JPH03191097A - 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 and iron-base metal and to which organic polymer is added. CONSTITUTION:In a composite electroplated steel sheet plated with Zn containing 5-30% Cr and 1-10% iron-base metal, 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, a 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.

(Traditional technique ti!#) 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 response to requests for improved corrosion resistance of galvanized steel sheets.

There is a way to increase the amount of zinc plating (adhesion amount),
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.

In addition, attempts are made to obtain highly corrosion-resistant electroplated copper sheets by incorporating Cr into Zn or Zn-based alloy plating.
Publications, JP-A-61-270398 and JP-A-62-5
Publication No. 4099 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.Also, since the plating bath consists of metal ions or metal ions and oxides, the Cr source in the plating bath is Some Cr" or Cr3+ is not sufficiently reduced and Cr is a mixture of metal and oxide and tends to precipitate into the plating layer. Therefore, while improving corrosion resistance, it is small and the workability is not good. Even if the Cr concentration in the plating bath is increased for the purpose of increasing the Cr content, normal plating with good workability cannot be obtained, and there are many problems such as a significant decrease in current efficiency. In order to improve this, the Cr content must be increased to an unprecedented level,
There was a need for a method for obtaining an electroplated steel sheet with good workability. As a result, we obtained a prospect of obtaining a Zn-based composite electroplated steel sheet containing a high content of Cr and a trace amount of organic polymer and having excellent corrosion resistance and workability. However, for long-term stable production on an industrial scale,
It is necessary to consider the effects of impurities that inevitably enter the plating bath from plating chemicals, electrodes, rolls, and other plating machines. This is because impurities may eutectoid in the plating layer during the plating process and impair quality such as corrosion resistance. Z
The influence of impurities on plating systems such as n-plating and Zn-iron group alloy plating is known to some extent, such as in Japanese Patent Publication No. 58-36070, but the new Zn-based composite electrical with high Cr content targeted by the present invention As for plating, the influence of impurities is, of course, 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. In view of the above 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 discovered that 5 to 30% by weight of Cr, 0.0% of organic polymer
01 to 5% by weight, or 5 to 30% by weight of Cr, 1 to 10% by weight of iron group metals, and 0.001 to 5% by weight of organic polymers.
In a new Zn-based composite electroplated steel sheet containing
As a result of intensive study on the influence of impurities on corrosion resistance, it was found that corrosion resistance is reduced when a certain amount of a specific heavy metal is contained in the plating layer. The present invention was made based on this knowledge, and its gist is that Cr5-
In a Zn-based composite electroplated steel sheet containing 30% by weight and 0.001 to 5% by weight of organic polymer, Cu in the plating layer
is 800 ppm or less, Sn is 200 ppm or less, Sb
is 200PP11 or less, Pb is 1100pp or less, Bi
is 100PPat or less, and the total amount of these is 1000PPat or less
Composite electroplated steel sheet characterized by having ρρm or less, 5 to 30% by weight of Cr, 1 to 10% by weight of iron group metal,
In a Zn-based composite electroplated steel sheet containing 0.001 to 5% by weight of an organic polymer, Cu in the plating layer is 800% by weight.
PPM or less, Sn is 200 pp+s or less, Sb is 20
0 ppH or less, Pb 1100 pp or less, Bi 11
00 ppm or less, and the total amount thereof is 100 ppm 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 amount is 5% by weight or more, the occurrence of red rust is suppressed and the corrosion resistance is greatly improved.For example, salt spray test,
Red rust does not easily occur even after 500 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 is difficult to practically apply this method because it cannot prevent the deterioration of the powdery peeling w1).

The composite electroplated steel sheet may further contain an iron group metal. The iron group metal here is Ni.

It refers to Co and Fe, and the content of these is preferably 1 to 10% by weight in the total amount of one or more types. The function of iron group metals is mainly to improve spot weldability.

Composite plating consisting of Zn-high Cr-organic polymer that does not contain iron group metals has slightly inferior spot weldability compared to conventional Zn-Ni and Zn-Fe alloy plating. The reason for this is not clear, but the composite plating has a low electrical resistance and is easily melted by heat generated by electricity, or the plating layer is soft and easily deforms under the pressure of the welding tip, making it difficult for current to concentrate at the welding area. etc. are possible. When iron group metals are included, the electrical resistance increases and the plating layer becomes appropriately hard, so spot weldability is reliably improved. If the iron group metal is less than 1% by weight, the above effect is not noticeable, and if the iron group metal is less than 1% by weight,
If it exceeds Cr, the properties of the iron group metal will become stronger and the effect of Cr will be reduced. Considering powdering properties during processing, the total amount of Cr and iron group metals is preferably 30% by weight or less. In addition,
Among the iron group metals, Ni is effective for corrosion resistance and is the most advantageous.

The organic polymer contained in the composite electroplated steel sheet of the present invention is added to the plating bath as a Cr deposition accelerator, and by eutectoiding a small amount of this together with Cr into the plating layer, it is can improve the powdering resistance of This eutectoid effect of organic polymers is
It is presumed that Cr inhibits the 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.
If the content is less than 0.001% by weight, the effect of improving powdering resistance will be poor, and if the content exceeds 5% by weight, it will not only be difficult to obtain even if the concentration of organic polymer in the plating bath is increased, but if a large amount is eutectoid, the plating will be adversely affected. Adhesion deteriorates. In order to ensure powdering resistance, the Cr content must be 1/1000
It is desirable to eutectoid the organic polymer at the above content rate.

