JPH05171491A - Double layer plated steel excellent in corrosion resistance after coating - Google Patents

Abstract

PURPOSE:To improve the corrosion resistance of steel after coating by constituting lower layer plating and upper layer plating of specified alloy layers. CONSTITUTION:Lower layer plating is constituted as a Zn or Zn alloy layer and upper layer plating is constituted as a porous Al or Al alloy layer with 0.1 to 10g/m<2> coating weight. In this way the objective double layer plated steel excellent in corrosion resistance after coating as well as corrosion resistance in an uncoating and flawless state and hard to produce the blister of a coating film even if the coating film is flawed after coating can be manufactured.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a two-layer plated steel material having excellent coating film adhesion suitable for fields such as automobiles, building materials and home appliances.

[0002]

[Prior Art] Zn-based plated steel as a base material
The present inventors have previously developed a double-layered steel material plated with Al and applied for a patent (Japanese Patent Laid-Open No. 61-119693).
No. 61-261496). These two-layer plated steel materials have even better corrosion resistance than the Zn-based plated steel material of the base material, and particularly have very high corrosion resistance in the unpainted and intact state. However, as a result of a more detailed study, it was found that in some of the double-layer plated steel materials of this type, blistering is likely to occur at the coating film cut portion.

This coating film swelling occurs not at the interface between the coating film and the upper Al-based plating but at the interface between the Al-based plating and the Zn-based plating, and is similar in appearance to a general coating film swelling. It becomes a situation. Usually, the plated steel material is often used by painting, and such a blister of the coating film is not preferable.

[0004]

SUMMARY OF THE INVENTION An object of the present invention is to provide a two-layer plated steel material which is excellent not only in unpainted corrosion resistance but also in coating adhesion and corrosion resistance after painting.

[0005]

As a result of investigating the cause of coating film blistering in two-layer plating, the present inventors found that the amount of upper Al-based plating adhered was 10 g / m ² or less and the upper-layer plating film It was found that the above-mentioned swelling of the coating film was suppressed when it was porous, and the present invention was completed.

In the present invention, the lower layer plating is Zn or
Zn alloy layer, upper layer plating is 0.1-10 g / m ² porosity
The gist is a two-layer plated steel material having excellent corrosion resistance after painting, which is an Al or Al alloy layer.

[0007]

According to the investigation by the present inventors, the blisters in the coating film cut portion have an adhesion amount of the upper Al-based plating of more than 10 g / m ² , or an adhesion amount of the upper-layer Al-based plating of 10 g / m ^2. It has been found that the denser the plating film, the more likely it is to generate even if it is m ² or less. This is considered to be due to the difference in the elution rate between the upper Al-based plating and the lower Zn-based plating. When the adhesion amount of the upper Al-based plating exceeds 10 g / m ² or when the plating film is dense, the corrosion potential of the obtained two-layer plating shows almost the same potential as the single-layer Al-based plating (−0.7 V, vs Ag / AgCl), lower layer Z
It is significantly more noble than n-type plating. for that reason,
Zn-based plating is preferentially eluted over Al-based plating in the coating cut portion, and as described above, blistering occurs at the interface between the Al-based plating and the Zn-based plating. On the other hand, when the adhesion amount of the upper Al-based plating is 10 g / m ² or less and the coating is porous, the corrosion potential of the two-layer plating is as low as the lower Zn-based plating. (-1.0 to -1.1 V, vs Ag / AgCl), and preferential elution of the lower Zn-based plating is suppressed. Therefore, it is considered that the above-mentioned blistering is prevented.

The base material of the two-layer plated steel material of the present invention is typically a cold rolled steel sheet, but the steel type and shape are not particularly limited. A base material may be a steel material having an arbitrary shape such as a plate material, a bar material, a wire material, a pipe material, and a mold material, which is made of an arbitrary steel type including carbon steel, various alloy steels, and stainless steel.

