JPS61194195A - Highly-corrosion resistant two-layer plated steel plate - Google Patents

Abstract

PURPOSE:To provide a two-layer plated steel plate having excellent corrosion resistance against the rust on the inside and outside surfaces by forming a specific Zn-Ni alloy plating layer on the surface of the steel plate and forming a specific ratio of the specific composite zinc plating layer consisting essentially of Zn thereon. CONSTITUTION:The Zn-Ni alloy plating layer contg. 7-20wt% Ni is formed as a lower layer on the surface of the steel plate and the deposition thereof is adjusted to about 5-60g/m<2> per side. The upper layer of 1-10g/m<2> deposition consisting of the composite zinc plating layer consisting essentially of Zn and contg. >=0.1wt% and <7wt% 1 or >=2 kinds among Ni, Co, Mn and Sn is formed thereon. Further 1 or >=2 kinds among Mo, W and Cr can be incorporated at >=0.1wt% and <7wt% into the upper layer according to need. The steel plate having the excellent performance as an outside plate panel for an automobile is thus obtd.

Description

DETAILED DESCRIPTION OF THE INVENTION (Field of Industrial Application) The present invention relates to a surface-treated steel sheet that is useful as an outer panel of an automobile and has excellent corrosion resistance against internal rust and external steel.

(Prior art) With the increase in the amount of antifreeze salts used in cold regions, various zinc alloy plated steel sheets have been developed to prevent rust on car bodies, especially to improve corrosion resistance against internal rust. It has already been put into practical use.

Internal rust occurs when corrosion progresses from areas where the electrodeposited coating does not spread sufficiently, such as the joints of the bag-like structure of the car body, and eventually leads to holes. As a countermeasure against such pitting corrosion, a surface-treated steel sheet is required that has excellent bare corrosion resistance and corrosion resistance when electrodeposition coating is insufficient. A Zn-Ni alloy plated steel sheet has been proposed to meet such requirements.

However, in recent years, there has been an increasing demand for anti-rust properties for the outer surfaces of automobiles, and the use of double-sided plated steel sheets with surface treatment applied to the outer surfaces, which were conventionally left as cold-rolled surfaces, is being considered. Since the external surface is electrodeposited after press forming, a surface-treated steel sheet is used for the external surface, which has not only bare corrosion resistance but also corrosion resistance against external surface rust, that is, excellent post-painting corrosion resistance and paint film adhesion. required.

External rust occurs when sand and pebbles, which are sprayed together with antifreeze to prevent slippage, hit the moving car body and damage the electrodeposited coating.

If the base material is a bare cold-rolled steel plate, the scratches will corrode the base material, resulting in red rust-like corrosion. Therefore, it is thought that using a plated steel plate that has sacrificial anti-corrosion ability, such as a galvanized steel plate, to suppress corrosion of the base material from the vortices is effective in improving the corrosion resistance after painting.

In conventional galvanized steel sheets, red rust from the base material is suppressed due to the sacrificial anti-corrosion ability of Zn, but because the dissolution rate of Zn is fast, corrosion under the coating progresses from the vortices, and extensive coating creep tends to occur. There were drawbacks.

To compensate for these drawbacks, iron-zinc alloy plated steel sheets in which zinc is alloyed with iron have been developed, but these also have the drawbacks of poor bare corrosion resistance and susceptibility to internal rust.

On the other hand, the Zn-Ni alloy plated steel sheet mentioned earlier, which has excellent internal rust resistance, has good corrosion resistance in the coating layer itself (
Therefore, it has the advantage that paint film creep is less likely to occur even if it is painted, but the sacrificial anticorrosion ability is not sufficient and red rust is likely to occur from uneven parts of the paint film. If an attempt is made to compensate for this by increasing the basis weight, the advantages in workability and weldability that a thin coating plated steel sheet exhibits will be lost.

In this way, bare corrosion resistance (inner surface rust resistance) and post-painting corrosion resistance (
Since single-layer plating has its limits, two different Zn-based platings are used.
Dual-layer plated steel sheets were developed.

