JPH0390591A - Method for electroplating zinc-manganese alloy with high productivity - Google Patents

Abstract

PURPOSE:To form a plating film excellent in corrosion resistance with high productivity by incorporating tetrafluoroborate ion in a Zn-Mn-based electroplating bath and further adding polyethylene glycol to the bath. CONSTITUTION:Zinc tetrafluoroborate is used as the Zn ion in an electroplating bath and manganese tetrafluoroborate as the Mn ion. The total amt. of both components in the bath is controlled to 0.25-2.5mol/l, and the molar ratio of the metallic Mn to Zn is adjusted to 5-50. One or more kinds between polyethylene glycol and polypropylene glycol are added to the bath by <=0.05-20g/l in total, and the pH is limited to 0.5-6.0. A plating film of the Zn-Mn alloy contg. 5-95wt.% Mn is formed on the surface of a material to be plated by using the bath. Boric acid is added, as required, to the bath by <= about 50g/l.

Description

[Detailed Description of the Invention] [Industrial Application Field] This invention relates to an electroplating method for forming a zinc-manganese alloy plating film with excellent corrosion resistance on the surface of a plated object with excellent productivity. be.

[Conventional technology]

Galvanizing has traditionally been widely used as a corrosion-preventing method for steel sheets, and galvanized steel sheets are used in a wide range of applications, including building materials, home appliances, and automobiles. By the way, in recent years,
There is an increasing demand for improved product durability, and there is a strong need to further improve the corrosion resistance of galvanized steel sheets.

As a result of repeated research to meet the above requirements, the present inventors have developed a zinc-manganese alloy electroplated steel sheet that has significantly superior corrosion resistance compared to conventional zinc electroplated steel sheets, and have filed a patent application for this invention. The technology has already been published in Japanese Unexamined Patent Publication No. 58-3/
It is disclosed in Publication No. 188. However, zinc-
The cathode electrolytic efficiency during the production of manganese alloy electroplated steel sheets is considerably lower than that during the production of zinc-based alloy electroplating such as zinc-iron alloy plating and zinc-nickel alloy plating, and is only about 25 to 50%. do not have.

Low cathode electrolysis efficiency not only leads to poor productivity of electroplated steel sheets, but is also unfavorable from the viewpoint of energy saving. Furthermore, the maximum current density required to obtain a healthy zinc-manganese alloy plating film is about half that of zinc-iron alloy plating and zinc-nickel alloy plating, when the plating conditions such as stirring are the same. . A low maximum current density means a low cathode electrolysis efficiency as well as a poor productivity, which is not preferable in terms of industrial production.

As described above, in the production of zinc-manganese alloy coated steel sheets, the conventional technology has a drawback of poor productivity due to low cathode electrolysis efficiency and low maximum current density. The reason for the low cathode electrolysis efficiency is that citric acid is added as a complexing agent to deposit zinc and manganese simultaneously, and the zinc deposition is shifted to a base potential.
This is because the hydrogen generation reaction, which is a competitive reaction, was promoted.

Further, the reason why the limiting current density is low is considered to be that the metal ions form a complex with citric acid, so that the ion diffusion rate is low.

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

■ By adding a selenium compound or a tellurium compound to a zinc-manganese alloy plating solution containing zinc sulfate and manganese sulfate as main components and adding citric acid as a complexing agent, cathode electrolytic efficiency during zinc-manganese alloy plating can be improved. (Japanese Unexamined Patent Publication No. 60-52590,
JP-A-60-52591).

(1) A method for improving cathode electrolytic efficiency during zinc-manganese alloy plating by adding thiosulfate or salts of organic acids having thiol groups instead of selenium or tellurium compounds (Japanese Patent Laid-Open No. 62-44593).

In the above-mentioned conventional technology, although the cathode electrolysis efficiency was improved, the maximum current density at which a sound plating could be obtained did not increase, and Fe-Zn, which has been put into practical use as a rust-proof steel sheet for automobiles,
Alternatively, the productivity is still lower than that of Ni-Zn alloy plating, and problems still remain in this respect.

