JPS6134194A - Galvanized and zinc alloy electroplated steel sheet and its manufacture - Google Patents

Abstract

PURPOSE:To obtain a galvanized and Zn alloy electroplated steel sheet having high corrosion resistance and smoothness by galvanizing the surface of a steel sheet in a galvanizing bath to form a Zn layer, adding Mo and Ni to the bath, and forming a Zn-Mo-Ni alloy layer on the Zn layer by plating in the resulting bath. CONSTITUTION:The surface of a steel sheet is cleaned and immersed in a conventional acidic galvanizing bath to form the 1st Zn layer of about 0.01-0.1mum thickness, To the galvanizing bath are added a water soluble Mo compound by 6X10<-3>-12g/l (expressed in terms of Mo) and a water soluble Ni compound by 0.3-44g/l (expressed in terms of Ni), and the bath is adjusted to 2.0-6.0pH. The 2nd Zn-Mo-Ni alloy layer is formed on the 1st Zn layer by plating in the resulting 2nd plating bath. The preferred composition of the 2nd layer is composed of 0.05-50wt% Mo, 0.1-12wt% Ni and the balance Zn.

Description

[Detailed Description of the Invention] [Field of Application of the Invention] The present invention relates to a multilayer electrolytic zinc alloy plated steel sheet and a method for manufacturing the same, and in particular, a multilayer electrolytic zinc alloy plated steel sheet with excellent corrosion resistance and smoothness, and the production thereof. Regarding the method.

[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 good shapes without impairing the workability of the material. Therefore, electrogalvanized steel sheets are widely used in deep drawn sections and in applications where flatness is required. For example, building materials, automobiles, vehicles,
It can be used in a wide range of fields, including home appliances and iron furniture.

Conventionally, such electrogalvanized steel sheets have been made of binary or ternary alloys containing zinc as the main component and adding elements such as cobalt, molybdenum, tungsten, iron, nickel, tin, and lead to improve the corrosion resistance of zinc. It is widely known that a layer is formed on the surface.
No. 7-16522, Mesae 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, Fe, etc. are added, which is effective in further improving the corrosion resistance of the steel sheet.

However, when forming a Zn alloy plating layer on the surface of a steel plate by electroplating, hydrogen gas tends to be generated. This hydrogen gas causes the generation of small pits within the plating layer, and as a result, conventional Zn alloy plated steel sheets have had the problem that the smoothness, which is the greatest advantage of electroplating, is severely impaired.

[Purpose of the invention]

An object of the present invention is to provide a multi-layer electrolytic zinc alloy-plated steel sheet that has 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.

[Summary of the invention]

The first aspect of the present invention is characterized in that a first plating layer made of zinc is formed on a steel plate, and then a second plating layer containing zinc as a main component and molybdenum and nickel is formed. More specifically, the thickness of the first plating layer is set to 0.
MO:0.05-50%, Ni:O81-12% by weight on the first plating layer.
The second plated layer is formed of Zn and the remainder 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 first plating layer by electroplating. and then add an aqueous compound of molybdenum as molybdenum to an acid electrogalvanizing bath at 6X10-3 to 12 g/l
, 0.3 to 0.3 when the water-soluble compound of nickel is nickel
Zinc-molybdenum was added in a range of 44 g/l on the first glitter layer in a second plating bath in which the PR of the plating bath was kept in the range of 2.0 to 6.0. This manufacturing method is characterized by forming a second alloy plating layer made of Tsukel. The thus obtained multilayer electrolytic zinc alloy plated 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 double electrolytic zinc alloy plated steel sheet according to the present invention has a first plating layer made of zinc and a second plating layer mainly composed of zinc and containing molybdenum and nickel formed on the surface of the steel sheet. The thickness of the first plating layer is 0.
．． 01 to 0.1 μm, and the composition of the second plating layer is MO20.05 to 50 inches, Ni: 0.1 to
12% and the remainder zn. In this way the first
The reason for limiting the thickness of the plating layer is that the thickness is 0.01μ.
m or less, it is difficult to deposit zinc uniformly on the surface of the steel sheet, and therefore it is not possible to completely prevent blowholes from occurring when forming the second plating layer. Further, if the thickness is 0.1 μm or more, the following disadvantages occur. In other words, if damage such as scratches occurs on the first plating layer and the second plating layer, the potential will change between the steel plate, the second plating layer (Zn-MO-Ni) and the first plating layer (Zn)
There is a problem in that the potential of the first plating layer becomes the most base, and the zinc becomes more likely to be 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, when the thickness of the first plating is 0.1 μm or more, the gap is so large that the zinc corrosion product cannot completely fill it, resulting in crevice corrosion.

