JPH0826474B2 - High productivity zinc-manganese alloy electroplating method - Google Patents

Description

TECHNICAL FIELD The present invention relates to an electroplating method for forming a zinc-manganese alloy plating film having excellent corrosion resistance on the surface of an object to be plated with excellent productivity. is there.

[Conventional technology]

Galvanizing has been widely used as a corrosion protection method for steel sheets, and galvanized steel sheets are used in a wide range of applications such as building materials, home appliances and automobiles. By the way, in recent years, the demand for improving the durability of products has been increasing, and there is a strong demand for further improving the corrosion resistance of galvanized steel sheets.

As a result of repeated studies to meet the above requirements, the present inventors have developed a zinc-manganese alloy electroplated steel sheet having excellent corrosion resistance as compared with a conventional zinc electroplated steel sheet and applied for a patent, The technology is already JP-A-58-3718
No. 8 publication. However, the cathode electrolysis efficiency during the production of zinc-manganese alloy electroplated steel sheet is
It is considerably lower than the cathode electrolysis efficiency during the production of zinc-based alloy electroplating such as zinc-iron alloy plating and zinc-nickel alloy plating, and is only about 25-50%. The low cathode electrolysis efficiency is not only inferior to the productivity of the electroplated steel sheet, but also undesirable from the viewpoint of energy saving. Furthermore, the maximum current density for obtaining a sound zinc-manganese alloy plating film is as low as about half that of zinc-iron alloy plating and zinc-nickel alloy plating when plating conditions such as stirring are the same. . The low maximum current density means that the cathode electrolysis efficiency is low and the productivity is low, which is not preferable in industrial production.

As described above, in the related art, in the production of a zinc-manganese alloy-plated steel sheet, there is a disadvantage that the productivity is poor because both the cathode electrolytic efficiency and the maximum current density are low. The reason for the low efficiency of cathodic electrolysis is that citric acid is added as a complexing agent to precipitate zinc and manganese at the same time, and the precipitation of zinc shifts to a base potential, which promotes the hydrogen evolution reaction, which is a competitive reaction. Because it was done. Further, it is considered that the reason why the limiting current density is low is that the metal ion forms a complex with citric acid, so that the ion diffusion rate is low.

In order to solve these problems, the following techniques have been conventionally disclosed.

By adding a selenium compound or tellurium compound to a zinc-manganese alloy plating solution containing zinc sulfate and manganese sulfate as main components and citric acid as a complexing agent, the cathode electrolysis efficiency during zinc-manganese alloy plating is improved. What is improved (JP-A-60-52590, JP-A-6
0-52591).

A thiosulfate or a salt of an organic acid having a thiol group is added in place of a selenium or tellurium compound to improve the cathode electrolysis efficiency during zinc-manganese alloy plating (JP-A-62-44593).

In the above-mentioned conventional technology, the cathode electrolysis efficiency is improved, but the maximum current density at which sound plating can be obtained is not increased and is practically used as an anticorrosion steel sheet for automobiles, Fe-Zn or Ni-Zn alloy plating. The productivity is still low compared to the above, and a problem still remains in this respect.

Further, as in the present invention, a report on a plating bath using a tetrafluforoboric acid-based plating bath is described in "Battelle Memor
ial Institute ”Technical Report 5692 (November
1952). According to this report, various studies have been made on the plating composition, plating conditions, and additives, but even the one with the highest manganese content is 35.4 wt.%, And this coating is brown and partially powdered. It was reported that it was impractical.
Furthermore, the cathode electrolysis efficiency at this time is very low at 7.8%, and the limiting current density has already reached at 10 A / dm ² ,
It was completely unexpected that the tetrafluoroboric acid-based plating bath could be a highly efficient plating bath.

[Problems to be Solved by the Invention]

The cathode electrolysis efficiency during zinc-manganese alloy plating is
It is lower than zinc plating, zinc-iron alloy plating, or zinc-nickel alloy plating, and is only about 25 to 50%. Furthermore, the maximum current density at which a sound plating film can be obtained is also low. As described above, the zinc-manganese alloy plating has a drawback of low productivity, and a method of increasing productivity has been earnestly desired for industrial practical use.

