Final Report - Strategic Environmental Research and Development ...

expensive and extensive analyst training is needed to interpret the resulting data. For these
reasons EIS is not a routine tool for the coatings industry. Nonetheless, it is a method that offers
quantitative information and can be used in conjunction with field or accelerated exposure
protocols to characterize damage accumulation.
Equivalent circuit (EC) modeling is the basis for much interpretation of EIS data collected for
coated metals. The evolution in time of EC elements, such as resistors and capacitors, can give
information on the corrosion protection given by a coating and can be used to characterize
aspects of coating performance [19]. The circuit shown in Figure 6.1 can be used to model the
response of a bare metal protected by a porous coating.
To assess the extent of coating delamination, Mansfeld proposed that the delamination area, A del ,
could be extracted from a ratio of measured capacitances [20]:
C
C
0
dl
dl
A
del
(eq. 6.1.1)
0
where C is the area-specific value for the metal-coating interface capacitance (F/cm 2 ) and
dl
C
dl
(F) is extracted from the EC fitting of measured data from a coated metal sample experiencing
coating disbondment. The model proposed by Mansfeld assumes that the change in C
dl
is due to
a change in the delaminated area and that changes in solution chemistry or the metal interface
0
change C during the course of the disbondment process. This model was implemented by
dl
Deflorian and co-workers to monitor the adhesion of different aluminum pre-treatments [21].
The authors found a good correlation between dry adhesion (measured with a pull-off test) and
that given by A del. Also, they were able to establish a ranking among pre-treatments based on
A del .
Mansfeld and co-workers also used the change in the pore resistance (R pore ) to evaluate
breakdown of the corrosion protection by polymer coatings in aluminum and steel alloys [22].
They concluded that breakdown was caused by the short circuiting of the coating which is
measured by a decrease in the R pore value. In this method, R pore is used as the parameter to
measure coating degradation. Corrosion protection characterization by the R pore method and the
delaminated area method are based on EC modeling.
Using EIS data, collected after 24 hours of exposure in 0.5M NaCl in the input layer of an
Artificial Neural Network (ANN), Kumar and co-workers used ANN to predict visual
degradation of a set of coated metals exposed to 1500 hours in a salt spray chamber [23]. They
found a correlation between ANN predicted and visually observed times to failures with a R 2 of
0.90. The authors also found that information contained in the phase angle parameter at low or
intermediate frequencies was more relevant than any other EIS-derived parameter.
The objective of this work was to assess the protectiveness of several emergent coating systems
using EIS-based analysis of samples subjected to ASTM B117 exposure. This approach allowed
a detailed assessment of the relative protectiveness of emergent Cr-free coating systems and
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