ELSEVIER Progress in Organic Coatings 24 (1994) 309-322

Characterization of active pigments in damage of organic coatings on steel by means of electrochemical impedance spectroscopy

Ursula Rammelt, Georg Reinhard Dresden University of Technology, Institute of Physical Chemktty and Electrochemistry, Dresden,

Germany

Abstract

Active anticorrosive pigments are solid additives for primers which can give further protection for areas with coating damage in addition to their barrier effect. These pigments are expected to prevent corrosion of metal substrate in coating damage by build-up of permanently passive conditions at the metal surface (electrochemical protection) and/or by build-up of solid compounds which plug the coating damage (chemical protection). Electrochemical Impedance Spectroscopy (EIS) was applied to characterize the corrosion protection behaviour of alkyd primers containing different pigments. Impedance spectra were recorded in the frequency range 50 mHz&f<SO kHz at the open-circuit potential as a function of the type of pigment and the exposure time in different corrosive media. In general, two different parts can be distinguished in the impedance diagrams. The higher frequency part is related to the insulating properties of the primer and the lower frequency part can be attributed to electrochemical processes taking place within the coating defects. The parameters derived from EIS results show that the low frequency data can be used for characterization of the protective properties of anticorrosive pigments in the presence of defects in organic coatings.

Keywords: Organic coatings; Active pigments; Protective mechanism; Electrochemical impedance

spectroscopy

Introlhlction

Organic coatings have been used for a long time and will also be used in the future for the decoration and protection of many metallic structures. The corrosion protective properties are determined by a complex mechanism which

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310 U. Rammelt, G. Reinhard / Pmpss in Organic Coatings 24 (1994) 309-322

includes the action of different factors. The behaviour of a protective system will depend on [l-4]: the dielectric properties of the coating; the adhesion of the coating to the substrate; the water and oxygen uptake of the coating; ion penetration of the coating; pigments and inhibitive additives; mechanically weakened spots and pinholes; surface pretreatment; environmental conditions; and various com- plicated electrochemical corrosion reactions at the metal-coating interface after permeation of water and oxygen.

Hence, the protective properties of organic coatings on metal substrates may be attributed to both a barrier and an electrochemical/chemical mechanism. As organic coatings have a high resistance to ionic conductivity, they offer good barrier properties and retard the diffusion of chemical species to and from the metal surface. The corrosion protective properties of organic coatings can be enhanced by the use of anticorrosion pigments in the coating. A protective coating system usually consists of primer, filler and topcoat. Thus, the primer is especially responsible for corrosion protection. Water molecules inserted at the metal-primer interface may induce adhesion losses for the coating and electrochemical under-film corrosion may take place. In order to prevent this, typical barrier pigments are incorporated within the coating. Moreover, active pigments in primers often develop further protective action in addition to the barrier effect when local destruction of the organic coating occurs. The pigments may then protect the metal substrate electrochemically by either a galvanic or a passivating mechanism and/or chemically by build-up of solid compounds with barrier properties [4-lo].

The best method for testing coatings is by outdoor exposure of the coated specimen in the required environment of application. However, in this testing it takes a very long time to get information about the quality of the system. Another common method for the testing of coatings is accelerated weathering, for instance in salt spray or humidity chambers. These tests only allow a qualitative estimation of the protective coating properties and no information is given about the degradation mechanism. These methods do not give quantitative data for fundamental properties of the coating, like the transport rate of water and ions in the coating or the corrosion rate of the metal under the coating or in damage. As corrosion of coated metals is an electrochemical process, the use of electrochemical methods plays an important role in the evaluation of the corrosion performance and in providing a better understanding of how a coating protects a metal substrate. Of these methods, electrochemical impedance spectroscopy (EIS) is very useful because it provides the possibility of investigating systems with a high resistance owing to their dielectrical properties, and moreover, it is also possible to detect the start of the corrosion process at a very early stage and to derive the corrosion protective mechanism of active pigments.

EIS has been used for some years now, even in coating research [ll-151, however the development of electronics [16] and the optimization of the mea- surement setup has increased the applicability of the technique. Modern electronics are capable of covering broad frequency ranges from the ~I-Iz range up to the GHz range. Furthermore it is possible to measure very small currents down to the pA range. Also the interlxetation analysis and simulation of impedance

U. Rammelt, G. Reinhard / Progress in Organic Coaiings 24 (1994) 309-322 311

measurements is simplified by the broad availability of computers and adequate software for this purpose [17,X3].