The lower layer is Zn-based plating, that is, Zn plating or Zn
Alloy plating. The Zn alloy plating preferably has a sacrificial anticorrosive ability for steel. For example, Zn-Fe
Alloy plating (including galvannealing), Zn-Ni alloy plating, Zn-Co alloy plating, Zn-Mn alloy plating, Zn-
Examples include Mg alloy plating, Zn-Cr alloy plating, and Zn-Al alloy plating, but the invention is not limited to these.
For Zn alloy plating, additional alloying elements such as Ni, Fe, Mo,
One or more of various elements such as Cr, Co, Al and Mg may be further added. Also, the lower layer Zn plating or Zn
The alloy plating layer is made of Al ₂ O ₃ , SiO ₂ , TiO ₂ , Zr during plating.
A composite plating in which an oxide such as O ₂ , SnO ₂ or Sb ₂ O ₃ is codeposited may be used.

The amount of the lower Zn-based plating applied is not particularly limited, but it is preferably 100 in consideration of workability and weldability.
g / m ² or less, in particular 20 to 60 g / m ² is preferred. The method for forming the Zn-based plating is also not particularly limited, and an electroplating method, a hot dip plating method, a vacuum deposition method, an ion plating method or the like can be used.

The upper layer is Al-based plating, that is, Al plating or Al
It is an alloy plating, and the coating amount is 0.1 to 10 g / m ² porous film. If the coating amount of the upper Al-based plating film is less than 0.1 g / m ² , the corrosion resistance is unpainted and intact, and if it exceeds 10 g / m ² , sufficient coating film adhesion is achieved even with a porous film. ,
No corrosion resistance is obtained after painting.

The porosity of the upper porous Al-based plating film is
The optimum porosity (area ratio of holes per unit area) is 0.2 to 0.7. The porosity and the pore diameter can be directly determined by the measurement such as SEM (scanning electron microscope) observation. If the porosity is less than 0.2, the corrosion resistance after coating is lowered, and the coating film swells at the coating film cut portion, as in the case of the dense coating film. If the porosity exceeds 0.7, unpainted,
Corrosion resistance is reduced without damage. On the other hand, if the pore size is too small, the corrosion product will be clogged in the pores, and the effect will not be sufficiently exhibited. Therefore, the pore size is preferably about 0.01 to 3.0 mm.

The type of Al-based plating is not particularly limited as long as the above conditions are satisfied, but in order to most effectively exhibit the effect of improving the corrosion resistance after coating, Al-Mn alloy plating,
Al alloy plating such as Al-Cr alloy plating and Al-Mg alloy plating is suitable.

The upper layer Al-based plating is formed by a molten salt bath,
Various conventionally known methods such as an electroplating method using an organic solvent bath, a vacuum deposition method, a dry process method such as an ion plating method, and a thermal spraying method can be used. However,
In order to form the porous film, for example, the electroplating method uses a method such as increasing the current density or not stirring the plating solution. Further, in the case of the dry process, a means of lowering the substrate temperature or increasing the film forming speed is adopted within a range where the adhesiveness does not decrease. The porosity can be adjusted by controlling such plating conditions.

The two-layer plated steel material of the present invention may have the above-mentioned two-layer plating on at least one surface of the steel material. When the above two-layer plating is not applied to the entire surface, the remaining surface may be a non-plated surface or a surface on which another plating layer (eg, Zn-based plating single layer) is applied.

When coating the double-layer plated steel sheet of the present invention,
As the coating base treatment, a chemical conversion treatment such as a chromate treatment or a phosphate treatment may be performed by a conventional method, whereby the corrosion resistance and / or the adhesion is further improved.

[0017]

【Example】

As a basic bath (lower Zn-based plating) ZnSO _4, Zn plating bath composed mainly of Na ₂ SO _4, the alloying elements components using the added sulfate type plating bath as the sulphate if necessary, plate thickness 0.8 mm
Various cold-rolled steel sheets were plated with Zn or Zn alloy by electroplating to obtain various Zn-plated steel sheets. The plating current density was 20 A / dm ^2, and the current was applied so that a plating film with an adhesion amount of 40 g / m ² was formed.

