Galvanized fireproof iron sheets are widely used in building safety. The adhesion between the fire-retardant coating and the galvanized layer directly affects the material's structural stability and fire resistance under high-temperature environments. Insufficient adhesion can cause the coating to detach under thermal expansion and contraction or mechanical stress, leading to fire resistance failure and even safety hazards. Therefore, adhesion needs to be comprehensively improved from multiple dimensions, including surface treatment, coating selection, process optimization, and environmental control.
The surface characteristics of the galvanized layer are a fundamental factor affecting adhesion. Hot-dip galvanized layers have smooth surfaces and contain zinc-iron alloy phases. Without pretreatment, fire-retardant coatings cannot penetrate directly and form a mechanical anchor. For example, microscopic gaps easily form at the interface between the pure zinc phase and the iron-zinc alloy phase in the zinc layer. If surface roughness is not increased through grinding or sandblasting, coating molecules cannot fully fill these gaps, resulting in decreased adhesion. Furthermore, residual flux or oil from the galvanizing process can form an isolation layer, further hindering the bonding between the coating and the substrate. Therefore, before coating, surface impurities must be thoroughly removed using methods such as mechanical grinding, acid washing, or solvent cleaning. Phosphating treatment is then applied to generate a phosphate conversion film, enhancing the chemical adhesion between the coating and the zinc layer.
The formulation design of fire-retardant coatings must consider compatibility with the galvanized layer. Traditional oil-based or alkyd coatings react chemically with the zinc layer during curing, generating water-soluble zinc formate and other loose substances, leading to coating peeling. For example, the drying oil in alkyd coatings produces organic acids during oxidative drying; these acids corrode the zinc layer and generate corrosion products that expand in volume, damaging adhesion. To solve this problem, epoxy, inorganic zinc-rich, or water-based fire-retardant coatings should be selected. These coatings do not contain active double bonds or acidic substances and can bond with the zinc layer through both physical adsorption and chemical bonding. For example, the phosphate ions in epoxy zinc phosphate primers can react with the zinc layer to form insoluble complexes, creating a dense chemically bonded layer.
Precise control of coating process parameters is crucial for improving adhesion. During spraying, ensure the coating evenly covers the galvanized layer surface, avoiding areas that are too thick or too thin. For example, uneven coating thickness can lead to cracking or peeling during curing due to differences in shrinkage stress. Furthermore, curing temperature and time must be adjusted according to the coating type. Epoxy coatings typically require baking at 80-120℃ to promote cross-linking, while water-based coatings require controlled ambient humidity to prevent whitening. Inappropriate curing conditions will prevent the coating molecules from fully aligning to form a dense structure, reducing the bond strength between the coating and the zinc layer.
An intermediate layer design between the galvanized layer and the fire-retardant coating can significantly enhance adhesion. For example, applying an epoxy zinc phosphate primer to the galvanized layer allows the phosphate groups to react with the zinc layer to form a chemical bond, while the epoxy resin provides excellent physical adhesion. This "galvanized layer-primer-fire-retardant coating" sandwich structure effectively mitigates the difference in thermal expansion coefficients between the galvanized layer and the fire-retardant coating through the transition of the intermediate layer, reducing stress concentration caused by temperature changes. Furthermore, the anti-rust pigments in the primer can inhibit the corrosion of the zinc layer in humid environments, preventing coating peeling caused by the expansion of corrosion products.
Environmental factors have a significant impact on adhesion. In high-temperature and high-humidity environments, a water film easily forms on the surface of the galvanized layer, leading to a decrease in the bonding force between the hydrophilic groups in the coating and the zinc layer. For example, when water-based coatings are applied in environments with humidity greater than 85%, pinholes or whitening are likely to occur, severely affecting adhesion. Therefore, painting workshops need to control temperature and humidity within a suitable range and add moisture-proofing agents to the coating to improve its water resistance. In addition, after coating, it is necessary to avoid rain or contact with corrosive media to prevent premature coating failure due to external factors.
Long-term adhesion maintenance requires regular inspection and touch-up coating. During service, galvanized fireproof iron sheets may experience localized coating peeling due to mechanical damage or environmental corrosion. If not repaired in time, corrosive media will penetrate into the galvanized layer and cause electrochemical corrosion, further expanding the peeling area. Therefore, a regular inspection system is necessary to grind, clean, and reapply the same type of fire-retardant coating to damaged areas of the coating, ensuring the integrity of the coating system.
Improving the adhesion between the fire-retardant coating and the galvanized layer of the galvanized iron sheet requires a comprehensive approach throughout the entire process, including material selection, process design, construction control, and maintenance management. By optimizing surface treatment, selecting compatible coatings, precisely controlling process parameters, designing intermediate transition layers, managing environmental conditions, and establishing maintenance mechanisms, the bonding strength between the coating and the zinc layer can be significantly enhanced, thereby ensuring the long-term stability and fire-retardant performance of the galvanized fireproof iron sheet in extreme environments.
