Hot-dip galvanized bridges, a classic product in the field of metal corrosion protection, owe their core advantage to the formation of a dense zinc-iron alloy layer on the steel surface through a high-temperature molten zinc process. This structure not only endows the cable trays with excellent corrosion resistance but also exhibits strong resistance to detachment during long-term use due to its unique "self-healing" mechanism. However, whether the galvanized layer detaches is not determined by a single factor but is the result of the combined effects of material properties, process control, usage environment, and maintenance methods.
From the perspective of material properties, the formation process of the hot-dip galvanized layer determines its bonding strength with the substrate. When steel is immersed in molten zinc at temperatures above 600°C, zinc and iron atoms undergo a diffusion reaction at the interface, forming a three-layer structure consisting of a pure zinc layer, a zinc-iron alloy layer, and an iron substrate. The zinc-iron ratio in the alloy layer is gradient-distributed, and this metallurgical bonding method makes the adhesion between the coating and the substrate far superior to surface coating processes such as electroplating. Even if the coating is locally damaged, the zinc in the alloy layer will preferentially oxidize, forming a zinc oxide protective film that slows down the rate at which corrosion spreads to the substrate.
Process control is a critical factor affecting coating life. During the pretreatment stage, pickling and rust removal must thoroughly remove oxide scale and oil from the steel surface; otherwise, residual impurities will hinder zinc immersion, leading to decreased coating adhesion. The selection and concentration ratio of the flux are equally important; its function is to prevent re-oxidation of the steel after pickling and to enhance the wettability of the zinc immersion solution on the steel surface. Temperature and time control during the zinc immersion process directly affect the thickness and structure of the alloy layer—excessive temperature or time will result in an overly thick and brittle alloy layer, while insufficient temperature or time will lead to incomplete alloy layer formation.
The operating environment has a dual effect on the coating. In humid environments, water molecules in the air react with zinc to form zinc hydroxide, which further oxidizes to basic zinc carbonate, forming a dense, white corrosion product layer. Although this process consumes some of the zinc layer, the resulting corrosion products can actually prevent further penetration of oxygen and moisture, acting as a "self-sealing" barrier. However, the presence of acidic gases (such as sulfur dioxide) or chloride ions (such as salt spray in coastal areas) in the environment disrupts the structure of corrosion products, accelerating coating wear. Furthermore, prolonged water accumulation or poor ventilation can create localized corrosion cells, exacerbating coating peeling.
Mechanical damage is a common cause of coating detachment. Cable trays may be subjected to impacts, scratches, or pressure during installation, transportation, or use, causing localized damage to the coating. If the damage is not repaired promptly, the steel at the damaged area will be directly exposed to the corrosive environment, forming a corrosion cell. Because zinc has a lower electrode potential than iron, the zinc layer around the damaged area will act as the anode and be preferentially corroded; this phenomenon is called "sacrificial anode protection." However, as corrosion continues, the anode area will gradually expand, eventually leading to separation of the coating from the substrate.
Maintenance methods are crucial for extending coating life. Regular inspections can promptly detect signs of coating damage or rust. Minor damage can be repaired with zinc-rich coatings to restore its protective properties. For severely corroded areas, rust products must be removed before re-hot-dip galvanizing. In acidic or alkaline environments or high-humidity areas, applying an organic coating (such as epoxy resin) to the galvanized layer can be considered to form a composite protective system, significantly improving corrosion resistance.
Coating peeling from hot-dip galvanized bridges is not inevitable; their lifespan depends on the synergistic effects of materials, processes, environment, and maintenance. Through rigorous quality control, proper environmental adaptation, and scientific maintenance management, hot-dip galvanized bridges can maintain stable protective performance over long-term use, providing reliable support for cable lines.
