KR100578214B1 - The method of developing Zn-Mg alloy electroplated steel sheet - Google Patents

Abstract

The present invention relates to a method for manufacturing zinc-magnesium electroplated steel sheet for automobiles, and to the conventional galvanic steel by adding magnesium and antimony in the plating bath to the electro-galvanized steel sheet produced in zinc sulfide plating bath The purpose is to have a better corrosion resistance and uniform surface appearance than galvanized steel sheet. In order to achieve this purpose, magnesium hydrate is added in the plating bath to be electrodeposited in the plating layer, and alloyed magnesium improves the corrosion resistance of the plated steel sheet. Also, by adding antimony as an additive in the plating bath, the surface appearance is excellent even under high current density. Let's have

Zinc Sulfide Plating, Magnesium Sulfate, Antimony, Whiteness, Glossiness

Description

The method of developing Zn-Mg alloy electroplated steel sheet}

1 is a change in magnesium content in the plating layer according to the magnesium content in the plating solution

2 is a change diagram of the current efficiency according to the magnesium content in the plating solution

3 is a change in surface appearance characteristics according to the magnesium content in the plating layer

The present invention relates to a method for producing zinc-magnesium (Zn-Mg) electroplated steel sheet for automobiles, in particular, by the addition of magnesium and antimony in the plating bath to the galvanized steel sheet produced in the zinc sulfide plating bath By alloying magnesium (Mg), there is an object to have a superior corrosion resistance and uniform surface appearance than the conventional automotive galvanized steel sheet produced in a chloride plating bath using a hydrochloric acid bath.
In order to achieve the above object, the present invention adds magnesium hydrate in the plating bath to electrodeposit the plating layer, and alloyed magnesium (Mg) improves the corrosion resistance of the plating steel sheet and adds antimony (Sb) as an additive in the plating solution of the plating bath. Thus, it has excellent surface appearance even under high current density.

In general, electro-galvanized steel sheet is widely used as a material for home appliances, construction materials, automobiles, etc. because the surface is not only beautiful but also protects the base iron by the sacrificial effect of zinc to suppress corrosion. In particular, various post-treatments such as chromate treatment, phosphate treatment, and functional resin treatment are possible, and are being applied to many fields as various products.

At present, cold rolled surface treatment products are produced by continuous plating process, and hydrochloric acid bath is most used for various reasons, such as high speed production by high current density and relatively simple production and composition of plating solution.

The electroplating process applied to the present invention relates to electroplating zinc by employing an insoluble anode in a sulfuric acid bath in a vertical manner for the reasons described above. In general, sulfuric acid baths are more electrically conductive than hydrochloric acid baths, so overvoltage is greatly required. Therefore, it is necessary to minimize the distance between poles using an insoluble anode. However, insoluble anodes coated with IrO ₂ can be damaged by steel strips if the distance between the poles is too short, thereby limiting the distance between them.

In general, in the case of zinc-based alloy plating, current efficiency is low due to the non-ideal precipitation behavior of zinc. Therefore, a high current density is required, and a large amount of alloying elements must be added, which makes it difficult to secure economic feasibility. Therefore, many studies have been conducted to solve this problem by using a trace amount of organic / inorganic additives.

Recent domestic patents include 2000-062855,1991-008175. In this case, examples of increasing the electrodeposition amount and plating efficiency of magnesium using various organic additives based on a hydrochloric acid bath. However, in the case of the sulfuric acid bath, there is little published domestic and international for improving the physical properties using additives. Particularly, in the case of sulfuric acid bath, there is a disadvantage in that the addition of organic material is adsorbed on the insoluble anode, so there is a limitation in using an organic additive which shows a great effect in the hydrochloric acid bath.

The present invention relates to a method for producing Zn-Mg electrogalvanized steel sheet having excellent corrosion resistance in a sulfuric acid bath, and in particular, Zn-Mg electroaluminum having excellent corrosion resistance and excellent surface appearance by adding antimony, an inorganic additive, as well as magnesium in a zinc plating solution. It is possible to manufacture alloyed steel sheet.

