KR20130003576A - Plating method of magnesium alloy using alkali etchant - Google Patents

Abstract

The present invention relates to a plating method of a magnesium alloy sheet using an alkaline etching solution, a surface treatment processing technology used for decorative purposes that gives high corrosion resistance, high electrical conductivity, and beautiful appearance, which is applied to lightweight portable parts, products, and structural parts. The quality and durability of magnesium alloy can be improved, and environmentally friendly surface treatment without chromic acid is possible.

Description

Plating method of magnesium alloy using alkali etchant}

The present invention relates to a plating method of a magnesium alloy sheet material using an alkaline etching solution.

The lightest magnesium alloy among commercially available metals is an electrochemically very expensive metal and is a material to which surface treatment is essential because of its fast corrosion property even in the atmospheric environment. Among them, the plating method widely applied for excellent corrosion resistance and decoration has the advantage of obtaining mirror gloss and giving precious metal plating and welding property such as chrome plating and gold plating for decorative purpose to obtain high hardness by depositing different metals in multiple layers. There is a feature that can obtain a heterogeneous metal surface by tin plating for.

Zinc plating is commercially used rather than electroless nickel plating for the plating of magnesium alloys, and the general decorative plating processes used for plating magnesium are degreasing, etching, zinc substitution, copper cyanide, copper sulfate, nickel, chromium and It proceeds in the order of drying.

The most problematic process in the plating process of depositing dissimilar metals on magnesium is the correlation between the etching process and the underlying plating layer formed by zinc substitution and copper cyanide plating.

The etching process should apply a method for removing heterogeneous inorganic materials from the magnesium alloy material and stabilizing the surface electrode potential value of the magnesium alloy material. Magnesium should be cautious in the choice of etching solution because corrosion reaction proceeds below pH 11.5 and especially chlorides that promote grain boundary corrosion reactions rapidly develop filiform corrosion along the grain boundaries. In the case of magnesium alloy, the reduction potential is very low, so grain boundary corrosion and intragranular corrosion proceed by the electrode potential of the solution and adsorption of impurities, so it is necessary to closely examine the material and pH change.

In the case of zinc substitution process, zinc salt, a precious metal, is present as a chemical reaction product due to the reducing power of pyroan acid salt as a driving force for dissolution of magnesium, thereby inhibiting corrosion of magnesium and forming a precipitate as an inorganic substance in the zinc substitution layer. The zinc-substituted layer thus formed has a role of inhibiting the continuous dissolution of magnesium, inhibiting the corrosion reaction in the aqueous solution, and at the same time buffering the electrochemical reaction between the dissimilar metal and magnesium. In direct contact with a copper layer, a precious metal with a large electrode potential difference, magnesium rapidly dissolves into ions and generation of oxidized chemical reactions (smuts) leads to a problem of poor adhesion between the underlying layer and the magnesium material. . Therefore, in the case of the zinc-substituted layer present as a substitute layer of the inorganic material, it inhibits the corrosion of magnesium ions and has a buffering effect of suppressing the electrical corrosion reaction with the upper plating layer. When the zinc substitution layer is thin, the adhesion is degraded by the magnesium substitution reaction by the dissimilar metal, and when the zinc substitution layer is thick, the plating layer is dropped due to the low bonding strength between the zinc substitution layers. Therefore, the formation of a dense zinc substitution layer with stabilization of appropriate conditions can suppress local corrosion of the magnesium alloy.

Copper cyanide plating (strike) process is a process that gives the adhesion between the material and the upper plating layer can be obtained a dense metal layer by the method of electroplating. Particular attention should be paid to the copper cyanide plating process to suppress the erosion of the zinc-substituted layer and to have excellent uniform electrodeposition properties. In the alkali solution of copper cyanide plating, the substitution reaction of the zinc substitution layer and copper coexists, and thus, the use of electrical properties gives excellent adhesion. In addition, the erosion of the zinc-substituted layer should be suppressed, and the process after the base copper plating thus formed shows the same characteristics as the electrolytic plating of general dissimilar materials. Therefore, the plating process on magnesium gives electrochemically stable properties of the magnesium surface to uniformly form a zinc substituted layer using an electrochemical reaction, and forms a copper cyanide plated layer on which the magnesium material does not erode. Clarifying the mechanism is essential for the stabilization of commercial surface treatment techniques.

Therefore, the present inventors analyze the problems of the plating on the magnesium alloy to ensure corrosion resistance using the plating on the magnesium plate, and analyzes the characteristics of the etching and zinc substitution process that has the greatest effect on the post-plating properties of the plating pretreatment process In addition, the present invention was completed by examining the influence factors for the formation of a uniform zinc substitution layer and the effect of the formation of the zinc substitution layer on the electroplating process.

An object of the present invention is to provide a method for plating a magnesium alloy sheet material using an alkaline etching solution including an optimal etching step and a zinc replacement step.

