Application of film pretreatment technology in automotive coating

Phosphating treatment due to its mature and stable characteristics, has become the most widely used pre-coating technology, but because the phosphating liquid contains zinc, nickel, manganese and other heavy metal ions, as well as phosphate and sodium nitrite and other substances that are restricted from discharge, and the treatment temperature is higher, the harmless transformation process of wastewater and waste residue is more complex, etc.

1 Reaction mechanism of thin film pretreatment technology

Film pretreatment technology is mainly divided into two categories: zirconium thin film technology and silane thin film technology

Figure 1 Chemical film formation process of zirconia

The main component of zirconium films is fluorozirconate, and its reaction mechanism is mainly the chemical film-forming process of zirconia, as shown in

Fig.

After hydrolysis, condensation, chemical bonding and crosslinking, Si-O-Me covalent bonds

In recent years, due to increasingly stringent environmental protection requirements, both zirconium films and silane films have been promoted and applied

The combination of inorganic and organic is the development trend of thin film technology, such as the ultra-thin organic coating composed of zirconia/zirconium hydrate and silane lattice in silane film adsorbed on the metal surface, forming organic (Si-O-Me) and inorganic (Zr-O-Me) composite coatings on the metal surface, as shown in

Fig.

2 Point of difference analysis between thin film technology and phosphating technology

2.
1 Advantages of thin film technology

The advantages of thin-film technology are clear and are briefly summarized below:

(1) More environmentally friendly, no harmful heavy metals, no phosphorus, no nickel, low slag, less wastewater discharge, easy to
treat.

(2) Simplify the process, cancel the table blending and passivation process, simplify the process flow, and reduce investment costs
.

(3) Energy saving, the process temperature is generally 25 ~ 45 ° C, in addition to winter or long-term suspension of production, can be basically not heated, thereby reducing energy consumption
.
Electrical energy
can also be saved due to the reduction of pumps and the reduction of power in the circulation system of the thin film treatment tank.

2.
2 Difficulties of thin film technology

Thin film technology has unparalleled advantages in environmental protection and energy saving, but compared with phosphating, there are technical difficulties caused by the following differences:

(1) Covering ability
.

The film thickness of the phosphating layer is 1 to 2 μm, and the thin film layer is a nanoscale conversion film, and its film thickness is less than 1/10 of the phosphating layer, so the film process is more sensitive
to defects such as body-in-white polishing, mimeography, and stamping indentation.

(2) Adhesion
.

Due to the characteristics of the film nanoscale film thickness, so the anti-pollution ability is poor, if the oil is too much degreasing is not clean, the film can not be formed locally after the film treatment or the film is incomplete, which will affect the surface quality of the conversion film, and even affect the electrophoretic adhesion
.

Since the amount of erosion of the plate by thin film treatment is much smaller than that of phosphating, how to avoid the poor adhesion of electrophoresis due to the influence of alkali erosion and vapor erosion of the plate is a technical difficulty
that needs special attention in the film process.

(3) Process and equipment control
.

(a) The primary focus in the application of the film process is degreasing
.
Since phosphating is carried out at higher temperatures (about 45 °C) and lower pH (pH = about 3.
2), dirt and grease on the body surface can be further cleaned and removed
at this temperature and pH.
However, the process conditions of film treatment are very mild (generally 25 ~ 45 ° C, pH = about 4.
5), which basically does not have the possibility of
further cleaning.
The film forming after film treatment is very thin, and very little surface grease will also affect the binding force after film formation, thereby greatly reducing the anti-corrosion ability
after coating.
This means that the film process has higher
requirements for degreasing.

(b) The water washing process also has a great impact on the quality of
film treatment.
After degreasing and cleaning, the workpiece is easy to rust, especially the problem of cold-rolled plate is relatively large, and the corrosion inhibitor can be used in the washing process, which is effective
in preventing rust.

(c) The requirements for equipment on the thin film process are also relatively high
.
Equipment failure should be avoided as much as possible and lead to stalling of the conveyor chain, if it is stopped in the degreasing area and the washed area after degreasing, it is easy to cause corrosion of the plate; If it is stopped in the film treatment tank, the film weight will continue to increase, exceeding the control range will affect the adhesion of the film, and if the adhesion is not up to standard, it must be scrapped
.

(4) Matching
with electrophoretic paint.

As a nano-scale conversion film, the film has a greater impact on the conductivity of electrophoretic coating, and the swim permeability and adhesion of electrophoresis will be affected
.
Since the thickness of the thin film layer is significantly lower than the thickness of the phosphating layer, the resistance of the thin film layer is significantly lower than that of the phosphating layer, so the surface film forming thickness of cathodic electrophoresis will also be thicker
.
The effective voltage of the inner cavity of the body will decrease due to the Faraday effect, so electrophoresis will be difficult to apply a paint film
layer to the surface of the inner cavity.
When selecting a film process, it is necessary to consider whether it is compatible with the electrophoretic paint, and the matching of the film process and electrophoresis needs to go through multiple certifications and salt spray tests in the laboratory and on-site to select the appropriate electrophoresis material
.
Figures 4 and 5 show the matching
of the film process with two electrophoretic paints.

