Epoxy resin primer demonstrates the best adhesion on Galvanized steel plate. Laboratory tests show that its bonding strength reaches 15-20 megapascals, which is much higher than the 5-8 megapascals of alkyd paint. The key lies in the fact that the surface energy of the zinc layer is approximately 35 mN/m, while the polar groups of epoxy resin can increase the interfacial bonding force by 40%. Akzonobel’s 2023 research confirmed that G90 galvanized sheets treated with phosphate conversion (zinc coating weight ≥275g/m²) had a salt spray test life exceeding 1,200 hours after being coated with epoxy primer, with a peeling rate of only 0.1%. In the case of the construction industry, the steel structure of the Hong Kong-Zhuhai-Macao Bridge used an epoxy system, and the coating integrity rate remained at 98% six years later.
Waterborne acrylic modified polyurethane coatings have become an emerging choice due to their environmental friendliness, with VOC emissions below 100g/L, but they need to be used in conjunction with dedicated pretreatment agents. PPG Industries’ Chromate-free technology can enhance the adhesion between the coating and the zinc layer by 30%. It was applied to the roof of Tesla’s Berlin factory in 2022, with a weathering temperature range of -40°C to +120°C. Data shows that when the environmental humidity is higher than 85%, the curing time of water-based coatings needs to be extended by 50%, but innovative nano-additives (such as 3% silica content) can shorten the drying cycle to 45 minutes and increase the construction efficiency by 20%.
The height deviation of zinc flowers on the surface of hot-dip galvanized steel sheets should be controlled within ±5μm; otherwise, the zinc-rich areas (zinc content ≥99.9%) will cause pinholes in the coating. Dupont experiments found that when using a 30μm thick two-component epoxy mica intermediate paint, the incidence of pinholes decreased from 15% in conventional coating to 0.5%. In actual engineering, the Singapore Marina Bay project applied a three-layer coating system to galvanized steel plates (2mm thick) : 10μm phosphating primer + 60μm epoxy intermediate paint + 25μm fluorocarbon topcoat. Accelerated aging tests showed that the corrosion area rate was less than 3% within 25 years.
Although the cost of polyvinylidene fluoride (PVDF) topcoat is 40% higher than that of polyester, it stands out in terms of cost performance in areas with ultraviolet radiation greater than 2500 kJ/m². Data from Jotun Coatings’ Dubai project shows that the color difference ΔE value of the PVDF coating only increased by 0.8 after five years, which is far lower than the 2.5 of the polyester coating. For instance, in 2016, at the US Coast Guard facilities, 60% of the alkyd coating bubbling occurred after 18 months. After replacing it with an epoxy-polysiloxane system, the maintenance cycle was extended from 2 years to 10 years. Industry standard ISO 12944-5 clearly stipulates that 25% of the coating budget for galvanized parts should be reserved for surface treatment. Sandblasting to Sa2.5 grade can reduce the standard deviation of adhesion from ±3MPa to ±0.5MPa.
The latest trend is the penetration of smart coatings. Conductive polymer coatings (such as polyaniline with a concentration of 0.3%) can suppress the corrosion rate of the zinc layer from 5μm/ year to 0.2μm/ year. In 2024, Mitsubishi Heavy Industries will apply this technology in the LNG storage tank project, and it is expected that the full life cycle maintenance cost will be reduced by 50%. However, it should be noted that when the humidity exceeds 80%, the application window period for all coatings needs to be compressed to within 3 hours; otherwise, the risk of adhesion will increase by 30%.