1.What is zinc's natural weakness?
When exposed to air, the pure zinc layer reacts with oxygen, moisture, and carbon dioxide in the air, naturally forming a thin, loose, and porous layer of basic zinc carbonate (commonly known as "white rust"). This natural oxide film is very unstable and cannot effectively prevent further corrosion of the zinc within, resulting in limited corrosion protection.
2.What is the role of passivation treatment?
Passivation treatment is a chemical or electrochemical method that allows the galvanized surface to react with the passivation solution (usually containing chromates, phosphates or chromium-free compounds) under controlled conditions to artificially generate a dense, stable, and highly adhesive composite oxide film. This film is the "passivation film."
3.How does passivation treatment enhance corrosion protection?
Physical Barrier Effect:
The passivation film is very dense, acting like a layer of armor, effectively isolating the zinc layer from corrosive media such as moisture and oxygen, significantly slowing the rate of electrochemical corrosion.
Self-Healing Effect (especially Chromate Passivation):
If the passivation film is locally damaged, the chromate components in the film dissolve and migrate to the damaged area, forming a new protective film and preventing further corrosion of the zinc at the damaged site. This "self-healing" ability is the key to its excellent corrosion resistance.
Chemical Inertness:
The passivation film itself is extremely stable and chemically much more inert than even reactive metallic zinc, making it less susceptible to reaction with corrosive substances in the environment.
4.How does passivation compare to no passivation?
The galvanized coils that have undergone passivation treatment have extremely strong corrosion resistance, and the salt spray test time can be extended by several times to dozens of times; the galvanized coils that have not undergone passivation treatment have weaker corrosion resistance and are prone to white rust quickly.
5.What are the types of passivation treatments?
Chromate passivation: Traditional and most effective, with self-healing properties. However, due to the toxicity of hexavalent chromium and increasingly stringent environmental regulations, its use is restricted.
Chromium-free passivation: A mainstream technology today. This method uses environmentally friendly materials such as molybdates, silicates, and titanium/zirconium systems to form a passivation film. While its self-healing properties may not be as good as chromate, its corrosion resistance meets most application requirements and is more environmentally friendly.
Oil coating: Often used in conjunction with passivation treatment. The oil film provides additional physical isolation, synergizing with the passivation film to further enhance rust prevention.