JPH06235085A - Zn-cr base surface-back different plating rust preventive steel sheet excellent in corrosion resistance - Google Patents

Abstract

PURPOSE:To obtain a rust preventive steel sheet having excellent corrosion resistance both in the outer side and inner side by forming Zn-Cr base plating layers with different Cr content on both sides of a steel sheet. CONSTITUTION:One side of a steel sheet is applied with Zn-Cr base plating contg., by weight, 0.1 to 6% Cr and in which, as for the existing state of Cr, 0 valence is not substantially contained is applied on one side of a steel sheet. The other side is applied with Zn-Cr base alloy plating contg. 6 to 30% Cr. The plating layer on one side may contain 0.1 to 5% Co and 0.01 to 5% alumina sol. At the time of using it in such a manner that the one contg. lower Cr content as the outer side and the one contg. higher Cr content as the inner side, excellent corrosion resistance can be obtd. in corrosive environments on both sides.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a Z for automobile outer panel, which has excellent corrosion resistance under both the outer surface side and the inner surface side.
The present invention relates to an n-Cr-based front and back different plating anticorrosion steel plate.

[0002]

2. Description of the Related Art In recent years, various anticorrosion steel sheets have been used for the purpose of improving anticorrosive power of automobile bodies. The mainstream of rust-preventive steel sheets is Zn-based plated steel sheets that utilize the sacrificial anticorrosion effect of Zn on steel in the atmosphere. The simplest and clearest method for improving the rust preventive power is to increase the plating amount of Zn (hereinafter referred to as the basis weight). On the other hand, an increase in the basis weight of Zn deteriorates workability and weldability during vehicle body forming.

Therefore, attempts have been made to obtain good corrosion resistance with a smaller basis weight by adding other elements to the Zn-based plating layer. For example, Japanese Patent Publication No. 62-6758
No. 1, Co, Ni, Fe and C in the electroplated Zn layer
0.1 to 30 wt% of at least one element selected from the group consisting of r.
A Zn-based plated steel sheet characterized by containing 0.1 to 3 wt% is proposed. Certainly, with such measures, pure Z
Although it has better corrosion resistance than the n-plated steel sheet, it cannot be said that it has sufficient corrosion resistance when it is used as an automobile outer panel. The reason for this will be described in detail below.

[0004]

The progress of research in recent years has revealed that environmental factors greatly affect the corrosion behavior of rust-preventive steel sheets, and that even the same sample exhibits completely different corrosion behavior. It was For example: Kurokawa et al., "Iron and Steel" Vol. 70 (1984) A96. This is because the corrosion mechanism differs depending on the environment. As a form of corrosion that becomes a problem in the case of automobile outer panels, in a moist environment on the inner surface side, corrosion progresses from the part with poor adhesion of the electrodeposition coating and the part such as scratches, and finally it reaches perforation. Corrosion morphology (defined as perforated corrosion) and corrosion morphology where blister spreads under the coating film starting from the coating damage part of the three-coat coating film due to stone splashes etc. in the dry environment on the outer surface side (this Defined as external corrosion)
There is.

The most reliable test method for determining the corrosion resistance of an anticorrosive steel sheet for automobiles is an actual vehicle test, but it takes time to obtain a result, so an accelerated test using a test piece is required. It is generally done in. However, even in the case of performing such an evaluation, if corrosion on the inner surface side is assumed due to the above-mentioned reason, the plate thickness decreases,
It is considered desirable to evaluate the blister width when assuming corrosion on the outer surface side.

From the above viewpoints, the inventors of the present invention tried to add various elements and substances to the Zn plating layer, and as a result, it is essentially difficult to have both excellent corrosion resistance under the environment of the inner and outer surfaces. The conclusion was reached. It is considered that this is because the environmental factors such as temperature, humidity, and coating thickness are different between the inner surface side and the outer surface side as described above, and the resulting corrosion phenomenon is governed by completely different mechanisms.

