Organic Coatings for Corrosion Control - American Chemical Society

Chapter 3

Application of Localized Electrochemical Impedance Spectroscopy to the Study of the Degradation of Organic Coatings

Downloaded by COLUMBIA UNIV on July 20, 2012 | http://pubs.acs.org Publication Date: March 30, 1998 | doi: 10.1021/bk-1998-0689.ch003

F. Zou and D. Thierry Swedish Corrosion Institute, Roslagsvägen 101, Hus 25, 10405 Stockholm, Sweden A high resolution probe that allows localized Electrochemical Impedance Spectroscopy (LEIS) has been developed. In this paper some possible applications of the use of LEIS in the study of the degradation of organic coating on metal surfaces are presented. It is shown that LEIS can be successfully applied to locate micro-blisters on painted metal surfaces. It is also shown that the technique can be useful in order to obtain mechanistic information on the kinetics of activation and passivation of metals in the vicinity of a defect in an organic coating. During the last decades, Electrochemical Impedance Spectroscopy (EIS) has been extensively used to study the degradation of coated metals exposed to various environments However, the interpretation of the EIS data is generally difficult due to the complexity of the systems studied and to the fact that the impedance are averaged over the whole exposed area while the degradation generally occurs locally. Several attempts to develop a scanning impedance technique has been reported in the literature 3. However, the spatial resolution of these techniques was limited by the size of the probe and the mass transport process may be influenced by the cell geometry. A novel method of measuring local impedance spectra has been reported by Lillard et al.4 . In this method, local impedance spectroscopy (LEIS) was performed by determinating the local AC solution current density using a two-electrode microprobe. The probe consists in two platinum electrodes mounted in glass capillaries. The AC potential difference associated with the AC current flow in the solution was measured between the two electrodes. The local AC solution current density is then calculated using Ohm's law according to: i(co) = A V ( o »

prol)e

j

[1]

where i(co) is the local AC solution current density, AV(co)

is the AC potential

probe

difference at the probe, κ is the conductivity of the electrolyte, and d the distance between the two tips of the capillaries. The local impedance was thereafter calculated according to :

©1998 American Chemical Society

In Organic Coatings for Corrosion Control; Bierwagen, G.; ACS Symposium Series; American Chemical Society: Washington, DC, 1998.

23

24

Ζ ( ω )

Μ

[2]

= ^ «

i(co) The technique has been applied to study galvanic corrosion on an Al/Mo electrode4 and to study coating failure for thin coated mild steel cans^. In a more recent paper, Wittmann and Taylor 6 used Ag/AgCl micro-reference electrodes instead of Pt. The resolution of the system was 125 μπι and the impedance limit was 106 Ω . α η 2 which limits the frequency range over which impedance data can be collected. The authors applied the impedance probe to locate heterogeneities such as pinhole defects in organic coatings. In this paper, a newly designed high resolution probe 7-9 (i. . 30-40 μπι in lateral resolution) that allows localized impedance measurements has been applied to the study of the degradation of organic coatings on metal surfaces. The main objective of this work was to determine the feasibility of using a local electrochemical impedance technique to detect the formation of blisters on painted metals. The technique has also been applied to study the mechanisms of activation and passivation of iron in the vicinity of a defect in an epoxy primer.


e

Experimental Probe and Experimental Setup The experimental setup used to measure localized EIS data has been described in details elsewhere?- . The probe consists in a pair of microelectrodes (Pt-Ir with a tip of 10 μπι). The microelectrodes were plated in a PtC^ solution in order to decrease the interfacial impedance at the tip. The relative position of the two tips were adjusted so that a straight line passing through the two tips was perpendicular to the surface of the specimen. The local AC IR drop in solution was measured as the AC potential difference between the two microelectrodes. The applied voltage was 30 mV. The signal was first preamplified using a preamplifier (PAR model 5113 with an input impedance of 100 ΜΩ and a gain of 25). Both the in-phase and the quadrature of the signal associated with the AC current flow in the solution were measured using either a lock-in amplifier or a multichannel frequency response analyzer (Solartron 1254). The local AC solution current density and the local impedance were thereafter calculated using equations [1] and [2], The scanning of the probe was accomplished by using three stepper motors (spatial resolution 0.4 μπι). The EIS spectra were interpreted using the software "Equivalent Circuit " written by Boukamp 10. This software obtains from the Nyquist plots the approximate values of the passive elements in the equivalent circuit which are then fitted using a Non- Linear Least Squares (NLLS) method. 9

Specimens Carbon steel and hot-dip galvanized steel (zinc thickness 20 μπι) were used as substrates in this work. The pretreatment for the hot-dip galvanized steel panels consisted of alkaline cleaning, alkaline oxidation and a chromic acid after rinse. The panels were