In addition, a report on a plating bath using a tetrafluoroboric acid plating bath as in the present invention is published in "Battelle
Memorial Institute J Technical Report 5692 (NoveI!1ber 1
952). According to this report, various studies have been conducted on the plating composition, plating conditions, and additives, but the highest manganese content is 35.4 wt.%, and this coating is brown and partially powdered. It is reported that it was impractical. Furthermore, the cathode electrolysis efficiency at this time was 7°8%.
This is very low, and the critical current density has already been reached at IOA/drrr, so it was completely unexpected that a tetrafluoroboric acid-based plating bath could become a highly efficient plating bath.

[Problem to be solved by the invention]

The cathode electrolysis efficiency during zinc-manganese alloy plating is lower than that of 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 healthy plating film can be obtained is also low. As described above, zinc-manganese alloy plating has the drawback of low productivity, and a method of increasing productivity has been eagerly awaited for industrial practical use.

As a means to overcome this drawback, a technique of adding selenic acid or sodium fluorosulfate has been applied for as described above. However, although these interpositions improve the cathode electrolysis efficiency, the maximum current density does not increase.
Productivity has not yet reached a sufficient level.

Therefore, this invention was made to solve the above-mentioned problem of low productivity of zinc-magan alloy plating, and to increase the cathode defrosting efficiency and to form a healthy plating layer. An object of the present invention is to provide an electroplating method for forming a sub-manganese alloy plating film with excellent productivity, which can increase the maximum current density obtained and sufficiently increase productivity.

[Means to solve the problem]

In order to increase the cathode electrolysis efficiency and the maximum current density at which a healthy plating film can be obtained, the present inventors have conducted many studies on a method using a new plating bath, that is, a plating bath that does not use citric acid. I did it. Examples of plating baths considered here include sulfuric acid bath, chloride bath,
Inorganic acid plating baths such as sulfuric acid-hydrochloric acid mixed bath, perchloric acid bath, tetrafluoroboric acid bath, phenolsulfonic acid bath,
Organic acid plating baths such as salicylic acid baths.

As a result, industrially usable zinc-manganese alloy plating could not be obtained from almost all of these plating baths, but only the tetrafluoroboric acid bath was able to produce a healthy plating film with excellent productivity. We discovered that it can be manufactured.

This invention was made based on the above-mentioned findings, and it is possible to form a zinc-manganese alloy plating film containing a predetermined amount of manganese on the surface of the object to be plated in a plating medium containing zinc ions and manganese ions. In the electroplating method to form a The total amount of the zinc tetrafluoroborate and the manganese tetrafluoroborate in the inside is limited to 0.25 or more and 2.5 mol/l, and the metal molar ratio of zinc to manganese (M
n / Z n ) is limited to 5 or more and 50 or less, and furthermore, the total amount of one or two of polyethylene glycol and polypropylene glycol is 0.05 g / 1 or more and 20 g / 1 or less in the plating bath. and adjust the pH value of the plating bath to 0.5 or more and 6.0.
limited to the following, and thus, on the surface of the object to be plated, zinc-
Next, a method for forming a zinc-manganese alloy film according to the present invention, which is characterized by forming a manganese alloy film, will be explained.

The mechanism by which a healthy zinc-manganese alloy plating film can be obtained over a wide range of alloy compositions at high current density and high cathode electrolytic efficiency by using a tetrafluoroboric acid bath is not clear, but it is difficult to understand the mechanism by which a conventional plating bath can be used. In a sulfuric acid bath containing citric acid, the plating metal is deposited from complex ions of citric acid, zinc, and manganese, whereas the tetrafluoroboric acid bath of the present invention does not contain such stable complex ions. Since the precipitation is not from ions, the discharge reaction and movement speed of ions are fast, and it is thought that the limiting current density and electrolytic efficiency are high. Furthermore, in baths such as sulfuric acid baths, chloride baths, sulfuric acid-hydrochloric acid mixed baths, and perchloric acid baths that do not contain citric acid, zinc, which is a critical metal, precipitates preferentially, and manganese hardly precipitates. Although there were some cases in which manganese was precipitated, the resulting film was a black powder and could not be put to practical use industrially. On the other hand, in a tetrafluoroboric acid bath, the presence of tetrafluoroborate ions (BF) polarizes the zinc deposition, and as a result, the deposition potential of zinc approaches that of manganese, allowing both metals to be deposited simultaneously, allowing for a wide range of It can be inferred that a plating film can be obtained across all alloy compositions.

