1.What is the basic protection capability of galvanized Z275?
Zinc layer thickness and corrosion rate: Z275 means the weight of the double-sided zinc coating is 275g/m², corresponding to a thickness of about 38μm. In a marine salt spray environment (C5-M), the annual corrosion rate of the zinc layer is about 1.2μm, and the theoretical life span is about 31 years. However, in practice, the zinc layer will fail prematurely due to coating damage, mechanical scratches, etc.
Cathodic protection mechanism: The zinc layer corrodes first as an anode, and every 1μm of zinc consumed can protect 3μm of the steel substrate. When the coating is damaged, the zinc layer can delay the corrosion of the substrate, buying time for repair.
2.What is the performance of the coating system of the two-coat and two-bake process?
Typical supporting solutions:
Primer: Epoxy zinc-rich primer (zinc content ≥ 80%), providing strong adhesion and cathodic protection, salt spray resistance ≥ 1000 hours.
Topcoat: Fluorocarbon topcoat (PVDF), weather resistance of more than 20 years, salt spray resistance ≥ 1500 hours, UV resistance (xenon lamp ≥ 5000 hours).
3.How do zinc layers and coatings work together to provide protection?
The zinc layer compensates for coating defects through electrochemical protection;
The coating isolates the corrosive media from the environment and slows down the consumption of the zinc layer.
4.What are the technical measures to extend life?
Coating system optimization
Primer upgrade: Use chrome-free passivation primer to meet environmental protection requirements and enhance adhesion.
Topcoat reinforcement: Choose high weather-resistant polyester (HDP) or trifluoroethylene copolymer resin (FEVE), which has weather resistance close to PVDF but costs 30% less
Construction quality control
Surface treatment: The zinc coating needs to be phosphated or silane treated, with a roughness Ra of 3.2-6.3μm, to ensure that the primer adhesion is ≥5MPa.
Film thickness management: Primer dry film thickness 8-10μm, topcoat 15-20μm, total thickness ≥23μm (two coats) or ≥30μm.
Maintenance strategy
Regular inspection: Adhesion test (pull-off method ≥5MPa) and salt spray spot inspection (≥1000 hours rust-free) every 2-3 years.
Local repair: Grind the damaged coating to St3 level, apply epoxy zinc-rich primer + fluorocarbon topcoat, and ensure that the repaired area is compatible with the original coating.
5.What are the key life protection measures for Z275 substrate + two coatings and two bakings?
Coating system optimization
Primer must be: epoxy resin + zinc phosphomolybdate corrosion-inhibiting pigment (salt spray resistance > 1000h)
Topcoat must be: PVDF resin content ≥ 70%, film thickness ≥ 22μm ◦ Total film thickness: ≥ 30μm.
Edge protection enhancement
Apply epoxy zinc-rich repair paint after cutting (dry film > 60μm).
Use stainless steel fasteners for installation (to avoid galvanic corrosion).