JPH0211800A - Highly corrosion-resistance zn-based surface-treated steel sheet - Google Patents

Abstract

PURPOSE:To produce the title highly corrosion-resistant Zn-based surface-treated steel sheet by forming a Zn Ni electroplating layer on a steel sheet under specified conditions, and forming a Zn-Mn plating layer contg. a hydrochloric acid- soluble alumina sol thereon under specified conditions. CONSTITUTION:A Zn-Ni electroplating layer contg. 5-50wt.% Ni is formed at >=5g/m<2> coating weight on a steel sheet as a first layer. 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 then formed thereon at >=3g/m<2> coating weight as a second layer. By this method, a Zn-based surface-treated steel sheet having excellent corrosion resistance before and after coating and weldability is obtained, and advantageously used for an automobile body member.

Description

DETAILED DESCRIPTION OF THE INVENTION <Industrial Application Field> The present invention relates to a Zn-based surface-treated steel sheet having excellent properties such as corrosion resistance and weldability.

<Prior Art> Zn-plated steel sheets are widely used as anti-corrosion treated steel sheets for automobiles, home appliances, building materials, etc. that require corrosion resistance. This is because the pure Zn layer is base against the iron of the steel sheet, so there is a sacrificial corrosion protection effect where the Zn corrodes first against plating defects such as pin poles and exposed parts of the base steel caused by processing. This is because it prevents red rust from forming on the steel plate.

However, since pure Zn is active, it has the disadvantage that corrosion of the inner body of the Zn plating layer progresses significantly in a corrosive environment such as a salt water jet. In addition, pure Zn contains conductive Z as a corrosion product.
Since nO is easily generated, the protective effect of corrosion products existing on the surface is poor, which is considered to be one reason why the corrosion resistance is not sufficient.

Zn-Ni alloy plated steel sheets have been put into practical use as a material that suppresses the sacrificial anticorrosion ability of Zn plating and improves the corrosion resistance of the internal body of the plating layer, but depending on the corrosive environment, it has the disadvantage of being prone to red rust.

Since there are limits to such single layer plating, Zn-
A two-layer plated steel sheet has been proposed in which the lower layer is Ni-based plating and the upper layer is plated with Zn as the main component and a small amount of metal components (Japanese Unexamined Patent Publication No. 194195/1982).
However, it does not necessarily have sufficient corrosion resistance as a polished steel sheet for automobile bodies, and there are also problems with weldability.

<Problems to be Solved by the Invention> In other words, the deterioration in continuous spot welding in spot welding commonly used for welding automobile bodies is caused by the continuous welding of Zn-based plated steel sheets, especially when the coating layer is rich in Zn. When making a point, the tip of the welding electrode (made of Cu) is made of hard and brittle Cu.
- A Zn alloy layer (such as Cu5Zn6) is formed, and the tip of the electrode is crushed by repeating partial peeling and formation as the number of dots increases.

The present invention solves the above-mentioned drawbacks of the prior art, and uses a Zn-Mn-alumina composite electroplating layer that is difficult to form an alloy with Cu, which is the main component of welding electrodes, and is less noble than steel sheets and has sacrificial corrosion protection. It is an object of the present invention to provide a plated steel sheet having a Zn--Mn alloy plating layer as an upper layer and a lower layer of a Zn--Mn alloy plating layer which is more negative than the upper layer plating and less noble than the steel sheet.

Means for Solving the Problems> The present invention has an electrolytic Zn-Ni plating layer with a Ni content of 5 to 50 wt% as a first layer of 5 g/m' or more, and a second layer thereon with an Mn In the electrolytic Zn-Mn plating layer with a content of 3 to 60 wt%, an alumina sol soluble in hydrochloric acid with a concentration of 0.1 N or more is added in terms of AJ22O3 of 0.01 to 3.
The present invention provides a highly corrosion-resistant Zn-based surface-treated steel sheet having a plating layer containing 3 g/rn' or more of wt%.

The second layer further includes Co, Ni, FeCr and M
at least one element selected from the group consisting of o and/or 5i02Tie2, ZrO2, Nb2O5
It is preferable to contain 30 wt % or less and 6 wt % or less of one or more oxides of Ta and 05, respectively.

The present invention will be explained in more detail below.

