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Electrochemical techniques for delamination studies
Corrosion Science, Vol. 36, No. 4, pp. 643-652, 1994
~ )
Pergamon
Copyright O 1994 Elsevier Science ktd Printed in Great Britain. All rights reserved (~110-938X/94 $6.011+ 0.011
ELECTROCHEMICAL TECHNIQUES DELAMINATION STUDIES
FOR
D. H. VAN DER WEIJDE, E. P. M. VAN WESTINGand J. H. W. DE WIT Delft University of Technology, Laboratory for Materials Science, Division of Corrosion Technology and Electrochemistry, Rotterdamseweg 137, 2628AL Delft, The Netherlands *TNO Centre for Coatings Research, Division of Corrosion Prevention, P.O. Box 6034, 2600 JA Delft, The Netherlands
Abstract--Electrochemical impedance spectroscopy (EIS) has been described by several authors as a technique to measure delamination of organic coatings. The interpretation of EIS data can be performed in several ways. In this paper two methods are critically compared. These two methods are the breakpoint frequency method (BF) as described and used by Hirayama and Mansfeld and the most probablc impedance equivalent circuit method (MPI) as described by van Westing and Geenen. Experiments are presented which were designed for a proper evaluation of these two methods.
INTRODUCTION
ELECTROCHEMICALimpedance measurements on an intact or partly damaged coating can be performed relatively easily nowadays. A wide range of specialized equipment is available. As a result of this electrochemical impedance spectroscopy (EIS) is used increasingly. A proper interpretation is however difficult. This requires a suitable physical model of the system studied, and a suitable procedure for evaluation of the measured data with this model. In this paper the breakpoint frequency method (BF) as described by Hirayama I and Mansfeld2 and the most probable impedance equivalent circuit method (MPI) as described by Geenen 4 and van Westing3 will be critically compared for two important stages of degradation of barrier type coatings: delamination of an intact coating and delamination combined with a defect in the coating.
The breakpoint frequency method The BF method is based on measuring an impedance spectrum over a wide frequency range at regular intervals in time. This implies that the system should be stable in time during the measurements because the measurement of each impedance spectrum will take about 1 h. Hirayama I proposes the equivalent circuit of Fig. 1 to represent the impedance of an intact, partly delaminated coating. In this circuit the delamination component stands in parallel to the coating capacitance. A simulation of the impedance spectrum for this circuit from 1 MHz to 10 MHz produces the Bode and Nyquist plots of Fig. 2(a) and (b). The values for the components were taken from the Hirayama: a C o f = 2 . 4 n F , Ca1 = 1.6fzF, W = 2.92p~ -1, Rpf = 250 kfl, Rot = 650 kO, RsoI = 10 ~. Manuscript received 9 July 1993; in amended form 27 August 1993. 643
644
D . H . VAN DER WEIJDE, E. P. M. VAN WESTING and J. H. W. t)E WIT
C pf
Rct
W
FIG. 1. Proposed equivalentcircuitof HirayamaI for an intact, partly delaminated coating (breakpoint method). Cpe=paint film capacitance, Cd== double layer capacitance, W = Warburg impedance, Rpf = paint film resistance, Rc = charge transfer resistance and Rsol = solution resistance.
(a) id
Simulated Bode plot . . . . Iog(IZl) "=,. , % oO= °°° •
°
i -100 • alpha -80
105
(b) ira(z)
Simulated Nyquist-plot
-3.0 10~ -2.5 1 05
-60 -2.0 lo 6 -40
%°°,°.°°°
1000 ,
".
°
-20
o °°~
10 0.001 FIG. 2.
10
- I . 0 106 -5.0 105
0 0.1
-c51o6
1000
1 05
10 7
0
.~." 0
. . . . . . 1.0106 2.0106
3.0106
4.010~
(a) Simulated B o d e plot for the p r o p o s e d circuit, arrows indicate the breakpoints in the log [ZI curve; (b) simulated Nyquist plot for the p r o p o s e d circuit.
Normally the measurements cannot be performed above + 1 0 0 k H z . If the equipment is able to measure at higher frequencies the influence of cables and other parts of the equipment will be included in the measured impedance. 5 Using the proposed equivalent circuit it is possible to derive a quantitative relation between the frequency of the breakpoints in the log Izl curve and the values of the capacitors of the equivalent circuit. The values of the plateaux in the log IZI curve are related to the values of the resistors of this circuit. In case of actual measurements it is possible to obtain realistic values for the components of the circuit using the method described above. When this is done at regular time intervals after start of the exposure, one may monitor the changes of the components of the equivalent circuit. From these changes the delamination area can be determined quantitatively. According to Hirayama 1 the equivalent circuit of Fig. 3 represents a defective delaminated coating. When using this circuit it is again possible to derive the relationship between the breakpoint frequencies and the plateaux and the values of the components in the equivalent circuit. A remarkable feature of this circuit is that the delamination component and the defect component are completely separated. The authors I obviously assume that this will also be valid in practical situations.
Electrochemical techniques for delamination studies
645
C pf
m
i
W
Rot
R °°1
Rctp FIG. 3. Eqivalent circuit proposed by Hirayama I for a partly delaminated coating with a defect. C~lp = double layer capacitance of pore, Rotp = c h a r g c transfer resistance of pore and Rp,, = resistance of pore.