Therefore, the thickness of the first plating layer is 0.01 to 0.1 μm.
Within this range, the above-mentioned inconvenience does not occur.

In addition, in the case of high-speed plating, it is practically preferable that the film be as thin as possible within the above-mentioned range of thickness.

Next, the second plating layer will be explained.

The second plating layer is made of zinc as a main component, to which corrosion-resistant metal elements such as molybdenum and nickel are added, and is 6D.
It strongly protects the substrate in corrosive environments.

The amount of molybdenum added to the second plating layer is 0.05~
A range of 50% by weight is preferred. Molybdenum is an element that improves the corrosion resistance of the zinc layer, and if it is less than 0.05% by weight, its effect is small and it hardly contributes to the corrosion resistance. In addition, when the amount of molybdenum in the plating layer is 50% by weight or more,
The amount of molybdenum should be 0.05-5.
It was limited to the range of 0.

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 improving corrosion resistance. The nickel layer in this plating layer is 0.
If the weight is less than 1%, even if molybdenum is present in the plating layer, sufficient corrosion resistance will not be exhibited. In addition, if the amount of nickel in the plating layer is 12% by weight or more, the corrosion resistance of the plating layer tends to decrease, so the amount of nickel should be 0.1 to 12% by weight.
It was limited to a range of 12 inches. 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 plated steel sheet as described above will be described. In this manufacturing method, the electrogalvanizing bath used to form the first plating layer and the second plating layer may be a known acidic electrogalvanizing bath. For example, a chloride bath, a sul-7 amino acid salt bath, 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 the steel sheet, hydrogen gas with nickel particles as the nucleus is generated when the alloy plating layer is deposited. The generation of hydrogen gas prevents contact between the plating solution and the steel sheet and cuts off the current, thereby inhibiting the deposition of the plating metal. As a result, the plating layer is as shown in Figure 1 (
As shown in 4), bits called blowholes occur, causing loss of 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 blowholes from occurring, as shown in FIG. 1 (6). When forming a second zinc alloy plating layer on zinc,
The difference in potential between the steel sheet and zinc prevents formation of hydrogen generation nuclei due to nickel precipitation on the galvanized layer, so blowholes can be effectively prevented. Next, the electroplating conditions for forming the second zinc alloy plating layer will be described.

As a source of molybdate ions added to the electrogalvanized layer, soluble molybdates such as ammonium molybdate, sodium molybdate, potassium molybdate, and lithium molybdate are preferred. It should be noted here that molybdate ions form insoluble calcium molybdate with Cal-7-wam ions and precipitate, so it is necessary to remove Cal-7-wam ions in the plating bath to an extreme extent.

Molybdate ions are the source that forms molybdenum compounds in the second plated film, and in the presence of chloride and sulfate ions in the plating bath, the molybdate ions become It eutectoids as a water-use oxide of molybdenum, passivates the plating film, and greatly improves the corrosion resistance of the plating film itself. If the molybdate ion concentration in the plating bath is less than o, oxg/, the amount of molybdenum precipitated in the plating film is small, and therefore cannot contribute to improving corrosion resistance. On the other hand, the molybdate ion concentration is 20
If the ratio exceeds 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, it is necessary to add a water-soluble nickel compound to the electrogalvanizing bath in a range of 0.3 to 44 g/l of nickel. 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 more, the concentration of hydrogen ions in the plating solution increases, and the fluidity of the stain, which is less likely to generate hydrogen gas during plating, decreases.

If hydrogen gas is generated during plating, blowholes will occur in the plated film and it will be difficult to obtain a smooth plated film. Furthermore, if the pH of the plating solution is 6 or more, 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 is between room temperature and 90C, and is practically preferably in the range of 40 to 70C.

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 for normal zinc plating 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 solution 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, the second 7. By forming the n-alloy plating layer, a multilayer electrolytic zinc alloy-plated steel sheet with high corrosion resistance and excellent smoothness can be obtained.

The present invention will be explained in detail below.

[Example 1] A normal cold-rolled steel sheet was degreased with alkali, then electrolytically degreased in an aqueous alkaline solution, washed with water, pickled in a 5% by weight hydrochloric acid aqueous solution, washed with water, and dried to be plated. . This steel plate was subjected to acid zinc plating in a first plating bath having the following composition.

The first plating bath plating is carried out at a current density of IA/dm while air is stirred.
, plating was carried out at a bath temperature of 30'C for different times.

Next, nickel ions and molybdate ions were added to the next basic bath A at various ilA it, and then PHf,
Adjusted 4! , plating was carried out on the first plating film in the plating bath of step 2.

