For corrosion-resistant coatings, thickness of each single coating is a very important factor for determining quality. And measurement of that thickness is usually obtained by using non-destructive methods. Different kinds of duplex coatings are normally classified as per their measurability. They have been described below in detail:
- Those comprised of a zinc-nickel or zinc-iron alloy layer and an organic layer. Due to their high costs, they are usually found on components of automobiles for preventing corrosion entirely.
- Systems installed on piping like shopping carts, brake lines, certain household appliances having zinc thicknesses of 30 microns or less and organic coating of plastic or paint with thicknesses of 200 microns or less.
- Hot-dip galvanized steel components having zinc layers denser than 80 microns, paint layers with thickness of over 85 microns and distinguishing iron-zinc diffusion zones. They are generally found in steel construction.
- Versions having zinc thicknesses from 0 to 10 microns, deposited either in a dip bath or galvanically, and also paint thickness within 150 microns. They are common in vehicle body manufacturing.
As duplex systems combine metallic and organic coatings, the methods used for measuring them involve usage of economical and non-destructive instruments that are easy to handle. Such coating inspection techniques incorporate eddy current and magnetic induction test methods. The latter test procedure is normally performed as per DIN EN ISO 2178 for measuring nonmagnetic coatings primarily applied to iron, steel and other ferrous metals. On the other hand, the eddy current method test is conducted as per the DIN EN ISO 2360 to measure non-conductive, non-magnetic coatings on various non-ferrous metals.
Eddy current method, which is phase sensitive, necessitates the metal coating to have significant electrical conductivity so that the current can actually develop. However, zinc coatings often exhibit lesser electrical conductivity of below 1MS/m due to having crystalline structure. For carrying out thickness measurement on basis of phase measurement, frequencies above 10 MHz are used.
In general, large components present within practical measurement amalgamate magnetic induction with phase sensitive eddy current. With extra action input from the operator, the measurements are performed using both methods successively after placing a probe. After that, the data of the measurement is collected suitably.
Thus, these are the industrial processes used for measuring thickness of duplex coatings. The above explanation is a simple demonstration of the methods involved. If you want to know more about them, look up informative articles over the Internet or get in touch with an expert.