In order to achieve the above object, the present invention is a plating method of a magnesium alloy sheet material comprising a degreasing process, etching process, zinc substitution process, copper cyanide plating process, acid copper plating process and nickel plating process, the etching process is triethanol Provided is a plating method of a magnesium alloy sheet material comprising using an alkaline etching solution containing amine, ethylenediaminetetraacetic acid (EDTA) and sodium hydroxide.

The alkaline etching solution may include 65 to 75% by weight of triethanolamine, 15 to 25% by weight of ethylenediaminetetraacetic acid (EDTA), and 5 to 15% by weight of sodium hydroxide.

The etching process is preferably deposited by etching using the alkaline etching solution for 3 to 5 minutes at a pH range of 10.5 to 11.5, a temperature of 65 to 70 ℃.

The zinc substitution process is preferably using a zinc substitution bath containing 15 to 18% by weight of ZnSO ₄ , 75 to 80% by weight of Na ₃ PO ₄ , 3 to 10% by weight of Na ₂ CO ₃ and 0.5 to 5% by weight of NaF. .

The zinc substitution process is preferably carried out at a fluoride ion concentration of 2.5 to 3.5 mol, a pH range of 10.5 to 11.5 and a temperature of 65 to 70 ℃.

It is preferable that the zinc precipitation layer formed by the said zinc substitution process is 2-3 micrometers in thickness.

The magnesium alloy may be any one selected from AZ 31, AZ 61 or AZ 91, but there is no particular limitation as long as the plating method of the present invention can be applied.

In addition, the present invention provides a magnesium alloy sheet produced according to the above production method.

It will be described below in more detail based on one embodiment of the present invention.

According to one embodiment of the present invention, the etching and zinc substitution layer formation which has the greatest influence on the characteristics of the plating during the pretreatment process for the plating on the AZ31B magnesium alloy is performed. Confirmed. In the present invention, the effect of temperature, concentration, time, and pH on the formation of zinc-substituted (Zincate) layer in the process of degreasing-etching-zinc substitution, which is generally used for plating on magnesium alloy, was studied.

At this time, for wet surface treatment of magnesium alloy having the lightest feature among commercial metals, 0.67M triethanolamine, 0.1M EDTA, 0.1M potassium sodium tart as a pretreatment method for plating by etching the material surface and removing oxide and smut. Potassium Sodium Tartrate is dissolved in 0.2M sodium hydroxide for pH control and then deposited by 3 to 5 minutes etching at pH 10.5 ~ 11.5, 65 ~ 70 ℃ for temperature, magnesium corrosion products, oxides, inorganic contaminants, etc. P was removed to form a mirror plated layer without pit, void, and blister.

From the plating method of the present invention, first, in order to form a decorative electrolytic plating layer on the magnesium alloy, it was necessary to remove the impurities, the processing layer, and the oxide layer on the surface to obtain a decorative plating layer without a pit.

Second, the etching process and the zinc substitution process are the main processes that cause plating defects in the surface treatment characteristics of magnesium alloys, which are lightweight materials and are very high. The treatment time and temperature, especially pH condition management, were important for the reactivity of magnesium alloys and solutions. .

Third, alkali plating using triethanolamine, EDTA, and sodium hydroxide was very effective for electroplating on magnesium alloys, and it was important to use an immersion etching time of about 4 minutes and an alkaline solution having a pH of 10.8 to 11.2. .

Fourth, the zinc-substituted layer forms zinc complex salts while suppressing dissolution of the inferior magnesium alloy, and obtains the thickness of the substituted layer and the dense precipitated film at 70 ° C. at a pH of about 10.8 to 11.0 with 3 mol of [F ⁻ ]. Was very important.

Fifth, it is important to perform chemical and electrochemical reactions between pH 10.5 and 11.5 for alkali degreasing, alkali etching and base plating layer formation for the electrolytic plating layer formation on the AZ 31B, which is a rolled product. Among them, zinc substitution (zincate) treatment is a decorative electrolytic plating is possible by obtaining a dense and uniform zinc precipitation layer of 2-3㎛ in the solution containing fluoride.

The present invention relates to an etching process and a zinc substitution process applied for removing oxides and inorganic substances from the surface and providing and activating the anchor effect for the surface treatment of the magnesium alloy sheet, which provides high corrosion resistance, high electrical conductivity, and beautiful appearance. As a surface treatment technology used for decorative purposes, the quality and durability of magnesium alloys applied to lightweight portable parts or products and structural parts can be improved, and environmentally friendly surface treatment without chromic acid is possible.

