Linyi Dingsheng Powder Coating Co., Ltd.
Why Pretreatment is the Critical Step for Thermoset Electrostatic Powder Coating
Release time:
2025-12-16
Why Pretreatment is the Critical Step for Thermoset Electrostatic Powder Coating
Pretreatment plays a pivotal role in the electrostatic powder coating process, directly determining the final
coating's quality and performance. Here’s a detailed analysis of its importance:
Core Functions of Pretreatment
1.Fundamental Cleaning & Surface Activation: Pretreatment removes oils, rust, and contaminants from
the substrate surface through degreasing and derusting, providing a clean base. Studies show surface oil
residue can reduce coating adhesion by 30%-50%.
2.Enhanced Coating Adhesion: Phosphating creates a microcrystalline layer on the metal surface,
significantly improving the bond between the coating and substrate. Experimental data indicates adhesion
can increase by 2-3 times on phosphated parts.
3.Ensuring Coating Uniformity: The surface profile after pretreatment directly affects powder electrostatic
attraction. Improper pretreatment leads to uneven film thickness and defects like "orange peel."
Standard Pretreatment Process Flow
Degreasing: Uses alkaline cleaners (e.g., sodium hydroxide solution) at 50-70°C for 5-15 minutes to remove
oils.
Rinsing: Employs multi-stage counter-current rinses (typically 3-4 stages) to remove chemical residues,
controlling conductivity to <50 μS/cm.
Derusting: Removes rust via acid pickling (e.g., hydrochloric or sulfuric acid) or abrasive blasting.
Acid-pickled parts require thorough neutralization.
Phosphating: Zinc or iron phosphating forms a 3-5 μm conversion coating, dramatically improving
adhesion.
Final Rinse: Uses deionized water (resistivity >1 MΩ·cm) to prevent water spotting.Severe Consequences of
Improper Pretreatment
Coating Defects: Incomplete degreasing causes craters (incidence 15%-20%); incomplete derusting leads to
blistering and pinholes.
Adhesion Failure: Poor pretreatment can reduce adhesion by over 60%, making coatings prone to peeling
under stress.
Reduced Durability: Salt spray tests show corrosion resistance plummets from 1000+ hours to under 200
hours with poor pretreatment.
Rework Costs: Rework rates due to pretreatment issues can reach 5%-8%, with potential for complete
batch rejection.
Best Practice Recommendations
Strict Process Control: Monitor and record parameters (temp, time, concentration) in real-time; establish
Standard Operating Procedures (SOPs).
Equipment Maintenance: Clean pretreatment tanks regularly (recommended quarterly); replace filter
systems when differential pressure >0.3 MPa.
Quality Inspection: Perform cross-hatch adhesion tests per batch (ISO rating ≥4B is pass) and film
thickness checks (60±10 μm).
Environmental Control: Maintain pretreatment area temperature at 15-30°C and relative humidity <70%
to prevent recontamination.
Conclusion
: As the "critical step" in electrostatic powder coating, the importance of pretreatment
cannot be overstated. Only a well-established pretreatment system ensures coatings meet the desired
protective and decorative performance.
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E-mail：sales@dspowdercoating.com
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