As a means for overcoming this drawback, a technique of adding selenic acid and sodium thiosulfate has been filed as described above. However, according to these techniques, although the cathode electrolysis efficiency is improved, the maximum current density is not increased, and therefore the productivity is not yet at a sufficient level.

Therefore, the present invention is made to solve the problem that the zinc-manganese alloy plating has low productivity as described above, and increases the cathode electrolysis efficiency and obtains a sound plating film. An object of the present invention is to provide an electroplating method for forming a zinc-manganese alloy plating film having excellent productivity, which can also increase the maximum current density and can sufficiently enhance the productivity.

[Means for solving the problem]

In order to increase the cathode electrolysis efficiency and the maximum current density at which a sound plating film can be obtained, the inventors of the present invention have made many studies on a method using a new plating bath that has never existed before, that is, a plating bath that does not use citric acid. I went. Examples of plating baths studied here include inorganic acid plating baths such as a sulfuric acid bath, a chloride bath, a sulfuric acid-hydrochloric acid mixed bath, a perchloric acid bath, and a tetrafluoroboric acid bath, a phenolsulfonic acid bath, and a salicylic acid bath. It is an organic acid plating bath such as a bath.

As a result, industrially usable zinc-manganese alloy plating could not be obtained from almost all of these plating baths, but only tetrafluoroborate-based bath produced a sound plating film with excellent productivity. I learned that I can do it.

The present invention has been made based on the above findings, and in a plating bath containing zinc ions and manganese ions, a zinc-manganese alloy plating containing a predetermined amount of manganese on the surface of the object to be plated. In the electroplating method for forming a film, zinc tetrafluoroborate is used as the zinc ion in the plating bath, and manganese tetrafluoroborate is used as the manganese ion in the plating bath, In the above, the total amount of the zinc tetrafluoroborate and the manganese tetrafluoroborate in is limited to 0.25 or more and 2.5 mol / l, and a metal molar ratio of zinc and manganese (Mn / Zn).
Is limited to 5 or more and 50 or less, and one or two kinds of polyethylene glycol and polypropylene glycol are added to the plating bath in a total amount of 0.05 g / l or more and 20 g / l or less, and the plating bath. Is limited to 0.5 or more and 6.0 or less, thus forming a zinc-manganese alloy film having a manganese content of 5 wt.% Or more and 95 wt.% Or less on the surface of the object to be plated. .

Next, a method for forming a zinc-manganese alloy film of the present invention will be described.

The mechanism by which a zinc-manganese alloy plating film with a high current density and high cathode electrolysis efficiency can be obtained over a wide alloy composition by using a tetrafluoroborate-based bath is not clear, but is a conventional plating bath. In a sulfuric acid bath containing citric acid, the deposition of the plating metal occurs from the complex ions of citric acid, zinc and manganese, whereas the tetrafluoroborate bath of the present invention uses such a stable complex ion. Since it is not precipitation, it is considered that the ion discharge reaction and the moving speed are fast, and the limiting current density and the electrolytic efficiency are large. In addition, baths such as a sulfuric acid bath, a chloride bath, a sulfuric acid-hydrochloric acid mixed bath, and a perchloric acid bath that do not contain citric acid preferentially deposit zinc, which is a noble metal, and hardly deposit manganese. Among these, some were precipitated with manganese, but the obtained film was black powder and could not be industrially put to practical use. In contrast, tetrafluoroboric acid bath in tetrafluoroboric acid ion (BF ₄ ^-) presence by zinc precipitation is polarized, therefore the deposition potential of zinc approaches that of manganese, both metals can be deposited at the same time, wide It can be inferred that a plating film can be obtained over the alloy composition.

Next, the reasons for limiting the plating bath composition as described above will be described below.