Theoretical background and representation of EIS

This technique is based on the application of a perturbating voltage to a system and the subsequent measurement of the resulting current [19]. When a small sinusoidal voltage

U(f) = U, sin(ot + qp,)

is applied to a system, a sinusoidal current

I(t) =I, sin(wt + vi)

with a phase shift cp= 4pU - vi is observed. The magnitude of the amplitudes of perturbation and response, U,,., and I,,,, and the phase shift (p depends on the reactions taking place in the system under investigation [16,20,21].

To simplify further calculations, the perturbation and response are transformed from a function of time into the frequency domain via a Laplace transformation [22]. Then the perturbation and the current response can be written as complex functions:

The impedance is defined as the ratio between voltage and current:

Z(jm)= F d(V-*>= (zl@=Z'+jZ"

m

where: Z(jw), frequency-dependent electrochemical impedance; o, angular fre- quency (o= 24; j, imaginary quantity (- l)ln; [Z], modulus of the impedance; Z’, real part of the impedance; Z”, imaginary part of the impedance.

In the complex plane the impedance of a single frequency can be represented by a vector of length ]z/ with argument rp, where cp is the angle between the real axis of the impedance Z and the vector. Z’ is equal to the resistive part of the impedance, Z” represents the capacitive part of the impedance. As shown in Fig. 1 the relationship between these quantities are:

]Z] = [(Z’)Z + (jz”)z]ln

Z’ = ]Z/ COS rp; Z” = ]ZI sin cp

Analysis of impedance data is carried out over a wide range of frequencies in order to determine the coating properties and electrochemical processes. The impedance of a system can be described graphically by means of two types of diagram. In the Nyquist plot, (Fig. 2(a)), the impedance figure is constructed by

312 U. Rammelt, G, Reinhard 1 Pmgress in Oqanic Coatings 24 (1994) 309-322

_z ”

t,/

IZI eels Y ______-_____- Z(jwl

IZI ~IZlsln P

Y

’ 2’ Fig. 1. Representation of impedance as a complex quantity.

connecting the endpoints of the impedance vectors as functions of the frequency in the complex plane. In the so-called Bode plot, (Fig. 2(b)), the logarithm of the modulus of the impedance log ]ZI and the phase shift Q are plotted as a function of the logarithm of the frequency f.

EIS primarily characterizes a system in terms of its electrical properties. Hence, it is convenient to describe the measured phenomena by their electrical equivalent such as capacitances, resistances and distributed elements (Zdi, CPE). The combination of these elements having the same impedance as the measured system is called the equivalent circuit. The most important elements that can be used to construct an equivalent circuit are given in Table 1.

In Table 1 R is the resistor used to represent the resistance of a coating or the polarization resistance of a corrosion process. C is the capacitor describing the coating capacitance or the double layer capacitance. L is the inductance occurring at high frequencies and is mainly due to the current lead to the working electrode. This inductance is spurious and should be kept to a minimum. Zdiff is the impedance element resulting from a diffusion-controlled electrochemical process. The theoretical derivation of this complex impedance element has been given by Warburg [16,20,21]. CPE is the Constant Phase Element and is mainly used to describe nonideal capacitive behaviour. The understanding of this phenomenon is not complete, but generally the CPE is due to a distribution in the current density as a result of surface inhomogeneities [U-15,23-25].

Model conceptions for coated metals

An organic coating ideally behaves as a dielectric, giving a capacitive impedance characterized by an w-l frequency dependence. This behaviour is electrically equivalent to the capacitance C,, of the polymer layer in parallel with the resistance R, of the coating. However, most impedance spectra of coated metals immersed in an electrolyte solution with the resistance RE show deviations from this simple impedance behaviour as a result of the development of ionic conductivity paths. This can be described by the general model depicted in Fig. 3 [13-15,261, where the capacitance Co is replaced by a CPE, indicated by the CPE exponent a (see Table 1). Z, is a general impedance which occurs when an electrochemical

U. Rammelt, G. Reinhard / Pmgms in Organic Coatings 24 (1994) 309-322 313

- Imaginary part (l&l ,

Impedance (01 Phase

R=lOOkQ C= SOnF

@I Frequency (Hz1 +

Fig. 2. (a) Nyquist and (b) Bode plots of the impedance of an equivalent circuit consisting of a

resistor and a capacitor in parallel.

reaction takes place at the metal-coating interface. For many systems, Zr can be explained by a transfer resistance R, in parallel with a double-layer capacitance Cd, (model a) or by a diffusion impedance (model b).

