TESTING & MEASURMENT
Duplex coating system thickness measurement
DeFelsko ’ s PosiTector 6000 FNDS coating thickness gage nondestructively 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
Colour 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 eddycurrent 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 .
46 JUNE 2021 twitter : @ surfaceworldmag