JPS60128290A - Manufacture of zinc-alumina composite electroplated steel sheet having high corrosion resistance - Google Patents

Abstract

PURPOSE:To improve the corrosion resistance by carrying out electrolysis in a galvanizing bath contg. alumina sol having adsorbed Co ions and a chloride such as NH4Cl or KCl under prescribed conditions to deposit Zn, alumina and Co at the same time. CONSTITUTION:A galvanizing bath contg. alumina sol having adsorbed Co ions in >=1.4 molar ratio of Al2O3/Co and >=100g/l one or more among NH4Cl, KCl, LiCl, CaCl2, BaCl2, MgCl2, TiCl3 and TiCl4 is adjusted to 4-6.2pH. A steel sheet is electrolyzed as a cathode in the galvanizing bath at >=60A/dm<2> current density to deposit Zn, alumina and Co at the same time on the surfaces of the steel sheet. Thus, the corrosion resistance of the steel sheet is improved.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for producing a highly corrosion-resistant zinc-alumina composite electroplated steel sheet.

Galvanized steel sheets are widely used as anti-corrosion treated steel sheets for automobiles, home appliances, building materials, etc. that require corrosion resistance. This is due to sacrificial corrosion protection in which the pure zinc layer is base against the iron of the steel sheet, so the zinc corrodes first on exposed areas of the base steel caused by plating defects such as pinholes or processing methods. This is because it is effective and has the effect of preventing the occurrence of red rust on steel plates. However, pure zinc produces conductive corrosion products when exposed to salt spray or in a humid environment, resulting in extremely high corrosion rates.Furthermore, under the painted coating, the zinc corrosion products cause the coating to blister and eventually peel. There are drawbacks to pure zinc due to its active nature. On the other hand, in order to improve the corrosion resistance of zinc plating, metals that are more potent than zinc in terms of suppressing the activity of the galvanized layer,
For example, a method of precipitating an alloy of Go, Ni, Cr, Fe, etc. can be considered, and many documents and patents are available. However, because it is difficult to produce industrially while keeping the alloying ratio, which influences the quality of plating, constant, there are few examples of practical use.

The present invention does not significantly change the manufacturing conditions of electrogalvanized steel sheets, and minimizes the increase in manufacturing costs.
As a result of intensive research based on the point of improving corrosion resistance, we have found that positively charged alumina sol that has adsorbed a small amount of cobalt ions is added to a weakly acidic chloride-based zinc plating bath to improve corrosion resistance during plating. The present inventors have discovered that alumina and cobalt can suppress the progress of corrosion, and can improve desired performance such as corrosion resistance by forming and retaining a corrosion product (zinc hydroxide) with low conductivity.This has led to the present invention.

That is, in the present invention, the cobalt ion adsorption amount is A120.
1 to 100 g/u of positively charged alumina sol with a molar ratio of 1.4 or more and NH4C1,K[El, Na
C1゜LiC1, Gael2, BaCl2,
In a galvanizing bath with a pH of 4 to 6.2 to which one or more monomuides selected from the group consisting of MgCl2, TiCl3, and TiCl4 is added at a loog/i level, a current density of 60 A/drn' or higher is applied using a steel plate as a cathode. The present invention provides a method for manufacturing a highly corrosion-resistant zinc-alumina composite electroplated steel sheet, which is characterized by electrolyzing the steel sheet to eutectoid zinc, alumina, and cobalt on the surface of the steel sheet.

The present invention also has a cobalt ion adsorption amount of A1203/
Positively charged alumina sol with a Co molar ratio of 1.4 or more
100g/bun, NH4C1, KCI, Na1l.

