Dry-film thickness - ferrous and non-ferrous substrates (ASTM D7091 and SSPC-PA2)

There are many instruments that can be used to measure the thickness of coatings on metal substrates, these instruments are based on the principles of electromagnetic induction and eddy current. The use of instruments is covered by two standards:

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

SSPC-PA2, Procedure for Determining Compliance with Dry Coating Thickness Requirements

All instruments are calibrated by the gage manufacturer or an accredited calibration laboratory, but the accuracy of the instrument needs to be verified for each use and the instrument may need adjustment to compensate for substrate roughness. Manufacturer's specific instructions need to be followed, but the following steps apply to all Class 2 (electronic) instruments:

Step 1 - Use a certified coating standard over the intended range of use to verify that the instrument is functioning properly (known as accuracy verification).

Step 2 - Place a certified or measured shim (within intended use) on the prepared uncoated metal surface and adjust the instrument (if necessary) to closely match the value indicated on the shim near. This step effectively removes any influence of the base metal (metallurgy, roughness, curvature, etc.) on the coating thickness reading (step 3).

Step 3 - After verifying the accuracy of the instrument and adjusting its substrate properties, take at least 3 measurements within a 1.5" diameter circle and average them together to determine the thickness at a specific location. See Figure 7. This is called a point Measurements. Multiple point measurements were taken in an area of ​​100 square feet to determine the thickness of the coating applied.

NOTE: When measuring coating thickness over existing paint or galvanizing, the thickness of the existing paint or galvanizing needs to be measured and subtracted from the total reading (i.e. the gage measures the cumulative thickness of all coatings present on the substrate) ). One instrument manufacturer offers gages that measure the cumulative thickness of galvanized coatings and then display the thickness of each layer separately.