Coating Thickness Measurement
© 2017 DeFelsko Corporation
Galvanizing Thickness Measurement
This article details the use of DeFelsko Coating Thickness Gages within the zinc galvanizing industry. It describes different types of hand held gages, the measurement process, several precautions to be taken, and a Q&A section for the most commonly asked questions concerning this application.
Steel corrodes when an electrolyte (such as water) connects anodes and cathodes on a steel surface. The formation of the corrosion cell causes a flaky iron oxide known as rust.
To avoid rust, something must prevent the formation of the corrosion cell. Two common methods for preventing corrosion on steel are:
- Cathodic protection (by using a sacrificial anode).
- Creating a barrier to block electrolytes from contacting the steel.
Galvanizing is the process by which a sacrificial anode layer of zinc is applied to the surface of a fabricated steel part to provide corrosion protection. The final step in this process is the inspection of:
- Coating thickness
- Visual appearance
The thickness of the galvanized coating is directly related to:
- Service life
- Amount of corrosion protection
A thicker coating of galvanize results in a longer service life for the coated part. Consequently, inspection of coating thickness is the single most important step in determining the quality of a galvanized coating.
Measuring Galvanize Thickness
The size, shape and number of pieces to be tested will dictate the appropriate test method. Specified test methods are categorized as either destructive or non-destructive.
There are four ways to measure the thickness of galvanizing:
- magnetic principle thickness gages
- stripping and weighing
- weighing the part before and after galvanizing
- optical microscopy (ASTM B 487)
The most practical test is the non-destructive method utilizing the magnetic principle for determining coating thickness. This test is
Because it is non-destructive, magnetic thickness measurement is the most common method of evaluating galvanizing coating thickness.
Magnetic Principle Thickness Gages
Coating thickness gages that operate on a magnetic principle are designed to measure non-magnetic coatings applied to ferrous metals. The three most common types of magnetic principle thickness gages fall into one of two categories:
- Mechanical pull-off (sometimes referred to as "Type I") as shown above
- Electronic (sometimes referred to as "Type II") as shown to the right
Comparing Magnetic Gage Types
- Measure the change in flux density using electronic circuitry.
- Clear, digital readout
- Variety of specialized probe styles
- Many offer built-in memory
- Adjustments can be made for substrate conditions
- Measure the strength required to pull a magnet away from steel. The thicker the zinc, the weaker the attractive magnetic force.
- No calibration adjustment required
- Simple and rugged
Magnetic Gage Features
- No calibration adjustment required
- Very small, unique magnet allows pinpoint placement
- Ideal for use on small, hot, or difficult-to-access measurement locations
- ±10% accuracy
- No calibration adjustment required
- Simple, durable, universally accepted
- No batteries/electronics
- GO/NO-GO button can be preset for rapid measurement
- ±5% accuracy
- Fast and simple operation
- Manual calibration adjustments are possible for increase accuracy
- Easy-to-read digital display
- Versatile - variety of integral or cabled probes
- Connectivity options - direct printing, USB, WiFi, Bluetooth
- Statistical capabilities - averaging, min/max
- Powerful software for reporting measurement data
- Built-in memory
- ±1% accuracy
- Follow gage manufacturer's instructions
- Verify gage accuracy regularly using reference standards
- Ensure that the test surface is free from dirt, grease, oxide, and corrosion products
- Measurement points should be chosen to avoid obvious peaks or irregularities in the coating
- A sufficient number of readings should be taken to obtain a true average coating thickness
When utilizing a mechanical gauge, perform the following steps:
- To compensate for the influence of substrate conditions (including mass, metallurgy, roughness, temperature and curvature), measure the uncoated substrate/part at a number of spots to obtain a representative average value. This average value is called the “base metal reading” or “BMR”.
- Measure the zinc thickness at the number of spots required by the relevant procedure or standard.
- Subtract the base metal reading (BMR) from the gage reading to obtain the thickness of the galvanizing.
When utilizing a electronic gauge, perform the following steps:
- To compensate for the influence of substrate conditions (including mass, metallurgy, roughness, temperature and curvature), check zero on the uncoated substrate/part and adjust if necessary.
- Check by measuring shims placed on the uncoated substrate.
