Use of impedance spectroscopy to evaluate the durability of protective coatings after thermal shock

Corrosion Science, Vol. 40, No. 415, pp. 663-672, 1998 Q 1998 Published by Elsevier Science Ltd. All rights reserved. Printed in Great Britain. 0010-938X/98 $19.00+0.00

Pergamon

PII: SOOlO-938X(97)00168-6

USE OF IMPEDANCE SPECTROSCOPY TO EVALUATE THE DURABILITY OF PROTECTIVE COATINGS AFTER THERMAL SHOCK A. MISZCZYK Technical

University

of Gdansk,

Faculty

and K. DAROWICKI

of Chemistry,

1 l/12 Narutowicza

Str., 80-952 Gdansk,

Poland

Abstract-The properties of coatings have been measured before and after artificial thermal shock. Impedance results have shown that the thermal shock causes the development of coating delamination and porosity in the film, which were measured quantitatively. The possibility of employment of impedance spectroscopy has been indicated for evaluation of resistance of coatings to thermal shock. The effect of thermal shock has been analysed on protective coatings used for flue gas desulphurisation installations. 0 1998 Published by Elsevier Science Ltd. All rights reserved Keywords:

A. organic

coatings,

B. EIS, thermal

shock

INTRODUCTION Coatings used for protection of metal, concrete, wooden and other materials undergo many temperature changes during their service life. Due to different coefficients of thermal expansion of the substrate and coating materials, temperature changes generate internal stress in the coatings, which is transferred to the coating/substrate interface. Fortunately many coatings yield under stress, and, if properly formulated, will accommodate relatively large dimensional changes resulting from thermal effects. However, some coatings in specific conditions undergo accelerated degradation as the result of temperature changes, observable by loss of adhesion to the substrate and/or cracking of the coating. Usually in coatings there is residual internal stress created during formation of the coating (process of drying).’ The effect of various coating additives on the magnitude of residual stress had been investigated ’ 8 in solvent and non-solvent protective coatings. It has been stated that in non-solvent epoxide coatings internal stress increases with increase of coating thickness.’ Temperature and moisture content changes introduce additional stress to the coating. It has been stated that the history of temperature changes in the initial service life period has an effect on the durability of the coating.“’ Perera et al.’ investigated the effect of temperature and moisture content on the durability of organic coatings on metals. It has been stated that the generated stress has a negative effect on adhesion and cohesion of organic coatings and accelerates ageing of coatings. King et aL9 investigated thick (1.5 mm) non-

Manuscript

received

11 September

1997; in amended

form 13 November 663

1997

664

A. Miszczyk

and K. Darowicki

solvent amine hardened epoxide coatings on aluminum substrate, which were subjected to thermal shock. It has been stated that heating to a temperature above the glass transition point (T,) causes formation of irreversible stress on thick coatings. Glass beads’ and mica flakes of different size” are added to limit the magnitude of formed stress in thick coatings during temperature changes. At present thick coatings with filler in the form of glass flakes are being applied frequently.“‘* Due to their shape and placement in the coating (in parallel to the substrate) they partially compensate stress formed during thermal shock. This is of special importance in the case of thick coatings used in industrial applications. Coatings with glass flakes are being used for protection of tanks, pipelines, chemical and power installations, especially in flue gas desulphurisation plants. Internal stress measurements in coatings are mainly carried out by the bimetallic method, in which a single tested coating is applied on a standardised, thin metallic substrate. The formed stress causes mechanical deformation of the sample, which is measured, and in this way the stress value is determined,” Fig. 1. No possibility of observation of stress effects on the coating and its protective properties due to deformation of the substrate is the drawback of this method. In real conditions the metal substrate is rigid enough and is not deformed and the formed stress can cause delamination of the coating from the substrate and/or its cracking. Due to the above trials have been made to elaborate methods, enabling non-destructive determination of stress in real multilayer coatings.14 Due to the character of changes caused by thermal shock in coating/substrate systems (delamination, microcracks) it seems justified that they could be detected with the use of impedance spectrocopy. This technique has found application in testing the degradation of coatings in aggressive environments.” ” Impedance spectroscopy has not been used up till now for investigation of samples after thermal shock. The aim of this paper was to state if application of impedance spectroscopy for testing of protective layers subjected to thermal shock allows the determination of the degradation mechanism and estimation of the resistance of the coating to such types of interaction.

