JPH055914B2 - - Google Patents

Description

[Detailed description of the invention]

(Field of Industrial Application) The present invention relates to a method for manufacturing a composite electroplated steel sheet for rust prevention, which is used in automobiles, home appliances, building materials, etc. and has excellent corrosion resistance and surface gloss. (Prior Art) It is well known that electrogalvanized steel sheets are already widely used as surface-treated steel sheets that can be mass-produced without impairing the corrosion resistance of cold-rolled steel sheets, the corrosion resistance after painting, and workability. However, in recent years, attempts have been made to use galvanized steel sheets as anti-rust steel sheets for automobiles in response to the salt sprayed on roads in winter to prevent roads from freezing in cold regions, and there is an increasing demand for corrosion resistance in harsh corrosive environments. In response to these demands for improving the corrosion resistance of galvanized steel sheets, it is known that the corrosion resistance can be improved by changing the amount of zinc plating (adhesion amount). Many have been proposed. Many of these are
It contains iron group elements such as Fe, Ni, and Co as alloying components. These zinc-iron group electroplated steel sheets are characterized by excellent corrosion resistance unpainted or after painting, as seen in Japanese Patent Publication No. 50-29821 and Japanese Patent Publication No. 57-61831, and are used industrially. Although it has been produced and put into practical use, it is strongly desired to further improve its corrosion resistance. On the other hand, as an electroplated steel sheet containing Cr during Zn or Zn-based alloy plating, for example,
Publications Nos. 38313 and 59-40234, JP-A-61-130498,
61-270398, 62-54099, etc. are disclosed. All of these have a trace amount of Cr content,
The effect of Cr on corrosion resistance can only be incidental. Therefore, for the purpose of improving corrosion resistance, it is strongly desired to eutectoid a higher content of Cr. Furthermore, in addition to corrosion resistance, requirements for the appearance after coating are becoming increasingly strict for the exterior surfaces of automobile bodies and for home appliance applications, and from this point of view, it is strongly desired that the plated surface be made glossier. (Problems to be Solved by the Invention) However, conventionally there has been no technology for producing Zn-Cr electroplating which has a high Cr content and excellent surface gloss. In other words, even if the Cr ³⁺ ion concentration in the plating bath is simply increased, normal plating with good surface gloss and workability cannot be obtained, and there are problems such as a sudden decrease in current efficiency, and it is difficult to increase the Cr content industrially. It has been extremely difficult to obtain high-quality electroplated steel sheets. In view of the above problems, the present invention has developed an unprecedented high Cr
The present invention provides a method for producing a highly practical composite electroplated steel sheet with excellent corrosion resistance and surface gloss. (Means for Solving the Problems) The present invention introduces a water-soluble cationic polymer into a Zn-Cr plating bath, and this action promotes Cr precipitation, thereby achieving an unprecedentedly high Cr content. By anodic oxidation of Cr ³⁺ ions, the ratio of generated Cr ⁶⁺ ions to Cr ³⁺ ions is suppressed to below a certain value, resulting in excellent corrosion resistance and surface gloss with high Cr content. The present invention provides a method for stably manufacturing composite electroplated steel sheets. The gist of the invention is as follows. (1) Contains Cr ions and cationic polymers,
Production of composite electroplated steel sheets with excellent corrosion resistance and surface gloss, characterized by electroplating using an acidic Zn-based plating bath with a Cr ⁶⁺ ion/Cr ³⁺ ion ratio of 0.1 or less method, (2) the above-mentioned first method containing one or more of Ni ²⁺ , Fe ²⁺ , and Co ²⁺ ions in the acidic Zn-based plating bath;
(3) Composite electroplating with excellent corrosion resistance and surface gloss as described in Items 1 and 2 above, wherein the cationic polymer is a quaternary amine polymer. This is a method for manufacturing steel plates. (Function) The corrosion resistance of the composite electroplated steel sheet obtained by the manufacturing method of the present invention is mainly due to the effect of Cr. The Cr content is preferably 5 to 30% by weight. If it is less than 5% by weight, the effect of improving corrosion resistance is not observed, but the tendency to easily generate red rust still remains, and the corrosion resistance is not sufficient. When the content is 5% by weight or more, the occurrence of red rust is suppressed even after a 500-hour salt spray test, and an epoch-making effect appears. Such high corrosion resistance cannot be achieved by conventionally known Zn plating or alloy plating such as Zn-Fe or Zn-Ni. When Cr coexists with Zn, it does not become passivated, and together with Zn it participates in the sacrificial anticorrosion effect, and the corrosion products of Cr deposit a poorly soluble protective film on the corroded areas, thereby suppressing corrosion. This is thought to be the reason why it exhibits high corrosion resistance. If the Cr content exceeds 30% by weight, corrosion resistance is good, but even with the eutectoid effect of the cationic polymer described later, the plating layer peels off during processing such as pressing, resulting in a so-called deterioration of powdering property. cannot be prevented, making it difficult to apply in practice. The first gist of the present invention is to introduce a cationic polymer, typified by a quaternary amine polymer, into the plating bath when producing such a composite electroplated steel sheet with a high Cr content. Cationic polymers act as Cr precipitation accelerators and also exhibit a type of gloss effect, making it possible to form a composite electroplated layer with a high Cr content and excellent surface gloss. Become. When no cationic polymer is added,
Although plating with a high Cr content can be obtained under certain conditions at a high current density of 200 A/dm ² or more, there are problems such as a significant decrease in current efficiency and blackening of the surface. By adding a cationic polymer, the current density range can be expanded, and a composite electroplated steel sheet with a high Cr content and excellent surface gloss can be produced with practical current efficiency. A quaternary amine polymer is effective as the water-soluble cationic polymer used in the present invention, and the molecular weight is preferably 10 ³ to 10 ⁶ in this case. Among the following amine polymers, polyamine sulfone (abbreviated as PAS) and polyamine (PA) are Cr
Most effective as a precipitation promoter. It is thought that the adsorption effect by the amine group and the bond between the sulfone group and the metal ion or metal contribute. Basically, the following quaternary amine salts (ammonium salts)

