JPH0477078B2 - - Google Patents

Description

[Detailed description of the invention]

[Industrial Field of Application] The present invention relates to a chrome plating method using a lead dioxide electrode as an anode, and particularly to a chrome plating method for protecting the anode when the plating current is stopped. [Prior Art and Problems] Chrome plating is used in a wide range of applications because it not only has a beautiful surface, but also has hardness and good corrosion resistance. For example, they are commonly used for industrial and decorative purposes, such as engine parts for automobiles, various cylinders, rolls for gravure printing, tableware, and cans for canning. Various plating baths have been used to perform chrome plating, but the typical ones are a method using a silicate bath and a method using a sergeant bath. The former has the advantage of a good finish and relatively high current efficiency, but on the other hand, it is difficult to manage the liquid, and the plating bath contains fluoride ions, which are corrosive and cause difficulties in maintenance and safety of the plating equipment. be. On the other hand, although the latter method using a sergeant bath generally has a problem of somewhat low current efficiency, it is easy to handle and is widely used. Generally, in the chromium electroplating method using a sergeant bath (Cro ₃ +H ₂ SO ₄ ), lead or a lead alloy is conventionally used as an anode. Although lead or lead alloy anodes maintain a moderate concentration of trivalent chromium ion Cr ³⁺ , lead or lead alloy components elute during use, and the rate of elution is slow.
It is extremely large, ranging from mg to several tens of mg/AH. Therefore, there are disadvantages in that eluted lead or lead alloy components have an adverse effect on chromium plating and lead chromate precipitates are formed in the bath. In this way, when lead chromate precipitation occurs, there are problems such as the need to make the electrolytic cell deeper and to periodically remove precipitates and replace the liquid in order to avoid its negative effects. In order to reduce the effects of lead, methods of using ferrite, magnetite electrodes, or platinum-plated titanium electrodes are known, but the former electrodes are extremely brittle and lack mechanical strength, so they must be handled with care. Also, due to the low conductivity of the electrode material, it cannot be used at high current densities;
The concentration of Cr ³⁺ increases, which has drawbacks such as a decrease in current efficiency and a decrease in plating quality. Currently, lead dioxide-coated electrodes are attracting attention as the most suitable electrodes for chrome plating. Unlike lead or lead alloy electrodes, lead dioxide coated electrodes elute into the electrolyte at 0.1 to 1 mg/AH during use as anodes.
or less, and there is almost no liquid contamination or precipitate formation. However, as mentioned above, this type of electrode is extremely stable and exhibits excellent performance during plating electrolysis, but when the plating is not energized or is not electrolyzed, it reacts with the electrolytic bath to form lead chromate, resulting in short-term damage. It has the disadvantage that it becomes unusable over time. To prevent this, it is conceivable to take the electrode out of the plating bath immediately after the plating is energized and wash it, or to remove the plating bath, but in reality, such operations are performed several times to several dozen times a day. The problem was that the operation was extremely complicated and difficult, and the efficiency was extremely low. [Object of the invention] The present invention was made to solve the above problems, and uses a lead dioxide electrode as an anode.
To provide a chrome plating method that does not require complicated operations and can be operated stably for a long period of time. [Means for Solving the Problems] The present invention, in a chrome plating method using a lead dioxide electrode as an anode, uses an auxiliary cathode installed in the plating tank to continue flowing a protective current to the anode even when the plating current is stopped. This is a chrome plating method characterized by the following. By doing so, the various difficulties described above are solved, and chrome plating operation can be carried out stably and without difficulty for a long period of time even when the anode is immersed in the plating tank. The present invention will be explained in detail below. The lead dioxide electrode is extremely stable as an anode, and can be used stably for a long period of time as an excellent insoluble electrode as long as electricity is continued. For example, under normal sergeant bath chrome plating conditions, the anode current density is 30A/
When dm ² , the amount of electrode coating loss is 50mg/KAH
The electrode has 10Kg of lead oxide per ^m2 ,
Assuming that 50% of that is available, the calculation is approximately
Withstands continuous use for 10,000 hours, or more than 1 year.
If the annual usage time is 1000 to 2000 hours, you can expect a lifespan of 5 to 10 years. However, in actual operations, it is unavoidable to frequently stop the electricity supply, such as when replacing the plating material or stopping at night. It will be damaged and become unusable in a short period of time. Therefore, it was discovered that the above-mentioned difficulties could be solved by providing an auxiliary cathode connected to a separate power supply in the plating bath, and by passing current through the lead dioxide electrode to continue to polarize the lead dioxide electrode even when the plating current was stopped, and the present invention was made based on the present invention. It came to this. That is, when a lead dioxide electrode is immersed in a chrome plating bath, the immersion potential is usually 1.55 to 1.65V vs NHE.
This potential corresponds to the corrosion region on the phase diagram of PbO ₂ and is the region where Pb ⁴⁺ or Pb ²⁺ is stable, so the reaction in which lead components are eluted to form lead chromate occurs. is thought to occur. On the other hand, when a current is passed through lead dioxide as an anode, its potential becomes 1.8 to 1.85V vs NHE,
This is thought to be because PbO ₂ is stable. In this way, in chrome plating that uses a lead dioxide electrode as the anode, by continuously passing a small current to the anode that does not cause chrome plating on the cathode even when non-electrolytic, the lead dioxide electrode is reliably protected and the electrolyte is It is possible to continue the next plating operation without the trouble of removing the electrode or pulling up the electrode. In addition, in general, electrodes for electrolysis are subject to accelerated wear when the power source is repeatedly turned on and off due to energization and de-energization, but the method of the present invention allows the anode to continue to be in an on-on state, resulting in only a change in current density. So,
It has been found that the acceleration of the wear caused by repeated on-off cycles is prevented, and the degree of wear of the lead dioxide electrode is reduced to about one-fifth or less compared to when the lead dioxide electrodes are cycled on-off repeatedly. Furthermore, when the chrome plating is not energized, some of the Cr ³⁺ in the plating solution is oxidized to Cr ⁶⁺ by the small current applied to the anode, so by appropriately setting the size of the auxiliary cathode to that of the anode, , in Metsuki liquid
It has the advantage of being able to control the Cr ³⁺ ion concentration. The amount of protective current applied to the lead dioxide anode is
There is no particular limitation as long as a potential of about 1.8 V vs NHE is applied, but 0.01 to 1 A/dm ² is usually sufficient. If it is less than 0.01 to 1A/dm ² , the potential may partially fall below 1.8V vs NHE depending on the shape and arrangement of the anode, and if it exceeds about 1A/dm ² , the amount of gas generated will increase. It is not desirable because it On the other hand, since the main purpose of the auxiliary cathode is to supply a protective current to the anode, the amount of current applied is not particularly limited, but it is desirable that chrome plating does not substantially form on the auxiliary cathode, and the cathode current density is is preferably 10 A/dm ² or less.
In order to control the Cr ³⁺ ions described above, it is desirable that the cathode current density be greater than that of the anode, that is, the electrode area of the anode should be smaller than that of the anode. The shape and material of the auxiliary cathode are not particularly limited, but there is a possibility that a slight amount of chromium may be deposited on the cathode, so in order to prevent damage to the cathode due to distortion of the electrodeposition, it should be made into a rod shape, a blind shape, or Reticulated metals such as Nb, Ta, Ti, and Zr are suitable. In order to energize the lead dioxide anode using the auxiliary cathode, it is sufficient to energize the anode using a separate power supply circuit at least when the plating is not energized or during non-electrolysis. However, as mentioned above, the protective current is about 0.01 to 1A/ ^dm2 , so even if the current circuit is closed and a small current is kept flowing while the plating is being energized, it will not have a big effect on the plating. It can be left unturned regardless of whether the power is turned on or off. In this way, if a method is adopted in which the protective current is constantly passed through the anode, the operation is simple and the operation can be carried out more safely. [Examples] Hereinafter, the present invention will be specifically explained with reference to Examples, but these are not intended to limit the present invention. Example 1 Chromic acid (CrO ₃ ) 220g/l, H ₂ SO ₄ 3g/l,
Corrosion tests were conducted on lead dioxide anodes using a sergeant chrome plating bath made of residual pure water. A niobium wire strand with a diameter of 1 mm was used as a cathode, and current was applied at a temperature of 50° C. at a cathode current density of 5 A/dm ² and an anode current density of 0.01 to 2 A/dm ² for 300 hours. For reference, the results are shown in Table 1, along with the case where the same lead dioxide electrode was kept immersed in the same plating bath without electricity.