EXPERIMENTAL Coatings have been investigated for protection of flue gas desulphurisation installations by the wet limestone method. Coatings from different manufacturers have been investigated: vinylester, four-layer, 2 mm in thickness, containing glass flakes, vinylester, four-layer, 4 mm in thickness, containing glass flakes of various dimensions, fluoropolymeric, monolayer, 1.2 mm in thickness, containing graphite flakes.

Fig.

1.

Schematic

illustration

of internal

stress measurement method.

within

coatings

by the bimetallic

Use of impedance

spectroscopy

to evaluate

the durability

of protective

coatings

665

Thermal shock was realised according to the schematic diagram in Fig. 2. Before shock the sample was exposed in 1% H,SO,+0.2% Cll solution at 80°C. The solution simulates the condensate deposited on the walls of the installation in the purified gas zone. The temperature of 80°C is characteristic for purified flue gases. The temperature of 170°C is typical for non-purified gases. In the case of break-down, starting or stopping of the functioning of the installation the protection is subject to short-term interactions of such temperatures. Impedance measurements were carried out at 20 f 1“C in 1% H2S04+ 0.2% Cll solution. A Schlumberger Model 1255 frequency response analyzer with a high-input impedance adapter was used. The coated metal was the working electrode and platinum mesh was used as the counter electrode. The exposed surface of the coating was 50 cm2. The frequency used ranged between 1 MHz and 10 mHz (10 points for each decade). The amplitude of the sinusoidal signal was 100 mV because of the high impedance of the measured system and to improve the ratio of the effective signal to noise.

RESULTS

AND

DISCUSSION

In Fig. 3 results have been presented of impedance measurements of vinylester 2mm thick coating in the Bode system. Curve 1 presents the impedance spectrum of vinylester coating (2 mm in thickness) before exposure, curve 2 after 12000 h of exposure in 1% H,S04+0.2% Cl- solution at 80°C (before thermal shock), curve 3 presents the same sample after thermal shock and 24 hours of immersion in 1% H2S04+0.2% Cl- solution at 80°C curve 4 shows the same sample after a further 3000 h of exposure in 1% H,SO,+0.2% Cl- solution at 80°C. Curve 5 presents another sample of the same coating which was not subjected to thermal shock after exposure lasting 15000 h, i.e., after the same period as the sample subjected to shock. Comparison of curves 2 and 3 shows that the impedance spectrum has changed as the result of thermal shock and short-term interaction at a 170°C temperature, especially in the frequency range below approx. 1 kHz. From Fig. 3 one can conclude that in the high-frequency range a decrease is observed of the impedance modulus slope in relation to the frequency axis. The sample exposed further for another 3000 h exhibits after this time a significantly different spectrum (curve 4). In Fig. 3 the spectrum has been presented for comparison (curve 5) of another sample not subjected to interaction of thermal shock. The remaining factors were identical. Thus one can conclude that the observed changes were the result of thermal shock. The relationship between the

4h

4h

24 h

immersion in

1%H,SO, + 0.2%CI-

measurement

measurement

time Fig. 2.

Schematic

diagram

of thermal

shock realization.

666

A. Misrczyk

and K. Darowicki

(a) 12 -

-2

0

2

4

6

log f (Hz)

A

60-

E

z. 2g

40-

CT 20 -

0;

I

-4

,

I

/

,

,

,

,

4

-2 I:g

,

,

6

f (H:)

Fig. 3. Bode plots for glass Rake vinylester coating. 2mm in thickness: (1) before exposure. (2) after 12000 h of exposure; before thermal shock, (3) after thermal shock. (4) after further 3000 h of exposure: (5) presents another sample after 15000 h of exposure without thermal shock.

