JPH03191096A - Composite electroplated steel sheet - Google Patents

Abstract

PURPOSE:To improve corrosion resistance under severe conditions by limiting respective contents of Cu, Sn, Sb, Pb, and Bi as impurities in a plating layer of a composite plated steel sheet plated with Cr-containing Zn to which organic polymer is added. CONSTITUTION:In a composite electroplated steel sheet plated with Zn containing 5-30%, by weight, of Cr, an organic polymer is incorporated as a Cr precipitation accelerator by 0.001-5%. Further, the contents of Cu, Sn, Sb, Pb, and Bi in the plating layer are limited to <=800ppm, <=200ppm, <=200ppm, <=100ppm, and <=100ppm, respectively, and also the total content of the above is limited to <=1000ppm. As the organic polymer, water-soluble cation polymer can effectively be used.

Description

DETAILED DESCRIPTION OF THE INVENTION (Field of Industrial Application) The present invention relates to a Zn-based composite electroplated steel sheet with a high Cr content and excellent corrosion resistance, which is used for automobiles, home appliances, building materials, etc.

(Conventional technology) Improves the corrosion resistance of cold-rolled steel sheets and the corrosion resistance after painting.

It is well known that electrogalvanized steel sheets are widely used as surface-treated steel sheets that can be mass-produced without impairing workability.

In recent years, attempts have been made to use galvanized steel sheets as a rust-proofing measure for automobiles against salt sprayed to prevent roads from freezing during the winter in cold regions, and a high degree of corrosion resistance is required in harsh corrosive environments.

In order to meet the demand for improving the corrosion resistance of galvanized steel sheets, one method is to increase the amount of zinc plating (adhesion), but this has many problems in terms of weldability and workability.

Therefore, many alloy platings have been proposed as a method of suppressing the dissolution of zinc itself and extending the life of zinc plating.

Many of these contain iron group metals such as Fe, Co, and Ni as alloy components.

Further, it is an attempt to obtain a highly corrosion-resistant electroplated steel sheet by incorporating Cr into Zn or Zn-based alloy plating,
For example, Japanese Patent Publications No. 59-38313 and No. 59-40234, Japanese Patent Application Publication Nos. 61-270398 and 62-54.
No. 099 and the like are disclosed. All of these have a low Cr content of 5% or less, and the effect of Cr on corrosion resistance is only additive. In addition, since the plating bath consists of metal ions or metal ions and oxides, C
Cr'' or Cr'+, which is rg, is not sufficiently reduced and Cr tends to precipitate in the plating layer in a mixed state of metal and oxide, so during the improvement of corrosion resistance, it is small and workability is not good. Even if the Cr concentration in the plating is increased in order to increase the Cr content based on these conventional techniques, a normal plating with good workability cannot be obtained, and there are many problems such as a significant decrease in current efficiency. In order to increase the width, a method was needed to increase the Cr content to an unprecedented level and to obtain an electroplated steel sheet with good workability.

(Problems to be Solved by the Invention) By adding a specific organic polymer to the plating bath as a Cr precipitation accelerator, the present inventors have achieved corrosion resistance and workability that contain a high Cr content and a trace amount of organic polymer. We have obtained a prospect of obtaining a Zn-based composite electroplated steel sheet with excellent properties. However, in order to produce it stably on an industrial scale over a long period of time, it is necessary to consider the influence of impurities that inevitably enter the plating bath from the plating machine, such as plating agents, electrodes, and rolls. This is because impurities may eutectoid in the plating layer during the plating process and impair quality such as corrosion resistance. Zn
Although the influence of impurities on plating systems such as plating and Zn-iron group alloy plating is known to some extent, such as in Japanese Patent Publication No. 58-36070, the novel Zr-based composite electroplating with a high Cr content targeted by the present invention Of course, the influence of impurities is unknown. However, in order to maintain the high corrosion resistance of Zn-based composite electroplated steel sheets and to stably manufacture them over a long period of time, it is necessary to clarify the relationship between the impurity concentration in the plating layer and corrosion resistance, and to reduce the impurity concentration in the plating layer. It is extremely important to limit In view of these circumstances, the present invention provides a Zn-based composite electroplated steel sheet with excellent corrosion resistance in which the concentration of specific impurities in the plating layer is limited.

