KR890002496B1 - Process for preparing zn-ni-alloy-electroplated steel sheets excellent in corrosion reisstance - Google Patents

Abstract

Zn-Ni alloy plated steel sheet is produced by electroplating onto surface of a steel sheet from a plating bath containing Zn 10-40 g/l, Ni 15-160 g/l, Ti 0.2-10 g/l, Co 0.1-5 g/l, and at least one Al 0.1-5 g/l and Mg 0.2-4 g/l at pH 1.5-2.5. The steel sheet may be heat treated at 60-200 deg.C. after the application of the Zn-Ni alloy plating layer. In further detail the source compounds for the Co, Al and Mg are for example the sulphates. The source compound for the Ti is for example K Ti fluoride or Na Ti fluoride etc. The heat treatment is effected using hot water or steam.

Description

Manufacturing method of electroplated steel sheet of zinc-nickel alloy with excellent corrosion resistance

Correlation between the amount of aluminum added to the plating bath containing 20 g / l of potassium-titanium fluoride and the amount of titanium expressed by X-ray fluorescence simultaneously attached to the plating layer.

The present invention relates to a method for producing an electroplated steel sheet of zinc-nickel alloy excellent in corrosion resistance. Zinc galvanized steel sheet is widely used for various purposes because of its excellent beauty and corrosion resistance. In general, the corrosion resistance of the plating layer is enhanced by forming a chemically treated coating.

For the formation of chemically treated coatings, it was effective to use chromate or phosphate chemical treatment solutions. Although many efforts have been made to improve such chemical treatment solutions by changing their composition, it is difficult to obtain a dense and uniform coating having excellent corrosion resistance by chemical treatment. That is, although the chemically treated coating was improved, the coating layer was easily peeled off or scratched to expose the plating layer to the outside. Therefore, the improvement of corrosion resistance using chemical treatment has reached the limit.

In recent years, the method of improving the corrosion resistance of a well-known plated zinc layer is used by forming the zinc layer containing a component other than zinc which improves corrosion resistance. A typical example is zinc-nickel alloy electroplating.

The developed zinc-nickel alloy electroplating layer contains expensive nickel in a high proportion of 8 to 16%. Therefore, the unit cost of the plating layer is about twice that of the conventional zinc electroplating layer of the same coating weight. In order to ensure that the zinc-nickel alloy electroplating layer has inherent corrosion resistance, that is, to have corrosion resistance 4 to 6 times higher than that of the conventional zinc electroplating layer, the zinc-nickel alloy electroplating layer should be thickly plated at 30 g / m ² or more per electrode plate. Is quite thick. If only stable corrosion resistance is required, a coating weight of 20 g / m ² per pole plate is required, which does not mean that the coating weight can simply be reduced because it presupposes low corrosion resistance. Therefore, it is true that the plating cost is considerably higher than that of the conventional zinc electroplating. The reason why the plating layer should be thickened to 30 g / m ² per electrode plate in order to exhibit the intrinsic corrosion resistance of the plating layer is that corrosion products having high corrosion resistance are not formed on the surface of the plating layer until the plating layer is significantly corroded. When the coating amount is 30 g / m ² or less per electrode plate, the plating layer remaining as a metal after the corrosion product is formed becomes too thin, so that corrosion resistance cannot be maintained for a long time due to the reduction of the corrosion product. In addition, when the coating amount is 30 g / m ² or less per electrode plate, the generation of pinholes is remarkable, and the corrosion solution comes into contact with the steel substrate through the pinhole, and corrosion occurs at the interface between the steel substrate and the plating layer, and as a result, the plating layer is peeled off. . This phenomenon is called " corrosion peeling " of the plated layer and is a major disadvantage of conventional electroplated steel sheets of zinc-nickel alloys. The steel plate electroplated with zinc-nickel alloy at a coating amount of 20 g / m ² per electrode plate was exposed to an adhesive tape peel-off test after being exposed to a temperature of 70 ° C. and a humidity of 98% or higher for 96 hours. In the peeling test, a fatal corrosion peeling phenomenon occurs that the plating layer is easily peeled off.

