JPS63310991A - Chromium plating method - Google Patents

Abstract

PURPOSE:To enable stable Cr plating with a lead dioxide electrode as the anode over a long period without requiring troublesome operation by continuously supplying protective electric current to the anode with an auxiliary cathode placed in a plating tank even after the supply of electric current for plating is stopped. CONSTITUTION:When Cr plating is carried out with a lead dioxide electrode as the anode, an auxiliary cathode is placed in a plating tank and protective electric current is continuously supplied to the anode even after the supply of electric current for plating is stopped. At this time, cathode current density is regulated to <=about 10A/dm<2> and the quantity of the protective electric current supplied to the anode to about 0.01-1A/dm<2>. Thus, the lead dioxide electrode is surely protected during a cessation of electrolysis, so subsequent plating work is successively carried out without drawing out an electrolytic soln. or pulling up the electrode.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to a chromium plating method using a lead dioxide electrode as an anode, and particularly to a chromium plating method for protecting the anode when plating current is stopped.

[Conventional technology and problems]

Chrome plating is used in a wide range of applications because it not only has a beautiful surface, but also has hardness, good corrosion resistance, etc. For example, engine parts for automobiles, various cylinders,
It is generally used for industrial and decorative purposes, such as rolls for gravure printing, tableware, and cans for canning.

Various plating baths have been used to perform chromium plating, but typical ones include a method using a silicofluoride bath and a method using a Sargent 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 is corrosive and makes maintenance and safety of the plating equipment difficult. be.

On the other hand, although the latter method using a Sargent 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 Sargent bath (Crys+)lzsO4), lead or a lead alloy is conventionally used as an anode. Although lead or lead alloy anodes maintain a moderate concentration of trivalent chromium ions Cr, lead or lead alloy components elute during use, and the rate of elution is extremely high, ranging from several mg to several tens of mg/AH. Disadvantages include the adverse effect of leached lead or lead alloy components on chromium plating and the formation of lead chromate precipitates in the bath.In this way, in order to avoid the adverse effects when lead chromate precipitation occurs, There are also other problems, such as the need to make the electrolytic cell deeper and to periodically remove precipitates and replace the liquid.

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, making them difficult to handle. In addition, due to the low conductivity of the electrode material, it cannot be used at high current densities; furthermore, the latter electrode increases the concentration of Cr in the plating bath, resulting in a decrease in current efficiency and poor plating performance. It has drawbacks such as deterioration of quality.

Currently, lead dioxide-coated electrodes are attracting attention as the most suitable electrodes for chromium plating. Unlike lead or lead alloy electrodes, lead dioxide-coated electrodes elute into the electrolyte during use as an anode, which is extremely small at 0.1 to 1 mg/A)l or less, preventing contamination of the solution and formation of precipitates. There aren't many. However, as mentioned above, this type of electrode is extremely stable and exhibits excellent performance during plating electrolysis, but when the plating current is stopped or when no electrolysis is applied, 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. In order to prevent this, it is conceivable to immediately remove the electrode from the plating bath and wash it after the plating energization ends, or to remove the plating bath from the plating bath, but in reality, such operations are performed several to several dozen times a day. There was a problem that the process was repeated, the operation was very complicated and difficult, and the efficiency was extremely reduced.

[Purpose of the invention]

The present invention was made to solve the problem of ridges, and it is a chrome plating method that uses a lead dioxide electrode as an anode, does not require complicated operations, and can be operated stably for a long period of time. The purpose is to provide.

[Means for solving problems]

The present invention is a chromium plating method that uses a lead dioxide electrode as an anode, and is characterized in that a protective current is continued to flow through the anode even when the plating current is stopped using an auxiliary cathode installed in the plating tank. be. By doing so, the various difficulties described above are solved, and chrome plating operations can be carried out stably and without difficulty for a long period of time even when the anode is immersed in the plating bath.

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 current is continued. For example, under normal Sargent bath chrome plating conditions, the anode current density was set to 3OA/ds+" In this case, the amount of electrode coating loss is about 50 mg/KA).
Assuming that it has 10 kg of lead oxide per 1 kg and that 50 χ of it is usable, it can withstand continuous use for about 10,000 hours, that is, more than one year. And the annual usage time is 1,
000 to 2,000 hours, a lifespan of 5 to 10 years can be expected.

However, in actual operations, it is unavoidable to frequently stop the power supply, such as when replacing the plated items 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 positively polarize the electrode even when the plating current was stopped, and the present invention was made based on the present invention. reached.

That is, when a lead dioxide electrode is immersed in a chromium plating bath, the immersion potential is usually 1.55 to 1.65 V vs NHE.
This potential corresponds to the corrosion region on the phase diagram of PbO□, and since pb" or pb" is a stable region, a reaction occurs in which the lead component is eluted to form lead chromate. it is conceivable that.

On the other hand, when a current is passed through lead dioxide as an anode, the potential becomes 1.8 to 1.85 V vs NHE,
This is thought to be because Pb0z is stable.