The most probable impedance equivalent circuit method The MPI method starts with an intact, dry coating without delamination. When this coating is exposed to a solution it will take up water. With a large number of short impedance measurements (10-50 kHz, this takes about 2 min) it is possible to measure the small changes caused by the uptake of water. Analysis is done by fitting a R(RCpO circuit on these data, using the Boukamp program EQUIVCRT. 6 From the Cpf data obtained as a function of time it is possible to calculate the uptake of water in the coating. This method had been described by Geenen and van Westing in several publications. 3,4,7.s The simple circuit used describes the behaviour of an intact coating in this frequency range very well. From a theoretical point of view this circuit cannot describe the impedance of a delaminated coating. However, it appears that for a defect-free barrier coating the changes caused by the delamination are very small: the system remains high in impedance and the simple circuit can still be fitted. Only in the curve of Cpf against time will a small step indicate the onset of delamination (see Fig. 6 later). This step does not give quantitative information. With the circuit from Fig. 1 it is not possible to explain these observations; therefore, other equivalent circuits are needed. Figure 4 shows a possible circuit. The precise configuration of the delamination part is less important. Most important is the fact that this part is no longer in parallel to the coating, but is placed in series with a part of the coating impedance. This is logical, because it is assumed that the coating is defect free so the delamination will occur under the coating, leaving the coating itself unchanged. When a simulation is made with this circuit the Bode and Nyquist plots of Fig. 5 are created (for these simulations the same values as for Fig. 2 were used with • = 0.1). Although Fig. 5 is a simulation, it is representative for actual measurements. It appears that the delamination components are even in a simulation completely hidden in the coating capacitance. So it is obvious that there is a large difference with the foregoing method. Apparently the equivalent circuit proposed in Fig. 1 describes another situation, and is not the most probable equivalent circuit for a defect-free, delaminated barrier type coating. Therefore the circuit of Fig. 4 is preferred. It
646
D.H. VANDER WEIJDE, E. P. M. VANWESTINGand J. H. W. D~ WIT (1 -¢~)Cpf
W
Rc~
¢~Cpf ~pf
FIG. 4.
Equivalent circuit that m a y r e p r e s e n t an intact, partly delaminated coating ( M P I
method). • = delaminated fraction of surface.
10~ Iog(IZI) .......................... , .~ ........... . 10s " o Simulated Bode-plot =-.
10'
-100 alpha -80
Im(Z)
Simulated Nyquist-plot
-I,I lO5
:60 -7.5 I1)4
103
•
-40
10~
-20
-33 104
oO
10'
..........
0.001
..0,,°
0.1
1 01 log(frequency)
FI6.5.
1 03
1 05
0
0
0
5.0 104
1.0 105 1.5 105
2.0 105 2,5 105
Re(Z)
Simulated B o d e and Nyquist plot of a delaminated coating. At this point the Cpf-time curve already shows a step.
should be stressed that the delamination part of this circuit is rather arbitrary. It is possible that this has another configuration, but this combination is chosen because it is comparable with that of Fig. 1. When an R(RC) circuit is fitted to the simulation of Fig. 5 (using the B o u k a m p p r o g r a m E Q U I V C R T ) the simple circuit is still able to fit on the simulation of the complex circuit. Only the values of the capacitor and resistors change a little. This explains the small step in the capacitance-time curve. A representative example of this step is given in Fig. 6. This was measured on a transparent coating so that corrosion, and the preceding delamination, could also be observed visually. Obviously the delamination part of the circuit is almost invisible. Only in a plot of the difference between simulation of the complex circuit and the R(RC)fit are some differences visible, as can be seen in Fig. 7. For actual m e a s u r e m e n t s these errors fall within the noise of the measurement. In the foregoing part it was m a d e clear that it is only possible to obtain qualitative information on delamination as long as the coating is defect free. The M P I method can also be extended to other situations, e.g. a coating with delamination and a defect. When defects are f o r m e d in a partly delaminated coating it is logical to assume that these defects are f o r m e d in the coating above the delaminated area, for
Electrochemical techniques for delamination studies
647
Cp~F) 8.0 10 [ ° ,. 7 £ 10 -I°
7,0 I0 "1°
i 6.5 IO-]°
4 I
Ji
6.0 I0 "t0 0
200
400
600
8001000
1200
1400
Time (min) FIG.
Coating capacitance as a function of time. At point 1 no corrosion is visible, at points 2 and 3 small corrosion spots are visible.
6.
131 I0 4 6.0 lff 5 2.0 11)5
[
1.11 lO ~
~ Relative difference between simple and extended equivalent circuit -
6.0 IU 5
L t
~
-2.0 10 5
~
-6.0 l(t 5
~ ~ f
2.0 I0 5 -2.11 I(15
- -
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-lmaginairen-or Real error
-1.0 I(I ~
-I.(1 IO 4
II.{X) 1
(I. I
1111
I0 t
I(15
log(frequency) FIG.
Difference between simulation of a complex circuit and a fit with a simple circuit as a function of frequency.
7,
Cpf
W FIG.
8.
Rc t
po
Equivalent circuit that represents a partly delaminated coating with a defect.
e x a m p l e , w h e n blisters burst due to o s m o t i c pressure, or w h e n blisters are d a m a g e d m e c h a n i c a l l y . O n l y w h e n defects are m a d e m e c h a n i c a l l y (scribed) is it possible that defect and d e l a m i n a t e d area are separated. H o w e v e r , d e l a m i n a t i o n m a y also occur at a scratch, y i e l d i n g the s a m e result. In practice m o s t defects will occur in the d e l a m i n a t e d area. T h e r e f o r e the circuit o f Fig. 8 s e e m s better than the circuit o f Fig. 3. T h e exact configuration o f the d e l a m i n a t e d area and the defect part m a y differ
648
D.H. VANDER WEIJDE, E. P. M. VANWEST1NGand J. H. W. DEWIT
coatin~ I
salt-layer , /t
I
metal substrate
FIG. 9.