The present inventors previously discovered that among alloy plating, Zn-Ni
Focusing on the fact that the system exhibits particularly excellent corrosion resistance, we used two-layer plating that uses this as a lower layer and then Zn plating on top.
proposed that a double-layer plated steel sheet with improved electrodeposition coating properties and post-coating corrosion resistance could be obtained without a significant increase in the amount of coating (Japanese Patent Application No. 59-97580); however, even with this 2N steel sheet, It was found that the coating film creep was likely to occur in the uneven parts of the coating film due to corrosion under the coating film of Zn, and the corrosion resistance was still insufficient.

Other known double-layer plated steel sheets include Zn-Ni/Zn-Fe (F
e-5 to 30%) (Japanese Patent Application Laid-Open No. 57-207194),
Zn-Ni/Zn-Fe-Ni or Co
(Fe215%, Nj or Co=0°5-8.5%)
(Japanese Unexamined Patent Publication No. 51-145996), and Zn-Ni/
Zn-Cr (Cr=0.005-0.5%) (JP-A No. 57-070291), all of which have Zn-N in the lower layer.
I-series metal coatings, metal layers, and some have a zinc alloy or composite plating layer on the upper layer.

However, even these known two-layer plated steel sheets are still not sufficient. For example, when the upper layer is plated with a Zn-Fe alloy, as described in JP-A-57-207914 and JP-A-57-145996, if the Fe content is low, the plating film becomes Corrosion rate is high and corrosion resistance is poor. Therefore, although the range of Fe content is relatively high, the alloy phase in the plating layer increases accordingly, and coating film defects called craters are more likely to occur during cationic electrodeposition coating, and the appearance after coating increases. and corrosion resistance deteriorates.

When the upper layer is a Zn-Cr alloy plating, as described in JP-A-57-070291, the higher the Cr content, the better the corrosion resistance, but if the Cr content is 0.5% or more, the appearance of the plating will deteriorate. Cr deteriorates, so there is a limit to the improvement of corrosion resistance by Cr.

(Problems to be solved by the invention) Therefore, by having sufficient bare corrosion resistance and appropriate sacrificial prevention power, coating film creep is less likely to occur, and the coating film has excellent red rust resistance in uneven parts. There is still a need for a surface-treated steel sheet (that is, sufficient corrosion resistance after coating), and which is less likely to cause craters during electrodeposition coating. An object of the present invention is to provide such a surface-treated steel sheet.

(Means for Solving the Problems) The present inventors have proposed the patent application filed in 1983-
In the double-layer plated steel sheet No. 97580, the upper layer of pure Zn
Adding a small amount of a specific metal component to the plating layer is effective in suppressing the dissolution rate of Zn, resulting in excellent bare corrosion resistance, improved corrosion resistance after painting, and good plating appearance and crater resistance. It has been discovered that ring properties can be obtained, and the present invention has been achieved.

'Here, the present invention provides Ni 7 to 20-t on the surface of the steel plate.
A lower layer consisting of a Zn-Ni alloy plating layer containing 0.1 wt % or more of 7 wt % or more of Zn as a main component and one or more of old, C0lMn and Sn.
% and/or TiSIgl, qg and Si
0.1 wt% or more 10-t% of one or more of the following
If desired, MOLW and Cr] iM or two or more of 0.1 wt% or more and 7 wt%
This is a highly corrosion-resistant two-layer plated steel sheet characterized by forming an upper layer consisting of a composite galvanized layer which may contain less than 10 g/rd of adhesion.

In this way, only a small amount of the metal component may be contained in the upper layer composite galvanizing, but it produces the above-mentioned excellent effects.

The two-layer plating according to the present invention is particularly suitable for external surface treatment of automobile outer panels. In this way, when this two-layer plating is used on the outer surface, the back side, that is, the inner side, is coated with an alloy plate such as Zn-Nj, which has excellent corrosion resistance, or other mesoplating, or is further coated with a metal such as Zn. The surface may be coated with an organic coating containing powder.

(Function) Next, the reason why the components of each plating layer are limited as described above in the present invention will be explained. In addition, in the following explanation,
All percentages are -t% unless otherwise specified.

