JPH0765225B2 - Molten salt electroplating method for steel materials - Google Patents

Description

TECHNICAL FIELD The present invention relates to a molten salt electroplating method for steel materials, and particularly to a molten salt electroplating method for Al having excellent plating adhesion.

(Prior Art) Conventionally, for example, molten salt aluminum electrolytic plating has been known as a plating method for steel materials, but it has hardly been put to practical use. This is because the steel material (steel strip, steel wire, etc.) had to be previously treated and activated by an appropriate means before electrolytic plating in the AlCl ₃ mixed molten salt.
In particular, it is difficult to completely remove the surface oxide film, and development of an efficient means therefor is desired. On the other hand, the molten salt electroplating bath easily deteriorates when water is mixed in, and the water management in the plating line must be strictly controlled together with the management of the plating atmosphere.

Conventionally, the surface of a steel material to be plated has been surface-activated by a wet method such as pickling and then water washing. Therefore, it is necessary to provide a drying step to dry the steel material prior to plating. However, high temperatures in the atmosphere (eg 180
If heated and dried at (.degree. C.), the plating adhesion of the molten salt electrolytic plating to be performed next will be significantly reduced.

(Problems to be solved by the invention) Therefore, the inventors of the present invention have made various studies on the cause of such a decrease in plating adhesion, and as a result, an oxide film is formed again on the surface of the steel material during heating and drying of the steel material. I knew that it was because I would end up.

In order to prevent the formation of such an oxide film, it is necessary to perform the drying at a low temperature and raise the temperature by preheating in an inert gas. On the other hand, however, the oxide film that has once been formed must be removed by some means.

(Means for Solving Problems) Here, the inventors of the present invention continued various research and development for making molten salt electroplating an industrially practicable process. -When performing Mn alloy plating, after anodic dissolution of the steel surface in the pretreatment bath, Al-Mn
We have found that a plating film with good adhesion can be obtained by performing alloy plating.

Therefore, we continued our research on this anodic dissolution.
When anodic dissolution is performed in the molten salt plating bath, aluminum metal or alloy is electrodeposited on the counter electrode, and long-term operation cannot be performed.
A method using alternating current was also considered, but it was found to be inefficient. Therefore, in order to solve such a problem, after repeated studies, in the case of Al plating, especially, the composition of the pretreatment bath is AlCl ₃ 50 to 54 mol%, and more preferably by using Al or Ti as the counter electrode. , It was learned that the electrodeposition of metal or alloy on the counter electrode can be prevented and stable operation can be performed for a long time.

Here, the gist of the present invention is that Al or
In the molten salt electroplating method of a steel material for activating the surface of a steel material by subjecting the steel material to an anodic dissolution treatment prior to the molten salt electrolytic plating of an Al alloy, the anodic dissolution treatment is provided separately from the bath for performing the molten salt electrolytic plating. Molten salt electroplating of steel material, characterized in that AlCl ₃ is 50 to 54 mol% as a composition of the anodic dissolution treatment salt bath, which is performed in a molten salt bath consisting of AlCl ₃ and an alkali metal chloride. Is the way.

Further, according to another aspect of the present invention, the counter electrode when performing the anodic dissolution treatment is made of Al or Ti or an alloy thereof to prevent electrodeposition on the counter electrode, and Stable processing becomes possible. The anodic dissolution treatment can be performed at a temperature within the range of 70 ° C from the melting point of the molten salt bath.

The "steel material" refers to a steel strip, a steel plate, a steel wire, or the like.

In the present invention, the plating metal is typically Al and its alloys, but Ti, Zr, etc. may be mentioned.

(Operation) The present invention will now be described in further detail with reference to the accompanying drawings.

FIG. 1 of the accompanying drawings is a schematic explanatory view of a molten salt electrolytic Al plating apparatus for steel products for carrying out the method according to the present invention.

