1.Where does the role of enhanced corrosion resistance manifest itself?
Physical barrier effect: The phosphate film is composed of a large number of tiny phosphate crystals, which are closely arranged to form a porous structure. It can fill the microscopic defects on the surface of the steel plate, isolate the external corrosive medium (such as moisture, oxygen, salt spray, etc.) from direct contact with the metal substrate, and delay the electrochemical corrosion process.
Electrochemical protection effect: The phosphate film itself has a certain electrochemical inertness, which can reduce the electrode potential difference on the metal surface and inhibit the micro-battery reaction. In addition, if the phosphating solution contains metal ions such as zinc and manganese, the formed phosphate film can also form a "sacrificial anode" effect with the zinc layer and the substrate, further improving the anti-corrosion ability.
Synergistic protection with the galvanized layer: The galvanized layer itself protects the substrate by sacrificing the oxidation of zinc, and the phosphate film covers the surface of the galvanized layer, which can reduce the direct contact between the zinc layer and the external environment and reduce the corrosion rate of zinc. The combination of the two forms a composite protection system of "zinc layer + phosphate film", and the anti-corrosion effect is significantly better than that of a single coating.
2.What effect does it have on coating adhesion?
Microscopic anchoring effect: The porous structure of the phosphate film can form a mechanical bite with the coating (such as paint, electrophoretic paint), which is equivalent to "gripping" the coating on the surface of the steel plate to prevent the coating from falling off due to force or environmental changes. For example, during electrophoretic coating, the pores of the phosphate film can adsorb paint particles and enhance the bonding between the coating and the substrate.
Surface activity regulation: Phosphating treatment can eliminate the oxide film or oil stains on the surface of the galvanized layer, forming a uniform and rough surface, increasing the specific surface area, and at the same time, the chemical composition of the phosphate film (such as phosphate groups) can react chemically with the resin in the coating (such as hydrogen bonding, covalent bonding), further improving the adhesion of the coating.
Improved coating consistency: The phosphate film can evenly cover the surface of the steel plate, avoiding the difference in coating distribution caused by the uneven surface state of the galvanized layer, making the coating thickness more uniform and the appearance quality (such as gloss and flatness) more stable.
3.What effect does it have on processing performance?
Lubrication and friction reduction: In stamping, drawing and other forming processes, phosphate film (especially saponified phosphate film) can be used as a solid lubricant to reduce the friction between the metal mold and the steel plate surface, reduce processing resistance, prevent plate cracking or mold wear, and extend the service life of the mold.
Balance between insulation and conductivity: Although the phosphate film is an insoluble crystal, it has a certain insulation property, which can reduce dust pollution caused by electrostatic adsorption during processing; at the same time, its porous structure retains the conductivity of part of the metal surface, does not affect the current conduction of subsequent welding processes (such as spot welding), and avoids poor welding due to excessive insulation.
4.What other auxiliary functions are there?
Surface cleaning and activation: Degreasing, pickling and other pretreatments are usually required before phosphating, which can completely remove the oil, rust and scale on the surface of the galvanized layer, activate the metal surface, and lay the foundation for the uniform growth of the subsequent phosphating film.
Appearance and identification function: Different types of phosphating films (such as zinc series and manganese series) can present different colors such as gray-black and light gray, which can improve the appearance of the steel plate to a certain extent; some phosphating processes can also form specific surface textures by controlling the film thickness and crystal structure, which can be used for product identification or quality differentiation.
5.What is the core value of phosphating?
Phosphating treatment builds a transition layer on the surface of galvanized steel sheet through multiple mechanisms of "physical barrier + chemical synergy + interface optimization", which has the functions of protection, coating adaptation and process assistance. It is indispensable in the fields of automobile manufacturing, home appliance housing, steel structure and other fields with high requirements for corrosion protection and coating. Its functions can be summarized as: improving corrosion resistance, strengthening coating adhesion and optimizing processing performance. It is a key pretreatment process in the subsequent application of galvanized steel sheet.