crystallization reaction. It is possible that the surface of point C is attached to other substances before entering the phosphate tank, which prevents or delays the phosphation crystallization reaction of the substrate surface in the phosphate tank. In the normal phosphation reaction, under the effect of the immersion reaction (1) and the promoter (2), the H-plus in the microencrystic environment of the steel plate surface is quickly consumed, which causes the hydrolytic balance reaction of phosphoric acid to move to the right and dissodiate po
4
3-
ions, when the concentration of PO
4
3-
and Zn
2 plus
, Fe
2 plus
metal ions reaches Zn
3
(PO
4
)
2
.4H
2
O and Zn
2
Fe (PO
4
)
2
.4H
2
O Ksp solubility constant, a phosphate crystal is generated attached to the surface of the steel plate to form a phosphate film .4, type (5). Combined with previous experience, the attachment to the surface of point C may be iron oxide, in the acidic phosphate liquid, the surface of the steel plate will first carry out ferric acid oxide reaction ( type (6) ) , delaying the normal phosphation reaction. In order to verify this hypothesis, the rust-proof test of the pre-treatment washing process was carried out below.
Normal phosphating reaction type
3.5 anti-rust verification and analysis between washing processes
In order to verify whether the iron oxide generated by rust occurring between the front treatment process of the body plate is the cause of the white dot of phosphating film, the inter-process rust test was designed for pre-wash, degreasing after washing 1 and washing 2. Add sodium nitrite (indicated by the concentration of the added amount of promoter) and degreaser A (the main component is sodium hydroxide, the addition of which is expressed by free alkalinity) respectively to improve the rust resistance of the washing process, the test results are shown in Table 4.
From Table 4, it can be seen that adding a certain amount of degreasing agent A to the white dot of phosphate film on cold-rolled plate has an improved effect, when the free alkalinity in pre-washed ≥0.8 can inhibit the whitening point of phosphate film on cold-rolled plate, while the free alkalinity ≤0.6 is not obvious. The inhibition effect of sodium nitrite on white spots of phosphidized membrane alone is not as good as that of degreaser A.
Table 4 water washing process rust control on phosphating film white dot impact test results
According to the test results, to the pre-water wash liquid added degreasing agent A, so that the free alkalinity of 0.8 to 1.0, body roof, beams, tailgates and other cold-rolled plate parts of the phosphating film white dot disappeared. In order to further verify the inhibition effect of adding a degreaser A to the pre-water sink to the white dot of the phosphating film, replace the pre-water sink with fresh water during the workshop weekend break without adding the degreasing agent A. At monday's production, the white spot of the phosphate film at the body cold-rolled plate site was found to be reproduced and consistent with the pre-adjustment. Subsequently, the addition of degreasing agent A to the pre-water sink makes the free alkalinity of 1 (corresponding to pH 10.5), the body roof, beams, tailgates and other cold-rolled plate parts of the phosphating film white spots disappear again. It is explained that the anti-rust treatment between the washing process has obvious improvement effect on the white dot defect of phosphating film on the body cold-rolled plate, and the previous guess is verified.
addition of degreaser A to the pre-water wash tank can make the free alkali content reach a certain concentration, can play a rust-proof effect on the cold-rolled plate. The reasons for the analysis may be: cold-rolled steel plate in the pre-wash this high temperature and high humidity environment through the original battery principle is prone to oxygen absorption corrosion. When the degreaser A is added to the pre-wash to make the aqueous solution reach a certain alkalinity, the OH-can inhibit the positive reaction in the solution (8) to move to the right, slowing down the rate of the reduction reaction of O
2
, so that the flow of electricity in the positive and negative circuit of the original battery is reduced, thereby slowing down the rust rate of the Fe negative pole.
iron oxygen absorption corrosion reaction type:
4
countermeasure implementation and effect confirmation
for the cold-rolled plate of phosphate film white dot defects, in the pre-water wash tank to add degreasing agent A, so that the free alkalinity remains at 0.8 to 1.0 (corresponding pH of 10.3 to 10.5).
the implementation of countermeasures, on-site tracking confirmed 2 months, body cold-rolled plate phosphate film white.