1 is a schematic view of a deposition treatment tank,
2 is a schematic diagram of an electrolytic plating experiment apparatus,
3 is a plating problem on the magnesium alloy,
4 is a plating defect on the Mg alloy according to the galvanized layer formation,
5 is a surface shape and surface roughness characteristics after the material and the etching,
6 is a surface impurity distribution in a raw material state,
7 shows surface impurity distribution after etching;
8 shows the formation of zincate layers under etch conditions (zincate standard conditions),
9 is the precipitation of the zinc substitution layer with the addition of the amount of [F ⁻ ],
10 is [F ^-] a surface shape of the precipitated zinc complex salt according to the concentration gradient,
11 is [F ^-] the microstructure of the zinc substitution layer according to the concentration gradient,
12 shows the amount of precipitation of zinc-substituted layers at different concentrations of [F ⁻ ];
13 is the surface shape (pH 10.8, 4min. [F-] 2.5mol) according to the zinc substitution treatment temperature,
14 is a zinc substitution layer precipitation amount according to the zinc substitution treatment temperature,
15 is a zinc substitution layer precipitation amount according to the zinc substitution treatment time,
16 is a zinc substituted layer fine shape according to the zinc replacement treatment time,
17 is a comparison of the amount of zinc substitution layer precipitation according to the treatment time in the zinc substitution treatment base bath,
18 is a zinc substitution layer precipitation amount according to the zinc substitution treatment time,
19 is the thickness observation of the zinc substitution layer (2.2 ~ 2.6㎛),
20 is a zinc substitution treatment surface characteristics according to the pH change,
21 relates to plating on a magnesium plate using a zinc substitution method.

The present invention will be described in more detail by way of the following examples. However, the present invention is not limited by these examples.

≪ Example 1 >

I. Experimental Method

1. Preparation of Specimen

In this example, AZ31B magnesium plate produced by POSCO was used as a standard specimen, and a specific lot drawn specimen was used to develop a surface treatment process. The size of the specimen was applied by a method of immersion and electroplating by applying a width of 50mm × length 50mm × 0.5mm thickness. At this time, the composition of the AZ31B alloy components is shown in Table 1 below.

element weight % Mg 88.15-94.3 Al 2.15-3.21 Zn 0.5 to 1.5 Mn 0.31 to 0.85 Si 0.54-0.88 total 100.00

2. Experiment apparatus

In the present embodiment, as shown in Figure 1 in the process proceeded to the general plating process of magnesium alloy [degreasing → zinc substitution → copper cyanide → electroplating (Cu, Ni, etc.)] etching and zinc in the deposition treatment tank Zincate treatment was performed, and concentration gradients were minimized using a bath and agitator to uniformize the temperature change and the interfacial reaction of the product surface. The solution of the immersion tank was prepared in the same material and concentration at the beginning of the experiment to minimize chemical reaction and to give uniformity of concentration material, and to keep the solution amount / surface treatment area at 10 for the application of uniform test method.

The degreasing and electrolytic plating bath removes hydrogen and oxygen gas from the surface of the product by applying filtration, flow, and air agitation to remove constant temperature and foreign substances as shown in FIG. The rectifier, which is an external power supply, adopts an IGBT type with a capacity of 15V and 20A. In addition, for the preservation of the solution by applying a general wet solution analysis method was used to correct the concentration of the initial solution of the experiment.

3. Degreasing

The first step for the surface treatment of magnesium alloys is to remove oxidants, mold release agents, and organic / inorganic impurities on the surface to enhance adhesion.As a general rule, magnesium oxide materials are generally deposited or degreased using sodium hydroxide. It is common. The material applied to degreasing is to remove foreign substances by swelling and emulsifying organic materials using sodium hydroxide, sodium cyanide, sodium silicate, sodium phosphate, sodium carbonate and surfactants and the like by mechanical or electrical methods.

In the present embodiment, the electrolytic degreasing applied to the die-casting process is applied in consideration of the characteristics of the magnesium plate that may be contaminated during the molding process although the contamination of the substrate is small. Table 2 shows the degreasing liquid composition and conditions for plating of the AZ31B magnesium alloy.

Furtherance amount Condition NaOH
Na ₂ SiO ₃
Na ₃ PO ₄
Additive 100 g
60 g
50 g
30 g 4min, 60 ℃,
cathodic electro degreased at 8V

4. Etching

Etching process is a process of removing 3 ~ 10㎛ surface layer on the surface for fine iron and anchor effect and surface homogenization on the surface of organic material removed by degreasing process. Removal was applied to give uniformity of the surface reaction layer. The nature of the magnesium etching reaction is a process in which reactivity with the aforementioned solutions is very important. In this example, the solution consisting of EA-100, sodium hydroxide, and EDTA studied as a result of the technology development was applied to the deposition process for 70 minutes at an optimum condition for 4 minutes.

