1.How does the fatigue performance of different galvanizing processes compare?
Hot-dip galvanizing: micro cracks induced by brittle Zn-Fe alloy layer
Electrogalvanizing: Hydrogen embrittlement + surface pore stress concentration
Mechanical galvanizing: cold work hardening + particle embedding micro defects
Zinc-aluminum-magnesium coating: flexible MgZn₂ phase inhibits crack growth
2.How does the surface stress state change with different galvanizing processes?
Hot-dip galvanizing
430℃ zinc immersion causes annealing and softening of the substrate, yield strength ↓10%
Compressive stress gain: Cooling shrinkage produces 50-80MPa compressive stress on the surface (beneficial)
Tensile stress hazard: The difference in thermal expansion coefficient of the Zn-Fe alloy layer (δ phase) causes 200MPa tensile stress (harmful)
Electrogalvanizing
The cathode hydrogen evolution reaction produces 400-600MPa tensile stress on the surface
Porosity in the coating (density 10³-10⁴/cm²) becomes the source of fatigue cracks
3.What are the measures to strengthen the substrate before plating?
Shot peening:
Steel shot diameter 0.3mm, coverage 200%, introduce 200-300MPa surface compressive stress
Can offset the tensile stress of HDG alloy layer, and restore fatigue strength to 95% of the base material
Laser shock peening
Power density 5GW/cm², shock wave produces 1.2mm deep 500MPa compressive stress layer
Fatigue limit increased by 22% (SAE J1099 standard)
4.How to deal with it after plating?
Micro-arc oxidation
Generates a 10μm ceramic film on the surface of the galvanized layer, and the crack growth rate da/dN↓50%.
Roller polishing:
20mm diameter ceramic roller, 50N pressure, surface roughness Ra reduced from 3.2μm to 0.4μm
10⁷ cycles fatigue limit increased by 30%.
5.What is the best practice path?
High fatigue demand scenario:
Substrate shot peening → zinc-aluminum-magnesium coating (Mg≥5%) → micro-arc oxidation
Fatigue strength can reach 105% of the substrate
Economical solution (building steel structure):
Hot-dip galvanizing (Ni modification) → gradient cooling → rolling of key parts
Fatigue attenuation is controlled within 10%
Direct electrogalvanizing is strictly prohibited: it must be combined with pulse electroplating + 230℃×2h to remove hydrogen, otherwise the fatigue life will decrease by more than 40%.