(Means for Solving the Problems) The present inventors have found that Cr is 5 to 30% by weight, organic polymer is 0.
As a result of intensive study on the influence of impurities on corrosion resistance in a new Zn-based composite electroplated steel sheet containing 0.001 to 5% by weight, it was found that if a certain amount or more of a specific heavy metal is contained in the plating layer, the corrosion resistance will be reduced. I found out.

The present invention was made based on this knowledge, and its gist is that 5 to 30% by weight of Cr and 0.001 to 30% of organic polymer
In a Zn-based composite electroplated steel sheet containing 5% by weight, Cu in the plating layer is 800ppm or less and Sn is 200ppm or less.
PpH or below, Sb below 200ppm, pb 110
0pp or less, Bi is looppm or less, and the total amount of these is 11000pp or less.

(Function) The high corrosion resistance of the composite electroplated steel sheet of the present invention is expressed by the action of Cr. The Cr content is preferably 5 to 30% by weight. If it is less than 5% by weight, a slight improvement in corrosion resistance is observed, but the tendency for red rust to occur remains, and the corrosion resistance is not sufficient. When the content is 5% by weight or more, the occurrence of red rust is suppressed and corrosion resistance is greatly improved. For example, the salt spray test
Red rust does not easily occur even after 00 hours or more. Such high corrosion resistance is at a level that cannot be obtained not only with conventionally known Zn plating, but also with alloy plating such as Zn-Ni and Zn-Fe.

When Cr coexists with Zn, it does not become passivated and exhibits a sacrificial anticorrosion effect together with Zn, and corrosion products form a poorly soluble protective film to cover the surface and suppress the progress of corrosion. It is presumed that this is the reason why it exhibits revolutionary high corrosion resistance.

High corrosion resistance is maintained even when the Cr content exceeds 30% by weight, but even with the eutectoid effect of organic polymers that promote Cr precipitation, such as cationic polymers described below, powdering during processing (plating layer It cannot prevent the deterioration of powdery peeling), making it difficult to apply in practice.

The organic polymer contained in the composite electroplated steel sheet of the present invention is added to the plating bath as a Cr precipitation accelerator, and by eutectoiding a small amount of this together with Cr into the plating layer, Powdering resistance can be improved. Such eutectoid effect of organic polymers is due to C
It is presumed that r inhibits uniform electrodeposition growth of Zn and prevents a plating structure lacking in uniformity and smoothness. That is, it is thought that a dense plating layer in which Zn and Cr are uniformly mixed or alloyed is formed via the eutectoid organic polymer. The content of the organic polymer is preferably 0.001 to 5% by weight. 0.0
If the content is less than 0.01% by weight, the effect of improving powdering resistance will be poor, and if the content exceeds 5% by weight, it will not only be possible to obtain it even if the concentration of organic polymer in the plating bath is increased, but if a large amount is eutectoid, it will result in Plating adhesion decreases. In order to ensure powdering resistance, the Cr content should be reduced to 1. It is desirable to eutectoid the organic polymer at a content of /1000 or more.

As the organic polymer used in the present invention, water-soluble cationic polymers are effective, and among them, polymers of quaternary amines are particularly effective polymers. In this case, the molecular weight is 10
3 to 10" is preferable. Specifically, among the following amine polymers, polyamine sulfone (abbreviated as PAS) and polyamine (abbreviated as PA) are most effective as Cr precipitation promoters.

This is thought to be due to the adsorption effect of the amine group on the cathode surface and the coordination bond of the Cr3+ ion to the sulfone group. These are basically composed of homopolymers or copolymers containing the following quaternary amine salt (ammonium salt) in the main chain.

Some compounds will be specifically listed below.

First, the following polymers obtained from diallylamine are mentioned.

/ \ 1R2 or / \ 1R2 CI+7 N...X / \ R, R2 or Re R2 R,, R, represents a lower alkyl group, and
-, H2PO4-, R-8O3- (R is a C-C4 alkyl group), and NO3- are anions.

Another example is a polymer synthesized from vinylbenzyl.

R1, R,, R3 represent a lower alkyl group, and X is Cα-
, H8O, -, H2PO4, R-8O, - (R is C□~
C4 alkyl group), NO3- anion.

Further examples include allylamine polymers.

(-CH2-CHl.

N...X -) Abbreviated as PAR ／　＼ RIR2R.

R□, R2, R3 represent lower alkyl groups, X is CQ
-, H3O4-, H2PO4, R-8O3- (R is C1
~C4 alkyl group), represents an anion of NO3-.

