Hot-dip galvanized bridges are the core structure of electrical systems, carrying cables. Loss of zinc coating at welds directly undermines the corrosion protection system, exposing the steel substrate to humid or corrosive environments and accelerating corrosion. The key to repairing lost zinc coating at welds is to restore corrosion protection comparable to the original coating while ensuring the bond strength and appearance consistency between the repaired layer and the substrate, thereby extending the overall service life of the hot-dip galvanized bridge.
Before repair, thorough pretreatment of the welds is essential for ensuring repair quality. Use an electric angle grinder or wire brush to remove weld slag, spatter, scale, and loose rust, ensuring the metal substrate is fully exposed and free of residual impurities. For oily or stubborn stains, organic solvents can be used, but be mindful of the solvent's compatibility with the zinc coating to avoid secondary contamination. After pretreatment, the weld surface roughness should reach Sa2.5 to enhance the adhesion of the repair material and provide a good base for subsequent repairs.
The selection of repair materials must balance corrosion resistance with ease of application. Zinc-rich paints are preferred due to their high zinc content. Epoxy and inorganic zinc-rich paints are widely used for their excellent weather resistance and adhesion. For emergency repairs or small defects, single-component zinc-rich paints can quickly form a protective layer. Their dry film zinc content can reach up to 96%, providing cathodic protection similar to hot-dip galvanizing. If the zinc coating is severely missing at a weld, thermal spraying can create a dense zinc layer on the defective surface using molten zinc wire. However, this requires specialized equipment and operator qualifications and is suitable for applications with high corrosion protection requirements.
The application process directly impacts the quality of the repair. When applying by brush or spray, a two-pass coat is required: the first coat primarily covers the substrate, maintaining a controlled thickness within a certain range; the second coat focuses on filling and leveling the surface. The total thickness should be thicker than the original coating to compensate for application losses. When spraying, maintain an appropriate distance between the spray gun and the surface to avoid excessive coating, which can cause sagging or cracking. For complex areas such as corners, manual touch-up is required to ensure that no gaps are missed and the repair layer is uniform and continuous.
Appearance and performance inspection are required after the repair. The repaired layer should be uniform and free of pores, with a color close to the original coating. The color can be adjusted using a zinc colorant, allowing it to oxidize and darken simultaneously with the original coating after exposure to sunlight. Regarding performance, salt spray testing is used to verify corrosion resistance. The repaired area should show no red rust formation during a long-term salt spray test, demonstrating corrosion resistance comparable to that of hot-dip galvanizing. A magnetic thickness gauge can be used to measure coating thickness to ensure compliance with standard requirements.
Differentiated repair strategies are required for specific weld environments. If the weld is located in a humid or chemically exposed environment, zinc-based alloy coatings can be used to enhance corrosion resistance. For high-temperature areas, a repair material with a higher melting point should be selected. If the weld also has cut damage, the cut surface should be pretreated similarly before repair to ensure continuity of the overall corrosion protection system and avoid failure of the overall protection due to local defects.
After the repair is completed, a long-term maintenance program should be established. Regularly inspect the weld repair layer, focusing on coating integrity and signs of corrosion. If coating flaking or extended corrosion is observed, a secondary repair should be performed immediately. During transportation or installation, protective measures should be taken at the welds, such as wrapping with anti-corrosion tape, to prevent mechanical damage that could lead to further damage to the zinc layer. A systematic repair process and maintenance mechanism ensures that the corrosion resistance of the hot-dip galvanized bridge welds remains consistent with the overall structural lifespan.