Simplified sketch of the tested system.
from this m o d e l . I n Fig. 3 the blister is in parallel to the d e l a m i n a t i o n a n d the coating, in Fig. 8 the d e l a m i n a t i o n a n d the defect are p u t in series with each o t h e r a n d in parallel to the rest of the coating. This has a b e t t e r r e l a t i o n to the actual s i t u a t i o n as was e x p l a i n e d above. A circuit like that of Fig. 8 can again b e fitted o n m e a s u r e d d a t a using the p r o g r a m E Q U I V C R T . I n s o m e cases o n e m i g h t b e able to get q u a n t i t a t i v e i n f o r m a t i o n a b o u t the defect a n d the d e l a m i n a t e d area. E X P E R I M E N T A L METHOD To verify and compare the two methods several experiments were performed. For this purpose a pigmented alkyd coating was applied to a steel substrate that was partially contaminated with NaCl. This contamination was made by applying a drop of 5/,d of a 3% NaCl solution on the substrate. After drying a thin salt film covers about 0.5 cm2 of the substrate. When the substrate is coated, the situation shown in Fig. 9 results. It is obvious that in this situation delamination will occur as soon as the first water reaches the interface between salt and coating. In principle the (thin) coating above the formed salt solution will remain undamaged. The coatings were studied with EIS during immersion in a 3% NaC1 solution at 20°C. During the first few hours the measurements were performed over a limited frequency range (5-65 kHz), because the system was changing rapidly due to water uptake of the coating. After several hours this frequency range was extended until after some days a range from 10 mHz to 100 kHz was measured. EIS measurements were carried out using a Schlumberger FRA-1255 and a Solartron ECI-1286. Both were controlled with a DOS-PC using the Interactive Software for Impedance Spectroscopy (ISIS) software9 that was developed within this group. Details on the cell that was used can he found in Geenen.4 The height of the solution above the coating was about 7 cm. The steel substrates were type S Q-panels from the Q-panel company (Cleveland, Ohio, U.S.A.). E X P E R I M E N T A L RESULTS T w o essentially identical p a n e l s were tested, a n d will be r e f e r r e d to as case A a n d B. D i r e c t l y after i m m e r s i o n case A s h o w e d a N y q u i s t a n d B o d e plot that d e v i a t e d from the e x p e c t e d plots for intact coatings. A r e p r e s e n t a t i v e m e a s u r e m e n t after I h is given by Fig. 10(a). A f t e r 10 days, Fig. 10(b) results. A t that time b r o w n c o r r o s i o n p r o d u c t b e c a m e visible o n the edge of the c o n t a m i n a t e d area. A p p a r e n t l y the c o a t i n g h a d a defect. This m a y b e caused by a p o o r w e t t i n g of the c o a t i n g o n the salt film. O n the edges of the c o n t a m i n a t e d area, w h e r e the salt film m a y b e s o m e w h a t thicker, this results in a p e r f o r a t i o n of the c o a t i n g by the salt film which f u r t h e r results in a d e l a m i n a t e d area with a defect a b o v e it. W h e n Fig. 10(b) is a n a l y s e d using the B o u k a m p p r o g r a m it a p p e a r s that at least four time c o n s t a n t s are p r e s e n t in the m e a s u r e m e n t . This m e a n s that t h e r e should be four b r e a k p o i n t s in the B o d e plot (Fig. 10c). T h e r e are, h o w e v e r , only t h r e e b r e a k p o i n t s visible. F u r t h e r m o r e these b r e a k p o i n t s are n o t easy to d e t e r m i n e b e c a u s e they are n o t sharp d u e to s o m e o v e r l a p p i n g time constants. Case B has a different start. H e r e a n o r m a l , capacitive b e h a v i o u r is seen after I h as s h o w n in Fig. 11(a). A f t e r 2 days this has still n o t c h a n g e d as can be seen in the N y q u i s t plot of Fig. 11(b). This b e h a v i o u r is o b s e r v e d for several days. H o w e v e r ,
Electrochemical techniques for delamination studies
(a)
(b)
649
lm(Z) -5.0 104
Im(Z) -3.5103
Nyquist-plot of coating A after 1 hour immersion
Nyquist plot of coating A after 10 days immersion
-4.0 104
-30103 3.0 104 ?
J
2.5103 -2.0 104 ~
2.0 103
S
-10 104 ~ / / / "
-I.5 103 -I.0 103 2.0103
3.0 103
4.0 103 Re(Z)
(C)
0.0 10°
5.0103
6.01() ~
0
3.0 104
6.0 104
9.0 104
1.2 105
Re(Z)
i -IOR) - alpha
lO~ I •
;
Bode-plot of coating A 10 days after immersion
Iog(IZl) I
-80
I05 L -60 -40 104 L
-20
E 10 3 [
°.o~""
0.001
0.1
101
I(}5
103
0
log(frequency) FIG. I0.
Two Nyquist plots of measurement on coating A: (a) after 1 h and (b) after ten days. (c) Bode plot of Fig. 10(b).
(a)
(b) Ira(Z)
Im(Z) -3o ]o 5
-2.5 106
Nyquist plot of coating B after 1 hour immersion
Nyquist plot of coating B after 2 days immersion
-2.0 106 - 2 0 I()5
-1,5 106
$~'
q.O I06 1.0l~ -5.0 105
2.5 104
5.0 104 Re(Z)
Fro. 11.
7.5 104
l.O 105
0
1.0106
2.0106 Re(Z)
3.0106
4.0 I0 ¢'
Two Nyquist plots of measurement on coating B.
after 2 days delamination is already clearly visible because a small blister is formed at the contaminated area. The corrosion potential appeared not to have a stationary value. Several times the o p e n circuit potential drifted to - 3 0 0 0 m V ( S C E ) . Every time this happened it was polarised at a normal potential for steel in this solution. This behaviour is due to
650
D . H . VANDER WEIJDE, E. P. M. VANWESTING and J. H. W. DE WIT
Cpf(F)
6.5 10 -l°
f
6.2 10 l °
j
J
.,f e
5.9 10-1° 5.6 lff l° 5.3 10 -I°
,/
/
5.0 10-j° 0
100
200
300
400
Time (rain)
FIG. 12.
