Influence of Scribing Treatment on Corrosion Propagation of Electrophoretic Paint Film in Salt Spray Test

introduction

When evaluating the corrosion resistance of paint films in the laboratory, it is usually necessary to pre-treat the painted samples before the salt spray test

In this study, common tools were selected to perform different scribing treatments on the electrophoretic paint films of iron-based, aluminum-based, and iron-based galvanized sheets.

test

The samples were scribed according to the following five knife shapes.

Figure 1 Schematic diagram of 5 kinds of tool marking

1.

The galvanized sheet was marked with c and d in Figure 1 and subjected to 840h neutral salt spray (NSS) test

Figure 3 shows the scribing morphology of the sample surface with corrosion products removed after the neutral salt spray test

 Figure 2 Scribing micrographs of two V-shaped tools

Figure 3 The spread of paint film corrosion under two V-shaped tools a: V-shaped tip; b: V-shaped arc tip 

At present, the evaluation of corrosion spread in domestic and foreign standards is mainly divided into two types: one is to calculate the average value according to ISO 4628-8, and the other is to directly measure the maximum spread width

2.
The influence of different marking methods

At present, the major OEMs have not unified the shape and angle of the scribing, and the influence of different scribing methods on the spread of corrosion has not been studied in depth
.
Figure 4 shows 5 ways of scribing on the same galvanized sheet using cutters of the same width.
Among them, a, b, and c are the most commonly used scribing methods at present
.

Figure 4 Corrosion spread in different scribing methods

It can be seen from Figure 4 that the corrosion spread at the horizontal lines in c and e is significantly smaller than in other directions, and at the same time, due to rust in c and d, the scribe line below it is disturbed
.
Ignore the horizontal lines in c and e, and take the average of the spread at the five scribes.
The results show that the spread of a, b, c, and e is basically the same.
The difference between a and d is analyzed.
The larger spread difference in oblique direction is mainly due to the existence of cross position in a, which leads to heavier corrosion and easier coating peeling
.
Test two boards at the same time, and the rules obtained are consistent
.
Therefore, in the actual scribing process, a line in the vertical direction can be used to characterize the anti-scribing spread of the paint film
.
The reason for the large difference between the spread of horizontal and vertical lines may be that corrosion products and salt are more likely to accumulate in the horizontal direction.
High-concentration corrosion products and salt have a competitive relationship with adsorbed oxygen, which can inhibit the continued expansion of the corrosion reaction.

.

3.
The influence of different substrates and test conditions

The base metal materials commonly used in the automotive industry include iron-based, aluminum-based, and iron-based galvanizing
.
The resistance to salt spray corrosion spread at the scribe line of the electrophoretic paint film of the three materials is very different
.
In order to study the influencing factors of the corrosion propagation performance of the marking, the representative test conditions are selected and the tests are carried out at the same time
.
Figure 5 shows the corrosion spread data of different substrates under different salt spray test conditions
.

Figure 5 Corrosion propagation values ​​under different substrates and test conditions

It can be seen from the figure that the corrosion spread of the galvanized electrophoretic plate under NSS conditions is obvious, and there is no obvious corrosion spread of the aluminum-based electrophoretic plate under the three conditions
.
In the SAE J2334 condition, the corrosion spread of the galvanized electrophoresis plate<the iron-based electrophoresis sheet, which is also the iron-based electrophoresis sheet in the ISO 11997-1 cycle B of the circulating salt spray<the galvanized electrophoretic sheet
.
First, analyze the obvious reason why the corrosion spread of the galvanized layer under the NSS condition is that the neutral salt spray test continues to spray.
The Cl- concentration and relative humidity in the box are always high.
The corrosion products of the galvanized sheet are loose and cannot prevent the entry of moisture and Cl-.
The corrosion products in the iron-based electrophoresis plate are relatively dense, resulting in slower corrosion progress
.

Analyze the opposite performance of zinc electrophoresis and iron-based electrophoresis under the two cyclic salt spray conditions, which may be caused by different test solutions and test conditions
.
The solution used in SAE J2334 contains CaCl2 and NaHCO3, and involves a high temperature stage; while ISO 11997-1 cycle B uses a common NaCl solution and does not have a high temperature stage above 50°C
.
The solution used in GMW 14872 is similar to SAE J2334 and the temperature in the high temperature stage is maintained at 60°C.
Galvanized and iron-based electrophoresis sheets were used for the same cycle of GMW 14872 test.
The results are shown in Figure 6
.
It can be seen from the figure that the corrosion spread of the galvanized electrophoretic plate after the GMW 14872 test is significantly smaller than that of the iron-based electrophoretic plate
.

In summary, the corrosion spread of the galvanized substrate is greater than that of the iron matrix in the non-high temperature stage and the use of NaCl solution in the cyclic corrosion process, and the corrosion spread of the iron-based electrophoretic paint film in the high temperature stage and the corrosion process of the composite salt The value is greater than the galvanized substrate
.
It can be seen that the extent of corrosion spread has a greater relationship with the electrophoretic matrix and corrosion conditions
.