Next, the reason why the plating bath composition was limited as described above will be described below.

10 The total molar concentration of zinc tetrafluoroborate and manganese tetrafluoroborate in the bath is O025 mol/I! If it is less than that, the critical current density will be drastically reduced, making it impossible to obtain sound plating at high current densities, and the feature of the high critical current density of the present invention will disappear. On the other hand, even if it exceeds 2.5 mol/l, the effect of improving the limiting current density is almost saturated, and no great advantage can be obtained when considering the cost of the chemical solution. Therefore, the total molar concentration of zinc tetrafluoroborate and manganese tetrafluoroborate in the bath is 0.2
It should be limited to 5 or more and 2.5 mol/l or less.

(2) Metal molar ratio of zinc and manganese (M n / Z
When n ) is less than 5, the preferential precipitation tendency of zinc is strengthened, and the precipitation of manganese is suppressed, and the manganese content is 5 w t
.

% or more alloy film cannot be obtained. On the other hand, when it exceeds 50, the concentration of zinc ions in the plating bath is quite low, making it extremely difficult to control the concentration of the plating bath.

Therefore, the metal molar ratio of zinc to manganese (M n /Z
n) should be limited to 5 or more and 50 or less.

(3) Polyethylene glycol and polypropylene glycol have the effect of improving corrosion resistance. Even if polyethylene glycol and polypropylene glycol are not added, the productivity is sufficiently high compared to the conventional technology. However, when these chemicals are not added, the plating crystals are not dense and the plating film cannot completely cover the base. For this reason, the high corrosion resistance characteristic of zinc-manganese alloys cannot be sufficiently achieved. Therefore,
As a result of repeated studies on methods for improving corrosion resistance, it was discovered that adding polyethylene glycol and/or polypropylene glycol makes it possible to obtain dense and fine crystals, thereby improving corrosion resistance. It is presumed that such an effect due to the addition of polyethylene glycol and/or polypropylene glycol occurred because these chemicals were adsorbed to the plated surface and increased the deposition overvoltage. Furthermore, the addition of these chemicals slightly improves the cathode electrolysis efficiency and changes the plating appearance from matte to semi-glossy. However, if the total amount of one or two of polyethylene glycol and polypropylene glycol is less than 0.05 g/l2,
The desired effect of improving corrosion resistance as described above cannot be obtained. On the other hand, if it is added in an amount exceeding 20 g/l, there will be no problem with productivity or corrosion resistance, but the plating solution will tend to foam rapidly, which will cause problems in continuous operation. Therefore, the total amount of one or two of polyethylene glycol and polypropylene glycol is 5 g/l or more.
It should be limited to a range of 0 g/l or less.

(41 Decreasing the pH value tends to suppress the precipitation of manganese. If the pH value is less than 0.5, it is difficult to precipitate manganese, and in addition, the hydrogen generation reaction tends to occur, resulting in a decrease in cathode electrolysis efficiency, which is preferable. On the other hand, if the pH value exceeds 6.0, zinc ions and manganese ions will generate hydroxide and precipitate. Therefore, the pH value should be limited to 0.5 or more and 6.0 or less. be.

(5) When the manganese content of the zinc-manganese alloy plating film is less than 5 wt.%, there is no difference in corrosion resistance from that of the zinc plating film. On the other hand, 95 wt.

If it exceeds %, the plating film will be easily oxidized and the surface will easily turn brown during storage, so it should be preferably limited in terms of appearance.