- Measure the galvanized part. The gage reading indicates the thickness of the applied galvanizing.
Measuring Duplex Coating Systems
Duplex coating systems use a combination of two corrosion protection systems – typically paint or powder coating over galvanized steel (hot-dip, electro or zinc spray metallizing). The corrosion protection which results from a duplex coating system is superior to either protection system used independently.
DeFelsko’s PosiTector 6000 FNDS coating thickness gage non-destructively measures the individual thicknesses of both the paint and the zinc galvanize layers in a duplex coating system with a single reading.
See our Duplex Coating System Thickness Measurement application note to learn more.
Magnetic coating thickness gages report measurement values in units of linear distance, not coating weight. However, the gage reading can easily be converted to an expression of coating weight.
Example A: Imagine you take a measurement on a steel coil coated with zinc galvanize using a PosiTector 6000 F gage and obtain a single-side thickness reading of “0.35 mils.” This can be easily converted to oz/ft² using the following method:
- Multiply the gage reading of 0.35 mils by 2 to account for both sides of the panel (0.70 mils)
- Multiply 0.70 by a factor of 0.5938 to convert to oz/ft² (0.4157 oz/ft²)
- 0.4157 oz/ft² of zinc galvanize indicates G40 weight (0.40 oz/ft² minimum average, total of both sides per ASTM A 653)
Example B: Alternately, you can quickly confirm that a galvanized steel sheet meets a specific coating weight. According to ASTM A 653, a G90 coating designation means the weight of zinc on both sides of a steel sheet is 0.90 oz/ft².
However, a magnetic thickness gage measures only one side. Therefore:
0.45 oz/ft² x 1.684 = 0.76 mils per side, or 0.45 oz/ft² x 42.78 = 19 microns per side
Example C: To calculate grams/meter² from a result displayed in microns, first multiply the gage reading (or the average of a series of readings) by a factor of 2, and then multiply that result by 7.133. The final calculation will indicate the coating weight for both sides of the coated part.
Q & A
Q: Why does it seem that I never obtain the same gage reading twice?
A: Although the galvanized surface may appear smooth, microscopic surface roughness exists on both the zinc and steel. Therefore, the best representation of coating thickness is obtained by averaging a series of readings as per ASTM A123.
Q: Can I use a magnetic coating thickness gage to measure coating weight?
A: Magnetic coating thickness gages report measurement values in units of linear distance, not coating weight. However, the gage reading can easily be converted to an expression of coating weight using a conversion table or multiplication factor.
Q: What should a coating thickness gauge read on a G90 galvanized coating?
A: According to ASTM A 653, a G90 coating designation means the weight of zinc on both sides of a steel sheet is 0.90 oz/ft2
A magnetic thickness gauge measures only one side.
- 0.45 oz/ft2 x 1.684 = 0.76 mils per side
- 0.45 oz/ft2 x 42.78 = 19 microns per side
Excerpts from ASTM A123 Standard Specification for Zinc (Hot-Dip Galvanized) Coatings on Iron and Steel Products:
- The specimen coating thickness shall be the average of a minimum of 5 readings at widely dispersed points.
- This average value shall be not less than one coating thickness grade lower than the value listed in the appropriate specification.
- Thickness should be between 1.4 and 3.9 mils (35 and 100 µm) depending on the coating grade.
ASTM E 376 Standard Practice for Measuring Coating Thickness by Magnetic-Field or Eddy-Current (Electromagnetic) Examination Methods
ASTM A123/A123M Standard Specification for Zinc (Hot-Dip Galvanized) Coatings on Iron and Steel Products
ASTM A153/A153M Standard Specification for Zinc Coating (Hot-Dip) on Iron and Steel Hardware
ASTM A653/A653M Standard Specification for Steel Sheet, Zinc-Coated (Galvanized) or Zinc-Iron Alloy-Coated (Galvannealed) by the Hot-Dip Process
ASTM A767/A767M Standard Specification for Zinc-Coated (Galvanized) Steel Bars for Concrete Reinforcement
ASTM D7091 Standard Practice for Nondestructive Measurement of Dry Film Thickness of Nonmagnetic Coatings Applied to Ferrous Metals and Nonmagnetic, Non-conductive Coatings Applied to Non- Ferrous Metals