eis equivalent circuit model: electrolytic damage...

American Journal of Materials Science 2013, 3(5): 149-161 DOI: 10.5923/j.materials.20130305.07

EIS Equivalent Circuit Model: Electrolytic Damage Assessment of the Scribed Coated Steel Surface Embedded with Corrosion Inhibitor Capsules

Narinder K. Mehta

Department of Chemical Engineering, University of Puerto Rico, Mayagüez, PR 00681 USA

Abstract Electrochemical Impedance (EIS) experimental data recorded on an intentionally inflicted scribe on Q-steel coated panels exposed in a laboratory environment was analyzed using equivalent electrical circu it-analog model technique for the corrosion-inhibition performance. The sand-blasted steel panels were pre-painted with a non-chromate industrial epoxy primer having dispersed corrosion-inhibitor capsules. A polyurethane topcoat layer was applied to the dried panel and the air-cured scribed panels were placed in contact with an ASTM D 5894 electro lyte. As oxidation kinetics init iate, embedded capsules leachates and oxides deposits are released into the scribe-area resulting in the addition of a sub-circuit component to the model for optimization-fitting of the experimental data. It is concluded that EIS modeling is a useful method and can be extended to study the corrosion-inhibition performance and the mode of corrosion react ion mechanism for the growth of iron oxide film on the scribe-area of a damaged coated film on the metal substrate.

Keywords Steel, EIS, Equivalent Circuit Modeling, Po lymer Coatings, Corrosion Inhibition, Self-healing Inhibitor Microcapsules

1. Introduction Surface treatment of metals by various techniques to

inhibit the corrosion augmentation for advance-technology applications is a necessity and is in demand to protect the hardware. Micro-capsulated corrosion inhibitors mixed into a paint system[1-5] and nano-structured surface modification [6, 7] are the new state-of-the-art approaches to combat damage created by the development of the galvanic circuit. In the past, technologies included utilization of heavy metals like chromium and cadmium which is toxic to humans performed well and protected the metal surface, but have been eliminated under EPA guidelines because they are toxic to humans. An example is the utilization of non-toxic metal-based epoxy primer pre-treatment systems[8, 9], zinc-titanium-silica based nanostructured coatings[6] and multifunctional micro capsulated corrosion-inhibitor system[10] to protect aluminium alloys and steel surface from corrosion.

Several early p ioneers [11-14] have developed and discussed the science of ut ilizing EIS fo r the po lymer degradat ion p rocess. Instrument manufacturers[15] and

* Corresponding author: [email protected] (Narinder K. Mehta) Published online at http://journal.sapub.org/materials Copyright © 2013 Scientific & Academic Publishing. All Rights Reserved

others[16] have developed software programs to model the experimental data to electrical circuit-analog models. Polymer film capacitance (CPEcoat) and high frequency phase angle ( ) have demonstrated their usefulness in comparing the dielectric coefficient of the failed coatings to a standard film[17]. The technique has also been used to study the system for environmental degradation of polymer-graphite composite[18] and also when attached to a 7075-T6 aluminum panel with a titanium bolt and exposed to salt water at room and elevated temperatures[19].

Recent work using the EIS technique[1] demonstrated the effectiveness of the self-healing property of the corrosion-inhibiting microcapsules on coated steel in contact with the electrolyte test solution exposed to the laboratory environment and to the acerbic sea-shore environment. However, work was needed to advance the interpretation and understanding of the vast EIS experimental data using the equivalent electrical circuit-analog modeling technique to study the corrosion inhibition mechanis m of the embedded inhibitor capsules on a scribed coated metal film. In this paper, efforts are made to extend its utilizat ion[20] to study corrosion reaction mechanism on the growth of iron oxide film as part of the reaction products on and around the scribe on a coated Q-steel surface. Specifically, concerted efforts were focused to calculate various model-circu it parameters for the corrosion-inhibiting protection of the self-healing capsules leachates on the exposed scribe-area.

