The term coating thickness refers to the thickness of zinc applied to steel, while coating weight refers to the amount of zinc applied to steel for a given surface area. Two different methods are used in order to measure the coating thickness of hot-dip galvanized steel.
The first method to measure coating thickness involves using magnetic thickness gauges. There are three different types of magnetic thickness gauges and all can be used quite easily in the galvanizing plant or in the field.
The first type of magnetic thickness gauge is very small and utilizes a spring-loaded magnet encased in a pencil-like container, as seen in Figure 14. The tip of the gauge is placed on the surface of the steel and is slowly pulled off in a continuous motion. When the tip of the gauge is pulled away from the surface of the steel, the magnet, near the tip, is attracted to the steel. A graduated scale indicates the coating thickness at the instant immediately prior to pulling the magnet off the surface of the steel. The accuracy of this gauge requires it to be used in the true vertical plane because, due to gravity, there is more error associated with measurements taken in the horizontal plane or overhead positions. The measurement should be made multiple times because the absolute accuracy of this type of gauge is below average and it is difficult to determine the true coating thickness when only one reading is taken.
A banana gauge, as seen in Figure 15 is the second type of thickness gauge. With this gauge, coating thickness measurements are taken by placing the rubber magnet housing on the surface of the product with the gauge held parallel to the surface. A scale ring is rotated clockwise to bring the tip of the instrument in contact with the coated surface and rotated counter-clockwise until a break in contact can be heard and felt. The position of the scale ring when the magnetic tip breaks from the coated surface displays the coating thickness. This type of gauge has the advantage of being able to measure coating thickness in any position, without recalibration or interference from gravity.
The electronic or digital thickness gauge, as seen in Figure 16 is the most accurate and arguably, the easiest thickness gauge to operate. The electronic thickness gauge is operated by simply placing the magnetic probe onto the coated surface and then a digital readout displays the coating thickness. Electronic gauges have the advantage of not requiring recalibration with probe orientation, but do require calibration with shims of different thicknesses in order to verify the accuracy of the gauge at the time it is being used. These shims are measured and the gauge is calibrated according to the thickness of the shim, and then this process is repeated for shims of different thicknesses until the gauge is producing an accurate reading in all ranges of thickness.
ASTM E 376
The specification ASTM E 376 contains information for measuring coating thickness using magnet or electromagnetic current. It also provides some tips for obtaining measurements with the greatest accuracy, as well as describing how the physical properties, the structure, and the coating can interfere with the measurement methods. The requirements for ASTM E 376, as seen below, are intended to make the coating thickness measurements using magnet or electromagnetic current as accurate as possible.
ASTM E 376 Requirements
- Measurements on large products should be made at least four inches from the edge to avoid edge effects
- Measurement readings should be as widely dispersed as possible
There are some general guidelines, as seen below, for reducing error and ensuring the most accurate readings are being collected when using magnetic thickness gauge instruments.
Guidelines for Reducing Error
- Recalibrate frequently, using non-magnetic film standards or shims above and below the expected thickness value
- Readings should not be taken near an edge, a hole, or inside corner
- Readings taken on curved surfaces should be avoided if possible
- Test points should be on “regular areas” of the coating
- Take at least five readings to obtain a good, “true” value which is representative of the whole sample
The second method used to measure the coating thickness involves optical microscopy, as seen in Figure 17. This is a destructive technique and is typically only used for inspection of the coating of single specimen samples that have failed magnetic thickness readings or for research studies. Since it is not a common method, the accuracy is highly dependent on the expertise of the operator.