coating interface

Corrosion Science, Vol. 26, No. 9, pp. 735-742.1986 Printed in Great Britain

001&938X/86$3.00+ 0.00 @ 1986Pergamon Journals Ltd.

AN ELECTROCHEMICAL IMPEDANCE SPECTROSCOPY STUDY OF REACTIONS AT THE METAL/COATING INTERFACE F. MANSFELD,* Rockwell

International

S. L. JEANJAQUET

Science Center,

1049 Camino

and M. W. KENDIG

DOS Rios. Thousand

Oaks,

CA 91360, U.S.A.

Abstract-A

segmented two electrode system has been used to obtain impedance spectra under a polymer coating. These spectra have been compared with those obtained across the coating between the short circuited steel electrodes and a counter electrode. A transmission line model has been used to obtain estimates for the delaminated volume and the polarization resistance of the steel surface as a function of exposure time to a NaCl solution. INTRODUCTION

of electrochemical impedance spectroscopy (EIS) to polymercoated metals has resulted in new information concerning the interaction of the metal/coating system with corrosive environments. The model in Fig. 11.2suggests that the properties of the coating and its changes during exposure to electrolytes can be determined at relatively high frequencies, while the corrosion reaction at the metal surface can be evaluated at relatively low frequencies.3 The coating capacitance C, has been found to increase with time due to water uptake. The authors have used the pore resistance Rpo as a relative measure of the corrosion protection provided by the coating. 24 It has been sugg ested earlier that the build-up of corrosion products at the metal/coating interface produces stresses in the coating which in turn decrease R .2s This suggestion explains why different surface treatments can have such a layge effect on the lifetime of a metal/coating system such as steeYpolybutadiene. While EIS has provided useful new information concerning reactions in the coating, as expressed by C, and Rpo in Fig. 1, it has been difficult to determine values of the polarization resistance Rp and double layer capacitance C,, as a function of exposure time. This is due to the fact that R, and C,, are accessible only at very low frequencies for the systems studied so far. At these very low frequencies, experimental difficulties and time restraints make accurate measurements of R, and C,, problematic. If R, is lower than Rpo, then its accurate determination becomes difficult, if not impossible. The study of the reactions occurring at the metal/coating interface, such as corrosion and loss of adhesion, is very important from both a fundamental and practical standpoint. The present results are based on a variation of the experimental technique for obtaining EIS data in coated metals in which impedance measurements have been carried out under the coating. These EIS data are being compared with those for the more traditional approach of measuring impedance data across the coating. The former data have been obtained with the

THE APPLICATION

* Present address: Department CA 90089-0241, U.S.A. Manuscript received

Materials Science,

University

of Southern

31 July 1985; in amended form 18 April 1986. 735

California,

Los Angeles,

736

F. MANSFELD,S. L. JEANJAQUETand M. W. KENDIG Ce

RO

GENERAL MODEL

Cl-

R po Rp MODEL I

Fro. 1.

Model for impedance of polymer-coated metals.

electrode system which is used in the atmospheric corrosion monitor (ACM). It consists of a number of parallel steel plates which are isolated from each other. 6 The a.c. signal is applied between alternate plates. EIS data have been obtained as a function of exposure time /corr to an aerated 0.5 M NaCI solution for clean steel surfaces and for surfaces which were exposed to diluted NaCI before coating. EXPERIMENTAL METHOD EIS data have been obtained with the optimized experimental approach described earlier 7 which allows accurate determination of the electrode impedance over a wide frequency and impedance range. A steel/steel ACM was used as the substrate which consisted of 20 steel plates 1 mm wide and 6.5 cm long. The steel (A366) plates are separated by a 50/.tm thick mylar foil. Alternate steel plates are connected to establish a two electrode system. The epoxy coating was 20 p.m thick. It consisted of 0.7 weight fraction epoxy and 0.3 weight fraction polyamide. After curing of the coating an electrochemical cell was attached to the active part of the ACM which contained the saturated calomel electrode (SCE) reference electrode and a large Pt counter electrode. The test electrolyte was an aerated 0.5 M NaC1 solution. The counter and reference electrodes were used for measurement across the coating which was carried out at the open circuit potential of the coated steel using a PAR potentiostat Model 173 with a Model 276 interface. The measurement under the coating was carried out at 0 mV applied potential. A Solartron 1174 transfer function analyzer was used with a 10 mV a.c. signal. EXPERIMENTAL

RESULTS AND DISCUSSION

Figure 2 gives a typical result for the changes of the electrode impedance IZ I as a function of tcorr for measurements across the coating and under the coating, respectively. In the former case, the capacitive behavior observed at frequenciesf = ~o/2~r exceeding 10 Hz corresponds to that of a 20 ~m epoxy film for e = 4 and A = 13 c m 2 (Co in Fig. 1). At lower frequencies, the pore resistance Rpo becomes apparent. It slowly decreases during exposure, but remains at the high values observed earlier for 20/~m coatings. 2 For measurements under the coating, a much lower impedance, which changes significantly with tcorr, is observed. The initial capacitance at high frequencies

Study of reactions at the metal/coating interface

737 SCB4-26566

I EPOXY CO/TING/STEE~/O.5N NaCI

-

-

"

- - ~ .