physical model for a film with pores and the corresponding equivalent circuit model (Fig. 4) has been the subject of discussion.‘h-‘x.‘y Nevertheless, it is generally agreed that the high frequency impedance response comes from a parallel combination of coating capacitance (or CPE) and pore resistance. Nonetheless, the interpretation of the low frequency impedance is not, very clear, but low frequency impedance is derived from the wetted metal area at the bottom of pores and usually the wetted area is not generally the same as the porosity,“’ Fig. 5. If there is a loss of adhesion to the substrate, then the wetted area is much larger than the pore area. The obtained results show that the disadvantageous interaction of thermal shock on protective parameters of the coating is connected in the initial period

Use of impedance

spectroscopy

to evaluate

the durability

of protective

coatings

667

..~_~_~.~_~_~.~_~.~_~.~. .. .. .... :,~i.-.~.~.~.S~::. .~.~_~.~.~_~_~.~.~.~.~.

.~.~.~.~.~.~.~_~.~,~.~. . .. ... .. . .... . .. .. ... . :....:............:. .‘.‘.‘.‘.‘.....‘.....‘. _..~....i_.i.......~. .~.~.~.~,~.~.-_~.~.~.~. _..._....~...~.......~. .~.~.~.~.~.‘.~.~.~.~.~. .‘.‘.‘.‘.~.‘.‘.~.‘.‘.~. . .. .. .. .. ... . .... ..

electrolyte Fig. 4.

Fig. 5.

Equivalent

coating

circuit for the impedance

metal behavior

of a polymer

coated

metal

Two models for pores in protective coatings: (a) pore area is equal to the wetted metal area and (b) the wetted metal area is greater than the pore area.

with loss of adhesion. This is indicated by the decrease of the impedance modulus in the low frequency range (curves 2 and 3). Simultaneously, the high frequency part underwent relatively small changes, pointing to minimal changes of the porosity of the coating. In this range only a decrease is observed of the slope of the impedance modulus curve in relation to the horizontal axis, pointing to the increase of the non-homogenity of the coating, resulting from formation of microcracks. After a further 3000 h of exposure we observe an increase of the porosity of the coating, indicated by changes of the high-frequency part of curves 3 and 4. They are exhibited as a decrease in the pore resistance (formation of new ion conducting paths) and a further increase of the substrate area available to the electrolyte. Barrier parameters of the coating change in a direction characteristic for accelerated degra-

668

A. Miszczyk Table I. CPE (Z,,,

and K. Darowicki

Parameters of vinylester, 2 mm thick, coating (0) = Y ’ (jw)-“, where w-angular frequency; Y, n-parameters of CPE).

Plot (Fig. 3) I 2 3 4

Y [F cm-%“-’ 4.91 x 10 ‘I 9.75 x IO “I 1.38 x IO my 2.69 x IO-’

]

n 0.926 0.625 0.605 0.702

dation. Information on the direction of change is obtained from comparison of spectra obtained at the beginning of exposure (curve 1) and after 12000 h of exposure (curve 2). In Table I parameters have been presented of the constant phase element describing the dielectric properties of a coating determined in the 1 kHz to 1 MHz range using the Boukamp software*’ during different stages of exposure. A gradual increase is observed of the Y parameter values of the constant phase element. Simultaneously a decrease is observed of coefficient n, with the exception of curve 4. The observed increase of coefficient n in curve 4 in comparison with curve 3 should be explained by transformation of some microcracks into pores reaching the substrate. Therefore their presence is recorded as a decrease of resistance in the pores of the coating and is not recorded as a property described by the constant phase element. In Fig. 6 a schematic diagram has been presented of the mechanism of degradation of coating A subjected to thermal shock. After thermal shock microcracks are formed in the coating, not causing increase of the porosity of the coating, and we observe a loss of adhesion of the coating (stage 1). During further exposure the microcracks cause formation of additional ion conducting paths (pores) and an increase of the porosity of the coating is observed (stage 2). A further increase also is observed of the delamination area. In Fig. 7 impedance spectra have been presented of vinylester 4 mm thick coating before and after thermal shock. No changes are observed, pointing to resistance of the given coating to thermal shock. In Fig. 8 analogous spectra have been presented of the fluoropolymeric coating with no glass flakes. Thermal shock causes loss of adhesion of the coating to the substrate and a decrease in porosity.