Fig.
4 Comparison of outer surface electrophoresis film thickness

Fig.
5 Electrophoretic roughness comparison

As can be seen from Figure 4 and Figure 5, the film process and the 2# electrophoresis paint are poorly matched, the roughness of local electrophoresis is as high as 0.
5 to 0.
6 μm, and the film thickness uniformity is poor, some electrophoresis films are as thick as 35 to 40 μm (including galvanized layer), and some electrophoresis film thickness is not up to standard
.

(5) Requirements for the surface of the body of white
.

Since the film layer is a nano-scale conversion film, the surface defects of the body are required to be higher, and the body is required to minimize sanding printing, mimeograph and indentation printing
.
Body exposed sanding prints on hot-dip galvanized sheets may result in sanding marks and "angel ring" defects
after film treatment and electrophoresis.
For the stamping skin damage on the galvanized sheet, due to the extrusion during the stamping process, the surface of the workpiece produces irregular bright stripes, which may cause changes in the surface state, and the resistance difference is generated after the film treatment, which affects the film forming speed at the beginning of electrophoresis, resulting in visual defects
.

3 Process management

3.
1 Process Management

In addition to the same laboratory feeding as the phosphating process, the pretreatment of the film process applied to the automobile body has increased the detection of the membrane weight of the first car per shift, and the detection of the membrane weight is crucial
.

(1) Process monitoring
.

(a) According to the requirements of the process materials and parameter range of the tank liquid analysis and monitoring, each shift in the pretreatment of each tank liquid test work before entering the car, while taking the film treatment tank liquid in the laboratory to do the sample film weight measurement, only on the basis of the sample film weight qualification can be arranged into the car
.

(b) Check all tank parameters, according to the process requirements in time to adjust to the best settings, the first film treatment body must be detected according to different plates and corresponding parts of the membrane weight, membrane weight in line with the process requirements to mass production, on the contrary, must be added to adjust the parameters
.

(2) Equipment monitoring
.

(a) According to the process requirements, the liquid level, temperature and spray pressure of each process tank are inspected and the relevant records
are made.

(b) The circulation pump opening state of the film treatment tank directly affects the membrane weight, so special attention is paid to the circulation of the film treatment tank
.

(3) Quality control
.

Comprehensively supervise and inspect the quality of pretreatment, ensure that the quality of degreasing and film meets the requirements of process control, and do a good job in comprehensive performance analysis and evaluation of film, such as membrane weight, content of chemical components, and detection of bacteria, to meet anti-corrosion and stone impact quality standards
.

3.
2 Control of membrane weight

The film is chemically converted into a film, and the film-forming reaction involves complex inorganic and organic composite reactions, and the chemical and physical parameters will interact and influence
each other.
Under the premise of ensuring that the membrane weight meets the specifications, we take into account other influencing factors (such as plate, degreasing, etc.
), and seek the best membrane weight to achieve the best performance
.
The factors affecting the film formation rate of silane films are divided into two categories: physical factors and chemical factors
.
Physical factors include cycle conditions, temperature, treatment time, and treatment method (spray or dip); Chemical factors include sheet type, pH, mass concentration
of free fluorine, Zr, Si, Cu ions, etc.
The effect of each parameter on the film weight is shown in
Table 1.

Table 1 The influence of control parameters on film weight

Membrane weight is an important indicator to evaluate the quality of film pretreatment, and the bath parameters are regulated to ensure that the membrane weight is within the process range
.
Within the scope of membrane weight control, the higher the membrane weight, the stronger
the corrosion protection.
When the film weight is higher than the upper limit, the adhesion of the film layer decreases, resulting in poor corrosion protection; When the film weight is below the lower limit, the film layer cannot play an antiseptic effect
.

4 Common quality problems and improvements

4.
1 "Angel Ring"

(1) Problem description
.

After the film is formed, the conductivity of the surface greatly affects the appearance of the electrophoretic coating, such as the body-in-white sanding mark (sanding to expose the steel plate substrate) on the outer surface of the body, and many appearance defects are produced after film treatment and electrophoresis, such as the shade of color, the sense of concave and convex, orange peel-like ripples, "angel rings" and so on
.