Regarding the Ni content of the lower layer Zn-Ni alloy plating, corrosion resistance is best within the range of 7 to 20%, and when it is outside this range, corrosion resistance deteriorates significantly. In addition, rZn-Ni
Plating containing trace amounts of alloying elements other than Zn and Ni, such as Japanese Patent Publication No. 57-33347 and Japanese Patent Application Laid-open No. 57-671.
As is known from Japanese Patent Publication No. 88 and Japanese Patent Publication No. 58-6796, it is meant to include metals containing trace amounts of Co or Cr.

According to one feature of the double-layer plated steel sheet of the present invention,
The upper composite galvanized layer contains one or more of Ni, Co, Mn, and Sn. Ni, Co
, Mn and Sn, when present in a zinc electroplating bath, are deposited as metals and either form a solid solution in Zn or form an alloy with Zn, thereby moderately slowing down the melting rate of Zn. It exhibits a suppressing effect and contributes to improving the corrosion resistance of the plating film. Each of these elements is 0.
It exhibits the best effect when it is present in Zn in an amount ranging from 1% to less than 7%; outside this range, coating film creep and red rust occur easily in uneven areas, resulting in good corrosion resistance. is not obtained. Although these elements exhibit effects when added alone to Zn, they may be added in combination of two or more.

Good corrosion resistance can also be obtained by adding one or more of Mo, W and C to the above metal components. Mo and W eutectoid with Zn as oxides or hydroxides. By doing so, it is thought that C' is electrodeposited as a metal and dissolved in Zn as a solid solution, thereby contributing to the formation of a protective layer against the corrosion reaction (melting) of Zn, thereby improving corrosion resistance.

Note that when Cr is added alone to Zn, the plating appearance deteriorates as described in the above-mentioned Japanese Patent Application Laid-Open No. 57-070291, but according to the present invention, one of Ni, Co, Mn, and Sn or When eutectoid with two or more types, the deterioration of the plating appearance is improved.

Mo, W, and Cr are not sufficiently effective in suppressing the progress of Zn corrosion if they are simply combined with these elements and Zn, but if they are eutectoid with 1ai or more of any of Ni, Co, Mn, and Sn. By doing so, a remarkable effect of improving corrosion resistance is exhibited. When one or more of Mo, W, and Cr are added, each of these elements is most effective in improving post-painting corrosion resistance when used in a metal amount of 0.1% or more and less than 7%. It was found that it had excellent effects.

According to another feature of the present invention, the upper plating layer is composed of a composite zinc plating in which Zn contains at least IN of Ti, AQ-Mg, and Si. Since Ti, 8Q, Mg, and Si are not electrodeposited from aqueous solutions, they must be added in the form of oxides, hydroxides, metal powders, etc., and eutectoid in the plating film. These components also form a protective layer on the surface together with the corrosion products of other components, and have the effect of appropriately suppressing the corrosion rate of Zn, and are therefore considered to contribute to improving the corrosion resistance of the coating.

Unlike MOLW and Cr mentioned above, Ti, Mg, and Si sufficiently improve the corrosion resistance after coating even when they are contained alone in Zn. The optimum content of these components in the plating film is 0.1% or more and less than 10% as metal. If it exceeds 10%, the corrosion inhibiting effect of Zn becomes excessive, making it difficult to obtain the desired sacrificial rust prevention ability.

These components may be contained in Zn alone, or two or more of them may be added in combination.

In addition to one or more of Ti, AQlMg and Si, the above-mentioned Nis Co, Mns Sn
, Mo, W and Cr 1! Alternatively, if two or more types are added to each in a specified amount, that is, an amount of 0.1% or more and less than 7%, and eutectoid with Zn, Ti, A.
It has been found that the effect of improving corrosion resistance is further improved compared to the case where only one or more of Mg, Mg and Si are added.

As mentioned above, various additive elements can be used singly or in combination in the upper composite galvanized layer of the double-layer plated steel sheet of the present invention, and in any case, although there are differences in degree, Z
It is effective in suppressing corrosion of n. However, when two or more alloying elements are used together within the scope of the present invention, if their total content becomes excessive, the corrosion inhibiting effect becomes too severe, so generally the total content of alloying elements other than Zn is
It is preferable to set it to about less than 0%. Note that the selection of the type of additive element and its content within the scope of the present invention can be appropriately determined by those skilled in the art through experiments.