It is composed of a homopolymer or copolymer containing [Formula] in the main chain. Some compounds are listed below as specific examples. Examples include polymers obtained from diallylamine. R ₁ and R ₂ represent a lower alkyl group or an alkyl group,

Abbreviated as [formula]. It is abbreviated as X is Cl ^- , HSO ₄ ^- , H ₂ PO ₄ ^- , R-SO ₃ ^- (R is
C ₁ -C ₄ alkyl group), NO ₃ ^- anion. Another example is a polymer synthesized from vinylbenzyl. R ₁ , R ₂ , R ₃ represent hydrocarbons,

[Formula] and abbreviation X is Cl ^- , HSO ₄ ^- , H ₂ PO ₄ ^- , R-SO ₃ ^- ,
NO ₃ ^- indicates the anion of ,,. Alternatively, allylamine polymer can be mentioned.

Abbreviated as [formula]. R ₁ , R ₂ , R ₃ represent hydrocarbons, X is Cl ^- ,
It represents an anion of HSO ₄ ^- , H ₂ PO ₄ ^- , R-SO ₃ ^- , NO ₃ ^- . 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. The amount of cationic polymer added is 0.01 to 20g/
is preferred. If it is less than 0.01 g/l, it will have little effect on Cr deposition, and if it exceeds 20 g/l, the Cr precipitation promoting effect will be saturated and there will be a risk of causing a voltage increase during electroplating. The above cationic polymers may be used alone or in combination of two or more. Cationic polymers as Cr precipitation promoters
If the amount added is large or the current density is high, a small amount will precipitate within the plating layer. Co-deposition of cationic polymers improves press workability, but has little effect on corrosion resistance. In terms of improving press workability, the eutectoid rate of the cationic polymer should be 0.005 to 5% by weight and 1/1000 of the Cr content.
It is preferable that it is above. The second gist of the present invention is that Cr ³⁺ of Cr ⁶ + ions that are inevitably generated by anodic oxidation of Cr ³⁺ ions
The point is that the ratio to ions is 0.1 or less.
That is, when electroplating is performed continuously in a plating bath consisting of Zn ²⁺ ions, Cr ³⁺ ions, and cationic polymers, Cr ⁶⁺ ions are generated by anodic oxidation of Cr ³⁺ ions. It's coming. This phenomenon is caused by Pb,
This is particularly noticeable when using a Pb-based insoluble anode containing a small amount of Sn, In, Ag, etc. In the case of insoluble anodes having a Pt or ceramic layer on their surface, or soluble anodes such as Zn, there is a tendency for Cr ⁶⁺ ion generation, although it is milder than in Pb-based insoluble anodes. Figure 1 shows the Cr ⁶⁺ ion/Cr ³⁺ ion ratio and
The relationship between Cr content and surface gloss is shown. As a plating bath, Zn ²⁺ 70g/, total Cr (Cr ³⁺ + Cr ⁶⁺ )
Electroplating was carried out at a current density of 150 A/dm 2 and a deposition amount of 20 g/m ² using a sulfuric acid acidic bath containing 10 g of cationic polymer (polyamine sulfone) and 2 g of cationic polymer (polyamine sulfone) at a pH of ² and a bath temperature of 60°C. As the ratio of Cr ⁶⁺ ions increases, as shown in FIG. 1, the Cr precipitation rate decreases, the surface gloss decreases (matting, and even blackening), and the current efficiency also decreases. From this point of view, Cr ⁶⁺
The ion/C ³⁺ ion ratio needs to be 0.1 or less. More preferably, it is 0.05 or less. Methods of suppressing the Cr ⁶⁺ ion/Cr ³⁺ ion ratio to 0.1 or less include adding a reducing agent such as sulfite, Fe ²⁺ ions that have a reducing effect on Cr ⁶⁺ ions, or using a plating solution. Cr ⁶⁺ in contact with metal Zn
A practical method is to reduce ions to Cr ³⁺ ions. Next, if a plating bath containing one or more of Ni ²⁺ , Fe ²⁺ , and Co ²⁺ ions in addition to Zn ²⁺ ions, Cr ³⁺ ions, and cationic polymers is used.
A composite plated steel sheet is obtained in which Ni, Fe, and Co are also eutectoid, and when these are contained in an amount of 1% by weight or more, spot weldability can be improved. in this case,
The total amount of Cr and one or more of Ni, Fe, and Co is desirably 30% by weight or less from the viewpoint of press workability. Among Ni, Fe, and Co, Ni in particular contributes to improving corrosion resistance in a relatively low Cr content of 5% by weight or less. Note that even in baths containing Ni ²⁺ , Fe ²⁺ , and Co ²⁺ ,
