The harsh corrosive factors in coastal environments cause significant differences in the lifespan of galvanized steel sheets. According to the ISO 9223 corrosion classification, the area within 500 meters of the coastline is defined as C5-M grade (extremely corrosive), where the average annual salt spray deposition reaches 300-1500 mg/m²/day. In this environment, the measured corrosion rate of the standard 85μm hot-dip galvanized coating is approximately 5-8μm per year, which means that the first appearance of base red rust usually takes 10-15 years. Data from the Florida Corrosion Test Field shows that the average time for Galvanized steel plate samples exposed to the coastal front to reach a 20% surface rust rate is 12.3 years, which is approximately 67% shorter than the lifespan in inland environments.
Specific corrosive factors have a decisive impact on the service life. When the concentration of chloride ions exceeds 56mg/m³, the loss rate of the zinc layer intensifies nonlinearly. In areas frequently hit by typhoons (such as the southeast coastal areas of China), the peak instantaneous concentration of salt spray caused by storms can reach 50 times the daily value, accelerating the corrosion process by an average of 8.5μm per year. Monitoring by the National Atmospheric Deposition Program of the United States shows that in the Gulf Coast region, due to the combined effect of high temperature (average annual 26℃) and high humidity (RH 85%), the rate of zinc layer depletion has increased by 40% compared to the theoretical value. In actual engineering, the first maintenance cycle of steel structures is only 5 to 7 years.
The upgrade of protective technology is significantly extending service life. The Setonaikai Bridge project in Japan used components with a 275g/m² ultra-high zinc coating (equivalent thickness 39μm), combined with an epoxy resin sealing layer. After 17 years of service in an environment with a salt spray concentration of 1423mg/m²/day, the remaining zinc coating still reached 71%. The new European standard EN 10346:2015 stipulates that coastal buildings should adopt aluminum-zinc-magnesium alloy coatings (such as ZnAl4Mg0.3B). In the simulated wave splash acceleration test, the service life of such materials is 2.3 times longer than that of traditional galvanized sheets, and the expected service life can reach more than 30 years.
Engineering practice verifies the multi-factor synergy effect. The underground power distribution system of the Marina Bay Financial Centre in Singapore adopts a dual protection design: 120μm hot-dip galvanizing +100μm epoxy mica coating. In a highly corrosive environment with a soil resistivity of 12Ω·m and a chloride ion concentration of 0.8%, 10-year testing shows only 6μm loss. The Norwegian Oil Authority’s report indicates that the North Sea oil platforms, which use hot-dip galvanized steel plates, have a service life of 25 years in splash zones (where the proportion of days with wave height ≥3 meters is 45%), which is 60% longer than the budgeted maintenance cycle and reduces the total life cycle cost by 42%. It should be emphasized that ISO 14713 suggests that in areas less than 1km from the sea, a minimum coating thickness of 100μm should be ensured and regular testing (measuring the remaining zinc coating every five years) should be carried out to achieve reliable protection for more than 20 years.