®

~-'~(~

Q

DAY 1

®

DAY 7

o,~, , ,

,c.oss

co,m,G

.

._.E

o=

2

FIG. 2.

I 0

I 1

1 2 log ~ ( w in rad/s)

I 3

I 4

Bode plots for measurements across and under an epoxy coating on a steel ACM as a function of tcorr.

corresponds to that of the bare steel/mylar/steel system (Fig. 4). After longer exposure times, a - 0 . 5 frequency dependence of the impedance is observed at intermediate frequencies which points to transmisson line effects. A photograph of the ACM surface for tcorr = 14 days is shown in Fig. 3. The rust spots were initially green. After longer exposure times, corrosion spots cover several adjacent steel plates providing a path for the current flow under the coating. Considering the relatively long exposure time, such spots are not too numerous. As observed earlier with EIS 2-4 and well known in practice, contamination of the steel surface before coating can lead to rapid corrosion and loss of adhesion of the coating. This effect was simulated by covering the ACM surface with a solution of 1 mM NaC1 containing some ethanol which was allowed to dry. The epoxy coating was applied to the resulting surface. As Fig. 4 shows, much more rapid changes of the impedance occur in this case for both types of measurements. After tcorr = 1 day, Rpo is already lower than the value for tcor~ = 7 days for an uncontaminated ACM surface. For measurements under the coating rapid changes occur even after Rpo has become constant. The slope of - 1 / 2 is found at intermediate frequencies. Figure 4 also shows an impedance spectrum for the bare surface. The much larger extent of corrosion covering a number of neighboring plates can be seen in the photograph in Fig. 5 for tcorr = 7 days. The underfilm impedance spectra can be evaluated in terms of a transmission line model. 2"
738

F. MANSFELD, S. L. JEANJAQUETand M. W. KENDIG 8

~ I

I "~--BLANK

3h

6

I

I STEEL/STEEL

I

5h

o

~" 5

24h

4

72h

3

2 -2

Fl~. 4.

-1

~ 0

1 2 Log oJ(oJ in rad/s)

3

4

5

Bode plots for measurements across and under an epoxy coating on a steel ACM with NaCI pretreatment as a function of to,,,,.

tance C and parallel resistance Rp with a series ohmic element R due to the finite resistance of the electrolyte layer resulting from lift-off of the coating by a distance x (Fig. 6). In actual fact, only a fraction, f, of the total ACM length will be delaminated. A capacitor Ci,s due to the mylar separators and an ohmic resistance R a due to leakage across the insulator completes this transmission line model (Fig. 6). Since the separator capacitance has values of several nF (Fig. 4), it will influence the observed impedance under the coating only above 10 2 rad s -l. Hence the impedance measured under the coating at low frequencies equals:

,Z,= R~ + R

+

~R_~

RRp + 1 + jwRpC"

(1)

R, Rp and C depend on the differential 6 as follows: R-

p6 xLf

(2)

Rp = R°/6Lf C = COSr~ d ~J,

(3) (4)

where R ° and C~ are the specific capacitance (F cm -2) and corrosion resistance ( ~ cm 2) for the interface. The term p (ohm cm) is the bulk resistivity of the delaminated area of thickness x (Fig. 6). In the limit 6 --~ 0, lim ]ZI = Ra +

a~o

~/

RRe 1 + joJRpC'

(5)

Equation (5) may be fitted by computerized adjusting of three constants Ki for a properly weighted least squares fit according to a procedure described elsewhere: 9

Izl = K, + K2/V1 + joJK3.

(6)

8C88-31477

S T E E L I S ~ E L / 2 0 ~ E ~ X Y COATING tcorr = 14 DAYS FIG. 3.

Photograph of epoxy-coated ACM surface for tcorr = 14 days.

739

STEEL/STEEL/20 ~m EPOXY COATING NaCI PRETREATMENT tcorr = 7 DAYS FIG. 5.

Photograph of epoxy-coated A C M surface with NaC1 pretreatment for tcorr = 7 days.

740

Study of reactions at the metal/coating interface

741

COATING

--I

R~

R

Cins T FIG. 6.

Model of delaminated area and corresponding equivalent circuit.

