The core differences between Galvanized plates and color-coated plates are first reflected in their protection mechanisms and structures. The base plate is a hot-dip galvanized sheet made of cold-rolled steel coil, with a zinc coating on its surface typically ranging from 60 to 275g/㎡ (national standard GB/T 2518). It can provide a corrosion resistance life of 15 to 20 years in C3 industrial corrosive environments (ISO 9223 accelerated corrosion test shows no red rust for 4800 hours). The base layer of color-coated steel sheets is often a galvanized or aluminum-zinc alloy substrate (such as AZ150), with a 20-25μm thick organic coating (epoxy primer + polyester/fluorocarbon topcoat) superimposed on the surface. This composite structure gives it an advantage in resisting ultraviolet degradation – according to the AAMA 2605 standard High-quality fluorocarbon color plates still have a light retention rate of over 80% after being exposed to the sun in Arizona for 10 years. Meanwhile, more than 200 color options can be achieved through roller coating printing. However, during the inspection of the container yard at Qingdao Port in 2020, it was found that in the salt spray environment near the sea, the inferior polyester color-coated steel sheets had only been in service for seven years, with 50% of their area powdered, while the 120g/㎡ zinc coating hot-dip galvanized steel sheets in the same area only had slight whitening at the edges.
The production process flow significantly affects the physical performance parameters. The processing temperature of the hot-dip galvanizing line (CGL) is approximately 460° C. The aluminum content in the zinc bath needs to be precisely controlled within the range of 0.15-0.25% (DIN EN 10346). The microhardness of the formed Zn-Fe alloy layer reaches 150HV, which is crucial for scratch resistance in subsequent processing. Color-coated steel sheets add a coating section on the basis of galvanizing: the initial coating roller speed is approximately 90m/min, and the baking furnace is set at 200°C±5°C to make the solvent evaporation rate > 99.8%. However, high-temperature baking will reduce the yield strength of the substrate by 3% to 5% (Baosteel’s experimental data shows that the S350GD+AZ substrate has dropped from 350MPa to 330MPa). This means that when used for the roof panels of cold storage, the 0.5mm thick galvanized sheet can bear a load of up to 12kN/㎡, while the upper limit of the load of the color-coated sheet with the same thickness after coating processing is reduced to 10.8kN/㎡. It is worth noting that when complex shapes are required (such as hydraulically formed automotive fuel tanks), galvanized plate without coating are more suitable for deep stamping processes than color-coated plates due to their ductility of over 30% (in accordance with ISO 10113).
Chemical resistance and maintenance cycle are the key bases for material selection. Tests conducted by the laboratory in accordance with ASTM B117 standard show that the hot-dip galvanized sheet with a zinc coating of 160g/㎡ only shows 5% red rust after 4000 hours in a 5%NaCl salt spray. Although the organic coating of color-coated steel sheets has good initial barrier properties (the adhesion by the cross-cutting method reaches grade 0), once the base material is scratched by a hard object, the corrosion rate at the cut edge can reach 0.3mm per year (the self-healing property of the cut edge of hot-dip galvanized steel sheets can reduce the corrosion rate to 0.08mm per year). The material selection for the pipeline system of the 2022 FIFA World Cup venues in Qatar verified this feature: the DX54D galvanized sheet used for the fire sprinkler brackets was maintenance-free for 8 years in an environment with sulfur-containing condensate water, while the coating of the color-coated pipe fittings in the adjacent areas hydrolyzes due to frequent condensation and shows bubbling and peeling within just 36 months. However, color-coated steel sheets perform exceptionally well in the food and beverage industry: The top plate of a certain dairy factory’s fermentation tank uses PVDF fluorocarbon color steel sheets, which can withstand daily erosion with chemical cleaning agents ranging from pH2 to pH12. The amount of bacteria adhering to its surface is 85% lower than that of ordinary galvanized steel sheets (verified by ATP fluorescence detection method).
The economic benefits throughout the entire life cycle need to be quantitatively evaluated. The current market price for basic hot-dip galvanized steel sheets (DX51D+Z) is approximately 6,100 yuan per ton, while the premium for polyester color-coated steel sheets with the same specification base is 23% to 30% (about 7,500 to 7,900 yuan per ton). However, in scenarios where both anti-corrosion and decorative properties are required, color-coated steel sheets can save the cost of secondary coating: taking the exterior walls of steel structure factories as an example, the total cost of subsequent spraying of epoxy zinc-rich primer and fluorocarbon topcoat on bare galvanized steel sheets is 95 yuan per square meter, while directly using pre-rolled color-coated steel sheets only costs 78 yuan per square meter (saving 17.8%). In terms of maintenance costs, the 2023 Shandong Wind Power Tower Maintenance report shows that the average annual maintenance cost of galvanized tower bodies with on-site touch-up painting is 1.2% of the original value of the equipment, while the maintenance cost of prefabricated color-coated maintenance plates (requiring additional zinc layer protection) is only 0.3%. However, in extreme temperature scenarios (such as the equatorial region), the stability of hot-dip galvanized sheets is more prominent – the container quay cranes at the Port of Johor in Malaysia use S550GD high-strength galvanized sheets and have not experienced coating failure after 12 years of use in an environment with 85% humidity and +45°C. During the same period, the coating bubbling rate of color-coated metal blinds due to damp heat reached 22%.