Zinc layer thickness measurement

The thickness of a zinc coating (e.g., galvanized steel) is critical for corrosion protection and compliance with standards (e.g., ASTM A123, ISO 1461). Several methods are used to measure zinc thickness, each suited for different applications.

 

Author: Anna

 

The thickness of a zinc coating (e.g., galvanized steel) is critical for corrosion protection and compliance with standards (e.g., ASTM A123, ISO 1461). Several methods are used to measure zinc thickness, each suited for different applications.

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1. Non-Destructive Testing (NDT) Methods

A. Magnetic Induction (Coating Thickness Gauge)

• Principle: Measures magnetic flux changes caused by the zinc layer.

• Best for: Hot-dip galvanized steel (HDG), electroplated zinc.

• Pros:

  • Fast, portable, and easy to use.

  • Works on flat and curved surfaces.

• Cons:

  • Requires calibration with a known standard.

  • Less accurate on rough surfaces.

B. Eddy Current Testing

• Principle: Uses electromagnetic induction to detect coating thickness.

• Best for: Non-ferrous substrates (e.g., aluminum with zinc coating).

• Pros:

  • Works on conductive substrates.

  • No surface damage.

• Cons:

  • Sensitive to surface roughness.

C. Ultrasonic Testing (UT)

• Principle: Measures the time delay of ultrasonic waves reflected from the coating-substrate interface.

• Best for: Thick zinc coatings (>100 µm) or multi-layer coatings.

• Pros:

  • Works on non-metallic substrates (e.g., plastics with zinc plating).

• Cons:

  • Requires couplant gel.

  • Less common for standard galvanizing.

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2. Destructive Testing Methods

A. Cross-Sectional Microscopy (Metallography)

• Principle: Cuts a sample, polishes it, and measures under a microscope.

• Best for: Lab verification, research, and failure analysis.

• Pros:

  • Highly accurate.

  • Reveals coating structure (e.g., alloy layers in HDG).

• Cons:

  • Destructive (sample is ruined).

  • Time-consuming.

B. Coulometric Method (ASTM B504)

• Principle: Electrochemically dissolves the zinc coating and measures the charge required.

• Best for: Small parts, electroplated zinc.

• Pros:

  • Precise (measures in µm).

  • Works on complex shapes.

• Cons:

  • Destructive (small area is stripped).

C. Weighing (Gravimetric Method, ASTM A90)

• Principle: Weighs the sample before and after stripping zinc (using acid).

• Best for: Batch testing, verification.

• Pros:

  • Highly accurate.

  • Accepted by standards.

• Cons:

  • Destructive.

  • Slow process.

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3. Factors Affecting Measurement Accuracy

✔ Surface Roughness – Can interfere with magnetic/eddy current readings.
✔ Substrate Material – Ferrous vs. non-ferrous affects method choice.
✔ Coating Type – Hot-dip vs. electroplated zinc may require different techniques.
✔ Calibration – Must use standards with known thickness.

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4. Industry Standards for Zinc Coating Thickness

Standard	Application	Typical Thickness Range
ASTM A123 (HDG)	Structural steel	45–85 µm (varies by steel thickness)
ISO 1461 (HDG)	General galvanizing	50–150 µm
ASTM B633 (Electroplated Zn)	Automotive, fasteners	5–25 µm

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5. Best Practices for Measurement

1. Clean the Surface – Remove dirt, oil, or oxidation before testing.

2. Calibrate the Gauge – Use a certified reference standard.

3. Take Multiple Readings – Average at least 3–5 measurements per area.

4. Check for Uniformity – Test different spots (edges, corners, flat areas).

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Conclusion

• For quick field checks: Use a magnetic induction or eddy current gauge.

• For lab precision: Use cross-section microscopy or Coulometric testing.

• For compliance testing: Follow ASTM/ISO standards (gravimetric or microscopy).