The effect of promoting Cr precipitation by the cationic polymer does not change at all, and it is also essential that the ratio of Cr ⁶⁺ ions and Cr ³⁺ ions be 0.1 or less. The metal ion concentration in the plating bath is not particularly limited, but the concentration of Zn ²⁺ ions is 10 to 150 g/,
The concentration of Cr ³⁺ ions may be set in the range of 5 to 100 g/h to obtain the desired composition. Zn ²⁺ ions less than 10g/and Cr ³⁺ ions 5g/
If it is less than that, the respective current efficiencies are too low to be practical. In addition, Zn ²⁺ ions exceed 150g/
If the Cr ³⁺ ion content exceeds 100 g/L, the loss of metal ions due to dragout of the plating solution increases and precipitation is likely to occur. Furthermore, if one or more of Ni ²⁺ , Fe ²⁺ , and Co ²⁺ ions are added, 5 to 100 g/
is preferred. Note that the total amount of metal ions involved in plating is preferably 20 to 200 g/. In addition, if necessary, for example, increasing the alkali resistance of the plating layer,
In order to improve corrosion resistance after painting, SiO ₂ ,
Oxide particles such as TiO ₂ and Al ₂ O ₃ are added to the plating bath, and Na ⁺ , K ⁺ ,
NH ₄ ⁺ ions or the like may be added to the plating bath. In addition to the above substances, Fe ³⁺ , Pb, Sn, Ag, In,
Even if Bi, Cu, Sb, AsAl, Ti, etc. are unavoidably mixed into the plating bath, the effects of the present invention essentially remain the same. The pH of the plating bath is preferably 0.5 to 3. If the pH is less than 0.5, the Cr content and current efficiency will be too low, which is disadvantageous, and if the pH is more than 3, plating burn will occur at a high current density of 150 A/dm ² or more, and the appearance will turn black. The plating bath may be a sulfate bath, a chloride bath, or a mixed bath thereof. Next, regarding the plating conditions, the current density is 20A/
dm ² or more is preferred. If it is less than 20 A/dm ² , the Cr content is too low and is not effective in improving corrosion resistance. The temperature of the plating bath is suitably 40 to 70°C, and the flow rate of the plating liquid relative to the steel strip is suitably 1 to 200 m/min. Examples are given below. Example 1 A cold-rolled steel sheet was degreased with alkali, pickled with 5% sulfuric acid, washed with water, and prepared using an anode Pt/Ti electrode and an interelectrode distance of 10.
Electroplating was carried out with a coating weight of 20 g/m ² in a mm circulation plating cell. Table 1 summarizes the test results for plating bath composition, plating conditions, plating composition, corrosion resistance, and surface gloss. Cationic polymers include polyamine sulfones with average molecular weights of 5,000, 100,000, and 300,000, respectively, as PAS-1, PAS-2, and PAS-3, and polyamines, polyvinylbenzylamine, and polyallylamine with average molecular weights of 100,000, respectively, as PA, PB, and
Abbreviated as PAR. Corrosion resistance was determined by salt spray test (based on JIS Z2371,
(abbreviated as SST) was evaluated based on the area where red rust occurred after 500 hours. 1% or less: ◎ More than 1% to less than 10%: ○ More than 10% to less than 30%: △ More than 30%: × Surface gloss is the gloss level (JIS Z8741Gs (60°), black glass plate standard value 93) evaluated. Glossiness 50 or more: ◎ Glossiness 20 or more: ○ Glossiness less than 20: △ Blackening: × Among the examples of the present invention, those containing 5% by weight or more of Cr had particularly good corrosion resistance. Even if the Cr content is less than 5% by weight, Nos. 25 and 26 containing Ni,
Corrosion resistance was better than Nos. 16 and 17, which did not contain Ni. Furthermore, all of the examples had excellent surface gloss. Regarding Comparative Examples, Nos. 31 and 33 did not contain a cationic polymer in the plating bath, and the current density was not high enough, so the Cr content was very small and the corrosion resistance was poor. Nos. 32 and 34 also contained cationic polymers, but had sufficiently high current density, high Cr content, and good corrosion resistance.
The surface appearance was poor. Comparative Examples 35 and 36 had a high Cr ⁶⁺ /Cr ³⁺ ratio in the plating bath, so the Cr content was low, the corrosion resistance was insufficient, and the surface appearance was poor. In addition, the current efficiency is 60 in all examples.
% or more, whereas in all comparative examples it was 50%.
It was as low as below.

【table】

[Table] Example 2 Using a Pb-5%Sn electrode as the anode, No. 1 of Example 1.
Continuous electrolysis was performed using the plating bath No. 1, and the Cr ⁶⁺ /Cr ³⁺ ratio in the plating bath was periodically measured.
Plating with a deposit of 20 g/m ² was created at a current density of 150 A/dm ² . Until the Cr ⁶⁺ /Cr ³⁺ ratio reaches 0.1,
When the Cr content was 6% by weight or more, a glitter with excellent surface gloss was obtained, but when it exceeded 0.1, the Cr content decreased, the surface gloss decreased significantly, and blackening occurred.
When the plating solution was passed through the tank filled with metallic Zn when the Cr ⁶⁺ /Cr ³⁺ ratio reached 0.5, the Cr ⁶⁺ /Cr ³⁺ ratio decreased. After Cr ⁶⁺ /Cr ³⁺ decreased to 0.1, plating was performed under the above conditions, and the Cr content was 7.
An excellent finish with a surface gloss of % by weight was again obtained. Thereafter, when the same process as above was repeated periodically by continuously applying electricity while passing the plating solution into the metal Zn filling tank, Cr was removed under the condition that the Cr ⁶⁺ /Cr ³⁺ ratio was 0.1 or less. When the content was 6% by weight or more, plating with high surface gloss was obtained. Example 3 No. 1 of Example 1, using a Pb-5%Sn electrode as the anode.
When the same treatment as in Example 2 was carried out using plating bath No. 7, the same results as in Example 2 were obtained up to a Cr ⁶⁺ /Cr ³⁺ ratio of over 0.1. Cr ⁶⁺ /Cr ³⁺ ratio is
When 5g/Fe ²⁺ was added when the value reached 0.5, the Cr ⁶⁺ /Cr ³⁺ ratio decreased to 0.05 and the Cr content was 10.
A flash with excellent surface gloss in weight percent was again obtained. (Effects of the Invention) As described above, the present invention makes it possible to stably produce a Zn-based composite electroplated steel sheet with a high Cr content and excellent surface gloss, which has been extremely difficult in the past. This method is suitable for producing rust-proof steel sheets for use in automobiles, home appliances, building materials, etc., which require a high degree of corrosion resistance and surface gloss.

[Brief explanation of drawings]

Figure 1 shows the relationship between the Cr ⁶⁺ ion/Cr ³⁺ ion ratio, Cr content ^, and surface gloss in ^a typical plating bath used in the present invention. This indicates that it is essential that the value be 0.1 or less.

Claims (1)

[Claims] 1 Contains Cr ions and a cationic polymer,
Production of composite electroplated steel sheets with excellent corrosion resistance and surface gloss, characterized by electroplating using an acidic Zn-based plating bath with a Cr ⁶⁺ ion/Cr ³⁺ ion ratio of 0.1 or less Method. 2. A composite electric metal plate having excellent corrosion resistance and surface gloss according to claim 1, which contains one or more of Ni ²⁺ , Fe ²⁺ , and Co ²⁺ ions in an acidic Zn-based plating bath. Method for producing steel plate. 3. The method for producing a composite electroplated steel sheet with excellent corrosion resistance and surface gloss according to claims 1 and 2, wherein the cationic polymer is a quaternary amine polymer.