500
Capacitance as a function of time duringwater uptake of coating B.
-90
l0 s
Ira(Z)
Bode-plot of coating B one h o u r after perforation
N y q u i s t plot of coating B one h o u r after perforation
-I.1 104 -60 104
-7.5 103
~
~
-
-30 -3.7 10 3
0
0
1.0 104
Fie. 13.
2.0 104 3.0 104 4.0 l04 5.0 l04 Re(Z)
l0 s 0.1
l01 103 log(frequency)
l0s
Nyquist and Bode plots of measurements on coating B directlyafter perforation.
the very high resistance of the coating. The equipment is not able to handle this accurately. It is obvious that the impedance that was observed is different from the behaviour presented by Hirayama 1 in Fig. 2. So the BF method is based on a wrong assumption for the equivalent circuit and therefore cannot give any information about the starting delamination under a barrier coating. The MPI method does in this case give some qualitative information. From the Cpf-time plot of Fig. 12 it is clear that the coating is not stable, but is delaminating. In this case the expected step at the start of the delamination is not clearly visible. This is because the delamination is forced and is starting when the coating is still taking up water. Therefore water uptake and the step are not separated. From these measurements it is clear that the intact film indeed dominates the impedance of the total system as was predicted by the circuits and simulations of Figs 4 and 5. To test the MPI method the blister of Case B was deliberately perforated with a needle. Directly after that the impedance spectrum of Fig. 13 was measured.
Electrochemical techniques for delamination studies - -
651 IZI
al aha O
25t0S Nyquist plot of coating B 2o l c~ s o m e d a y s af ter p e r f o r a t i o n .
~ 107
• • " , 40
10105
f'
~
\..
," -SO
5 c)104 0 2O]O ~
4q)l(] ~
60 It} =
B
flOl( 15
1011)6
1211c~
-1.0 10;0 0 0 ]
: I0 s
1 O4
B o d e plot o f c o a t i n g B some days after perforaUon 0.1
10
103
1
O5t 0 0 0
log(frequency)
FlG. 14. Nyquist and Bode plot of measurements on coating B some days after perforation of the blister. Because it is known that defect and delamination are combined, an equivalent circuit like that of Fig. 8 is used to describe this m e a s u r e m e n t instead of using Fig. 3. After some days the spectrum of Fig. 14 was recorded. Qualitatively this is the same as in case A. Quantitatively they are different which can be expected because the shapes of the defects in the delaminated area are also different. When this m e a s u r e m e n t is analysed according to the M P I method, again at least four time constants are found. In the Bode plot, however, it is not even possible to determine three breakpoints, because two breakpoints are nearly combined at the edge of the curve. In Case B also some brown corrosion products are visible some hours after perforation. The impedance in both cases (A and B) increased with time. Apparently the defect was becoming blocked by corrosion products. This will increase the resistance of the defect and thereby the impedance of the system. A complete analysis of all m e a s u r e m e n t s is beyond the scope of this article and will be published later. CONCLUSION From the results of the m e a s u r e m e n t s described above it is clear that quantitative m e a s u r e m e n t s of delamination under an intact coating are not possible with both methods. Only the M P I method will give a qualitative indication of the onset of delamination. Also on a theoretical basis this method is preferred above the BF method, especially when breakpoints are so close together it is not possible to distinguish between the separated points. This method may work in a few, simple cases but it is not generally applicable. The MPI method is more generally applicable. Besides the difference in the method there is also a difference in the equivalent circuits used. F r o m the experiments it was proven that the circuits for a delaminated coating as presented by H i r a y a m a cannot describe the measurements on delaminated barrier type coatings. The delamination part of the circuit should be in series with a part of the coating. In case of a delaminated and defect containing coating it is preferable to put the delamination and the defect part of the circuit in series instead of putting everything in parallel.
Acknowledgements--The authors wish to thank AKZO corporate research Arnhem, Hoogovens IJmuiden, DSM Research Geleen, Shell Research/Billiton Arnhem, SigmaCoatings Amsterdam and the Dutch Ministry of Economic Affairs for financial support of this research.
652
D . H . VANDER WEIJDE, E. P. M. VArqWESTIr~Gand J. H. W. DE WIT
REFERENCES 1. R. HIRAYAMAand S. HARUYAMA,Corrosion 42,952 (1991). 2. F. MANSFELDand C. H. TSAI, Corrosion 42,958 (1991). 3. E. P. M. VAN WEST1NG, Determination of coating performance with impedance measurements. PhD. Thesis Tech. Univ. Delft (1992). 4. F. M. GEENEN, Characterisation of organic coatings with impedance measurements. PhD. Thesis Tech. Univ. Delft (1991). 5. E. P. M. VAN WESTING, G. M. FERRARI, F. M. GEENEN and J. H. W. DE WIT, Prog. Org. Coat., accepted. 6. B. A. BOUt,AMP, Solid State Ionics 23, 89 (1986). 7. F. M. GEENEN, J. H. W. DE WIT and E. P. M. VANWESTING, Prog. Org. Coat. 18 (1990). 8. E. P. M. VAN WESTING, D. H. VAN DER WEIJDE, G. M. FERRARIand J. H. W. DE WIT, Proc. lOth Eur. Cong. Corrosion, Barcelona, Spain (1993). 9. H. J. W. LENDERINK, M. M. DlNX and D. H. VAN DER WEIJDE, Interactive Software for Impedance Spectroscopy (ISIS). Tech. Univ. Delft, The Netherlands (1991).
