JPH0211799A - Zn-based surface-treated steel sheet having excellent corrosion resistance after coating - Google Patents

Abstract

PURPOSE:To produce the title Zn-based surface-treated steel sheet having excellent corrosion resistance after coating by forming a Zn-Mn electroplating layer contg. a hydrochloric acid-soluble alumina sol on a steel sheet under specified conditions, and forming an Fe-Zn plating layer thereon under specified conditions. CONSTITUTION:A Zn-Mn electroplating layer contg. 3-60wt.% Mn and further contg. 0.01-3wt.% of an alumina sol soluble in >=0.1 N hydrochloric acid expressed in terms of Al2O3 is formed at 5g/m<2> coating weight on a steel sheet as a first layer. An Fe-Zn plating layer contg. >=30wt.% Fe is formed thereon at >=1g/m<2> coating weight as a second layer. By this method, a Zn-based surface- treated steel sheet coated with a tightly adhered film and having excellent corrosion resistance after coating is obtained, and advantageously used for an automobile body member, etc.

Description

DETAILED DESCRIPTION OF THE INVENTION <Industrial Application Field> The present invention relates to a surface-treated steel sheet suitable for automobile steel sheets, etc., and particularly to a Zn-based surface-treated steel sheet that has excellent corrosion resistance after painting.

<Conventional technology> Due to the anticorrosion function of Zn, Zn-plated steel sheets have been widely used as anti-rust steel sheets for automobiles, home appliances, construction materials, etc. that require corrosion resistance. There is a problem that the anti-corrosion function of the coating film cannot be fully demonstrated.

In particular, in the phosphate treatment as a base for electrodeposition coating in the painting process of automobile manufacturing, Zn-rich plated steel sheets are treated with hovite Zn+ (PO4), which is easily attacked by alkali.
)2.4H20 precipitates as a chemical conversion film, so it does not function sufficiently to suppress corrosion reactions.

By forming an Fe-rich FeZn alloy plating layer on top of the Zn-based plating layer, it is difficult to attack by alkali.
4 H2O can be formed as a conversion coating. However, if the first layer has Fe-Zn plating and the second layer has normal alloy plating such as ZnNi or Zn-Fe, when paint film damage (chipping) occurs due to flying stones etc. in automotive applications, the damaged part will be damaged. There is a problem that red rust easily occurs.

To solve this problem, Ni-Zn alloy plating (N i + co / Mn = 16 to 80 wt%) as the second layer is disclosed in JP-A-61-170593, but the phosphate treatment liquid type or Depending on the treatment process, the uniformity and adhesion amount of the phosphate film tend to vary, leading to problems with stability and versatility.

<Problems to be Solved by the Invention> (1) When Fe-Zn plating with good phosphate treatment property is used as the second layer, the above-mentioned normal alloy plating has insufficient sacrificial corrosion protection ability as the first layer plating. It is not sufficient and can easily cause red rust. Furthermore, since pure Zn plating is active, it has the disadvantage that corrosion of the Zn plating layer significantly progresses in a corrosive environment such as a salt water jet.

(2) First layer Zn alloy plating, for example Zn-Ni
, Zn-Fe, Zn-Co, Zn-Cr, Zn-Mn,
Zn-3n and the like have a disadvantage that the internal stress of the plating film is greater than that of pure Zn plating, and therefore the adhesion of the plating layer to the steel plate base is poor.

(3) In addition, it is common to apply three coats of cationic electrodeposition coating, intermediate coating, and top coating to a total thickness of approximately 100 μm or more on the exterior surfaces of automobiles, and these bake-offs occur due to shrinkage stress during shrinkage. Acting on the galvanizing layer, plating adhesion is lower than when unpainted. Furthermore, in cold regions during the winter, the temperature drops far below the freezing point, causing shrinkage of the coating film, resulting in large stress on the plating layer. If the steel sheet is chipped in such a state, the plating layer of the Zn-based alloy plated steel sheet will peel off. As a countermeasure against this, a plated steel sheet (Japanese Patent Application Laid-open No. 62-267491) has been disclosed in which thin coating of one or more of Fe, Ni, and CO is applied between the first layer and the steel sheet, but this requires an additional treatment process. 3 layers), it becomes complicated and impractical.

Therefore, an object of the present invention is to eliminate the various drawbacks of the prior art described above, and to provide a Zn-based surface-treated steel sheet that has excellent coating film adhesion and corrosion resistance after coating.

<Means for Solving the Problems> The present invention provides an electrolytic Zn--Mn plating layer having an Mn content of 3 to 60 wt% as a first layer, in which an alumina sol soluble in hydrochloric acid with a concentration of 0.1 N or more is mixed with AfL20. 0.01 to 3 in conversion
Corrosion resistance after painting, characterized by having a plating layer containing 5 g/m2 or more containing Fe at 5 g/m2 or more, and having, as a second layer thereon, a Fe-Zn plating layer containing 30 wt% or more Fe at 1 g/m2 or more. The purpose of the present invention is to provide a Zn-based surface-treated steel sheet with excellent properties.