(1) If the total molar concentration of zinc tetrafluoroborate and manganese tetrafluoroborate in the bath is less than 0.25 mol / l, the limiting current density is drastically reduced, and sound plating cannot be obtained at high current density. The feature of the present invention of high limit current density disappears. On the other hand, even if it exceeds 2.5 mol / l, the effect of improving the limiting current density is almost saturated, and a great merit cannot be obtained in consideration of the chemical solution cost. Therefore, the total molar concentration of zinc tetrafluoroborate and manganese tetrafluoroborate in the bath should be limited to 0.25 or more and 2.5 mol / l or less.

(2) If the metal molar ratio of zinc to manganese (Mn / Zn) is less than 5, the preferential precipitation of zinc is enhanced, the precipitation of manganese is suppressed, and an alloy film having a manganese content of 5 wt.% Or more cannot be obtained. On the other hand, if it exceeds 50, the zinc ion concentration in the plating bath is considerably low, and it becomes very difficult to control the concentration of the plating bath. Therefore, the metal molar ratio of zinc and manganese (Mn / Z
n) should be limited to 5 or more and 50 or less.

(3) Polyethylene glycol and polypropylene glycol have a function of improving corrosion resistance. Polyethylene glycol and polypropylene glycol are
Even if it is not added, the productivity is sufficiently high as compared with the prior art. However, when these chemicals are not added, the plated crystal is not dense and the plated film cannot completely cover the base. Therefore, the high corrosion resistance, which is a characteristic of zinc-manganese alloys, cannot be sufficiently obtained. Therefore, as a result of repeated studies of a method for improving corrosion resistance, it was found that when polyethylene glycol and / or polypropylene glycol is added, dense and fine crystals can be obtained and the corrosion resistance is improved. It is speculated that such an effect due to the addition of polyethylene glycol and / or polypropylene glycol was caused by the fact that these chemicals were adsorbed on the plating surface and increased the deposition overvoltage. Further, the addition of these chemicals has the characteristics that the cathode electrolysis efficiency is slightly improved and the plating appearance is changed from dull to semi-gloss. However, if the total amount of one or two of polyethylene glycol and polypropylene glycol added is less than 0.05 g / l, the desired effect cannot be obtained for the above-mentioned corrosion resistance improving action. On the other hand, if it is added in excess of 20 g / l, there will be no problems in productivity and corrosion resistance, but the plating solution will be prone to foaming rapidly, which will be a problem in continuous operation. Therefore, the total addition amount of one or two of polyethylene glycol and polypropylene glycol should be limited to the range of 5 g / l or more and 20 g / l or less.

(4) When the pH value is lowered, the precipitation of manganese tends to be suppressed. If the pH value is less than 0.5, it is difficult to precipitate manganese, and furthermore, the hydrogen generation reaction is apt to occur, which undesirably lowers the cathode electrolysis efficiency. On the other hand, when the pH value exceeds 6.0, zinc ions and manganese ions form hydroxides and precipitate. Therefore, the pH value is 0.5 or more and 6.0
Should be limited to:

(5) If the manganese content of the zinc-manganese alloy plating film is less than 5 wt.%, There is no difference in corrosion resistance from the zinc plating film. On the other hand, if it exceeds 95 wt.%, The plating film is likely to be oxidized, and the surface thereof tends to turn brown during storage, which is not desirable in appearance. Therefore, the manganese content of the zinc-manganese alloy plating film should be limited to 5 wt.% Or more and 95 wt.% Or less.

(6) Boric acid has an effect of suppressing the generation of free fluorine ions due to hydrolysis of tetrafluoroborate ions. The fluorine ions are undesirable because they form precipitates with the plating metal ions. That is, it is desirable to add boric acid in order to suppress decomposition of tetrafluoroborate ions and prolong the life of the plating solution. On the other hand, however, if the amount of boric acid added exceeds 50 g / l, the cathode electrolysis efficiency will drop sharply.
Therefore, the amount of boric acid added should be limited to 50 g / l or less.

As described above, the present invention greatly enhances productivity as compared with the prior art. Here, considering power costs, it is effective to improve the conductivity of the plating bath for power saving. For that purpose, addition of sodium tetrafluoroborate, ammonium tetrafluoroborate or the like is effective. In addition, a very small amount of fluorine is detected in the film obtained from the tetrafluoroboric acid-based plating bath,
This is not an amount of a problem in handling the coating obtained from the tetrafluoroboric acid-based plating bath, and does not affect various performances at all.