In addition, polymers of primary, secondary, and tertiary amines are also effective as Cr precipitation promoters, although they are not as effective as the above-mentioned quaternary amine polymers. In addition to these cationic polymers, polyoxyalkylene derivatives, especially polyethylene glycol (
PEG) is effective.

A coating weight of 10 to 50 g/rrf can ensure sufficient corrosion resistance.

Composite electroplated steel sheets having the above-mentioned composition have unprecedented high corrosion resistance and good workability, but if specific impurities are contained in the plating layer, the corrosion resistance will decrease. These impurities include Cu, Sn, Sb, Pb,
Heavy metals such as Bi, particularly elements with large atomic weights and large atomic radius, have an effect even in trace amounts. The concentration ranges in which these impurities do not reduce corrosion resistance are 800 ppm or less for Cu, 200 ppm or less for Sn, and 200 ppm or less for Pb.
pb is 1100pp or less, Bi is 1100pp or less,
And the total amount of these is 11,000 pp or less. These upper limit concentrations are based on experimental results, and the experimental method and results will be explained below.

First, as for the experimental method, a horizontal circulation type plating equipment was used, the cathode was a cold-rolled steel plate, the anode was a platinum plate, and the distance between the electrodes was 10 m.
During this time, a sulfuric acid acidic composite plating solution was circulated at a constant flow rate to produce a composite electroplated steel sheet with a coating weight of 20 g/r/, a Cr content of 10%, and an organic polymer content of 0.1%. The plating bath conditions are as follows.

Zn”: 50-65 g/Q Cr”=1.5-26 g/Q Na”: 16 g/Q Organic polymer = 1-2g/Q (PAS, average molecular weight 3500) pH: 2 Bath temperature: 50℃ Impurities such as Cu, Sn, and Sb are present in the plating bath.

Adding Pb and Bi as aqueous solutions of copper sulfate, stannous sulfate, antimony sulfate, lead acetate, and bismuth sulfate, respectively,
The impurity concentration in the plating layer was varied by adjusting the amount of impurity added and the liquid flow rate, and the relationship between the concentration of each impurity and corrosion resistance was investigated. In addition, special grade reagents are used for plating chemicals and impurities.
The impurity concentration in the plating layer was determined by atomic absorption spectrometry.

The results are shown in FIGS. 1 to 5. In each of Figures 1 to 5, the horizontal axis represents the concentration of each impurity in the plating layer (pp
m), the vertical axis is the salt spray test (JIS Z-2371
) is the number of days (days) until red rust occurs. Figure 1 shows the case of impurity Cu, and Cu is in the plating layer.
From cases containing no Cu to cases containing 11,000 ppp, the number of days until red rust occurs is approximately 70 days, showing extremely high corrosion resistance, but when the Cu concentration in the plating layer exceeds 11,000 pp, a tendency for corrosion resistance to deteriorate is observed. FIGS. 2 and 3 show the impurity Sn.

In the case of Sb, if the Sn and Sb concentrations in the plating layer exceed 200 ppm, the corrosion resistance deteriorates.

Figures 4 and 5 show the cases of impurities Pb and Bi, respectively, and the Pb and Bi concentrations in the plating layer are both 1100 pp.
If it exceeds this, corrosion resistance will deteriorate rapidly. The reason for the decrease in corrosion resistance due to such impurities is presumed to be that the uniformity of the composite plating layer is locally disturbed due to the eutectoid deposition of these impurities, and that area tends to become a starting point for corrosion. Especially Sn,
Elements with large atomic radii, such as Sb, Pb, and Bi, greatly disturb the uniformity of the composite plating layer, so even a trace amount is thought to degrade corrosion resistance. Although the reason for the above-mentioned limitation is for each impurity alone, the impurities are usually contained in the plating layer in a complex state. From this point of view, the total amount of these impurities needs to be 11,000 pp or less. for example,
Cu750ppm, Snl 90ppm, Sbl
When the plating layer contains a combination of 80 ppm, 90 ppm of Pb, and 80 ppm of Bi, the concentration of each impurity is below the upper limit, but the total amount exceeds 11,000 ppm, so the corrosion resistance deteriorates. Cu 800ppm or less, S
n is 200 ppm or less, Sb is 200 ppm or less, pb is 1100 ppm or less, Bi is looppm or less, and the total amount of these is 1. It is necessary to set it to OO0pprrI or less.