150 Narinder K. Mehta: EIS Equivalent Circuit Model: Electrolytic Damage Assessment of the Scribed Coated Steel Surface Embedded with Corrosion Inhibitor Capsules

2. Experimental 2.1. Materials and Methods

2.1.1. Preparation and Identification of the Test Specimens

The steel test coupons (specimens or panels) of dimensions 22.9 cm. x 7.6 cm. x 0.06 cm were sand-blasted to a white finish before application of a primer with or without microcapsules and a polyurethane top-coat layer. Figure 1 shows the schematic depicting the studied coating system on the test panels. The details on the preparation of

the test panels are described elsewhere[1]. As shown in Figure 1, the notation ‘X’ was attached to the set of coupons PT (primer + topcoat) and PCT (microcapsules embedded in the primer + topcoat) to identify ‘scribe-area’ of a part icular test panel. A deep-cut scribe within the O-ring of the EIS cell was made for the containment of the electrolyte without any seepage. An EIS cell containing the electrolyte was permanently attached over the scribe for the room temperature exposure study. Figures 2A and 2B demonstrate the setup of the test panels for exposure in the laboratory environment.

Primed and Top Coated Specimen Preparation Q-steel 9 in x 3 in x 0.025 in panels

Silvery/white sand-blasted bare steel surface

2-component mixed BLUE polyurethane top-coat brush -application;(~5-12 mils dry film)

Microcapsules (~12% wt) mixed into white non-chromate based 2-component epoxy primer for 15-mils metal applicator-application;(~12-13 mils dry film)

Scribed top coated pre-primed panels without embedded inhibitor capsules

Scribed top coated pre-primed panels embedded with inhibitor capsules

X stands for a test panels with an inflicted scribe and the corresponding data

(PT #X) Primer + topcoat

(PCT #X) Capsules mixed Primer +

topcoat

# is the identification number of a test panel

Panel Description Identification

Figure 1. Primed and top coated Q-steel test panels and classification for identifying the scribed panels (PT #X & PCT #X)

(A) (B)

Figures 2. (A) and (B) Setup of the test panels in the laboratory room temperature environment

American Journal of Materials Science 2013, 3(5): 149-161 151

2.1.2. Instrumentation for EIS Study

The Potentiostat/Galvanostat/ZRA EIS[15] system was checked, recalibrated and installed in a personal computer workstation before recording the frequency dispersion spectrum. The test cell consisted of a glass cylinder clamped with an O-ring seal at one end of the scribed specimen film surface. The exposed surface area of the cell was 7.544 cm2 and an ASTM D 5894 electrolyte was used to make the impedance measurements. The counter electrode was a platinum d isc dipped into the solution and a saturated calomel electrode was used as the reference electrode. A sine wave of ± 10 mV amplitude was applied to the cell at the open-circuit potential (OCP), and measured frequencies ranged from 0.1 Hz to 105 Hz. After record ing the EIS spectra, the specimens were returned back to the room temperature exposure area.

A commercial software p rogram[16] was used to evaluate the EIS experimental data for interpretation, modeling and evaluation of the damage to the exposed coated scribed film-area of test panels.

3. Results and Discussions 3.1. Fundamental Theory

Electrical resistance is well known (Ohm’s law) to be the ability of an electrical circu it to resist the flow of current. For a resistor (one circuit element), which is frequency independent, it is R=V/I. However, for complex systems, we use impedance, Z a frequency dependent parameter and a measure of a circuit ’s tendency to impede the flow of an alternating electrical current. The new relation is Z=Vac/Iac.

When a sine wave voltage (ac) is applied to an electrochemical cell with resistors only, the current sign wave is exactly in phase with the voltage sine wave. There is no time lag and the phase angle of a resistor’s impedance is zero degree at all frequencies (ideal resistor). However, in case of a capacitor, the current response is shifted in time due to the slow response of the system. This shift is 900 out of phase with voltage and is known as phase angle shift. So a sine wave voltage waveform becomes a cosine current waveform. Hence for this system we specify the magnitude of impedance written as IZI, phase angle ( ) and frequency (Hertz). These three parameters will be used to show Bode plot for the EIS experimental data[21].