Corrosion peeling can be prevented to some extent by chromate treatment after plating, but this is not a complete solution. Once chromate treatment of the plated, there is a problem that the phosphate treatment can not be properly performed, so that the plated plate can only be used in a limited range.

It is known that the plating layer of the zinc-nickel alloy containing a small amount of titanium is superior to the plating layer containing no titanium (Japanese Patent Laid-Open No. 104194/83). Titanium is added to the plating bath in the form of a titanium salt (sodium or potassium-titanium fluoride, also known as sodium or titanium hexafluoride salt). However, this has a difficulty in that a significant amount of titanium is simultaneously attached to the plating layer of the zinc-nickel alloy. The reason is assumed that titanium is present in the plating bath in the form of a complex anion and is not easily hydrolyzed.

The present inventors endeavored to develop an electroplated steel sheet of zinc-nickel alloy having a very stable and high corrosion resistance with an adhesion amount of 30 g / m ² or less. It has been found to be significantly improved. In addition, it was found that the corrosion resistance of the plating layer is further improved by heating the electroplated steel sheet in an atmosphere having a temperature of 60 to 200 ℃. Based on these facts, the present inventors researched and developed the average composition which improves corrosion resistance and suppresses pinhole generation, and completed as this invention.

The inventors of the present invention, 8 to 16% by weight of nickel, 0.005 to 1% by weight of titanium, 0.01 to 0.5% by weight of cobalt, 0.001 to 2% by weight of aluminum and the balance of zinc, or 8 to 16% by weight of nickel, 0.005 to 1% by weight of titanium, cobalt It has been found that plating layers of alloys composed of 0.05 to 0.5% by weight, 0.001 to 1% by weight of magnesium and zinc residues have superior corrosion resistance and corrosion peeling resistance under exposure to conventional zinc-nickel alloys.

An object of the present invention contains 10 to 40 g / l zinc, 15 to 160 g / l nickel, 0.2 to 10 g / l titanium, 0.1 to 5 g / l cobalt and 0.1 to 5 g / l aluminum or 0.2 to 4 g / l magnesium and PH An acid plating bath of 1.5 to 2.5, to provide a method for producing an electroplated steel sheet of zinc-nickel alloy having excellent corrosion resistance having the composition comprising an electroplated steel sheet. Of course, the metal exists as ions in the plating bath, but the metal does not necessarily exist as simple metal ions. Thus only the names of the metals are mentioned herein. It will be appreciated that the metal is added in the form of salts in the plating bath.

It has been described that the corrosion resistance of the electroplated steel sheet according to the above method can be improved by heating the plated steel sheet in an atmosphere having a temperature of 60 to 200 ° C. The present invention provides a first feature of mixing aluminum or magnesium with titanium in the plating layer to improve the corrosion resistance, and to cope with cobalt to suppress the generation of pinholes, and to plate the pinholes that may occur even if cobalt is added. A second feature of heating the steel sheet is a summary.

Titanium, as you know, has excellent corrosion resistance and, when added to a zinc-nickel alloy, improves corrosion resistance. However, in the case of a plating bath containing 8 to 16% of titanium, only a very small amount of titanium is simultaneously deposited. In addition, the amount of titanium attached at the same time is unstable, so that it is impossible to produce a product having a uniform corrosion resistance. In the present invention, aluminum or magnesium is added to the plating bath to increase the amount of titanium attached simultaneously and to stabilize the corrosion resistance of the product. In other words, aluminum or magnesium is added to the plating bath to increase the amount of titanium simultaneously attached to the plating layer.

The reason why aluminum or magnesium increases the deposition amount of titanium is that aluminum ions or magnesium ions can be considered to promote hydrolysis of the titanium composite at the surface where electrodeposition occurs.

X-ray fluorescence analysis of the plated layer produced when aluminum was added and the composition was used as a titanium source found simultaneous attachment of sodium or potassium with aluminum in addition to titanium. In the same way, when magnesium was added, adhesion of sodium or potassium with magnesium was found in addition to titanium. Based on this fact, the attached titanium can be judged to be a hydrolysis product comprising titanium and the metal ion added to increase the amount of titanium attached, the metal and the cationic metal of the added titanium composition.