The double-layer plated steel sheet of the present invention can be manufactured using a known composite zinc electroplating method. For example, after the surface of a cold-rolled steel sheet is degreased, pickled, cleaned, and activated, a lower layer of Ni--Zn alloy plating is first applied. This is because in known sulfate bath or chloride bath Zn plating, the Zn in the bath
It can be carried out by replacing a part of the amount (for example, about 40 to 90%) with nickel sulfate or nickel chloride, and the plating conditions are, for example, pi i, o ~ 3.0, bath temperature 50
~70°C and a plating current density of 50 to 100 A/dm2, as usual.

The amount of plating deposited on the lower layer is N
It may be the same as in the case of the i-Zn-based plating layer, and is within the range of about 5 to 60 g/rrl per side, for example.

If the amount of plating deposited on the lower layer is too small, the excellent corrosion resistance inherent to Ni-Zn-based plating cannot be ensured. On the other hand, 60g/
A thickness exceeding rrf is not practically necessary and is only uneconomical.

On the thus formed Ni-Zn alloy plating layer,
A composite zinc plating containing at least one metal component defined in the present invention is applied to form an upper layer. This plating can also be carried out by a normal composite zinc electroplating method in which a part of the Zn amount in the bath is replaced with a desired metal component compound or metal powder.

For example, chloride or sulfate for Ni, Co, Mn and Sn, metal acids or their salts for Mo, W and Cr, and Ti, Al, Mg
and Si can be added to the bath in the form of oxides. The amount of metal components added to the galvanizing bath varies depending on the type of compound added, and is adjusted so as to obtain the desired metal content.

The coating amount of the upper layer of composite galvanizing is 1 to 1 per side.
0g/n? It is necessary to have a light weight within the range of .

Is this adhesion amount Ig/n? If it is less than this, the occurrence of red rust in the vortex becomes noticeable, and the corrosion resistance after painting deteriorates. On the other hand, if this exceeds 10 g/m, coating film creep is particularly likely to occur, which also leads to deterioration of corrosion resistance. In addition,
As for the plating conditions, normal Zn plating conditions can be used as in the case of the lower layer Ni-Zn plating.

Hereinafter, the effects of the present invention will be illustrated by examples.

After electrolytically degreasing and pickling a 70x150m cold-rolled steel plate, Zn was applied by beaker plating under the following conditions.
-Ni alloy electroplating was applied to one side to form plated coatings of varying coverage and Ni content.

Plating bath composition: Zinc sulfate 60-250g/j! Nickel sulfate 260g/f Plating bath temperature: 50℃ Plating bath pH: 1.5 Current density: 40A/d 2 After washing the Zn-Ni plated test piece with water, the electrogalvanizing bath shown below was prepared with the basic composition. and Ni, Co,
Mn, Sn, M0. W% Cr, Tis AQ
s Using a plating bath containing one or more metal compounds of Mg and Si, electroplating was performed under the following conditions to coat the previously formed Zn-Ni alloy plating layer with various compositions. An upper layer composite galvanized coating was formed.

Electrogalvanizing bath: Zinc sulfate 400g//! Plating bath temperature: 50°C Plating bath pH: t, s Current density: 40A/d1112 The specific compound species of each of the above metals added to the bath are Ni, Co,
, Mns Sn is sulfate, Cr is dichromic acid, MO is ammonium molybdate, W is sodium tungstate, and Ti, Al, Mg, Si are oxides, i.e., titanium dioxide (average particle size 0.1μ ),
Alumina sol (average particle size 0.1μ), magnesium oxide (average particle size 0.1μ), silica sol (average particle size 0.05
μ).

The two-layer plated steel sheets thus obtained all exhibited a good plating appearance of gray to grayish white color.

For comparison, various double-layer plated steel sheets (comparative examples) in which the coating composition or coating amount of any layer is outside the range specified by the present invention, as well as the Various known double layer or partially plated steel plates (conventional examples)
was prepared.

The plating performance of the surface-treated steel sheet thus obtained was evaluated by a composite corrosion test, a cratering test, and an atmospheric exposure test. Note that the same test was conducted on the bare cold-rolled steel sheet used in this example, and the results were included in the conventional example.