~ )
Pergamon
Copyright O 1994 Elsevier Science ktd Printed in Great Britain. All rights reserved (~110-938X/94 $6.011+ 0.011
ELECTROCHEMICAL TECHNIQUES DELAMINATION STUDIES
FOR
D. H. VAN DER WEIJDE, E. P. M. VAN WESTINGand J. H. W. DE WIT Delft University of Technology, Laboratory for Materials Science, Division of Corrosion Technology and Electrochemistry, Rotterdamseweg 137, 2628AL Delft, The Netherlands *TNO Centre for Coatings Research, Division of Corrosion Prevention, P.O. Box 6034, 2600 JA Delft, The Netherlands
Abstract--Electrochemical impedance spectroscopy (EIS) has been described by several authors as a technique to measure delamination of organic coatings. The interpretation of EIS data can be performed in several ways. In this paper two methods are critically compared. These two methods are the breakpoint frequency method (BF) as described and used by Hirayama and Mansfeld and the most probablc impedance equivalent circuit method (MPI) as described by van Westing and Geenen. Experiments are presented which were designed for a proper evaluation of these two methods.
INTRODUCTION
ELECTROCHEMICALimpedance measurements on an intact or partly damaged coating can be performed relatively easily nowadays. A wide range of specialized equipment is available. As a result of this electrochemical impedance spectroscopy (EIS) is used increasingly. A proper interpretation is however difficult. This requires a suitable physical model of the system studied, and a suitable procedure for evaluation of the measured data with this model. In this paper the breakpoint frequency method (BF) as described by Hirayama I and Mansfeld2 and the most probable impedance equivalent circuit method (MPI) as described by Geenen 4 and van Westing3 will be critically compared for two important stages of degradation of barrier type coatings: delamination of an intact coating and delamination combined with a defect in the coating.
The breakpoint frequency method The BF method is based on measuring an impedance spectrum over a wide frequency range at regular intervals in time. This implies that the system should be stable in time during the measurements because the measurement of each impedance spectrum will take about 1 h. Hirayama I proposes the equivalent circuit of Fig. 1 to represent the impedance of an intact, partly delaminated coating. In this circuit the delamination component stands in parallel to the coating capacitance. A simulation of the impedance spectrum for this circuit from 1 MHz to 10 MHz produces the Bode and Nyquist plots of Fig. 2(a) and (b). The values for the components were taken from the Hirayama: a C o f = 2 . 4 n F , Ca1 = 1.6fzF, W = 2.92p~ -1, Rpf = 250 kfl, Rot = 650 kO, RsoI = 10 ~. Manuscript received 9 July 1993; in amended form 27 August 1993. 643
644
D . H . VAN DER WEIJDE, E. P. M. VAN WESTING and J. H. W. t)E WIT
C pf
Rct
W
FIG. 1. Proposed equivalentcircuitof HirayamaI for an intact, partly delaminated coating (breakpoint method). Cpe=paint film capacitance, Cd== double layer capacitance, W = Warburg impedance, Rpf = paint film resistance, Rc = charge transfer resistance and Rsol = solution resistance.
(a) id
Simulated Bode plot . . . . Iog(IZl) "=,. , % oO= °°° •
°
i -100 • alpha -80
105
(b) ira(z)
Simulated Nyquist-plot
-3.0 10~ -2.5 1 05
-60 -2.0 lo 6 -40
%°°,°.°°°
1000 ,
".
°
-20
o °°~
10 0.001 FIG. 2.
10
- I . 0 106 -5.0 105
0 0.1
-c51o6
1000
1 05
10 7
0
.~." 0
. . . . . . 1.0106 2.0106
3.0106
4.010~
(a) Simulated B o d e plot for the p r o p o s e d circuit, arrows indicate the breakpoints in the log [ZI curve; (b) simulated Nyquist plot for the p r o p o s e d circuit.
Normally the measurements cannot be performed above + 1 0 0 k H z . If the equipment is able to measure at higher frequencies the influence of cables and other parts of the equipment will be included in the measured impedance. 5 Using the proposed equivalent circuit it is possible to derive a quantitative relation between the frequency of the breakpoints in the log Izl curve and the values of the capacitors of the equivalent circuit. The values of the plateaux in the log IZI curve are related to the values of the resistors of this circuit. In case of actual measurements it is possible to obtain realistic values for the components of the circuit using the method described above. When this is done at regular time intervals after start of the exposure, one may monitor the changes of the components of the equivalent circuit. From these changes the delamination area can be determined quantitatively. According to Hirayama 1 the equivalent circuit of Fig. 3 represents a defective delaminated coating. When using this circuit it is again possible to derive the relationship between the breakpoint frequencies and the plateaux and the values of the components in the equivalent circuit. A remarkable feature of this circuit is that the delamination component and the defect component are completely separated. The authors I obviously assume that this will also be valid in practical situations.
Electrochemical techniques for delamination studies
645
C pf
m
i
W
Rot
R °°1
Rctp FIG. 3. Eqivalent circuit proposed by Hirayama I for a partly delaminated coating with a defect. C~lp = double layer capacitance of pore, Rotp = c h a r g c transfer resistance of pore and Rp,, = resistance of pore.
