JPH052745B2 - - Google Patents

Description

[Detailed description of the invention]

[Field of Application of the Invention] The present invention relates to a multilayer electrolytic zinc alloy coated steel sheet and a method for manufacturing the same, and more particularly to a multilayer electrolytic zinc alloy coated steel sheet with excellent corrosion resistance and smoothness and a method for manufacturing the same. [Background of the Invention] Electrogalvanized steel sheets are made from cold-rolled continuous steel sheets, and because the plating temperature is low, it is easy to obtain a good shape without impairing the workability of the material. Therefore, electrogalvanized steel sheets have
Widely used in applications requiring flatness.
For example, it can be used in a wide range of fields, including building materials, automobiles, vehicles, home appliances, and iron furniture. Conventionally, such electrogalvanized steel sheets have been produced using binary or ternary alloys containing zinc as the main component and elements such as cobalt, molybdenum, tungsten, iron, nickel, tin, and lead added to improve the corrosion resistance of zinc. It is widely known that a layer is formed on the surface (Special Publication No. 47-16522,
Special Publication No. 49-19979). In addition to the cathodic protection properties of zinc, the Zn alloy plating layer formed on the surface of the steel sheet contains corrosion-resistant metal elements such as Co, W, and Fe, which is effective in further improving the corrosion resistance of the steel sheet. be. However, by electroplating on the surface of steel plate,
When forming a Zn alloy plating layer, hydrogen gas tends to be generated. This hydrogen gas causes pits to form within the plating layer, and as a result, conventional Zn alloy plated steel sheets have had the problem that smoothness, which is the greatest advantage of electroplating, is severely impaired. [Object of the Invention] An object of the present invention is to provide a multi-layer electrolytic zinc alloy plated steel sheet with high corrosion resistance and smoothness by forming a multi-layer alloy on the plating layer covering the steel sheet, and a method for manufacturing the same. It is in. [Summary of the invention] The first invention is to form a first plating layer made of zinc on a steel plate, and then to form a second plating layer containing zinc as a main component and molybdenum and nickel. More specifically, the thickness of the first plating layer is 0.01 to 0.1 μm, and on the first plating layer, Mo: more than 2% and less than 50% by weight, It is characterized by forming a second plating layer consisting of 0.1 to 12% Ni and the balance Zn. Furthermore, the second invention maintains the surface of the steel plate normally, immerses the steel plate in a first plating bath consisting of an acidic electrogalvanizing bath, and applies the electroplating method to form the first plating layer. Then, a water-soluble compound of molybdenum was added in an amount of 0.006 to 12 g/as molybdenum and a water-soluble compound of nickel as nickel was added in an amount of 0.3 to 44 g/in terms of nickel to an acidic electrogalvanizing bath. A manufacturing method characterized by forming a second alloy plating layer made of zinc-molybdenum-nickel on the first plating layer in a second plating bath whose pH is maintained in the range of 2.0 to 6.0. It is. The thus obtained multilayer electrogalvanized steel sheet has excellent corrosion resistance and smoothness without forming small pits in the plating layer. The present invention will be explained in detail below. The multi-layer electrolytic zinc alloy coated steel sheet according to the present invention includes:
A first plating layer made of zinc and a second plating layer mainly composed of zinc and containing molybdenum and nickel are formed on the surface of a steel sheet, and the thickness of the first plating layer The thickness is 0.01 to 0.1 μm, and the composition of the second plating layer is Mo:
More than 2% and less than 50% Ni, 0.1 to 12% Ni, and the balance Zn. The reason why the thickness of the first plating layer is limited in this way is that if the thickness is 0.01 μm or less, it is difficult to deposit zinc uniformly on the surface of the steel sheet. It is not possible to completely prevent blowholes from occurring when forming layers. In addition, if the thickness is 0.1μm or more,
The following inconveniences occur. In other words, if damage such as scratches occurs on the first plating layer and the second plating layer, the electric potential will change between the steel plate and the second plating layer (Zn-Mo-