Basic bath A Nickel ions are N i CLxΦ6 H20 and molybdate ions are (NH4)6 Mo, 024 ・4
The required amount of each was added in OQ form. Plating was carried out at a current density of 10 A/dml 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 spent in the pit where red rust occurred.

Table 1 shows the test results. In Table 1, in the appearance column, a mark of 0 indicates good, a mark Δ indicates normal, and a mark X indicates poor.

In addition, in the column of corrosion resistance, in the white rust occurrence section, ◯ indicates no white rust, Δ indicates partial white rust, and X indicates full white rust.

In Table 1, sample numbers 1 to 7 are smooth highly corrosion-resistant zinc alloy plated steel sheets according to the present invention. Sample numbers 8 to 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 bits in the plating film, and exhibits extremely excellent corrosion resistance.
It has corrosion resistance 10 times or more than the pure galvanized steel sheet of Comparative Example 9 in which N1 and MO are not present in the plating film. Moreover, it has a corrosion resistance three times or more that of the zinc-nickel alloy coated steel plate of Comparative Example 10. Furthermore, in Comparative Examples 11 to 14, although the corrosion resistance was good, bits were generated in the plating film. On the other hand, by forming the second zinc alloy plating film on the first plating film in A1 to A7 of the present invention, a smooth plating film without bits can be obtained.

[Example 2] Using the same cold-rolled steel sheet as in Example 1, after undergoing the same pretreatment, using the following basic bath B, nickel ions and molybdate ions were added at various concentrations, Zinc alloy plating was performed in a second plating bath whose pH was adjusted. In addition,
The first plating was the same as in Example 1.

Basic Bath B Nickel ions and molybdate ions were added in required amounts in the form of (NH4)aMo4z<-4H20. Plating current density xoA/dm”
The bath temperature was set to 3 (l) 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 the second page.

Sample numbers 16 to 18 in Table 2 are related to the present invention!
11 flats 19 to 25 are comparative examples. As is clear from Table 2, the zinc alloy plated 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 occurrence of white rust and red rust. On the other hand, the comparative example Haruka 19-20
Even if the first plating film exists, the second plating film is 14+ because the added ion # degree and pH of the second plating bath are not appropriate.
The plated gold 11 particles in the plated film are coarse and have a nice appearance.

Also, Satoru didn't spend as much time in the red rust building as expected.

A23 to 25 have no or insufficient first plating film, and the pH of the bath is still low, so pits appear in the second plating film, resulting in poor smoothness and poor appearance and corrosion resistance. Not enough.

Note that the meanings of the O, Δ, and X 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 in the same manner as in Example 1 to form the second plating syllable. did.

As a comparative example, Zn-Co-M with the following bath composition.

Alloy plating was applied and the corrosion resistance was compared.

Plating conditions were plating bath pH 2,0, bath temperature 35C1.
'Rfi density 10A/dm' for 2 minutes a6) Kit, *.

Table 3 shows the corrosion resistance test results by salt spray test.

Table 3 As is clear from Table 3, the thrust lead alloy plated steel sheet according to the present invention has superior corrosion resistance up to the onset of the red zone compared to the comparative example.

〔Effect of the invention〕

In the zinc alloy coated steel sheet of the present invention, a smooth plated film is obtained without blowholes etc., and the corrosion resistance of the plating tlJ itself is significantly improved, so a thinner plating film is sufficient than before. Corrosion resistance is obtained,
This has the effect of improving the productivity of plated steel sheets.

[Brief explanation of drawings]

Figure 1 (A) [F]) shows the occurrence of pits in the plating film due to blowholes. This is a mirror photograph.

Claims (1)

[Claims] 1. Forming a first plating layer made of zinc on a steel plate,
A multilayer electrolytic zinc alloy plated steel sheet comprising a second plating layer containing zinc as a main component and molybdenum and nickel. 2. A multilayer 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. In claim 1, the second plating layer has a weight ratio of Mo: 0.05 to 50% and Ni: 0.1.
A multilayer electrolytic zinc alloy plated steel sheet comprising ~12% Zn and the balance Zn. 4. Keeping the surface of the steel plate clean, immerse the steel plate in a first plating bath consisting of an acidic electrogalvanizing bath to form a first plating layer by electroplating; A water-soluble compound of molybdenum was added to the galvanizing solution in an amount of 6 x 10^-^3 to 12 g/l as molybdenum, and a water-soluble compound of nickel was added in the range of 0.3 to 44 g/l as nickel, and the plating solution was added. A second alloy plating layer made of zinc-molybdenum-nickel is formed on the first plating layer in a second plating solution whose pH is maintained in the range of 2.0 to 6.0. A method for manufacturing a multi-layer electrolytic zinc alloy plated steel sheet.