The most probable impedance equivalent circuit method The MPI method starts with an intact, dry coating without delamination. When this coating is exposed to a solution it will take up water. With a large number of short impedance measurements (10-50 kHz, this takes about 2 min) it is possible to measure the small changes caused by the uptake of water. Analysis is done by fitting a R(RCpO circuit on these data, using the Boukamp program EQUIVCRT. 6 From the Cpf data obtained as a function of time it is possible to calculate the uptake of water in the coating. This method had been described by Geenen and van Westing in several publications. 3,4,7.s The simple circuit used describes the behaviour of an intact coating in this frequency range very well. From a theoretical point of view this circuit cannot describe the impedance of a delaminated coating. However, it appears that for a defect-free barrier coating the changes caused by the delamination are very small: the system remains high in impedance and the simple circuit can still be fitted. Only in the curve of Cpf against time will a small step indicate the onset of delamination (see Fig. 6 later). This step does not give quantitative information. With the circuit from Fig. 1 it is not possible to explain these observations; therefore, other equivalent circuits are needed. Figure 4 shows a possible circuit. The precise configuration of the delamination part is less important. Most important is the fact that this part is no longer in parallel to the coating, but is placed in series with a part of the coating impedance. This is logical, because it is assumed that the coating is defect free so the delamination will occur under the coating, leaving the coating itself unchanged. When a simulation is made with this circuit the Bode and Nyquist plots of Fig. 5 are created (for these simulations the same values as for Fig. 2 were used with • = 0.1). Although Fig. 5 is a simulation, it is representative for actual measurements. It appears that the delamination components are even in a simulation completely hidden in the coating capacitance. So it is obvious that there is a large difference with the foregoing method. Apparently the equivalent circuit proposed in Fig. 1 describes another situation, and is not the most probable equivalent circuit for a defect-free, delaminated barrier type coating. Therefore the circuit of Fig. 4 is preferred. It
646
D.H. VANDER WEIJDE, E. P. M. VANWESTINGand J. H. W. D~ WIT (1 -¢~)Cpf
W
Rc~
¢~Cpf ~pf
FIG. 4.
Equivalent circuit that m a y r e p r e s e n t an intact, partly delaminated coating ( M P I
method). • = delaminated fraction of surface.
10~ Iog(IZI) .......................... , .~ ........... . 10s " o Simulated Bode-plot =-.
10'
-100 alpha -80
Im(Z)
Simulated Nyquist-plot
-I,I lO5
:60 -7.5 I1)4
103
•
-40
10~
-20
-33 104
oO
10'
..........
0.001
..0,,°
0.1
1 01 log(frequency)
FI6.5.
1 03
1 05
0
0
0
5.0 104
1.0 105 1.5 105
2.0 105 2,5 105
Re(Z)
Simulated B o d e and Nyquist plot of a delaminated coating. At this point the Cpf-time curve already shows a step.
should be stressed that the delamination part of this circuit is rather arbitrary. It is possible that this has another configuration, but this combination is chosen because it is comparable with that of Fig. 1. When an R(RC) circuit is fitted to the simulation of Fig. 5 (using the B o u k a m p p r o g r a m E Q U I V C R T ) the simple circuit is still able to fit on the simulation of the complex circuit. Only the values of the capacitor and resistors change a little. This explains the small step in the capacitance-time curve. A representative example of this step is given in Fig. 6. This was measured on a transparent coating so that corrosion, and the preceding delamination, could also be observed visually. Obviously the delamination part of the circuit is almost invisible. Only in a plot of the difference between simulation of the complex circuit and the R(RC)fit are some differences visible, as can be seen in Fig. 7. For actual m e a s u r e m e n t s these errors fall within the noise of the measurement. In the foregoing part it was m a d e clear that it is only possible to obtain qualitative information on delamination as long as the coating is defect free. The M P I method can also be extended to other situations, e.g. a coating with delamination and a defect. When defects are f o r m e d in a partly delaminated coating it is logical to assume that these defects are f o r m e d in the coating above the delaminated area, for
Electrochemical techniques for delamination studies
647
Cp~F) 8.0 10 [ ° ,. 7 £ 10 -I°
7,0 I0 "1°
i 6.5 IO-]°
4 I
Ji
6.0 I0 "t0 0
200
400
600
8001000
1200
1400
Time (min) FIG.
Coating capacitance as a function of time. At point 1 no corrosion is visible, at points 2 and 3 small corrosion spots are visible.
6.
131 I0 4 6.0 lff 5 2.0 11)5
[
1.11 lO ~
~ Relative difference between simple and extended equivalent circuit -
6.0 IU 5
L t
~
-2.0 10 5
~
-6.0 l(t 5
~ ~ f
2.0 I0 5 -2.11 I(15
- -
~6~ io 5
-lmaginairen-or Real error
-1.0 I(I ~
-I.(1 IO 4
II.{X) 1
(I. I
1111
I0 t
I(15
log(frequency) FIG.
Difference between simulation of a complex circuit and a fit with a simple circuit as a function of frequency.
7,
Cpf
W FIG.
8.
Rc t
po
Equivalent circuit that represents a partly delaminated coating with a defect.
e x a m p l e , w h e n blisters burst due to o s m o t i c pressure, or w h e n blisters are d a m a g e d m e c h a n i c a l l y . O n l y w h e n defects are m a d e m e c h a n i c a l l y (scribed) is it possible that defect and d e l a m i n a t e d area are separated. H o w e v e r , d e l a m i n a t i o n m a y also occur at a scratch, y i e l d i n g the s a m e result. In practice m o s t defects will occur in the d e l a m i n a t e d area. T h e r e f o r e the circuit o f Fig. 8 s e e m s better than the circuit o f Fig. 3. T h e exact configuration o f the d e l a m i n a t e d area and the defect part m a y differ
648
D.H. VANDER WEIJDE, E. P. M. VANWEST1NGand J. H. W. DEWIT
coatin~ I
salt-layer , /t
I
metal substrate
FIG. 9.
