1.What is the core reason for the zinc layer to fall off at the corners of galvanized steel?
Wetting and reflux of zinc liquid are unbalanced
Sharp corner effect:
The curvature radius of the corner is small, and the surface tension of the zinc liquid prevents it from completely wrapping the substrate → forming micro air gaps.
Excessive reflux of zinc liquid:
When the workpiece leaves the zinc pot, the zinc liquid at the corners drips faster due to gravity, resulting in the actual zinc layer thickness being only 30%~50% of that of the flat part
Deterioration of alloy layer structure
ζ phase has high hardness (>300HV) but is brittle and easily cracked by external forces
Cooling stress concentration
The heat dissipation rate of the corners is 3~5 times that of the flat area → The shrinkage stress of the zinc layer is as high as 150~200MPa
Processing damage amplification
Shear fracture exists Cold work hardening layer (hardness increased by 20%) → hinders zinc and iron diffusion
Stamping burrs (height > 0.1mm) become stress crack sources
2.What are the strategies for optimizing the process?
Prevention at the design stage
Chamfering: Minimum radius of curvature R ≥ 2 times the plate thickness
Pre-punch stress relief hole inside the bending angle
Avoid sharp angle welding:
Replace the fillet weld with a K-type groove (angle ≥ 60°) to replace the right-angle overlap
Pre-treatment reinforcement
Sandblasting:
Sandblasting strength of the corner area is increased by 20% (using 0.3mm cast steel shot)
Electrolytic activation:
Direct current (5A/dm²) is passed through the corners to promote the exposure of active particles
3.What materials can be substituted?
Local toughening coating
Pre-coating of corners and edges Zinc-nickel alloy (electroplating, Ni content 12~15%)
Spraying after galvanizing Epoxy zinc powder primer + polyurethane topcoat (total thickness 120μm)
Substrate upgrade • Boron microalloyed steel (B=0.002%) is used for the 20mm range of corners and edges → inhibiting the formation of ζ phase
4.What are the testing items and methods?
Zinc layer thickness: magnetic thickness gauge
Bonding strength: cross-cut method
Brittleness assessment: cupping test
5.What is the emergency repair plan for on-site problems?
Treatment of dezincification area:
Grinding with angle grinder to St3 level (exposing metallic luster)
Acetone cleaning and degreasing
Repair materials: ◦ Scraping zinc-aluminum pseudo alloy paste (containing 85% Zn + 10% Al) ◦ Spraying cold galvanizing paint (zinc content > 96%)
Protection enhancement: Repair layer thickness = 1.5 times of original zinc layer
External coating of epoxy micaceous iron intermediate paint