When a coating is exposed to an aqueous solution, the resistance R, cannot be obtained initially owing to limitations in the measuring equipment (- 10” a). Under such circumstances, the only parameter than can be determined is Co, which is given by the expression

314 U. Rammelt, G. Reinhard I Pmgress in Organic Coatings 24 (1994) 309-322

TABLE 1 Impedance elements and their expression

Element Symbol

Resistance R

Capacitance C

Inductance L

Warburg Z dlff Constant phase element CPE

Impedance expression

R

t&O-’ jd K&jw) -05*

K(jo)**

*k;l,p RT z*F~c~(D)‘~

; where cO = concentration of the diffusing species and D = diffusion coefficient.

**0.5 <crQ 1.0.

model o model b

Cdl 102

CD lo’ h I’ 10’ 103 10’ 105 f lo‘

Fig. 3. Equivalent circuit for the impedance of coated metals.

Fig. 4. Theoretical Bode plots for coated metals at different stages of deterioration during load.

(~,=permittivity of vacuum; l =relative dielectric constant of the coating with the area A and the thickness d). The value of C, depends on the characteristics of the coating, particularly its thickness, composition and structure. As the relative dielectric constant E of an organic coating is small (3-8) compared to that of water (-SO), the value of C, will change with exposure time as the coating is penetrated by water. The increase of C,, the deviation of the capacitive part of the impedance from the w-l dependence and the situation that a general impedance 2, is observed with increasing exposure time can be used as an indication of the degradation of the coating and the start of the corrosion process.

In Fig. 4 the changes occurring in an insulating coating from the start of exposure to a test medium to longer time periods are illustrated schematically [27]. Initially, the polymer coating gives a straight line plot with a slope of - 1, indicating that the modulus \Z( is inversely related to w and that the phase shift

U. Rammelt, G. Reinhard I F’rogress in Organic Coatings 24 (1994) 309-322 31s

9 is -90’ at all frequencies. The impedance behaviour is purely capacitive with the upper limit ]Z],,,= (&)-I. The capacitance Co can be determined by ex- trapolation of the straight line to w= 1.

With increasing exposure time, a shift of the straight line is observed depending on the water uptake into the coating, giving higher values of Co. At time fz, the low frequency part of the Bode plot is a straight line parallel to the log f axis. The value of the layer resistance R, is obtained by extrapolation of this frequency independent line to the log ]Z] axis. If water and aggressive ions penetrate to the metal-coating interface, corrosion reactions can start after delamination of the coating. This can be seen at time L,. The Bode plot is split into two sections, one at high frequencies, corresponding to coating parameters (Co, R,), and one at low frequencies, corresponding to electrochemical corrosion reaction at the substrate. The upper limit (Zlmin represents the ohmic resistance of the entirely delaminated coating saturated with electrolyte solution.

The impedance behaviour strongly depends on intrinsic polymer properties such as polymer structure and composition and can be influenced by addition of pigments.

Pigment function in corrosion-protective primers

The protective action of pigments in corrosion-protective primers may be attributed to a barrier and to an electrochemical and/or chemical mechanism. The pigments may contribute to a barrier mechanism by its shape. By use of plate- or flake-shaped pigments the pathways of diffusion through the coating is lengthened and hence the water permeability is decreased significantly. The barrier effect of such substances can usually be optimized by employing the most favourable values of the particle size and the pigment volume concentration (PVC) [4,7,10,28-301.

In addition to their barrier effect, active anticorrosion pigments can give further protection for areas with coating damage. There, pigments have some solubility in water and when water enters a coating and reaches the metal substrate the corrosion inhibiting action starts. Active anticorrosion pigments are expected to prevent corrosion of metal substrate in coating damage by build-up of permanently passive conditions at the metal surface (electrochemical protection) and/or by build-up of solid compounds which plug the coating damages (chemical protection).

In order to distinguish between both mechanisms of protection, that of passivation by anticorrosion pigments and that by barrier action, the studies were made with primers with an artificial defect. Through variation of the aggressive test medium the protection mechanism of an anticorrosion pigment and the requirements involved in its action can be studied very well.

