SVET method for characterizing anti-corrosion performance of metal-rich coatings

Corrosion Science 52 (2010) 2636–2642

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SVET method for characterizing anti-corrosion performance of metal-rich coatings Maocheng Yan *, Victoria J. Gelling **, Brian R. Hinderliter, Dante Battocchi, Dennis E. Tallman, Gordon P. Bierwagen Department of Coatings and Polymeric Materials, North Dakota State University, Fargo, ND 58108, USA

a r t i c l e

i n f o

Article history: Received 17 January 2010 Accepted 12 April 2010 Available online 28 April 2010 Keywords: A. Metal-rich coating A. Steel B. SVET B. Microelectrode technique C. Galvanic interaction

a b s t r a c t The galvanic interaction between a metal-rich coating and the underlying metal substrate was characterized by a new analysis method based on the scanning vibrating electrode technique (SVET). The total anodic current at various immersion periods was evaluated by integrating the anodic current density on SVET maps. Zinc-rich paints (ZRPs) coated on a steel panel were used to demonstrate the experimental approach. The anti-corrosion performance of the ZRP was analyzed based on the integrated anodic current and the experimental EOC–iInt diagram. Closely correlative behaviour was found between the integrated anodic current and the open-circuit potential. Published by Elsevier Ltd.

1. Introduction As one of the most cost effective methods for corrosion protection of metallic objects, organic coatings provide corrosion protection mainly by four ways: a barrier effect, sacrificial cathodic protection, corrosion inhibitor release, and anodic protection. Metal-rich coatings (MRCs) [1,2] are a class of corrosion protection coatings containing sacrificial metal pigments that are more electrochemically reactive than the underlying metal substrate, which inhibit corrosion by providing sacrificial/cathodic protection to the metal substrate. MRCs are generally designed with high volume fraction of metal pigment (near critical pigment volume concentration, CPVC) dispersed in non-conductive polymer or inorganic matrix. The most effective and commonly used MRCs for steels are Zn-rich primer (ZRP) coatings [3–5]. Most recently, Mg-rich coatings have been developed and found to provide similar protection to aerospace Al alloys [6–8]. Various electrochemical methods have been employed to assess the anti-corrosion performance of metal-rich coatings, such as corrosion potential measurements, electrochemical impedance spectroscopy (EIS) [3,9–12], electrochemical noise methods (ENM) and galvanic coupling measurement [3,13]. Murray [9,14] reviewed electrochemical methods used for evaluating organic anti-corrosion coatings. Sekine [15] gave a review on characteristics of various electrochemical measurement methods and even their correlations.

* Corresponding author. Tel.: +1 701 231 8027; fax: +1 701 231 8439. ** Corresponding author. E-mail addresses: [email protected] (M. Yan), [email protected] (V.J. Gelling). 0010-938X/$ - see front matter Published by Elsevier Ltd. doi:10.1016/j.corsci.2010.04.012

The development of microelectrode techniques and scanning electrode techniques has made it possible to measure electrochemical processes on a local scale, which has yielded new types of information relevant to the local electrochemical processes on corroding surfaces and advanced the investigations of localized corrosion. Among all the techniques are the Scanning Vibrating Electrode Technique (SVET), the Scanning Reference Electrode Technique (SRET), Local Electrochemical Impedance Spectroscopy (LEIS), and the Scanning Kelvin Probe (SKP). SVET was originally devised for detecting the extra-cellular current near living cells in the 1970s [16]. It was firstly developed to study localized corrosion processes by Isaacs in the 1980s [17,18]. The electrochemical process of corrosion contains an ionic current flow in the electrolyte balanced by the electron flow through the metal. The ionic current flow causes a potential gradient to exist in the solution at the electrochemically active site. SVET was designed to detect the potential gradient via a movable vibrating microelectrode. The electrode potential difference between the two extreme points of its vibration, r/, is recorded at the extremes of the vibration amplitude, generating a sinusoidal AC signal. The AC signal is then converted to the ion current density (i) by a calibration procedure [10,19]. The local current is related to r/ and the electrolyte conductivity k by Ohm’s law [20]

i ¼ jr/

ð1Þ

SVET systems are designed to oscillate the probe in a Lissajous mode so that both parallel component ix and perpendicular component iz of the current can be obtained by partial differentiation of (1) with respect to x or z.

