1. Core goals of pre-treatment
Remove impurities: thoroughly remove grease, rust, scale, dust, etc., to prevent impurities from causing blistering and shedding of the coating.
Surface activation: Increase the surface roughness of the workpiece by chemical or physical methods (such as the porous structure of the phosphating film) to improve the adhesion of the coating.
Anti-corrosion pre-treatment: Form a conversion film to enhance the corrosion resistance of the workpiece itself and extend the overall service life of the coating.
2. Select pre-treatment steps according to the material of the workpiece
The surface properties of different materials (steel, aluminum alloy, zinc alloy, etc.) vary greatly, and the pre-treatment steps need to be designed specifically:
1. Steel parts (most common)
The surface of steel is prone to rust and often carries rolling oil, cutting oil and other grease. The pre-treatment needs to take into account degreasing, rust removal and conversion film treatment.
Typical steps:
Pre-degreasing: Remove a large amount of floating oil (using spraying or immersion, using alkaline degreasing agent, temperature 50-70℃).
Main degreasing: Deeply remove residual grease (combination of spraying + immersion to enhance the degreasing effect).
Water washing 1: Rinse the residual degreasing agent (flowing clean water to avoid acid and alkali residues affecting subsequent steps).
Rust removal (if rusted): Use hydrochloric acid, sulfuric acid or organic acid (such as citric acid) to remove scale and rust (immerse or spray, control the time to avoid over-corrosion).
Water washing 2: Rinse the rust remover to prevent acid residue.
Surface adjustment: Adjust the surface activity through colloidal titanium phosphate solution to make the subsequent phosphating film more uniform and fine (immerse for 1-3 minutes, room temperature).
Phosphating: Form a phosphate conversion film (common zinc series, zinc-manganese series), improve adhesion and corrosion resistance (immerse or spray, temperature 30-50℃, film weight 1-5g/m²).
Water washing 3: Remove residual phosphating solution.
Pure water washing: Rinse with deionized water to avoid calcium and magnesium ions affecting the gloss of the coating (conductivity needs to be ≤50μS/cm).
Drying: Remove surface moisture (temperature 80-120℃, ensure thorough drying to avoid blistering of the coating).
Note: If the workpiece is free of rust (such as cold-rolled steel plate), the "rust removal" step can be omitted; for scenes with extremely high requirements for corrosion resistance (such as automotive parts), phosphorus-free conversion treatment can be used (environmentally friendly, replacing traditional phosphating).
2. Aluminum alloy parts
The surface of aluminum alloy has a natural oxide film (Al₂O₃), but it is thin and uneven. The pretreatment needs to remove oil and form a stable chemical conversion film (such as chromate passivation, chromium-free passivation).
Typical steps:
Degreasing: Use a weak alkaline or neutral degreasing agent (avoid strong alkaline corrosion of the aluminum surface) to remove grease and cutting fluid.
Water washing 1: Rinse the degreasing agent.
Alkali etching (optional): Use a dilute sodium hydroxide solution to remove the surface oxide film and burrs to make the surface uniform (control the time to avoid excessive corrosion causing surface roughness).
Brightening: Use nitric acid or hydrofluoric acid to neutralize the residual alkali solution and restore the gloss of the aluminum surface.
Water washing 2: Rinse thoroughly.
Passivation: Form a corrosion-resistant conversion film (traditional chromate passivation has a good effect, but is not environmentally friendly; chromium-free passivation, such as zirconium and titanium, is now mostly used to meet environmental protection requirements).
Pure water washing: Rinse with deionized water.
Drying: Low temperature drying (60-80℃ to prevent deformation of aluminum alloy).
3. Zinc alloy parts
Zinc alloy surfaces are prone to white rust, and the material is relatively soft, so the pretreatment needs to be gentle to avoid corrosion.
Typical steps:
Degreasing: Neutral or weak acid degreasing agent to remove oil stains (avoid strong alkaline solution from corroding zinc).
Water washing 1: Rinse.
Activation: Use dilute nitric acid or organic acid to slightly etch the surface and remove the oxide layer (short time, about 30 seconds - 1 minute).
Water washing 2: Rinse.
Passivation: Use chromate or chromium-free passivation (such as silane treatment) to form a protective film.
Pure water washing + drying: Same as aluminum alloy treatment.
3. Adjust steps according to the surface state of the workpiece
New workpieces without rust and less oil pollution: can be simplified to "degreasing → water washing → surface adjustment → phosphating → water washing → drying".
Old workpieces with severe rust: need to add "sandblasting / shot blasting" physical rust removal steps (suitable for large workpieces), or extend the chemical rust removal time.
Precision parts / thin-walled parts: avoid long-term immersion in strong acids and alkalis, give priority to environmentally friendly degreasing agents and low-temperature phosphating processes to prevent workpiece deformation.
4. Optimize according to coating performance requirements
High corrosion resistance requirements (such as outdoor equipment): must include phosphating or passivation steps, and the phosphating film must be uniform and dense (film weight ≥ 3g/m²).
High decorative requirements (such as furniture, home appliances): strengthen degreasing and washing to avoid impurities causing particles and shrinkage holes on the coating surface; add "electrolytic polishing" steps to improve surface finish.
Areas with strict environmental protection requirements: chromate passivation and phosphorus phosphating are prohibited, and phosphorus-free phosphating, silane treatment or ceramic treatment (zirconium conversion film) are used instead.
V. Key points
Agent concentration and temperature: strictly control the concentration and temperature of degreasing agent and phosphating solution (such as free acidity and total acidity of phosphating solution), and regularly check and adjust to ensure stable treatment effect.
Washing quality: Each washing process must be thorough to avoid the previous agent from being brought into the next process (such as degreasing agent residue will cause uneven phosphating film, and acid residue will corrode the coating).
Drying thoroughness: The moisture content of the workpiece surface must be ≤0.5%, otherwise the evaporation of water during powder curing will cause blistering and pinholes in the coating.
In short, the selection of pre-treatment steps for electrostatic powder spraying needs to be "treated according to the material", and the core is to match the workpiece material, surface state and final performance requirements, while taking into account production efficiency and environmental protection requirements. Reasonable pre-treatment can increase the adhesion of powder coating by more than 30% and extend the salt spray resistance to more than 500 hours (far exceeding the 100 hours of untreated workpieces).