[Table] From the results in Table 1, the lead dioxide electrode (reference) that was left immersed without a protective current was found to have a markedly yellowed surface, which is thought to be due to lead chromate precipitation, and to become brittle. , the weight reduction of the electrode is very large. On the other hand, if a small current is continued to flow through the anode, wear on the electrode is significantly reduced, and it is clear that the present invention provides a longer service life for lead dioxide electrodes in chrome plating. Note that no deformation was observed in the niobium Sudare cathode. However, when a niobium plate was used, some deformation was observed in which the plate warped toward the side opposite to the anode. Example 2 In the same plating bath as in Example 1, the anode current density was
Turn on the lead dioxide electrode by energizing at 30 A/dm ² for 10 minutes, turning off the power, and then de-energizing for 20 minutes.
An off test was conducted for 900 hours and the amount of electrode wear was measured. At this time, a current of 0.1 A/dm ² was continued to flow through the anode even when the power was off, and a comparison was made with a case where no current was passed. As a result, when a protective current was applied according to the present invention, the amount of loss of the electrode was 90 g/m ² , but when no current was applied, the amount of loss of the electrode reached 860 g/m ² . Example 3 Chromic acid 220g/l and sodium silicate
A corrosion test was conducted by immersing a lead dioxide electrode as an anode in a chrome plating bath consisting of 5 g/l and the balance being water, and applying a current of 0.1 A/dm ² . A niobium mesh was used as the cathode, and the effect of changing the current density was investigated. The results obtained are shown in Table 2.

〔Effect of the invention〕

(1) According to the present invention, protection of the lead dioxide electrode during non-electrolysis is ensured, and chrome plating operation can be carried out stably for a long period of time without complicated operations using the anode. (2) The production of lead chromate at the anode, which was previously unavoidable, is virtually eliminated, and deterioration of the electrode and contamination of the plating bath can be prevented. (3) It is possible to significantly reduce the consumption of the anode due to repeated energization and de-energization. (4) The Cr ³⁺ ion concentration in the plating bath can be controlled by applying a small current using the auxiliary cathode.

Claims (1)

[Scope of Claims] 1. A chrome plating method using a lead dioxide electrode as an anode, characterized in that a protective current continues to flow through the anode even when the plating current is stopped using an auxiliary cathode installed in the plating tank. . 2 The protective current flowing to the anode when electricity is stopped is
The method according to claim 1, wherein the electric current is 0.01 to 1 A/dm ² at the anode surface. 3. The method according to claim 1, wherein the auxiliary cathode is a rod-shaped or mesh-shaped metal. 4. The method according to claim 1, wherein the current density of the auxiliary cathode is 10 A/dm ² or less.