resistance R2 is a measure of corrosion resistance of the
material in the environment; a constant phase element
(CPE) was used in the equivalent circuit instead of a
capacitance because often the measured capacitance is not
ideal. The impedance representation of CPE is given by:
ZðCPEÞ ¼ 1=½QðjxÞ
n
ð1Þ
Q is a constant phase element and x is the angular
frequency. The number n is an empirical exponent, which
can vary between 1 for a perfect capacitor and 0 for a
perfect resistor. A value of n less than one would represent
a somewhat capacitor and it is generally thought to
arise from the presence of heterogeneities both laterally
and within the depth of the coating. If the values of the
exponent n are approximately one, Q can be said to
behave similar to a pure capacitor and the Eq. 2 is valid
that defines the capacitance of the parallel plate
capacitor:
C ¼ e0eA=dox ð2Þ
where C is the capacitance, e0 is the permittivity of vacuum,
e is the dielectric constant, A the effective area and