1.How to classify according to the phosphating solution system?,
Film-forming ingredients: Zinc phosphate (Zn₃(PO₄)₂・4H₂O) is the primary component. The phosphating solution contains Zn²⁺ and PO₄³⁻, with nitrates and nitrites added as oxidants (accelerators).
Film characteristics: 1-5μm thickness, uniform and fine needle- or dendritic-shaped crystals, moderate porosity (facilitating paint penetration), and a good balance of adhesion and corrosion resistance.
Applications: Widely used in applications such as automotive bodies and appliance exteriors where corrosion resistance and coating appearance are paramount (for example, this system is often used for phosphating sedan body-in-white).
Process parameters: Temperature 40-60°C (medium temperature), treatment time 2-5 minutes, pH 2.5-3.5.
Zinc-Nickel Phosphating (High Corrosion Resistance Requirements)
Film-forming Ingredients: Ni²⁺ (concentration 1-5g/L) is added to a zinc-based coating to form a zinc-nickel phosphate composite film.
Film Characteristics: The film is denser (porosity is reduced by over 30%), exhibits superior corrosion resistance compared to pure zinc-based coatings (bare film corrosion resistance is improved by 50% in salt spray testing), and exhibits excellent compatibility with cathodic electrophoretic coatings (electrophoretic coatings deposit more evenly on nickel-containing coatings).
Applications: Pre-coating applications requiring stringent corrosion resistance, such as high-end automotive parts and marine engineering.
Process Parameters: Temperature 50-70°C, treatment time 3-6 minutes, pH 2.0-3.0.
Iron-Based Phosphating (Low Cost, Low Corrosion Resistance Requirements)
Film-forming Ingredients: Primarily composed of iron phosphate (FePO₄), it contains no zinc ions and relies on Fe₂⁺ (formed by the reaction of zinc and iron) dissolved in the substrate (zinc layer) to form the film.
Film Characteristics: Thin film (0.5-2μm), blue or gray-black in color, with good adhesion but weak corrosion resistance, low cost, and fast processing speed.
Applications: Low-demand coating of household appliance parts, simple structural parts, or as a pre-phosphating agent before electrophoresis.
Process Parameters: Room temperature (20-30°C), processing time 1-3 minutes, pH 3.5-5.0.
2.How to classify by treatment method?
Spray phosphating, immersion phosphating, spray-immersion combination
3.What are the process characteristics of different methods?
Spray Phosphating (High-Efficiency Continuous Production)
Process Features: Phosphating solution is evenly sprayed onto the galvanized sheet surface via a high-pressure spray (0.1-0.3 MPa). Mechanical scouring accelerates the film-forming reaction, resulting in high process efficiency (line speeds of up to 10-30 m/min).
Advantages: Suitable for large-scale, continuous production (such as automotive sheet and coil phosphating), with excellent film uniformity (no blind spots in the spray coverage).
Precautions: Ensure stable spray pressure to avoid uneven pressure that could result in thin or thick film layers.
Immersion Phosphating (Treatment of Complex Parts)
Process Features: The galvanized part is completely immersed in the phosphating solution, allowing the surface to fully contact and react with the solution. This process is suitable for workpieces with complex shapes (such as pipes and special-shaped parts) or deep holes or grooves.
Advantages: It can reach areas difficult to reach with spraying, and the film has stronger adhesion (more complete reaction).
Notes: The treatment time is relatively long (5-10 minutes), and the phosphating solution must be replaced regularly to prevent impurities from accumulating and affecting the film quality.
Combined Spray-Impregnation Process (Balancing Efficiency and Coverage)
Process Features: Spraying is used for pretreatment (degreasing and pickling), while the phosphating stage utilizes spraying (for rapid film formation) followed by impregnation (to fill dead corners and optimize the film layer), combining the advantages of both.
Applications: Automotive parts (such as doors and chassis), where both efficiency and high-quality coatings on complex surfaces are crucial.,
4.What is the typical process flow of phosphating treatment?
Pre-degreasing: Removes surface oil and contaminants (by spraying or immersion, using an alkaline degreaser at 50-70°C).
Main degreasing: Deep degreasing (used with a pre-degreaser to ensure residual oil is less than 5mg/m²).
Water washing: Removes residual degreaser (2-3 passes, with the final pass using deionized water to prevent impurities).
Surface conditioning: Activates the zinc surface before phosphating (using a titanium salt or sodium phosphate solution, pH 8-9) to promote crystallization and refinement of the phosphate film (a key step for a more uniform film).
Phosphating: Performs treatment according to the selected system (controlling temperature, time, and concentration).
Water washing: Removes residual phosphating solution (2 passes, with the final pass using deionized water to prevent chloride ion residue).
Passivation/sealing (optional): Provides additional protection for the phosphate film (e.g., a chromium-free passivator to enhance corrosion resistance).
Drying: Removes surface moisture (80-120°C to prevent cracking caused by high temperatures).,
5.What conditions must be met for the selection of phosphating process for galvanized sheet?
High corrosion resistance + efficient production → Zinc-nickel system + spraying;
Low cost + simple parts → Iron system + dipping;
High environmental requirements → Chromium-free phosphating;
Complex structural parts → Combined spraying and dipping.