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, one molecular weight is 10
1 to 10' is desirable. 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 made 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.

CI(2 R engineering 2 or R. 2 CH.

Alternatively, R2 represents a lower alkyl group, and X represents C2H3O,
-, H, PO, -, R-3o3- (R is a C-c4 alkyl group), NO, - represents an anion.

Another example is a polymer synthesized from vinylbenzyl.

R1, R2, R, represent lower alkyl groups, and X is C4-
, H3O4-, H2PO4, R-8o3- (R is C~
C4 alkyl group), NO3- anion.

Further examples include allylamine polymers.

N...X / \ R11t, RJ R4, R,, R, represents a lower alkyl group, and X is CQ-
, H3O4-, H2PO4, R-3o, -(R is C1~
C4 alkyl group), No3- anion.

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/rd 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 as follows: Cu: 800 ppm or less, Sn: 200 ppm or less, Sb: 200 ppm or less, Pb: 10,100 pp or less, and Bj: 100 ppm or less.
pm 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 1olW.
! During this period, a sulfuric acid acidic composite plating solution was circulated at a constant flow rate to give a coating weight of 20 g/m and a Cr content of 1.
A composite electroplated steel sheet with a Ni content of 0%, a Ni content of 2%, and an organic polymer content of 0.1% was produced. The plating bath conditions are as follows.

Zn”: 50-65g/fl Ni”: 20-29g/fl Cr”: 15-26 gIQ 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
-) and the vertical axis is the salt spray test (JIS Z-2371).
The number of days (days) until red rust appears is shown.

Figure 1 shows the case of impurity Cu, and shows extremely high corrosion resistance, with the number of days until red rust appears approximately 70 days from when the plating layer does not contain Cu to when it contains 1000 pρ■.
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
80pp+*, Pb90ppm, Bi80p
When PM is contained in the plating layer in combination, the concentration of each impurity is below the upper limit, but the total amount exceeds 11,000 pp, resulting in deterioration of corrosion resistance. The same applies to composite electroplated steel sheets that do not contain iron group metals, and C
u is 800PP- or less, Sn is 200pp@ or less, S
b is 200PP- or less, Pb is 100PPII or less,
Bi is 1100pp or less, and the total amount of these is 100pp
It is necessary to set it below OPρho.

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 may be applied to the base plating of an organic composite plated steel sheet having 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, phosphorus, etc. can also be used.

(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,
PAS-+PAS-H with an average molecular weight of 100,000), N
a″″, PH2 containing divalent iron group metal ions, bath temperature 5
A 0°C sulfuric acid acid bath was used. By changing the composition and current density of the above-mentioned plating bath, composite electroplated steel sheets with different plating compositions and a coating weight of 20 g/r4 were produced. At this time, Cu was added to the plating bath.

Sn, Sb, Pb, and Bi were added as aqueous solutions of copper sulfate, stannous sulfate, antimony sulfate, lead acetate, and bisnus sulfate, respectively, and the impurity concentration in the plating layer was changed by adjusting the amount of impurity addition and the liquid flow rate. .

Special grade reagents were used for plating chemicals and impurities, and 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 water jet test (JIS Z-2371
) to determine the number of days until red rust occurs, and compared to a reference material with the same -Cr content. The ratio of the number of days in which red rust occurred was calculated.

In Table 1, Invention Example 1, Invention Example 2, and Comparative Example 1.
This is the reference material for No. 2. In Comparative Example 1, the C'r content is too low, and in Comparative Example 2, the concentration of Cu among the impurities is too high, so the corrosion resistance ratio is low, that is, the corrosion resistance is reduced. Invention Example 3 is a reference material for Invention Examples 4 to 8 and Comparative Examples 3 to 6.

Comparative Examples 3 to 6 contain impurities S no S b, P
Corrosion resistance is reduced because the concentration of b, B x is too high0 Inventive example 9゜10.
It is a reference material of 8,9°10. In Comparative Example 7, the total amount of impurities is too high, and in Comparative Examples 8, 9, and 10, the concentrations of impurities Pb, Sb, and Sn are too high, and the total amount of impurities is also too high, so the corrosion resistance is reduced. 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. Figure λ Figure Thou desire V 〆〃 Like/Wθ Gauze, Shiba layer sn 4jl (rtxt > Figure Figure 21! 70 4 de υM Lη Me, Ki layer, ?shi] Shi] Shi]shi fly (PPM') 5/Figure 2〃 φr lθ0 Skin v10oθ Me?In color layer, s;11 ((pna) Procedural amendment March 14, 1990

Claims (3)

[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) 5 to 30% by weight of Cr, 1 to 10% by weight of iron group metals,
In a Zn-based composite electroplated steel sheet containing 0.001 to 5% by weight of organic polymer, Cu in the plating layer is 800p.
pm or less, Sn 200ppm or less, Sb 200pp
m or less, Pb is 100 ppm or less, Bi is 100 ppm
A composite electroplated steel sheet having the following properties and a total amount of 1000 ppm or less. (3) The composite electroplated steel sheet according to claims 1 and 2, wherein the organic polymer is a cationic polymer.