LiC1, CaC12, BaCl2, MgCl2
100 g/u or more of chloride of "one or more selected from the group consisting of , TiCl3 and T.614", and one or more selected from the group consisting of boric acid, amino acids, organic carboxylic acids, and salts thereof. In a galvanizing bath with a pH of 4 to 6.2 to which 10 g/u or more of pH buffering agent has been added, electrolysis is carried out at a current density of 60 A/drn' or more using the steel plate as a cathode, and zinc, alumina, and cobalt are co-deposited on the steel plate surface. The present invention provides a method for producing a highly corrosion-resistant zinc-alumina composite electroplated steel sheet.

The present invention will be explained in more detail below.

In the present invention, an alumina sol on which Co2+ ions are adsorbed is used, and the amount of Co2+ ions adsorbed on the alumina sol is preferably set to a molar ratio of A1203/Co of 1.4 or more. This is because if this value is less than 1.4, a sufficient amount of Go will not be incorporated into the Zn plating layer and will not contribute to the corrosion resistance.

The reason why a composite zinc plating in which Zn contains alumina and cobalt exhibits excellent corrosion resistance in a corrosive environment is not necessarily clear, but it is thought to be as follows.

When plating with a chloride-based Zn plating bath to which alumina sol adsorbing cobalt ions is added, the cobalt adsorbed on the alumina eutectoid in the Zn plating layer may not be sufficiently reduced and may remain as ions. There is sex.

Also, depending on the plating conditions, it is highly possible that cobalt is sufficiently reduced and exists around alumina as metallic cobalt. In this case, as the corrosion of the Zn-alumina composite plating layer progresses in a corrosive environment, both alumina and cobalt act as a cathode and Zn acts as an anode in the initial stage, so Zn2+ ions are generated and then zinc hydroxide is formed by hydrolysis. is generated. It is thought that when the alumina particles are covered with this zinc hydroxide, the alumina becomes a cathode, the cobalt becomes an anode, and cobalt ions are generated.

In either case, cobalt ions penetrate into the zinc hydroxide layer and the zinc hydroxide is modified.

This modification mainly means to suppress the transformation of zinc hydroxide into highly conductive zinc oxide in the corrosive environment of salt spray (immersion) and drying. In addition, the hydroxide formation pH of Zn2+ and Co2+ ions is Zn(OH)2
In this case, the pH is 5.25, and the pH of Go(OH)2 is 8.54, and 00 is clearly higher, and cobalt can sufficiently diffuse as ions in the above-mentioned corrosive environment.

Furthermore, another reason for improving corrosion resistance is that the eutectoid inert alumina occupies a part of the plated surface, reducing the active exposed area and the steric effect of alumina. Zinc hydroxide).

Furthermore, the present invention is characterized by the fact that by using a chloride-based zinc plating bath which is an alumina sol that has adsorbed cobalt ions, alumina and cobalt can be easily eutectoided into the plating layer instead of on the plating surface. It is in. JP-A-54-159342 describes a method for eutectoiding an oxide sol into a galvanized layer, in which heavy metal ions such as Ni2+ and Cu2+ are used to cationize the oxide sol.
H+ ion or cationic surfactants are used.

Thus, it is easily inferred that the positively charged colloid particles migrate to the cathode part in the electroplating bath and eutectoid. However, with this method, almost no alumina particles were eutectoided into the plating layer, and although it was thought that the amount of eutectoid was increasing in proportion to the amount of alumina added, it was simply the amount of eutectoid deposited on the surface of the plating layer. It was found that there was an increase in

Therefore, as a result of extensive research in order to reliably eutectoid alumina particles in the galvanized layer, it was discovered that the reason why alumina particles were not eutectoid in the galvanized layer was the zinc hydroxide film that formed at the cathode interface. The reduction in current efficiency at the cathode interface is a well-known phenomenon, resulting in the generation of H2 gas, which increases the pH near the interface, reaching a pH of about 5.
In 25, zinc ions in zinc hydroxide form a film that acts as a barrier to the formation of alumina particles. Therefore, in order to reliably eutectoid alumina particles, it is necessary to break or loosen this film and incorporate it during the reduction and precipitation of zinc.