Two critical parameters can be expressed as a function of the resulting values KI, K2 and K 3. The corrosion resistance Rp° of the delaminated region is calculated as: R~ - K3 C]"

(7)

An effective 'volume of delamination' Veff can be obtained as

Ve# = xLdf2 = K3 p d

c',l L"

(8)

where d is the width of each ACM plate. Assuming that the electrolyte within the delaminated region has a resistivity equal to that for the bulk electrolyte (O = 20 ohm cm) and that Cat = 30/~F cm -2, the values for R ° and Ve#can be calculated using the set of fitted parameters (K1, K2, K3). For these calculations, L is taken as 20 x 6.5 cm and d is 0.1 cm. As can be seen from Table 1, a rapid increase in V4f for the contaminated surface occurs from 4.2 x 10 -9 cm 3 at 5 h to 3.8 x 10 -6 cm 3 at 146 h, while the corrosion resistance decreases slowly in the first 72 h, but then drops by an order of magnitude between 72 and 146 h once delamination has occurred. The clean surface also exhibits delamination, but only at much later times. In general, the corrosion resistance for the clean surface at comparable delamination is higher than for the NaCI contaminated surface. This suggests that the surface treatment and contamination have a significant influence on corrosion rates at the earliest stages of the hydrolytically induced adhesion loss. An important point to note is that the observed corrosion resistances for both surface preparations are of the order of 104-105 ohm cm 2, an order of magnitude or more higher than the observed values for steel in aerated 0.5 M NaCI solution. This suggests that less corrosive conditions occur at the metal/coating interface despite initial debonding of the coating.

742

F. MANSFELD,S. L. JEANJAQUETand M. W. KENDIG TABLE 1. CORROSIONRESISTANCERp0 AND EFFECTIVE DELAMINATED VOLUME Veff FOR COATED STEEL A C M s EXPOSED TO 0.5 M NaCI

Time (h)

R ° (ohm cm 2)

Veff(cm 3)

24 168 336

I, clean surface 2.7 x 105 9.0 x 10-9 6.9 x 105 9.3 × 10-8 1.1 x 105 3.1 x 10-6

5 24 72 146

1.4 1.7 1.4 1.5

II, NaCl treated surface x 105 4.2 x x 105 6.9 x × 105 1.5 x × l04 3.8 x

10-9 10 -9 10 -6 10-6

The effective delaminated volume Vefyisvery small, which could be due to the fact that in these experiments most of the corrosion activity was located along the steel plates, but did not overlap two adjacent plates and contribute to the measured impedance (see Figs 3 and 5). For the untreated surface, only a few areas exist where the corrosion spots cover adjacent plates. In the case of the NaCl pretreatment, many more such areas have developed despite the shorter tcorr. The assumed values of C O and p might not be entirely correct and contribute to some error in the calculated values of R ° and V~fp CONCLUSIONS

Impedance spectra obtained under a polymer coating and their comparison with spectra determined across the coating provide new information concerning the reactions at the metal/coating interface. The segmented two electrode technique is useful for laboratory studies, but could also be used in outdoor studies of atmospheric corrosion of polymer-coated metals. Results obtained with Cu/steel and Zn/steel electrodes will be reported elsewhere. 10 Acknowledgement--This work has been funded by the Office of Naval Research under Contract N00014-79-C-00437. REFERENCES 1. L. BEAUNIER,I. EPELBOIN,J. C. LESTRADEand H. TAKENOUTI,Surf. Technol. 4, 137 (1976). 2. F. MANSFELD,M. W. KENDIGand S. TSAI, Corrosion 38, 478 (1982); M. W. KENDIG, F. MANSFELD and S. TSAI, Corros. Sci. 23,317 (1983). 3. F. MANSFELD and M. W. KENDIG, Proc. Int. Congress Metallic Corrosion, Vol. 3-74. Toronto, Canada, June (1984). 4. F. MANSFELDand M. W. KENDIG, ASTM, STP 866, 122 (1985). 5. M. W. KENDIG, F. MANSFELDand A. ARORA, Proc. Int. Congress Metallic Corrosion, Vol. 4-73. Toronto, Canada, June (1984). 6. F. MANSFELDand J. V. KENKEL,Corrosion 33, 13 (1977). 7. M. W. KENDIG,A. T. ALLENand F. MANSFELD,J. electrochem. Soc. 131,935 (1984). 8. C. FIAUD, R. CHAHROURI, M. KEDDAM, G. MAURIN and H. TAKENOUTI, Proc. 8th Int. Cong. on Metallic Corrosion, Vol. 1-18. Mainz, West Germany, Sept. (1981). 9. B. BOUKAMP,in Computer Aided Acquisition and Analysis of Corrosion Data (M. KENDIG, U. BERTOCCIand J. E. STRUT'r,eds), PV 85-3. The Electrochem. Soc. 10. F. MANSFELD, S. L. JEANJAQUETand M. W. KENDIG, to be submitted to Corrosion.