Excellent anticorrosion pigments are available, however those such as red lead and several chromates are subjected to restrictions for their detrimental influence on the environment and health, and new types have to be developed. To evaluate the usefulness of substitutes of anticorrosion pigments it is important to know which mechanism is responsible for protection, especially in damage.

316 U. Rammelt, G. Reinhard I Progress in Otganic Coatings 24 (1994) 309-322

In the present work the effects of two pigments at the same PVC levels in an alkyd-based primer have been compared directly in order to determine the protection mechanism of active pigments with the help of EIS.

Experimental

Two series of alkyd primers containing either zinc chromate as an active pigment or mica as an inactive pigment were applied by conventional spray techniques to a thickness of 45 f 5 pm onto mild-steel panels. All samples were damaged by a mechanical scribe and then exposed to several traditional test media: condensation chamber, salt spray and exterior exposure to atmosphere. After different exposure times, the samples were characterized by impedance measurements. Impedance data have been obtained at the open-circuit potential using the IM5d impedance measurement system of Zahner-MeBtechnik. The frequency range analyzed was 50 kHz to 0.05 Hz, and the amplitude of the superimposed a.c. signal was 20 mV. The interpretation of the impedance data was carried out by combining the results of the impedance and potential mea- surements and visual inspection of the coated metal. The electrochemical cell, as shown in Fig. 5, was especially made for measurements on coatings.

In the cell a platinium gauze was used as the counter electrode. The saturated Hg/Hg,Cl,/KCl reference electrode was provided with a condenser of 10 PF and coupled to a platinium wire in order to prevent high frequency distortion. The working electrode area was 0.5 cm’ with a defect size in this area of 0.03 mm’. The impedance measurements were carried out in 0.005 M KN03 solution.

Results and discussion

Fig. 6 shows spectra typical for samples with and without a defect prior to exposure. The Nyquist and the Bode plots exhibit several important features.

Fig. 5. Cell configuration for impedance measurements on coated substrates. 1: Electrolyte container; 2: coated metal as working electrode; 3: Pt gauze as counter electrode; 4: reference electrode (SCE).

U. Rammelt, G. Reinhard / Pmgms in Organic Coatings 24 (1994) 309-322 317

(4 Real part (kQ) -

Fig. 6. (a) Nyquist; (b) and (c) Bode plots of pigmented primers prior to exposure. 1: Undamaged

primer; 2: zinc chromate pigmented primer with an artificial defect; and 3: mica pigmented primer

with defect.

No difference in impedance response is apparent for zinc chromate and mica pigmented primers without an artificial defect. Both primers give a capacitive impedance, characterized by a vertical line in the Nyquist plot (Fig. 6(a)), a straight line of slope - 1 in the log 121 Bode plot (Fig. 6(b)) and a value of cp of almost -90’ at all frequencies (Fig. 6(c)).

In contrast, for the samples with an artificial defect, two different parts can be distinguished in the impedance response. The high frequency part is related to the dielectric properties of the primer with the capacitance Co corresponding to that of the undamaged coating and the layer resistance R, depending on the size of the defect. From Fig. 6 very little difference in impedance response is apparent for either the zinc chromate or the mica pigmented primer. However, the low frequency part, which can be attributed to electrochemical processes taking place within the coating defect, presents a different behaviour pattern. From this

318 U. Rammelt, G. Reinhard 1 Progress in Organic Coatings 24 (1594) 309-322

part, it can be seen that zinc chromate as the active pigment is able to repair the defect by formation of a passive film at the metal surface, which correlates well with the noble open-circuit potential of the coating.

The low frequency data for the mica pigmented primer exhibit considerably reduced values of C and R showing that the inactive pigment mica is not able to develop any corrosion protection in the defect.

Impedance responses obtained from the coated panels containing mica after a longer period of outdoor exposure are shown in Fig. 7.

For these samples, a clear trend towards progressively increasing deterioration of the coating can be observed. In contrast to the data prior to exposure, the 14 day exposure data show an additional diffusion term at low frequencies owing to electrochemical dissolution of the metal substrate and build-up of corrosion products within the defect. After longer test periods, the diffusion term is diminished and a nearly capacitive behaviour is observed owing to the fact that corrosion products

- Imaginary part (Wzl -W., I Impedance (Ql

Frequency (Hz) -*

Fig. 7. (a) Nyquist; (b) and (c) Bode plots of damaged primers containing mica at different times of outdoor exposure. 1: Prior to exposure; 2: 14 days; 3: 42 days.

