Electrophoretic coating technology has been widely adopted in China's coating industry. The characteristic of electrophoretic paint lies in its electrodeposition coating process. Although different types of coatings and construction process conditions will lead to different results, generally speaking, compared with the immersion coating method, it has the following characteristics:

(1) The quality of the film obtained by electrodeposition is roughly proportional to the amount of electricity passed, so the coating deposition amount can be adjusted by increasing or decreasing the amount of electricity.

(2) Electrodeposition coating can obtain relatively uniform films on sharp edges or corners of complex-shaped objects, welding seams, and both inner and outer surfaces of box-shaped bodies, significantly improving anti-corrosion performance.

(3) The water content of the electrodeposited film is very low before drying. It is insoluble in water, non-flowing, and less prone to film defects such as drips, runs, and stagnation marks. It also avoids the solvent gas washing phenomenon often occurring in the drying process of immersion-coated films (inside box-shaped or pipe-shaped parts) and can significantly shorten the pre-drying time for water evaporation.

(4) Due to the directional deposition of negatively charged polymer particles under the electric field, the electrodeposited film has excellent water resistance and higher adhesion than other construction methods.

(5) The paint solution used in electrodeposition coating has low concentration and viscosity, so less paint is carried out by the object due to immersion. Especially after the application of ultrafiltration technology in electrodeposition coating, the paint utilization rate can even approach 100%.

(6) Similar to the construction of general water-based paints, electrodeposition coating eliminates the risks of fire and benzene poisoning.

Although electrophoretic coating has obvious advantages over other coating methods, due to the uniqueness of the electrophoretic coating method, the causes and prevention methods of film defects, though similar to those of general film defects, are different, and some defects are unique to electrophoretic coating. The following introduces common film defects in electrophoretic coating, their causes, and prevention methods:

1. Particles

Particles are hard particles that are rough to the touch (or visible to the naked eye) on the surface of the electrophoretic film after drying, which is the most prone defect in the coating process.

Causes:

(1) Precipitates, aggregates, or other foreign matters in the electrophoretic bath solution, with poor filtration.

(2) Dirty post-electrophoresis rinsing solution or excessively high paint concentration in the rinsing water.

(3) Unclean workpieces entering the electrophoretic bath, incomplete washing after phosphating.

(4) Dirty environment in the coating production area and many contaminants in the drying oven.

Prevention Methods:

(1) Reduce dust intake, strengthen filtration of the electrophoretic bath solution. All circulating paint solution should be fully filtered through a 25μm precision filter bag. Strengthen stirring to prevent precipitation, eliminate "dead corners" and exposed metal parts in the tank, and strictly control pH value and alkaline substances to prevent resin precipitation and aggregation.

(2) Improve the cleanliness of post-rinsing water, keep the solid content of post-electrophoresis rinsing water as low as possible, and maintain overflow replenishment from the rear tank to the front tank. The cleaning solution should be filtered to reduce foam.

(3) Clean the drying oven, check the air filter, and inspect the balance system and air leakage.

(4) Strengthen washing after phosphating to remove phosphating residues on the workpiece surface. Check whether the filter of the deionized water circulation washing tank is blocked to prevent secondary pollution of the coated object surface.

(5) Keep the coating production area clean. Drain thoroughly between phosphating and electrophoretic bath, as well as after electrophoresis (before entering the drying oven), and check and eliminate air dust sources.

2. Craters (Sinks)

Craters are volcanic 口 - shaped pits with a diameter of usually 0.5–3.0mm, caused by dust, oil, etc., adhering to the surface of the coated object, phosphating film, or wet electrophoretic film, or mixing incompatible particles with the electrophoretic coating in the film, which become the center of craters and cause uneven flowability at the initial stage of drying. Those exposing the substrate are called craters, and those not exposing the substrate are called sinks.

Causes:

(1) Foreign matters (oil, dust) mixed in the bath solution, with oil floating on the surface or emulsified in the bath solution.

(2) Coated workpieces contaminated by foreign matters (such as dust, lubricating oil falling from the conveyor chain, oily iron powder, topcoat dust, oil in the compressed air for drying).

(3) Poor degreasing in pretreatment, with oil on the phosphating film.

(4) Foreign matters (oil, dust) mixed in the rinsing solution after electrophoresis; poor purity of pure water.

