Painting Technology: Automotive Pre-Treatment Process

Ran Minghao is a pretreatment and electrodeposition process engineer at the Lingang Base Paint Shop of SAIC Motor's passenger vehicle division. He was awarded the "Second Prize for Rationalization Suggestions" and honored with the title of "Star of Lingang."

Pretreatment is the first process in the paint shop of an automotive assembly plant. A proper pretreatment process not only ensures excellent corrosion resistance of the vehicle body but also provides a high-quality substrate surface for subsequent painting. From a functional perspective, the pretreatment process can be divided into two parts: "cleaning the metal body" and "phosphating the metal body." "Cleaning the metal body" includes removing stamping oils, rust-preventive oils, and various other oils from the panel surface, as well as welding slag, welding spatter, metal grinding debris, and other contaminants remaining inside the body. The phosphating process includes surface conditioning before phosphating and passivation after phosphating.

1. Degreasing

Degreasing utilizes saponification and the roll-up mechanism of surfactants to remove various oils from the body-in-white surface. The degreasing process in the paint shop consists of two stages: spray degreasing and immersion degreasing. In the spray degreasing stage, oil on the exterior surface of the body-in-white is removed by spraying from four directions (up, down, left, and right). In the immersion degreasing stage, the vehicle is completely submerged in the degreasing solution to dissolve and remove oil from the body cavity, including sheet metal gaps. The key control points of the degreasing process are as follows.

1) Degreasing temperature: Maintaining an appropriate temperature of the degreasing solution promotes saponification reactions and oil dissolution. The degreasing temperature is (55 plus or minus 5) degrees C.

2) Degreasing time: The immersion degreasing tank is a rectangular tank with a volume of 225 cubic meters. The time the vehicle spends passing through the immersion degreasing tank via the swing chain is approximately 4 minutes.

3) Chemical composition of the degreasing solution: To remove various types and properties of oils from the panel surface and ensure good degreasing effectiveness, the degreasing solution is composed of multiple components. Inorganic components include alkalis, silicates, phosphates, borates, and carbonates. Organic components mainly include defoamers, corrosion inhibitors, chelating agents, and surfactants. The condition of the degreasing solution is monitored by testing the total alkalinity parameter (8 to 10) and the oil content of the tank solution. Regular solution transfer is conducted to maintain a certain level of solution renewal.

2. Surface Conditioning

The main component of the surface conditioner is titanium phosphate, which can improve the roughening effect caused by alkaline cleaning of the metal surface, resulting in finer phosphating crystals. Since the powder surface conditioning system does not have liquid filtration equipment such as filter tanks, special attention must be paid to the smoothness of the circulation pipeline of the surface conditioning tank to prevent concentration inconsistencies caused by poor solution circulation. Additionally, the spray pressure of the surface conditioning stage should not be too high, as excessive spray pressure may locally disperse the titanium phosphate colloid adsorbed on the body surface, leading to uneven phosphating film formation.

3. Phosphating

The phosphate film is a layer of inorganic salt crystals between the metal substrate (metal) and the electrodeposition paint (organic matter), playing a crucial bridging role. The specific functions of the phosphate film include: improving the rust and corrosion resistance of the substrate; enhancing the adhesion between the metal and the coating; and improving the electrical insulation of the vehicle body. The morphology of the phosphate film is an important criterion for evaluating its quality. A good phosphate film should be closed and dense, uniform and fine, free of residual sludge, and free of secondary phosphating. The key control points of the phosphating process are as follows.

1) Phosphating temperature: Tank solution temperature is a critical process parameter in phosphating. If the phosphating temperature is too low, the phosphate film will be too thin and the corrosion resistance will be weakened. If the phosphating temperature is too high, it will lead to excessive sludge formation in the tank solution.

2) Phosphating sludge removal: As the phosphating reaction proceeds, the byproduct of phosphate sludge is continuously generated. If not removed in time, it may cause surface roughness on the electrocoated vehicle (fine phosphate sludge depositing on the body surface). In addition, excessive sludge content in the tank solution may also cause blockage of circulation pipelines, laminar nozzles, and plate heat exchangers.

3) Phosphating accelerator: The function of the accelerator is briefly described as depolarization of hydrogen gas, preventing the formation of hydrogen bubbles on the workpiece surface that would hinder phosphate film formation, thereby making the phosphate film uniform and dense.

4) Chemical parameters of the phosphating solution: The composition of the phosphating solution is very complex, including nearly 10 types of cations and anions. In daily production, the condition of the phosphating solution is mainly monitored by testing parameters such as total acid, free acid, accelerator points, and sludge content of the tank solution, and dosing is performed based on the test results.

4. Passivation

The main component of the passivation solution is hexavalent chromate solution. On one hand, it can dissolve the peak areas of the phosphate film, reduce the porosity of the phosphate film, and make the surface properties of the phosphate film more uniform. On the other hand, it removes calcium and magnesium ions brought in by water rinsing and provides a slightly acidic surface for the subsequent electrodeposition paint, thereby improving the corrosion resistance and adhesion of the paint film.

5. Water Rinsing

The functions of water rinsing include cleaning iron filings and welding slag from the vehicle body, as well as removing residual degreasing solution, phosphating solution, and passivation solution. The pretreatment line in this shop has a total of 6 water rinsing stages: hot water rinse, first and second water rinse after degreasing, and third and fourth water rinse after phosphating followed by pure water rinse. Among them, the hot water rinse, first water rinse, and third water rinse are spray tanks, while the second water rinse, fourth water rinse, and pure water rinse are immersion tanks.

Improving the water rinsing effect is mainly approached from two aspects: first, how to wash off the contaminant particles from the body-in-white; second, how to remove the washed-off contaminant particles from the tank solution. For the former, the key is controlling the impact force of the spray water flow. To achieve good rinsing results, it is necessary not only to ensure adequate spray pressure and water flow rate but also to focus on the distance between the nozzle and the body surface, as the impact force of the spray decreases rapidly with increasing distance. For removing contaminant particles from the tank solution, various methods can be used, including liquid filter bags, magnetic separators, hydrocyclones, and paper belt filters. Additionally, appropriate slag and debris removal methods should be selected based on the characteristics of each water rinsing stage. For example, for the first water rinsing stage in pretreatment, the hot water rinse, the particle content in the tank solution is much higher than in other tanks. If only the method of increasing the accuracy and replacement frequency of liquid filter bags is used, it would significantly increase production costs. Therefore, we modified the hot water rinse tank by adding a combined filter and a settling basin. The honeycomb-shaped deflector in the combined filter allows particles such as welding slag and spatter in the tank solution to settle to the cone bottom. In addition, 20 magnetic rods are installed in the combined filter to adsorb metal debris. By regularly discharging the wastewater from the cone bottom and cleaning the magnetic rods, the cleanliness of the tank solution can be achieved without consuming liquid filter bags. The settling basin uses the principle of static settling to deposit large particles at the bottom of the tank, which can then be cleaned during the weekly solution transfer, providing a simple and efficient method for removing settled contaminants.

For details, please refer to "Modern Paint and Finishing," Issue 5, 2015.