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

(1) The Ni content of the first layer Zn-Ni alloy plating is 5
A range of ~50 wt% is appropriate. If it is less than 5 wt%, it hardly contributes to improving corrosion resistance, and if it exceeds 50 wt%, it becomes too much of a liability and sacrificial corrosion protection ability decreases, which is not desirable.
The amount of Zn--Ni plating deposited must be 5 g/m' or more, and if it is less than 5 g/m', the steel plate will not be sufficiently coated and there will be a problem in corrosion resistance.

Note that a small amount of Co, Fe,
It may also contain Cr.

(2) Mn in the second layer of Zn-Mn-alumina composite electroplating is difficult to form an alloy with the Cu-based welding electrode, and has sacrificial corrosion protection and exhibits excellent corrosion resistance.

(3) Including An in the Zn-Mn plating layer improves corrosion resistance. This is because Aj2 ions are generated during the corrosion of ZnMn and A4 in the plating layer, and corrosion products with excellent protective effects, such as Zn (OH)2. This is because it contributes to the production and stability of Mn2O3.

(4) By containing an acid-soluble An compound in the Zn-Mn plating layer, A4 ions can be present in the corrosion products. That is, corrosion progresses through an anode reaction and a cathode reaction, but in the anode part, for example, Z n -y Z n ''+ 2 e Zn '' + 2H2O-4Zn (OH) 2 + 2H'''M
n -4Mn"+2e Mn 2'"+ 2 H2O-Mn (OH)
2+ 2 H"F e →F e ν+
2eFe”+2H2o = Fe(OH)2
+2H" is produced, acidified by dissolution and hydrolysis of Zn, Mn and Fe (base steel), and dissolves the ρ compound present in the plating layer (ionizes An).

On the other hand, since the cathode part becomes alkaline or no dissolution reaction of Mn occurs, the cathode reaction that causes coating blistering is suppressed.

(5) Depending on the corrosive environment, 51o2 T i 02 , Z r 02 , N b2
By containing one or more oxides among 0s and Ta2O5 in the Zn-Mn alumina plating layer, these particles retain corrosion products, resulting in even better corrosion resistance. This shows the effect of

(6) Furthermore, Co, NiFe, and C are more sensitive than Zn and Mn.
One or more elements among r and Mo are replaced with Zn-
By including it in the Mn plating layer, the activity of the Zn-Mn plating layer is controlled.

(7) The Zn-Mn-alumina composite plating coating amount of the second layer needs to be 3 g/m2 or more, and is 3 g/rr? If it is less than that, the first layer will not be sufficiently covered and there will be problems in corrosion resistance and weldability.

As the Ajl compound added to the Zn-Mn plating solution,
A positively charged alumina sol is desirable. Positive-Il
This is because the l' electron tends 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 Au2O3, and most of it is produced as AJ2 hydroxide, hydrated oxide, or a composite with AJi+salt. Existing.

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 HCJit is generated at the anode part in a corrosive liquid mainly composed of common salt, p
H decreases and becomes acidic. Therefore, A used in the present invention
The fl 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 Al2
001 to 3 wt% in terms of O3 is desirable. This is 0.
If it is less than 0.01 wt%, the corrosion resistance improvement effect will not be sufficient, and if it exceeds 3 wt%, the plating appearance will turn black and the plating adhesion will tend to decrease, and it will be necessary to add a high concentration of alumina sol to the plating bath. However, the viscosity of the bath increases, causing manufacturing problems.

The preferred range of the Mn content in the Zn-Mn-alumina composite plating layer is 3 to 60 wt%.

As will be detailed in the Examples below, if it is less than 3 wt%, there is no effect of improving weldability, and if it exceeds 60 wt%, corrosion resistance is saturated, and depending on the type of chemical conversion treatment, there is a tendency for corrosion resistance after painting to decrease. It was done.

This is thought to be because the uniformity of the chemical conversion film deteriorates, resulting in variations in the amount of the film.

5102, TiO in Zn-Mn-alumina plating layer
The content of one or more oxides among 2, ZrO2, Nb2Og and Ta,05 is preferably 6 wt% or less.

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

In the present invention, such 5in2, TiO2, ZrO
2. Preferred plating properties can be obtained within the range of the content of one or more oxides among Nb2O5 and Ta, 05, but if the content is 0.01 wt% or more, corrosion may occur, especially in the case of a thin basis weight. The high product retention effect is advantageous because high corrosion resistance is obtained.

The SiO2, TiO2, ZrO2Nb2O5 and Ta2o5 used preferably have a fine particle size of about 1 to 500 mm. When these particles are added to the Zn--Mn plating solution, they may be added separately from the alumina sol, or they may be added to these particles to which alumina sol has been adsorbed or bonded in advance.