As a result of extensive research into the destruction or loosening of zinc hydroxide coatings, we found that the first method is effective.

(1) Even if a zinc hydroxide film is formed on the plating cathode interface due to high current density, if the current density is high, the Zn ions in the zinc hydroxide will be reduced compared to the replenishment rate of Zn ions from the bath. Since the rate of conversion into metallic zinc is fast, the thickness of the coating tends to decrease as a result. It has been found that the current density in this case is preferably 60 A/drn' or more.

(2) High concentration addition of chloride as a conduction aid When chloride ions are present in a bath at a high concentration, chlorine ions easily enter the zinc hydroxide film formed at the cathode interface, making the film loose, and It was found that the thickness of the coating was reduced because it also had the effect of dissolving the coating. As the chloride, it is desirable that even if it is added to the galvanizing bath, the metal will not be easily discharged and will not easily enter the plating layer. The reason for this is that if heavy metals such as Cu and Pb, which are free from discharge, enter the plating layer, the corrosion resistance will be significantly reduced.

Specifically, NH4CI, KCI.

y, (+1 1Hp1 rsCl a R1”: Iri
Advantages: It is desirable to add at least one chloride selected from the group consisting of TiC13 and TiCl4 to the galvanizing bath in an amount of 100 g/Jlj or more. The concentration of chloride added is 100g/41 or more.
If it is less than g/min, the chlorine ion concentration in the bath is close and the zinc hydroxide film cannot be sufficiently destroyed or loosened.

(3) Use of pH 1 buffer We have found that it is effective to add a pH buffer to suppress the rapid increase in pH at the plating bath and cathode interface. Although this pH buffer is effective even when added alone to the galvanizing bath, it has been found that when added in combination with the chloride described in (2) above, the effect of suppressing the formation of a zinc hydroxide film is even more remarkable. did. As a pH buffer, boric acid,
one or more selected from amino acids such as glycine, organic carboxylic acids such as citric acid, and salts thereof;
It is desirable to add 0g 1 sentence or more. If it is less than 10 g/M, the pH buffering property is not sufficient.

(4) Plating in a Weakly Acidic Zinc Plating Bath Furthermore, we have found that it is desirable to perform Zn-alumina composite plating using a weakly intoxicating zinc plating bath with a pH of 4 to 6.2. The reason for this is that when the pH is low, the H+ ion concentration is high, so discharge of H+ ions at the cathode is likely to occur, leading to an increase in pH at the cathode interface, promoting the formation of a zinc hydroxide film, and increasing the H+ concentration. This is because if it is too high, the plating precipitation efficiency will decrease. Incidentally, when the concentration of chloride becomes high, almost no zinc hydroxide is generated in the bath even at a pH of about 8.2, which is not a practical problem. Therefore, it is preferable that the pH of the zinc plating bath be slightly acidic, ranging from 4 to 6.2.

In addition, p disclosed in Japanese Patent Application Laid-Open No. 54-159342
The method of the present invention is completely different from the method of using a zinc plating bath with a low pH of H4 or less to adsorb H+ on the oxide sol surface and positively charging it.The method of the present invention is that Co2+ ions are adsorbed on the alumina sol in advance. It is clear from the fact that

FIG. 1 shows a depth profile of alumina and cobalt eutectoid in the Zn-alumina composite plating layer produced by the method of the present invention as determined by glow discharge optical emission spectroscopy (GDS analysis). This figure shows that alumina and cobalt are eutectoid uniformly over the entire plating layer, not on the plating surface.

The composite plating according to the present invention has improved corrosion resistance without impairing the characteristics of pure zinc plating, and also exhibits excellent properties in chemical conversion treatment and chromate treatment.

