1.How do chloride ions penetrate and destroy each other?
Passivation film destruction: Zinc will form a dense basic zinc carbonate passivation film in the air to protect the substrate. However, the high concentration of Cl⁻ in saline-alkali land has strong penetrability and can be adsorbed on the surface of the passivation film, replacing oxygen atoms to form soluble zinc chloride, destroying the integrity of the film.
Local corrosion initiation: Cl⁻ accumulates in the defects of the zinc coating, forming a "chloride ion channel" and initiating pitting corrosion. Local acidification accelerates the dissolution of zinc.
2.What are the negative effects of a highly alkaline environment?
Amphoteric dissolution of zinc: Zinc undergoes an amphoteric reaction under strong alkaline conditions to generate soluble zincates, which causes the passive film to dissolve.
Interference from carbonate deposition: CO₃²⁻ in saline-alkali land reacts with Zn²⁺ to form loose basic zinc carbonate (Zn₅(CO₃)₂(OH)₆), but its structure is porous and easily broken, which cannot provide effective protection. Instead, it hinders the formation of a dense passivation film.
3.What are the consequences of increased galvanic corrosion?
Microscopic galvanic cell:
Impurities in the zinc coating or the exposed steel substrate form galvanic pairs with zinc. Zinc dissolves preferentially as the anode and the steel substrate is protected as the cathode, but in saline-alkali soil:
High conductivity medium: The high ion concentration of the soil/water solution increases the conductivity of the electrolyte and accelerates electron transfer.
Cathode reaction acceleration: The oxygen reduction reaction (cathode reaction) is easier to carry out in saline-alkali soil, and the high dissolved oxygen content further promotes the dissolution of the anode (zinc).
4.What are the consequences of physical destruction of salt crystals?
Capillary penetration and crystallization pressure:
The salt solution in the saline-alkali land penetrates through the microcracks of the galvanized layer, and the salt crystallizes after the water evaporates. The crystallization process generates huge expansion pressure, which squeezes the coating and causes it to peel off, exposing the base steel.
Dry-wet alternating cycle:
The temperature difference between day and night in the saline-alkali land is large, and the frequent dry-wet cycle intensifies the salt crystallization-dissolution process and accelerates the mechanical failure of the coating.
5.What are the negative effects of corrosion products?
Loose product layer:
The corrosion products of zinc in saline-alkali environment are flocculent or porous, which cannot prevent the invasion of corrosive media.
Volume expansion effect:
When zinc is converted into corrosion products, the volume expands, generating internal stress, which causes the coating to crack and peel off.