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 and Ni, which are more harmful than Zn and Mn, are alloying elements that further suppress the activity of the Zn-Mn plating layer. Fe, Cr
and Mo are selected. One or two of these elements
The content in the plating layer above the seed is desirably 30 wt% or less.

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.
This is also because the cost increases significantly.

Furthermore, the above-mentioned oxides such as 8102 and elements such as Go 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 plated 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 20mm, and the liquid temperature is 5.
It was carried out at 0°C.

(1) First layer plating conditions Bath composition Zinc sulfate 10-300 g/fl Nickel sulfate 50-300 g/l Sodium sulfate 50
g/u pH 1 to 2.5, current density 100 A/m2m' For comparison of first layer plating, a plated steel sheet for quality evaluation test was prepared with plating conditions outside the scope of the present invention.

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

As the alumina sol added to the Zn--Mn plating solution, #52O 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.

AfL2O3 content 2Owt% Dispersion medium Water pH + 4.5 Specific gravity: 1.23 (25℃) Viscosity: 8cp (2O℃) Surface area: z45rn'/g Stabilizer Nitric acid particle shape: rod to granular particle charge Positive crystal form Agglomerated boehmite Particle size: 5 to 2 Onm Appearance: Milky colloid liquid For sealing Zn-Mn eyelids, a sulfate bath, a chloride bath, a mixed bath thereof, or other acidic baths may be used. The following example mainly shows a sulfate bath, but is not limited to this.

Here, Mn contained in the plating layer, the amount of alumina sol, SiO2, TiO2, ZrO2Nb2 05 , Ta
2 os amount and Co, Ni Fe, Cr,
The total amount of Mo was appropriately controlled by the concentration added in the plating solution.

(2) Second layer plating conditions Bath composition Zinc sulfate 50-10 g/4 Manganese sulfate 30-12 O g/Sodium sulfite 05-5 g/J2 glycine
5-50g/β alumina sol 1-100
mIL/jlSin2. TiO2, ZrO2, Nb
One or more of 2O5, Ta2O5 01-50g/fl Cobalt sulfate Loog/JIL or less Nickel sulfate 100g/JZ or less Ferrous sulfate 100g/fL or less Chromium sulfate 100g/A or less Ammonium molybdate 100g/fL or less A Zn-Mn-alumina composite electroplated steel sheet was produced using a bath in which two or more of the above were appropriately added.

pH 2-4, current density 75A/m2m' (comparative example) For comparison of the second layer plating, the concentration of Mn and alumina sol added was 5in2. Addition concentration of one or more of TiO2Zr02, Nb2O5 and Ta2O5,
Plated steel sheets for quality evaluation tests were prepared with the concentration of one or more ions added among Co, NiFe, Cr, and Mo outside the range of the present invention.

(1) Weldability evaluation According to the above plating conditions, the first layer was ZnNi alloy plating (Ni content 30 wt%) 10 g/m2, and the second layer was Zn-Mn-alumina composite plating +t (Mn40 wt%).
t%, AfL2O30. 5wt%, 5iQ21.
5wt%, Co2wj%) 1-60 g/rr? A plated steel plate with a 100% carbon steel plate was prepared, and its continuous weldability was examined under the following spot welding conditions. The results are shown in Figure 1.

Electrode Cu-1%Cr alloy, tip diameter 4, 8mm welding time. 12 cycles (60 Hz) Welding current + 8 KA Pressure force 200 kgf In the tensile shear test every 500 points, the point at which peeling breakage occurred was defined as the continuous dot life.

[Evaluation] 0 6500 points or more Q +5000 to 6000 points △ 3500 to 4500 points
It can be seen that the weldability is clearly improved when the weldability is greater than / m'.

Note that FIG. 1 merely illustrates the case of a plating layer containing SiO2 and CO gold, and 5j02. TiO2, ZrO
,,, one or two of Nb2O5 and Ta2O5
Similar results were obtained in terms of weldability when the Zn-Mn-alumina plating layer contained one or more of Co, Ni, Fe, Cr, and Mo.

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

The sample was prepared using a single-sided plated steel plate, with the non-plated side as the punch side and the plated side as the die side, under the following cylindrical drawing conditions.