25 thereafter painted with an epoxy primer and a top-coat of polyester to a total thickness of 35 μπι. The sample was thereafter exposed to a 0.3 M HC1 solution as this procedure has shown to produce fast blistering on these samples . The surface of carbon steel was partially contaminated by dropping approximatly 10 μΐ of a 3% NaCl on the substrate and then forced drying prior to paint with an epoxy primer at a thickness of 25 μιη. This procedure was used in order to obtain fast blistering at the paint/metal interface. The impedance measurements were performed in a 10 mM NaCl solution. n

Results and Discussion


Detection of Micro-blister Figure 1 shows the local AC solution current density over painted electrogalvanized steel exposed for different times to 0.3 M HC1. The measurements were performed at a frequency of 890 Hz. Figure lb shows a slight increase in the AC IR drop at

Figure 1: Current density maps for electrogalvanized steel exposed to 0.3 M HC1. a) 1 hour, b) 3hours, c) 5 hours, d) 7 hours and e) 20 hours.



26 one location near the center of the sample (-2368,2457). The increase of the local current density is due to the creation of a low impedance site at this location (see equations [1] and [2]). At this stage the surface of the organic coating shows no visible changes using a microscope (magnification χ 100). Figure lc, Id and le show a clear peak at this location, and in this case a small micro-blister was also visible under a microscope (magnification X 100). It is well known that in the case of intact paint films the impedance response in the high frequency region will be dominated by the contribution of the capacitance of die paint layer. Hence, the results shown in Figure 1 c and 1 d seems to indicate that the increase in the local current density over the blistered region may be related to an increase in the capacitance of the organic coating in this region (i.e. as the capacitance is directly proportional to the AC current measured by the probe in the case of a pure capacitor). This is in agreement with the results of Van der Weijde and al. showing using traditional EIS measurements that the curve of the capacitance of the organic coating against time for a barrier type coating shows a small step indicating the onset of delamination 12. However, one cannot exclude the possibility that microcracks were formed during the blistering process which may lead to a decrease of the local impedance at these locations. After 20 hours of exposure (Figure le) in 0.3 M HC1 the paint layer over the micro-blister was visually ruptured resulting in the formation of a macroscopic defect down to the metal substrate. As noted in the experimental section, a traditional EIS measurement was performed immediately following each LEIS map. Figures 2 shows the evolution of Bode plots obtained using traditional EIS measurements on painted electrogalvanized steel exposed for different time in 0.3 M HC1. The impedance spectra change rapidly to the behavior typical of intact coatings (Figure 2a) to that of deteriorated coated with spectra exhibiting two time constants after 20 hours of exposure (Figure 2e). It should be noted that for "intact" blisters (i.e. Figure la-d), the EIS spectra only show one visible time constant. A comparison of Figure 1 and 2 indicates that the visual rupture of the micro-blister corresponds to a large decrease in the impedance of the system. The EIS data shown in Figure 2a-d were interpreted using the simple equivalent circuit shown in Figure 3 whereas the EIS data displayed in Figure 2e were interpreted using the general equivalent circuit given for painted metals 1. Cpf was modeled as constant phase elements (i.e. CPE for which the impedance may be expressed as Ζ(ω) =

) representing the non-ideal dielectric behavior of this Υ„0ω)η

element. The variation of C f with the time of exposure is shown in Table 1 together with an estimation of the size of the micro-blister obtained from the current density maps shown in Figure 1. The coating capacitance increases largely during the first stage of exposure (i.e. in connection with the development of the microblister at the metal/paint interface), and then remains almost constant until the rupture of the blister. It should also be noted that the η value of the CPE associated with the organic coating decreases largely due to the formation of the blister. This may be related to an increase distribution of this passive element due to blister formation. p



Figure 2: Bode plots obtained by traditional impedance measurements on electrogalvanized steel exposed to 0.3 M HQ., a) 1 hour, b) 3hours, c) 5 hours, d) 7 hours and e) 20 hours.

Figure 3: Simple electrical equivalent circuit. R is the electrolyte resistance, C f the capacitance of the organic coating and R the pore resistance (see reference 2 for more details). u

p

p o


28


Table 1: Variation of Cpf and the size of the blister with the time of exposure to 0.3 M HQ. Time of exposure, hours