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1.1. SVET for characterizing anti-corrosion performance of coatings SVET has enjoyed wide acceptance as a powerful electrochemical technique for evaluation of corrosion inhibitor, detection of corrosion activity and quantification of corrosion defects in coatings. SVET has been used in the research of various types of corrosion, such as pitting [21], cut-edge corrosion [22–24], galvanic corrosion [18], microbiologically influenced corrosion (MIC) [25], weld corrosion, and stress corrosion cracking (SCC) [26]. In the case of corrosion of a coated metal, SVET is able to give detailed insights into the electrochemical interactions between a coating and its substrate at a defect, which has been provided valuable information on the anti-corrosion mechanism by a coating, including the generation and development of defects, and the influence of pigments/ inhibitors on corrosion of substrate at a defect [27,28]. In previous studies from this laboratory, series of coatings for Al alloy (AA) 2024-T3 has been characterized by SVET. To monitor both the corrosion activity of the substrate and the possible galvanic interaction between the coating and the substrate, the measurement was conducted in the vicinity of a scratch exposing the underlying substrate. The SVET results for polypyrrole deposited on AA 2024-T3 showed that a large anodic current occurred at the defect due to anodic dissolution of the alloy and that the cathodic current was rather uniformly distributed over the polymer surface, which implies that a p-doped CP would promote active dissolution of the AA 2024-T3 substrate at the defect if the passivation could not be obtained [29,30]. Most recently, interesting interactions between neutral or n-doped poly(2,3-dihexylthieno[3,4-b]pyrazine) and AA 2024-T3 has been demonstrated by SVET [31]. The n-doped conjugated polymer exhibited the ability to sacrificially protect the exposed Al alloy in a defect. For a redox inactive barrier coating, such as a plain epoxy coating on steel or AA 2024-T3, the SVET showed that both anodic current and cathodic current were located at the scribe [32]. Due to the high impedance/low-conductivity of the intact barrier coating, a complete corrosion cell, if any, would be established within the defect, and no current was distributed on the coating. In the case of metal-rich coatings, such as Mg-rich primer coated on Al alloys, the SVET results exhibited a well-defined cathodic current peak above the scratch. The anodic current (related to the anodic dissolution of the sacrificial pigment) distributed on the primer, which demonstrates that the sacrificial pigment functions by the cathodic protection mechanism [1,33,34]. In this work, a SVET method is provided for further understanding the galvanic interaction between a metal-rich coating and the metal substrate, and evaluating the anti-corrosion performance of the metal-rich coating. A zinc-rich primer (ZRP) coated on steel was examined to demonstrate the efficacy of the method. Several characteristic indexes are obtained from the SVET current density map to characterize the galvanic interaction between the ZRP and its substrate. The total anodic current (and hence the corrosion rate) over the scan area is evaluated by integration of the overall anodic current density on the SVET maps. The variations of the total anodic current and that of open-circuit potential (EOC) were analyzed as a function of the immersion time. Additionally, the SVET current indexes were compared with the galvanic current obtained by zero resistance amperometry (ZRA).

