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Coating thickness refers to the thickness of the final hot-dip galvanized coating. Two different methods are used to measure the coating thickness of hot-dip galvanized steel; a magnetic thickness gauge and microscopy.

  • Utilizing a magnetic thickness gauge is a non-destructive, simple way to measure coating thickness. There are three different types of magnetic thickness gauges. The first type of magnetic thickness gauge is pocket-size and employs a spring-loaded magnet encased in a pencil-like container. The accuracy of the pencil-style gauge depends on the skill of the inspector, thus the measurement should be made multiple times.
  • A banana gauge is another tool used to measure coating thickness. Banana gauges can measure coating thickness in any position, without recalibration or interference from gravity.
  • The most accurate gauge, and easiest to use,  is the electronic or digital thickness gauge. Electronic gauges can also store data and perform averaging calculations.

The specification ASTM E 376 contains information on measuring coating thickness using a magnet or electromagnetic current to make coating thickness measurements as accurate as possible.

The other method to measure coating thickness is a destructive technique that exposes the edge of a coating under an optical microscope. The sample must be sectioned then mounted and polished to show the exposed edge of the hot-dip galvanized coating. The calibrated eyepiece of an optical microscope can then determine the thickness of the coating. Since this technique destroys the part being measured, it is only used as a reference method for resolving measurement disputes.

See Also:
The Inspection of Hot-Dip Galvanized Steel Products

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