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Impedance measurements for the evaluation of protective nonmetallic coatings
Progress in Organic Coatings, 10 (1982)
171
171 - 163
IMPEDANCE MEASUREMENTS FOR THE EVALUATION OF PROTECTIVE NONMETALLIC COATINGS
T. SZAUER Jnetitute of Inorganic Chemistry and Technology, 80-952 Gdairsk (Poland)
Technical University of Gdarisk,
Contents Summary ..........................................................
171
1 2 3 4 5
171 173 177 179
Introduction .................................................... Insulating and barrier acting coatings ................................. Coatings with conductive pathways. .................................. Adaptation of Randles equivalent circuit for MCE systems ................. Impedance 2” versus 2’ data for hydrocarbon based temporary protective coatings. ....................................................... 6 Discussion. ..................................................... References ........................................................
180 181 182
Summary Questions regarding the interpretation of impedance data obtained with metal-coating-electrolyte solution systems are discussed on the basis of theoretical considerations and experimental results. The analysis of plots in the form of Nyquist diagrams leads to the conclusion that the method provides valuable infkrnation about protective properties of coatings and enables identification of the mechanism of protection.
1. Introduction Impedance measurements of protective coatings on metals supply valuable information about the protective properties and the mechanism of action. In spite of some disadvantages associated with the simplified nature of the test, carried out mostly by immersion of coated electrodes, measurements of the impedance or its components have found many practical applications [ 1 -161. These measurements are being accomplished in various instrumental variants and data are represented in a number of ways. For instance, dielectric and barrier characteristics of. coatings are commonly tested by impedance measurements as functions of angular frequency o and time of immersion T, data being frequently plotted as lg e” = f(lg w) and 2’ = 0033-0656/82/010171-13/$03.50
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f(r), where e” and 2’ are the imaginary component of the complex permittivity E and real component of the impedance 2, respectively. For instance, such data representation for polymer coatings may be found in interesting work of Leidheiser et aZ. [ 17 - 211. Simplified tests for the presence or absence of conductive paths through the coating consist of the determination of functions 2 and C = f(o) [4,8,14,22] or the famous Bode dependence 2 = f(o) [5,10,23]. Another variant arises from the graphical representation of impedance data in the form of the Nyquist diagram [23 - 251, being the parametric function 2” = f(Z), with w as a parameter, 2” being the imaginary component of the impedance. The last procedure, based on special measuring techniques [ 11, 261, appears particularly interesting since it allows more comprehensive evaluation of corrosion protective properties of coatings and the mechanism of their action. This approach, moreover, enables simultaneous assessment of the actual eligibility of the main concepts of protection by coatings, namely, the surface insulation or partial blocking and the kinetic hindering [27] . The first two concepts relate in a direct manner dielectric, barrier as well as adhesive, properties of the coating to obstructed diffusion of the solution through the coating and limited area of the immediate metal-solution contact. In the case of the kinetic hindrance, protection is mainly linked to, the enhanced charge transfer resistance R, , or to retarded transport of reagents for the electrode reactions. Experimental data of the function of 2” = f(P), =VBrfor real metal-coatingelectrolyte solution (MCE) systems exhibit quite a complex nature and a number of variations [ 5,11,12] . Interpretation of some of these still remains non-evident and ambiguous. The aim of this communication is an effort to systematize questions falling within the scope of the evaluation of coating protective abilities by the analysis of the function of 2” = f(Z’ ), =VBr.The key for such a systematic treatment should be a possibly general and adequate electrical equivalent circuit, comprising most existing MCE systems. One can find in the literature a number of propositions in this matter, more or less satisfactorily describing the real MCE or electrochemically related systems, when subject to the oscillating signal V = V,-, exp@T) [2,4,11,18,21,22,28 - 301. One of the most general and comprehensive solutions has been given by Menges and Schneider [5] . The authors considered separately the group of coatings whose degradation is governed by the second Fick law, describing diffusion of the solution to the substrate, and the group of porous and defective coatings with electrolytic, conductive paths allowing a direct contact between the metal and solution. The first group has received a modified treatment by Leidheiser et al. [17,21], while the second has been specifically described by Wormwell and Brasher [l] , Okamoto and Morozumi [ 21 and Klark and Mikhajlovskaja [22] , as well as a general treatment by Beaunier et al. [ll]. Allowing for the variety of protective coatings encountered in practice, it seems useful to introduce three complementary equivalent circuits, facilitating the interpretation of 2” = f(Z), EVllldiagrams.
173
,
,,
Fig. 1. Equivalent circuit for insulating and barrier acting coatings. RE resistance of electrolyte solution between working and counter-electrode; RjE resistance of penetrated part of coating; Rc resistance of intact part of coating; Cc, capacity of intact part of coating; CE, capacity of penetrated part of coating.
2. Insulating and barrier acting coatings Eligible coatings are those lacking continuous conductive paths, which usually allow immediate contact between -metal and electrolyte solution. Corrosion protection afforded by these coatings is predominantly due to dielectric and barrier properties of the coating material. The main evaluating factor of the protection is thus the time-related resistance of the coating to become penetrated with conductive solution. The equivalent circuit, which has been found to describe adequately the MCE system in question, is shown in Fig. 1, being a double terminal network built up of two reactive segments in series representing penetrated and non-penetrated fractions of the coating [ 51. Regarding both homogeneous [ 51 or random, heterogeneous diffusion of the solution into the coating [ 17,211, the allowance for metal-coating interface parameters seems to be negligible for common industrial metals. It stems from the fact that for barrier acting coatings on active metals, the total penetration with electrolyte solution and development of electrolytically conducting paths usually signifies the onset of electrochemical processes at the metal substrate and the necessity to adopt a different equivalent circuit. MCE systems liable to such representation consist mainly of those based on synthetic resins like epoxy and polyurethane types [5], and other polymers [17 - 211, applied in practice in relatively thick layers. It is worthy of note that some hydrocarbon based oils and protective coatings, even if in thin layers, behave similarly due to their hydrophobic nature and dielectric properties [ 27,311. Consider, now, the case of insulating and barrier acting coatings at the commencement of testing, provided that there is no electrolytic phase within the coating. In terms of impedance measurements with low amplitude sinusoidal signal, the applicable equivalent circuit may be that of Fig. 1, without the segment representing the penetrated fraction of the coating. Thus, one may consider the relevant MCE system as a leaky condenser and the frequency response will depend on the dielectric characteristic of the coating material. This characteristic is defined by dielectric losses, specifically by d.c. losses as well as processes such as dielectric dispersion and dielectric absorption [ 32 - 351. Schematic representations of 2” versus 2’ data for coatings
174
R' r
RE
w
2
Fig, 2. A. Schematic representation of the plots of 2” us. 2’ for insulating and barrier acting coatings. B. Schematic plots ‘a’, ‘b’ and ‘b” of lg 6’ us. lg o and ‘c’ of Ig E’us. lg O. Z”, E” and 2’. e’ are the imaginary and real parts of the impedance and permittivity, respectively.