The first layer further includes Co, Ni, and Fe.

At least one selected from the group consisting of Cr and Mo
Seed elements and/or 5i02, TiO2, ZrO2
, Nb2O5 and Ta205 in an amount of 30 wt% or less and 6 wt% or less, respectively.

The present invention will be explained in more detail below.

The present invention has been made based on the findings described below.

(1) In order to suppress the occurrence of red rust on the second L'e-Zn plating layer, the first plating layer must have sufficient sacrificial anticorrosion ability. , Zn and Mn are selected which can be electroplated from an aqueous solution. Mn is Zn
It is even more vulgar and exhibits stronger sacrificial anti-corrosion ability.

The Mn content in the Zn-Mn alloy plating layer is 3 to 60 wt.
% is a suitable range. If it is less than 3 wt%, there is no sacrificial corrosion protection effect, and if it exceeds 60 wt%, it becomes susceptible to chemical dissolution by the Fe-Zn plating solution during the second layer Fe-Zn plating,
This causes inconveniences such as a rough surface.

(2) Corrosion resistance is improved when Afl is contained in the Zn-Mn plating layer. This is Zn, Mn and A in the Metsuki calendar.
In the process of corrosion of fL, ρ ions are generated, which are corrosion products with excellent protective effects, such as Zn (OH) 2 and Mn.
This is because it contributes to the generation and stability of =03.

(3) AfL ions can be present in corrosion products by including an acid-soluble A°C compound in the Zn-Mn plating layer. That is, corrosion progresses through an anode reaction and a cathode reaction, but in the anode part, for example, according to the following formula, Zn-4Zn''+2e Z n''' + 2 H20 = Z n (
OH) 2 + 28 "Mn-+Mn" + 2e M n'" + 2 H20= M n (OH
) 2 + 2 H”F e −+ F e ”+
2 e Fe"+2H20-eFe (OH) 2 +2H
'' is generated, acidified by dissolution and hydrolysis of Zn, Mn and Fe (base steel), and dissolves the compound present in the plating layer (ionization of 8β).

On the other hand, although the cathode part becomes alkaline, the dissolution reaction of Mn does not occur, so the cathode reaction that causes paint film blistering is suppressed.

(4) Depending on the corrosive environment, by including one or more oxides among 5i02 Ti02, ZrO2, Nb2O5 and TaJs in the ZnMn-alumina plating layer, these particles can retain corrosion products. Because of this, it exhibits even better corrosion resistance.

Moreover, the above-mentioned particles seem to have the effect of relaxing the internal stress of the Zn--Mn plating layer, and exhibit particularly excellent effects in suppressing plating peeling during chipping.

(5) Furthermore, Co, Ni, Fe, which are more sensitive than Zn and Mn,
11m or two or more elements in Cr and Mo with Z
By incorporating it into the n-Mn plating layer, Z
The activity of the n-Mn plating layer is controlled.

(6) The amount of plating deposited on the first layer is preferably 5 g/l112 or more. If it is less than 5 g/m2, the corrosion resistance is not sufficient (7) The Fe content of the second layer Fe-Zn plating is 30w
t% or more is preferable. If it is less than 30 wt%, it is difficult to form a good phosphate film. In addition, the plating amount is 1
g/m' or more is preferable.

If it is less than 1 g/m2, the formation of a phosphate film is insufficient.

Note that a small amount of Co is contained in the Fe-Zn plating layer.

It may contain N1 and Gr.

As the An compound added to the Zn--Mn plating solution, positively charged alumina sol is desirable. This is because positive charging makes it easier to eutectoid in the plating layer. In addition, since alumina sol is generally produced by a wet method at a low temperature below 100°C, it is not completely converted into AfL203, and most of it is composed of Al1 hydroxide, hydrated oxide, or a composite with A°C salt. It exists as.

When such alumina sol is eutectoid on the plating layer, 0
．． It dissolves relatively easily in acids stronger than 1N. It is particularly soluble in hydrochloric acid. In addition, since HCA is generated in the anode part of the corrosive liquid mainly composed of common salt, the pH decreases and becomes acidic. Therefore, AfL used in the present invention
The compound is preferably an alumina sol that is soluble in 0.1N or higher hydrochloric acid.

The content of alumina sol in the Zn-Mn plating layer is Ait
0.01 to 3 wt% in terms of 203 is desirable. If this is less than 0.01wt%, the corrosion resistance improvement effect is not sufficient,
In addition, if it exceeds 3 wt%, not only does the appearance of the plating tend to turn black and the plating adhesion decreases, but it also requires the addition of a high concentration of alumina sol during plating, which increases the bath viscosity and makes manufacturing difficult. This also causes problems.

