1.What effect does phosphating have on improving coating adhesion?
While the zinc coating on galvanized steel sheets offers some corrosion resistance, its smooth surface is prone to forming oxide films (such as zinc oxide and zinc hydroxide). When applied directly, the paint and zinc layer primarily rely on physical adsorption, resulting in weak adhesion and prone to coating flaking.
Phosphating enhances adhesion through the following methods:
Mechanical bonding: The phosphate film has a porous honeycomb structure (pore size approximately 0.5-5μm), allowing the paint to penetrate these pores. After curing, this creates an "anchoring effect," significantly enhancing the mechanical bond between the coating and the substrate.
Chemical bonding: The phosphate groups in the phosphate film can chemically adsorb or bond with the resin in the coating (such as epoxy resin or polyurethane), forming chemical bonds and further strengthening adhesion.
2.What effect does phosphating have on corrosion resistance?
Surface Cleaning and Activation: Degreasing and pickling before phosphating remove oil, oxide film, and impurities from the zinc surface, making the surface more uniform. During the phosphating process, the zinc surface is evenly dissolved and a phosphate film is formed, concealing original defects (such as zinc spangles) and providing a smooth base for painting.
Promoting Paint Leveling: The porous structure of the phosphate film absorbs solvents in the paint, reducing coating shrinkage caused by surface tension differences, resulting in a smoother, pinhole-free coating. This is particularly important for highly decorative coatings (such as automotive exterior panels).
3.What impact does it have on the weather resistance and mechanical properties of the coating?
Weather Resistance: Phosphate coatings reduce interfacial stress between the coating and the zinc layer (caused by temperature fluctuations or mechanical deformation), minimizing cracking or warping caused by thermal expansion and contraction, and improving stability in outdoor exposure and high- and low-temperature cycling.
Mechanical Properties: Phosphate coatings exhibit a certain degree of flexibility (elongation at break approximately 10%-20%), which alleviates stress concentration between the coating and the substrate during bending and stamping of galvanized sheet metal, reducing coating cracking. (For example, after stamping automotive parts, the cracking rate of phosphate-coated and painted coatings is over 50% lower than that of unphosphated coatings.)
4.What are the negative effects of improper phosphating?
Excessive film thickness (>5μm): The phosphate coating becomes more brittle and prone to cracking due to impact or bending after application. Thick films also weaken the adhesion between the coating and the substrate (pores are filled, weakening the mechanical bond).
Coarse or uneven crystals: Coating coverage can easily result in cosmetic defects such as "orange peel" and "pitting," and localized weak spots in the coating can become starting points for corrosion.
Residual chemicals (such as acid radicals and chloride ions in the phosphating solution) can cause under-coating corrosion (blistering and peeling), especially in high-temperature and high-humidity environments.
5.Why is phosphating treatment of galvanized sheet metal the "key process for improving quality" before painting?
By strengthening adhesion, enhancing corrosion resistance, and improving coating appearance and mechanical properties, the service life of coated products can be significantly extended. In practical applications, strict control of the phosphating process (such as temperature, time, and phosphating solution concentration) is required to ensure a uniform film (1-3μm) and fine crystals to maximize its positive effects on the coating.