showing no d.c. but Debye and/or Maxwell, Wagner and Sillars (MWS) losses are plots ‘a’and ‘b’in Fig. 2(A). The linearity of these plots might be accounted for on the ground of a correlation between the impedance and permittivity of relevant material. In the case of real polymers, Debye or MWS losses, brought about by dipole relaxation and interspatial polarization processes [32,36,37] , are featured by considerable broadening of the relaxation time and resultant linearity of plots of Ig e” versuslg o. Mathematically, this relation has been derived by Fuoss and Kirkwood [38], being illustrated by plot ‘a’in Fig. 2(B). Furthermore, it is of interest to express the impedance 2 of the leaky condenser, represented by the parallel equivalent circuit [39] , with use of the complex permittivity. Combination of eqns. (1) and (2) leads to the definition of real 2’ and imaginary 2” parts of the impedance as well as the slope (tan a) of the impedance vector, with use of real e’and imaginary e” parts of the complex permittivity = 2’ -
2
jz"
RC
=
1
E
=
e’ -
je"
=
Cc _ -
i
=
(1)
(2)
e” UC* [ (e”)2+ (e’)2]
2”
Rz
+ 02C2R2 c c - 1 + 02C2R2 e c
CO oCoRc 2’ =
jwC,
(3)
e’ WC, [ (ep2 +
(e’)2]
(4)
175
(5)
tanaL E”
where C,-,is the capacity of the spatial condenser. Thus, independence of lg E”and lg E’uersus lg w for real polymers displaying Debye and/or MWS losses [ 401, as visualized by plots ‘a’and ‘c’in Fig. Z(B), leads to the conclusion that plots of 2” uersus 2 should be straight lines, the slope being governed by the relationship between E’and E”. One may point out the rule that the greater the dielectric losses e”, the lower the slope of the lines. The limiting cases for these lines will be the coordinate origin at o + 00 and the infinity at w + 0. However, due to the definite conductivity of the testing solution, the intercept with the abscissa will be in practice determinative of RE. In the rather rare case of coating materials showing specific Debye relaxation time, resulting in the Cole-Cole semicircle in lg e”-1g E’coordinates [ 41,421, there wil1 occur some deviations from the linearity of the plots of 2” uersus 2’ within o values defining ELand ek quantities, i.e. the real permittivity at 0 and at the greatest frequency influencing its values, respectively. The apparent discrepancy between the assumed parallel equivalent circuit and the non-semicircle impedance response on the 2”-2’ plane arises from the fact that the impedance data relate not to ohmic conductivity, i.e. the migration of charges as ions or electrons, but to dielectric losses of nonconductive nature. For the same conditions as above, one may note a bent plot in 2”-2’ coordinates, as shown by curve ‘c’in Fig. 2(A), for coatings displaying ionic or electronic, and hopping conductance. The former case deals with electronconducting coating materials [ 321 or those possessing intrinsic, ionic charge carriers, being the frequent case with real polymers [43 - 451. For most real polymers, with natural, not purposely admixed, intrinsic carriers, the plots of lg e” and Ig E’uersus lg o remain at relationships schematically depicted by curves ‘b’and ‘c’in Fig. 2(B) [40], the slope of the plot of lg e” uersus lg o being -1 and lg c’uersus lg o dependences showing in practice some deviations from the invariant run. Hence, with allowance for eqns. (3) - (5), the plot of 2” uersus 2’ will be an ascending curve, as is the ‘c’plot in Fig. 2(A). This curve will also assume a linear shape at high frequencies with a ’ slope governed by the relationship between invariable E” and e’values at these frequencies. For coatings featured by considerable inherent conductivity, a more realistic representation might be by curves ‘c’and ‘b” in Fig. 2(B) [ 461, resultant of more semicircle-like plots of 2” uersus 2’ than curve ‘c’in Fig. 2(A). Similar bent plots will be obtained with coating materials showing the hopping mechanism of conduction [47 - 491. For such materials, plots of lg E”uersus lg o are straight lines with slopes ranging in practice between -1 and 1 [ 501. This derives from the general relationship (6), describing the hopping mechanism of conduction:
176 (Thop =
Ad
(6)
with 0 < n < 1 for most real polymers. In conjunction with the relation u = oe”, one arrives at eqn. (7): lg 6’ = lgA+(n-1)lgo
(7)
explaining the dispersion of slopes. A further consequence of relation (7) is the occurrence of curved plots of 2” uers’8us 2’. Somewhat different plots of 2” versus 2’ will be noted for coatings partly penetrated with electrolyte solution due to either originally present capillaries and pores or proceeding, homogeneous diffusion of the solution into the coating phase. Such coatings are available for modelling with the equivalent circuit shown in Fig. 1, without any simplifications. Thus, the circuit consists of two capacitive segments characteristic of individual time constants RC. Both of these circuit segments should undergo detection on frequency-related diagrams of 2” uersus Z’, the range of frequency depending on the present time constant value. For coating materials showing Debye or MWS losses at the initial period of testing one obtains plots like ‘d’ in Fig. 2(A). At high frequencies a barely distinctive curved part of the plot is noted, related to the electrolytically penetrated fraction of the coating. This is caused by relatively lower values of CERk products in comparison with those of C, R,, at the initial instant of testing. Along with growing values of the electrolytically originated time constant t&R;,, being a consequence of progressive penetration of the coating with electrolyte solution, the plots of 2” uersus 2’ become more like ‘e’ in Fig. 2(A), i.e., detection of the relevant segment of the equivalent circuit starts at lower frequencies. In the extreme case of the totally penetrated coating, a representative plot of 2” uersus 2’ is the semicircle ‘f’ in Fig. 2(A), and the equivalent circuit reduces to the combination of RE, RI, and Cx factors from Fig. 1, being characterized by only one time constant R&. By analogy, one may apply the same reasoning to the coatings displaying inherent d.c. losses or hopping conductance. Thus, for insulating and barrier acting coatings an analysis of impedance spectra enables qualitative studies of the dielectric properties of the coating material to be made; moreover, diffusion of the corrosive media into the coating phase may be followed up to total penetration of the coating. A graphical illustration of the state of an entirely penetrated coating, shown by plot ‘f’ in Fig. 2(A), becomes, however, strongly limited ,due to questionable compliance to the assumption that there is no contribution from the electrical double layer and electrochemical processes parameters to the impedance measured. In practice, for steel and other active metals, the state of total penetration of the protective coatings with a corrosive environment is tantamount to initiated electrochemical processes, and 2” versus 2’ plots require a different interpretation on the basis of modified equivalent circuits, to be introduced later. Nevertheless, there remains the possibility of determining the incubation period of the corrosion on finding the time interval between the test initiation up to the first occurrence of semicircle plots of 2” uersus 2’.