Ni), the first plating layer (Zn) increases in order,
Since the first plating layer has the most base potential, there is a problem in that zinc is easily eluted (corroded). While the zinc in the first plating layer is eluting, the steel sheet and the second plating layer are electrolytically protected, but conversely, the first plating layer is eluting and the steel sheet and the second plating layer are leaching out. A gap is formed in between. Therefore, if the thickness of the first plating layer is 0.1 μm or more, the gaps are so large that the corrosion products of zinc cannot completely fill the gaps, resulting in crevice corrosion. Therefore, when the thickness of the first plating layer is in the range of 0.01 to 0.1 μm, the above-mentioned disadvantages do not occur. Note that in cases such as high-speed plating, it is practically preferable that the film be as thin as possible within the above-mentioned thickness range. Next, the second plating layer will be explained. The second plating layer is mainly composed of zinc to which are added corrosion-resistant metal elements such as molybdenum and nickel, and strongly protects the substrate in a corrosive environment. The amount of molybdenum added to the second plating layer is 2
% to 50% by weight is preferred. Molybdenum is an element that improves the corrosion resistance of the zinc layer, and if it exceeds 2% by weight, a good corrosion resistance effect can be obtained.
Furthermore, if the amount of molybdenum in the plating layer exceeds 50% by weight, the precipitated metal will become particulate and the density of the plating layer will be poor, resulting in a rough plating layer. limited to a range. Next, nickel in the plating layer forms an intermetallic compound with zinc, which has the effect of reducing the corrosion rate of the plating layer and increasing its corrosion resistance. If the nickel layer in the plating layer is less than 0.1% by weight, sufficient corrosion resistance will not be exhibited even if molybdenum is present in the plating layer. Also, the amount of nickel in the plating layer is
If it exceeds 12% by weight, the corrosion resistance of the plated layer tends to deteriorate, so the amount of nickel was limited to a range of 0.1 to 12% by weight. The multilayer electrolytic zinc alloy plated steel sheet of the present invention constructed as described above has excellent corrosion resistance and smoothness. Next, a method for manufacturing a multilayer electrolytic zinc alloy coated steel sheet as described above will be described. In this manufacturing method, the electrogalvanizing bath for forming the first plating layer and the second plating layer may be a known acidic electrogalvanizing bath. For example, chloride baths, sulfamate baths, etc. can be used. Here, the role of the first plating layer is to serve as a base plating for the second plating layer, which has extremely excellent corrosion resistance. When this second plating layer with excellent corrosion resistance, for example, an alloy plating layer, is formed directly on the surface of a steel sheet, nickel particles become nuclei and hydrogen gas is generated when the alloy plating layer is deposited. The generation of hydrogen gas prevents contact between the plating solution and the steel plate and cuts off the current, thereby inhibiting the deposition of the plating metal. As a result, pits called blowholes are formed in the plating layer, as shown in FIG. 1A, which impairs corrosion resistance and smoothness. In the present invention, the first plating layer previously formed on the surface of the steel sheet before forming the Zn alloy plating layer effectively acts to prevent the occurrence of blowholes, as shown in FIG. 1B. When forming a second zinc alloy plating layer on top of zinc, the difference in potential between the steel sheet and zinc prevents the generation of hydrogen nuclei on the galvanized layer due to the precipitation of nickel, effectively preventing blowholes. can do. Next, the electroplating conditions for forming the second zinc alloy plating layer will be described. As the source of molybdate ions added to the electrogalvanized layer, soluble molybdates such as ammonium molybdate, sodium molybdate, potassium molybdate, and lithium molybdate are preferred. Note that since molybdate ions form insoluble calcium molybdate with calcium ions and precipitate, it is necessary to remove the calcium ions in the plating bath in a polar manner. Molybdate ions are the source for forming molybdenum compounds in the second plated film, and are X-ray-amorphous in zinc-nickel in the presence of chloride and sulfate ions in the plating bath. It