(5) Unclean drying oven or oil in the circulating air.

(6) Imbalance of pigment-to-binder ratio in the bath solution.

(7) Poor dissolution of replenished paint or resin (insoluble particles).

Prevention Methods:

(1) Install oil-removing filter bags in the bath solution circulation system to remove contaminants.

(2) Keep the coating environment clean. The conveyor chain and fixtures should be clean, and the compressed air used should be oil-free to prevent dust, topcoat mist, and oil from falling on the coated workpieces. Coated workpieces with oil and dust are not allowed to enter the electrophoretic bath; set up partitions.

(3) Strengthen the degreasing process in pretreatment to ensure no pollution on the phosphating film.

(4) Maintain the water quality of post-electrophoresis rinsing, strengthen filtration of the rinsing solution, and set up a dust-proof corridor from rinsing to the drying oven.

(5) Keep the drying oven and circulating hot air clean, and avoid too rapid initial heating.

(6) Maintain the correct pigment-to-binder ratio and solvent content in the electrophoretic bath.

(7) Stir thoroughly when adding new paint to ensure good dissolution and neutralization, and filter it.

3. Pinholes

Pinholes refer to needle-like small pits on the film, which differ from craters in that the latter generally have foreign matters as the core in the center of the pit, with the surrounding film 堆积凸起 (accumulated and raised).

Causes:

(1) Redissolution pinholes: The wet film coated on the workpiece surface is redissolved due to delayed post-electrophoresis rinsing, resulting in pinholes.

(2) Gas pinholes: Excessive bubbles generated by intense electrolysis during electrophoresis with poor bubble release; pinholes appear due to the rupture of film bubbles during drying caused by too low bath solution temperature or insufficient stirring.

(3) Step-like pinholes during charged tank entry: Occur in severe cases of step defects during charged tank entry, with pinholes exposing the substrate along the tank entry diagonal; additionally, bubble pinholes are formed when bubbles are trapped in the film due to poor wetting of the object surface by the bath solution during charged tank entry, or foam on the bath solution surface adheres to the workpiece surface, prone to occur at the lower part of the coated workpiece.

Prevention Methods:

(1) Immediately rinse the workpiece surface with ultrafiltration (UF) solution (or pure water) after film formation to eliminate redissolution pinholes.

(2) Control the concentration of impurity ions in the paint solution during electrophoretic coating, regularly test the content of various ions in the tank, and discharge ultrafiltrate if exceeding the standard; also control the polar solution within the specification. Pinholes are prone to occur when the phosphating film has high porosity, so the process-specified temperature (generally 28–30°C for cathodic electrophoresis) should be observed.

(3) To eliminate step-like pinholes during charged tank entry, ensure the flow rate of the bath solution surface is greater than 0.2m/s to remove accumulated foam; prevent too low conveyor chain speed during charged tank entry.

(4) To eliminate washing pinholes, first ensure good electroosmosis of the film, control the solvent content (not too high) and impurity ion content in the tank, and obtain a dense film. The rinsing water pressure should not exceed 0.15MPa.

4. Thin Film

The film thickness on the workpiece surface after coating and drying is lower than the process-specified thickness.

Causes:

(1) Too low solid content in the bath solution.

(2) Too low voltage or too short coating time in the electrophoretic bath.

(3) Bath solution temperature lower than the process-specified range.

(4) Too low organic solvent content in the bath solution.

(5) Aging of the bath solution, resulting in too high wet film resistance and low bath solution conductivity.

(6) Poor contact or loss of the plate, low anolyte conductivity.

(7) Too long UF solution rinsing time after electrophoresis, causing redissolution.

(8) Too low pH value of the bath solution.

Prevention Methods:

(1) Maintain the solid content within the process-specified range, with fluctuations preferably controlled within ±0.5%.

(2) Adjust the coating voltage and time to appropriate ranges.

(3) Regularly clean the heat exchanger, check for blockages, and ensure the heating system and temperature indicators are in good condition; control the bath solution temperature within or at the upper limit of the process-specified range.

(4) Add organic solvent regulators to bring the content to the process-specified range.

(5) Accelerate bath solution renewal or add regulators to improve bath solution conductivity and reduce wet film resistance.