3.2. EIS

The EIS technique is widely used for evaluating corrosion protective layers on metals and alloys. The advantage of EIS over measurements in the time domain is that the measured dispersion data can be described analytically employing an equivalent electrical circuit-analog as a model. The elements of the circuit represent the microscopic processes involved in the transport of mass and charge. If a constant phase element (CPE) is present, a more sophisticated analysis procedure is required as the variation of one circuit parameter may

influence a large part of the frequency dispersion data affecting the parameters of the sub-circu its. The non-linear least square fit (NLLSF) technique[22] ad justs all equivalent circuit parameters simultaneously, thus obtaining the optimum fit to the measured EIS dispersion data. The program uses a circu it description code, which allows the use of various equivalent circuits for simulation and analysis.

The results presented here are derived from the EIS data obtained using models for various circuit elements. Rct and CPEdl are parameters related to the corrosion process while parameters related to the inhibitor film are Rpore and CPEcoat. Efforts were made to interpret and correlate the modeled circuit data with microscopic processes involved in the transport of mass and charge on and around the scribe area.

3.2.1. Pure Capacitance (Q when n = 1) versus Constant Phase Element (CPE when n>0 and <1)

Exposure of a scribe on a coated metal system to an electrolyte results in a dynamic reaction situation. Since the properties of the system change drastically as a function of small time interval, they result in a non-ideal dielectric property situation. For this reason, it is always advisable to use CPE instead of Q as a circu it element. Actually, there is nothing like a perfect capacitance (phase angle = -900) in a real word scenario, a deviation from ideal behavior. CPE may be regarded as an imperfect capacitor where surface in-homogeneity may cause deviation from perfect parallel plates. According to the relation CPE = Qn, CPE is like a perfect capacitor and n is its corresponding variable. If n = 1, then the CPE element is an ideal capacitor as Q. However, water and chemicals d iffuse within the dielectric , and magnitude of n starts decreasing for a coated metal substrate exposed to a degrading environment. A typical example is when n ~0.5, and chemical diffusion with in the d ielectric is probably taking place. Creation of an interface between the coating and the metal substrate will result in delamination of the coating.

The magnitude of a capacitor for a coated film increases with electro lytic solution contact time as a result of water absorption-swelling[23] and will significantly increase the dielectric coefficient of a coating[17]. However, in this work, leaching out of the inhibitor from embedded microcapsules by the attack of the acidic electrolyte will decrease the reaction velocity and thus will result in the reduction of the swelling capacity of the fine compact deposits on the scribe; in other words, it will make them hydrophobic. As a function of exposure time, more and more inhib itor leachates and oxides will deposit on to the scribe. Since these particles are highly fine and porous, the thickness of the deposits will not change significantly. Though hydrophobic, these fine particles are highly porous to water penetration and hence the CPE of these porous deposits may be similarly defined as that of a coating[23, 24].

3.3. Evaluation of an Inflicted Damaged Area

For evaluating a scribe (damage) area on a coated metal


surface with inhibitor capsules embedded in the primer, CPEcoat becomes a parameter of vital interest especially when the scribe and its coated surroundings are exposed to a corrosive electrolyte. For an excellent (undamaged) polymer coating on a metal surface, CPEcoat is of the order of >10-9 farads/cm2 and impedance is of the order of >109 ohms/cm2. As coating degrades on exposure, CPcoat magnitude increases while impedance and phase angle decrease. The same is true for a scribe and the surrounding coated-area with embedded inhibitor capsules. Inhibitors are basic compounds used to reduce the intensity of acidity (pH) to corrode the metal surface on and around the scribe. If there are no corrosion inhibitor compounds available e.g. leachates to protect the scribe area, impedance and phase angle will decrease drastically due to fast kinetics to corrode the scribe-area. It will further delaminate the coated surface near the scribe and will expose the new metal surface augmenting the overall corrosion reaction rate. However, in the presence of inhibitor containing leachates seeping out of the embedded capsules, the oxidation reaction kinetics will be greatly reduced and will be reflected in the gradual increase of CPEcoat with exposure time. Seeping of the leachates in to the scribe-area will form a hydrophobic film. However, the film does not act as a barrier protector since the film is permeable to the acidic electrolyte; it only reduces the attack by increasing pH around the scribe.