(6) Boric acid has the effect of suppressing the generation of free fluorine ions through hydrolysis of tetrafluoroborate ions. This fluorine ion is an undesirable ion because it forms a precipitate with the plating metal ion. That is, it is desirable to add boric acid in order to suppress the decomposition of tetrafluoroborate ions and extend the life of the plating solution. However, on the other hand, the amount of boric acid added is 50g.
If it exceeds zl, the cathode electrolysis efficiency will drop sharply. Therefore, the amount of boric acid added should be limited to 50 g/l or less.

As mentioned above, the present invention greatly increases productivity compared to the prior art. In consideration of power costs, it is effective to improve the conductivity of the plating bath in order to save power. For this purpose, it is effective to add sodium tetrafluoroborate, ammonium tetrafluoroborate, and the like. In addition, minute amounts of fluorine are detected in films obtained from tetrafluoroboric acid plating baths, but this amount is at a level that causes problems in the handling of films obtained from tetrafluoroboric acid plating baths. Rather, it has no effect on various performances.

Next, the present invention will be explained in more detail with reference to examples.

[Example 1] After degreasing and pickling a cold-rolled steel sheet, the cold-rolled steel sheet was electroplated using a plating bath composition and electrolytic conditions within the range of the present invention shown in Table 1.

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 it.

The composition and cathode electrolysis efficiency of the Zn-Mn alloy obtained at this time are shown in Table 3 as Examples Nos. 1 to 3. Further, as a comparative example, the cold rolled steel sheet was electroplated using a citric acid-sulfuric acid plating bath composition and electrolytic conditions that are outside the scope of the present invention as shown in Table 2. The composition and cathode electrolysis efficiency of the Zn-Mn alloy obtained at this time are shown in Table 3 as Comparative Examples Nos. 1 to 3.

As is clear from Tables 1 to 3, Example N was electroplated using the tetrafluoroboric acid plating bath of the present invention.
o1-3 are current density 80A/dn? It can be seen that no plating or burning was observed in this case, indicating that the limiting current density was extremely high. Furthermore, the cathode electrolysis efficiency was also 37 wt,%
It can be seen that high efficiency and healthy plating can be obtained. On the other hand, in the comparative example of the conventional bath, plating burn was already observed at a current density of 45 A/d%, and the cathode electrolysis efficiency was 40% at 40 A/dn (which is considered to be the almost limit current). 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 and pickling a cold-rolled steel sheet, the total number of moles of zinc tetrafluoroborate and manganese tetrafluoroborate in the plating bath was adjusted to a plating bath composition within the range of the present invention. 0.25
The cold rolled steel sheet was electroplated with 2.5 mol. 25 g/l of boric acid/2 g/It of polyethylene glycol was added to the plating bath, and the temperature was set to 5.
The temperature was 0°C. To adjust the pH value of the plating bath, add tetrafluoroboric acid to lower the pH value, and
Sodium hydroxide was used for raising. The composition and cathode electrolysis efficiency of the Zn-Mn alloy obtained at this time are shown in Table 4 as Examples Nos. 1 to 3.

In addition, as a comparative example, the total number of moles of zinc tetrafluoroborate and manganese tetrafluoroborate in the plating bath was set to 0.2 mol/l and 3.0 mol/l, and the cold rolled steel sheet electroplated. Z obtained at this time
The composition of the n -M n alloy and the cathode electrolysis efficiency are
These are shown in the table as Comparative Examples Nos. 1 and 2.

As is clear from Table 4, in the example, 55A/
No plating abnormality was observed even at the current density of 7.
A plating film is formed with a high cathode electrolysis efficiency of 9% or more. On the other hand, in Comparative Example No. 1, in which the total number of moles of zinc tetrafluoroborate and manganese tetrafluoroborate in the plating bath was 0.2 mol/l, plating abnormalities were observed. It was found that the application of current density was not possible. 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 healthy plating film was obtained with high current density and high cathode electrolytic efficiency. However, the result is the same as the cup electrolysis efficiency at a lower total mole number within the range of the present invention, and no advantage can be found in increasing the concentration. Furthermore, if the cost of the chemical solution is taken into account, there is little significance in increasing the concentration to such a high level.