177
Finally, some ambiguities may arise when interpreting plots of 2” uers’8us 2’ for originally relatively well-conducting coatings due to resemblance between arc-shaped curves representing unpenetrated and entirely penetrated coatings. In this case, it might be helpful to carry out complementary impedance measurements with coating material separated from contact with electrolyte solution. Some confusion in interpretation might also be the case, when a stable and tight oxide layer on the metal substrate is present. Generally, such layers reveal semiconductive or dielectric properties [ 51 - 531, and for correct interpretation of plots of 2” uers’sus 2’ there must be introduced an additional, capacitive or inductive, segment to the equivalent circuit in Fig. 1. One may find in the literature [ 12) a different interpretation of linear plots ‘a’ and ‘b’ from Fig. 2(A), basing on supposed infinite values of the charge transfer resistance Rt of the electrochemical processes taking place at the metal substrate-solution interface. As the preceding discussion showed, such plots should rather be considered in terms of dielectric and insulating properties of the coating material. On the contrary, the complexity of a metal-solution interface must be allowed for, when closed arcs are observed, mainly due to such quantities involved as the faradaic impedance ZF and the double layer capacitance Cn with their specific dependence on o, all these factors being different in numerical value and nature if compared with parameters describing intact coatings.
3. Coatings with conductive pathways A number of commonly used coatings fall within this group. Solvent- or water-based commercial paints usually produce coatings with pores and fissures originated during evaporation of solvents and atmospheric exposure. All these discontinuities have to be considered as potential conductive paths, when in contact with electrolyte solution. Other coatings are pertinent, when conductive paths have already been formed in effect of the diffusion process of the solution into the coating phase. An equivalent circuit proposed by Beaunier et al. [ 111, shown in Fig. 3, seems to model these coatings fairly. Based on this model one can successfully interpret most real 2” uers’sus 2’ plots for the coatings in question. Some of the most characteristic are shown schematically in Fig. 4. Assuming the existence of liquid paths of the electrolytic conductance in the coating and the electrochemical reactions proceeding at the metallic substrate, the protection of metal in terms of separate contributions of conductance limitations related to effective surface blocking and hindering of electrochemical processes has to be considered. The relationship between these aspects of protection will significantly affect the shape of plots of 2” uersus 2’. A difference between plots ‘a’ and ‘b’ in Fig. 4 may be illustrative, which indicates a growing function of direct hindering of electrode processes, on going from plot ‘a’to ‘b’. In both cases the extraction of R& and Rt values from the plots is difficult, and if made, is susceptible to a considerable error due to the long extrapolation required.
178
CE I I
Fig. 3. Equivalent circuit for coatings with conductive pathways. CE, electrical capacity of coating; CD, capacity of electrical double layer; ZF, Faradaic impedance; Rh, electrolytic resistance of coating; RE, resistance of electrolyte solution between working and counter-electrodes.
Fig. 4. Schematic representation of the plots of Z” us. Z’ for coatings with conductive pathways. R,, charge transfer resistance; R,(f), R&, polarization resistance variants.
In general, such inconvenient plots with poorly developed separate, conductively and electrochemically originated arcs will be noted, when relation (8) is fulfilled.
This relation expresses a general rule that the less difference between time constants of both capacitive elements in the equivalent circuit shown in Fig. 3, the more difficult is determination of individual quantities involved. In the case of coatings suffering from conductive pathways, this state is usually achieved at the beginning of testing, displaying a transitional nature. Later on, more convenient plots are observed, like curves ‘c’ and ‘d’ in Fig. 4, enabling ready determination of RL and Rt values, especially as the
179
Fig. 5. The Randles equivalent circuit adopted for the MCE system.
inequality RI, CE Q R,C,-, becomes greater. Segments of plots ‘e’, ‘f’ and ‘g’ in Fig. 4 depicted with dashed lines could be detected at subacoustic frequencies [ 11,53,54]. If one of the steps of the electrochemical process at the coated metal surface is adsorption, a third capacitive arc ‘e’ with the farthest 2’ intercept is observed, being the polarization resistance R,. Multistep nature of electrode reactions might introduce additional inductive arcs, as shown by plot ‘f’ in Fig. 4, while diffusion limitations in the form of the Warburg impedance are noted as the fragment ‘g’, represented by a straight line with the slope unit. Plots similar to ‘d’, ‘c’ from Fig. 4 have been found by Beaunier et al. [ 1 l] , when testing an epoxy coating on steel and iron in 0.5 M HzS04. They were able to state that the corrosion mechanism of iron was the same for painted and bare electrodes. Therefore the protection has been linked to the blocking mechanism, interpreted as effective elimination of a part of the metal surface from contact with the corrosive medium. In summary, the plots of 2” uersus 2’ discussed above may serve as an indication of the present mechanism of protection, providing the possibility of distinguishing between contributions of the surface blocking and direct hindering of electrode processes. The determined R, values enable calculation of the corrosion rate, using the well-known Stern-Geary relation [ 551. However, this rate has to be related to the real surface area on which the corrosion proceeds. Finally, the procedure appears valuable in verifying the protective capability of coatings as well as the selection of inhibitive additives. 4. Adaptation of Randles equivalent circuit for MCE systems As the limiting case of the insulating and barrier acting coatings was the porous coating model, the limiting case of the last one seems to be a model, represented by the Randles equivalent circuit [ 561 with a minor modification, shown in Fig. 5. Here, the resistance denoted by R& is understood to be more complex, consisting not only of the bulk solution resistance but also the electrolytic resistance of the coating and that of eventual oxides, adsorption layer and corrosion products on the metal surface. This concept has been utilized for interpretation of impedance data obtained with chlorinated rubber coated steel [ 121. It is believed that the above simplified model is
180
-jr (
Fig. 6. Schematic representation liable MCE systems.
