Discussion on Cyclic Corrosion Resistance Test of Electrophoretic Coating Film of Steel and Aluminum Colinear Coating

0 Preface

With the rapid development of the whole industry of new energy vehicles, the demand for lightweight vehicles is becoming more and more urgent

1 trial

1.

In this test, 6-series aluminum alloy outer plate, GA zinc-iron alloy hot-dip galvanized plate, and DC04 cold-rolled steel plate were selected.

Table 1 Test board parameter information

1.

6 series aluminum alloy outer plate, GA zinc-iron alloy hot-dip galvanized plate, and DC04 cold-rolled steel plate are all carried out two kinds of pre-treatment plate-making work in the laboratory, namely traditional phosphating pre-treatment and film pre-treatment, two kinds of pre-treatment process degreasing The procedures are the same, using an alkaline degreasing agent, model S5166, the operation mode is dipping, and the degreasing time is 3 min

The two pretreatment processes are as follows:

(1) Pretreatment process of phosphating: degreasing→water washing→surface conditioning→phosphating→water washing

(2) Film pretreatment process: degreasing→water washing→film treatment→pure water washing

1.
3 Electrophoresis experiment

6 series aluminum alloy sheet, GA zinc-iron alloy hot-dip galvanized sheet, and DC04 cold-rolled steel sheet are all carried out electrophoretic sheet-making work in the laboratory
.
The prepared pretreatment test plate is wetted with pure water and then electrophoresed, and the electrophoretic paint is CG-800 electrophoretic paint with high electrophoresis penetrability
.
Electrophoresis process parameters: the electrophoresis voltage is set between 100 and 320 V according to the voltage and film thickness of different plates; the temperature of the electrophoresis bath is 30 °C; the electrophoresis time: soft start 30 s, and the electrophoresis time is 3 min in total
.
After electrophoresis, the test plate was washed with pure water, baked in an oven at 170 °C for 20 min, and then placed in a desiccator after natural cooling to control the thickness of the electrophoretic coating film of the test plate (18±2) μm
.

1.
4 Test performance and technical indicators

The performance testing of the two pretreatment films and the electrophoretic coating films of the 6-series aluminum alloy sheet, the GA zinc-iron alloy hot-dip galvanized sheet, and the DC04 cold-rolled steel sheet was carried out according to the technical indicators and detection methods in Table 2
.

Table 2 Detection technical indicators and detection methods

2 Test results and discussion

2.
1 Preprocessing performance

The pretreatment properties of 6 series aluminum alloy sheets, GA zinc-iron alloy hot-dip galvanized sheets, and DC04 cold-rolled steel sheets were tested, mainly from the appearance and film weight of the two pretreatment films
.
The appearance, crystal size, crystal shape and film weight of the film of the pre-phosphating test plate were tested.
The visual appearance showed that the phosphating film of several test plates was a gray film, and the surface was smooth, uniform, fine, and the film was complete.
Defects such as yellow rust
.
The SEM 2000 times mirror image shows that the crystal shape of DC04 cold-rolled steel sheet is granular, the crystal size is 2-5 μm, the crystal shape of galvanized sheet is columnar, the crystal size is ≤10 μm, and the crystal size of aluminum alloy sheet is ≤10 μm
.
The film weight detection was carried out on the phosphate film with grain structure mesh film formed by the pretreatment of 6 series aluminum alloy plate, GA zinc-iron alloy hot-dip galvanized sheet, and DC04 cold-rolled steel sheet by the stripping method.
The film weight of the galvanized sheet is about 3.
01 g/m2, which meets the technical requirements of the film weight (1.
8～3.
5) g/m2, and the phosphate film weight of the galvanized sheet is about 2.
7 g/m2, which meets the film weight (2.
5～4.
5) g/m2 According to the technical requirements of the aluminum alloy sheet, the film weight of the aluminum alloy sheet is 2.
78 g/m2, which meets the technical requirements of the film weight (1.
5-3.
5) g/m2
.
The film weight of the mesh film without grain structure formed by the pretreatment of 6 series aluminum alloy sheet, GA zinc-iron alloy hot-dip galvanized sheet, and DC04 cold-rolled steel sheet was tested by XRF, and the pretreatment performance all met the technical requirements.
The specific data are shown in the table.
3
.

Table 3 Testing data of two pretreatment films

2.
2 Electrophoresis performance

Laboratory electrophoresis was performed on the pre-treated panels prepared from 6 series aluminum alloy sheets, GA zinc-iron alloy hot-dip galvanized sheets, and DC04 cold-rolled steel sheets, and the thickness of the electrophoretic paint film was controlled to be (18±2) μm.
performance is checked
.