2. Experimental

properties and hardness of the primer by giving near to the maximum amount of crosslinking. Methyl isobutyl ketone (MIBK) was used as solvent. Then, zinc powder was added to the solution and was stirred to form a thick mortar-like mixture. A steel panel (R-35, from Q-Panel) was pretreated by grinding with 400and 600-grit SiC sandpaper, followed by degreasing with hexane. The coatings were applied using a drawn down bar at a wet thickness of 200 lm. The coated panels were placed in a convection oven at 70 °C for 24 h after flashing off for approximately 30 min. For the primers of pigment volume concentration (PVC) lower than 25%, the zinc pigment was firstly dispersed in MIBK for full dispersion. The dry film thicknesses were in the range of 140–190 lm. 2.2. SVET measurement and data analysis The current distribution over the interface of solution/ZRP (67% PVC, subsequently referred to as ZRP67) was measured using a SVET system from Applicable Electronics (USA). The Pt–Ir microelectrode (Microprobe Inc.) with a 10 lm diameter tip which was platinized to a 20 lm diameter sphere. The microprobe was vibrated 200 lm above the samples with the amplitude 20 lm along the X and Y directions. A pair of platinized platinum wires was used as both the reference and bath ground electrodes. The probe made 20  20 measurements in each scan (600 s), generating a 400-point mesh across the surface. Scans were initiated 5 min after immersion and repeated every 60 min. The ZRP67 sample (1  1 cm2) was masked by a polyester tape, exposing an open area of 3  3 mm2 as the scanning area. An artificial scratch was introduced in the center of the scanning area. For comparison, the same SVET measurement was also conducted on an unscratched ZRP67. All SVET measurements were performed at the free-corrosion condition in a cell containing 5 mL 3.5 wt.% NaCl aqueous solution. The SVET current density mapping and the statistical analysis of the data were performed with Origin software. The current densities were displayed in three-dimensional (3D) maps, showing the spatial distribution of the current density as a function of the (x, y) position in the scan region on ZRP. The current values in the SVET map are positive for anodic currents and negative for cathodic currents. The contour map of the current densities is at the bottom of the 3D map. The SVET current density vector images superimposing the measured current vector onto an optical image of the sample showed images of the sample surface as well as the locations of anodic/cathodic area. Based on the SVET current density map, the anodic current density peak (iA,max), the cathodic current density peak (iC,max), the average current density (iAve) and the integrated anodic current (IInt) were used to characterize the anti-corrosion properties of the coating. IInt was evaluated by integration of the overall anodic current (IA) on a SVET current density map, which is theoretically equal to the total cathodic current (IC) over the ZRP surface. Splitting the scan area (S, 3  3 mm2) into 20  20 small squares, we calculate the anodic or cathodic current on each square, and sum all the resulting currents to obtain IInt (lA) on the scan area, as shown by

IInt ¼

S

P n

iA

¼

S

P

iC

n

ð2Þ

where iA is the anodic current density (iA P 0), iC the cathodic current density (iC < 0) and n the number of measurement points in each scan (n = 400).

2.1. Materials and electrode preparation 2.3. Galvanic coupling measurement An epoxy resin (Epon 828, from Hexion) and a modified polyamide (Epikure 3175, from Hexion) curing agent were mixed in a 1:1.1 stoichiometric ratio. This ratio results in the optimal barrier

Galvanic coupling (both the mixed potential, EMix, and the coupling current) between the coated metal and the bare substrate

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was measured using a Gamry PC4/300 potentiostat in a zero resistance ammeter (ZRA) mode. The experiments were carried out in a two-compartment enclosed cell described in our previous work [29]. The two-compartment cell permitted careful control of the atmospheric conditions in each compartment. The working electrode in one compartment was ZRP67-coated on the steel (subsequently referred to as the ZRP-compartment) and the working electrode in the other compartment was the bare steel (subsequently referred to as the steel-compartment). The exposed area of ZRP67 was 1.0 cm2 and the bare steel was in a pinhole of 0.04 cm2 (simulating a coating defect), yielding an area ratio (ZRP67 to steel) of ca. 25. To simulate the condition for a topcoated sample, where a topcoat would protect the primer from direct oxygen access, the solution in the coating-compartment was purged with N2, while the solution in the alloy-compartment was purged with air.