In this way, in chromium plating using a lead dioxide electrode as the anode, the lead dioxide electrode is reliably protected by continuously passing a small current to the anode that does not cause chromium plating on the cathode even when non-electrolytic. It is possible to continue the next plating operation without the trouble of draining the electrolyte 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. This prevents the acceleration of wear due to repeated on-off cycles, and the lead dioxide electrode
It has been found that the level of wear is reduced to about one-fifth compared to when the device is repeatedly turned off.

Furthermore, when chromeme plating is not energized, a small amount of current applied to the anode oxidizes a portion of Cr in the plating solution to Cr, so by appropriately setting the size of the auxiliary cathode relative to that of the anode, There is an advantage that the Cr'' ion concentration in the plating solution can be controlled.

The amount of protective current applied to the lead dioxide anode is 1.8V.
There is no particular limitation as long as a potential of the order of vs NHE is applied, but 0.01 to IA/d+a'' is usually sufficient.

If it is less than 0.01A/dn+", the potential may partially fall below 1.8V vs. NHE depending on the shape and arrangement of the anode, and if it increases beyond about IA/dI11", the amount of gas generated will increase. undesirable as it increases

On the other hand, since the main purpose of the auxiliary cathode is to conduct 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 The density is preferably 10 A/d2 or less. In order to control the Cr'' ions mentioned above, it is desirable that the cathode current density be larger than that of the anode, that is, the electrode area of the cathode should be smaller than that of the anode.The shape and material of the auxiliary cathode are not particularly limited. However, there is a slight possibility that chromium may be plated on the cathode, and in order to prevent damage to the cathode due to distortion of the electrodeposition, Nb, Ta, or Ti in the form of rods, blinds, or nets is used.
+ Metals such as Zr are preferred.

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 plating current is stopped or when electrolysis is not performed. However, as mentioned above, the protective current is about 0.01 to IA/dm'', so even if the current circuit is closed and a small current is kept flowing during plating current, it will not have a big effect on the plating, so the protective current circuit can be done without turning off regardless of whether the plating is energized or stopped.

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.

〔Example〕

EXAMPLES Hereinafter, the present invention will be specifically explained with reference to Examples, but these are not intended to limit the present invention.

Chromic acid (CrOi) 220g/E, HzSO*-3
Corrosion tests were conducted on lead dioxide anodes using a (Sargent) chromium plating bath consisting of 50 g/l, the remainder pure water.

A niobium wire with a diameter of 1 + wm was used as a cathode, and current was applied at a temperature of 50°C with a cathode current density of 5A/d11z and an anode current density of 0.01 to 2A/d+*'' 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 1 From the results in Table 1, the lead dioxide electrode (reference) that was immersed without applying a protective current had a markedly yellowed surface, which is thought to be due to lead chromate precipitation, and was 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, the wear of the electrode is significantly reduced, and it is clear that the present invention provides a longer service life of the lead dioxide electrode in chrome plating.

It should be noted that no deformation was observed in the niobium Sudare cathode. However, when a niobium plate was used, some deformation was observed in which the niobium plate was warped in the opposite direction to the anode.

In addition, in the same plating bath as in Example 1, the anode current density was 30 A/d.
The lead dioxide electrode was subjected to an on-off test for 90 minutes, with a cycle of energizing for 10 minutes at "A", turning off the power, and stopping the current for 20 minutes.
The test was carried out for 0 hours, and the amount of electrode consumption was measured. At this time, even when the power is off, the current of O, IA/dw+" continues to flow through the anode,
Comparison was made with the case where no current was applied. As a result, when a protective current is applied according to the present invention, the amount of electrode wear is 90g 7m
t, but the amount of wear when no current is applied is 8
60g/s”.

1 ship 5-primary chromic acid 220g/l and sodium silicofluoride 5871
A lead dioxide electrode was immersed as an anode in a chromium plating bath consisting of water and water, and a corrosion test was conducted by passing a current of O, LA/da''. A niobium mesh was used as the cathode, and the current density was varied. The results were shown in Table 2.

Table 2 From the results shown in Table 2, when the cathode current density increases, it has almost no effect on the anode, but it causes an increase in weight due to cathode plating, so it is appropriate that the current density of the auxiliary cathode is 10 A/dm or less. I understand that.

〔Effect of the invention〕

(1) According to the present invention, protection of the lead dioxide electrode during non-electrolysis is ensured, and chromium plating operations can be carried out stably for a long period of time without complicated operations using the anode.

(2) The generation of lead chromate at the anode, which was conventionally unavoidable, is substantially eliminated, and deterioration of the electrode and contamination of the plating bath can be prevented.

(3) It is possible to significantly reduce consumption of the anode due to repeated plating energization/stopping.

(4) The Cr'' ion concentration in the plating bath can be controlled by applying a small current using the auxiliary cathode.

Claims (4)

[Claims] (1) A chromium plating method using a lead dioxide electrode as an anode, which is characterized by using an auxiliary cathode installed in a plating bath to continue to flow a protective current to the anode even when plating current is stopped. (2) The method according to claim (1), wherein the protective current applied to the anode when the plating current is stopped is 0.01 to 1 A/dm^2 on 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.