The structure of the present invention may be used on both sides of a steel plate, or may be applied to only one side and the other side may be left as the steel plate surface or coated with another plating layer. Further, it is also possible to apply the base plating to an organic composite plated steel sheet with an organic coating applied to the upper layer.

The base steel plate to which the present invention is applied is usually a mild steel plate that has been subjected to dull finish rolling.
, S, Mn, etc., as well as Cr, Cu, N, etc.
Highly corrosion-resistant steel plates containing a large amount of i, P, etc. can also be applied.

(Example) A cold-rolled steel sheet was degreased with alkali, pickled with a 5% aqueous sulfuric acid solution, and then electroplated under the following conditions. As a plating bath, z n2 +, Cr3 +, organic polymer (
PA with an average molecular weight of 10,000, PAS with an average molecular weight of 3,500,
average molecular weight 100,000 (7)PAS-)PASI],
A sulfuric acid acidic bath containing Na"+ at pH 2 and bath temperature of 50° C. was used. By changing the composition and current density of the plating bath, composite electroplated steel sheets with different plating compositions and a coating weight of 20 g/bo were produced.

At this time, Cu, Sn, Sb, Pb,
Bi was added as an aqueous solution of copper sulfate, stannous sulfate, antimony sulfate, lead acetate, and bismuth sulfate, respectively, and the impurity concentration in the plating layer was changed by adjusting the amount of impurity added and the liquid flow rate. We use special grade reagents for plating chemicals and impurities.
The impurity concentration in the plating layer was determined by atomic absorption spectrometry.

Table 1 shows the relationship between the plating composition, impurity concentration in the plating layer, and corrosion resistance. In the impurity concentration column, Tr represents below the detection limit. Corrosion resistance was determined by salt spray test (JIS Z-2371
) was used to determine the number of days until red rust appeared, and the ratio of the number of days until red rust appeared relative to a reference material with the same -Cr content was calculated.

In Table 1, Invention Example 1 is the reference material for Comparative Examples 1 and 2. In Comparative Example 1, the Cr content is too low, and in Comparative Example 2, the concentration of Cu among impurities is too high, so the corrosion resistance ratio is low, that is, the corrosion resistance is reduced. Invention Example 2 is a reference material for Invention Example 3 and Comparative Example 3. Comparative Example 3 contains impurities Sn and Sb, respectively.

Corrosion resistance is reduced because the concentrations of Pb and Bi are too high. Inventive Examples 4 and 5 are reference materials for Comparative Examples 4 and 5, respectively. In Comparative Examples 4 and 5, the concentrations of impurities Pb, Sb, and Sn were too high, and the total amount of impurities was also too high, resulting in poor corrosion resistance. As described above, when the concentration of specific impurities is high and exceeds the range of the present invention, the corrosion resistance clearly decreases, but when the impurity concentration is within the range of the present invention, good corrosion resistance is maintained.

(Effects of the Invention) As described above, by regulating the concentration of specific impurities contained in the plating layer in a composite electroplated steel sheet with a high Cr content within a permissible range, the composite electroplated steel sheet inherently has This can be provided without any loss in ground-breaking corrosion resistance. Therefore, the present invention exhibits excellent effects in the long-term industrial production of composite electroplated steel sheets.

[Brief explanation of drawings]

Figures 1 to 5 are diagrams showing the results of an experiment to find the permissible range of impurity concentration contained in the plating layer of a composite electroplated steel sheet.
The cases of b, Pb, and Bi are shown. Fig. 2 Suna, Hosae middle 5/2; J) lol (f'F) It ) Fig. 3 Fig. 2 strange layer FbJ degrees P) n) Fig. 2 I〉Woo! During the two-it, B ``fj & (pP7L)
1-,'! 1-

Claims (2)

[Claims] (1) Cr5-30% by weight, organic polymer 0.001-5
In a Zn-based composite electroplated steel sheet containing % by weight,
Cu in the plating layer is 800ppm or less, Sn is 200ppm
pm or less, Sb is 200 ppm or less, Pb is 100 ppm
A composite electroplated steel sheet, characterized in that Bi is 100 ppm or less, and the total amount of Bi is 1000 ppm or less. (2) The composite electroplated steel sheet according to claim 1, wherein the organic polymer is a cationic polymer.