of the plots of 2” us. 2’ for Randlee equivalent
circuit
applicable mostly to very porous, fissured or well-conducting coatings ineluding those of advanced degradation, when the capacitive effect of the coating may be disregarded. Impedance response variations, comprising relevant coatings, are schematically shown in Fig. 6. Unlike plots in Fig. 4, here the Rt values canbe simply determined due to absent, interfering effects of the parameter Ck. Interpretation of ‘b’, ‘c’and ‘d’fragments of plots remains consistent with that given for respective plots in Fig. 2(B). 5. Impedance 2” versusZ’ data for hydrocarbon based temporary protective coatings
The considerable dielectric and insulating properties of hydrocarbon based temporary protectives has already been mentioned, associated with their pronounced hydrophobic nature as well as the lack of pores and fissures. Some of the impedance data for such coatings are presented below. A more comprehensive communication is to be published soon. Impedance measurements have been carried out with a sinusoidal signal of 10 mV amplitude, utilizing Transfer Function Analyser Type 272, manufactured by Unipan-Poland. Wire electrodes made of carbon steel were covered with a composite coating based on petrolatum and ceresin wax, 20 + 2 pm in thickness, and immersed in 0.5 M HzSd, with 0.01 M KsFe(CN)s as an indicator of Fe2+ ions, at 20 “C. The projected area of the electrode was 1.5 cm2. . Representative data, plotted in Fig. 7, permit the conclusion that within the space of the first five days the coating performed as a dielectric with D or/and MWS losses, disclosing no symptoms of penetration by the solution. Deviations from the linear plots for times of 6 a days and 6 $ days of immersion indicated diffusion of the solution into the coating phase at a relatively
181
-j& 3.
I-5d
6fd
5
IO
Fig. 7. Impedance data as plots of 2” us. 2’ for hydrocarbon based temporary protective coating, applied in 20 * 2 pm thick layers on steel electrodes with projected area of 1.5 cm2, tested in 0.5 M H$304 at 20 “C.
high rate. In consequence, already after 6 % days, a semicircle plot of 2” uersus 2’ was obtained and the corrosion products became visible. One may conclude that the coating performed as a barrier for a relatively long time, but when this feature was lost it became penetrated quite rapidly. Afterwards there was no significant electrolytic resistance noted, which could be justified by the fact that the coating underwent mechanical damage due to evolution of gaseous hydrogen. As far as representation of the MCE system by the equivalent circuit is concerned, a time-related degradation of the coating may be described as a transition from the series model shown in Fig.‘1 to the Randles circuit. Finally, the data evidenced that protection by the coating tested was exclusively due to its dielectric and barrier properties. The loss of these attributes appeared tantamount to the vanished protection, which is a frequent case with coatings devoid of corrosion inhibitors. 6. Discussion With respect to the analysis of possible plots of 2” uersus 2’ for most practical non-metallic coatings, accomplished on the basis of the available literature and our own experiments, it seems justified to state that the procedure, i.e. the testing method and data representation, is a valuable tool for the identification of protective capabilities of coatings and the mechanism of protection. In the case of insulating and barrier acting coatings, the procedure enables studies on dielectric properties of the coating material to be made, as well as the control of the destructive influence of the contacting solution.
182
As far as porous coatings are concerned, the measurement makes possible evaluation of the metal surface blocking and kinetic aspects of the protection. For some real MCE systems, however, interpretation of the plots of 2” versus 2 is somewhat difficult and only qualitative. For instance, one may recall difficulties in the accurate determination of Rk and Rt values, when time constants of electrolytically and electrochemically originated segments of the equivalent circuit are approximately equal. Such cases, however, are rather incidental and do not depreciate the method. In summary, the method discussed provides valuable information about coatings in terms of the corrosion protection of metals and should find wide application in development and evaluation of new protective, nonmetallic coating materials. So far, major obstructions on this road have been tedious measurements and data analysis. It is believed that automation and computerization should increase the popularity of the method.
References 1 F. Wormwell and D. M. Brasher, J. Iron Steel Inst., 164 (1950) 141. 2 G. Okamoto and T. Morozumi, J. Electrochem. Sot. Jpn., 23 (1) (1955) 15. 3 N. D. Tomashov, Yu. N. Mikhailovskii and V. V. Leonov, Corrosion, 20 (1964) 125t. 4 I. L. Rozenfeld, V. N. Buryanenko and K. A. Zhigalova,,Lakokras. Mater. Zkh Primen, (3) (1966) 62; (5) (1966) 52. 5 G. Menges and W. Schneider, Kunststofftechnik, 12 (1973) 265,316, 343. 6 V. D. Gerber, V. J. Eliseeva, K. A. Zhigalova and I. L. Rozenfeld, Zashch. Met., 10 (1974) 71. 7 D. J. Gusakova, L. S. Nosova, A, J. Plyplina, V. J. Novozhilova and T. A. Naumova. Lakokras. Mater. Zkh Primen.. (5) (1974) 50. 8 G. Reinhard, D. Scheller and K. Hahn, Plaste Kautsch., 22 (1976) 57. 9 K. 5. Rajagopalan, S. Guruviah, M. Sundaram and V. Chandrasekaran, Sci. Znd. Res., 34 (1975) 482. 10 H. Potente and F. Stoll, Farbe Lack, 81 (1975) 701. 11 L. Beaunier, I. Epelboin, J. C. Lestxade and H. Takenouti, Surf. TechnoZ., 4 (1976) 237. 12 J. D. Scantlebury and K. N. Ho, J. Oil Colour Chem. Assoc., 62 (1979) 89. 13 W. M. Groenewoud, ibid., 62 (1979) 10. 14 K. S. Rajagopalan, S. Guruviah and C. S. Rajagopalan, ibid., 63 (1980) 144. 15 H. Leidheiser, Jr., Prog. Org. Coat., 7 (1979) 79. 16 I. Epelhoin, M. Keddam and H. Takenouti, J. Electrochem. Sot., 120 (1973) 230C. 17 R. E. Touhsaent and H. Leidheiser, Jr., Corrosion, 28 (1972) 435. 18 H. Leidheiser and D. Wallace, J. Electrochem. Sot., 120 (1973) 239C. 19 M. W. Kendig and H. Leidheiser, Jr., ibid., 121 (1974) 275C. 20 H. Leidheiser, Jr. and M. W. Kendig, Corrosion, 32 (1976) 69. 21 M. W. Kendig and H. Leidheiser, Jr., J. Electrochem. SOC., 123 (1976) 982. 22 Kh. B. KIark and M. I. Mikhajlovskaja, Tr. Inst. Fis. Khim. Akad. Nauk SSSR, (5) (1959) 145. 23 H. W. Bode, Network Analysis and Feedback Amplifier Design, Van Nostrand, Princeton, NJ, 1959. 24 J. L. Dowson, L. M. Callow, K. Hladky and J. A. Richardson, The International Corrosion Forum, Houston, 1978.