The etching process on the magnesium alloy is a very important process for imparting uniform adsorption of the zinc layer and removing oxides of the material in the zinc substitution process. If the etching process fails to remove the oxides or inorganic impurities, a local galvanic cell of magnesium is formed, which promotes corrosion of the portion acting as the anode, which causes internal pits, voids, and protrusions even when the plating layer is formed. In addition, since the heterogeneous adsorption of the zinc-substituted layer is a process that causes an erosion reaction in the plating process, care must be taken in the pretreatment process. In the past, the process using chromic acid, which has been applied for surface uniformity in die-casting processes, has been a problem in application due to environmental hazards in recent years. It is always important to keep in mind that, especially at low pHs, local corrosion is often observed in the final plated product, which is not observed with the naked eye depending on the treatment conditions. The solution composition and process conditions of the etching process are shown in Table 3.

Furtherance amount Condition EA-100
EDTA
NaOH 100 g
30 g
15g 5 ~ 8min
70 ° C,
pH 11.2

5. Zinc Substitution

Zinc sulfate and pyrophosphate baths were used as the basic solution for forming the zinc-substituted layer. The basic solution composition was a solution generally applied to form a plating layer on a die-casting alloy (AZ 91D) material for magnesium zinc replacement. The solution composition most influences the control of the growth of zinc particles, but the results of the development showed that the magnesium zinc substitution treatment greatly influenced the decorativeness of the product according to the amount of fluoride.

Therefore, in this embodiment, the relationship between the electrolytic plating layer and the electrodeposition properties of the zinc-substituted layer by the characteristics of the amount of the fluoride in the base solution composition and the process influence factors such as temperature, pH, and time were studied. Magnesium zinc substitution treatment forms a substitution reaction by the relationship between the ionization characteristics of magnesium and the precipitation characteristics of zinc, but electrons generated by the oxidation of pyrophosphoric acid with zinc complex salt and dissolution of magnesium ions It was confirmed that the precipitated properties affect the precipitates by reacting with zinc complex salts. Among them, [F ^-] concentration is in the suppression of the dissolution of magnesium ions to the formation of MgF ₂ was given the uniformity of the zinc ion to precipitate with the role to uniformly maintain the electrochemical properties of the magnesium surface. Therefore, [F ^-] was focused on the study of the characteristics according to the amount of the study was to carry out for forming a good quality of decorative plating layer characterized in the time, pH, temperature, etc. Depending on the characteristics of the chemical reaction.

PH, temperature, time, the basic solution of the composition in order to determine the throwing power properties of the zinc substitution layer, [F ^-] was to determine the characteristics of the amount of the exhibited the conditions shown in Table 4.

Furtherance amount Condition ZnSO ₄
Na ₃ PO ₄
Na ₂ CO ₃
NaF 50 g
300 g
30 g
8g / L pH
time
Temperature
[F ^-] 9.0, 10.8, 12.0
10 sec to 10 min.
60 ℃, 70 ℃, 80 ℃
6.2 mol (kind of five)

6. Electro Cu strike and electro plating

Plating method is a process generally used for decorative plating to form a multi-layer plating layer to impart functionality.

1) Alkaline Copper Plating (Cyanide Plating)

Processes essential for improving adhesion and under plating in decorative plating are applied to the general bath properties and conditions, and the copper cyanide plating bath composition and conditions are shown in Table 5.

compound density
(g / L, ml / L) Condition
(temp, sec, pH etc.) Copper cyanide 20 temp: 55 ℃
time: 8min, pH 12.5, agitation: air bubble,
voltage: 5V (3.5ASD) Sodium cyanide 40 Sodium hydroxide 30 Rotsel salt 30 Sodium carbonate 20

For the purpose of copper cyanide plating, it is essential to perform plating by a certain thickness in order to improve uniform application and adhesion of the material. Solution management is concentration control by general analysis and proceeds with variable current and time depending on the product. In this embodiment, the shape and size of the specimens were applied in the same manner, and only the concentration management was considered.

2) Acid copper plating (copper sulfate plating)

Copper sulfate plating is a plating method that is generally used for mirror polish or leveling. The bath compositions used for general decorative plating are shown in Table 6.

compound density
(g / L, ml / L) Conditions (temp, sec, pH etc.) Copper sulfate 200 temp: 30 ℃,
time: 25min,
agitation: air bubble,
Current density: 3ASD
Anode: Ham In-dong Sulfuric acid 55 Goat 80 additive Proper amount

7. Polished nickel plated

Nickel plating is a plating process that is mainly applied for protecting copper plating, giving corrosion resistance to products, and not for chrome plating. General plating solution conditions are shown in Table 7. The advantages of nickel plating are the most widely used plating methods among current plating products because of uniform color, good corrosion resistance, no corrosion, and moderate hardness and mechanical properties. Usually, it is possible to maximize the corrosion resistance by applying double nickel plating or triple nickel plating and to change various textures and colors with satin nickel or velours nickel. In this embodiment, the general polished nickel was applied to the surface treatment of the magnesium alloy.

