KR102153162B1 - Method for treating surface of aluminum alloy plated steel sheet and aluminum alloy plated steel sheet using the same - Google Patents

Abstract

The present invention comprises the steps of preparing an alkaline electrolyte solution by adding alkali to distilled water; And forming an oxide layer on the surface of the steel sheet by immersing the aluminum alloy plated steel sheet in the alkaline electrolyte and performing micro-discharge oxidation treatment, wherein the plating layer of the aluminum alloy plated steel sheet contains 55 to 70% by weight of aluminum and 30 to 45 of zinc. It relates to a method for surface treatment of an aluminum alloy plated steel sheet, which is contained in weight percent. According to the present invention, an aluminum oxide layer having excellent corrosion resistance can be formed on the surface of the steel sheet by applying the micro-discharge oxidation technique to the aluminum alloy plated steel sheet. In addition, it is possible to control the thickness and density of the oxide layer through current density, application time, and electrolyte control, so it can be applied not only to construction fields such as roofs and walls, but also to fields requiring corrosion resistance such as agricultural pipes and barns.

Description

Surface treatment method of aluminum alloy plated steel plate and aluminum alloy plated steel plate manufactured accordingly {METHOD FOR TREATING SURFACE OF ALUMINUM ALLOY PLATED STEEL SHEET AND ALUMINUM ALLOY PLATED STEEL SHEET USING THE SAME}

The present invention relates to a method for surface treatment of an aluminum alloy plated steel plate and an aluminum alloy plated steel plate manufactured accordingly.

Among the various metal wet surface treatment methods, micro arc oxidation (MAO) or plasma electrolytic oxidation (PEO) is generated near the surface of the metal by applying electricity while the target metal is positioned as an anode in the electrolyte. Numerous micro-discharges generated at the moment dielectric breakdown occurs in oxygen gas bubbles reacts with target metals and electrolytes located on the anode to form an oxide film.

This micro-discharge oxidation method is effective in improving corrosion resistance due to excellent adhesion between the coating layer and the metal base material because an electrochemical reaction occurs together with the anodic oxidation to form an oxide film.

Galvanized steel sheets are widely used throughout the industry, from automobiles and construction materials as well as home appliances. In recent years, research to extend the life of the plating layer without deteriorating the sacrificial corrosion resistance of zinc is actively underway.

In particular, by adding aluminum to the zinc plating bath, the focus is on simultaneously realizing the durability, heat resistance, and heat reflection of aluminum and the sacrificial anticorrosive effect of zinc. However, this aluminum-plated steel sheet is inferior to Zn-Mg-Al ternary alloy-plated steel sheet in corrosion resistance, especially in cross-section corrosion resistance, so research on surface treatment to improve this is required.

The present invention has been conceived to solve the above problems, and is to provide a method for surface treatment of a high aluminum alloy plated steel plate having improved corrosion resistance properties by locally inducing micro-discharge to the aluminum alloy plated steel plate.

According to an aspect of the present invention, the step of preparing an alkaline electrolyte solution by adding an alkali to distilled water; And forming an oxide layer on the surface of the steel sheet by immersing the aluminum alloy plated steel sheet in the alkaline electrolyte and performing micro-discharge oxidation treatment, wherein the plating layer of the aluminum alloy plated steel sheet contains 55% by weight or more, less than 70% by weight of aluminum, and zinc. It is contained in an amount of 30 to 45% by weight, a method for surface treatment of an aluminum alloy plated steel sheet is provided.

The alkali concentration in the alkaline electrolyte may be 0.1 to 1.25 mol/L.

The alkali may be at least one selected from potassium hydroxide, sodium hydroxide and sodium silicate.

The alkaline electrolyte may further include one or more metal salts selected from manganese salts, vanadium salts, and tungsten salts.

The alkaline electrolyte is TiO ₂ , ZrO ₂ And it may further include at least one metal oxide selected from CeO ₂ .

The temperature of the alkaline electrolyte may be 80°C or less.

The current density applied during the micro discharge oxidation treatment may be 50 to 500 mA/cm ² .

The voltage applied during the micro-discharge oxidation treatment may be 180 to 350V.