Combined corrosion test (corrosion resistance after painting): The test piece of the plated steel plate obtained above was subjected to the steps (11 to (6)) shown in Table 1 below in order according to the automotive exterior painting process. Treatment and 3-coat painting were applied to the plated side.

fl + Degreasing Ridrin SD 200 (Spray one-way method) (2) Surface conditioning Fixozin 5TO (dip method) (3) Phosphate treatment Granozin SD 20
00 (dip method) % formula %) *I S Yes Day Paint Tokuzu 1 Next, on the painted surface of the obtained painted steel plate, cross-cut scratches reaching the substrate were made on the painted surface, and salt water immersion (5% NaCl, Room temperature 15 minutes) - Drying (50°C, 19 hours 45 minutes) - Warm humidity (
A composite corrosion test was conducted by repeating the treatment at 50° C. and a relative humidity of 90% or higher for 4 hours for 30 days. Corrosion resistance was evaluated by examining the creep width of the coating film from the cross-cut scratches and the rate of occurrence of red rust at the cross-cut vortices.

The creep width of the coating film was determined by measuring the creep width on one side over the entire length of the cut scratch, and the maximum value was taken as the measured value.

In addition, the red rust occurrence rate is the area ratio (%) in which red rust occurs over the entire length of the cross-cut wound.

Cratering property test: After degreasing, surface conditioning, and phosphate treatment, the bare steel sheet test piece obtained above was subjected to cationic electrolysis under the conditions shown below. The material was coated and the cratering property during electrodeposition was investigated.

Paint system used: Nippon Paint ■ cationic electrodeposition paint Powercoat U-50 (trade name) (2 weeks after bath preparation) Temperature: 28±1℃ Electrodeposition voltage: 3oov Current application time: 2 minutes Electrodeposition film thickness: 20μm Sample area/counter electrode area = 1/2 (sample area 1.5 dm2) Baking: 170°C x 20 minutes Evaluation was performed by visually determining the number of craters with a diameter of 0.1 m or more on the electrodeposition coating, and when the number of craters was 10. A case of less than 10 pieces/d+m2 was rated A (good), and a case of 10 or more (11 or more and less than 100 pieces was rated B (poor)).

Atmospheric UAn test (resistance to corrosion after painting): After making a cross-cuff (cross-cuff) scratch on the coated surface of the test piece that reached the base material, a part of the electrodeposited plated steel plate obtained in the above-mentioned combined corrosion test was applied to the waterfront area of Wakaryo City. An atmospheric exposure test was conducted in which corrosion was accelerated by spraying 5% saline twice a week.

After 2 years had elapsed, the creep width of the coating film from the cross-cut scratches and the rate of occurrence of red rust on the cross-cut vortices were investigated using the same method as the composite corrosion test, and the post-painting corrosion resistance was evaluated.

The above test results are summarized in Tables 2 to 7.

Table 2 shows the results of a composite corrosion test and a cratering property test during cationic electrodeposition of a double-layer galvanized steel sheet whose upper layer is a composite galvanized layer containing one or more of Ni, Co, Mn, and Sn. Table 3 shows the results of atmospheric exposure tests for the same two-layer plated steel sheets.

The following can be seen from Table 2. As in Comparative Examples 81 to B8, when the amount of plating deposited on the upper layer or the content of metal components added to Zn is out of the range of the present invention, the creep width and the occurrence rate of red rust are large. Also, like B9~BIO, the lower layer Z
Regarding the n-Ni coating, good wearability after coating is not obtained outside the range of Ni content of 7 to 20%.

On the other hand, in the examples of the present invention shown in A1-A18, both the creep width and the incidence of red rust are small, and the comparative examples and conventional examples (C
1 to C6) exhibits better corrosion resistance.

In addition, regarding cratering property, Zn-Fe is used in the upper layer.
In the conventional examples (C2, C3) in which alloy plating was applied, the number of craters generated was more than 10 pieces/dI+12, but in all the examples of the present invention, the cratering property was improved.

Atmosphere 11J in Table 3! Similar to the above-mentioned composite corrosion test results, the dew test results demonstrate that the double-layer plated steel sheet of the present invention has excellent post-painting corrosion resistance in terms of both creep width and red rust incidence even when exposed to the atmosphere. It shows. On the other hand, both the comparative example and the conventional example have insufficient corrosion resistance.