Simplified sketch of the tested system.
from this m o d e l . I n Fig. 3 the blister is in parallel to the d e l a m i n a t i o n a n d the coating, in Fig. 8 the d e l a m i n a t i o n a n d the defect are p u t in series with each o t h e r a n d in parallel to the rest of the coating. This has a b e t t e r r e l a t i o n to the actual s i t u a t i o n as was e x p l a i n e d above. A circuit like that of Fig. 8 can again b e fitted o n m e a s u r e d d a t a using the p r o g r a m E Q U I V C R T . I n s o m e cases o n e m i g h t b e able to get q u a n t i t a t i v e i n f o r m a t i o n a b o u t the defect a n d the d e l a m i n a t e d area. E X P E R I M E N T A L METHOD To verify and compare the two methods several experiments were performed. For this purpose a pigmented alkyd coating was applied to a steel substrate that was partially contaminated with NaCl. This contamination was made by applying a drop of 5/,d of a 3% NaCl solution on the substrate. After drying a thin salt film covers about 0.5 cm2 of the substrate. When the substrate is coated, the situation shown in Fig. 9 results. It is obvious that in this situation delamination will occur as soon as the first water reaches the interface between salt and coating. In principle the (thin) coating above the formed salt solution will remain undamaged. The coatings were studied with EIS during immersion in a 3% NaC1 solution at 20°C. During the first few hours the measurements were performed over a limited frequency range (5-65 kHz), because the system was changing rapidly due to water uptake of the coating. After several hours this frequency range was extended until after some days a range from 10 mHz to 100 kHz was measured. EIS measurements were carried out using a Schlumberger FRA-1255 and a Solartron ECI-1286. Both were controlled with a DOS-PC using the Interactive Software for Impedance Spectroscopy (ISIS) software9 that was developed within this group. Details on the cell that was used can he found in Geenen.4 The height of the solution above the coating was about 7 cm. The steel substrates were type S Q-panels from the Q-panel company (Cleveland, Ohio, U.S.A.). E X P E R I M E N T A L RESULTS T w o essentially identical p a n e l s were tested, a n d will be r e f e r r e d to as case A a n d B. D i r e c t l y after i m m e r s i o n case A s h o w e d a N y q u i s t a n d B o d e plot that d e v i a t e d from the e x p e c t e d plots for intact coatings. A r e p r e s e n t a t i v e m e a s u r e m e n t after I h is given by Fig. 10(a). A f t e r 10 days, Fig. 10(b) results. A t that time b r o w n c o r r o s i o n p r o d u c t b e c a m e visible o n the edge of the c o n t a m i n a t e d area. A p p a r e n t l y the c o a t i n g h a d a defect. This m a y b e caused by a p o o r w e t t i n g of the c o a t i n g o n the salt film. O n the edges of the c o n t a m i n a t e d area, w h e r e the salt film m a y b e s o m e w h a t thicker, this results in a p e r f o r a t i o n of the c o a t i n g by the salt film which f u r t h e r results in a d e l a m i n a t e d area with a defect a b o v e it. W h e n Fig. 10(b) is a n a l y s e d using the B o u k a m p p r o g r a m it a p p e a r s that at least four time c o n s t a n t s are p r e s e n t in the m e a s u r e m e n t . This m e a n s that t h e r e should be four b r e a k p o i n t s in the B o d e plot (Fig. 10c). T h e r e are, h o w e v e r , only t h r e e b r e a k p o i n t s visible. F u r t h e r m o r e these b r e a k p o i n t s are n o t easy to d e t e r m i n e b e c a u s e they are n o t sharp d u e to s o m e o v e r l a p p i n g time constants. Case B has a different start. H e r e a n o r m a l , capacitive b e h a v i o u r is seen after I h as s h o w n in Fig. 11(a). A f t e r 2 days this has still n o t c h a n g e d as can be seen in the N y q u i s t plot of Fig. 11(b). This b e h a v i o u r is o b s e r v e d for several days. H o w e v e r ,
Electrochemical techniques for delamination studies
(a)
(b)
649
lm(Z) -5.0 104
Im(Z) -3.5103
Nyquist-plot of coating A after 1 hour immersion
Nyquist plot of coating A after 10 days immersion
-4.0 104
-30103 3.0 104 ?
J
2.5103 -2.0 104 ~
2.0 103
S
-10 104 ~ / / / "
-I.5 103 -I.0 103 2.0103
3.0 103
4.0 103 Re(Z)
(C)
0.0 10°
5.0103
6.01() ~
0
3.0 104
6.0 104
9.0 104
1.2 105
Re(Z)
i -IOR) - alpha
lO~ I •
;
Bode-plot of coating A 10 days after immersion
Iog(IZl) I
-80
I05 L -60 -40 104 L
-20
E 10 3 [
°.o~""
0.001
0.1
101
I(}5
103
0
log(frequency) FIG. I0.
Two Nyquist plots of measurement on coating A: (a) after 1 h and (b) after ten days. (c) Bode plot of Fig. 10(b).
(a)
(b) Ira(Z)
Im(Z) -3o ]o 5
-2.5 106
Nyquist plot of coating B after 1 hour immersion
Nyquist plot of coating B after 2 days immersion
-2.0 106 - 2 0 I()5
-1,5 106
$~'
q.O I06 1.0l~ -5.0 105
2.5 104
5.0 104 Re(Z)
Fro. 11.
7.5 104
l.O 105
0
1.0106
2.0106 Re(Z)
3.0106
4.0 I0 ¢'
Two Nyquist plots of measurement on coating B.
after 2 days delamination is already clearly visible because a small blister is formed at the contaminated area. The corrosion potential appeared not to have a stationary value. Several times the o p e n circuit potential drifted to - 3 0 0 0 m V ( S C E ) . Every time this happened it was polarised at a normal potential for steel in this solution. This behaviour is due to
650
D . H . VANDER WEIJDE, E. P. M. VANWESTING and J. H. W. DE WIT
Cpf(F)
6.5 10 -l°
f
6.2 10 l °
j
J
.,f e
5.9 10-1° 5.6 lff l° 5.3 10 -I°
,/
/
5.0 10-j° 0
100
200
300
400
Time (rain)
FIG. 12.
