Hot-Dip Galvanized Coating

Hot-dip galvanized coatings present a multi-layer preparation challenge. The outer eta (pure zinc) layer is very soft (50-70 HV) and smears easily, while the inner intermetallic layers (delta and gamma) are progressively harder (200-300 HV), and the steel substrate is harder still. This hardness gradient creates differential polishing rates and relief at layer boundaries. Cross-section preparation must preserve all layers, especially the thin gamma phase (often only 1-2 μm thick) at the steel interface. Edge retention at the outer zinc surface is critical for total coating thickness measurement.

Use a low-speed precision wafering saw with a thin diamond blade and continuous coolant. Cut perpendicular to the coated surface. The soft zinc outer layer will smear if cutting is too aggressive. Cutting speed: 100-200 RPM with very low feed rate. Alternatively, use a fine abrasive cut-off wheel for non-ferrous metals. Avoid clamping directly on the coated surface. For thin sheet material, sandwich between support plates before cutting. Leave 2-3 mm allowance for grinding.

Cold mounting with edge-retaining epoxy is strongly recommended. The soft eta (zinc) layer at the outer surface is easily rounded during grinding, making thickness measurement inaccurate. Use a hard epoxy compound with mineral filler for best edge support. Vacuum impregnation helps fill any gaps between layers, especially if the coating has cracks from service. Orient the coated surface face-down in the mount. Hot compression mounting can be used but may alter the zinc layer structure near the mounting temperature range. For best results, apply a thin layer of nickel plating (electroless Ni) over the zinc surface before mounting to protect the soft zinc edge.

The soft zinc layer (50-70 HV) requires careful grinding to avoid smearing. Start with medium grit SiC to avoid embedding coarse particles in the zinc.

Grinding sequence:

• 320 grit SiC: Remove sectioning damage (20-40 seconds). Light pressure (15-25 N). Monitor for zinc smearing.
• 400 grit SiC: Remove previous scratches (20-30 seconds). Maintain light pressure.
• 600 grit SiC: Refinement (20-30 seconds). Fresh paper essential.
• 800 grit SiC: Prepare for polishing (20-30 seconds).
• 1200 grit SiC: Final grinding (15-30 seconds). Very light pressure.

Disc speed: 200-250 RPM. Use fresh papers at every step; worn papers cause more smearing than cutting in the zinc layer. Rotate specimen 90° between steps. Thorough cleaning between steps to remove any embedded SiC particles.

Use napless cloths throughout to minimize relief between the soft zinc layer and harder intermetallic and steel layers.

Diamond polishing sequence:

• 6 μm diamond: 2-4 minutes on a napless synthetic pad with light pressure (15-20 N). Monitor the zinc layer for smearing.
• 3 μm diamond: 2-3 minutes on a napless pad (12-18 N). The intermetallic layer structure should begin to be visible.
• 1 μm diamond: 2-3 minutes on a napless pad (10-15 N). Check all layer boundaries for relief.

Final polishing:

• 0.05 μm colloidal silica: 1-2 minutes on a soft pad with very light pressure. Minimize time to prevent excessive relief between the soft zinc and hard steel layers.

If the zinc surface is electroless nickel plated before mounting, edge retention is greatly improved and more aggressive polishing can be used.

Etching reveals the individual Fe-Zn intermetallic layers. Each layer responds differently to etchants, creating natural contrast.

Nital 0.5-2% (Chemical Etching) - Primary choice:

• Composition: 0.5-2 ml HNO3, 98-99.5 ml ethanol
• Application: Immerse for 3-10 seconds. Start with 0.5% and increase concentration if needed.
• Reveals: All four Fe-Zn layers with good contrast: eta (bright, pure zinc), zeta (columnar structure), delta (compact, darker), gamma (thin dark line at substrate). Also reveals substrate microstructure.
• Rinse: Ethanol, then dry with warm air.

Palmerton's Reagent (Chemical Etching) - For enhanced layer differentiation:

• Composition: 200 g CrO3, 15 g Na2SO4, 1000 ml water
• Application: Immerse for 5-15 seconds.
• Reveals: Excellent contrast between all intermetallic layers. Particularly useful for differentiating zeta and delta phases. The chromic acid preferentially attacks different Fe-Zn phases at different rates.
• Note: Chromic acid is carcinogenic. Handle with extreme care. Use fume hood and full PPE.