In the present invention, titanium is co-attached to the zinc-nickel alloy electroplating layer by the above method. In detail, 0.005 to 1% by weight of titanium is simultaneously attached by adding aluminum or magnesium to the plating bath. The plating layer formed according to the present invention has excellent corrosion resistance with much lower corrosion weight loss in the exposed state than electroplating layers of conventional zinc-nickel alloys.

Although the generation of pinholes can be reduced by an increase in the amount of simultaneous deposition of titanium to be lower than in conventional electroplating methods of zinc-nickel alloys, the corrosion resistance peeling resistance is so markedly improved since it is impossible to completely suppress the generation of pinholes. It doesn't work. In order to improve this point and improve the corrosion resistance peeling resistance, 0.01 to 0.5% by weight of cobalt is added to the plating layer.

In the present invention, the pinholes generated even though measures are taken as described above to suppress the occurrence of the pinholes are sealed by heating the plated steel sheet. The heating method is carried out in a gas stream, a liquid or an atomized atmosphere. However, heating in hot water, steam or atomizing atmosphere is suitable. The heating temperature of the plated steel sheet should not be less than 60 ° C and no more effect will be obtained if it is more than 200 ° C. The heating time is enough for about 60 seconds.

Sealing of the pinhole by heating is accomplished by hydrolysis of the co-bonded titanium compound remaining undigested. That is, the hydrolysis product seals the hole of the pinhole. This is effective only for plating layers containing titanium, and particularly for plating layers containing titanium in an increased amount by the addition of aluminum or magnesium to the plating bath according to the invention. There is no sealing effect at temperatures below 60 ° C., which is presumed to not cause hydrolysis at such low temperatures.

To the water used for the sealing treatment, it is possible to add components which increase the sealing effect such as phosphate or chromate.

There are various compounds in the elements constituting the plating bath of the present invention. Titanium stannate, titanium oxalate, sodium-titanium fluoride, potassium-titanium fluoride, etc. are used as the titanium source, and are suitable because sodium-titanium fluoride and potassium-titanium fluoride are chemically stable in the plating bath. Examples of aluminum ions or magnesium ions include chlorides, sulfates, nitrates and acetates, but aluminum sulfates and magnesium sulfates that are inexpensive and easy to use are suitable. Zinc, nickel and cobalt sources include chlorides, sulfates, nitrates, acetates, etc., but sulfates are suitable.

Additives used to increase the adhesion of titanium are not limited to aluminum and magnesium. Chemical components with equivalent effects, such as iron ions or boron ions, can be used. In the present invention, as described above, 8 to 16% nickel is attached to the plating layer. This is the point of adhesion of nickel with good corrosion resistance. In this range, in order to obtain a Zn-Ni alloy layer by electroplating, the plating bath must contain 10 to 40 g / l zinc and 15 to 160 g / l nickel. At concentrations lower than the lower limit, the amount of zinc ions and nickel ions is too small compared to the amount consumed for adhesion, the amount of zinc ions and nickel ions in the plating bath is large, and the amount of zinc and nickel in the above range is always It is almost impossible to keep it constant. In the range exceeding an upper limit, although the average plating amount of nickel is 8 to 16%, it becomes a cause which the composition of a plating layer becomes nonuniform. In addition, it is not economical to maintain this high nickel content in the plating bath. Suitable nickel content is 20 to 60 g / l and optimal content is 30 to 50 g / l. The preferred zinc content range is 12 to 25 g / l, more preferably 13 to 21 g / l.