As an example, FIG. 1 shows the results of examining the corrosion resistance of the inner and outer surfaces when Cr (metal state) is codeposited in the Zn plating layer. The outer surface corrosion resistance was judged by blister width by an air exposure test with knife scratches on a test piece coated with 3 and the pitting corrosion resistance was found in JP-B-62-675.
Under the same test conditions as No. 8, under a cyclic corrosion test environment with a wet rate of 67%, it was judged by the amount of reduction in the thickness of the corroded portion near the cross cut portion of the sample having an electrodeposition coating film of 10 μm. All test materials used were low carbon cold-rolled steel sheets having a plate thickness of 0.7 mm and plated with a basis weight of 20 g / m ² . Moreover, it has been confirmed by ESCA analysis that the presence state of Cr in the plated layer is all zero-valent. As a comparison material, a commercially available alloyed hot-dip galvanized steel sheet for automobiles having a basis weight of 60 (60 g / m ² ) (referred to as GA60) and an electric Zn-Ni alloy coated steel sheet for automobiles having a basis weight of 30 (N
An i content of 13%) (referred to as NZ30) was used.

From FIG. 1, the puncture resistance is improved by increasing the Cr content in the plating layer.
It is found that when the Cr content is equal to Z30 and the Cr content is 10 wt%, it is almost equal to that of GA60, and when the Cr content is ≧ 15 wt%, the plate thickness is hardly reduced.

With respect to the external surface rust resistance, however, it can be seen that the blister width becomes smaller by the addition of Cr until the Cr content = 5 wt%, but rather deteriorates when the Cr content is further increased. From the above results, it has been clarified that when the perforation resistance is improved, the outer surface rust resistance is deteriorated.

Therefore, the present invention provides a Z for automobile outer panel which has excellent corrosion resistance under both the outer surface side and the inner surface side environment.
It is an object of the present invention to provide an n-Cr-based front and back dissimilar plating rustproof steel plate.

[0011]

[Means for Solving the Problems] As described above, since the corrosive environment is different between the inner surface side and the outer surface side of the automobile outer plate, the plating species exhibiting the best corrosion resistance in each environment are not always the same. Of. As a result of further studies to solve the above problems, it became clear that by applying different kinds of plating on the front and back sides, it is possible to obtain a plated steel sheet that satisfies the corrosion resistance required on the inner and outer surfaces of the outer surface of the automobile. It was.

That is, according to the present invention, 0.1
Zn-Cr system containing ~ 6wt% Cr, the Cr existing state has a Zn-Cr system plating layer that does not contain substantially zero valence, and 6-30wt% Cr on the other surface. Z with excellent corrosion resistance characterized by having an alloy plating layer
The present invention provides an n-Cr-based front and back dissimilar plating anticorrosion steel plate. Further, the plating layer on the one side is 0.1 to 5 wt.
% Co and / or 0.01-5 wt% alumina sol.

[0013]

The present invention will be described in more detail below. The rust preventive steel sheet of the present invention has a Zn-Cr alloy plating layer containing 6 to 30 wt% of Cr on one surface of the steel sheet, and the other surface contains 0.1 to 6 wt% of Cr. The presence state of has a Zn-Cr-based plating layer that does not substantially contain zero valence. Further, 0.1-5 wt% Co and / or 0.01-5 wt% alumina sol may be contained on the opposite surface side. The reasons for each limitation will be described below.

As described above, since the corrosive environment is different between the inner surface side and the outer surface side of the automobile outer plate, the plating species having the best corrosion resistance in each environment do not necessarily match. Therefore, it is possible to impart excellent corrosion resistance to both the inner and outer surfaces by using different kinds of plating on the front and back and performing the best plating under each environment.

As shown in FIG. 1, increasing the Cr content is effective for improving the puncture resistance. In order to obtain puncture resistance higher than that of NZ30, which is generally used at present, with a smaller basis weight, Cr content ≥ 6 wt% is desirable. However, if the Cr content rate> 30 wt%, the plating adhesion deteriorates, so it is considered that the Cr content rate in the plating layer corresponding to the inner surface side is preferably 6 to 30 wt%.

