1.What is the mechanism by which the flanging process affects the zinc coating?
Outer Flanged Side (Tensile Stress Zone):
This is the most affected area. During bending, the outer material is forcibly stretched.
The zinc layer itself has a certain degree of plasticity and ductility, allowing it to undergo some plastic deformation along with the steel substrate. If the deformation is within the zinc layer's ductility range, the zinc layer will only thin, but will not peel off.
The Problem: When the flange radius (R angle) is too small or the deformation is too severe, the applied tensile stress exceeds the plastic limit of the zinc layer (especially in brittle iron-zinc alloy layers), causing micro-cracks to form. If the situation is more severe, these micro-cracks can connect, causing the zinc layer to lift or slightly peel off from the steel substrate.
Inner Flanged Side (Compressive Stress Zone):
The material in this area is compressed. The zinc layer generally does not have problems under pressure; it may develop slight wrinkles, but usually will not peel off.
Flanged Edge (Cut Surface):
This is the weakest point. At the cut edge, the steel substrate is directly exposed. Flanging deformation will exacerbate stress concentration in this edge area, which may cause the zinc layer at the edge to separate from the substrate.
2.How does the flange radius affect zinc layer peeling?
This is the most important factor! The smaller the flange radius, the greater the stretching of the outer material, and the easier it is for the zinc layer to crack or peel off.
How to optimize: Increase the flange radius. Avoid sharp-cornered flanges during design. It is generally recommended that the flange radius R ≥ 2 times the material thickness (t).
3.What impact do the type and material of the zinc coating have on zinc coating peeling?
Pure zinc layers (such as electroplated zinc) have better ductility and crack resistance than zinc-iron alloy layers (such as those obtained after hot-dip galvanizing). Alloy layers are harder and more brittle.
How to optimize: For parts that require drastic forming, prioritize galvanized coils with better ductility, such as pure electro-galvanized (EGI) or certain grades of hot-dip galvanized (GI).
4.Does the appearance of microcracks in the zinc coating equate to failure?
Corrosion resistance remains: Even with visible microcracks on the outer edge of the flange, the sacrificial anodic protection of the zinc coating on the steel substrate persists. Zinc preferentially corrodes, thus protecting the exposed steel substrate. As long as the cracks do not cause large areas of zinc coating to peel off, its corrosion resistance is only slightly reduced, but still far superior to ungalvanized steel sheets.
Appearance may be affected: From an aesthetic point of view, microcracks or slight peeling may be considered defects.
5.What are some summaries and suggestions?
Conclusion: Completely avoiding any zinc layer damage after galvanized coil flanging is difficult. However, through proper process control, damage can be limited to micro-cracks, preventing severe peeling and flaking.
How to minimize peeling:
Design Priority: Increase the flanging radius (R-angle).
Material Selection: For complex forming, choose galvanized coils with good substrate formability and zinc layer ductility.
Process Assurance: Ensure mold quality, proper clearance, and adequate lubrication.
How to View This Issue: In most industrial applications, such as chassis and cabinets, automotive parts, and appliance structural components, minor zinc layer cracks at the flanging are acceptable because they do not affect the core structural function and overall corrosion resistance. Only in applications with extremely high aesthetic requirements (such as high-end appliance panels) is it necessary to invest more in optimizing or even adding post-processing steps.