compound density
(g / L, ml / L) Conditions (temp, sec, pH etc.) Nickel Sulfate 300 temp: 55 ℃
time: 15min,
agitation: air bubble,
Current density: 5ASD
Anode: Nickel, pH 4.8 Nickel chloride 50 Boric acid 50 additive Proper amount

8. Analysis of Test Samples

In the case of metal products, dense film formation and coating thickness have a significant effect on the corrosion resistance. The thickness measurement by zinc substitution treatment on magnesium is performed by ingredient mapping using a microanalyzer (EPMA Electron Probe Micro Analyzer, Cameca SX-100). The surface observation was performed by using a scanning electron microscope (SEM: Scanning Electron Microscopy, Hitachi S-4200, Japan) to observe the fine surface shape. In addition, surface roughness measurement was observed using a scanning probe microscope (AFM: Atomic force microscopy, Nanoscopellla, Di instrument).

The component characteristics of the film forming layer were analyzed by the energy dispersive spectroscopy (EDS), and the concentration of the components was analyzed to observe the formation of uniform film. In particular, the observation of the characteristics according to the zinc substitution conditions and the characteristics of the plating layer are examined. The observation of the microstructure and the cross-sectional view of the plating layer focus on identifying the influence factors of the zinc substitution layer on the plating characteristics of magnesium by the phenomenon of zinc substitution treatment. Put it.

II. Experimental Method

1. Problems with decorative plating on magnesium

As magnesium alloy is used as a portable electronic component, it is applied to a cell phone exterior case with a mirror polish for decorative purpose. However, many studies have been conducted because the plating process of magnesium phase was not established, but there were many difficulties in commercialization. In particular, in 2002, the world's first commercialization of magnesium alloy products made of the plating process was made, but it is a surface treatment method that has many problems because the process is not stabilized by the casting process. After that, the magnesium plate (AZ31B) shows more problems than the product due to casting molding due to the mixing of impurities and the effect of oxide during rolling.

To summarize the problems of the plating process on magnesium, pit due to the corrosion of the material shows the most problems. 3 is a result showing the plating problem on the magnesium alloy. By this feature, the problem of wet surface treatment process of magnesium alloy, which is classified as a difficult material for plating, was investigated.

2. Plating phenomenon by zinc substitution treatment

Problems that occur during the plating on the magnesium alloy is caused by the correlation between the underlying plating layer, such as etching, zinc replacement, guri like, as described above. Through the preliminary experiments, the effect of the zinc substitution layer formation on the plating was confirmed through the experiments.

Figure 4 shows the plating defect phenomenon according to the zinc substitution treatment. When the zinc-substituted layer is formed thin, it is confirmed that the corrosion of the front surface of the material occurs due to the penetration of acid. If the zinc-substituted layer is locally unevenly formed, it causes local erosion of Mg material to form MgS. It was confirmed that it will generate. In addition, it was confirmed that the peeling phenomenon due to the lack of bonding strength between the zinc-substituted layer due to the increase in the thickness of the zinc-substituted layer. As a result of the formation of the zinc substitution layer, it was confirmed that plating defects occurred, and at the same time, when a uniform zinc substitution layer was formed, it was confirmed that a high quality plating layer was formed in the gulistake process.

3. Material Condition and Etching Surface Comparison

In degreasing, which is the first process for surface treatment on a magnesium alloy (AZ 31B), degreasing of the organic material was carried out at a solution composition having a pH of 13 or higher using sodium hydroxide as a base to remove organic matter. The degreasing process removes contaminants such as organic matter and does not affect the surface shape of the material. Therefore, the characteristics of the material and the etching process are shown in FIG. 5. Specimens showed different oxidation characteristics according to the surface roughness according to the rolling direction generated in the sheet manufacturing process.

Since the roughness difference was large depending on the rolling grain direction, the roughness was measured by scanning 5 mm using a surface roughness measuring instrument. As a result, Ra = 14.39 µm and Rz = 96.24 µm. In chemical surface treatment, the surface shape after the etching process is shown in FIG.

As shown in FIG. 5, the surface shape after etching was similar to that of the raw material, but the difference in the fine shape of the surface roughness and the surface roughness was apparent. It was found that the surface inclusions and the oxide film existing during the processing process were dissolved in the etching process and the surface electrode potential values were similar due to the surface homogenization. The difference in the characteristics according to the zinc substitution treatment after the surface process is that the zinc substitution layer is formed locally, where the dissolution of magnesium proceeds rapidly, while the precipitation of zinc is relatively high, but the presence of oxide or Mg (OH) ₂ exists. The high electrode potential of the material was determined to be because the zinc substitution reaction is relatively suppressed. Therefore, it was found that the surface roughness after the etching process was lower than the surface roughness of the material state at Ra = 6.8 µm and Rz = 46.3 µm. In addition, the etching process dissolves the surface coating of about 8 ~ 13㎛ in the part of the surface inclusion or natural oxide film, and gives the characteristic of showing the uniform energy level value. This is caused by the uniform precipitation of zinc in the zinc substitution process. It was related to the characteristics of having.