[Example 3] After degreasing and pickling a cold-rolled steel sheet, the molar concentration ratio (molar ratio) of zinc tetrafluoroborate and manganese tetrafluoroborate in the plating bath was determined using a plating bath composition within the range of the present invention.・M
The cold-rolled steel sheets were electroplated while changing n/Z n ) from 5 to 50. During the plating process, 2 boric acids were added.
5g/j? , 2 g/l of polyethylene glycol was 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 it. The composition and cathode electrolysis efficiency of the Zn-Mn alloy obtained at this time are shown in Table 5 as Examples Nos. 1 to 4. Further, as a comparative example, the cold rolled steel sheet was electroplated with the concentration ratio of zinc tetrafluoroborate and manganese tetrafluoroborate in the plating bath set to 2 and 60, respectively. The composition and cathode electrolysis efficiency of the Zn--Mn alloy obtained at this time are shown in Table 5 as Comparative Examples No. 1 and 2.

As is clear from Table 5, by setting the concentration ratio to 5 or more, a film with a manganese content of 5% or more has a high current density,
It was found that it was formed with high cathode electrolysis efficiency. On the other hand, in Comparative Example No. 1 where the concentration ratio is 2, it is very difficult to precipitate manganese, and the manganese content in the film is only 2 wt.%, which is a high concentration characteristic of zinc-manganese alloy plating. Corrosion resistance cannot be obtained. Furthermore, in Comparative Example No. 2 with a concentration ratio of 60, a film with a manganese content of 96% was obtained with a cathode electrolysis efficiency of 77%, but the amount of zinc ions in the plating bath was as low as 1.4 g/l, and the plating It is extremely difficult to manage the bath and is not practical as a plating bath.

[Example 4] After degreasing and pickling a cold-rolled steel sheet, the cold-rolled steel sheet was subjected to electricity with a plating bath composition within the range of the present invention and with the pH value of the plating bath varied from 0.5 to 6.0. Plated. During the plating, 25 g/l of boric acid and 2 g/l of polyethylene glycol were added, and the temperature was set at 50°C. pH of plating bath
To adjust the pH value, tetrafluoroboric acid was added to lower the pH value, while sodium hydroxide was used to increase it. The composition and cathode electrolysis efficiency of the Zn-Mn alloy obtained at this time are shown in Table 6 for Examples N01 to
Shown as 3. In addition, as a comparative example, p of the plating bath
The H value was set to 0.2 and 6.5, and electroplating was applied to the cold rolled steel sheet. Z n −M obtained at this time
The composition of the n-alloy and the cathode electrolysis efficiency are shown in Table 6 as Comparative Example No. 2.

As is clear from Table 6, the pH value of the plating bath was 0.5.
~6.0, the manganese content is 5wt%
It was found that the above film was formed with high current density and high cathode electrolysis efficiency. On the other hand, in Comparative Example No. 1 with a pH value of 0.2, the precipitation of manganese was suppressed and 4
wt.%, and the high corrosion resistance characteristic of zinc-manganese alloy plating cannot be obtained. In addition, the pH value is 6.
In Comparative Example No. 5, which is Comparative Example No. 5, the productivity is very high as in the Examples, but a precipitate that appears to be zinc or manganese hydroxide occurs in the plating bath, making continuous operation impossible.

[Example 5] After degreasing and pickling a cold rolled steel sheet, plating was performed with varying concentrations of polyethylene glycol and polypropylene glycol. The concentration of zinc tetrafluoroborate in the plating bath was 0.08M, the concentration of manganese tetrafluoroborate was 1.20M, and the amount of oxalic acid added was 25g/1SpH value was 3.0. The composition and cathode electrolysis efficiency of the Zn-Mn alloy obtained at this time are shown in Table 7. Example N
Shown as o 1-27. In addition, the corrosion resistance of each of the obtained test materials was investigated and the results are shown in Table 7. Here, the corrosion resistance is a coating weight of 30g/rr? A salt spray test specified in JIS Z 23/l was performed on a plated steel plate, and the evaluation was based on the time until red rust appeared. For comparison, there were samples in which polyethylene glycol and polypropylene glycol were not added, and samples in which one or both of polyethylene glycol and polypropylene glycol were added in excess at a total amount of 30 g/l, which is outside the range of the present invention. Comparative examples No. 1 to 1
It is also shown in Table 7 as 1. As shown in Table 7, 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. It took 1500 hours.