Regarding the corrosion resistance on the outer surface side, as shown in FIG. 1, it is the best in the vicinity of Cr content = 5 wt%, but it does not reach the commercially available NZ30. Therefore, the original purpose of improving the corrosion resistance with a smaller basis weight cannot be achieved. However, as a result of an examination focusing on the existence state of Cr in the plating layer, it was discovered that the outer surface rust resistance can be further improved by containing substantially no zero valence. The presence state of Cr is confirmed by ESCA, and it means that the peak intensity derived from 0 valence in the spectrum is ⅓ or less of trivalence in the spectrum when it does not substantially contain 0 valence. If the Cr concentration is less than 0.1 wt%, the corrosion resistance on the outer surface side will be insufficient, and if it exceeds 6 wt%, the same will result.
Limited to 6 wt%. It is more preferably 1 to 6 wt%.

The factors that mainly determine the corrosion resistance are the Cr concentration and the state of existence in the plating layer.
Further effects may be obtained by containing and / or alumina. Co is effective in increasing the corrosion resistance of the plating layer itself by making the corrosion potential of the plating layer noble, and the effect is exhibited at 0.1 wt% or more. Since it deteriorates, it is limited to 0.1 to 5 wt%. Alumina sol is effective in hindering the progress of corrosion of the plating layer by stably holding the corrosion product. The effect is exhibited at 0.01 wt% or more, but when it exceeds 5 wt%, the plating adhesion is still high. Deteriorates, so 0.01 to 5 wt
Limited to%.

A method for obtaining these plating layers will be described below. First, the plating layer on the inner surface side will be described. The plating layer containing 6 to 30 wt% of Cr corresponding to the inner surface side contains Zn ²⁺ and Cr ³⁺ as main components,
Electrodeposition is carried out from a bath to which a conduction aid is added. It can be obtained from either a sulfuric acid bath or a chloride bath, but particularly in the case of a sulfuric acid bath, it is desirable to add an appropriate amount of acetylene glycol in order to promote Cr precipitation.

The reason for this is that although the equilibrium potentials of Zn and Cr are close to each other in the plating bath, the exchange current density of Zn is high, whereas the exchange current density of Cr is low, that is, polarization on the cathode side during plating. This is because Zn is immediately electrodeposited when it is treated, but a large overvoltage is required to deposit Cr. Therefore, in order to promote the co-deposition of Cr, by adsorbing on the cathode surface, Zn
It is desirable to add a chemical that increases the activation overvoltage of precipitation and brings the precipitation potentials of Zn and Cr close to each other. When acetylene glycol is not added, there is also a method of increasing the concentration polarization by using a remarkably high current density to promote the co-deposition of Cr, but in this case, the plating adhesion is significantly deteriorated, which is desirable. Absent. In order to increase the Cr content, the Cr ³⁺ / Zn ²⁺ molar ratio in the bath may be increased or the current density may be increased. The presence state of Cr in the plated layer electrodeposited by the above method is almost zero.

Next, the plating method on the outer surface side will be described.
The component of the plating bath may be almost the same as the plating on the inner surface side, except that acetylene glycol is not added. Of particular note is that organic additives such as acetylene glycol should not be incorporated. As described above, when an organic additive such as acetylene glycol is mixed, the reduction precipitation of Cr is promoted and the existing state in the plating layer becomes zero valence, and the effect of improving the external surface rust resistance is suppressed. This is because When the Cr content is about 0.5 wt% or less, the electrodeposition from the above-mentioned bath results in substantially 3
It is possible to co-deposit Cr existing in the valence state.
To further increase the Cr content, Co sulfate or C
It is effective to add o chloride and / or alumina sol. In this case, of course Co in the plating layer
And / or alumina sol is codeposited.

[0021]

EXAMPLES The present invention will be specifically described below based on examples. (Example) Outer side and inner side plating having the composition shown in Table 1 was performed by appropriately selecting plating conditions from an acidic plating solution containing a plating species on a cold-rolled steel sheet according to a conventional method, and outer surface rust resistance and hole resistance. A test for openness and plating adhesion was conducted. The evaluation results are also shown in Table 1.

The test piece for evaluating the outer surface rust resistance was prepared by a conventional method, in which a zinc phosphate conversion treatment was performed, followed by cationic electrodeposition coating of 20 μm and intermediate coating and top coating of 35 μm each. After making scratches up to 0, they were exposed to the outdoors, sprayed with salt water once a week, and evaluated by measuring the blister width one year later. The judgment criteria are shown in Table 2. The EG20 in Table 2 is an electric Zn plated steel sheet having a basis weight of 20 (20 g / m ² ).