Next, the present invention will be described in more detail with reference to Examples.

[Example 1] After the cold-rolled steel sheet was degreased and pickled, according to the plating bath composition within the scope of the present invention shown in Table 1 and the electrolysis conditions,
The cold-rolled steel sheet was electroplated. To adjust the pH value of the plating bath, tetrafluoroboric acid was added to lower the pH value, while sodium hydroxide was used to raise the pH value. The composition and cathode electrolysis efficiency of the Zn-Mn alloys obtained at this time are shown in Table 3 as Examples Nos. 1 to 3. In addition, as a comparative example, the cold rolled steel sheet was electroplated with a citric acid-sulfuric acid plating bath composition and an electrolysis condition, which are outside the scope of the present invention, as shown in Table 2. The composition and cathode electrolysis efficiency of the Zn-Mn alloys obtained at this time are shown in Table 3 as Comparative Examples Nos. 1 to 3.

As is apparent from Tables 1 to 3, Examples in which electroplating was performed in the tetrafluoroboric acid-based plating bath of the present invention
Nos. 1 to 3 show no plating burn even at a current density of 80 A / dm ² , indicating that the critical current density is extremely high. Furthermore, the cathode electrolysis efficiency is as high as 87 wt.%, And it can be seen that sound plating can be obtained with high efficiency. On the other hand, in the comparative example of the conventional bath, the plating burn was already seen at the current density of 45 A / dm ² , and the cathode electrolysis efficiency at 40 A / dm ² , which is considered to be almost the limiting current, was 40%. It can be seen that both the limiting current density and the cathode electrolysis efficiency are low, and the productivity is significantly inferior to the examples of the present invention.

[Example 2] After degreasing a cold-rolled steel sheet and pickling, the total number of moles of zinc tetrafluoroborate and manganese tetrafluoroborate in the plating bath was 0.25 to with a plating bath composition within the scope of the present invention.
The cold-rolled steel sheet was electroplated in place of 2.5 mol. 25 g / l of boric acid and 2 g / l of polyethylene glycol were added to the plating bath, and the temperature was set to 50 ° C. To adjust the pH value of the plating bath, tetrafluoroboric acid was added to lower the pH value, while sodium hydroxide was used to raise the pH value. The composition and cathode electrolysis efficiency of the Zn-Mn alloys obtained at this time are shown in Table 4 as Examples Nos. 1 to 3. Further, as a comparative example, the total number of moles of zinc tetrafluoroborate and manganese tetrafluoroborate in the plating bath was 0.2 mol / l, and 3.0 mol / l, and the cold-rolled steel sheet was electroplated. . Zn− obtained at this time
Table 4 shows the composition of Mn alloys and cathode electrolysis efficiency.
Shown as No. 1-2.

As is clear from Table 4, in the example, 55 A / dm
^No abnormal plating was observed even at the current density of ² , and the plating film was formed with a high cathode electrolysis efficiency of 79% or more. In contrast, the total number of moles of zinc tetrafluoroborate and manganese tetrafluoroborate in the plating bath is
In Comparative Example No. 1 containing 0.2 mol / l, abnormal plating was observed, and it was found that application of high current density was impossible. In Comparative Example No. 2 in which the total number of moles of zinc tetrafluoroborate and manganese tetrafluoroborate in the plating bath was 3.0 mol / l, a sound plating film was obtained with high current density and high cathode electrolysis efficiency. However, the results are the same as the electrolysis efficiency of caustic and so on at a lower total number of moles within the range of the present invention, and the merit of increasing the concentration cannot be found. Further, considering the cost of the liquid medicine, it is not significant to make such a high concentration.