The zinc plating bath used in the present invention is preferably a chloride zinc plating bath containing zinc chloride as a main component, but sulfate baths, pyrophosphate baths, sulfamine #salt baths, or mixed baths thereof can also be used, and It is effective as long as it has a weak acidity of 4 to 6.2. Under these conditions, zinc can be used as a soluble anode and dissolves with nearly 100% efficiency, which has the advantage of easier bath management. In addition, the stability of some colloids also depends on pH, so pH 4
It is best to use it at a weak acidity of ~8.2. Bath temperature is 30-7
0°C is preferred.

In the present invention, 1 to 100 g/u (
Alumina 20wt%), preferably to-100g/4
1. Add 10 to 60 g as a more appropriate range.

If the colloidal solution of alumina particles is less than Ig/J1, the amount of eutectoid alumina will be small and it will not be effective. Also, 100gIn
If the plating solution is exceeded, the viscosity of the plating solution becomes large and stirring becomes difficult, resulting in a decrease in cathode current efficiency and an increase in bath voltage.
Unfavorable from an economic point of view. In addition, brightening agents such as acrylamide and dextrin and auxiliary agents such as tungstic acid and molybdic acid can be added, and their types,
There is no particular limitation on concentration etc.

The present invention will be specifically described below with reference to Examples and Comparative Examples.

A cold rolled steel plate (SPCC) was subjected to alkaline electrolytic degreasing, pickled with 5% hydrochloric acid, washed with water, and plated under the following conditions. Stirring is performed by a pump, and the liquid flow *l + extraction Rnm/+s
in-7S-mmIj Hatake-tt*e*t-4-1f11
,.

The distance between the electrodes was 20+am, and the liquid temperature was 50°C. The basis weight was 20 g/m'. Incidentally, unless otherwise specified, the alumina sol used was an alumina sol with an alumina content of 20 wt % to which Co2+ ions were adsorbed.

[Example 1] Bath composition: Zinc chloride 200g/Ammonium chloride 300g/Alumina sol 50g/1mm (Al103/Go='1.8) pH 5.0',
Current density: 100 Alt& In the resulting plating layer, alumina particles and cobalt were eutectoid uniformly throughout the depth direction, and the content was 0.8 wt% alumina and 0.5 wt% cobalt. .

[Example 2] Bath composition Zinc sulfate 300g/Possium chloride 200g/Alumina sol 50g/Also (A1203/Go=2.0) pH 4.5, current density 100 A/d, alumina was present in the resulting plating layer. Particles and cobalt are uniformly eutectoid throughout the depth direction, and the content is 0.6% of alumina.
wt%, cobalt 0.3vt%.

[Example 3] Bath composition: Zinc chloride 200g / ammonium chloride 300g / boric acid 15g 1-part alumina sol 50g / mass (Al103 / Go = 1.4) pl (5.0, current density 150A / drn') Obtained plating In the layer, alumina particles and cobalt are eutectoid uniformly throughout the depth direction, and the content is 0.5% of alumina.
8 wt%, Kobal) was 0.7 wt%.

[Example 4] Bath composition Zinc pyrophosphate 200g/u Sodium chloride 5J/u Magnesium chloride 100g/u Sodium citrate 30g/u Alumina sol 50g/u (Al103/Go=1.7) pH 5.2, current density 80A/d In the resulting plating layer, alumina particles and cobalt were eutectoid uniformly throughout the depth direction, and the content was 1.0 wt% alumina and 0.6 wt% cobalt. 5] Bath composition Zinc sulfamate 200g/u Titanium chloride (7i3+) 30g/Bunkaba calcium 150g/Uglycine 20g/U Alumina sol 50g/U (Al103/Co=2.0) pH 4.2, current density 120 A/drn' In the resulting plating layer, alumina particles and cobalt are eutectoid uniformly throughout the depth direction, and the content is alumina 1.
[Comparative Example 1] Bath composition Zinc chloride 200g/Sodium chloride 50g/Alumina sol 50g/text (Al103/Go=4.0) 5. 0. The contents of alumina and cobalt in the plated layer obtained at a current density of 100 A/d were each 0.
04 wt%, 0. The old wt% was low, and eutectoid was unevenly eutectoid in the depth direction.

