Electromagnetic Testing - Eddy Current Testing Applications Chapter 5 & 6

A typical operating frequency to measure a thickness of 0.15 mm of paint on
an aluminium alloy substrate, conductivity approximately 30% IACS would be
300 kHz to measure the same thickness of paint on a magnesium alloy,
conductivity approximately 15% IACS, a higher frequency, up to 1 MHz,
would be preferred. This is because the operating point depends not only on
frequency, but also on conductivity (and coil diameter). Lower conductivity
materials therefore require higher frequencies to bring the operating point to a
similar location as for high conductivity materials. Generally, as the thickness
required to be measured increases, the coil diameter should be increased, as
stated above. This moves the operating point further down the impedance
curve, and therefore lower frequencies may be used. There is no difficulty
with this because, in general, larger diameter coils are designed to operate at
lower frequencies. In fact, lower frequencies are recommended for thicker
coatings in order to avoid excessive sensitivity to lift off and to obtain a more
linear response. For example, for a thickness of 1 mm on an aluminium alloy
substrate, a frequency of approximately 80 kHz should be used.
Charlie Chong/ Fion Zhang