183 25 I. Epeiboin, M. Keddam and P. Morel, Proc. 3rd Znt. Congr. on Metallic Corrosion, Moscow, 1966. 26 R. D. Armstrong, M. F. BeIi and A. A. Metcalfe, J. Electroanal. Chem., 77 (1977) 287. 27 T. Szauer, Corros. Sci., submitted for publication. 28 P. J. Body, J. Electroanal. Chem.. 10 (1965) 199. 29 K. A. Holzman, J. Paint Technol., 43 (1971) 47. 30 V. V. Shmirev and B. N. Egorov, Lakokras. Mater. Zkh Prtmen., (4) (1973) 63. 31 B. Carrel, Physical Methods in Macromolecular Chemistry, Vol. 1, Marcel Dekker, New York, 1969. 32 L. H. K. Van Beek, Prog. Dielectr., 7 (1967) 69. 33 A. I. Lukomskaja and B. A. Dogadkin, Kolloidn. Zh., 15 (1953) 183,259; 16 (1954) 36; 22 (1960) 576. 34 A. M. North, J. Polym. Sci. Polym. Symp., 50 (1975) 354. 35 J. T. Iux, R. Kay and A. M. North, Polymer, 15 (1974) 799. 36 U. Beguin and T. Gaumann, Hew. Chim. Acta, 41 (1958) 1951. 37 A. J. Curtis, Progress in Dielectrics, Heywood, London, 1960. 38 R. M. Fuoss and J. G. Kirkwood, J. Am. Chem. Sot., 63 (1941) 385. 39 A. Von Hippel, Dielectrics and Waves, Wiley, New York, 1959. 40 G. L. Link, in Jenkins (Ed.), Polymer Science, North-Holland Publ. Co., Amsterdam, 1972, p, 1281. 41 K. S. Cole and R. H. Cole, J. Chem. Phys., 9 (1941) 341. 42 R. H. Cole, J. Chem. Phys., 23 (1955) 493. 43 D. A. Seanor, J. Polym. Sci., A2, 6 (1968) 463; Cl 7 (1967) 195. 44 J. J. O’Dwyer, The Theory of Electrical Conduction and Breakdown in Solid Dielectrics, Clarendon Press, Oxford, 1973. 45 S. Reich and I. Michaeli, J. Polym. Sci., A2, 13 (1975) 9. 46 R. Von Appel, Dielectric Materials and Applications, Wiley, New York, 1954. 47 A. Argall and A. Jonscher, Thin Solid Films, 2 (1968) 185. 48 N. F. Mott and E. A. Davies, Elektronnyje procesy u niekristalicheskikh uieshchestvakh, Izd. ‘Mir’, Moscow, 1974. 49 M. Pollak and T. H. Geballe, Phys. Rev., 122 (1961) 1742. 50 J. E. Lewis and E. Ed&, Phys. Rev., B, I1 (1975) 4033. 51 N. Sato, Electrochim. Acta, 16 (1971) 1683. 52 K. E. Heusler, 2. Electrochem., 62 (1958) 582. 53 I. Epelboin and M. Keddam, Electrochim. Acta, 17 (1972) 177. 54 I. Epelboin and M. Keddam, J. Electrochem. Sot., 117 (1970) 1052. 55 M. Stern and A. L. Gear-y, J. Electrochem. Sot., 104 (1957) 56. 56 J. E. B. Randles, Discuss. Faraday Sot., 1 (1947) 11.
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171 - 163
IMPEDANCE MEASUREMENTS FOR THE EVALUATION OF PROTECTIVE NONMETALLIC COATINGS
T. SZAUER Jnetitute of Inorganic Chemistry and Technology, 80-952 Gdairsk (Poland)
Technical University of Gdarisk,
Contents Summary ..........................................................
171
1 2 3 4 5
171 173 177 179
Introduction .................................................... Insulating and barrier acting coatings ................................. Coatings with conductive pathways. .................................. Adaptation of Randles equivalent circuit for MCE systems ................. Impedance 2” versus 2’ data for hydrocarbon based temporary protective coatings. ....................................................... 6 Discussion. ..................................................... References ........................................................
180 181 182
Summary Questions regarding the interpretation of impedance data obtained with metal-coating-electrolyte solution systems are discussed on the basis of theoretical considerations and experimental results. The analysis of plots in the form of Nyquist diagrams leads to the conclusion that the method provides valuable infkrnation about protective properties of coatings and enables identification of the mechanism of protection.
1. Introduction Impedance measurements of protective coatings on metals supply valuable information about the protective properties and the mechanism of action. In spite of some disadvantages associated with the simplified nature of the test, carried out mostly by immersion of coated electrodes, measurements of the impedance or its components have found many practical applications [ 1 -161. These measurements are being accomplished in various instrumental variants and data are represented in a number of ways. For instance, dielectric and barrier characteristics of. coatings are commonly tested by impedance measurements as functions of angular frequency o and time of immersion T, data being frequently plotted as lg e” = f(lg w) and 2’ = 0033-0656/82/010171-13/$03.50
0 Elsevier Sequoia/Printed in The Netherlands
172
f(r), where e” and 2’ are the imaginary component of the complex permittivity E and real component of the impedance 2, respectively. For instance, such data representation for polymer coatings may be found in interesting work of Leidheiser et aZ. [ 17 - 211. Simplified tests for the presence or absence of conductive paths through the coating consist of the determination of functions 2 and C = f(o) [4,8,14,22] or the famous Bode dependence 2 = f(o) [5,10,23]. Another variant arises from the graphical representation of impedance data in the form of the Nyquist diagram [23 - 251, being the parametric function 2” = f(Z), with w as a parameter, 2” being the imaginary component of the impedance. The last procedure, based on special measuring techniques [ 11, 261, appears particularly interesting since it allows more comprehensive evaluation of corrosion protective properties of coatings and the mechanism of their action. This approach, moreover, enables simultaneous assessment of the actual eligibility of the main concepts of protection by coatings, namely, the surface insulation or partial blocking and the kinetic hindering [27] . The first two concepts relate in a direct manner dielectric, barrier as well as adhesive, properties of the coating to obstructed diffusion of the solution through the coating and limited area of the immediate metal-solution contact. In the case of the kinetic hindrance, protection is mainly linked to, the enhanced charge transfer resistance R, , or to retarded transport of reagents for the electrode reactions. Experimental data of the function of 2” = f(P), =VBrfor real metal-coatingelectrolyte solution (MCE) systems exhibit quite a complex nature and a number of variations [ 5,11,12] . Interpretation of some of these still remains non-evident and ambiguous. The aim of this communication is an effort to systematize questions falling within the scope of the evaluation of coating protective abilities by the analysis of the function of 2” = f(Z’ ), =VBr.The key for such a systematic treatment should be a possibly general and adequate electrical equivalent circuit, comprising most existing MCE systems. One can find in the literature a number of propositions in this matter, more or less satisfactorily describing the real MCE or electrochemically related systems, when subject to the oscillating signal V = V,-, exp@T) [2,4,11,18,21,22,28 - 301. One of the most general and comprehensive solutions has been given by Menges and Schneider [5] . The authors considered separately the group of coatings whose degradation is governed by the second Fick law, describing diffusion of the solution to the substrate, and the group of porous and defective coatings with electrolytic, conductive paths allowing a direct contact between the metal and solution. The first group has received a modified treatment by Leidheiser et al. [17,21], while the second has been specifically described by Wormwell and Brasher [l] , Okamoto and Morozumi [ 21 and Klark and Mikhajlovskaja [22] , as well as a general treatment by Beaunier et al. [ll]. Allowing for the variety of protective coatings encountered in practice, it seems useful to introduce three complementary equivalent circuits, facilitating the interpretation of 2” = f(Z), EVllldiagrams.