[Comparative Example 2 Co-bath composition: Zinc sulfate 200g / magnesium chloride 80g / boric acid 5g / fL aluminium 50g / fL (Al103 / Go = 2.0) pH 3.0, current density 4 OA / dm' Obtained plating layer The content of alumina and cobalt in each is 0.
．． The content was as low as 0.02 wt% and 0.01 wt%, and eutectoid was unevenly eutectoid in the depth direction.

[Comparative Example 3] Bath composition Zinc sulfamate 200g/u glycine
8g/l Alumina sol 50g 1 sentence (-, 111203/, Go= 2.0) pH 5, (
1. Current density 4OA/drn' The contents of alumina and cobalt in the resulting plating layer were each 0. Old wt
% or less, 0.005 wt% or less.

[Comparative Example 4] Bath composition: Zinc chloride 200g/potassium chloride 350g/sL Alumina sol 50g/sL (Alumina content not adsorbing CO2+: 20%)
In the obtained plating layer, alumina particles were eutectoid uniformly in the depth direction, and the content thereof was 0.8 wt%.

The following corrosion resistance test was conducted on the composite plated steel sheets obtained in the above Examples and Comparative Examples. A bare (unpainted) laminated board material as shown in Figure 2 was made, and a corrosion resistance test was carried out using a cycle of immersion in 1% saline solution (35°C) for 4 hours, drying at room temperature for 4 hours, and drying at a high temperature of 80°C for 4 hours. 100 cycles were carried out, and the plate thickness reduction value was evaluated. As is clear from FIG. 3 showing the results, the Zn-alumina composite plated steel sheet containing appropriate amounts of alumina and cobalt exhibits excellent corrosion resistance.

[Brief explanation of the drawing]

Figure 1 is a diagram showing the profile of each component in the depth direction of the plating layer by GDS (glow discharge optical emission spectroscopy), Figure 2 is a perspective view of the corrosion test material, and Figure i3 is the corrosion resistance evaluated by plate thickness reduction value. It is a graph showing test results. Explanation of symbols 1...Cold rolled steel plate, 2...Composite plated steel plate, 3...
Insulating spacer (0.5mm thickness) Patent applicant Kawasaki Steel Corporation Figure 1 Depth direction □ Figure 2

Claims (2)

[Claims] (1) Positively charged alumina sol with a cobalt ion adsorption amount of 1.4 or more in Al2O3/Go molar ratio from 1 to 100 g/f
L, NH4C1, KCI, Na1l, Li(El
, CaCl2. BaCl2, MgCl2, TiCl3 and T
One or more chlorides selected from the group consisting of iG14
oog/J1 or more in a pH 4 to 6.2 galvanizing bath, using a steel plate as a cathode, with a current density of 60 A/drn'
A method for producing a highly corrosion-resistant zinc-alumina composite electroplated steel sheet, which comprises electrolyzing the above to eutectoid zinc, alumina, and cobalt on the surface of the steel sheet. (2) 1 to 100 g of positively charged alumina sol with cobalt ion adsorption amount of A1203 /Co molar ratio of 1.4 or more
n, NH4C1, KCI, NaC1, LiG1.
Ca1l 2 . BaCl2, MgCl2, TiG13 and T
One or more chlorides selected from the group consisting of iG14
oog, one or more p selected from the group consisting of boric acid, amino acids, organic carboxylic acids, and salts thereof
In a galvanizing bath with a pH of 4 to 6.2 to which 10 g or more of H buffer was added, a current density of 60 A/m was applied using a steel plate as a cathode.
A method for producing a highly corrosion-resistant zinc-alumina composite electroplated steel sheet, which comprises electrolyzing at a temperature higher than drn' to eutectoid zinc, alumina, and cobalt on the surface of the steel sheet.