eutectoids as a hydrated oxide of high quality molybdenum, passivates the plating film, and greatly improves the corrosion resistance of the plating film itself. If the concentration of molybdate ions in the plating solution is 0.01 g/or less, the amount of molybdenum precipitated in the plating film is small, and therefore cannot contribute to improving corrosion resistance. On the other hand, if the molybdate ion concentration exceeds 20 g/g, hydrogen gas will be generated in large quantities during plating and the current efficiency will decrease, which is not preferable. On the other hand, nickel in the plating film forms an intermetallic compound with zinc and has the effect of reducing the corrosion rate of the plating film. Therefore, in the electrolytic galvanizing bath, a water-soluble compound of nickel is used in the electrolytic galvanizing bath.
It is necessary to add within the range of 44g/. In this way, the amount of nickel in the film can be in the range of 0.1 to 12% by weight. Next, it is necessary to adjust the pH of the Zn plating bath. If the pH of the plating bath is 2 or less, the concentration of hydrogen ions in the plating solution increases, hydrogen gas is likely to be generated during plating, and the current efficiency decreases. If hydrogen gas is generated during plating, blowholes will occur in the plated film and a smooth plated film cannot be obtained. In addition, if the pH of the plating solution exceeds 6, the nickel in the plating film tends to decrease, which is undesirable, and the precipitated particles of the plating metal become coarse, resulting in an extremely poor appearance and smoothness, which is undesirable. . This pH adjustment is preferably carried out using acids such as hydrochloric acid and sulfuric acid, aqueous ammonia, and alkali metal hydroxides other than calcium salts. Next, other electroplating conditions will be described.
In the method of the present invention, the bath temperature during plating ranges from room temperature to 90°C.
Practically speaking, the temperature is preferably in the range of 40 to 70°C. In this temperature range, the molybdenum content is advantageously high. Current densities for both the anode and cathode can be set under the conditions used in conventional galvanizing or zinc:nickel alloy plating.
Furthermore, it is necessary to stir the plating solution during plating. As a stirring method, there is a method in which the plating liquid is circulated and flowed by air blowing or a pump. For the anode during plating, a combination of a zinc plate and a nickel plate or an insoluble electrode such as a platinum-coated titanium plate can be used. After forming the first plating layer on the steel plate surface under the above conditions, a second Zn alloy plating layer is formed to create a multilayer electrolytic zinc alloy with high corrosion resistance and excellent smoothness. A plated steel plate can be obtained. Examples of the present invention will be described below. Example 1 An ordinary cold-rolled steel sheet was degreased with alkali, then electrolytically degreased in an alkaline aqueous solution, washed with water, pickled in a 5% by weight aqueous hydrochloric acid solution, washed with water, and dried to form a covered body. This steel plate was subjected to acid zinc plating in a first plating bath having the following composition. First plating bath Plating is performed at a current density of 1A/while performing air agitation.
Plating was carried out at different times at dm ² and bath temperature of 30°C. Next, nickel ions and molybdate ions were added at various concentrations to the next basic bath A, and then the pH
Electroplating was carried out on the first plating film in a second plating bath in which the following conditions were adjusted. Basic bath A ZnCl ₂ ...100g/ NH ₄ Cl...200g/ NH ₄ OH...For PH adjustment HCl...For PH adjustment Nickel ion is NiCl _{2.6H 2} O and molybdate ion is (NH ₄ ) ₆ Mo The required amount of each was added in the _{form of 7O24.4H2O} . Plating current density 10A/d
m ² and a bath temperature of 40°C for 2 minutes. After plating, the appearance of the plated steel sheets was observed with the naked eye and with a microscope, and a salt spray test was conducted to examine the occurrence of white rust over a 24-hour period and the time required to produce red rust. Table 1 shows the test results. In Table 1, in the appearance column, ○ indicates good, △ indicates fair, and × indicates poor.
In addition, in the column of corrosion resistance, in the white rust occurrence section, ◯ indicates no white rust, △ indicates some white rust has occurred, and × indicates full white rust.