In recent years, development of surface-treated steel sheets having higher corrosion resistance than conventional ones, including the increase in the rust-proof life of automobiles, etc. has been required. Accordingly, the development of zinc-based alloy plated steel sheet having better corrosion resistance and economical efficiency than the conventional zinc plating has been continued. In particular, magnesium has been studied as an alloying element.

However, in the case of hot-dip plating, the plating adhesion amount is large, and the workability is poor, and the plating adhesion is also poor. In addition, in the case of evaporation plating, the cost of facility cost is very high. Since magnesium is a metal that is sublimated directly without going through the liquid phase, it is difficult to control the coating amount when it is added in the plating solution, and thus, the problem of supply of raw materials is caused. Therefore, magnesium such as magnesium sulfate is mainly used. It is added to the sulfuric acid bath in the form of a hydrate.

In the case of electroplating, plating efficiency is very low due to abnormal deposition of zinc (Zn) or anomalous electrodeposition (Anomalous Deposition). However, when the current density is increased to increase the magnesium content in the plating layer, the appearance of the plating layer becomes poor. There was.

Accordingly, the present invention aims to develop a zinc-magnesium alloy plated steel sheet having excellent corrosion resistance and uniform surface appearance in an electroplating line using a sulfuric acid bath. To this end, the present invention is characterized by producing an electro-galvanized steel sheet having excellent corrosion resistance and uniform surface appearance by adding antimony as an additive while controlling the magnesium component ratio and current density conditions in the plating layer.

The present invention for achieving the above object is a method for producing a zinc-magnesium electroalloy plated steel sheet with zinc sulfide plating solution made by adding magnesium sulfate as an alloying element to a sulfuric acid bath, the zinc sulfide plating solution in The magnesium ion content is 30 to 90 g / l, so that the magnesium ion concentration is 30 to 90% by weight relative to the zinc ion concentration, and antimony is added as an additive. Zinc is excellent in corrosion resistance and uniform surface appearance. The method of manufacturing magnesium electroplated steel sheet is provided as a summary.

In the present invention, the zinc-magnesium alloy plating is performed by changing the addition ratio of the plating element added to the zinc plating solution and the current density during electroplating. Also, by adding antimony (Sb) as an additive, the surface is uniform even at high current density. It had an appearance.

The amount of the inorganic additive added to the zinc sulfide plating solution was 300-1200 g / l and the magnesium ion content in the plating solution was 30-90 g / l as described above. If the magnesium ion content is less than 30 g / l, the vacancy reaction in the plated layer does not occur, and a predetermined effect is less likely to occur. In addition, the addition of more than 90g / l decreases the current efficiency and does not bring any more effects, it only results in expensive material waste. Therefore, when the zinc ion concentration of the zinc plating solution is fixed to 30g / l to 90g / l, the magnesium ion concentration (Zn ion / Mg ion) compared to the zinc ion concentration in the plating solution is 30 to 90% by weight. Increasing the magnesium content in the plating layer by more than 90 wt% by increasing the actual magnesium concentration ratio may result in higher corrosion resistance due to an increase in the magnesium content in the plating layer, but the coating adhesion amount is reduced, thereby decreasing the current efficiency and the adhesion. Too much magnesium content limits the magnesium content in the plating layer as described above because of poor workability.
In addition, reducing or increasing the concentration of zinc ions increases or decreases the magnesium ion concentration of the electrodeposited layer, which may change the surface properties. Since it cannot be achieved, it is difficult to improve the surface properties. Therefore, in the present invention, the plating experiment was performed by fixing the zinc ion concentration of the zinc plating solution to 90 g / l suitable for magnesium electrodeposition.

1 is a graph showing a change in magnesium content in the plating layer according to the magnesium content of the plating element in the plating solution, Figure 2 is a graph showing a change in current efficiency according to the magnesium content of the plating element in the plating solution. In addition, Figure 3 is a graph showing the surface appearance characteristics change according to the magnesium content in the plating layer.

