Surface anti-corrosion technology and standards for railway fasteners
- What is the anti-corrosion performance difference between hot-dip galvanizing and Dacromet coating?
Hot-dip galvanizing (85μm zinc) passes 720-hour salt spray tests, suitable for temperate zones. Dacromet (zinc-chromium coating 6-8μm) resists salt spray over 1,000 hours, with no hydrogen embrittlement, ideal for high-humidity areas (e.g., southern coasts). Dacromet is thin, uniform, and withstands 200℃, while hot-dip galvanizing melts above 150℃. An African project's hot-dip bolts had 30% red rust after 1 year, replaced by Dacromet with 5-year corrosion-free performance, saving 40% in maintenance despite 15% higher costs.
- What are the advantages of zinc-nickel alloy plating in marine environments?
Zinc-nickel alloy (12-15% Ni) passes 1,500-hour salt spray tests, twice that of hot-dip galvanizing, with hardness HV≥500 (vs HV≤200 for galvanizing), more impact-resistant. A South China Sea cross-sea railway's zinc-nickel fishplates lasted 8 years without corrosion, vs 5 years for ordinary galvanized parts. Control current density 10-15A/dm² and pH 4.5-5.5 during plating to ensure uniformity; insufficient Ni content reduces corrosion resistance.
- How is the "adhesion test" for anti-corrosion coatings performed?
Use the cross-cut method (GB/T 9286): score a 10×10mm grid (to substrate), apply 3M tape, and peel quickly. ≤5% coating detachment is qualified. A batch of elastic clips failed due to low sandblasting roughness (Ra≤1.6μm), with 15% detachment. Sandblast to Ra≥50μm for better mechanical bonding.
- Why has chromium-free passivation become a mandatory requirement in the EU market?
The EU REACH regulation bans hexavalent chromium. Chromium-free passivation (e.g., silane treatment) forms a nano-protective film (1-2μm), passing 720-hour salt spray tests, eco-friendly. EU-exported fasteners must comply with EN 10343. A company was fined €100,000 for using chromium passivator. Control silane passivation pH 5.5-6.5 and temp 20-30℃ to prevent film dissolution.
- What are the key points of "repair technology" for anti-corrosion coatings in on-site installation?
For coating damage, repair with zinc-rich paint (Zn≥95%), dry film thickness ≥80μm: ① clean rust to St2 grade; ② apply epoxy zinc-rich primer; ③ topcoat. A site using ordinary anti-rust paint saw corrosion spread within 3 months. Use a magnetic thickness gauge to match original coating thickness and avoid electrochemical corrosion.