[Example 3] After degreasing a cold-rolled steel sheet and pickling, the molar ratio of zinc tetrafluoroborate to manganese tetrafluoroborate in the plating bath (molar ratio / Mn
/ Zn) was changed to 5 to 50, and the cold rolled steel sheet was electroplated. In the plating bath, 25 g / l of boric acid and 2 g / l of polyethylene glucol were added, and the temperature was 50 ° C. To adjust the pH value of the plating bath, tetrafluoroboric acid was added to lower the pH value, while sodium hydroxide was used to raise the pH value. The composition and cathode electrolysis efficiency of the Zn-Mn alloys obtained at this time are shown in Table 5 as Example No.
Shown as 4. As a comparative example, the concentration ratio of zinc tetrafluoroborate to manganese tetrafluoroborate in the plating bath was set to 2 and 60, and the cold rolled steel sheet was electroplated. The composition and cathode electrolysis efficiency of the Zn-Mn alloys obtained at this time are shown in Table 5 as Comparative Examples Nos. 1 and 2.

As is apparent from Table 5, it was found that by setting the concentration ratio to 5 or more, a film having a manganese content of 5% or more can be formed with high current density and high cathode electrolysis efficiency.
On the other hand, in Comparative Example No. 1 in which the concentration ratio is 2, precipitation of manganese becomes very difficult, the manganese content of the coating is only 2 wt.%, And the high corrosion resistance characteristic of zinc-manganese alloy plating is high. I can't get it. In Comparative Example No. 2 with a concentration ratio of 60, a coating with a manganese content of 96% was obtained with a cathode electrolysis efficiency of 77%, but the zinc ion content in the plating bath was as low as 1.4 g / l, It is extremely difficult to control and is not practical as a plating bath.

[Example 4] After degreasing a cold rolled steel sheet and pickling, the pH value of the plating bath was changed to 0.5 to 6.0 with the plating bath composition within the range of the present invention, and the cold rolled steel sheet was electroplated. . 25 g / l boric acid and 2 g / l polyethylene glycol were added to the plating bath,
The temperature was 50 ° C. To adjust the pH value of the plating bath, tetrafluoroboric acid was added to lower the pH value, while sodium hydroxide was used to raise the pH value. The composition and cathode electrolysis efficiency of the Zn-Mn alloys obtained at this time are shown in Table 6 as Examples Nos. 1 to 3. As a comparative example, the pH values of the plating baths were set to 0.2 and 6.5, and the cold rolled steel sheet was electroplated. Zn− obtained at this time
Table 6 shows the composition of the Mn alloy and the cathode electrolysis efficiency.
No. 1 and 2 are shown.

As is clear from Table 6, the pH value of the plating bath is 0.5-
It was found that by setting the ratio to 6.0, a film having a manganese content of 5 wt.% Or more can be formed with high current density and high cathode electrolysis efficiency. On the other hand, Comparative Example No. 1 having a pH value of 0.2
In the case of Mn, the precipitation of manganese is suppressed and only 4 wt.% Is deposited, and the high corrosion resistance characteristic of zinc-manganese alloy plating cannot be obtained. Further, in Comparative Example No. 2 having a pH value of 6.5,
The productivity is very high as in the case of the example, but continuous operation is not possible due to the occurrence of zinc or manganese hydroxide precipitates in the plating bath.

[Example 5] After degreasing and pickling a cold-rolled steel sheet, plating was performed by changing the concentrations of polyethylene glycol and polypropylene glycol. The concentration of zinc tetrafluoroborate in the plating bath is 0.08M, and the concentration of manganese tetrafluoroborate is
1.20M, the amount of boric acid added was 25 g / l, and the pH value was 3.0. The composition and cathode electrolysis efficiency of the Zn-Mn alloy obtained at this time are shown in Table 7 as Examples No. 1 to 27. Further, the corrosion resistance of each of the obtained test materials was examined, and the results are also shown in Table 7. Here, the corrosion resistance was evaluated by subjecting a plated steel sheet with a coating weight of 30 g / m ² to a salt spray test specified in JIS Z 2371, and the time until red rust occurred. For comparison, polyethylene glycol and polypropylene glycol were not added, and one or two of polyethylene glycol and polypropylene glycol was added in an excessive amount outside the range of the present invention, which was 30 g / l in total. Those are shown in Table 7 as Comparative Examples No. 1 to 11. The red rust generation time in the salt spray test tends to be longer as the manganese content of the zinc-manganese alloy is higher, as shown in Table 7. However, in the examples, the manganese content is 38%.
Even with wt.%, the red rust generation time was as long as 1500 hours.
The test was continued up to 2000 hours, but only those having a manganese content of about 43 wt.% Or less produced red rust by 2000 hours. On the other hand, Comparative Example No1 without addition of polyethylene glycol and polypropylene glycol
It can be seen that in the samples Nos. 5 to 5, even if the manganese content is 62 wt. Furthermore, as compared with these comparative examples, the cathode electrolysis efficiency of the examples is slightly higher, and the appearance has changed from matte to semi-gloss, which shows that the plating appearance is improved. Comparative Examples Nos. 6 to 11, in which one or two of polyethylene glycol and polypropylene glycol were added in excess of 30 g / l in total outside the scope of the present invention, were corrosion resistance, cathode electrolysis efficiency, and plating appearance. However, it was not suitable for continuous plating because the foaming of the plating bath was remarkable.