The titanium content of the plating bath should be 0.2 to 10 g / l. At concentrations below the lower limit, titanium is not easily attached to the plating layer even though titanium is contained in the plating bath. When the content of titanium exceeds 10g / l occurs a problem that the titanium compound is not easily dissolved. In addition, when titanium in the plating bath is at a high concentration, it is unstable and easily hydrolyzed. A suitable content is 1 to 8 g / l, but an optimum content is 3 to 7 g / l. The content of aluminum and magnesium should be 0.1 to 5 g / l and 0.2 to 4 g / l, respectively. If the concentration of aluminum and magnesium is lower than the lower limit, there is no effect of increasing the amount of titanium attached. On the other hand, when the concentration of aluminum is 5 g / l or more, the upper limit, a precipitate of titanium ions and aluminum ions is formed during the initial formation of the plating bath, and the stability of the plating bath is reduced. In the case of magnesium ions, when the magnesium concentration reaches 4 g / l or more, the pattern of the local flow in the plating bath is formed on the surface of the plating layer, thereby degrading the appearance of the product. Suitable contents of aluminum and magnesium are 0.15 to 3 g / l and 0.3 to 3 g / l, respectively, but optimum contents are 0.2 to 2 g / l and 0.5 to 0.2 g / l, respectively.

The content of cobalt is 0.1 to 5 g / l. When the concentration of cobalt is 0.1 g / l or less, the effect of cobalt does not appear. The saturation concentration of cobalt is 5 g / l and there is no benefit in excess of this concentration. A suitable range is 0.5 to 3 g / l and an optimum range is 1 to 2 g / l.

The pH of the plating bath should be maintained at 1.5 to 2.5. When the acid becomes strong and has a pH of 1.5 or less, zinc preferentially adheres to nickel so that a large amount of expensive nickel must be kept in a dissolved state at all times to maintain a nickel layer of 8 to 16%. On the other hand, if the pH is 2.5 or more, the nickel content is more than 16% locally generated deposits become difficult to form a uniform plating layer, so the corrosion resistance is lowered and the corrosion resistance of the product is damaged. Therefore, the pH of the plating bath should always be adjusted to 1.5 to 2.5.

In practicing the method of the present invention, the current density is between 5 and 150 A / dm ² . In this range, balanced attachment of plural chemical components is achieved. If the current density is 5A / dm ² or less, the nickel content of the plated layer is more than 16%, whereas if the current density is 150A / dm ² or more, a higher voltage is required, so it is economically lost.

Suitable current densities range from 10 to 100 A / dm ² and the optimum range is 20 to 80 A / dm ² .

With reference to the accompanying drawings, the present invention will be described with examples of experimental work and work.

Specimens of cold-rolled steel sheets with a thickness of 0.8 mm are degreased and pickled with dilute acid solution in a conventional manner, and the coating amount is 30 g / m ² only in the test used in the third comparative bath. Electroplating was carried out using a plating bath of the composition shown in 1 so that the coating amount was 15 g / m ² . The zinc source was zinc sulfate, the nickel source was nickel sulfate, the cobalt source was cobalt sulfate, the titanium source was potassium-titanium fluoride, the aluminum source was aluminum sulfate, and the magnesium source was magnesium sulfate.

Electroplating was carried out at a current density of 20 A / dm ² .

Separately, regarding the plating bath No. 1 of the present invention, aluminum was added in various amounts, and the amount of titanium attached simultaneously was measured by X-ray fluorescence analysis.

The results are shown in the accompanying drawings. The amount of titanium attached is expressed by the fluorescence of X-rays. Looking at the figure, by the addition of a small amount of aluminum, it was increased by 3 to 10 times than without adding titanium. It was confirmed that the amount of co-attached titanium was likewise increased with the addition of magnesium.

Specimens of most of the electroplated steel sheets obtained as described above were heated under the conditions indicated in Table 1.

TABLE 1

The specimens were subjected to a corrosion test and a corrosion peeling test. The corrosion test was carried out according to the JIS (Japanese Industrial Standard) Z-2371 method, and the time until the red rust occurred in the salt spray test at the time intervals indicated in each table was measured. The corrosion peeling test was performed as follows. That is, after exposing the specimen for 96 hours in an atmosphere having a temperature of 70 ° C. and a relative humidity of 98%, each specimen was inserted between the two specimens of the same thickness steel plate, bent in a U-shape, and the adhesive tape was attached to the bent portion and peeled off therefrom. . It was observed whether the plating layer was peeled off by the tape. The results are summarized in Table 2. Specimen No. 1 is plated using the plating bath No. 1 and specimen No. 2 is the same as the plating bath No. 2 below.

