Final Report - Strategic Environmental Research and Development ...

|Z| (ohm)
Phase angle (degrees)
10 6
0
10 5
5 h
23 h
52 h
73 h
168 h
336 h
-80
-60
10 4
-40
10 3
-20
10 2
10 -2 10 -1 10 0 10 1 10 2 10 3
Frequency (Hz)
10 4 10 5
Figure 3.39. Bode magnitude and phase angle plots of Neat Epoxy coating upon static
exposure to 0.5M NaCl solution for a period of 2 weeks (336 h). The area exposed to the
solution was 5.55 cm 2
Figure 3.40a shows the total magnitude of impedance at 0.01 Hz of the coatings relative to the
SrCr.Ep standard. Better response was seen from the bentonite pigmented coatings at the end of
680 h with the impedance of 5ZnB.Ep (1.51 x10 6 Ω.cm 2 ) and 5LaB.Ep (1.79 x10 6 Ω.cm 2 ) in the
same order of magnitude as SrCr.Ep (2.48 x 10 6 Ω.cm 2 ). Interestingly, Ce and Pr bentonite
pigments showed inferior performance compared to the other bentonites. The water uptake by
the pigmented coatings is causing the release of the Zn 2+ and La 3+ cations from the pigments and
promoting passivity of the interface.
Figure 3.40b shows the variation in coating capacitances with time over 680 h relative to the
SrCr.Ep control. There is some incongruence in the interpretation as the coating capacitances of
the bentonites are too high on the order of 10 -7 to 10 -8 , which cannot be coating capacitances. As
a check, the dielectric constant of a coating with thickness, t, is given by ε = Ct/Aε o , where C is
capacitance, A is area and ε o is 8.85 x 10 -12 F/m. The thickness of the plain epoxy coating was
measured to be approximately 35 μm and the area of the exposed coating was 5.55 cm 2 .
Assuming the increase in capacitance during initial exposures up to 2 days is due to diffusional
water uptake, the capacitance vs. t 1/2 curve was extrapolated to obtain the capacitance of the
epoxy coating before exposure (t=0). The capacitance of the unpigmented epoxy coating was
found to be 7.8715 x 10 -11 F/cm 2 . The dielectric constant of the epoxy coating was found to be
3.1 which lies in the range of dielectric constants of polymers of 3 – 10 [108]. The thickness of 5
wt% Ce bentonite pigment loaded epoxy coating was measured to be 55 μm and the capacitance
of 5 wt% Ce bentonite pigment loaded epoxy coating was determined to be 2.162 x 10 -9 F/cm 2 .
This gave a dielectric constant 130.43, which suggests that this value of capacitance does not
correspond to that of the coating. Therefore, this implies that the high frequency capacitive
response from the bentonite coatings except 5LaB.Ep was from the mixed capacitances of the
coating and defects in the coating. It has to be noted that, despite bentonites degrading the barrier
properties, Zn bentonite and La bentonite performed comparably to SrCr.Ep.
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