3.4. Equivalent Electrical Circuit-analog Models

The scribe on a coated metal surface will not behave like a pure capacitor since the scribe will have hydrolyzed accumulated oxide and reaction products including leachates as a film with cracks and defects; a deviation from ideal behavior. Hence, the utilization of CPE will become a paramount factor for providing a better fit to the model for fitting the EIS data. Simple 3- , 5- and 7-element circu it models were evaluated to describe the scribe-film with or without embedded inhibitor capsules in the primer. Since the scribe exposes the bare metal, a simple 3-element Randles circuit init ially was used to fit the EIS experimental data. As expected, the data did not fit well with the circu it at the low and high ends of the EIS spectrum as shown in Figure 3A for a 39-day exposed PCT 32X specimen.

Since most of the area under the O-ring is coated except for a scribe, a sub-circuit CPEcoat and Rcoat was added to compensate for the coated area in contact with the electrolyte within the O-ring. To stress the point, EIS experimental data fitted well with the 5-element circuit model except for the high frequency range as shown in Fig. 3B for a 39-day exposed PCT 32X specimen. This augmented the need to add a new sub-circuit to optimize the fitting of the experimental data in the high frequency CPEcoat range.

In general, and as reported in the literature[20], it is very difficult to get a perfect fit to the phase angle data points especially in a high frequency coating capacitance area where the corresponding n component may have a value from ~0-1. If the EIS experimental data has a lot of hash or

the software program cannot model the data precisely, then it results in n > 1; in other words the program has reached its limit to precisely model the experimental data. The deviation in high frequency area suggests that on exposure to the electrolyte, the scribe as well as the coated area under the O-ring probably started accumulating micro-reaction product particles as kinetic react ion increased with exposure time and resulted in the format ion of a new infinitesimal layer; the accumulat ion probably was more toward the filling of the scribe line than the coated surface due to driving atomic/electron charge affinity on the bare metal exposed surface. This added another venue suggesting an argument that there must be a different type of R and CPE sub-element in the analog-circuit causing the deviation in high frequency range. It p rompted the addition of new sub-circu its CPEoxide/capsule ppt and Roxide/capsule ppt to compensate for the corrosion reaction products. A better fit of the new 7-element model was obtained in the high frequency range as shown in Figure 3C for a 39-day exposed PCT 32X specimen.

To provide the factual statistical support of the above argument, ‘weighted sum of squares’ and ‘chi-squared’ were found to be 2.0439x10-2 and 1.8925x10-4 respectively for the 7-element circuit versus 6.5638x10-2 and 5.9133x10-4 respectively for a 5-element circu it as obtained with the software program. ‘Weighted sum of squares’ and ‘chi-squared’ measurements are particularly useful parameters when comparing ‘goodness of fit’ of different circuit models with the same set of data. The excellent fit obtained with the 7-element model with a modified sub-circuit component did emphasize and suggests the existence of an oxide/leachate layer with d ifferent properties. The magnitude of other important sub-circuit parameters (CPE oxide/capsule ppt and CPEdl) was approximately constant and thus validated the credibility of the model used in the work.

