1.What is the core mechanism of passivation
The nature of corrosion of zinc coating and the need for passivation
The zinc coating is prone to electrochemical corrosion in a humid environment: zinc, as an anode, loses electrons in the electrolyte solution to generate Zn²⁺, while the cathode gains electrons to undergo a reduction reaction, causing the zinc layer to gradually dissolve.
The essence of passivation treatment is to build a dense and stable passivation film on the surface of the zinc layer through chemical or electrochemical methods, isolating the direct contact between the electrolyte and the zinc matrix, and inhibiting the occurrence of electrochemical reactions.
2.What is the formation mechanism of passivation film?
Chemical passivation: The zinc layer undergoes an oxidation-reduction reaction with the passivation solution (such as chromate, chromium-free passivation agent) to form a metal oxide, hydroxide or composite salt film. For example, in chromate passivation, Cr⁶⁺ reacts with Zn to form Cr³⁺ and Zn²⁺, which combine to form a Cr₂O₃・ZnO・nH₂O composite film with a film thickness of usually 0.5~2μm.
Electrochemical passivation: In an electrolyte containing a passivator, an external current is applied to cause the surface of the zinc layer to undergo anodization to form a more uniform oxide film, such as anodization passivation process.
3.What is the influence of film composition on corrosion resistance?
Chromate passivation film: Cr³⁺ provides the film skeleton, and Cr⁶⁺ is embedded in the film pores in the form of anions (such as CrO₄²⁻). When the film is damaged, Cr⁶⁺ dissolves and oxidizes the surrounding zinc matrix to form a new passivation zone, realizing the "self-repair" function.
Chromium-free passivation film: Trivalent chromium film relies on fluoride to inhibit the dissolution of the film, and organic components (such as carboxylic acid) fill the pores to reduce the permeability of the film; Silane film prevents water and oxygen from penetrating through the chemical stability of Si-O-Zn bonds.
4.What is the role of the film microstructure?
The density and porosity of the passivation film are key: the porosity of the chromate film is < 5%, while the porosity of high-quality chromium-free passivation film can be reduced to less than 10% through multi-layer film forming technology. For example, the molecular layers of the silane passivation film are arranged in an orderly manner, and the pore size is < 2nm, which is much smaller than the permeation path of water molecules (0.3nm), but is easily affected by pH.
Film-base bonding: The higher the interfacial bonding strength between the passivation film and the zinc layer, the less likely the film layer will fall off. The chromate film achieves strong bonding through the Cr-O-Zn chemical bond, while the Si-O-Zn bond of the silane film also has excellent interfacial compatibility.
5.What are the technical bottlenecks and breakthroughs of chromium-free passivation?
Corrosion resistance gap: Chromium-free passivation film lacks the self-repairing ability of Cr⁶⁺, and its corrosion resistance is still lower than that of chromate passivation in harsh environments (such as oceans and industrial atmospheres).
Solution: Composite passivation technology: such as "trivalent chromium passivation + silane sealing", through inorganic membranes to provide barriers and organic membranes to fill pores, so that the salt spray resistance performance is close to the level of hexavalent chromium.
Nano-composite film: Add nano-SiO₂ and TiO₂ particles to the passivation solution, and use the "maze effect" of nano-particles to extend the penetration path of corrosive media. For example, the zirconium salt passivation film modified by nano-TiO₂ can increase the salt spray resistance time by 40%.