ASTM D7091-22 Standard Practice for Nondestructive Measurement of Dry Film Thickness of Nonmagnetic Coatings Applied to Ferrous Metals and Nonmagnetic, Nonconductive Coatings Applied to Nonferrous Metals

The American Society for Testing and Materials (ASTM) standard "ASTM D7091-22 Standard Practice for Nondestructive Measurement of Dry Film Thickness of Nonmagnetic Coatings Applied to Ferrous Metals and Nonmagnetic, Nonconductive Coatings Applied to Nonferrous Metals" describes the use of magnetic and eddy current measuring instruments for dry film thickness measurement. This practice is intended to supplement, and not replace, the manufacturer's instructions for manual operation of the measuring instruments.

This standard includes definitions of key terms, reference documents, application significance and use, advantages and limitations of coating thickness measuring instruments, and description of test specimens. It describes methods and recommended frequencies for verifying the accuracy of measuring instruments and adjusting equipment, and lists reporting recommendations.

These procedures do not apply to coatings that will deform easily under the load of measuring instruments and probes, because the measuring instrument probe must be placed directly on the coating surface to obtain a reading. Provisions for making measurements on soft or sticky coatings are also described in this standard.

Coating thickness can be measured using a variety of measuring devices. These measuring devices are categorized as “magnetic pull” and “electronic.” These devices use a sensing probe or magnet to measure the gap (distance) between the base metal and the probe. This measured distance is displayed by the measuring devices as the coating thickness.

Coating thickness can vary greatly across a surface. As a result, obtaining single-point measurements may not accurately represent the actual coating system thickness. The SSPC-PA 2 standard specifies a coating thickness measurement frequency based on the size of the coated area. A measurement frequency for coated steel beams and coated test panels is also provided in the annexes to this standard. The governing specifier is responsible for providing the user with the minimum and maximum coating thickness for each layer and the total coating system.

For the purposes of this standard, the following definitions apply:

• Accuracy is a measure of the magnitude of error between the measurement result and the actual thickness of the item being measured. An accuracy statement predicts the ability of a coating thickness gage to measure the actual thickness of the coating to be measured. Accuracy statements provide the performance capability of the gage over the entire functional measurement range. Accuracy statements often include a fixed portion that remains constant throughout the measurement range and a variable portion that is related to the measurement result for a given thickness.
• Adjustment (optimization) is the physical act of aligning the thickness readings of a gage to match those of a sample of known thickness in order to increase the accuracy of the gage on a particular surface or over a particular portion of the measuring range. An adjustment affects the result of subsequent readings.
• A base metal reading is a measurement obtained on an uncoated substrate using a coating thickness gauge. A base metal reading is the determined effect of substrate roughness on a coating thickness gauge created by the manufacturing process (e.g. castings) or processes that produce the surface profile (roughness) (e.g. power tool cleaning, abrasive blast cleaning). Failure to compensate for the base metal effect may result in an overestimation of the actual thickness of the coating.
• Calibration is the process of obtaining measurements on traceable calibration standards throughout the meter's entire operating range and then making any necessary meter adjustments to correct out-of-tolerance conditions.
• Certification may be accompanied by documentation of the gauge condition, along with any corrective action required to correct out-of-tolerance conditions (such as adjustment or calibration, or both, or replacement of components).

This standard includes three operational steps necessary to ensure accurate coating thickness measurement: calibration, verification, and adjustment of coating thickness measuring instruments, and appropriate methods for obtaining coating thickness measurements on both ferrous and nonferrous metal substrates.

Many specifications for commercial and industrial coating projects specify minimum and maximum dry film thicknesses for each layer in a coating system. In addition, many manufacturers of high-performance coatings warrant their coating systems based in part on achieving the proper thickness of each layer and the total coating system. Even if a project specification is not provided, the coating manufacturer's recommendations published in the product data sheets may become the governing documents. Equipment manufacturers produce nondestructive coating thickness testing instruments that are used to measure the cumulative or individual thickness of coating layers after they have dried. Manufacturers typically provide instructions for use for setting up and using these instruments. The user of this equipment should be knowledgeable in the proper operation of these instruments, including methods and measurement procedures for verifying the accuracy of the equipment before, during, and after use.

Among the numerous testing, measurement, analysis and evaluation studies that our organization has provided for businesses in various sectors, our organization also provides testing services defined in the standard “ASTM D7091-22 Standard practice for non-destructive measurement of dry film thickness of non-magnetic coatings applied to ferrous metals and non-magnetic, non-conductive coatings applied to non-ferrous metals” with its trained and expert staff and advanced technological equipment.