【table】

[Table] In Table 1, sample numbers 1 to 6 are smooth highly corrosion-resistant zinc alloy coated steel sheets according to the present invention. Sample number 7
-14 are comparative examples. As is clear from Table 1, the zinc alloy coated steel sheet of the present invention has a smooth and good appearance with no pits in the plating film, and exhibits extremely excellent corrosion resistance.
It has corrosion resistance more than 10 times that of the pure galvanized steel sheet of Comparative Example 9, which does not contain Ni and Mo. Also, comparative example
It has more than three times the corrosion resistance of No. 10 zinc and nickel alloy plated steel sheets. Furthermore, in Comparative Examples 11 to 14, although the corrosion resistance was good, pits were formed in the plating film. On the other hand, the first in Nos. 1 to 6 of the present invention
By forming the second zinc alloy plating film on the plating film, a smooth plating film without pits can be obtained. Example 2 The same cold-rolled steel sheet as in Example 1 was used, and after the same pretreatment, the following basic bath B was used, and nickel ions and molybdate ions were added at various concentrations to adjust the pH. Zinc alloy plating was carried out using the second plating bath. Note that the first plating was the same as in Example 1. Basic bath B (ZnSO ₄・7H ₂ O...240g/ NH ₄ Cl...15g/ Al ₂ (SO ₄ ) ₃ ...30g/ NH ₄ OH...for PH adjustment H ₂ SO ₄ ...for PH adjustment Nickel Ions were added in the required amounts in the form of NiSO ₄ .6H ₂ O and molybdate ions in the form of (NH ₄ ) ₆ Mo ₇ O ₂₄ .4H ₂ O.Plating was carried out at a current density of 10 A/dm ²
The bath temperature was set to 30°C and the bathing was carried out for 2 minutes. The evaluation method after plating is the same as in Example 1.
The results are shown in Table 2.

【table】

[Table] Sample numbers 16 to 18 in Table 2 are according to the present invention, and samples No. 19 to 25 are comparative examples. As is clear from Table 2, the zinc alloy coated steel sheet according to the present invention has no pits in the plating film, and is smooth and has a good appearance. Furthermore, the plating film itself has good corrosion resistance and is excellent in preventing the formation of white rust and red rust.
On the other hand, in Comparative Examples Nos. 19 to 20, even if the first plated film was present, the second plated film was obtained because the added ion concentration and pH of the second plating bath were not appropriate. The plated metal particles are coarse and have a poor appearance. In addition, the time required for red rust to occur was not as long as expected. No.23～25
Since the first plating film is absent or insufficient, and the pH of the bath is low, pits appear in the second plating film, resulting in poor smoothness and poor appearance.
Corrosion resistance is also insufficient. Note that the meanings of the ○, △, and × marks in Table 2 are the same as in Table 1. Example 3 The same cold-rolled steel sheet as in Example 1 was used as a sample. After forming the first plating film (Zn: 0.08 μm film thickness), using basic bath A, nickel ions and molybdate ions were added thereto, and a second plating film was formed in the same manner as in Example 1.
I gave it a thumbs up. As a comparative example, Zn-Co-Mo alloy plating was performed using the following bath composition and the corrosion resistance was compared. ZnSO ₄・7H ₂ O……240g/ NH ₄ Cl……15g/ CoSO ₄・7H ₂ O……20g/ (NH ₄ ) ₆ Mo ₇ O ₂₄ 4H ₂ O……3g/ Plating conditions are plating Bath PH2.0, bath temperature 35℃,
Plating was carried out for 2 minutes at a current density of 10 A/dm ² . Table 3 shows the corrosion resistance test results by salt spray test.

〔Effect of the invention〕

In the zinc alloy coated steel sheet of the present invention, the plating film has no pits such as blowholes and a smooth plating film is obtained, and the corrosion resistance of the plating film itself is significantly improved, so that the plating film is thinner than before. The film has the effect of providing sufficient corrosion resistance and improving the productivity of galvanized steel sheets.

[Brief explanation of drawings]

FIGS. 1A and 1B are microscopic photographs of the metal structure showing the occurrence of pits in the plated film due to blowholes.

Claims (1)

[Claims] 1. A first plating layer made of zinc is formed on a steel plate, and then molybdenum is added in a weight ratio exceeding 2%.
% or less, a second plating layer consisting of 0.1% or more and 12% or less of nickel, and the balance zinc. 2. The multilayer electrolytic zinc alloy plated steel sheet according to claim 1, wherein the first plated layer has a thickness of 0.01 to 0.1 μm. 3. Holding the surface of the steel plate normally, immersing the steel plate in a first plating bath consisting of an acidic electrogalvanizing bath to form a first plating layer by electroplating; Add a water-soluble compound of molybdenum to the plating solution as molybdenum, 0.006 to 12 g/
0.3 to nickel water-soluble compound as nickel
Add in a range of 44g/ to adjust the pH of the matting solution.
Multilayer electrolytic zinc characterized by forming a second alloy plating layer made of zinc-molybdenum-nickel on the first plating layer in a second plating solution maintained at a temperature in the range of 2.0 to 6.0. A method for producing alloy plated steel sheets.