Can coating thickness measurements be taken on or near edges or corners?

Coating thickness measurements are commonly taken to verify a protective coating meets the specification requirements. For structural steel that has been galvanized, the most commonly used specification is ASTM A123. One of the methods this specification defines for verifying coating thickness is using a magnetic thickness gauge. Magnetic thickness gauges are popular because they are easy and quick to use, accurate, and can store many different measurements, which can be accessed at a later time. The specification that describes how to use a magnetic thickness gauge is ASTM E376 Standard Practice for Measuring Coating Thickness by Magnetic-Field or Eddy-Current (Electromagnetic) Examination Methods.

Like any coating thickness gauge, magnetic thickness gauges must be used correctly or they will not give an accurate reading. ASTM E376 discusses one potential area for misuse of magnetic thickness gauges in Section 5.5. Namely, this section explains taking coating thickness measurements too close to an edge or corner (1/8 to 1/2 inch from an edge or corner) can result in an inaccurate reading unless the probe is designed specifically for measuring those types of areas/surfaces. In other words, using a magnetic thickness gauge that is not specifically designed for taking readings at corners and edges to measure the coating thickness at an edge or corner will give an inaccurate reading. Furthermore, the curvature of a surface being measured can also affect coating thickness measurements (see Section 5.6 of ASTM E376). Coating thickness measurements will be most accurate when they are taken on flat surfaces and at least a 1/2 inch away from an edge or corner.