1.How to enhance coating performance?
Zinc-aluminum (Zn-Al) alloy coating upgrade
Principle: Add 5%~10% aluminum to the hot-dip galvanizing solution to form a Zn-Al alloy coating. The aluminum element will form a dense Al₂O₃ oxide film on the surface to prevent the penetration of corrosive media.
Effect: The corrosion resistance is 2~3 times higher than that of ordinary hot-dip galvanizing, and the service life can reach more than 30 years in a salt spray environment (ordinary hot-dip galvanizing is about 10~15 years).
Advantages of multi-element alloying coating: The corrosion resistance of Zn-Ni coating (nickel content 8%~15%) in acidic media is increased by 40%, which is suitable for chemical wastewater pipelines with pH <4.
Zn-Mg coating forms an insoluble Mg (OH)₂ passivation layer in a chloride ion environment to inhibit pitting corrosion, and is often used in coastal power plant pipelines.
2.How to provide composite protection for the surface?
Hot-dip galvanizing + epoxy resin coating (EP)
Process: Spray liquid epoxy resin after galvanizing, and after curing, form a dense coating of 300~500μm thick to isolate the chemical medium from the zinc layer.
Hot-dip galvanizing + polytetrafluoroethylene (PTFE) spraying
Features: PTFE coating is temperature-resistant - 200~260℃, resistant to strong acids and alkalis (including aqua regia), has a low friction coefficient, and is suitable for high-temperature corrosive media (such as hydrochloric acid steam pipelines below 150℃).
Hot-dip galvanizing + zinc-based zinc-rich coating
Principle: The coating contains more than 90% zinc powder, which forms a "double sacrificial anode" protection with the hot-dip galvanized layer, and the damaged part can self-repair.
3.How to optimize structural design?
Increase the coating thickness and substrate wall thickness
Coating thickness standards:
General environment: ≥55μm (ISO 1461);
Strongly corrosive environment: Customized ≥85μm (such as marine engineering), even 100~120μm (chemical high-salt wastewater).
Upgrade substrate wall thickness: Use SCH80 or SCH160 grade thick-walled elbows (such as DN100 elbow wall thickness increased from 4.5mm to 8.18mm), even after the coating corrodes, it can still maintain structural strength.
Streamlined design reduces erosion corrosion
Measures: Use long radius elbows (R=1.5D) instead of short radius elbows (R=1D) to reduce the erosion and wear of the fluid on the coating, especially suitable for pipelines containing granular media (such as slurry, sewage).
4.How does environmental corrosion regulate the medium?
Adding corrosion inhibitors to inhibit electrochemical corrosion
Types:
Anodic corrosion inhibitors (such as chromates, molybdates): form a passivation film on the zinc surface;
Cathode corrosion inhibitors (such as organic amines, thioureas): prevent hydrogen ions from discharging at the cathode to generate hydrogen.
Application: Adding 0.1%~0.5% sodium molybdate to the circulating water system can reduce the corrosion rate of hot-dip galvanized elbows from 0.2mm/year to below 0.05mm/year.
Control medium temperature and pH value
Temperature: Control the medium temperature below 60℃ (the corrosion rate of zinc in seawater at 60℃ is 2.8 times that of 20℃).
pH value: Add a neutralizer (such as sodium hydroxide) to the acidic medium (pH＜4) to maintain the pH in the neutral range of 6~8 to avoid direct reaction of the zinc layer with hydrogen ions.
5.How to perform regular maintenance and monitoring?
Regular thickness monitoring and zinc supplementation, cathodic protection (CP) system assistance, corrosion early warning system