CONCLUSION Thermal shock causes degradation of the barrier properties magnitude of the thermal shock effect depends on the composition extent on the contents of fillers in the flake form (mica, glass), generated by temperature changes. On the basis of analysis of mination is possible of the degradation mechanism of coatings as well as assessment of their resistance to such interaction.

Ac,knolck~~~~nzc~7/~This

work was supported

by Grant

of organic coatings. The of the coating, to a large which compensate stress impedance spectra detercaused by thermal shock.

Nr 7 T08C 059 12 financed

by KBN

Use of impedance

spectroscopy

to evaluate

the durability

of protective

.-

coatings

669

coating

thermal

shock

1

2 .... ... . . . .... ............................. . .......... .................. ........................ ......................................................... .:::::.:.>:.: ............................. ...................... ..................... ::.:.:.:.:.:. ....:.:.:. Fig. 6.

Schematic

diagram

of degradation

of 2 mm thick vinylester shock.

coating

subjected

to thermal

670

A. Miszczyk (a)

and K. Darowicki

14

Afl er shock

0

6

1

44 -2

0

4

6

log ;(Hz) (b)

1

l

Before shock

O After shock

00 -2

;(Hz)

0

4

6

log Fig. 7.

Bode plots for vinylester 4 mm thick coating after 120 h exposure to solution at 8O‘C and before and after thermal shock.

1% H,SO,+

0.2% Cl

Use of impedance

spectroscopy

to evaluate

the durability

of protective

671

coatings

(4 9-

8-

6-

4

;

I

.

Before shock

0

After shock

I

I

I 6

.

80 -

Before shock

O After shock

I -4

-2

0

2

4

6

log f (Hz) Fig. 8.

Bode plots

for fluoropolymeric coating after 120 h exposure to 1% H,SO,+0.2% solution at 80°C and before and after thermal shock.

Cl

672

A. Miszczyk

and K. Darowicki

REFERENCES 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18.

Sato, K. Prog. Org. Coat., 1980,8, 143. Croll, SC. J. Appl. Polym. Sri., 1979, 23, 847. Croll, S.G. J. Coat. Technol., 1979, 51(659), 49. Croll, SC. J.O.C.C.A., 1980, 63, 230. Croll, SC. J. Coat. Technol., 1981, 53, 85. Perera, D.Y. and Eynde, D.E. J. Coat. Technol., 198 I, 53, 39. Perera, D.Y. and Eynde, D.E. J. Coat. Technol., 1981,55, 37. Perera, D.Y. and Eynde, D.E. J. Coal. Technol., 1983, 57, 37. King, D. and Bell, J.P. J. Adhesion, 1988, 26, 37. Gupta, V.B., Brahatheeswaran, C. J. Appl. Polym. Sci., 1994, 52, 107. Fenner, J. and Schedlitzki, D. VGB Krufrwerkstechnik, 1992, 72, 502. Hendry, C.M. J. Prof. Coat. Lin., 1993, 50. Corcoran, E.M. J. Paint Technol., 1969,41, 635. Shiga, T., Narita, T., Tachi, K., Okada, A., Takahashi, H. and Kurauchi, T. Polym. Eng. Sci., 1997, 37, 24. Mansfeld, F., Kendig, M.W. and Tsai, S. Corrosion, 1982, 38, 478. Walter, G.W. Corros. Sci., 1986, 26, 681, Kendig, M. and Scully, J. Corrosion, 1990,46, 22. de Wit, J.H.W. Progress in the Understanding and Prevention qf Corrosion,Vol. I. The Institute of Materials, London, 1993, p. 240. 19. Mansfeld. F. J. Appl. Electrochem., 1995, 25, 187. 20. Armstrong, R.D. and Wright, D. Electrochim. Acta, 1993, 38, 1799. 21. Boukamp, B.A. Equivalent Circuit v. 3.99, University of Twente, 1992.