The micro discharge oxidation treatment may be performed for 0.1 to 10 minutes.

The plating layer of the aluminum alloy plated steel sheet may further include 2% by weight or less of one element selected from Si, Mg, and Ni.

According to another aspect of the present invention, there is provided an aluminum alloy plated steel sheet having excellent corrosion resistance, which is surface-treated by the above method to form an oxide layer on the surface of the steel sheet.

The thickness of the oxide layer may be 1 to 10 μm.

According to the present invention, an aluminum oxide layer having excellent corrosion resistance can be formed on the surface of the steel sheet by applying the micro-discharge oxidation technique to the aluminum alloy plated steel sheet. In addition, it is possible to control the thickness and density of the oxide layer through current density, application time, and electrolyte control, so it can be applied not only to construction fields such as roofs and walls, but also to fields requiring corrosion resistance such as agricultural pipes and barns.

1 is a process flow diagram schematically showing a method for surface treatment of an aluminum alloy plated steel sheet according to an embodiment of the present invention.
2 is a schematic diagram of a micro-discharge oxidation treatment step of the present invention.
3 is a scanning electron microscope photograph of an aluminum-plated steel sheet surface-treated according to Example 1. FIG.

Hereinafter, preferred embodiments of the present invention will be described with reference to various examples. However, embodiments of the present invention may be modified in various other forms, and the scope of the present invention is not limited to the embodiments described below.

The present invention relates to a method for surface treatment of an aluminum alloy plated steel sheet, and an aluminum alloy plated steel sheet manufactured thereby.

According to an aspect of the present invention, the step of preparing an alkaline electrolyte solution by adding an alkali to distilled water; And forming an oxide layer on the surface of the steel sheet by immersing the aluminum alloy plated steel sheet in the alkaline electrolyte and performing micro-discharge oxidation treatment, wherein the plating layer of the aluminum alloy plated steel sheet contains 55% by weight or more, less than 70% by weight of aluminum, and zinc. It is contained in an amount of 30 to 45% by weight, a method for surface treatment of an aluminum alloy plated steel sheet is provided.

In the present invention, it is preferable that the plating layer of the aluminum alloy plated steel sheet subject to surface treatment contains 55% by weight or more of aluminum and 45% by weight or less of zinc.

When aluminum is less than 55% by weight, there is a problem that micro-discharge does not occur in the micro-discharge oxidation treatment described later. However, if it is more than 70% by weight, micro-discharge occurs, but there is a problem that plating workability is inferior, so it is preferable that it is less than 70% by weight.

Further, it is preferable that zinc is contained in an amount of 45% by weight or less. When the plating layer contains 45% by weight or less of zinc and more than 55% by weight of aluminum, it is possible to produce excellent durability and inexpensively, as well as maximize the effect of the sacrificial method of zinc to the holding steel sheet.

On the other hand, the plating layer of the aluminum alloy plated steel sheet may further include one element selected from Si, Mg, and Ni, and this is to suppress the aluminum-zinc alloy solution from causing a violent alloy reaction with the base steel sheet. However, the content is preferably 2% by weight or less. This may add Si, Mg, Ni, etc. for the purpose of controlling the size of spangles in the plating layer and improving durability, and when the content exceeds 2% by weight, operability is deteriorated and physical properties such as corrosion resistance are rather inferior.

In the present invention, an alkali electrolyte is prepared by first adding alkali to distilled water. The type of alkali added at this time is not specifically limited, but for example, at least one selected from potassium hydroxide, sodium hydroxide and sodium silicate may be used.

On the other hand, the concentration of alkali in the alkaline electrolyte is preferably controlled to 0.1 to 1.25 mol/L. If it exceeds 1.25 mol/L, not only economic feasibility is deteriorated, but also literature (AL Yerokhin etal., Surf. Coat. Technol. (1999), https://doi.org/10.1016/S0257-8972(99)00441-7) As described in, this is because the micro-oxidation discharge treatment is not performed in the immersed high-concentration solution of a certain concentration or higher due to the high electrical conductivity of the solution.

On the other hand, if necessary, the alkaline electrolyte may further include at least one metal salt selected from manganese salt, vanadium salt and tungsten salt, and TiO ₂ , ZrO ₂ And it may further include at least one metal oxide selected from CeO ₂ .