Electrophoresis film thickness

The magnetic film thickness meter was used for the test to measure the coating film thickness of the test plate after the pretreatment electrophoresis, and the electrophoresis film thickness all met the design requirements
.

Table 4 Electrophoretic paint film thickness

Electrophoretic coating adhesion

The prepared electrophoretic coating test plate was crossed with a BYK crossbar (1 mm in size), and the adhesion of the coating film was tested with 3M tape
.
The test results show that the cutting edge of the cross part of the cross section of the electrophoretic coating film of the aluminum alloy plate, GA zinc-iron alloy hot-dip galvanized plate and cold-rolled plate is completely smooth, and there is no drop off, and the adhesion is all grade 0
.

Table 5 Adhesion of electrophoretic paint film

Cyclic Alternating Performance of Electrophoretic Paint Film

The CCX cyclic corrosion test chamber was used for the test.
The four sides of the electrophoretic coating test panels of the three types of plates were sealed with 3M waterproof tape.
The center of the test panels was scratched with a model 463 scratch tool produced by Erichsen Company with a width of 0.
5 mm.
, according to the QJQ 16113.
2-2018 Coating Cyclic Alternating Corrosion Determination Method Standard, the laboratory cyclic alternating corrosion test is carried out, and the corrosion resistance is evaluated by cyclic corrosion CCT (60 cycles)
.
The cyclic corrosion load is:

(1) According to the ISO 20567-1-2017 method, the stone chip resistance performance test is carried out, and on the remaining undamaged surface, a scratch about 10 cm long directly to the metal substrate is drawn;

(2) According to 4 hours of neutral salt spray conditions (GB/T 1771-2007 Determination of neutral salt spray resistance of paints and varnishes) → 4 hours of standard climatic conditions (temperature 23 ℃, humidity 50%) → 16 hours of damp heat Environmental conditions (temperature 40 ℃, humidity 100%, according to GB/T 13893-2008 Determination of the moisture resistance of paints and varnishes by continuous condensation method) experimental conditions for CCT cyclic alternating corrosion test
.
Test 5 cycles on 5 working days a week, and place them in a standard temperature environment of 23 ℃ and humidity of 50% on Saturday and Sunday;

(3) After 60 cycles of testing (84 days in total), the test panel must be removed from the test box and rinsed with running water to clean the surface of the test panel.
After 1 hour of conditioning, perform a second stone attack on the same surface that was previously stoned.
The corrosion resistance evaluation of the electrophoretic coating film was carried out
.
The corrosion resistance after mechanical damage was tested, and the test results are shown in Table 6
.

Table 6 CCT cyclic alternating corrosion test results

The experimental results show that after 60 cycles of alternating corrosion experiments on the electrophoretic coating sample of aluminum alloy plate, the surface of the test plate is evaluated.
Level 1.
5, all meet the technical requirements
.
The surface of the electrophoretic coating test plate has 0 mm of corrosion under the film, which meets the technical requirements.
The CCT cyclic alternating corrosion performance of the aluminum alloy test plate is the best, see Figure 1 and Figure 2 for details
.

Fig.
1 Corrosion diagram of aluminum alloy plate before phosphating

 Fig.
2 Corrosion pictures of aluminum alloy plate film pretreatment

3 Conclusion

In this paper, through the study on the properties of electrophoretic coatings prepared by 6 series aluminum alloy sheets, GA zinc-iron alloy hot-dip galvanized sheets, and DC04 cold-rolled steel sheets after two pretreatments The mechanical properties meet the technical requirements
.
In terms of cyclic corrosion resistance, when the current treatment process is the same and the electrophoresis material process is the same, the electrophoretic coating prepared by the aluminum alloy sheet has the best cyclic corrosion resistance; when the substrate is the same and the electrophoresis material is the same, the electrophoresis coating prepared by the phosphating pretreatment process is the best.
The coated sample has the best cyclic corrosion resistance
.
Through the research on the coating collinearity of aluminum alloy sheet, cold-rolled sheet and GA zinc-iron alloy hot-dip galvanized sheet material, the corrosion resistance of different materials is understood, and the CCT cyclic alternating corrosion performance of automotive coating products has a certain understanding
.

References (omitted)

Author | Liu Feng, Wu Jixia

(Anhui Jianghuai Automobile Group Co.
, Ltd.
, Hefei 230601)