3.4.1. 7-element Model CPEcoat and its Corresponding n Component Data

Figures 4A and 5A are the CPEcoat magnitude data fitted to the 7-element model of the EIS experimental data as a function of exposure t ime. As shown in Figures 4B and 5B, CPEcoat magnitude for all the replicates was of the order of ~10-3-10-10 farads.cm2 with the corresponding n component magnitude between 0-1 fo r the PT #X and PCT #X test panels respectively. Figure 4A reflects that CPEcoat for four out of the six scribe film-areas of control specimens (PT #X) had low CPEcoat magnitude, ~10-6 farads.cm2, init ially and increased to ~10-4 farads.cm2 for ~10 days of exposure. In general, the n component of CPEcoat for all PT #X specimens decreased (Fig. 4B) for various exposure times indicating moderate to extensive penetration of electrolytes between the metal-leachates/ppts-coating interface. The large variation in n component magnitude suggested that the scribe and the adjacent area reached water saturation point under the accumulated deposits on the scribe. One out of the six PT #X panels demonstrated inhibition by the deposits to the


penetration of electrolyte/water since CPEcoat reached a value of ~10-4 farads.cm2 after 27 days of exposure; the n

component value also decreased from ~1 in itially to ~0.3 for the same exposure time.

(A)

10-1 100 101 102 103 104 105102

103

104

Frequency (Hz)

|Z|

PCT32X RT 39D.DTAFitResult

-40

-30

-20

-10

0

theta

Rs CPEdl

Rct

Element Freedom Value Error Error %Rs Free(±) 348.8 3.0585 0.87686CPEdl-T Free(±) 0.0007309 2.9865E-05 4.0861CPEdl-P Free(±) 0.54501 0.015109 2.7722Rct Free(±) 23994 23030 95.982

Chi-Squared: 0.03005Weighted Sum of Squares: 3.4256

Data File: C:\06 Army RT EIS data for coated panels\PCT32X RT 39D.DTACircuit Model File: C:\SAI\ZModels\simple Randle model.mdlMode: Run Fitting / Freq. Range (0.1 - 100000)Maximum Iterations: 100Optimization Iterations: 0Type of Fitting: ComplexType of Weighting: Calc-Modulus


(B)

10-1 100 101 102 103 104 105102

103

104

Frequency (Hz)

|Z|PCT32X RT 39D.DTAFitResult

-40

-30

-20

-10

0

theta

Rs CPEcoat

Rcoat CPEdl

Rct

Element Freedom Value Error Error %Rs Free(±) -1890 4885.4 258.49CPEcoat-T Free(±) 5.9414E-05 0.00022492 378.56CPEcoat-P Free(±) 0.029838 0.018986 63.63Rcoat Free(±) 2731 4955.2 181.44CPEdl-T Free(±) 0.00044272 0.00026172 59.116CPEdl-P Free(±) 0.72787 0.0080183 1.1016Rct Free(±) 6049 10384 171.66


Data File: C:\06 Army RT EIS data for coated panels\PCT32X RT 39D.DTACircuit Model File: C:\SAI\ZModels\AppendixC Coated Metal.mdlMode: Run Fitting / Freq. Range (0.1 - 100000)Maximum Iterations: 100Optimization Iterations: 0Type of Fitting: ComplexType of Weighting: Calc-Modulus


(C)

Figure 3. (A) 3-element, (B) 5-element and (C) 7-element circuit model and the corresponding fitted graphs to the EIS experimental frequency dispersion data for PCT 32X specimen exposed for 39-day at room temperature

10-1 100 101 102 103 104 105102

103

104

Frequency (Hz)

|Z|PCT32X RT 39D.DTAFitResult

-40

-30

-20

-10

0

theta

Rs CPEcoat

Rcoat CPEoxide/capsule ppt

Roxide/capsule ppt CPEdl

Rct

Element Freedom Value Error Error %Rs Free(±) -290 826.75 285.09CPEcoat-T Free(±) 1.5863E-05 2.892E-05 182.31CPEcoat-P Free(±) 0.20431 0.060452 29.588Rcoat Free(±) 694 838.28 120.79CPEoxide/capsule ppt-TFree(±) 0.00022656 0.00013786 60.849CPEoxide/capsule ppt-PFree(±) 0.7454 0.10916 14.644Roxide/capsule pptFree(±) 142.9 70.963 49.659CPEdl-T Free(±) 0.00034819 0.00014463 41.538CPEdl-P Free(±) 0.74539 0.050411 6.763Rct Free(±) 3416 259.98 7.6107