The test continued for up to 2,000 hours, but the manganese content of those that developed red rust by 2,000 hours was approximately 43 wt.
,% or less. On the other hand, in Comparative Examples Nos. 1 to 5 without the addition of polyethylene glycol and polypropylene glycol, the manganese content was 62 wt.%, which took the longest time to generate red rust.
It can be seen that the test time was 650 hours, which is very short compared to the example. Furthermore, compared to these comparative examples, the cathode electrolysis efficiency of the examples was slightly higher, and the appearance changed from matte to semi-glossy, indicating that the plating appearance was improved. One or two of polyethylene glycol and polypropylene glycol in total sag/l
and Comparative Example No. 6 in which excessive addition was made outside the scope of the present invention.
- No. 11 were completely equivalent to Examples in terms of corrosion resistance, cathode electrolysis efficiency, and plating appearance, but foaming of the plating bath was significant.
It was unsuitable for continuous plating.

[Example 6] After degreasing and pickling a cold-rolled steel sheet, the cold-rolled steel sheet was electroplated with a plating bath composition within the range of the present invention and with the boric acid concentration of the plating bath varied from θ to 50 g/l. was applied. here,
The amount of polyethylene glycol (average molecular weight 3000) added was 2.0 g/It, 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 it. Z n − obtained at this time
The compositions and cathode electrolytic efficiencies of the Mn alloys are shown in Table 8 as Examples Nos. 1 to 9. In addition, as a comparative example, the boric acid concentration was set to 75 g/I!, which is outside the range of the present invention! Then, the cold rolled steel sheet was electroplated. Z n obtained at this time
-Mn alloy compositions and cathode electrolytic efficiencies are shown in Table 8 as Comparative Examples Nos. 1 to 3. As is clear from Table 8, as the boric acid concentration increases to 75 g/l2, the cathode electrolysis efficiency decreases. As mentioned above, addition of boric acid has the effect of suppressing the decomposition of tetrafluoroborate ions, but on the other hand, addition of a large amount exceeding 50 g/l has the disadvantage of reducing cathode electrolysis efficiency. Therefore, from the viewpoint of productivity, it is preferable that the amount of boric acid added be 50 g/l or less.

Table 1 Table 2 [Effects of the Invention] As explained above, according to the present invention, the current density and cathode electrolysis efficiency are much higher than that of the conventional method.
Zinc-manganese alloy plating can be formed on the surface of the object to be plated, and industrially useful effects such as extremely excellent productivity are brought about.

Claims (1)

[Claims] 1. An electroplating method for forming a zinc-manganese alloy plating film containing a predetermined amount of manganese on the surface of an object to be plated in a plating bath containing zinc ions and manganese ions, Zinc tetrafluoroborate is used as the zinc ion in the plating bath, manganese tetrafluoroborate is used as the manganese ion in the plating bath, and the tetrafluoroborate in the electroplating bath is The total amount of zinc and the manganese tetrafluoroborate is 0.25 or more and 2.5 mol/l.
and the metal molar ratio of zinc to manganese (M
n/Zn) is limited to 5 or more and 50 or less, and furthermore, one or two of polyethylene glycol and polypropylene glycol is added in a total amount of 0.0 in the plating bath.
5 g/l or more and 20 g/l or less, and the pH value of the plating bath is limited to 0.5 or more and 6.0 or less, and thus a manganese content of 5 wt.
% or more 95wt. 1. A method for electroplating a zinc-manganese alloy with excellent productivity, which is characterized by forming a zinc-manganese alloy film of less than 10%. 2. The method for 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.