time on the whole, except for two peaks appearing at 3 and 10 h. The SVET experiment was conducted under free-corrosion condition (without external polarization applied) where the anodic currents and cathodic currents above the ZRP67 are balanced and the net current should be zero. It should be note that, in a scanning plane above the free-corrosion surface, the integrated anodic current IA should be theoretically equal to the integrated cathodic current IC in the absolute value and hence the average current density (iAve) should be zero. But deviations between IA and IC are usually obtained by SVET, which causes iAve to deviate from zero. The deviations may be attributed to the fact that the current density on a SVET map was not taken at the same time. The corrosion behaviour and current distribution on the scan area are changing during scanning (one scan takes 10 min). 3.2. Integrated anodic current of the ZRP obtained by SVET

3. Results and discussion The ZRP paints under study here are heterogeneous systems with pores and zinc particles distributed randomly in the binder. The probable reactions on the primer in an electrolyte are as follows: Zinc dissolution to the oxide (ZnO)/polymeric-binder film, electrochemical dissolution of the active zinc particles, and oxygen reduction on zinc particles or on the substrate through pores [35]. A key aspect of the above mentioned mechanisms is the galvanic interaction between ZRP and the metal substrate. The galvanic interaction between ZRP and substrate may be influenced by any or all of the following factors: the electrochemical state of zinc particles at ZRP/solution interface, reactive Zn/Fe area ratio (SZn/Fe) interface, as well as the diffusion process through the coating and the deposit of Zn corrosion products [3,12]. The electrochemical behaviour and cathodic protection performance of ZRPs have been well studied by corrosion potential monitoring [3], EIS [3,12,36], conductive atomic force microscopy (AFM) [2], as well as the scanning electron microscopy (SEM) [4,12]. In this work, the electrochemical and corrosion performance of the ZRP was characterized by the SVET method. The integrated anodic current IInt above the ZRP67 obtained by the SVET method is presented in Fig. 4 as a function of the immersion time, together with EOC measured under the same conditions. For the scratched ZRP67, closely correlative behaviour was found between EOC and the total anodic current. Most obviously, two significant anodic current peaks appeared during the immersion exactly before and after the EOC peak occurred. For comparison, IInt and EOC for the unscratched ZRP67 are also presented in Fig. 4. The trend of IInt of the unscratched ZRP67 was similar to that of the scratched ZRP67 but with much lower amplitude. The most

3.1. SVET current density maps The SVET current density maps for the bare steel in 3.5% NaCl solution, as presented in Fig. 1, showed several anodic current peaks that appear after several minutes, due to possible pitting nucleation. After 30 min, these anodic current peaks combined into one broad anodic peak, where dark corrosion products began to appear on the steel surface. Fig. 2 displays SVET current density maps above the defect on the scratched ZRP67 (67% PVC) after various immersion periods in 3.5% NaCl solution. The cathodic current was mainly located at the scratch where a well-defined cathodic peak existed throughout the 5-day immersion period. The anodic area appeared at different sites on ZRP67 during the immersion. At the beginning of the immersion from 0.1 to 4 h, anodic areas were found to initiate only at the corners of the scratch, as shown in Fig. 2a and b. After 5 h of the immersion, anodic areas were scattered around the scratch (Fig. 2c). After 10 h of the immersion, significant changes occurred both in current distribution and in value, as presented in Fig. 2d. The anodic activity moved from one area to another near the scratch; the cathodic area included to almost all the scratch and a well-defined cathodic peak was observed. After 20 h of the immersion (Fig. 2e and f), the cathodic current decreased with time, and the anodic current was evenly distributed over the surface of the primer. The variations of the anodic current peak (iA,max), the cathodic current peak (iC,max) and the average current density (iAve) in SVET maps are shown in Fig. 3. The iA,max decreased over the immersion

19.0

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Fig. 1. SVET current density maps showing the anodic and cathodic current density distributions above the bare steel immersed in 3.5% NaCl solution for (a) 10 min and (b) 30 min.