Thickness of plated steel plate: 0.8 mm Oil/rust prevention oil (Oil Coat Z5 - manufactured by Idemitsu Kosan) Blank diameter: 60 mm Punch diameter: 33 mm Wrinkle presser + 200 kg Forming speed + 25 mm/m1n (2) Corrosion resistance evaluation No corrosion resistance Corrosion tests were conducted and evaluated on painted materials and 1-coat materials under the following conditions.

a) Uncoated material The following combined corrosion cycle was considered as one cycle, and the number of cycles shown in Note 2 of Table 1 was performed to evaluate the corrosion resistance.

Salt water fountain 'FM 6 hours → dry (60℃) 2 hours → wet (5
0°C, 95% relative humidity) 4 hours [Evaluation] ◎ No red rust ○ ・Red rust 1-10% △ ・Red rust 11-30% × Red rust 31% or more b) Chemical conversion of coating materials, painting The conditions are shown below. After painting, a cross cut was made to reach the base steel plate, and the state of blistering in the paint film was examined using the above-mentioned combined corrosion cycle.

Chemical treatment Phosphoric acid treatment (Nippon Parkerizing ■PB#
302O treatment liquid used) Painting/Cationic electrodeposition coating (Nippon Paint Power Top U-100, paint used) 20 μm The number of cycles shown in Note 2 of Table 1 was performed to evaluate corrosion resistance.

value] , no bulges Bulge 1-3mm :Bulge 4-8mm Bulge 9mm or more In Table 1

[Brief explanation of the drawing]

Note 1) -: Not added to the plating solution. Note 2) For unpainted corrosion resistance Nos. 1 to 29, the results are shown after 30 cycles. For No. 30 to 58, the results after 20 cycles are shown. For paint corrosion resistance Nos. 1 to 29, the results after 12O cycles are shown. For Nos. 30 to 58, the results after 90 cycles are shown. FIG. 1 is a graph showing the relationship between coating weight and weldability.

Claims (4)

[Claims] (1) The first layer has an electrolytic Zn-Ni plating layer with a Ni content of 5 to 50 wt% at a thickness of 5 g/m^2 or more, and the second layer thereon is an electrolytic Zn-Ni plated layer with a Mn content of 3 to 60 wt%. Concentration 0.1N in Zn-Mn plating layer
A plating layer containing 0.01 to 3 wt% of the above hydrochloric acid-soluble alumina sol in terms of Al_2O_3 was added at 3 g/m^2.
A highly corrosion-resistant Zn-based surface-treated steel sheet having the above. (2) As the first layer, have an electrolytic Zn-Ni plating layer with a Ni content of 5 to 50 wt% at a thickness of 5 g/m^2 or more, and on top of that, as a second layer, an electrolytic Zn-Ni plated layer with a Mn content of 3 to 60 wt%. The Zn-Mn plating layer contains 30 wt% or less of at least one element selected from the group consisting of Co, Ni, Fe, Cr and Mo, and further contains an alumina sol soluble in hydrochloric acid with a concentration of 0.1N or more. 0. in terms of Al_2O_3.
A highly corrosion-resistant Zn-based surface-treated steel sheet having a plating layer containing 01 to 3 wt% of 3 g/m^2 or more. (3) As the first layer, have an electrolytic Zn-Ni plating layer with a Ni content of 5 to 50 wt% at a thickness of 5 g/m^2 or more, and on top of that, as a second layer, an electrolytic Zn-Ni plated layer with a Mn content of 3 to 60 wt%. In the Zn-Mn plating layer, the concentration is 0.1
Contains 0.01 to 3 wt% of alumina sol soluble in hydrochloric acid of N or more in terms of Al_2O_3, and further contains SiO_2, T
iO_2, ZrO_2, Nb_2O_5 and Ta_2
A highly corrosion-resistant Zn-based surface-treated steel sheet having a plating layer containing 6 wt% or less of one or more oxides in O_5 at 3 g/m^2 or more. (4) As the first layer, there is an electrolytic Zn-Ni plating layer with a Ni content of 5 to 50 wt% at 5 g/m^2 or more, and on top of that, as a second layer, an electrolytic Zn-plated layer with a Mn content of 3 to 60 wt%. -Concentration 0.1 in Mn plating layer
Contains 0.01 to 3 wt% of alumina sol soluble in hydrochloric acid of N or more in terms of Al_2O_3, and contains SiO_2, TiO_
2, ZrO_2, Nb_2O_5 and Ta_2O_5
Contains 6 wt% or less of one or more oxides among Co, Ni, Fe, Cr, and Mo.
A highly corrosion-resistant Zn-based surface-treated steel sheet having a plating layer of 3 g/m^2 or more containing 30 wt% or less of a species or two or more elements.