In the present invention, the magnesium ions in the zinc plating solution have a very low standard electrode potential and have a very low electrodeposition amount in the plating layer under general conditions. Therefore, when plating at a high current density as shown in FIG. 1, it can be seen that as the magnesium concentration in the plating layer increases, the magnesium content in the plating layer increases. As such, high current density is generally required to increase the magnesium content in the plating layer, but when the plating layer grains are dendrite-grown, a 'burnt surface' phenomenon occurs and the surface appearance becomes uneven. . Therefore, when antimony is added as an additive to 0.1-20 mg / l in consideration of magnesium electrodeposition state, the amount of magnesium electrodeposition decreases in proportion to the amount of antimony added, but the surface appearance becomes uniform.

The lower limit of antimony addition is the lower limit of the effect. If the upper limit is exceeded, surface appearance defects and workability deterioration are caused due to agglomeration of the grains of the plating layer.

On the other hand, the current density was difficult to exceed 40 A / dm 2 by the conventional method. Increasing the current density, for example, about 70 A / dm 2 has the effect of increasing the amount of electrodeposited magnesium ions, but increases the power consumption and adversely affects the appearance of the surface. Therefore, additives must be used as in the present invention. In other words, the present invention can increase the electrodeposition amount of magnesium ions by using a current density exceeding 40 A / dm 2, which was almost impossible in the related art, thereby improving corrosion resistance and making the surface uniform and beautiful.

As shown in Table 1, in the surface appearance of magnesium alloy, it can be seen that when the addition of antimony, the surface appearance is superior to that of no addition, even if the current efficiency and surface appearance are increased as shown in FIGS. It can be seen that the degree is uniformly secured. When the added magnesium ion is vacant in the zinc plating layer, it forms a very stable magnesium oxide on the surface when corrosion behavior occurs, thereby acting as a barrier to prevent the progress of corrosion, resulting in excellent corrosion resistance. Looking at the results of Table 1 it can be seen that the higher the magnesium content is improved the corrosion resistance.

EXAMPLE

The cold rolled steel sheet was cut to 150 × 200 mm, pickled in 5-8% hydrochloric acid solution for alkali degreasing and oxide removal, and then electrogalvanized using electroplating simulator under the following solution composition and electrolytic conditions.

1) Plating solution temperature: 55 ℃

2) Plating amount: 20g / ㎡

3) anode and cathode gap: 10mm

4) pH: 1.8

5) Zinc ion concentration: 90g / l

6) Plating element (magnesium) ion concentration: 30-90g / l

7) Current Density: 10, 40, 70A / d㎡

The zinc ion concentration is kept constant at 90 g / l, the inorganic additive ion concentration is changed as described above, and the current density of 40 A / dm 2 is attached to 20 g / m 2 at 55 ° C., followed by the whiteness and glossiness, and the magnesium content in the plating layer. Was evaluated.

Glossiness was measured at an incident angle of 60 ° using a glossmeter (Tri-Micro gloss-60-85), and whiteness was measured using a color difference meter (Color Quest II Hunter Lab.). Magnesium content in the plating layer was analyzed by ICP-AES (Thermoelemetal IRIS) after dissolving the plating layer in 8% solution of hydrochloric acid. In the case of corrosion resistance, the specimen was cut to 150mm X 70mm and taped to the edge, and then, the salt spray test was performed for 24 hours according to ASTM B117 standard to determine the occurrence of surface red blue and white blue compared with the general galvanized steel sheet. Table 1 above shows the results.

As described above, in the present invention, the zinc-magnesium electroalloy plated steel sheet was manufactured by vaccinating magnesium in the plating layer by adding an appropriate amount of the plating elements magnesium sulfate and antimony in the electrogalvanizing solution. As a result, the zinc-magnesium electroalloy plate is generally used. It can be confirmed that it has a uniform surface appearance quality and excellent corrosion resistance even at higher current densities than electro zinc plating.

Claims (1)

In the method for producing a zinc-magnesium electroalloy plated steel sheet using a zinc sulfide plating bath obtained by adding magnesium sulfate and an additive to a sulfuric acid bath, The magnesium ion concentration in the zinc sulfide plating solution is 30 to 90 g / l, so that the magnesium ion concentration is 30 to 90 wt% compared to the zinc concentration, and antimony is added as an additive as 0.1 to 20 mg / l depending on the magnesium electrodeposition state. A method for producing a zinc-magnesium electro-alloy plated steel sheet having excellent corrosion resistance and uniform surface appearance.

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