For example, the coating layer may be colored by using the color of the metal oxide, or the coating layer having excellent wear resistance may be formed by using the high wear resistance of the metal oxide. Moreover, the adhesion between the coating layer and the base material can also be improved by adding a metal salt (oxide). On the other hand, the appropriate amount of the metal oxide or metal salt may be added differently depending on the purpose and application range.

It is preferable that the temperature of the alkaline electrolyte solution thus prepared is 80° C. or less (excluding 0). Micro-discharge oxidation treatment is a method in which an electrochemical reaction occurs by supplying electric charges based on an aqueous solution, so reaction heat is involved. For this reason, since it may cause evaporation of the aqueous solution during long-time work, it is preferable to perform it at 80°C or less in consideration of workability and replayability.

Next, the aluminum alloy plated steel sheet is immersed in the alkaline electrolyte and subjected to micro-discharge oxidation treatment to form an oxide layer on the surface of the steel sheet. In the micro-discharge oxidation method, a number of micro-discharges generated at the moment dielectric breakdown occurs in oxygen gas bubbles generated near the surface of the metal by applying electricity while the target metal is positioned as an anode in the electrolyte solution are located at the anode. As a technology for forming an oxide film by reacting with a target metal and an electrolyte, the principle thereof is widely known to a person skilled in the art, and thus a detailed description thereof will be omitted.

According to an embodiment of the present invention, an aluminum alloy plated steel sheet may be positioned as an anode, and stainless steel may be installed at the cathode, followed by micro-discharge oxidation treatment to form an oxide layer on the surface of the steel sheet. Hydrogen gas is generated by the decomposition reaction of water on the surface of stainless steel, which is the cathode in the electrolyte, and oxygen gas is generated on the surface of the aluminum alloy plated steel sheet, which is the anode, and an oxide film is formed. As the voltage increases, a large amount of electron emission by the breakdown voltage is generated from the electrolyte to oxygen gas, resulting in high-temperature micro-discharge (plasma) to form metal oxides.

The current density applied during the micro discharge oxidation treatment is 50 to 500 mA/cm ² (5 to 50 ASD) It is preferable to be. If it is less than 50 mA/cm ² , microdischarge may not occur, and if it exceeds 500 mA/cm ² , excessive microdischarge may cause corrosion of the base material.

In addition, the voltage applied during the micro-discharge oxidation treatment is preferably 180 to 350V. If it is less than 180 V micro-discharge may not occur, and if it is more than 350 V, excessive micro-discharge may cause corrosion of the base material.

Meanwhile, the micro-discharge oxidation treatment is preferably performed for 0.1 to 10 minutes. The micro-discharge oxidation treatment time, that is, the thickness of the oxide layer accordingly, is set differently depending on the product use and operating conditions. Products that require abrasion resistance properties such as bearings and gears that require a thick oxide film of tens to hundreds of micrometers are known to be treated for about 10 minutes, such as aluminum black masking used in UV processes that require electric current and coloring properties. In the product, it is processed within 1 minute to form within 10 microns of thickness. However, in the present invention, even when the treatment is performed for 10 to 30 seconds, differentiation can be secured in terms of surface shape and corrosion resistance.

According to another aspect of the present invention, there is provided an aluminum alloy plated steel sheet having an oxide film formed thereon by the surface treatment method described above.

As described above, the thickness of the oxide layer may vary depending on the micro-discharge oxidation treatment, but even in the case of 1 to 10 μm, differentiation in surface shape and corrosion resistance can be secured.

Example

Hereinafter, the present invention will be described in more detail with reference to examples. The following examples are for describing the present invention in more detail, and the present invention is not limited thereto.

Example 1

The aluminum alloy plated steel sheet was cut into a thin plate having a width of 50 mm, a length of 75 mm, and a thickness of 0.8 mm, washed with distilled water, and dried to prepare.

Thereafter, 0.125M NaOH was added to distilled water and stirred to prepare an alkaline electrolyte.