173
,
,,
Fig. 1. Equivalent circuit for insulating and barrier acting coatings. RE resistance of electrolyte solution between working and counter-electrode; RjE resistance of penetrated part of coating; Rc resistance of intact part of coating; Cc, capacity of intact part of coating; CE, capacity of penetrated part of coating.
2. Insulating and barrier acting coatings Eligible coatings are those lacking continuous conductive paths, which usually allow immediate contact between -metal and electrolyte solution. Corrosion protection afforded by these coatings is predominantly due to dielectric and barrier properties of the coating material. The main evaluating factor of the protection is thus the time-related resistance of the coating to become penetrated with conductive solution. The equivalent circuit, which has been found to describe adequately the MCE system in question, is shown in Fig. 1, being a double terminal network built up of two reactive segments in series representing penetrated and non-penetrated fractions of the coating [ 51. Regarding both homogeneous [ 51 or random, heterogeneous diffusion of the solution into the coating [ 17,211, the allowance for metal-coating interface parameters seems to be negligible for common industrial metals. It stems from the fact that for barrier acting coatings on active metals, the total penetration with electrolyte solution and development of electrolytically conducting paths usually signifies the onset of electrochemical processes at the metal substrate and the necessity to adopt a different equivalent circuit. MCE systems liable to such representation consist mainly of those based on synthetic resins like epoxy and polyurethane types [5], and other polymers [17 - 211, applied in practice in relatively thick layers. It is worthy of note that some hydrocarbon based oils and protective coatings, even if in thin layers, behave similarly due to their hydrophobic nature and dielectric properties [ 27,311. Consider, now, the case of insulating and barrier acting coatings at the commencement of testing, provided that there is no electrolytic phase within the coating. In terms of impedance measurements with low amplitude sinusoidal signal, the applicable equivalent circuit may be that of Fig. 1, without the segment representing the penetrated fraction of the coating. Thus, one may consider the relevant MCE system as a leaky condenser and the frequency response will depend on the dielectric characteristic of the coating material. This characteristic is defined by dielectric losses, specifically by d.c. losses as well as processes such as dielectric dispersion and dielectric absorption [ 32 - 351. Schematic representations of 2” versus 2’ data for coatings
174
R' r
RE
w
2
Fig, 2. A. Schematic representation of the plots of 2” us. 2’ for insulating and barrier acting coatings. B. Schematic plots ‘a’, ‘b’ and ‘b” of lg 6’ us. lg o and ‘c’ of Ig E’us. lg O. Z”, E” and 2’. e’ are the imaginary and real parts of the impedance and permittivity, respectively.
showing no d.c. but Debye and/or Maxwell, Wagner and Sillars (MWS) losses are plots ‘a’and ‘b’in Fig. 2(A). The linearity of these plots might be accounted for on the ground of a correlation between the impedance and permittivity of relevant material. In the case of real polymers, Debye or MWS losses, brought about by dipole relaxation and interspatial polarization processes [32,36,37] , are featured by considerable broadening of the relaxation time and resultant linearity of plots of Ig e” versuslg o. Mathematically, this relation has been derived by Fuoss and Kirkwood [38], being illustrated by plot ‘a’in Fig. 2(B). Furthermore, it is of interest to express the impedance 2 of the leaky condenser, represented by the parallel equivalent circuit [39] , with use of the complex permittivity. Combination of eqns. (1) and (2) leads to the definition of real 2’ and imaginary 2” parts of the impedance as well as the slope (tan a) of the impedance vector, with use of real e’and imaginary e” parts of the complex permittivity = 2’ -
2
jz"
RC
=
1
E
=
e’ -
je"
=
Cc _ -
i
=
(1)
(2)
e” UC* [ (e”)2+ (e’)2]
2”
Rz
+ 02C2R2 c c - 1 + 02C2R2 e c
CO oCoRc 2’ =
jwC,
(3)
e’ WC, [ (ep2 +
(e’)2]
(4)
175
(5)
tanaL E”
where C,-,is the capacity of the spatial condenser. Thus, independence of lg E”and lg E’uersus lg w for real polymers displaying Debye and/or MWS losses [ 401, as visualized by plots ‘a’and ‘c’in Fig. Z(B), leads to the conclusion that plots of 2” uersus 2 should be straight lines, the slope being governed by the relationship between E’and E”. One may point out the rule that the greater the dielectric losses e”, the lower the slope of the lines. The limiting cases for these lines will be the coordinate origin at o + 00 and the infinity at w + 0. However, due to the definite conductivity of the testing solution, the intercept with the abscissa will be in practice determinative of RE. In the rather rare case of coating materials showing specific Debye relaxation time, resulting in the Cole-Cole semicircle in lg e”-1g E’coordinates [ 41,421, there wil1 occur some deviations from the linearity of the plots of 2” uersus 2’ within o values defining ELand ek quantities, i.e. the real permittivity at 0 and at the greatest frequency influencing its values, respectively. The apparent discrepancy between the assumed parallel equivalent circuit and the non-semicircle impedance response on the 2”-2’ plane arises from the fact that the impedance data relate not to ohmic conductivity, i.e. the migration of charges as ions or electrons, but to dielectric losses of nonconductive nature. For the same conditions as above, one may note a bent plot in 2”-2’ coordinates, as shown by curve ‘c’in Fig. 2(A), for coatings displaying ionic or electronic, and hopping conductance. The former case deals with electronconducting coating materials [ 321 or those possessing intrinsic, ionic charge carriers, being the frequent case with real polymers [43 - 451. For most real polymers, with natural, not purposely admixed, intrinsic carriers, the plots of lg e” and Ig E’uersus lg o remain at relationships schematically depicted by curves ‘b’and ‘c’in Fig. 2(B) [40], the slope of the plot of lg e” uersus lg o being -1 and lg c’uersus lg o dependences showing in practice some deviations from the invariant run. Hence, with allowance for eqns. (3) - (5), the plot of 2” uersus 2’ will be an ascending curve, as is the ‘c’plot in Fig. 2(A). This curve will also assume a linear shape at high frequencies with a ’ slope governed by the relationship between invariable E” and e’values at these frequencies. For coatings featured by considerable inherent conductivity, a more realistic representation might be by curves ‘c’and ‘b” in Fig. 2(B) [ 461, resultant of more semicircle-like plots of 2” uersus 2’ than curve ‘c’in Fig. 2(A). Similar bent plots will be obtained with coating materials showing the hopping mechanism of conduction [47 - 491. For such materials, plots of lg E”uersus lg o are straight lines with slopes ranging in practice between -1 and 1 [ 501. This derives from the general relationship (6), describing the hopping mechanism of conduction:
