How to Select Anode Cells/ Anode Tubes in Electrocoating

The anode system refers to the set of devices in the electrophoretic bath that serve as the positive electrode (anode), primarily consisting of three types: anode cells, anode frames, and bare anodes.

Composition: Anode plates/rods, anode covers, circulation system, and power connections.

Main Functions:

• Completes the electrical circuit and provides the electric field.
• Adjusts bath liquid parameters.
• Protects the bath liquid from contamination.

Calculation of Anode Area and Selection of Anode Type:

The anode area is not directly related to the size of the electrophoretic bath but correlates with the cathode area (the coated object), typically at a cathode-to-anode area ratio of 4:1 to 6:1.

• For plate anodes, the area of a single anode plate is calculated as: S = d × h.
• For tubular anodes, the lateral area of a single anode cell is: S_side = π × d × h.

From the formula and diagrams, it is evident that, occupying the same space, the anode area of a tubular anode is theoretically π times (approximately 3 times) that of a plate anode.

How to Choose the Anode Type?

Anode cells typically use 1.9-inch (48mm) diameter 316 seamless stainless steel tubes as the anode, with an anode area of 0.15 m²/m per unit length. They are equipped with a 2.65-inch (67mm) diameter anion exchange membrane cover, with a membrane area of 0.21 m²/m per unit length. The anode can withstand a current density of 50 amps per square foot, with an anolyte flow rate of 1.12 L/min/m (equivalent to 7.5 L/min/m² per unit anode area). The anode area is recommended to be calculated based on the electrode area of the stainless steel tube (minimum area). The advantages and disadvantages are as follows:

Advantages:

1. Uniform electric field strength and high current density.
2. Controllable installation and arrangement, easy to relocate (adjusting anode cell density allows better control of film deposition rate and wetting time).
3. High mechanical strength of the outer tube, resistant to external damage.
4. Relatively simple maintenance and convenient replacement during continuous production.

Disadvantages:

1. High initial cost and significant investment.
2. Anode membranes have specific water quality requirements.

The appropriate anode system should be selected based on site-specific conditions, but the use of bare anodes is not recommended.

Inspection and Maintenance of the Anode System:

1. Check for Anode Membrane Leaks or Blockages: Leaks cause the anolyte to become turbid and discolored (matching the bath liquid color), while blockages reduce membrane permeability, leading to uneven electric field distribution.
2. Inspect Anodes (Stainless Steel/Plates) for Corrosion or Rust: Severe corrosion causes the anolyte to darken (typically yellow or reddish-brown), resulting in significant film thickness variations, reduced throw power, and defects like pinholes.
3. Check for Deformation or Damage to Tubes/Frames and Anolyte Circulation Flow: Ensure no abnormalities in flow rate and verify that all circuit connections are functioning properly.
4. Check for Bacterial Growth in the Anode System: Bacterial growth causes the anolyte to develop an odor, with bacterial clusters forming suspended matter that adheres to the system, leading to membrane blockages and reduced flow. Use hydrogen peroxide or Kathon for sterilization (with proper safety precautions).
5. During Production Downtime: Drain the anolyte, fill with pure water to keep the anode membrane wet, preventing anolyte leakage into the electrophoretic bath and membrane drying or blockages.
6. Daily Monitoring: Effectively monitor the appearance, conductivity, and pH of the anolyte during regular production.