6 and 7 compare the surface impurity state of the raw material state with the surface state after etching as a result of EPMA analysis, it can be seen that the distribution of impurities or oxides becomes uniform by etching. This phenomenon also means that the zinc substitution can be uniformly deposited by uniformizing the surface energy of the impurity or material by the etching process.

4. Formation of Zinc Substitution Layer According to Surface Condition

The driving force of the zinc substitution is a simultaneous reaction in which electrons generated by dissolving magnesium are precipitated by reacting with zinc ions complexed and precipitated by reaction with electrons generated by oxidation of a reducing agent. In the case of heterogeneous metals other than zinc, the substitution reaction proceeds rapidly and exhibits deterioration in adhesion. When the surface state is uneven due to the influence of the etching process, the substitution reaction in the zinc substitution solution composed of a standard solution is shown in comparison with FIG. 8.

According to the characteristics of the etching process, the deposition of zinc ions on the surface of the rolling direction was largely distributed, and as time elapsed, the portions peeled off by the growth of the zinc substitution layer on the scratch portions (pressure connection direction) where overreaction was generated could be confirmed. . In addition, since the zinc-substituted layer was formed thin in the part showing the anode characteristics relatively, nonuniform staining phenomenon could be confirmed by visual observation.

When the non-uniform zinc substitution layer is formed, the substitution reaction between copper and magnesium alloy and the substitution reaction between zinc and magnesium coexist in the copper strike process, and a local galvanic cell is formed, resulting in poor adhesion due to corrosion and substitution reaction. In addition, when the zinc substitution layer was not formed uniformly, it was confirmed that the occurrence of pit of the plating layer was increased by local sedimentation.

5. [F ^- Precipitation of zinc-substituted layers depending on the amount of]

The results of confirming the precipitation characteristics of zinc according to the amount of [F ⁻ ] added in the zinc substitution process are shown in FIG. 9. [F ^-] in the zinc substitution process, the influence of the precipitation of dissolved zinc and magnesium is to occur at the same time according to the activity of magnesium and reaction by forming an MgF ₂ while suppressing the dissolution of the abrupt magnesium zinc solution. In the reaction [F ^-] Properties of could be confirmed that the substance for controlling the precipitation of a substituted passivation of magnesium and zinc. In particular, [F ^-] it can be seen that in this case the magnesium surfaces is to occur to the zinc precipitation abruptly under the influence of pH and concentration exercise a negative effect on the deposition uniformity by a local difference in the reaction no. In addition, [F ^-] amount is too much, if [F ^-] a substitution reaction only due to affects and surface stabilization by MgF ₂ material solubility particular portion where the high energy of the substituted precipitation of zinc present is such as scratch in the I could see the progress. In order to understand these characteristics, the formation of zinc substitution layer was observed while adding the amount of [F ⁻ ] by 1.56 mol.

In the zinc replacement step [F ^-] For imparting the properties of controlling the deposition energy of the role and the zinc complex salt to inhibit the dissolution of the magnesium, but the solubility of the zinc-substitutional solution is influenced in a temperature that is no longer soluble in the 6mol outside Characterized. When the temperature was increased, the solubility increased, but it was confirmed that the adhesion of the plating was deteriorated due to the deposition non-uniformity of the zinc-substituted particles and the thickness of the substitution layer. Thus zinc substitution treatment solution of [F ^{-] -} control of the magnesium dissolution in the formation of MgF ₂ by the addition of an amount of the addition, by the rapid dissolution of the magnesium zinc complex salt is is non-uniformly deposited, if not added at all in the [F] It can be seen that the precipitation of the zinc complex salt is suppressed and a uniform film is formed. However, [F ^-] amount is increased to be more saturated [F ^-] of this due to the increased effect on the precipitation of a layer dense zinc substitution, but 6mol or more of [F ^-] a dissolution-precipitation of a zinc complex salt does not magnesium When there is It didn't happen. In [F ^-] AFM images of observing a fine structure, when the concentration of 2 ~ 3mol the order can be seen that the most uniform surface layer is formed of zinc substitution. The fine surface shape and structure according to the addition of [F ⁻ ] are shown in FIGS. 10 and 11. As seen [F ^-] in the absence of the fine surface observed as the amount of the formed precipitation of the zinc complex salt to the non-uniformity is the unreacted layer is present. In addition, [F ^-] a characteristic that dissolution of the magnesium surface is stable, even if the amount of the increased quality of course also be the zinc salt precipitation was observed. It can be seen that the occurrence of pit is accelerated by substitution reaction and dissolution reaction of copper and magnesium material of the underlying plating layer.