Cpf, F.cm-2

η of CPE

Blistered μπι2

1

2.10-10

0,98

-

3

610-9

0,96

60

5

5,810-9

0,96

80

7

710-9

0,955

900

20

2.510-8

0,89

Open blister 1200

area,

LEIS Spectra of Defected Painted Steel Samples Figure 4 shows the local AC solution current density over painted steel exposed for different times to 10 mM NaCl. The steel surface was contaminated by dropping NaCl on the surface prior to painting. This results in a very fast blistering and in the perforation of the organic coating. Figure 4a clearly shows that such defects in the organic coating can be monitored using LEIS. The size of the defect was estimated from the half height of the peak to be 0.06 m m 2 . The frequency of the applied signal was 890 Hz. As further shown in Figure 4 the current density decreases largely over the defect when increasing the exposure time in the NaCl solution. This corresponds to an increase of the impedance at the defect due to the formation of corrosion products that limits the diffusion of oxygen to the metal surface. Traditional and LEIS data have been recorded for defected painted steel after different time of exposure to 10 mM NaCl (i.e. corresponding to the AC JR drop maps shown in Figure 4). In this case, the localized impedance data were recorded at the defect located by scanning impedance measurements shown in Figure 4, whereas traditional impedance data represent an averaged impedance including the defected area and the intact coating. Figure 5 shows Nyquist plots generated by traditional (Figures 5a and 5c ) and from LEIS (Figures 5b and 5d) after different times of exposure to 10 mM NaCl. The traditional EIS spectra show a feature that has been reported previously for painted metals with defects. The contribution of the "intact" organic coating is found in the high frequency region, whereas the contribution of the corrosion processes in the defected area is found in the intermediate and low frequency regions. The Nyquist plots generated by LEIS show are very different from tha' obtained by traditional EIS. At early stage of exposure, three time constants are visible ( a high frequency capacitive loop, an intermediate frequency capacitive loop and a low frequency capacitive loop). The high frequency loop defined with a characteristic frequency of 30-40 Hz is probably due to charge transfer while the low frequency loops probably corresponds to the kinetic of intermediate species in the dissolution path of iron in the vicinity of the defect. From figure 5d, it is obvious that the low frequency region transforms in a negative resistance loop characteristic of the passivation after 5 hours of exposure in 10 mM NaCl. The present results are in good agreement to previous work performed with traditional EIS on iron in neutral and basic mediae.



Figure 4: Current density maps for contaminated and painted carbon steel exposed to 10 m M NaCl. a) 30 min, b) 150 min, c) 270 min and d) 470 min

0

10000 20000 30000 40000 50000 60000 RefZ^cm

0

40000

500

2

80000

1000 Re(Z).Û*cm

1500

2000

2

120000

•12000

-8000

-4000 0 ReiZJ^cm

4000

2

Figure 5: Nyquist plots for contaminated and painted carbon steel exposed to 10 m M NaCl. a) Traditional EIS (30 min) b) LEIS (30 min), c) Traditional EIS (300 min) and d) LEIS (300 min).


30 Hence from the data presented in Figure 5 to obtain an estimate of the local corrosion rate in the vicinity of the defect. However, local changes in conductivity of the electrolyte in the defect area due to the metal dissolution should be taken into consideration. This is the subject for further research in our laboratory.

Conclusions -LEIS is sensitive to follow localized variations in the dielectric properties of organic coatings due to blistering. -LEIS can be used to obtain qualitative information on the mechanisms of dissolution and passivation of iron in the vicinity of a defect in an organic coating.


References 1- A. Amirudin, Doctoral Thesis, Stockholm (Sweden), 1994. 2- A. Amirudin and D. Thierry, Prog. in Org. coatings, 26, p 1, 1995. 3- H.S. Isaacs and M.W. Kendig, Corrosion 36(6), p 269, 1980. 4- R.S. Lillard, P.J. Moran and H.S. Isaacs, J. Electrochemical Soc. 139(4), p 1007, 1992. 5- R.S. Lillard, J. Kruger, W.S. Tait and P.J. Moran, Corrosion Vol 51, No4 p 251, 1995. 6- M.W. Wittman and S.R. Taylor, in Advance in Corrosion Protection by Organic Coatings II, ECS proceedings, PV 95-13: 158, 1995. 7- H.S. Isaacs, A.J. Aldykiewicz Jr., D. Thierry and T.C. Simpson: Corrosion 95, ΝACE International, 1995. 8- F. Zou, D. Thierry, A. Annergren and H.S. Isaacs, submitted to J. of Electrochem. Soc., 1996 9- A. Annergren, D. Thierry and F. Zou, submitted to J. of Electrochem. Soc, 1996 10- Β. Boukamp, Proc. 9th Eur. Corr. Cong., Utrech, FU-252, 1989 11- A. Iversen, internal report, Swedish Corrosion Institute, 1996. 12- D.H. Van der Weijde, E.P.M. van Westing and J.H.W. De Wit, Corrosion Science, Vol 36, No 4, p 643, 1994. 13- I. Epelboin, C. Gabrielli, M. Keddam and H. Takenouti, in Comprehensive Treatise on Electrochemistry, Ed J.O'M Bockris, B.E. Conway and E.B. Yeager, Plenum Press (New York) Vol 4, p 151, 1981


Organic Coatings for Corrosion Control - American Chemical Society

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