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Duplex Coating System Thickness Measurement

DeFelsko’s PosiTector 6000 FNDS coating thickness gage non-destructively measures the individual thicknesses of both the paint and the zinc galvanize layers in a duplex coating system with a single reading.

BACKGROUND

Steel corrodes when an electrolyte (such as water) connects anodes and cathodes on a steel surface. The formation of the corrosion cell causes a flaky iron oxide known as rust.

To avoid rust, something must prevent the formation of the corrosion cell. Two common methods for preventing corrosion on steel are:

  1. Cathodic protection (by using a sacrificial anode).
  2. Creating a barrier to block electrolytes from contacting the steel.

Duplex coatings are a combination of both cathodic protection (zinc galvanizing) and a barrier layer (paint or powder coating).

THE APPLICATION

Duplex coating systems use a combination of two corrosion protection systems – typically paint or powder coating over galvanized steel (hot-dip, electro or zinc spray metallizing). The corrosion protection which results from a duplex coating system is superior to either protection system used independently.

Benefits of duplex coating systems:

  • Extended life of coating system and underlying substrate
  • Delayed maintenance cycle - less touch-up repairing, reduced maintenance costs
  • Color Coding
  • Aesthetics

MEASUREMENT CHALLENGES

Measuring the thickness of these layers with conventional instruments is challenging.

Typically, a magnetic gauge such as a PosiTector 6000 F1 or PosiTest DFT ferrous is first used to measure the zinc thickness. The same instrument then measures the total thickness of both the zinc and paint layers together. The difference between the two readings is taken to be the paint layer thickness. This method assumes the zinc thickness never varies.

In reality, zinc thickness is not always uniform over the entire part or structure. Coating thickness depends on the thickness, roughness, chemistry, and design of the steel being galvanized. Any or all of these factors could produce galvanized coatings of non-uniform thickness.

A second conventional method uses a combination magnetic and eddy-current probe such as the PosiTector 6000 FN3. Two readings are taken, the first in eddy-current mode to get the paint thickness, the second in magnetic mode to get the total coating system thickness.

The difference between the two readings is taken to be the zinc layer thickness. Again, this method assumes the zinc thickness does not vary over the structure.

Both methods are time consuming and error-prone. The PosiTector FNDS duplex probe performs this operation with one reading, simply and accurately.

DEFELSKO'S SOLUTION

The PosiTector 6000 FNDS duplex coating thickness gage measures the individual thicknesses of both the paint and the zinc layers in a duplex coating system with a single reading.

In Duplex mode, the PosiTector 6000 FNDS utilizes both magnetic (ferrous) and eddy current (non-ferrous) principles simultaneously 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 gauge will operate 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.

 

ADDITIONAL RESOURCES

Paint over Galvanized Steel Duplex Systems

Duplex Coatings

Duplex Coating: How it Works, How it's Applied