TABLE 2

As can be clearly seen from Table 2, the electroplated steel sheet produced according to the present invention is excellent in corrosion resistance and corrosion peeling, even though the adhesion amount is small as 15g / m ² compared to the conventional method.

As described above, it is possible to increase the amount of simultaneous deposition of titanium, to suppress the occurrence of pinholes and to seal the possible pinholes, by using the method of the present invention to increase the amount of simultaneous deposition of titanium which improves the corrosion resistance of the electroplated layer of the zinc-nickel alloy. Therefore, the corrosion resistance and the corrosion resistance peeling resistance can be significantly improved, and consequently, the amount of adhesion can be reduced to save. The electroplated steel sheet of zinc-nickel alloy having a coating amount of 10 g / m ² prepared according to the present invention has the same corrosion resistance as the electroplated steel sheet of a conventional zinc-nickel alloy having a coating amount of 20 g / m ² .

Claims (11)

10 to 40 g / l zinc, 15 to 160 g / l nickel, 0.2 to 10 g / l titanium, 0.1 to 5 g / l cobalt and 0.1 to 5 g / l aluminum or 0.2 to 4 g / l magnesium with a pH of 1.5 to 2.5 An electroplating steel sheet production method of zinc-nickel alloy excellent in corrosion resistance, characterized by electroplating the steel sheet with an acid plating bath. According to claim 1, wherein the acid plating bath is 15 to 25g / l zinc, 20 to 60g / l nickel, 1 to 8g / l titanium, 0.5 to 3g / l cobalt and 0.15 to 3g / l aluminum or 0.3 to 3 g / An electroplating steel sheet production method of zinc-nickel alloy having excellent corrosion resistance, characterized in that it contains l. According to claim 2, wherein the acid plating bath is zinc 13 to 21g / l, nickel 30 to 50g / l, titanium 3 to 7g / l, cobalt 1 to 2g / l and aluminum 0.2 to 2g / l or magnesium 0.5 to 2g / An electroplating steel sheet production method of zinc-nickel alloy having excellent corrosion resistance, characterized in that it contains l. The method of claim 1 wherein the zinc source is zinc chloride, sulfate, nitrate or acetate, the nickel source is nickel chloride, sulfate, nitrate or acetate and the titanium source is titanium-tartrate, titanium oxalate, sodium-titanium fluoride or potassium- Titanium fluoride, cobalt source is cobalt chloride, sulfate, nitrate or acetate, aluminum source is aluminum chloride, sulfate, nitrate or acetate, magnesium source is magnesium chloride, sulfate, nitrate or acetate -Production method of electroplated steel sheet of nickel alloy. The zinc source is zinc sulfate, nickel sulfate is used as nickel source, cobalt sulfate is used as cobalt source, sodium or potassium-titanium fluoride is used as titanium source, aluminum source or magnesium A method for producing an electroplated steel sheet of zinc-nickel alloy having excellent corrosion resistance, wherein aluminum sulfate or magnesium sulfate is used as a circle. The electroplating layer according to any one of claims 1 to 5, wherein the electroplating layer is made from 8 to 16 wt% nickel, 0.005 to 1 wt% titanium, 0.01 to 0.5 wt% cobalt, 0.001 to 2 wt% aluminum, or 0.001 to 1 wt% magnesium. And the remainder is made of zinc, an electroplated steel sheet manufacturing method of zinc-nickel alloy excellent in corrosion resistance. The method for producing an electroplated steel sheet of zinc-nickel alloy having excellent corrosion resistance according to any one of claims 1 to 5, wherein the electroplating layer is heated in an atmosphere having a temperature of 60 to 200 ° C. 8. The method of claim 7, wherein the atmosphere is hot water, steam, or atomization atmosphere. The method for producing an electroplated steel sheet of zinc-nickel alloy having excellent corrosion resistance according to any one of claims 1 to 5, wherein the electroplating is performed at a current density of 5 to 150 A / dm ² . 10. The method of claim 9, wherein the electroplating is performed at a current density of 10 to 100 A / dm ² . 12. The method of claim 10, wherein the electroplating is performed at a current density of 20 to 80 A / dm ² .

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