SiO2, TiO2, Zr0z, Nb2O5 and Ta in the Zn-Mn-alumina plating layer
The content of one or more oxides in 205 is 6
It is desirable that it is less than wt%.

If it exceeds 6 wt%, the corrosion resistance improvement effect of the plating material itself reaches saturation, but the plating layer becomes brittle, making plating defects such as cracks more likely to occur during processing, and the corrosion resistance of the plating as a whole decreases.

In the present invention, preferable plating characteristics can be obtained within the range of the content of one or more oxides among 5in2 T i 02 , Z r 02 , Nk2O5 and Ta205, but especially 0
．． When the content is 0.01 wt% or more, it is advantageous, especially in the case of a thin basis weight, because the effect of retaining corrosion products is high and high corrosion resistance can be obtained.

5in2 to use. The particle size of Tie, ZrO2, Nb2O5 and Ta2o5 is preferably fine particles of about 1 to 50 onm. When adding these particles to the ZnMn plating solution, they may be added separately from the alumina sol,
Furthermore, particles to which alumina sol has been adsorbed or bonded in advance may be added to these particles.

Particularly, particles that are adsorbed or bonded to positively charged alumina sol are likely to eutectoid in the plating layer by electrolysis and are effective.

Co, Ni, Fe, Cr, and Mo, which are more responsible than Zn and Mn, are used as alloying elements that appropriately control the activity of the Zn--Mn plating layer. The content of one or more of these elements in the plating layer is preferably 30 wt% or less. This is because if it exceeds 30 wt%, the sacrificial anticorrosion performance of Zn-Mn decreases, resulting in a decrease in corrosion resistance depending on the corrosive environment, and also significantly increasing costs.

In addition, oxides such as the above-mentioned S i 02 and Go
These elements may be added individually or both to the Zn-Mn-alumina plating layer.

<Example> The present invention will be specifically described below with reference to Examples.

A cold-rolled steel sheet was subjected to alkaline electrolytic degreasing, pickled with 5% hydrochloric acid, washed with water, and polished under the following conditions. Stirring was performed using a pump, and the liquid flow rate was approximately 60 m/min.
A PT plate is used for the anode, the distance between the electrodes is 20 mm, and the liquid temperature is 5.
It was carried out at 0°C.

As the alumina sol added to the Zn--Mn plating solution for the first layer, #520 manufactured by Nissan Chemical Co., Ltd. was used. Its properties are as follows. This alumina sol dissolves well in 0.1N or higher hydrochloric acid.

An 203 content/20wt% Dispersion medium/water pH: 4.5 Specific gravity 1. 23 (25℃) Viscosity +8
cP (20°C) Surface area: 245m27g Stabilizer Nitric acid particle shape: Rod to granular particle Charge: Positive crystalline Size of coagulated boehmite particles: 5 to 20 nm Appearance Milky colloid liquid Note that the Zn-Mn plating bath is a sulfate bath, a chloride bath A chemical bath, a mixed bath thereof, and other acidic baths can be used. The following example mainly shows a sulfate bath, but is not limited to this.

Here, the amount of Mn and alumina contained in the plating layer is 5in2. Tio2, zro2, Nb, 05, Ta,
The amount of 05 and the total amount of C01Ni, Fe, Cr, and Mo were appropriately controlled by the concentration added in the plating solution.

(1) First layer plating conditions Bath composition Zinc sulfate 50-100 g/u Manganese sulfate 30-2 Og/fL Sodium sulfite 0
．． 5-5 g/IL glycine 5-50
g/ρ alumina sol 1~100mβ/1Si
One or more of n2, TiO2, ZrO2, Nb, C6, TazOs 0.1 to 50 g/J2 Cobalt sulfate 100 g/It or less Nickel sulfate 100 g/42 or more Ferrous sulfate
100 g/f1 or less chromium sulfate 1
o0 g/JZ or less ammonium molybdate 1 o o g/fl A Zn-Mn-alumina composite electroplated steel sheet was produced using a bath to which two or more of the following were appropriately added.

pH 2-4, current density 75A/dm' For comparison of first layer plating, Mn, alumina sol addition concentration, Si
O2, Ti02Z r O2, Nb2O5 and Ta2
Addition concentration of one or more of 05, Co, Ni
, Fe.

Plated steel sheets for quality evaluation tests were prepared with the concentration of one or more ions added among Cr and Mo outside the range of the present invention.

After the first layer plating, water washing was performed, and second layer plating was performed under the following conditions.

(2) Second layer plating conditions Bath composition Ferrous sulfate 100-300 g/fL Zinc sulfate 10-200 g/J! sodium sulfate
50 g/J2 pH 1-2, current density 100 A/dm2 For comparison of second layer plating, a plated steel plate for quality evaluation test was prepared with a bath composition outside the range of the present invention.