After placing the sample of the aluminum-plated steel sheet on the anode in the alkaline electrolyte, installing stainless steel on the cathode, and applying DC power through a 1.5 kW power supply, microelectrolytic oxidation treatment was performed at a current density of 50 ASD. Then, by performing the surface treatment for 30 seconds to form an oxide film on the surface of the aluminum-plated steel sheet. At this time, the temperature of the electrolyte was maintained at 20 to 30.

Comparative Example 1

Micro electrolytic oxidation was performed in the same manner as in Example 1, except that 0.25M NaOH was used instead of 0.125M NaOH.

Comparative Example 2

Micro electrolytic oxidation was performed in the same manner as in Example 1, except that 0.5M NaOH was used instead of 0.125M NaOH.

A photograph of the microstructure of the aluminum alloy plated steel sheet subjected to the surface treatment as described above was taken with an electron scanning microscope and is shown in FIG. 3.

In general, as the electrical conductivity (EC) increases, the ignition voltage at which micro-discharge begins to decrease decreases, but in the case of Comparative Examples 2 and 3, the voltage does not reach the point at which micro-discharge occurs, so selective oxidation of aluminum in the plating layer is reduced. Occurred predominantly.

On the other hand, in the case of Example 1, it can be observed that a large number of pores having a size of several microns are randomly formed on the surface due to micro-discharge, and it can be seen that the size of the micropores gradually increased as time passed.

The specific surface area of the high aluminum alloy plated steel sheet according to the NaOH concentration in the electrolyte and the time change was measured, and the results are shown in Table 1. In the case of Comparative Examples 1 and 2 at the same time due to the occurrence of micro-discharge under the conditions of Example 1 It can be seen that the specific surface area is increased by more than two times, which is consistent with the content taught in FIG. 3.

10 seconds 20 seconds 30 seconds Example 1 3,057㎛ (22.3 %) 3,343㎛ (33.7 %) 3,116㎛ (24.7 %) Comparative Example 1 2,712㎛ (8.5 %) 2,746㎛ (9.9 %) 2,903㎛ (16.1 %) Comparative Example 2 2,618㎛ (4.7 %) 2,681㎛ (7.2 %) 2,854㎛ (14.2 %)

In order to evaluate the corrosion resistance of the high aluminum alloy plated steel sheet according to the NaOH concentration in the electrolyte, an electrokinetic polarization test was performed in a 3.5 wt.% NaCl solution, and the cathode and anode of the corrosion potential (Ecorr), current density (icorr) and tafel curves Table 2 shows the polarization resistance value (Rp) calculated by applying the slope to the Stern-Geary equation.

Concentration (M) Time(s) Ecorr (V) icorr (A/cm ² ) Rp (Ωcm ² ) bare -1.2 17 2.17 × 10 ³ Example 1 10 -1.1 5.3 6.83 × 10 ³ -1.1 13 2.78 × 10 ³ -1.1 8 4.52 × 10 ³ Comparative Example 1 20 -1.0 1.9 1.90 × 10 ⁴ -1.1 8 4.52 × 10 ³ -1.1 5 7.24 × 10 ³ Comparative Example 2 30 -0.9 0.33 1.21 × 10 ⁵ -1.1 4 9.05 × 10 ³ -1.1 One 3.62 × 10 ³

It is known that the higher the corrosion potential value or the lower the corrosion current density value, the higher the resistance to corrosion. In the case of Example 1 in which micro-discharge occurred, the polarization resistance values of all other samples within the same time were higher, and the values increased as the processing time elapsed. This is considered to be due to the high corrosion resistance of the Al ₂ O ₃ phase formed by micro-discharge, and as time increases, the thickness growth of the Al ₂ O ₃ layer by micro-discharge also supports this.

Test example

In order to confirm the effect of the micro-discharge oxidation treatment according to the metal element content in the plating layer, as shown in Table 3, the composition of the plating layer of the aluminum alloy-plated steel sheet was controlled to perform the micro-discharge oxidation treatment under the same conditions as in Example 1, and the result was obtained. It is shown in Table 3. The evaluation criteria are as follows.