Data File: C:\06 Army RT EIS data for coated panels\PCT32X RT 39D.DTACircuit Model File: C:\SAI\ZModels\AppendixC Coated Metal with Oxide 2.mdlMode: Run Fitting / Freq. Range (0.1 - 100000)Maximum Iterations: 100Optimization Iterations: 0Type of Fitting: ComplexType of Weighting: Calc-Modulus


Exposure Time in Days

0 10 20 30 40

CP

Eco

at /

fara

ds.c

m2

1e-12

1e-11

1e-10

1e-9

1e-8

1e-7

1e-6

1e-5

1e-4

1e-3

1e-2

PT 2X PT 3X PT 4X PT 5X PT 8X PT 10X

(A)


0 10 20 30 40

n co

mpo

nent

(CP

Eco

at-P

)

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

1.8

PT 2X PT 3X PT 4X PT 5X PT 8X PT 10X

(B)

Figure 4. (A) Coating capacitance, CPEcoat, and (B) the corresponding n component of CPEcoat data for a 7-element electrical circuit-analog model for the six PT #X test panels with no inhibitor microcapsules in the primer as a function of exposure time

As shown in Figure 5A, only one out of the six scribe film-areas (PCT #X) of the test panels with embedded microcapsules indicated a CPEcoat magnitude of ~10-4

farads.cm2 in itially and did not change for up to 17 days of exposure. The other five out of six scribe film-areas (PCT #X) of the replicates had initial CPEcoat in the range of ~10-6-10-10 farads.cm2. CPEcoat magnitude demonstrated a uniform increase for ~60 days of exposure which stabilized afterwards as shown in Figure 5A in the data for up to 87 days of exposure. As shown in Figure 5B, the n component data followed a similar trend of CPEcoat magnitude data. The

data reflected several-fold corrosion protection performance of the inflicted scribe-area on PCT #X steel panels attributable to the presence of inhibitor-embedded primer leachates seeping out of the self-healing inhib itor microcapsules. Subsequently, favorable constructive arguments may be made to interpret informat ion provided in Figures 5A and 5B that corrosion inhib itor microcapsules embedded into the primer were leaching out between the steel metal and film interface. The leachates provided the necessary corrosion-inhibition to dimin ish/stabilize the reaction kinetics to reduce and hold back the driving force


for oxide-augmentation and damage to the scribe-steel surface due to neutralization of the acid ic hydrated-electrolyte around the scribe as compared with the scribe on the control primed specimens (PT #X) without microcapsules (Figures 4A and 4B).

In general, as shown in Figure 4B, the decrease in the magnitude of the n component for most of the PT #X panels was erratic for up to ~12 days of exposure, a trend similar to the CPEcoat magnitude increase for these panels reflecting slight inhibition due to the deposit of paint-residue-oxide reaction products initially on to the scribe. Most of the six PT #X panels demonstrated typical diffusion process (n = ~0.5 range) for up to ~12 days of continuous exposure. For one

panel (PT 2X), the diffusion process continued for up to ~25 days. It is likely that the scribe cut was not made deep enough on this particular panel, although increase in n magnitude started reflecting inhibit ion from reaction products after this period. In comparison, as shown in Figure 5B, the n magnitude for PCT #X panels demonstrated decreasing trend for ~60 days of exposure, suggesting protection from corrosion augmentation diffusion-controlled kinetics on the scribe and delamination or seeping under the surrounding coated area. The stable-constant trend of n values after ~60 days demonstrates deposition of a homogeneous compact film of fine reaction products on to the scribe-area.