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Fig. 2. SVET current density maps showing the anodic and cathodic current density distributions over an artificial line scratch on ZRP (67% PVC) after (a) 0.1 h, (b) 3 h, (c) 5 h, (d) 10 h, (e) 20 h, and (f) 80 h immersion in 3.5% NaCl solution. The optical images superimposed with current vectors (right of a and f) show location of the defect.

positive potential of ZRPs (unscratched) reached at 3 h immersion and this potential was observed to depend significantly on the PVC, as shown in Fig. 5. The most positive potentials for ZRPs with PVC 5%,

15%, 45% and 67.5% were 0.166, 0.00, 0.60 and 0.91 V, respectively. The following several stages were clearly recognized from both IInt and EOC shown in Fig. 4 for the scratched ZRP67.

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iA, max

6

Current density (µ A/cm2 )

iC, max iAve

4

2

0

-2

-4

0.1

1

10

100

Time (h) Fig. 3. The anodic current density peak (iA,max), cathodic current density peak (iC,max) and average current density (iAve) obtained in SVET above the scratched ZRP67 in 3.5% NaCl solution.

EOC (V/SCE)

-0.6 -0.7

(a)

Scratched Unscratched

-0.8 -0.9 -1.0 25

(b)

15

-3

IInt (10 µ A)

20

10 5 0 0.1

1

10

10 0

Time (h) Fig. 4. The open-circuit potential EOC (a) and integrated anodic current IInt (b) obtained by the SVET method for both scratched and unscratched zinc-rich primers (67.5% PVC) as a function of immersion in 3.5% NaCl solution.

0.2

PVC of ZnRP 5% 15% 45% 67.5%

0.0

OCP (V/SCE)

-0.2 -0.4

of Zn particles through dissolving ZnO film or soaking the binder film, approximating 1.0 V (EOC of the zinc particles) at 1 h. In the following 3 h, the EOC shifted in the positive direction, implying that the steel substrate began to be wetted by the electrolyte through pores (decreasing the ratio of SZn/Fe). The IInt (1.2  102 lA initially) slightly decreased in the first 1 h immersion. Then, it gradually increased as a result of the dissolution of ZnO/zinc particles at the ZRP surface. From 3 h immersion, IInt decreased sharply accompanying with the rapid increase of EOC due to the wetting process of the steel substrate until EOC reaching a peak (0.71 V) at 5 h, where the steel substrate was expected to be totally wetted. A current valley (7.3 lA) exactly corresponded to the EOC peak at 5 h. Once the substrate was wetted, galvanic interaction between zinc particles and steel substrate would be expected to occur. The period of the beginning 5 h immersion can be referred to as the activating stage, where the main process was the dissolution of the ZnO film and then the zinc particles reaction with the electrolyte. In the activating stage, the electrochemical reaction process mainly occurred on the ZRP/solution interface. 3.2.2. The sacrificial protection stage Even after the steel surface became completely wetted at 5 h in Fig. 4, some zinc particles were still covered by thick oxides or by binder films. Zinc particles continued to be activated by the dissolution of the ZnO film and/or galvanically coupling to the substrate. At the beginning of the galvanic stage, corrosion product was formed in pores in the primer, which tended to seal the pores, reducing the number and size of pores [3]. This process shifted EOC toward negative directions and increased IInt sharply until attaining 2.3  102 lA at 11 h. The fluctuation of EOC in the range of 0.78 to 0.87 V in the vicinity of 9 h and the significant decrease in IInt at 10 h might be attributed to the accumulation of Zn corrosion product which improved the barrier property of the coating. For the unscratched ZRP67, the EOC fluctuation at 8 h immersion disappeared, which further implied that the EOC fluctuation might be related to the deposit of the zinc corrosion product in the defect and its adhesion to the surface. 3.2.3. The barrier stage The EOC continued to decay, with some fluctuations, beginning from 11 h (Fig. 4). IInt decayed to a low value of 1.2  102 lA, where it remained for the following duration of the experiment. The distinct decrease in IInt may be attributed to the zinc depleting and/or the improved barrier property of the primer. The barrier properties of the primer would improve by deposit of the corrosion product on the primer. The zinc corrosion product also trends to improve barrier effect by sealing pores in the primer, as has been proposed and observed by other authors [3,4,36].

