Case Study: Functions and Maintenance of Degreasing Solution in Electrophoretic Coating Pretreatment
Case Study: Functions and Maintenance of Degreasing Solution in Electrophoretic Coating
The purpose of degreasing is to remove grease and oil contaminants from workpiece surfaces, providing a clean, activated substrate for subsequent phosphating and passivation. Control of the degreasing solution directly determines the cleaning effect and the overall quality of the entire pretreatment chain. The procedure consists of several sub-steps, including hot water washing, pre-degreasing and main degreasing.
Characteristics of Two Types of Grease on Metal Surfaces
Grease adhered to metal surfaces can be divided into two categories according to their reaction with alkalis:
| Type | Description | Removal Mechanism |
|---|---|---|
| Saponifiable oils | Animal and vegetable oils that react with alkalis | Saponification |
| Non-saponifiable oils | Mineral oils: petrolatum, lubricating oil, paraffin wax | Emulsification |
Judgment Methods for Degreasing Cleanliness
- Droplet Test: apply a drop of degreaser to the surface; no residual marks after evaporation indicate thorough degreasing, while circular residues indicate incomplete oil removal.
- Wiping Test (commonly used): wipe the surface with white silk cloth or filter paper; a stain-free cloth indicates qualified degreasing.
- Water Wetting Test (most commonly used): immerse the workpiece and observe; a continuous, intact water film means no oil contamination, while droplet aggregation indicates residual grease.
- Breath Test: exhale onto the metal surface; a clean surface forms a uniform mist layer, while oily areas show no mist condensation.
Main Factors Affecting Degreasing Performance
- Concentration: alkaline content is measured by titration in "points". Insufficient alkalinity weakens saponification and emulsification; excessively high alkalinity may corrode non-ferrous metals such as aluminum and zinc and carry over into later processes.
- Temperature: proper heating accelerates saponification, reduces grease viscosity and strengthens surfactant penetration, improving efficiency. Excessive temperature causes rapid evaporation after workpieces exit the tank, leaving dried residues and floating oil that deteriorate the conversion film.
- Oil Contamination Content: the amount of emulsified and suspended grease in the solution. The upper limit is ≤1.0 g/L for spray degreasing and ≤3.0 g/L for immersion degreasing; excessive grease reduces efficiency and causes coating cratering defects.
Improvement and Maintenance Measures
- Stabilize the alkaline concentration to ensure sufficient degreasing while avoiding substrate corrosion from over-alkalinity. For aluminum and galvanized layers (amphoteric substances), neutral degreasing agents are recommended.
- In hot summer environments, closely monitor workpiece surfaces after exiting the tank to prevent solution drying and residue formation; in cold winter, heat the solution appropriately, since low temperature leaves oil incompletely removed and residual grease dissolves into the 28–35 °C electrophoretic bath, aggravating cratering.
- Conduct regular oil removal maintenance according to production output and thoroughly skim floating oil from the surface to maintain stable solution performance.