500
Capacitance as a function of time duringwater uptake of coating B.
-90
l0 s
Ira(Z)
Bode-plot of coating B one h o u r after perforation
N y q u i s t plot of coating B one h o u r after perforation
-I.1 104 -60 104
-7.5 103
~
~
-
-30 -3.7 10 3
0
0
1.0 104
Fie. 13.
2.0 104 3.0 104 4.0 l04 5.0 l04 Re(Z)
l0 s 0.1
l01 103 log(frequency)
l0s
Nyquist and Bode plots of measurements on coating B directlyafter perforation.
the very high resistance of the coating. The equipment is not able to handle this accurately. It is obvious that the impedance that was observed is different from the behaviour presented by Hirayama 1 in Fig. 2. So the BF method is based on a wrong assumption for the equivalent circuit and therefore cannot give any information about the starting delamination under a barrier coating. The MPI method does in this case give some qualitative information. From the Cpf-time plot of Fig. 12 it is clear that the coating is not stable, but is delaminating. In this case the expected step at the start of the delamination is not clearly visible. This is because the delamination is forced and is starting when the coating is still taking up water. Therefore water uptake and the step are not separated. From these measurements it is clear that the intact film indeed dominates the impedance of the total system as was predicted by the circuits and simulations of Figs 4 and 5. To test the MPI method the blister of Case B was deliberately perforated with a needle. Directly after that the impedance spectrum of Fig. 13 was measured.
Electrochemical techniques for delamination studies - -
651 IZI
al aha O
25t0S Nyquist plot of coating B 2o l c~ s o m e d a y s af ter p e r f o r a t i o n .
~ 107
• • " , 40
10105
f'
~
\..
," -SO
5 c)104 0 2O]O ~
4q)l(] ~
60 It} =
B
flOl( 15
1011)6
1211c~
-1.0 10;0 0 0 ]
: I0 s
1 O4
B o d e plot o f c o a t i n g B some days after perforaUon 0.1
10
103
1
O5t 0 0 0
log(frequency)
FlG. 14. Nyquist and Bode plot of measurements on coating B some days after perforation of the blister. Because it is known that defect and delamination are combined, an equivalent circuit like that of Fig. 8 is used to describe this m e a s u r e m e n t instead of using Fig. 3. After some days the spectrum of Fig. 14 was recorded. Qualitatively this is the same as in case A. Quantitatively they are different which can be expected because the shapes of the defects in the delaminated area are also different. When this m e a s u r e m e n t is analysed according to the M P I method, again at least four time constants are found. In the Bode plot, however, it is not even possible to determine three breakpoints, because two breakpoints are nearly combined at the edge of the curve. In Case B also some brown corrosion products are visible some hours after perforation. The impedance in both cases (A and B) increased with time. Apparently the defect was becoming blocked by corrosion products. This will increase the resistance of the defect and thereby the impedance of the system. A complete analysis of all m e a s u r e m e n t s is beyond the scope of this article and will be published later. CONCLUSION From the results of the m e a s u r e m e n t s described above it is clear that quantitative m e a s u r e m e n t s of delamination under an intact coating are not possible with both methods. Only the M P I method will give a qualitative indication of the onset of delamination. Also on a theoretical basis this method is preferred above the BF method, especially when breakpoints are so close together it is not possible to distinguish between the separated points. This method may work in a few, simple cases but it is not generally applicable. The MPI method is more generally applicable. Besides the difference in the method there is also a difference in the equivalent circuits used. F r o m the experiments it was proven that the circuits for a delaminated coating as presented by H i r a y a m a cannot describe the measurements on delaminated barrier type coatings. The delamination part of the circuit should be in series with a part of the coating. In case of a delaminated and defect containing coating it is preferable to put the delamination and the defect part of the circuit in series instead of putting everything in parallel.
Acknowledgements--The authors wish to thank AKZO corporate research Arnhem, Hoogovens IJmuiden, DSM Research Geleen, Shell Research/Billiton Arnhem, SigmaCoatings Amsterdam and the Dutch Ministry of Economic Affairs for financial support of this research.
652
D . H . VANDER WEIJDE, E. P. M. VArqWESTIr~Gand J. H. W. DE WIT
REFERENCES 1. R. HIRAYAMAand S. HARUYAMA,Corrosion 42,952 (1991). 2. F. MANSFELDand C. H. TSAI, Corrosion 42,958 (1991). 3. E. P. M. VAN WEST1NG, Determination of coating performance with impedance measurements. PhD. Thesis Tech. Univ. Delft (1992). 4. F. M. GEENEN, Characterisation of organic coatings with impedance measurements. PhD. Thesis Tech. Univ. Delft (1991). 5. E. P. M. VAN WESTING, G. M. FERRARI, F. M. GEENEN and J. H. W. DE WIT, Prog. Org. Coat., accepted. 6. B. A. BOUt,AMP, Solid State Ionics 23, 89 (1986). 7. F. M. GEENEN, J. H. W. DE WIT and E. P. M. VANWESTING, Prog. Org. Coat. 18 (1990). 8. E. P. M. VAN WESTING, D. H. VAN DER WEIJDE, G. M. FERRARIand J. H. W. DE WIT, Proc. lOth Eur. Cong. Corrosion, Barcelona, Spain (1993). 9. H. J. W. LENDERINK, M. M. DlNX and D. H. VAN DER WEIJDE, Interactive Software for Impedance Spectroscopy (ISIS). Tech. Univ. Delft, The Netherlands (1991).