-0.6 -0.8 -1.0 -1.2 0.1

1

10

100

Time (h) Fig. 5. Evolution of open-circuit potential for a series of ZRPs with different PVC during immersion in 3.5% NaCl solution.

3.2.1. The activating stage The EOC (ca. 0.9 V initially) shifted gradually towards negative direction during the first 1 h immersion as a result of the activation

3.2.4. EOC–iInt diagram The evolution of the EOC–iInt results for the scribed ZRP67 is shown in Fig. 6, with the numbers indicating the immersion time (in hour) and the dashed lines showing the shifting direction of the EOC–iInt points. Several stages (the wetting stage, the cathodic protection stage and the barrier stage) were demonstrated in the EOC–iInt evolution for the scratched ZRP67 during the immersion period. Almost all the conventional electrochemical measurements require an externally imposed polarization which would possibly have an unfavorable effect on the specimen. By contrast, SVET allows to measure integrated corrosion current at the free-corrosion condition (without external polarization applied) which is beneficial, especially for the highly active metal-rich coatings.

M. Yan et al. / Corrosion Science 52 (2010) 2636–2642

current (and therefore the corrosion rate) over the coating surface was obtained and used to evaluate the galvanic interaction between coatings and substrates. This SVET method allows the measurement of corrosion current at the free-corrosion condition (without external polarization applied), which is highly beneficial, especially for active metal-rich coatings. For the zinc-rich primer, several stages during immersion were distinctly recognized by the SVET method: the activating stage, the sacrificial protection stage and the barrier stage. Closely correlative behaviour was found between the total anodic current and the open-circuit potential. This SVET analysis method may provide a new insight for the corrosion protection performance and even the service life of metal-rich coatings.

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9

11

32 60-80

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3 2

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0.1

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iInt (µ A/cm ) Fig. 6. EOC–iInt plot for the scratched ZRP67 (67.5% PVC) in 3.5% NaCl solution. The iInt was obtained from integrating the SVET current density maps.

Potential (V/SCE) -3

Current (10 µ A)

Acknowledgment The authors would like to thank the US Army Research Laboratory (Contract # W911NF-04-2-0029) for sponsoring this research. References

-0.8

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2641

1

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10 0

Time (h) Fig. 7. Coupling current and mixed potential for the steel coupled with the ZRP67coated steel in 3.5% NaCl solution. Exposed areas: ZRP67 coating, 1 cm2; steel, 0.04 cm2. Positive coupling current signifies reduction in the steel-compartment.

3.3. Galvanic coupling between ZRP and the bare steel The coupling current and mixed potential for ZRP67 (1.0 cm2, N2 purged) coupled with bare steel (0.04 cm2, air purged) in 3.5% NaCl solution are shown in Fig. 7. The coupling current started at 4  103 lA and rapidly increased to 12  103 lA at 25 min. After immersion, the zinc particles were gradually activated through dissolution of the oxide (ZnO) surface and water penetration through polymeric-binder film. The zinc particles were fully activated at the end of 1 h, where the mixed potential attained the lowest value and the coupling current was highest. Then the mixed potential increased and the coupling current declined steadily. At 26 h, the EMix attained 0.85 V, and the coupling current was 6.9  103 lA. In the case of the galvanic coupling measurement in the twocompartment cell, the bare steel pinhole (simulating coating defect) is separated in another cell, where the bare steel was not affected by the deposit of the zinc corrosion product in the defect. The depletion of the zinc particles and the lack of the corrosion product deposit may lead to the steady decrease of the coupling current, which was a fundamental difference from the case of the SVET measurement, where the barrier stage was clearly observed as the result of a barrier effect of the Zn products deposit. 4. Conclusions By integrating the overall anodic current density measured by scanning vibrating electrode technique (SVET), the total anodic

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