PosiTector FNDS - Coating Thickness Measurement in Duplex Coating Systems

Coating Thickness Measurement
in Duplex Coating Systems with the PosiTector FNDS

The PosiTector 6000 FNDS  coating thickness gage measures the individual thicknesses of both the paint and the zinc galvanize layers in a duplex coating system, all in one go.

Background information

Steel corrodes when an electrolyte (such as water) connects anodes and cathodes on a steel surface. When the corrosion element arises, a iron oxide, better known as rust, forms. In order to avoid rust, the formation of corrosion has to be prevented.

The two most common methods of preventing corrosion on steel are:

• Cathodic protection (through the use of a sacrificial anode)
• Creating a barrier that prevents electrolytes from coming into contact with the steel

Duplex coatings are in fact a combination of cathodic protection (galvanizing) and a barrier layer (paint or powder coating)

Duplex coating systems combine two corrosion protection systems - usually paint or powder coating over galvanized steel (hot-dip galvanizing, electro- or spray-galvanizing). The corrosion protection resulting from a duplex coating system is far superior to either of the two protection systems used independently.

Advantages of duplex coating systems:

• Extended lifespan of the coating system and underlying substrate
• Delayed maintenance cycle - fewer touch-ups, lower maintenance costs
• Color coding
• Aesthetics
  

Measurement Challenges

Measuring the thickness of these layers with conventional inspection instruments is a challenge. Usually the zinc layer thickness is measured first with a magnetic gage, for example the PosiTector 6000 F1 or the PosiTest DFT ferrous. The same device is then used to measure the total thickness of the zinc and paint layers together. The difference between the two readings is regarded as the paint layer thickness. This method takes for granted that the zinc thickness never changes. But that´s not the case.

In reality, the zinc is not always evenly distributed over the entire part. The thickness of the coating depends on the thickness, roughness and the chemical composition of the steel that is being galvanized. Any of these factors can cause the galvanization to be uneven in thickness.

A second conventional method uses a combined magnetic and eddy current probe such as the PosiTector 6000 FN3. Two measurements are made, the first in eddy current mode to determine the paint film thickness, the second in magnetic mode to determine the total thickness of the coating system. The difference between the two measured values is assumed to be the zinc layer thickness. As mentioned before,, it is assumed that the zinc layer thickness does not vary across the structure. Both methods are time consuming and prone to errors. The PosiTector FNDS duplex probe performs this process with a single measurement, simply and accurately.

The innovative solution from mtv messtechnik & DeFelsko:

The PosiTector 6000 FNDS duplex coating thickness gage measures the individual thicknesses of the paint and zinc layers in a duplex coating system with a single measurement. In duplex mode, the PosiTector 6000 FNDS uses both the magnetic (fe / ferrous) and the eddy current principle (NFe / non-ferrous) at the same time to calculate and display the individual paint and zinc layer thicknesses.

The magnetic principle is used to measure the combined paint / zinc thickness over the ferrous substrate and the eddy current principle is used to measure the paint thickness over the non-ferrous zinc coating.

The zinc thickness is calculated by subtracting  the paint thickness from the combined paint/zinc thickness measurement.

When not in Duplex mode, the FNDS will behave like a conventional combination ferrous/non-ferrous instrument (similar to PosiTector 6000 FNS probe).
Ideal for measuring non-magnetic coatings over steel and non-conductive coatings over non-ferrous metal substrates.

The following video shows the PosiTector 6000 FNDS probe in action: