1.How are the excellent film performance and good coating compatibility reflected?
Phosphate coatings are primarily composed of zinc phosphate crystals with a needle-like or dendritic structure and moderate porosity. These films provide mechanical anchor points for the paint (a "bite effect") while allowing small paint molecules to penetrate and fill the coating, significantly improving coating adhesion (cross-cut adhesion can reach level 0, meaning no coating detachment).
They are highly compatible with all types of coatings (solvent-based, water-based, and electrophoretic paints), and are particularly well-suited for cathodic electrophoretic coating (the film's surface carries a slight negative charge, which facilitates the directional deposition of electrophoretic paint particles).
2.How is the balance of corrosion resistance reflected in meeting most industrial needs?
The bare film (uncoated) can withstand a neutral salt spray test for up to 24-48 hours (no red rust). After coating (e.g., electrophoresis plus topcoat), corrosion resistance can be increased 5-10 times (no blistering or rust in a salt spray test for 500-1000 hours), meeting medium-corrosion requirements in applications such as automotive bodies and appliance housings.
The film provides some protection for the zinc layer of the galvanized sheet, slowing down "secondary rust" (flash rust) that occurs between phosphating and coating.
3.How to demonstrate mature technology and controllable costs?
Phosphating solutions have a stable formula (primarily composed of zinc salts, phosphoric acid, and a nitrate accelerator), readily available raw materials, and are less expensive than zinc-nickel solutions (30%-50% lower per ton).
Process parameters are easily controlled (medium temperature 40-60°C, treatment time 2-5 minutes), and they are adaptable to various treatment methods, including spraying and immersion, making them suitable for large-scale continuous production (e.g., automotive assembly lines).
4.How is the strong adaptability to substrate reflected?
It is not only suitable for hot-dip galvanized sheets and electro-galvanized sheets, but is also compatible with a variety of substrates such as cold-rolled steel sheets and aluminum alloys. There is no need to switch processes when processing mixed-material workpieces (such as galvanized sheets + cold-rolled steel parts for automobile bodies), simplifying the production process.
5.What are the disadvantages of zinc phosphating process?
Corrosion resistance is inferior to high-end systems. ◦ Compared to zinc-nickel phosphating, the film has a higher porosity (approximately 1.5-2 times that of zinc-nickel systems), and the bare film corrosion resistance is significantly lower (the bare zinc-nickel film can withstand salt spray tests for over 72 hours, while the zinc system typically lasts ≤48 hours). In applications requiring high corrosion resistance (such as marine engineering and heavy machinery), additional sealing treatment (such as chromium-free passivation) is required. 2. Environmental performance has room for improvement. ◦ Traditional zinc-based phosphating solutions contain accelerators such as nitrates and nitrites, requiring nitrogen removal during wastewater treatment (making it difficult to meet total nitrogen standards). Phosphating slag (primarily zinc phosphate precipitate) is also produced in large quantities (approximately 1-3 kg of slag per ton of workpiece), requiring specialized treatment to prevent contamination. 3. Sensitive to pre-treatment cleanliness: Residual oil, oxide layers, or fingerprints (including sweat and salt) on the galvanized sheet surface can easily lead to defects such as "undercoating" and "mottling" in the phosphate coating (because oil hinders film formation and salt disrupts the local pH balance). Therefore, strict control of the degreasing and water washing processes is required, increasing pre-treatment costs. 4. Limited compatibility of the coating with some coatings: For demanding water-based coatings (such as low-temperature curing coatings), the low surface hydroxyl density of zinc-based phosphate coatings may affect coating adhesion (requiring surface conditioning to optimize crystal morphology and improve compatibility).