The evaluation of perforation resistance is the same as that shown in Japanese Examined Patent Publication No. 62-6758 after scratching a test piece which has been subjected to cationic electrodeposition coating in the same procedure as the evaluation of outer surface rust resistance. That is, after performing a phosphate treatment (Nippon Parker Lining Co., Ltd. Bonderite # 3004), cation electrodeposition coating (Nippon Paint Co., Ltd. Power Top U-
30) was subjected to a cross-cut until reaching the base steel plate, and the following cycle corrosion test was performed for 10 hours with salt spray (JIS Z2371), 2 hours with drying (60 ° C), and with wetness (50 ° C, 95%). % Relative humidity) 6
A cycle corrosion test was performed with one cycle consisting of 6 hours for drying and 6 hours for drying (at room temperature), and the maximum reduction in plate thickness after 3 months was evaluated. The judgment criteria are shown in Table 2.

The plating adhesion was judged by checking the presence / absence of plating separation by bending 90 ° with the evaluation surface of the test piece cut out to 100 mm × 30 mm inside and bending back and then peeling the cellophane tape.

In any of the tests, in the case of showing a corrosion resistance superior to that of NZ30, it was determined to pass in order to improve the corrosion resistance with a less basis weight.

Inventive Examples 1 to 9 all satisfy the requirements of the present invention, and both the corrosion resistance of the inner and outer surfaces pass. Comparative Examples 1 and 2 satisfy the requirements of the invention of the present invention on the outer surface side, but the Cr content on the inner surface side is low and the puncture resistance is insufficient. In Comparative Example 3, the Cr content is within the range of the present invention, but since the existing state of Cr is 0, the outer surface rust resistance is insufficient and the Cr content is a little low in terms of puncture resistance. This is an example that is insufficient. Comparative Example 4 and Comparative Example 5 have sufficient perforation resistance, but are high in the Cr content and also present as zero valent, so that the outer surface rust resistance is extremely poor. Comparative Example 6 is C
Porosity resistance is insufficient due to low r content, and Cr content is also within the range of the present invention in terms of external surface rust resistance, but insufficient due to zero-value Cr existing state. Is. Comparative Example 7, Comparative Example 8 and Comparative Example 9 are all acceptable in terms of corrosion resistance, but poor in practicability and poor in plating adhesion and poor in practical use.

[0027]

[Table 1]

[0028]

[0029]

As described above, according to the present invention, it is possible to obtain a rust-preventing steel sheet which exhibits excellent corrosion resistance in both the corrosive environment on the inner surface side and the outer surface side when it is used for an outer plate of an automobile body.

[Brief description of drawings]

FIG. 1 Perforation resistance, outer surface rust resistance and Cr in plating layer
The figure which shows the relationship with content rate.

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Kazuhiro Hasegawa, Kazuhiro Hasegawa, 1st Kawasaki-cho, Chuo-ku, Chiba City, Chiba Prefecture Technical Research Division, Kawasaki Steel Corporation (72) Shigeo Kurokawa, Kawasaki, Chuo-ku, Chiba-shi, Chiba Prefecture Kawasaki Steel Co., Ltd. Technical Research Headquarters (72) Inventor Kazuo Mochizuki Kawasaki-machi, Chuo-ku, Chiba City, Chiba Prefecture Kawasaki Steel Co., Ltd. Technical Research Headquarters

Claims (2)

[Claims] 1. A steel sheet containing 0.1 to 6 wt% of Cr on one side, and the presence state of Cr does not substantially include zero valence Z.
It has an n-Cr system plating layer, and 6 to 30 wt% on the other surface.
Zn-Cr type alloy rust-preventing steel sheet having excellent corrosion resistance, characterized by having a Zn-Cr type alloy plating layer containing Cr. 2. The plating layer on one side is 0.1 to 5 wt%.
2. The Zn-Cr-based front and back dissimilar plating anticorrosion steel sheet having excellent corrosion resistance according to claim 1, which contains Co and / or 0.01 to 5 wt% of alumina sol.