Coating thickness measurement methods

What is NDT?

Various methods of NDT are used for the measurement of coating thickness – paint thickness, plating thickness, anodising etc. These methods may be X-ray, electron backscatter, magnetic, etc and the Table gives a broad guide to the most appropriate methods for various coatings and substrates.

Appropriate NDT techniques for coating thickness measurement

Substrates / Coatings	Copper	Nickel	Chromium	Autocatalytic Nickel	Zinc	Cadmium	Gold	Palladium
Magnetic Steel	MX	X M¹	M X⁵	M¹ X⁶	B MX	B MX	B MX	B MX
Non-magnetic stainless steel	X E⁴	X M¹	X⁵	B M¹ X⁶	BX	BX	BX	BX
Copper and alloys	-	X M¹	X	M¹ X⁶	X	BX	BX	BX
Zinc and alloys	X	X M¹	BX	X⁶	-	BX	BX	BX
Aluminium and alloys	BX	BX M¹	BX	B E^1,2 X⁶	BX	BX	BX	BX
Magnesium and alloys	BX	BX M¹	BX	B X⁶	BX	BX	BX	BX
Nickel	X	-	BX	-	X	BX	BX	BX
Silver	BX	BX M¹	BX	B X⁶	BX	-	BX	X⁶
Ni-Co-Fe alloys	M	M¹	MX	M¹	BM X	BM X	B MX	BM X
Non-metals	BX E⁴	BX M¹	BX	B X⁶	BX	BX	BX	BX
Titanium	BX	BX M¹	X	B E^1,2 X⁶	BX	BX	BX	BX

Substrates / Coatings	Rhodium	Silver	Tin	Lead	Tin-lead alloys	Anodic oxides	Non-metals	Vitreous and porcelain enamels	Aluminium and alloys
Magnetic Steel	B MX	B MX	B MX	B MX	MX B³	-	BM X⁶	BM X⁶	BM X
Non-magnetic stainless steel	BX	BX E⁴	BX	BX	X B³	-	BE X⁶	E X⁶	BX
Copper and alloys	BX	BX	BX	BX	X B³	E X⁶	BE X⁶	E X⁶	X
Zinc and alloys	BX	BX	BX	BX	X B³	-	BE X⁶	X⁶	X
Aluminium and alloys	BX	BX	BX	BX	X B³	E X⁶	E	E	-
Magnesium and alloys	BX	BX	BX	BX	X B³	E X⁶	E	-	-
Nickel	BX	BX	BX	BX	X B³	-	BE X⁶	X⁶	X
Silver	X	-	X	BX	X B³	-	BE X⁶	E X⁶	X
Ni-Co-Fe alloys	BM X	BM X	BM X	BM X	MX B³	-	BM X⁶	X⁶	X⁶
Non-metals	BX	BX	BX	BX	X B³	-	-	-	BE
Titanium	BX	BX	BX	BX	X B³	-	BE	-	-

METHODS - KEY
B - beta back scatter
E - eddy current
M - magnetic/electromagnetic
X - X-ray fluorescence

NOTES
1. Method is sensitive to permeability variations in coating.
2. Method is sensitive to phosphorus content of coating.
3. Method is sensitive to alloy composition.
4. Method is sensitive to conductivity variation in coating.
5. Not high chromium steels.
6. Possible but not accurate for all purposes.