1. Corrosion resistance

In order to confirm the composite anticorrosive performance of the plated steel, a constant temperature and humidity test was conducted in accordance with KS M ISO 6270-1 standard to maintain a constant temperature and humidity. In the evaluation of the constant temperature and humidity test, the cut surface of the specimen before and after the test was photographed using an optical microscope, and the area of rust occurrence was measured and evaluated as follows.

○: Rust generation rate 0% or more and less than 5%

△: 10% or more and less than 25% of rust

X: rust generation rate 25% or more and less than 50%

Al (% by weight) Zn (% by weight) Mg (% by weight) Plated
Workability Processing time (s) Whether micro discharge occurs Corrosion resistance Comparative Example 3 70 30 - × 10 ○ ○ × 20 ○ ○ × 30 ○ ○ Example 2 55 45 - ○ 10 ○ ○ ○ 20 ○ ○ ○ 30 ○ ○ Comparative Example 4 10 90 - △ 10 × × △ 20 × × △ 30 ○ △ Comparative Example 5 3.0 94 3.0 ○ 10 × × ○ 20 × × ○ 30 × △ Comparative Example 6 1.7 96.6 1.7 ○ 10 × × ○ 20 × × ○ 30 × △

As can be seen in Table 3, in the case of Example 2 satisfying the aluminum and zinc contents of the plating layer in the present invention, it can be seen that plating workability and corrosion resistance are very excellent. On the other hand, in the case of Comparative Example 3 where the aluminum content is too high and the zinc content is relatively low, the plating workability is poor, whereas in Comparative Examples 4 to 6 where the aluminum content is too small and the zinc content is relatively high, micro-discharge occurs. It can be confirmed that the plating workability and corrosion resistance are poor even if it does not or occurs.

Although the embodiments of the present invention have been described in detail above, the scope of the present invention is not limited thereto, and various modifications and variations are possible without departing from the technical spirit of the present invention described in the claims. It will be obvious to those of ordinary skill in the field.

Claims (12)

Adding an alkali to distilled water to prepare an alkaline electrolyte solution; And
Immersing an aluminum alloy plated steel sheet in the alkaline electrolyte and performing micro-discharge oxidation treatment to form an oxide layer on the surface of the steel sheet,
The alkaline electrolyte contains at least one metal oxide selected from TiO ₂ , ZrO ₂ and CeO ₂ ,
The alkaline electrolyte further comprises one or more metal salts selected from manganese salts and tungsten salts,
The plating layer of the aluminum alloy plated steel sheet contains 55% by weight or more, less than 70% by weight of aluminum, and 30 to 45% by weight of zinc.
The method of claim 1,
Surface treatment method of an aluminum alloy plated steel sheet, characterized in that the concentration of alkali in the alkali electrolyte is 0.1 to 1.25 mol/L.
The method of claim 1,
Surface treatment method of an aluminum alloy plated steel sheet, wherein the alkali is at least one selected from potassium hydroxide, sodium hydroxide and sodium silicate.
delete delete The method of claim 1,
Surface treatment method of an aluminum alloy plated steel sheet, characterized in that the temperature of the alkaline electrolyte is 80° C. or less (except 0).
The method of claim 1,
Surface treatment method of an aluminum alloy plated steel sheet, characterized in that the current density applied during the micro-discharge oxidation treatment is 50 to 500mA/cm ² .
The method of claim 1,
Surface treatment method of an aluminum alloy plated steel sheet, characterized in that the voltage applied during the micro-discharge oxidation treatment is 180 to 350V.
The method of claim 1,
Surface treatment method of an aluminum alloy plated steel sheet, characterized in that the micro-discharge oxidation treatment is performed for 0.1 to 10 minutes.
The method of claim 1,
The surface treatment method of an aluminum alloy plated steel plate, characterized in that the plated layer of the aluminum alloy plated steel plate further comprises 2% by weight or less of one element selected from Si, Mg, and Ni (however, excluding 0).
An aluminum alloy plated steel sheet having excellent corrosion resistance, wherein an oxide layer is formed on the surface of the steel sheet by surface treatment by the method according to any one of claims 1 to 3 and 6 to 10.
The method of claim 11,
An aluminum alloy plated steel sheet having excellent corrosion resistance, characterized in that the thickness of the oxide layer is 1 to 10 μm.

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