Dry Film Thickness (DFT) in Protective Coatings: Measurement, Instruments & Standards

Protective coatings are widely used in the oil and gas industry to protect carbon steel, pipelines, tanks, vessels, structural steel, and other equipment from corrosion and environmental degradation. One of the most important inspection parameters after coating application is Dry Film Thickness (DFT).

Checking the DFT of each coating layer, as well as the complete coating system, is essential to achieve the desired coating performance and service life. Applying too little coating may result in inadequate protection, while excessive coating thickness can also lead to coating defects and premature failure, depending on the coating system.

This article will discuss DFT and the important parameters that anyone working in protective coatings or painting activities within the oil and gas industry should be aware of. It will cover what DFT means, why it is important, the instruments and gauges used to measure it, applicable industry standards, gauge calibration and verification, acceptance criteria, and good practices for accurate DFT measurement.

Let’s begin by understanding what Dry Film Thickness really is.

Dry Film Thickness (DFT) is the thickness of a coating after it has dried or cured, normally measured in micrometers (µm) or mils.

For example, if a coating system specifies a nominal DFT of 350 µm, the coating inspector must verify that the applied coating achieves the required thickness in accordance with the project specification and applicable inspection standard.

DFT may be measured after each coat and/or after completion of the complete coating system, depending on the coating specification and inspection procedure.

Correct DFT is essential because the coating thickness directly affects coating performance.

Insufficient DFT: Can provide inadequate barrier protection and may reduce the expected corrosion protection and service life.

Excessive DFT: Can also be undesirable. Depending on the coating system, excessive thickness can contribute to cracking, solvent retention, poor adhesion, embrittlement, or other coating defects.

Quality Control: DFT measurements demonstrate that the coating system has been applied within the specified thickness range.

Corrosion Protection: Maintaining the specified thickness helps ensure the coating provides the intended protection against the operating environment.

DFT can be measured using both non-destructive and destructive coating thickness gauges. The type of gauge used depends on the substrate, coating system, inspection requirements, and applicable standard.

Some of the commonly used instruments in industrial and oil & gas coating inspection include:

This is an electronic, non-destructive coating thickness gauge available with different probes for measuring coatings on ferrous and non-ferrous substrates. Before taking actual measurements, the gauge can be verified and adjusted using suitable calibration methods, such as zero, smooth surface, and rough-surface/2-point adjustment, depending on the application and substrate condition.

This is another widely used electronic, non-destructive DFT gauge manufactured by DeFelsko. Similar to the Elcometer 456, it can measure coating thickness on both ferrous and non-ferrous substrates when the appropriate probe is selected. The instrument also provides different verification and adjustment options to help achieve accurate measurements under actual site conditions.

The PIG (Paint Inspection Gauge) is a destructive coating thickness measurement instrument. Unlike electronic DFT gauges, it measures the coating by making a small cut through the coating system down to the substrate. The depth of the cut is then examined using the built-in microscope and graduated scale to determine the coating thickness. Because the coating is physically cut, the measurement location normally requires suitable repair after inspection.

Before measurements are taken:

• Verify the gauge using certified thickness standards/shims.

• Check the gauge on the actual or representative uncoated substrate where possible.

• Adjust the gauge when required to account for factors such as substrate material, thickness, geometry, surface profile, and curvature.

• Re-check the gauge at the start of the inspection, after a significant impact/drop, or whenever readings appear questionable.

This is particularly important on abrasive-blasted steel because surface profile can influence the gauge reading.

Two commonly referenced standards for DFT inspection are:

1. SSPC-PA 2– Procedure for Determining Conformance to Dry Coating Thickness Requirements

   1. Provides a procedure for determining whether coating thickness conforms to specified minimum and maximum requirements.

   2. Uses defined measurement locations and sampling procedures.

   3. Covers Type 1 magnetic pull-off and Type 2 electronic coating thickness gauges.

2. ISO 19840 – Measurement of, and acceptance criteria for, the thickness of dry films on rough surfaces

   1. Specifically addresses DFT measurement on rough surfaces such as blasted steel.

   2. Defines instrument adjustment, inspection areas, sampling, measurement, and acceptance/rejection criteria.

   3. The standard is particularly relevant where surface profile needs to be considered.

The project specification, coating manufacturer’s data sheet, and contractual requirements should always be checked first, because they may specify a particular standard, DFT range, or additional acceptance requirements.

Acceptance criteria depend on the specified coating system and the standard invoked by the project.

For example, under the commonly used ISO 19840 approach:

• The average DFT of the inspection area should be at least the Nominal Dry Film Thickness (NDFT).

• Individual readings should generally be at least 80% of the NDFT.

• Readings between 80% of NDFT and NDFT can be permitted within the limits defined by the standard.

• A specified maximum DFT must not be exceeded.

SSPC-PA 2 also establishes defined sampling and thickness-conformance requirements, including restrictions on individual spot measurements.

Important: The 80% / 120% figures should not simply be applied to every project without checking the specified standard, coating specification, and restriction level. The acceptance criteria must come from the applicable project requirements and invoked standard.

A competent DFT inspection should include:

• Confirming the required NDFT/minimum/maximum DFT before inspection.

• Confirming the gauge’s calibration/verification status.

• Performing field verification and adjustment where required.

• Using the correct probe for the substrate and coating.

• Taking the required number and distribution of readings.

• Recording individual readings and calculated averages.

• Identifying areas that do not meet the acceptance criteria.

• Documenting the gauge identification, inspection area, coating system, readings, and pass/fail result.

Build your knowledge in protective coating inspection and QA/QC with the IOGS Coating Inspector Level 2 online course. Learn about coating systems, surface preparation, DFT measurement, inspection instruments, applicable standards, and quality control practices.

DFT inspection is much more than simply placing a coating gauge on a painted surface and recording a number. Accurate DFT measurement requires the correct instrument, proper field verification/adjustment, appropriate measurement technique, and compliance with the specified standard and project requirements.

When properly controlled, DFT inspection provides confidence that the protective coating has been applied at the right thickness to deliver the intended corrosion protection and service life.