176 (Thop =
Ad
(6)
with 0 < n < 1 for most real polymers. In conjunction with the relation u = oe”, one arrives at eqn. (7): lg 6’ = lgA+(n-1)lgo
(7)
explaining the dispersion of slopes. A further consequence of relation (7) is the occurrence of curved plots of 2” uers’8us 2’. Somewhat different plots of 2” versus 2’ will be noted for coatings partly penetrated with electrolyte solution due to either originally present capillaries and pores or proceeding, homogeneous diffusion of the solution into the coating phase. Such coatings are available for modelling with the equivalent circuit shown in Fig. 1, without any simplifications. Thus, the circuit consists of two capacitive segments characteristic of individual time constants RC. Both of these circuit segments should undergo detection on frequency-related diagrams of 2” uersus Z’, the range of frequency depending on the present time constant value. For coating materials showing Debye or MWS losses at the initial period of testing one obtains plots like ‘d’ in Fig. 2(A). At high frequencies a barely distinctive curved part of the plot is noted, related to the electrolytically penetrated fraction of the coating. This is caused by relatively lower values of CERk products in comparison with those of C, R,, at the initial instant of testing. Along with growing values of the electrolytically originated time constant t&R;,, being a consequence of progressive penetration of the coating with electrolyte solution, the plots of 2” uersus 2’ become more like ‘e’ in Fig. 2(A), i.e., detection of the relevant segment of the equivalent circuit starts at lower frequencies. In the extreme case of the totally penetrated coating, a representative plot of 2” uersus 2’ is the semicircle ‘f’ in Fig. 2(A), and the equivalent circuit reduces to the combination of RE, RI, and Cx factors from Fig. 1, being characterized by only one time constant R&. By analogy, one may apply the same reasoning to the coatings displaying inherent d.c. losses or hopping conductance. Thus, for insulating and barrier acting coatings an analysis of impedance spectra enables qualitative studies of the dielectric properties of the coating material to be made; moreover, diffusion of the corrosive media into the coating phase may be followed up to total penetration of the coating. A graphical illustration of the state of an entirely penetrated coating, shown by plot ‘f’ in Fig. 2(A), becomes, however, strongly limited ,due to questionable compliance to the assumption that there is no contribution from the electrical double layer and electrochemical processes parameters to the impedance measured. In practice, for steel and other active metals, the state of total penetration of the protective coatings with a corrosive environment is tantamount to initiated electrochemical processes, and 2” versus 2’ plots require a different interpretation on the basis of modified equivalent circuits, to be introduced later. Nevertheless, there remains the possibility of determining the incubation period of the corrosion on finding the time interval between the test initiation up to the first occurrence of semicircle plots of 2” uersus 2’.
177
Finally, some ambiguities may arise when interpreting plots of 2” uers’8us 2’ for originally relatively well-conducting coatings due to resemblance between arc-shaped curves representing unpenetrated and entirely penetrated coatings. In this case, it might be helpful to carry out complementary impedance measurements with coating material separated from contact with electrolyte solution. Some confusion in interpretation might also be the case, when a stable and tight oxide layer on the metal substrate is present. Generally, such layers reveal semiconductive or dielectric properties [ 51 - 531, and for correct interpretation of plots of 2” uers’sus 2’ there must be introduced an additional, capacitive or inductive, segment to the equivalent circuit in Fig. 1. One may find in the literature [ 12) a different interpretation of linear plots ‘a’ and ‘b’ from Fig. 2(A), basing on supposed infinite values of the charge transfer resistance Rt of the electrochemical processes taking place at the metal substrate-solution interface. As the preceding discussion showed, such plots should rather be considered in terms of dielectric and insulating properties of the coating material. On the contrary, the complexity of a metal-solution interface must be allowed for, when closed arcs are observed, mainly due to such quantities involved as the faradaic impedance ZF and the double layer capacitance Cn with their specific dependence on o, all these factors being different in numerical value and nature if compared with parameters describing intact coatings.
3. Coatings with conductive pathways A number of commonly used coatings fall within this group. Solvent- or water-based commercial paints usually produce coatings with pores and fissures originated during evaporation of solvents and atmospheric exposure. All these discontinuities have to be considered as potential conductive paths, when in contact with electrolyte solution. Other coatings are pertinent, when conductive paths have already been formed in effect of the diffusion process of the solution into the coating phase. An equivalent circuit proposed by Beaunier et al. [ 111, shown in Fig. 3, seems to model these coatings fairly. Based on this model one can successfully interpret most real 2” uers’sus 2’ plots for the coatings in question. Some of the most characteristic are shown schematically in Fig. 4. Assuming the existence of liquid paths of the electrolytic conductance in the coating and the electrochemical reactions proceeding at the metallic substrate, the protection of metal in terms of separate contributions of conductance limitations related to effective surface blocking and hindering of electrochemical processes has to be considered. The relationship between these aspects of protection will significantly affect the shape of plots of 2” uersus 2’. A difference between plots ‘a’ and ‘b’ in Fig. 4 may be illustrative, which indicates a growing function of direct hindering of electrode processes, on going from plot ‘a’to ‘b’. In both cases the extraction of R& and Rt values from the plots is difficult, and if made, is susceptible to a considerable error due to the long extrapolation required.
178
CE I I
Fig. 3. Equivalent circuit for coatings with conductive pathways. CE, electrical capacity of coating; CD, capacity of electrical double layer; ZF, Faradaic impedance; Rh, electrolytic resistance of coating; RE, resistance of electrolyte solution between working and counter-electrodes.
Fig. 4. Schematic representation of the plots of Z” us. Z’ for coatings with conductive pathways. R,, charge transfer resistance; R,(f), R&, polarization resistance variants.