LI. Rammel& G. Reinhard / Progress in Organic Coatings 24 (1994) 309-322 319

plug the coating defect completely. The open-circuit potential shifts markedly to more positive values. However the high frequency part of the impedance spectra indicates that the coating capacitance is increased and the layer resistance is decreased. These effects are caused by the delamination of the coating adjacent to the damage.

The responses for the primers containing zinc chromate are very similar to those obtained prior to exposure (Fig. 8). The similarity of the results supported by a noble open-circuit potential at all exposure times indicates considerable stability of the passive state within the defect under these exposure conditions.

In the presence of chloride or sulphate anions, the passivating action of anticorrosion pigments is significantly impaired or completely lost. Both ions interfere with the repair of defects in the oxide layer of the metal substrate by forming soluble ion complexes of iron which may diffuse away and are converted

- Imaginary part Wl

0 . . -

*.

Impedance t&21

I.“-. 0 - * g 0

L.” .pzzt -. j

Phase’ .I-

Fig. 8. (a) Nyquist; (b) and (c) Bode plots of damaged primers containing zinc chromate at different times of outdoor exposure. 1: Prior to exposure; 2: 14 days; 3: 42 days.

320 U. Rammelt, G. Reinhard I Pmg~~s in Organic Coatings 24 (1994) 309-322

to insoluble rust products [7,31]. This behaviour is supported by investigation of the effect of both pigments on coating performance properties and corrosion protection by a traditional salt spray test. There is relatively little difference in the results as can be seen for example after 48 h exposure (Fig. 9). From these data, no specific electrochemical activity can be attributed directly to the zinc chromate, even at small defects.

Plotting the potential as a function of time provides an additional insight into the protective mechanism of the anticorrosion pigment zinc chromate and the requirements involved in its action. During the initial 1 hour immersion in 0.005 M KN03, Fig. 10 shows a noble potential for the damaged sample, reflecting the passive state within the defect.

If the KNO, solution is changed for a 3% NaCl solution, the potential rapidly drops down to values which correspond to the potential of the corroding iron electrode.

- Imaginary part lkQ1

Phase’

Impedance ( K Q ) 4

0, ,, (

(cl i*‘= * 1.. 2” 111 IX II IH( II

&e&c&y (Hz) -+

Fig. 9. (a) Nyquist; (b) and (c) Bode plots of damaged primers after 48 h salt spray test. 1: Zinc chromate; 2: mica.

U. Rammelt, G. Reinhard I P~~grtw in Organic Coatings 24 (1994) 309-322 321

100 , L

H z -200 - .

i

E -300 -

5 m

-400 -

I

-I

0 10 20 30 40 50 60 70 60 90 100 110 120

lime / min

Fig. 10. Potential vs. time for zinc chromate pigmented primer during the initial 1 h immersion period in 0.005 M MO3 and after changing the solution for 3% NaCL

Under these conditions, protection can only be given by the chemical reaction of zinc ions to produce solid compounds with barrier properties (for example metallic soaps or basic zinc carbonates). These compounds are able to prevent an increase in the pH of the aqueous solution within the defect and in that way delamination of the coating adjacent to the defect is reduced.

EIS was used to monitor the protection mechanism of two one with zinc chromate and the other with mica as pigment.

In this study, damaged coatings, With the help of EIS it is possible to distinguish between the inert pigment, mica, which only acts as a diffusion barrier and the anticorrosion action of zinc chromate, which consists in an electrochemical protection by the chromate anion and a chemical protection by the zinc cation.

Conclusions

From the high frequency part of the impedance spectrum, which is related to the coating properties, the start of delamination can be detected in a very early stage by decreasing of the layer resistance R,.

From the low frequency behaviour of the impedance spectrum it could be derived whether the pigment provides electrochemical or chemical protection or only gives a barrier effect. The results presented here have shown that EIS may be used successfully to evaluate the suitability of substitutes of anticorrosion pigments with environmentally harmless properties.

322 U. Rammelr, G. Reinhard / Progress in Organic Coatings 24 (1994) 309-322

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