Coating a surface without removing oil, rust, cutting residue, weld contamination or other soils may appear acceptable in the short term. Over time, however, blistering, peeling, under-film corrosion and surface defects can develop.
The powder coating process therefore begins at the surface treatment line—not inside the spray booth.
What is the primary purpose of surface treatment?
Surface treatment before powder coating has two main objectives:
- Help the coating adhere to the metal more strongly and uniformly.
- Improve the metal’s corrosion resistance together with the coating system.
When the metal is scratched or the coating is damaged, the exposed area may begin to corrode. A correctly applied pretreatment layer helps limit corrosion from spreading beneath and lifting the surrounding coating.
Actual performance varies with the substrate, service environment, coating system, process control and the customer’s technical requirements.
Which stages make up a surface treatment process?
Industrial lines vary by facility, but the basic process generally includes:
- Degreasing and surface cleaning
- Rinsing
- Rust removal, where required
- Conversion coating
- Final rinse or suitable post-treatment
- Drying
- Powder application and curing
The process may use spray or immersion. Whatever the method, the objective remains the same: create a clean, uniform metal surface ready for coating.
Why is degreasing critical?
Metal parts may carry process oils, drawing compounds, protective films, fingerprints and other organic contamination. Pretreatment and coating cannot perform as intended until these residues are removed.
Degreasing systems are broadly classified as alkaline, neutral or acidic. Selection should consider the metal, type of soil, part geometry, weld or laser-cut residue, application method, process temperature and the subsequent coating stage.
Cleaning performance is not determined by chemistry alone. A successful process combines the right system and concentration with sufficient contact time, controlled temperature and mechanical action.
Why must rinse stages be controlled?
Chemicals and loosened soils remaining after degreasing must be removed. If rinsing is inadequate, they are carried into the next bath.
This carry-over can disturb the conversion bath, cause staining, increase chemical consumption and harm coating adhesion. Rinse replenishment, pH and conductivity should be monitored regularly.
How should a conversion coating be selected?
The cleaned metal is prepared with a conversion coating suited to its service conditions and performance target. Iron phosphate, zinc phosphate and zirconium-based thin-film systems are widely used in industry.
Iron phosphate
Iron phosphate is widely used for general industrial surface preparation. Its relatively simple process structure is an important advantage. With a suitable coating system and proper control, it can deliver effective results where moderate corrosion performance is required.
Zinc phosphate
Zinc phosphate is a crystalline conversion coating used where high corrosion resistance is required. The process generally involves more stages, activation control and regular sludge management. Balanced bath parameters are essential for a uniform crystal structure.
Zirconium-based thin-film coatings
Zirconium-based thin-film systems are increasingly considered for modern lines because of their potential to operate at lower temperatures and generate less sludge. They require excellent cleaning: oil, rust or soil left on the metal can prevent the thin film from reacting with the surface.
Can the same process be used for every metal?
No. Steel, galvanised steel and aluminium do not behave the same chemically. Mixed-metal lines require a process compatible with every substrate being treated.
The right process is not simply the one with the highest performance. It is the one that delivers the required performance with sustainable cost and stable production conditions.
Common problems on surface treatment lines
- Insufficient degreasing or unsuitable bath temperature
- Incorrect or unstable concentration and contact time
- Blocked or misaligned spray nozzles
- High rinse contamination and carry-over between baths
- Uncontrolled pH and conductivity
- Inadequate drying or incompatibility between pretreatment and coating
Bath analysis alone is therefore not enough. Part condition, line mechanics, spray pressure, time, temperature and rinse quality must be evaluated together.
CONCLUSION
A quality coating starts with a properly prepared surface.
Powder coating quality is directly linked to the stability of the surface treatment line. Even a high-quality powder coating cannot deliver the expected performance on a surface that has not been properly cleaned and prepared with a suitable conversion coating.
Ferovant treats powder coating and surface treatment not as separate products, but as parts of the same performance chain.
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