Titanium anodes for electrophoretic coating

Titanium anodes for electrophoretic coating

Titanium anodes are commonly used in electrophoretic coating processes due to their excellent resistance to corrosion and high stability in acidic environments. They are preferred over other types of anodes because they do not contaminate the coating solution and have a long service life.

During electrophoretic coating, a voltage is applied to the titanium anode, which attracts the negatively charged paint particles towards the positively charged workpiece being coated. The result is a uniform coating that adheres well to the surface.

Titanium anodes can be coated with various materials to further enhance their properties. For example, Iridium-Tantalum
coated titanium anodes have improved catalytic activity, making them more efficient in the electrophoretic coating process.

Specification of titanium anode

Automotive coating technology: Titanium anode systems in cathodic electrophoresis

Cathodic electrophoresis is a coating process commonly used in car body manufacturing, and it is the main means to improve the service life (corrosion resistance) of the car body. In cathodic electrophoresis, the electrophoretic paint used is cationic (positively charged) resin and pigment paste, and the coated object (body) is used as the cathode. The initial reaction at the cathode is the electrolysis of water to form hydrogen and hydroxide ions (OH-), this reaction results in an over-based interfacial layer on the cathode surface, allowing positively charged particles to agglomerate on the cathode, cations (resins and pigments) ) reacts with hydroxide ions (OH-) to produce coating deposition.

Organic acids are continuously produced in the anode zone:

If these acids are not removed in time, they will enter the bath to reduce the pH value, affect the stability of the process parameters, affect the penetrability and coating performance, and increase the resolubility. There are two ways to remove free acid from the bath: adding an unneutralized or partially neutralized cathodic electrocoat and using an anode diaphragm system. The anode diaphragm system method is generally used, especially in large cathodic electrophoretic coating lines.

Composition of the anode system

The anode system consists of two parts: an anolyte system and an anode power supply. The anolyte system is the main part of the anode system. It consists of an anode diaphragm system, an anolyte round-trip circulation pipeline, a pump, an anolyte tank, a conductivity, and turbidity controller, a Deionized water supply pipeline, and other components.

The anode diaphragm system is to seal the anode on a washable anode cover. The pole cover is made of non-conductive material. The open side (the side of the plate electrode cover facing the object to be coated. The surrounding area of the tubular electrode can be regarded as an open surface) is equipped with Ion selective membrane. Anodes are available in the form of plate type, tubular type, arc type anode, etc. Among them, the tubular type anode is currently the most widely used, and the following highlights:

The tubular anode unit is composed of an anode cover (including an ion exchange membrane) and an anode and is an alternative to the plate anode in the past. Its structure is compact, the membrane resistance is small, the power consumption is low, the swimming force is high, the unit membrane area is large, the service life is long, and the maintenance and management are convenient. Due to its small size and lightweight, it can be installed on the side, bottom, or upper part of the electrophoresis tank to meet the requirements of different coating processes. The anode electrode material is generally made of 316L stainless steel, and the thickness is preferably not less than 3.2mm. Under normal circumstances, the anode dissolves slowly, and its life depends on the productivity of the electrophoresis tank.

FAQ

1. What is the standard content of ruthenium or iridium coating on titanium anodes used for electrophoretic coating?

• A mixture of ruthenium and iridium with 6g/m² of iridium and 14g/m² of ruthenium. This ratio may be adjusted based on specific solution conditions and usage requirements

2. What is the expected service life of titanium anodes with ruthenium oxide coating for electrophoretic coating?

•  It is typically 24 months, with a voltage of 300V and a maximum current density of 50A/m².

3. When the titanium anode coating is depleted, how can it be determined?

• It can be determined by observing a rapid increase in voltage within a short period of time. The decrease in current may also occur. There is a possibility of corrosion on the titanium substrate.

4. What tools are used to test the titanium anode coating?

• It can be evaluated and tested using X-ray fluorescence spectrometer (XRF) tools.

5.  After the coating is completely depleted, how should the titanium substrate be handled? Should it be sent back for re-coating?

• It can be re-coating when the titanium substrate is not corroded. 

6. How can the depletion of the coating be slowed down?

• It is recommended to maintain a stable current and voltage state for the titanium anode, without interruptions. High current/voltage or unstable current/voltage can accelerate the depletion of the coating.

7.  Considering the pH value of the anode solution and the ppm of fluorine and chlorine being around 2, is it appropriate to use titanium anodes with such a solution?

• The presence of fluoride ions can cause corrosion on the titanium substrate. Titanium metal is not resistant to corrosion by hydrofluoric acid. If the titanium substrate is corroded, it may not be suitable for re-coating after the coating is depleted.