Figure 6 Corrosion morphology of different substrates obtained under GMW14872 cyclic salt spray conditions

4.
The effect of scribing depth

Use the same scoring knife to make two sets of scribing on the same galvanized board.
One set of scribing depth touches but does not penetrate the galvanized layer; the other set of scribing lines penetrates the galvanized layer, as shown in Figure 7
.

Figure 7 Microscopic images of different scribing depths

Perform 5 cycles of ISO 11997-1 cycle B corrosion test on the scribed test board.
The 3 bars on the left of Figure 8 are the test results of penetrating the zinc layer, and the 3 bars on the right are the test results of not penetrating the zinc layer.

.
It can be clearly seen from the figure that when the depth of the scribe line is deeper, the corrosion spread width is larger than that of the shallower line
.

Figure 8 Corrosion spread of different scribe depths

This is because the iron matrix leaks out when the line is deep.
The iron matrix and the zinc coating form a galvanic pair, which accelerates the corrosion of zinc.
The corrosion of zinc will form hairy 2ZnCO3•3Zn(OH)2, ZnO, ZnCl2•Zn(OH) ) 2 and so on
.
These corrosion products are hygroscopic.
In the process of salt spray and high humidity in the cyclic alternating salt spray test, corrosion products gradually accumulate at the scribe line, and the corrosion product fills the groove of the scribe line, and the corrosion gradually develops into the horizontal corrosion under the film.
Extension
.
Since galvanic acceleration is more likely to form corrosion product accumulation, the corrosion spread is more serious when the scribe is deeper than when the scribe is shallow
.

5.
The influence of different line widths

Scribe lines with a width of 0.
5 mm and 1 mm were performed on the same galvanized sheet, respectively, and the microscopic morphology of the line was shown in Figure 9
.
It can be seen that the difference between the two knife-shaped scribe lines is only in the width of the scribe line
.

Figure 9 Microscopic images of different scribing widths

Figure 10 shows the topography after the corrosion test.
The left 3 bars are 0.
5 mm wide, and the right 3 bars are 1 mm wide
.
Table 1 shows the spread results of the two scribing methods through corrosion tests
.
It can be seen that the 0.
5mm scribing method has a larger average value in terms of average value, and the 1mm scribing method has greater corrosion in terms of maximum spread
.

Figure 10 Corrosion spread of different scribe widths

Table 1 Corrosion spread of paint film under different line widths

The 0.
5 mm scribe line has a strong continuity of paint film peeling off visually.
Because the counting method of average corrosion spread is calculated according to the left and right maximum values ​​measured at a fixed distance in ISO 4628-8, this calculation method better reflects the overall Corrosion spread, but there is a problem of insufficient reflection of the severity of corrosion spread
.
When the maximum spread is used for evaluation, the ability of the paint film to resist the spread of scratches and corrosion can be clarified, but there is also the problem of data contingency.
Therefore, a better evaluation method should be to record and evaluate both evaluation methods at the same time
.

In the galvanic corrosion pair formed by the galvanized layer and the iron substrate, zinc is used as the anode and iron is used as the cathode.
The two scribing methods with different widths have the same exposed area of ​​the galvanized layer.
The exposed area of ​​the iron substrate is 0.
5 mm which is half of that under 1 mm.

.
In this corrosion system, the electrode reaction in the early stage of corrosion is mainly controlled by the cathode.
At this time, the electrode reaction speed increases with the increase of the exposed area; when there are more corrosion products, the corrosion system becomes resistance controlled, and the corrosion products formed by the galvanized layer at this time The number and distribution of the corrosion will completely determine the corrosion reaction speed
.
The corrosion products that accumulate when the scribe line is wider are more than those when the scribe line is narrower.
The flocculent corrosion product has stronger water absorption and induces strong under-film corrosion.
Therefore, the maximum corrosion spread value is larger when the scribe line is wider.
However, in the scribing method with a narrower scribe line, it seems that the corrosion spread is more uniform
.

Concluding remarks

(1) The use of different knife-shaped markings has different effects on the spread of corrosion, and there is little difference between the V-shaped tip and the V-shaped arc tip;

(2) With different scribing angles, the horizontal scribing spread is small, and the vertical and tilt angles are not much different;

(3) The degree of corrosion spread is largely related to the substrate and corrosion conditions, and the aluminum alloy substrate electrophoretic film has the best corrosion resistance performance;

(4) The depth of the scribing affects the corrosion resistance of the paint film, and the corrosion spreads when the scribing is deep;

(5) The impact of different scribe line widths on the paint film is more complicated.
When the scribe line is wider, the maximum corrosion spread value is larger, and the scribe line width is more uniform than h corrosion spread.
The actual test evaluation should be based on the average spread value and the maximum spread value.
The evaluation of each dimension is more reasonable
.

Authors | Song Hailin, Lu Chenghuan, Chen Yongbin, etc.

(CSTC Standard Technical Service Co.
, Ltd.
, Automotive Corrosion Protection Testing Technology Center)