Portal steel frames, widely used load-bearing systems in industrial and civil buildings, require corrosion protection that balances durability, economy, and construction feasibility. In harsh environments such as marine atmospheres and industrial corrosion, the main causes of portal steel frame corrosion include oxygen, moisture, acidic gases, and salt erosion. The protective system needs to form a multi-layered barrier to block contact with corrosive media. Among current mainstream corrosion protection processes, hot-dip galvanizing, thermal spraying of metal coatings, heavy-duty anti-corrosion coating systems, and cathodic protection technology each have their advantages, but their durability varies.
Hot-dip galvanizing involves immersing the rust-removed steel component in 600°C molten zinc, forming a dense zinc layer on the surface. Zinc has a lower electrode potential than iron, allowing it to preferentially corrode and form a zinc oxide protective film. This process is highly industrialized, and the zinc layer bonds firmly to the substrate, providing effective protection for over 10 years under normal atmospheric conditions. It is particularly suitable for outdoor structures that are difficult to maintain, such as transmission towers and communication towers. However, hot-dip galvanizing has limitations on component size, requires factory installation leading to increased transportation costs, and is difficult to repair after coating damage, necessitating the use of supplementary processes. Thermal spraying of zinc/aluminum coatings uses flames or electric arcs to melt metal wires, which are then sprayed onto the surface of steel components using compressed air. This forms a honeycomb coating, which is then filled with sealants such as epoxy resin. Aluminum has better corrosion resistance than zinc, making it particularly suitable for marine atmospheric environments. The coating thickness is controllable and it adapts well to complex shapes. This process extends the lifespan through a dual mechanism of sacrificial anodic protection and physical barriers. However, the spraying process is labor-intensive and has high initial costs, so it is mostly used in projects with extremely high durability requirements, such as bridges and offshore platforms.
Heavy-duty anti-corrosion coating systems achieve stepped protection through multi-layer coatings. A typical solution is an epoxy zinc-rich primer + epoxy micaceous iron oxide intermediate coat + fluorocarbon/polyurethane topcoat. The primer provides electrochemical rust prevention, the intermediate coat increases film thickness and shielding, and the topcoat imparts weather resistance and decorative properties. The total coating thickness for outdoor applications needs to reach at least 150μm, resisting ultraviolet radiation, salt spray, and temperature variations, with a lifespan of 20-30 years. This process offers flexible application and is suitable for various indoor and outdoor structures, but requires strict control over surface treatment quality. Sandblasting and rust removal must reach Sa2.5 level to ensure adhesion.
Cathode protection technology uses a metal with a more negative potential than iron (such as magnesium or zinc) as the anode, making the portal steel frame the cathode and protecting it from corrosion. It is commonly used in underground or underwater structures. This technology requires regular monitoring of anode consumption, resulting in higher maintenance costs. It is usually used in conjunction with coatings to reduce overall costs. For example, buried pipelines use coatings to isolate corrosive media while providing supplementary protection through sacrificial anodes, forming a "double line of defense."
Weathering steel, by adding elements such as phosphorus, copper, and nickel to form a dense oxide film, can be used directly exposed to the atmosphere and exhibits better low-temperature impact toughness than ordinary steel. However, its corrosion resistance improvement is limited, and coating protection is still required in marine environments, which is costly. It is mostly used in landscape architecture or structures with special aesthetic requirements.
The durability of portal steel frame corrosion protection needs to be determined based on a comprehensive consideration of environmental conditions, expected lifespan, and economic factors. For structures exposed to marine atmospheres or industrial corrosive environments for extended periods, thermally sprayed zinc/aluminum composite heavy-duty anti-corrosion coating systems offer the longest service life. Through the synergistic effect of metal sacrificial protection and polymer barriers, they effectively slow down the corrosion process. For larger projects or those requiring on-site application, hot-dip galvanizing combined with localized repair processes is more feasible. Cathodic protection is suitable as a supplementary protection method for underground or underwater structures. In the future, the development of new technologies such as nano-modified coatings and self-healing coatings will further enhance the durability and environmental friendliness of portal steel frame corrosion protection.