The electrophoretic paint film at the bottom of the polishing dew of the body-in-white body is rough and raised, and the thickness of the electrophoresis film is 5 to 8 μm higher than that of the normal part, and an obvious "angel ring"
is formed at the junction of Zn and the polished dew bottom Fe of the galvanized sheet.
This defect is particularly pronounced on hot-dip galvanized sheets, as shown in Figure
6.

Fig.
6 Hot-dip galvanized sheet sanding printing to form an "angel ring"

(2) Cause analysis
.

The main cause of this defect is that the different electrochemical potentials at the junction of Zn and the grinding dew bottom Fe of the galvanized plate affect the deposition of Cu during the film treatment process and the current distribution of subsequent electrophoresis, resulting in the difference in the thick growth of the electrophoresis film, and the electrophoresis at the junction of Zn and Fe on the polishing edge is rough, forming an "angel ring", and the electrophoresis film thickness of the polished bottom area is high and convex, called "plateau", as shown in
Figure 7.

Figure 7 "Angel Ring" and "Plateau"

(3) Improvement measures
.

(a) Body-in-white: Try to avoid sanding the dew bottom, and use fine sandpaper around the sand bottom area to smooth over.

(b) Water washing: In order to avoid the impact of micro-corrosion on the grinding area, increase the number of corrosion inhibitor points in the washing tank, and increase the pH
of the washing tank after the film process.

(c) Film: Reduce the Cu ion content
of the film treatment tank.

(d) Electrophoresis: On the basis of ensuring the thickness of the electrophoresis film, reduce the voltage of the slot section and extend the soft start time
.

After taking the above measures, the body-in-white polishing print can be well covered by electrophoresis and topcoat, without special treatment, without "plateau", without "angel ring", and smooth and flat, stable and acceptable
.

4.
2 Poor adhesion by electrophoresis

Electrophoretic adhesion is a quality indicator
that needs to be paid attention to in the film process.
Since the film forming after film treatment is very thin, very little surface grease will also affect the binding force after film formation, thereby greatly reducing the anti-corrosion ability after coating, so the degreasing agent supporting the film process is very important
.

Some degreasers perform well in the laboratory, but they often have problems when they are applied in the field, which is mainly due to the lack of full recognition of the corrosion
of the hot and humid environment at the production site.
Therefore, the development of degreasing agent should establish a field working condition simulation laboratory, for a variety of different plate conditions, such as cold-rolled plate, hot-dip galvanized, electro-galvanized, aluminum plate, zinc-iron alloy and other plates mixed, as well as the body-in-white state, on-site process equipment, the corresponding development verification and control, to find the balance point
of the degreaser formula and parameter control.

A coating line using the film pretreatment process was discontinued for two months, and after the resumption of production, it was found that the electrophoretic adhesion of the front longitudinal beam parts was found, which was mainly due to the long time of the parts inventory, the stamping oil on the surface of the parts formed oil spots, and the high temperature of the welding oxidized the surface of the zinc layer, and the degreasing process was not fully removed and affected the electrophoretic adhesion
.
Therefore, degreasing agents containing more active hydroxide components should be selected to more fully remove natural loose zinc oxide/hydroxide residues
generated during stamping, welding, storage and transportation.

4.
3 Membrane heavy failure analysis

The film process has very high requirements for film weight, and when the conveying equipment fails, the body stays in the film treatment tank, which will lead to an increase
in film weight.
Increasing the weight of the membrane to a certain range will seriously affect the adhesion and anti-corrosion performance
.

In the laboratory, by increasing the time gradient, increasing the film weight, and performing anti-corrosion detection after electrophoresis, the impact of the increase in film weight on the anti-corrosion performance was analyzed; After the topcoat, the stone strike adhesion is tested to analyze the effect of the increase in film weight on
the adhesion performance.
Through laboratory analysis, combined with the actual situation on site, the upper limit
of film film weight is determined.

Simulate conveyor equipment failures online and track vehicle immersion in film treatment tanks
.
Reduce the risk
of end-of-life by optimizing the start-stop procedure of the circulating pump.
When the pretreatment conveying is stopped, the circulation pump automatically shuts down after 1min, and when the conveying is restored, the circulation pump automatically opens, reducing the increase rate
of film film weight.

5 Conclusion

Compared with the phosphating process, the film treatment process has great advantages in environmental protection, and it is also the direction
of the development of automotive coating pretreatment in the future.
Only by deeply realizing the differences between thin film technology and phosphating technology can we make corresponding improvements
in a targeted manner.
While continuously optimizing and improving the film process materials, how to formulate a precise parameter control range, maintain the stability of the bath parameters to ensure product quality, effectively monitor in the mass production process, and strictly control the risk points of the film processing process to avoid risks, which is not an easy task to achieve overnight, and the task is heavy and long
.

Source of this article: Paint Industry, No.
12, 2021

This article is written by Zhou Jie, Cheng Yajun