In the figure, the steel strip 2 from the recoiler 1 is passed through a degreasing tank 3, a water washing tank 4, and a pickling tank 5 that perform a pre-cleaning step, and a water washing tank 6 that performs a water washing step, and then a drying step of a drying chamber 7. It is sent continuously. In this drying chamber 7, first, heating gas,
For example, the steel strip 2 is heated by being blown with heated air,
To be dried. Next, the steel strip 2 is subjected to a preheating step, heated and dried in an inert atmosphere 9 which is shielded from the outside by a seal roll 8, and then sent into an anode electrolysis cell 10 in the same atmosphere, for example AlCl ₃ -alkali. Anodic electrolysis is performed in a molten salt electrolytic bath of metal chloride.
This is an activation process. Reference numeral 11 indicates a conductor roll for anode electrolysis, and reference numeral 12 indicates a sink roll.

The steel strip 2 thus pretreated exits from the inert gas atmosphere through the compartment rolls 13, enters the molten salt electrolytic plating tank 14, and is subjected to predetermined plating. It should be noted that molten salt electroplating, for example, molten salt electroplating of aluminum itself is already known and well understood by those skilled in the art, so further reference is omitted for the sake of brevity.

As described above, when drying is performed in the air, an oxide film is formed on the steel sheet even at a low temperature of about 80 ° C., and the adhesion of the plating film decreases, but according to the present invention, the pretreatment of plating is performed. As a result, a steel material is anodically melted and its surface is activated, whereby plating with good adhesion is performed. Moreover, when anodic dissolution is carried out in the plating bath, it is difficult to carry out continuous plating for a long time because of electrodeposition on the counter electrode, but according to the present invention, AlCl ₃ 50 to 54 mol%,
It was found that in the balance of the alkali metal chloride bath, powdery electrodeposition was formed and almost no plating was performed.

When the AlCl ₃ concentration is less than 50 mol%, the melting point of the mixed molten salt becomes too high, which is not preferable. On the other hand, if it exceeds 54 mol%, the state of electrodeposition will be good, electrodeposition will proceed, and continuous operation will not be possible, such being undesirable.

Next, it was found that when Al and Ti or their alloys were used as the counter electrode material, the plating did not adhere easily.

As described above, by performing anodic dissolution in the pretreatment bath and immediately performing Al plating in the plating bath, the adhesion between the plating film and the base material becomes very good.

About 20 [C / dm ² ] is sufficient as the electrolysis condition in the pretreatment bath, but a smaller amount of electricity may be applied depending on the dry state of the steel sheet, that is, the degree of surface oxidation.

Thus, according to the present invention, the oxide film formed in the drying and preheating process is efficiently removed and the surface is activated, and the one having excellent adhesion can be obtained. In addition, continuous processing is efficiently performed.

Next, the present invention will be further described with reference to examples.

Example 1 In order to simulate the method shown in FIG. 1, a flow cell (made by Inconel 600) dedicated to molten salt was prepared, and an Al plate having a purity of 99.8% was installed as an anode in the flow channel.
Al electroplating was performed in a molten salt under the conditions shown in Table 1 using SPCD grade 0.8 mm thick mild steel plate.

First, the pretreatment of the raw steel sheet is performed by electrolytically cleaning the steel sheet sample in a 5% sodium orthosilicate solution as an anode at 10 A / dm ² for 15 S, followed by washing with water, then pickling with a 10% HCl aqueous solution for 20 S, and then again. Washed with water. The steel plate thus obtained is 180 to 200 ° C
It was dried for 30 seconds in a dryer (wind speed 10 m / sec) whose temperature was set in step _2. After completion of the drying, it was immediately switched to N ₂ gas, cooled, taken out, and inserted into a flow cell. Using this steel plate as an anode, anodic electrolysis was performed for 2 seconds under the conditions shown in Table 2, and then the above-described molten salt electrolytic plating was performed.

The plated sample was washed with water and dried, and then the adhesion of the plated film was evaluated by a DuPont impact test (ball head diameter 1/2 inch, potential energy 0.8 kgf-m). The results are summarized in Table 3.