Table 4 shows Ni, Co, Mn and S as upper layers.
In addition to L species of n or more, Mo, W and Cr
When forming a composite galvanized layer containing more than one species,
The results of the composite corrosion test and cratering property during cationic electrodeposition are shown. Similar to the results in Table 2 above, even when one or more of MO%W and Cr was further added to the upper composite galvanized layer, the examples of the present invention showed good post-painting corrosion resistance and cratering resistance. ing.

However, in the comparative example in which the content of the additive metal component in the upper plating layer was outside the range of the present invention, the corrosion resistance deteriorated.

Table 5 shows the results of a composite corrosion test and a cratering property test during cationic electrodeposition of a double-layer plated steel sheet whose upper layer is a composite galvanized layer containing one or more of Ti, A(!, Mg, and Si). Table 6 also shows the results of the atmospheric exposure test for the same two-layer plated steel sheets.

As can be seen from Table 5, when the content of the additive metal component in the upper layer or the amount of upper layer plating is out of the range of the present invention, the creep width of the coating film and the incidence of red rust deteriorate as in Comparative Examples G1 to G5. However, the examples of the present invention show good results for both of these.

From the atmospheric exposure test results in Table 6, the above-mentioned composite corrosion ii
t: Similar to the experimental results, it can be seen that the double-layer plated steel sheet of the example of the present invention shows good results in terms of corrosion resistance when exposed to the atmosphere. On the other hand, in the comparative example, both the creep width of the coating film and the incidence of red rust were not good.

Table 7 shows the composite corrosion test and cratering property during cationic electrodeposition when a composite galvanized layer containing one or more of Ti5N2, Mg, and Si as well as an additional metal component is formed as an upper layer. Show the results. Compared to the case of adding only one or more of Ti5Al, Mg, and Si shown in Table 5, further improvement in post-painting corrosion resistance was observed by adding metal elements such as Ni, Co, and MnsMo. It will be done.

(Effect) As is clear from the above results, the double-layer plated steel sheet according to the present invention has excellent effects on external rust evaluation in different corrosive environments.
Creep occurrence and red rust occurrence in uneven parts are both good, showing excellent post-painting corrosion resistance. In addition, since craters are less likely to occur during cationic electrodeposition coating, a suitable surface treatment with good bare corrosion resistance (e.g. Ni
If coated with Zn-based alloy plating, it will exhibit excellent performance as an automobile exterior panel.

Claims (4)

[Claims] (1) Zn containing 7 to 20 wt% Ni on the steel plate surface
- A lower layer consisting of a Ni-based alloy plating layer and a Zn layer on top of the lower layer.
and an upper layer with a coating weight of 1 to 10 g/m^2 consisting of a composite galvanized layer containing 0.1 wt% or more and less than 7 wt% of one or more of Ni, Co, Mn, and Sn as main components. Highly corrosion-resistant double-layer plated steel sheet. (2) The upper layer contains 0.1 wt% or more and less than 7 wt% of one or more of Ni, Co, Mn, and Sn, and further contains 0.1 wt% or more and less than 7 wt% of one or more of Mo, W, and Cr. The highly corrosion-resistant double-layer plated steel sheet according to claim 1, comprising a composite galvanized layer containing .1 wt% or more and less than 7 wt%. (3) Zn containing 7 to 20 wt% Ni on the steel plate surface
- A lower layer consisting of a Ni-based alloy plating layer and a Zn layer on top of the lower layer.
A composite galvanized layer containing 0.1 wt% or more and less than 10 wt% of one or more of Ti, Al, Mg, and Si, with a coating weight of 1 to 10 g/m^2
A highly corrosion-resistant double-layer plated steel sheet characterized by forming an upper layer of. (4) The upper layer contains 0.1 wt% or more and less than 10 wt% of one or more of Ti, Al, Mg, and Si, and further contains Ni, Co, Mn, Sn, Mo,
The highly corrosion-resistant double-layer plated steel sheet according to claim 3, comprising a composite galvanized layer containing at least 0.1 wt% and less than 7 wt% of one or more of W and Cr.