Prepainted GL Steel Coils

Chemical Pretreatment Conversion
Prior to painting, steel coils undergo sophisticated chemical pretreatment to create a nano-scale conversion layer optimizing paint adhesion and corrosion resistance. The 6-stage process includes: alkaline degreasing (pH 11.5, 60°C) removing rolling oils; acid pickling (H₂SO₄/HF, pH 1.5) eliminating surface oxides; rinsing; chromate-free conversion coating (zirconium/titanium fluorocomplexes forming 50-200nm amorphous layers); final rinsing; and deionized water sealing. The conversion coating chemically bonds to metallic substrates via M-O-M bonds (M=Fe, Zn, Al) while providing reactive sites for primer cross-linking. Coating weights are controlled at 5-20 mg/m² via conductivity meters and titration, ensuring complete coverage without crystalline buildup that could cause paint defects.

This nanotech surface engineering is critical for performance. Zirconium oxide layers exhibit Cr(VI)-equivalent corrosion inhibition by forming insoluble ZrO₂·nH₂O barriers and suppressing cathodic reactions. FTIR analysis confirms covalent bonding between conversion coating silanes and epoxy primer resins, increasing paint adhesion from 2B to 5B per ASTM D3359 Tape Test. Cyclic corrosion testing (GM9540P) shows pretreatment extends paint system life by 200% versus untreated controls. Environmental compliance is paramount: modern non-chromate systems reduce hazardous waste disposal costs by 90% versus chromates while meeting REACH/ELV regulations. The pretreatment’s amorphous structure also enables uniform electrodeposition of primers in CCL processes, with roughness (Ra) maintained at 0.5-1.2µm to enhance mechanical interlocking.