Electroplating is an electrochemical process as well as a REDOX process. During electroplating, the plated part is used as the cathode and the metal or insoluble material as the anode. In electroplating production, cathode process and anode process are a pair of interdependent contradictions. The cathode process is often the main aspect of the contradiction, the research work is more, but the anode process is often due to the lack of sufficient attention to plating faults.

Classification, selection and function of anodes

1.1 Classification of anodes

In electroplating production, the anode can be divided into insoluble anode and soluble anode. The role of an insoluble anode is to conduct electricity and control the distribution of current on the cathode surface. In addition, the soluble anode also has the role of adding discharge metal ions to the plating solution. Soluble anode is used in most kinds of plating and plating solution, the reason is: (1) the mass concentration of metal ions in the coating is easy to control; (2) the composition of the bath is stable; (3) the pH value of the bath is stable; (4) Low maintenance cost of bath. Some special plating solution uses insoluble anode, such as hexavalent chromium plating.

1.2 Conditions for an ideal anode

Soluble anode should have the following conditions: (1) good solubility, high limit current density; (2) The anode and cathode current efficiency are close; (3) uniform dissolution, as far as possible to avoid anode mud; (4) There should be enough purity so as not to pollute the plating solution with soluble impurities.

The insoluble anode should have good electrical conductivity and high chemical and electrochemical stability.

1.3 The role of anode

The role of the anode is reflected in the following aspects: (1) forming a closed circuit of the electrolytic cell with the cathode and plating solution, conducting current and oxidation reaction; (2) Chemical and electrochemical dissolution occurs, producing metal ions to supplement the metal ions deposited by the cathode; (3) keep the plating solution stable; (4) Change the cathode process parameters, improve the distribution of power lines, the dispersion ability of plating solution and reduce the production cost of plating.

2 Characteristics of the anode process

The anode process has different rules in different potential range. With the change of potential, the dissolution rate of the anode changes. When the critical passivation current density is reached, the anode passivation will occur and the dissolution rate will decrease. Passivation process is a special rule of anode process, which can be activated and restored to normal by adjusting the process in electroplating production.

3. Factors affecting anode process

(1) Metal nature

Some metals are easy to passivate, such as chromium, nickel, titanium and so on; Some metals are not easily passivated, such as copper, silver, etc.

(2) composition of bath

Some components in the plating solution can activate the anode and promote the anode dissolution. Some components, such as carbonates in cyanide baths, can blunt the anode.

(3) acid base

Generally, metal anodes are not easy to passivate in acid plating solution.

(4) Working conditions

Increasing the current density can accelerate the anode dissolution, but when it is greater than the critical value, it will significantly accelerate the anode passivation. At low temperature, the anode is prone to passivation. When anode area is small, it is easy to passivate.

4 Control and maintenance of the anode

(1) The calendering anode with high purity should be selected, which can not only improve the anode current density, but also achieve the uniformity of the anode dissolution, reduce the anode mud production, so that the anode is difficult to be passivated.

(2) The main surface of the plating should be parallel to the anode, and the distance between the anode and the different parts of the cathode should be reduced. If necessary, pictographic or auxiliary anode can be selected.

(3) In production, the anode layout is determined according to the cathode current density and the ratio of anode and cathode area. The current density distribution in the cathode is improved by the anode so as to keep the plating solution stable reasonably. Generally, the anode area of plating should be greater than the cathode area.

(4) The area ratio of soluble anode and insoluble anode is an effective method to ensure the normal dissolution of anode, prevent anode passivation and keep the plating solution stable.

(5) To eliminate hook and other adverse phenomena, strengthen the cleaning and maintenance of the anode plate and pocket is an important measure to ensure the normal work of the anode.

5. Prevention and analysis of anode passivation methods

(1) Master the area ratio and current density of anode and anode. This is an effective measure to prevent anode passivation and ensure the normal dissolution of anode. Only when the area ratio of anode, anode and current density are in accordance with the process specification, can the anode be dissolved normally and prevent passivation.

(2) Anode passivation can be detected by a sudden increase in the rectifier voltage. In the same tank, a sudden increase in the tank voltage indicates abnormal plating, and after excluding the bad contact of the conductive rod, the anode passivation can be inferred.

(3) Passivation can be identified from the surface condition of the anode. Such as galvanized anode zinc plate should be gray black, and white is passivation; Acid tin plating passivated into black film gray; Nickel is grayish white when normal, after passivation is reddish-brown, etc.

(4) Various electroplating processes have different temperature requirements. Too high or too low temperature has a direct effect on the anode dissolution and passivation, it should be controlled within the working range.

(5) Select high purity calendering anode, alloy anode should be selected according to the process requirements.

6 Conclusion

The behavior of anode is much more complicated than that of cathode in the process of electroplating. It is necessary to control the production process reasonably, ensure the plating bath process specification, and arrange the area and quantity of anode and cathode reasonably. According to different process requirements, using anode characteristics to stabilize the process is also an effective way to improve the quality of electroplating and reduce the failure and loss.

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