Also in the case of the three-component bath, there was no electrodeposition up to 50 to 54 mol% at 200 ° C. If it is less than 50 mol%, a solid phase appears and the electrolysis voltage becomes large. Electrodeposition was observed on the counter electrode for SUS and Fe.

Example 2 Example 1 was repeated, but in this example, the steel plate heating temperature in the drying step was variously set in the range of 20 to 180 ° C., and the anodic dissolution treatment was performed at 30 A / dm ² for 0 to 10 seconds.

The adhesion of the obtained Al plating film was evaluated in the same manner as in Example 1. The results are shown graphically in FIG.

As is clear from the results shown in the figure, poor adhesion occurs due to the drying of the steel sheet base material. This is largely influenced by the temperature of the drying plate, but can be effectively solved by the anodic electrolysis method according to the present invention.

Example 3 In this example, the state of electrodeposition on the counter electrode during the anodic dissolution treatment using an AlCl ₃ —NaCl 2 component system molten salt bath was examined. Example 1 was repeated, but in this example the composition of the anodic dissolution bath and bath temperature were varied. Also, using a 99.5% Al plate as the counter electrode,
The flow rate was 20 A / dm ² and the flow rate was 0.3 m / sec.

The results are shown graphically in FIG.

It can be seen that almost no electrodeposition occurs on the counter electrode within 70 ° C of the melting point (solid line part).

[Brief description of drawings]

FIG. 1 is a schematic explanatory view of an apparatus for carrying out the method according to the present invention; FIG. 2 shows the relationship between the steel plate surface temperature in the drying step and the required treatment time in the anodic electrolytic treatment depending on the plating adhesion. The summarized graphs; and FIG. 3 are graphs in which the relationship between the AlCl ₃ composition ratio of the anodic dissolution treatment bath and the anodic dissolution treatment temperature is summarized by plating adhesion, and the state diagram is superposed on this. 1: Recoiler, 2: Steel strip 3: Degreasing tank, 4: Washing tank 5: Pickling tank, 6: 〃 7: Drying room, 8: Seal roll 10: Anode electrolysis tank

 ─────────────────────────────────────────────────── ─── Continuation of front page (72) Inventor Junichi Uchida 1-3-3 Nishi-Nagasumoto-dori, Amagasaki-shi, Hyogo Prefecture Sumitomo Metal Industries, Ltd. Research Institute (72) Toshio Nakamori Nishi-Nagasumoto, Amagasaki-shi, Hyogo Prefecture Sumitomo Metal Industry Co., Ltd., Comprehensive Technical Research Institute (72) Inventor, Minato Shimozato 4-6-22 Kannon Shinmachi, Nishi-ku, Hiroshima City, Hiroshima Prefecture Mitsubishi Heavy Industries, Ltd. Hiroshima Research Laboratory (72) Inventor Yanagi Kenichi 4-6-22 Kannon Shinmachi, Nishi-ku, Hiroshima City, Hiroshima Prefecture Mitsubishi Heavy Industries Hiroshima Works

Claims (3)

[Claims] 1. A molten salt electrolytic plating method for a steel material, which comprises subjecting a steel material to an anodic dissolution treatment to activate the surface of the steel material prior to the molten salt electroplating of the steel material with Al or an Al alloy. AlCl ₃ and an alkali metal chloride are provided separately from the bath for salt electroplating, and the composition of the anodic dissolution treatment salt bath is AlCl 3.
_3. A molten salt electrolytic plating method for steel material, wherein ₃ is 50 to 54 mol%. 2. The method according to claim 1, wherein the counter electrode used for the anodic dissolution treatment is made of Al, Ti or an alloy thereof. 3. The anodic dissolution treatment is performed according to the melting point of the molten salt bath 70
Claim 1 which is carried out at a temperature within the range of ℃
The method according to Item 2 or Item 2.