The following tests were conducted on the plated steel sheets obtained in the above examples. The results are shown in Table 1.

The chemical conversion and painting treatments were performed in the following order, assuming the process of manufacturing an automobile body.

■Zinc phosphate chemical conversion treatment (using PB3020IA liquid manufactured by Nippon Barcalizing Co., Ltd.) ■Cationic electrodeposition coating (
Using Power Top U-100 paint manufactured by Nippon Paint Co., Ltd., 250cm, film thickness 20μm) ■Intermediate coating (using Amirac Chipping Sealer N3 manufactured by Kansai Paint Co., Ltd., film thickness 35-40μl11) ■Top coating (Kansai Paint Co., Ltd.) (Using Neoamilac 8002 manufactured by Paint Co., Ltd., film thickness 35-40 μm) The resulting 3-coat material was cooled to -20°C, chipped with 10 stones per test piece using a gravelometer, and then peeled off with tape. The peeling state of the paint film was observed and the adhesion of the paint film was examined.

[Evaluation] ◎ Slight peeling (average peeling diameter 3 mm or less)○ Peeling
Small (n4-6mm) △ Peeling Medium (〃
7-11mm) x Peeling (ll
12 mm or more) The above test piece was subjected to a 30-day combined corrosion test with the following combined corrosion cycle as one cycle,
The appearance of red rust was observed and the corrosion resistance was determined.

Salt water spray n 2 hours - dry (60℃) 2 hours - wet (50℃
, 95% relative humidity) 2 hours - 2 hours left indoors [Evaluation] ◎・No red rust, area rate 0%) O4 red rust, little (area rate 1-10%), medium red rust (area rate 11) ~30%) ×: A lot of red rust (occurrence area rate: 31%)
(Effects of the Invention) As detailed above, the Zn-based surface-treated steel sheet of the present invention has excellent paint film adhesion and corrosion resistance after painting, and has excellent properties when used in applications such as automobile body parts. Demonstrate performance.

Claims (4)

[Claims] (1) As the first layer, an alumina sol soluble in hydrochloric acid with a concentration of 0.1 N or more is added in an electrolytic Zn-Mn plating layer with a Mn content of 3 to 60 wt% in terms of 0.01 to 3 in terms of Al_2O_3.
A coating characterized by having a plating layer containing 5 g/m^2 or more containing Fe at a thickness of 5 g/m^2 or more, and having, as a second layer thereon, a Fe-Zn plating layer containing 30 wt% or more Fe at a thickness of 1 g/m^2 or more. Zn-based surface-treated steel sheet with excellent post-corrosion resistance. (2) As the first layer, at least one element selected from the group consisting of Co, Ni, Fe, Cr, and Mo is added to 30 wt% or less in an electrolytic Zn-Mn plating layer with a Mn content of 3 to 60 wt%. Contains an alumina sol that is soluble in hydrochloric acid with a concentration of 0.1 N or more, calculated as 0.0 in terms of Al_2O_3.
It is characterized by having a plating layer containing 1 to 3 wt% of Fe at 5 g/m^2 or more, and having a Fe-Zn plating layer containing 30 wt% or more of Fe as a second layer thereon at 1 g/m^2 or more. Zn-based surface-treated steel sheet with excellent corrosion resistance after painting. (3) As the first layer, an alumina sol soluble in hydrochloric acid with a concentration of 0.1 N or more is added in an electroplated Zn-Mn layer with a Mn content of 3 to 60 wt% from 0.01 to 0.01 in terms of Al_2O_3.
Contains 3wt% and further contains SiO_2, TiO_2, Zr
1 among O_2, Nb_2O_5 and Ta_2O_5
5 g/m^2 or more of a plating layer containing 6 wt% or less of a species or two or more oxides, and a 1 g/m2 Fe-Zn plating layer containing 30 wt% or more of Fe as a second layer thereon. A Zn-based surface-treated steel sheet with excellent corrosion resistance after painting, characterized by having ^2 or more. (4) As the first layer, an alumina sol soluble in hydrochloric acid with a concentration of 0.1 N or more is added in an electrolytic Zn-Mn plating layer with a Mn content of 3 to 60 wt% from 0.01 to 0.01 in terms of Al_2O_3.
Contains 3wt%, SiO_2, TiO_2, ZrO_2
, contains one or more oxides of Nb_2O_5 and Ta_2O_5 at 6 wt% or less, and further contains Co
, Ni, Fe, Cr, and Mo.
A Zn-based surface-treated steel sheet with excellent post-painting corrosion resistance, characterized by having an Fe-Zn plating layer containing 30 wt% or more of Fe at a thickness of 1 g/m^2 or more as a second layer thereon. .