What Does Dry Film Thickness (DFT) Mean?
Dry film thickness (DFT) is the thickness of a coating as measured above the substrate. This can consist of a single layer or multiple layers. DFT is measured for cured coatings (after the coating dries). The thickness of a coating depends on the application and type of process employed. Proper thickness should be determined by recommended coating system parameters.
A coating is the most critical component in an anticorrosive coating system. The main function of a coating is to provide corrosion protection by adhesion to the substrate. It is critical that the applied thickness be sufficient to ensure corrosion protection as well as proper adhesion.
Film thickness is measured to ensure its compliance with ASTM, ISO and other specifications. There are coating inspection tools that are used at every stage of the coating process to ensure DFT meets correct specifications.
Corrosionpedia Explains Dry Film Thickness (DFT)
The maximum coating thickness recommended by a manufacturer is often based on a coating's resistance to sagging. DFT is a parameter that is considered during quality control or inspection processes. The durability of a coating is function of DFT, whereas the thickness of an individual coating layer in a coating system and the total system DFT reported specifies the coating's durability.
Types of DFT gauges include:
- Magnetic pull-off (Type I)
- Constant pressure probe (Type II)
- Electronic magnetic flux
- Eddy current gauge
The standards concerning DFT measurements include:
- ISO 2808
- BS 3900 Part C5
- BS 5494
- ASTM D1005
- SIS 184160
Even the most basic protective coating specification requires that the DFT is measured. It is considered to be the most important factor that determines the durability of a coating system. Each coating layer thickness in a system and the total system DFT has to be measured and recorded to show that the specified system has met the desired durability. As a result, both a contractor and inspector need to know how many measurements to make as part of their inspection. Other aspects of DFT measurement that are done according to standards include the calibration of gauges and the effect of surface profile on thickness measurements. The number of areas that are to be measured for coating thickness varies depending on the size of the area that has been coated.
There are two tools that are used when measuring coating thickness - a wet film thickness (WFT) gauge and dry film thickness (DFT) gauge. A WFT gauge is used to test the thickness of a liquid-based wet coating and should be used as the coating is being applied to ensure proper measurement of the wet film before there is significant solvent evaporation. In order to ensure the most accurate readings, a DFT gauge is only used once the film has achieved thumbprint hardness.
X-ray fluorescence, coulometric, beta backscatter, magnetic induction, amplitude eddy current, and phase-sensitive eddy current are popular and complex plating and other coating thickness applications