12 is [F ^-] is shown in Figure using the measured weight after concentration increases the amount of precipitated zinc-substituted and substituted zinc layers the thickness of the zinc-substituted former according to the. As for the thickness measurement of the zinc substitution layer, as shown in FIG. 11, the zinc substitution layer is formed in a porous form, and thus, it is difficult to analyze the cross section, and the degree of dissolution of magnesium during the substitution reaction with magnesium in solution is limited at the surface. The weight increase of the zinc-substituted layer was reduced and precipitated with the substitution reaction. [F ^-] decrease the amount of precipitated zinc-substituted, as the concentration is increased, and it can be seen that the reduction is also thinning the thickness of the zinc substitution layer. 12 also shows that when the concentration of [F ⁻ ] is about 2 to 3 mol, a zinc substitution layer having a thickness of about 2 to 3 μm is formed.

6. Precipitation shape according to zinc substitution temperature

The formation characteristics of the zinc substitution layer with temperature affects the reactivity of the solution with magnesium together with the dense zinc ion substitution reaction. Below pH 11.5, the magnesium ion deposited in the solution dissolves and reacts with zinc. If the driving force of zinc substitution falls, the corrosion reaction proceeds under the influence of the local galvanic cell. As the corrosion reaction proceeds, the oxide is formed by the erosion reaction of magnesium, and as the deposition of zinc is slowed down, the pit phenomenon due to local corrosion increases. In particular, when the temperature is low, since the corrosion rate is faster than the zinc substitution layer formation rate, the presence of the unreacted portion or the progress of corrosion are simultaneously reacted, so that a dense zinc substitution layer is not formed. In addition, when the reactivity of the zinc-substituted layer is too high, the bonding strength between the zinc-substituted layer is lowered, causing adhesion problems. Therefore, the uniform precipitation and precipitation thickness of the zinc replacement layer are the driving force for forming around 2 to 3 μm. In the case of the basic solution concentration characteristic, the treatment is performed for about 4 minutes at 70 ° C. to suppress the compaction and the progress of the corrosion reaction. It is considered a condition.

14 is a graph showing the amount of zinc substitution precipitation with increasing zinc substitution treatment temperature. As the zinc substitution treatment temperature is increased to 60 ~ 80 ℃ it can be seen that the amount of zinc substitution precipitation also increases. However, it can be seen that the zinc-substituted precipitates showed similar values at 70 ° C and 80 ° C. Even in the surface shape of FIG. 13, it could be seen that a uniform zinc substitution layer was formed at both 70 ° C and 80 ° C. Therefore, it is judged that the temperature condition of zinc substitution treatment is about 70 degreeC.

7. Surface comparison of zinc substitution layer over time

The influence of precipitation and thickness of the zinc substitution layer in the decorative plating process on magnesium is the pit phenomenon of the plating layer due to the adhesion between the material and the upper plating layer, the substitution reaction in forming the upper plating layer, and the corrosion reaction of the magnesium alloy material. The effect of time in chemical reactions is the ability to control the amount of chemical reactions, which can control the amount of chemical reactions over time. The zinc substitution treatment has a function of maintaining the adhesion of the upper plating layer by formation of a uniform substitution treatment layer of the underlying magnesium alloy material and precipitation of a predetermined thickness or less.

15 and 16 show the shape of the zinc substitution layer over time. As the zinc substitution treatment time increases, the zinc substitution amount formed on the surface increases, and when the treatment is performed for 4 minutes or more, the surface becomes rough as the thickness increases. Increasing the thickness or rough surface of the zinc replacement layer may cause plating failure in the upper plating.

17 shows the precipitation amount of zinc complex salt according to zinc substitution treatment time in EDX mapping mode. Considering the volume effect according to the acceleration voltage of EDX analysis, the amount of metal ions was confirmed at the same time mapping. As a general feature of the chemical reaction, the longer the reaction time, the more the precipitation of the substitution layer increases. If the amount of substitution is too large, the adhesion between the zinc substitution layers is reduced when forming the upper plating layer, which causes peeling problems.

As shown in FIG. 17, the amount of the zinc replacement layer increases with treatment time, but the increase in the amount of zinc replacement layer is not clearly distinguished between 4 and 6 minutes. An increase in the amount of zinc substitution precipitates can be seen in FIG. 18. When the zinc substitution treatment time is 1 to 4 minutes, it increases in proportion to time, but it can be seen that the increase amount between 4 and 10 minutes is decreased. This is because the amount of the zinc substitution layer is deposited to a certain thickness is given a feature that can suppress the corrosion reaction under the influence of uniform precipitation with the magnesium base layer. From these results, the thickness of the zinc substitution layer on magnesium was analyzed by EPMA (FIG. 19). Although there is a difference, the thickness of the zinc substitution layer is generally 2.2 to 2.6㎛ by forming a zinc substitution layer to give the corrosion protection of the material and the adhesion between the substitution layer.