(6) Check whether the plates are damaged (corroded) or scaled, regularly clean and replace the plates, improve anolyte conductivity, and ensure good power supply to the coated workpieces and clean fixtures without paint accumulation.

(7) Shorten the UF solution rinsing time to prevent redissolution.

(8) Add coatings with low neutralization degree to bring the bath solution pH to the process-specified range.

5. Thick Film

The film thickness on the workpiece surface after coating and drying exceeds the process-specified thickness.

Causes:

(1) Too high solid content in the bath solution.

(2) Bath solution temperature higher than the process-specified range.

(3) Too high voltage during coating in the electrophoretic bath.

(4) Too long coating time in the electrophoretic bath (such as temporary production interruptions).

(5) Too high organic solvent content in the bath solution.

(6) High bath solution conductivity.

(7) Poor circulation around the workpiece.

(8) Improper cathode-anode ratio or anode position distribution.

Prevention Methods:

(1) Adjust the voltage to the process-required range during electrophoretic coating.

(2) Never exceed the process-specified bath solution temperature, especially for cathodic electrophoretic paints, as too high temperature will affect bath solution stability; maintain the bath solution temperature at the lower limit of the process-specified range.

(3) Keep the solid content within the process-specified range. Too high solid content not only causes thick film but also more paint 带出 (carried out) from the surface, increasing the difficulty of subsequent rinsing.

(4) Control the coating time within an appropriate range and avoid interruptions as much as possible in continuous production.

(5) Control the organic solvent content in the bath solution, discharge ultrafiltrate, add deionized water, and prolong the full dissolution time of the newly prepared bath solution.

(6) Timely repair pumps, filters, and nozzles if blocked, causing poor circulation around the workpiece.

(7) Discharge ultrafiltrate, add deionized water to reduce impurity ion content in the bath solution.

(8) Adjust the cathode-anode ratio and anode distribution positions.

6. Water Drop Marks on Workpiece Surface

After drying, there are uneven water drop marks on the electrophoretic film, caused by water drops on the film surface boiling during drying.

Causes:

(1) Water drops on the electrophoretic film surface before drying, with attached water drops after washing not dried (too high humidity in the production area) or blown off.

(2) Accumulated washing solution on the workpiece surface after electrophoresis washing.

(3) Water drops dripping from fixtures before drying.

(4) Insufficient final pure water washing volume.

(5) Poor water drop resistance of the undried electrophoretic film.

(6) Too rapid temperature rise after entering the drying oven, causing water drops to evaporate rapidly.

Prevention Methods:

(1) Blow off water drops before drying, and adjust the production area temperature to 30–40°C.

(2) Blow off water accumulated on the vehicle body and fixtures simultaneously.

(3) Blow off accumulated clean water, or open process holes or change the hanging method to solve water accumulation on the coated workpiece.

(4) Provide sufficient pure water.

(5) Adjust process parameters or coating composition to improve the water drop resistance of the wet film.

(6) Avoid too rapid temperature rise when entering the drying oven, or add preheating (60–100°C, 10min) to prevent water drops from boiling rapidly at high temperatures and leaving marks.

7. Abnormal Film Adhesion

Uneven conductivity of the coated object surface or phosphating film leads to concentrated current density in low-resistance areas during electrophoretic coating, causing film accumulation in these areas.

Causes:

(1) Uneven conductivity of the coated workpiece surface, leading to excessively high local current density:

① Pollution of the phosphating film (fingerprints, spot marks, pickling residues);

② Contamination of the workpiece surface (yellow rust, cleaning agents, welding flux);

③ Abnormal pretreatment process: poor degreasing, insufficient washing, residual degreasing solution and phosphating solution; blue or yellow rust spots on the phosphating film.

(2) Contamination of impurity ions in the tank, too high conductivity, too low organic solvent content or ash content in the bath solution.

(3) Too high coating voltage and bath solution temperature, causing film damage.

Prevention Methods:

(1) Strictly control the surface quality of the coated workpiece to be free of rust and residual welding flux.

(2) Strictly control the content of impurity ions in the bath solution to prevent contamination. Discharge ultrafiltrate and add deionized water to control impurity ion content. Add color paste if the ash content is too low.

(3) Do not exceed the process-specified coating voltage during normal production, especially control the workpiece tank entry voltage, reduce the bath solution temperature, and avoid too short electrode spacing.