In general, such inconvenient plots with poorly developed separate, conductively and electrochemically originated arcs will be noted, when relation (8) is fulfilled.
This relation expresses a general rule that the less difference between time constants of both capacitive elements in the equivalent circuit shown in Fig. 3, the more difficult is determination of individual quantities involved. In the case of coatings suffering from conductive pathways, this state is usually achieved at the beginning of testing, displaying a transitional nature. Later on, more convenient plots are observed, like curves ‘c’ and ‘d’ in Fig. 4, enabling ready determination of RL and Rt values, especially as the
179
Fig. 5. The Randles equivalent circuit adopted for the MCE system.
inequality RI, CE Q R,C,-, becomes greater. Segments of plots ‘e’, ‘f’ and ‘g’ in Fig. 4 depicted with dashed lines could be detected at subacoustic frequencies [ 11,53,54]. If one of the steps of the electrochemical process at the coated metal surface is adsorption, a third capacitive arc ‘e’ with the farthest 2’ intercept is observed, being the polarization resistance R,. Multistep nature of electrode reactions might introduce additional inductive arcs, as shown by plot ‘f’ in Fig. 4, while diffusion limitations in the form of the Warburg impedance are noted as the fragment ‘g’, represented by a straight line with the slope unit. Plots similar to ‘d’, ‘c’ from Fig. 4 have been found by Beaunier et al. [ 1 l] , when testing an epoxy coating on steel and iron in 0.5 M HzS04. They were able to state that the corrosion mechanism of iron was the same for painted and bare electrodes. Therefore the protection has been linked to the blocking mechanism, interpreted as effective elimination of a part of the metal surface from contact with the corrosive medium. In summary, the plots of 2” uersus 2’ discussed above may serve as an indication of the present mechanism of protection, providing the possibility of distinguishing between contributions of the surface blocking and direct hindering of electrode processes. The determined R, values enable calculation of the corrosion rate, using the well-known Stern-Geary relation [ 551. However, this rate has to be related to the real surface area on which the corrosion proceeds. Finally, the procedure appears valuable in verifying the protective capability of coatings as well as the selection of inhibitive additives. 4. Adaptation of Randles equivalent circuit for MCE systems As the limiting case of the insulating and barrier acting coatings was the porous coating model, the limiting case of the last one seems to be a model, represented by the Randles equivalent circuit [ 561 with a minor modification, shown in Fig. 5. Here, the resistance denoted by R& is understood to be more complex, consisting not only of the bulk solution resistance but also the electrolytic resistance of the coating and that of eventual oxides, adsorption layer and corrosion products on the metal surface. This concept has been utilized for interpretation of impedance data obtained with chlorinated rubber coated steel [ 121. It is believed that the above simplified model is
180
-jr (
Fig. 6. Schematic representation liable MCE systems.
of the plots of 2” us. 2’ for Randlee equivalent
circuit
applicable mostly to very porous, fissured or well-conducting coatings ineluding those of advanced degradation, when the capacitive effect of the coating may be disregarded. Impedance response variations, comprising relevant coatings, are schematically shown in Fig. 6. Unlike plots in Fig. 4, here the Rt values canbe simply determined due to absent, interfering effects of the parameter Ck. Interpretation of ‘b’, ‘c’and ‘d’fragments of plots remains consistent with that given for respective plots in Fig. 2(B). 5. Impedance 2” versusZ’ data for hydrocarbon based temporary protective coatings
The considerable dielectric and insulating properties of hydrocarbon based temporary protectives has already been mentioned, associated with their pronounced hydrophobic nature as well as the lack of pores and fissures. Some of the impedance data for such coatings are presented below. A more comprehensive communication is to be published soon. Impedance measurements have been carried out with a sinusoidal signal of 10 mV amplitude, utilizing Transfer Function Analyser Type 272, manufactured by Unipan-Poland. Wire electrodes made of carbon steel were covered with a composite coating based on petrolatum and ceresin wax, 20 + 2 pm in thickness, and immersed in 0.5 M HzSd, with 0.01 M KsFe(CN)s as an indicator of Fe2+ ions, at 20 “C. The projected area of the electrode was 1.5 cm2. . Representative data, plotted in Fig. 7, permit the conclusion that within the space of the first five days the coating performed as a dielectric with D or/and MWS losses, disclosing no symptoms of penetration by the solution. Deviations from the linear plots for times of 6 a days and 6 $ days of immersion indicated diffusion of the solution into the coating phase at a relatively
181
-j& 3.
I-5d
6fd
5
IO
Fig. 7. Impedance data as plots of 2” us. 2’ for hydrocarbon based temporary protective coating, applied in 20 * 2 pm thick layers on steel electrodes with projected area of 1.5 cm2, tested in 0.5 M H$304 at 20 “C.
high rate. In consequence, already after 6 % days, a semicircle plot of 2” uersus 2’ was obtained and the corrosion products became visible. One may conclude that the coating performed as a barrier for a relatively long time, but when this feature was lost it became penetrated quite rapidly. Afterwards there was no significant electrolytic resistance noted, which could be justified by the fact that the coating underwent mechanical damage due to evolution of gaseous hydrogen. As far as representation of the MCE system by the equivalent circuit is concerned, a time-related degradation of the coating may be described as a transition from the series model shown in Fig.‘1 to the Randles circuit. Finally, the data evidenced that protection by the coating tested was exclusively due to its dielectric and barrier properties. The loss of these attributes appeared tantamount to the vanished protection, which is a frequent case with coatings devoid of corrosion inhibitors. 6. Discussion With respect to the analysis of possible plots of 2” uersus 2’ for most practical non-metallic coatings, accomplished on the basis of the available literature and our own experiments, it seems justified to state that the procedure, i.e. the testing method and data representation, is a valuable tool for the identification of protective capabilities of coatings and the mechanism of protection. In the case of insulating and barrier acting coatings, the procedure enables studies on dielectric properties of the coating material to be made, as well as the control of the destructive influence of the contacting solution.
182
As far as porous coatings are concerned, the measurement makes possible evaluation of the metal surface blocking and kinetic aspects of the protection. For some real MCE systems, however, interpretation of the plots of 2” versus 2 is somewhat difficult and only qualitative. For instance, one may recall difficulties in the accurate determination of Rk and Rt values, when time constants of electrolytically and electrochemically originated segments of the equivalent circuit are approximately equal. Such cases, however, are rather incidental and do not depreciate the method. In summary, the method discussed provides valuable information about coatings in terms of the corrosion protection of metals and should find wide application in development and evaluation of new protective, nonmetallic coating materials. So far, major obstructions on this road have been tedious measurements and data analysis. It is believed that automation and computerization should increase the popularity of the method.
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