8. Comparison of surface of zinc substitution layer with pH change

The pH of the driving force of the chemical reaction has a great influence on the speed of the reaction. Particularly, in case of magnesium, as shown in FIG. 20, when Mg (OH) ₂ is formed of Mg (OH) ₂ , the surface of the magnesium is maintained at a stable state. do. In the zinc substitution process, it was confirmed that pH plays an important role as a driving force for the substitution of zinc salts. As a result, when the pH of the solution was low under certain conditions, the corrosion reaction of magnesium was promoted, resulting in excessive pit of the plated product. On the contrary, when the pH was relatively high, the reaction between zinc complex salt and magnesium was significantly reduced. This is because the precipitation of zinc complex salts is inhibited because the amount of electrons generated by the stabilization of the solution and the stabilization of the magnesium surface is small, as shown in the results according to the amount of fluoride.

9. Formation of Electrolytic Plating Layer

For the decorative plating on the magnesium alloy (AZ31B) material, the base treatment was performed by degreasing, etching, zinc substitution, and copper cyanide plating, and then a glossy copper plating and nickel plating process was applied. Decorative plating is a process of imparting decorativeness by multi-layer plating after the base plating treatment on various materials. Plating after the base plating is the same processing method as decorative plating of heterogeneous materials. The underlying plating layer is formed by using a basic solution composition of copper cyanide, which is the same as the general electrolytic plating. Underground plating on the magnesium alloy affects the electrochemical reaction between the copper cyanide plating solution and the magnesium material and zinc replacement layer. Therefore, in the copper strike plating process, the pH of the solution was maintained at 12.5 or more and the current density was plated using DC current for 10 minutes at a current density of 3.5ASD. The decorative plating property of magnesium phase is about 60% of defects caused by pit and void, and chemical stability such as corrosion resistance is a problem. However, it is difficult to check the appearance quality by the copper cyanide plated strike, so the appearance of the product was confirmed by applying copper sulfate plating and bright nickel plating. These results were summarized by preparing the specimen under the same conditions, and another specimen was found to be able to implement a beautiful glossy appearance by confirming the appearance of nickel plating proceeded to the glossy plating. It is difficult to commercialize the plating on the AZ 31B known to date, but in this embodiment, a high quality mirror polished plating layer was obtained by a pretreatment process on a magnesium alloy material, and the appearance of the plating layer without the pit was obtained. Figure 21 shows the image of the material surface by process in the plating on the magnesium plate using the zinc substitution method.

While the present invention has been particularly shown and described with reference to specific embodiments thereof, those skilled in the art will appreciate that such specific embodiments are merely preferred embodiments and that the scope of the present invention is not limited thereby. something to do. It is therefore intended that the scope of the invention be defined by the claims appended hereto and their equivalents.

Claims (8)

In the plating method of a magnesium alloy sheet including a degreasing process, an etching process, a zinc substitution process, a copper cyanide plating process, an acidic copper plating process, and a nickel plating process, the etching process includes triethanolamine, ethylenediaminetetraacetic acid (EDTA) and Plating method of magnesium alloy sheet material characterized by using an alkali etching solution containing sodium hydroxide. The method of claim 1, wherein the alkaline etching solution comprises 65 to 75% by weight of triethanolamine, 15 to 25% by weight of ethylenediaminetetraacetic acid (EDTA), and 5 to 15% by weight of sodium hydroxide. . The method of claim 1 or 2, wherein the etching step of the magnesium alloy plate material characterized in that the deposition process using the alkaline etching solution for 3 to 5 minutes at a pH range of 10.5 to 11.5, 65 to 70 ℃ Plating method. The process of claim 1 or 2, wherein the zinc substitution process comprises zinc comprising 15 to 18 weight percent ZnSO ₄ , 75 to 80 weight percent Na ₃ PO ₄ , 3 to 10 weight percent Na ₂ CO ₃ and 0.5 to 5 weight percent NaF. Plating method of magnesium alloy sheet material characterized by using a substitution bath. The method of claim 1 or 2, wherein the zinc substitution process is a plating method of magnesium alloy plate, characterized in that the carried out at a fluoride ion concentration of 2.5 to 3.5 mol, a pH range of 10.5 to 11.5 and a temperature of 65 to 70 ℃. The plating method for a magnesium alloy sheet material according to claim 1 or 2, wherein the zinc precipitation layer formed by the zinc substitution process has a thickness of 2 to 3 µm. The method of claim 1 or 2, wherein the magnesium alloy is any one selected from AZ 31, AZ 61 or AZ 91 plating method of the magnesium alloy sheet material. Magnesium alloy sheet material manufactured according to the method of any one of claims 1 to 5.

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