(Upper layer Al-based plating) Using various Zn-based plated steel sheets obtained above as a base material, Al-based plating is performed by a molten salt electrolysis method or a vapor deposition plating method under the following conditions to obtain various porosity and adhesion amount. An upper Al-based porous plating film was formed. Upper layer Al
The porosity of the system plating film was adjusted by controlling the plating current density and the liquid flow rate in the case of molten salt electrolysis and by controlling the substrate temperature in the case of vapor deposition plating.

(1) Molten salt electroplating method Basic bath: AlCl ₃ —NaCl—KCl ternary molten salt. In the case of Al alloy plating, the required amount of chloride of each alloy element is added.

Bath temperature: 150 to 270 ° C. Plating current density: 20 to 60 A / dm ² (Varies depending on alloying element, for example, 20 A / dm ² for pure Al plating; Al-Mn, Al-Cr
For alloy plating, 60 A / dm ² ) Electrode flow velocity: 0.4 to 0.7 m / sec (2) Vapor plating method Pretreatment: Ar sputter Vacuum degree: 10 ^-3 to 10 ^-4 Torr Substrate temperature: 200 ℃ or less Film formation Velocity: 10 μm / min or less The vapor deposition plating was performed by melting the metal to be alloyed with Al in separate crucibles and evaporating by a resistance heating method.

Using the obtained two-layer plated steel sheet, after the chromate treatment in the following manner, electrodeposition coating was carried out, and the corrosion resistance after coating was examined. (Test method for evaluating corrosion resistance after painting) Chromate treatment: Alchrome 713 manufactured by Nippon Parker was used, and after chromate treatment by immersion at room temperature, it was washed with water and dried. The amount of deposited Cr was 10 mg / m ² . Electrodeposition coating: Hold UD80 made by Nippon Paint at a liquid temperature of 28 ° C,
After energizing at a voltage of 210 V, it was washed with water and baked (175 ° C). The film thickness after baking was 20 μm.

Corrosion resistance after coating, the coating sample cut to reach the base steel, salt spray (35 ℃) → dry (50 ℃)
→ A complex corrosion test was conducted with wet (50 ° C) as one cycle (one cycle 24 hours), and the corrosion depth after 100 cycles and the blister width of the coating film were evaluated.

As a reference, the unpainted corrosion resistance was measured as the number of days until the occurrence of red rust in the salt spray test specified by JIS. The test results are summarized in the following Tables 1 and 2 together with the details of the plating of the lower layer and the upper layer.

[0024]

[Table 1]

[0025]

[Table 2]

[0026]

As can be seen from the results of the examples, the two-layer plated steel material having the lower Zn-based plating film and the upper Al-based porous plating film according to the present invention has an upper Al-based plating amount applied.
By using 0.1 to 10 g / m ² of porous plating, in the post-painting corrosion resistance test, both the coating run-out and the corrosion depth are very small, and the post-painting corrosion resistance is excellent. As a result, the phenomenon of coating film swelling when the coating film is scratched, which is a problem with this type of double-layer plated steel material, can be greatly reduced. The two-layer plated steel material of the type of the present invention is used for automobiles,
It is often used as a coating for building materials, home appliances, etc., and reducing the swelling of the coating film leads to a marked improvement in product quality and product value.

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Junichi Uchida 4-533 Kitahama, Chuo-ku, Osaka City Sumitomo Metal Industries, Ltd. (72) Inventor Kunihiro Fukui 4-53-3 Kitahama, Chuo-ku, Osaka City Sumitomo Metal Industries Co., Ltd.

Claims (1)

[Claims] 1. A two-layer structure excellent in corrosion resistance after painting, characterized in that the lower layer plating is a Zn or Zn alloy layer and the upper layer plating is a porous Al or Al alloy layer having an adhesion amount of 0.1 to 10 g / m ^2. Plated steel material.