[Example 6] After degreasing a cold rolled steel sheet and pickling, the boric acid concentration in the plating bath was changed to 0 to 50 g / l with a plating bath composition within the scope of the present invention, and the cold rolled steel sheet was electroplated. Was applied. Here, the addition amount of polyethylene glycol (average molecular weight 3000) is 2.
The temperature was 0 g / l and the temperature was 50 ° C. To adjust the pH value of the plating bath, adjust the pH
Tetrafluoroboric acid was added to lower the value, while sodium hydroxide was used to raise it. The composition and cathode electrolysis efficiency of the Zn-Mn alloys obtained at this time are shown in Table 8 as Examples Nos. 1-9. Also,
As a comparative example, the cold rolled steel sheet was electroplated with a boric acid concentration of 75 g / l, which is outside the range of the present invention. The composition and cathode electrolysis efficiency of the Zn-Mn alloy obtained at this time were
It is shown as Comparative Examples No. 1 to 3 in the table. As is clear from Table 8, the cathode electrolysis efficiency decreases as the boric acid concentration increases to 75 g / l. As described above, the addition of boric acid has the effect of suppressing the decomposition of tetrafluoroborate ions, while the addition of a large amount exceeding 50 g / l has the adverse effect of reducing the cathode electrolysis efficiency. Therefore,
From the viewpoint of productivity, it is preferable that the addition amount of boric acid be 50 g / l or less.

As described above, according to the present invention, it is possible to form zinc-manganese alloy plating on the surface of an object to be plated with much higher current density and cathode electrolysis efficiency than the conventional method. It is possible to obtain an industrially useful effect in which extremely excellent productivity can be obtained.

Front Page Continuation (72) Inventor Takayuki Urakawa 1-2-1, Marunouchi, Chiyoda-ku, Tokyo Nihon Kokan Co., Ltd. In-house (56) References Japanese Patent Publication Sho 53-28853 (JP, B2)

Claims (2)

[Claims] 1. An electroplating method for forming a zinc-manganese alloy plating film containing a predetermined amount of manganese on a surface of an object to be plated in a plating bath containing zinc ions and manganese ions. Using zinc tetrafluoroborate as the zinc ion in the bath,
Using manganese tetrafluoroborate as the manganese ion in the plating bath, the total amount of the zinc tetrafluoroborate and the manganese tetrafluoroborate in the electroplating bath, 0.25 or more 2.5 mol / l
And the metal molar ratio of zinc to manganese (Mn
/ Zn) is limited to 5 or more and 50 or less, and the total amount of one or two of polyethylene glycol and polypropylene glycol in the plating bath is 0,05 g / l or more 2
0 g / l or less, and limit the pH value of the plating bath to 0.5 or more and 6.0 or less, and thus the manganese content of 5 wt.% Or more and 95 wt.% Or less zinc-manganese alloy on the surface of the object to be plated. A method for electroplating a zinc-manganese alloy having excellent productivity, which comprises forming a film. 2. The method of electroplating a zinc-manganese alloy with excellent productivity according to claim 1, wherein boric acid is further added to the plating bath in an amount of 50 g / l or less.