Exposure Time in days

0 20 40 60 80 100

CP

Eco

at /

fara

ds.c

m2

1e-11

1e-10

1e-9

1e-8

1e-7

1e-6

1e-5

1e-4

1e-3

PCT 32X PCT 27XPCT 25X PCT 23XPCT 21XPCT 20X

(A)


0 20 40 60 80 100

n co

mpo

nent

(CPE

coa

t-P)

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

1.8

PCT 32X PCT 27X PCT 25X PCT 23 XPCT 21X PCT 20X

(B)

Figure 5. (A) Coating capacitance, CPEcoat, and (B) the corresponding n component of CPEcoat data for a 7-element model for the six PCT #X test panels with no inhibitor microcapsules in the primer as a function of exposure time


CPEoxide/capsules ppts. and CPEdl magnitude as obtained from the model basically remained constant in the range of 10-4-10-5 farads.cm2 (data not shown), a value expected for a bare steel-panel in contact with an electrolytic solution; it was expected since the system under study had an intentional in flicted scribe on the coated surface exposed to the attack of the acid ic electrolyte.

3.4.2. 7-element Model Rct Data

As shown in Figures 6A and 6B, Rct was of the order of ~1-10K ohms.cm2 for the scribed film-area (PT #X & PCT #X) of replicate test panels. Figure 6A reflects that Rct for five out of six scribe film-areas of control specimens (PT #X) decreased to less than 1K ohms.cm2 within 1-21 days whereas Figure 6B indicated a decrease in Rct of the same magnitude in approximately 17 days of exposure for only one out of the six scribe film-area (PCT #X) test panels with embedded microcapsules. The other five scribe film-areas (PCT #X) of the replicates had init ial Rct in the range of ~5-10K ohms.cm2. Rct stabilized to less than 4K ohms.cm2 after approximately 60 days of exposure (Fig. 6B demonstrates data for up to 87 days of exposure). The data demonstrated several-fo ld corrosion protection performance attributable to the presence of the inhibitor-embedded primer leachates seeping on to the scribe-film area of the coated panels.

(A)

(B)

Figure 6. Polarization resistance, Rct data for a 7-element model for the exposed coated panels as a function of exposure time. (A) Six PT #X panels without embedded inhibitor microcapsules in primer and (B) six RCT #X panels with imbedded inhibitor microcapsules in the primer

Continuous Exposure, Days

0 10 20 30 40

Rct

/ oh

ms.

cm2

1e+1

1e+2

1e+3

1e+4

1e+5

PT 10x PT 8xPT 5x PT 4xPT 3xPT 2x

Continuous Exposure, Days

0 20 40 60 80 100

R ct / o

hms.

cm2

1e+2

1e+3

1e+4

1e+5PCT 32xPCT 27xPCT 25xPCT 23xPCT 21xPCT 20x


Rct magnitude for the 7-element model data followed a similar pattern as previously detailed before for CPEcoat data of the scribed panels. This favors the constructive argument that corrosion-inhibitor microcapsules embedded into the primer were leach ing out of the embedded capsules to deposit on to the scribe-area for PCT #X panels and thus provided the necessary corrosion-inhibition to dimin ish /stabilize the react ion kinetics for reducing and holding back oxide-augmentation and damage to the steel surface as compared with the scribed film-area of control primed specimens (PT #X) without microcapsules.

4. Probable Corrosion Inhibition Mechanism on and Near the Scribe-area

Modeling of the EIS experimental data involves the explanation of mathematical relationship on the basis of a series of valid realities. If a coated metal surface is scribed, the metal will be exposed and a simple process of initiation of corrosion reactions to oxidize the metal surface will take place. Oxide film will be formed if there is no corrosion-inhibitor available to slow the oxidation kinetics. During the process, several properties/parameters of the coated system will be affected; it includes OCP, CPEcoat, and Rct among others. If corrosion-inhib itor protection is available, the kinetics of the oxide format ion will dimin ish and will also reduce the degradation of the coated film near the scribe. Since all the properties are related to the science of the metal-polymer system, it is possible to compare the inter-related properties on the basis of relationships between the rate of corrosion and order of chemical reaction including the nature of oxide film format ion.

In general, during electrochemical reaction on a scribed/damaged coated surface exposed to an acidic electrolyte, oxidation reaction products will continue to grow on and around the scribe due to the driving force attack of the acidic medium and will also force the delamination of the coated film-area around the scribe. As the oxidation process initiates, the oxide concentration starts to grow as a function of exposure time[25, 26] without affecting the ionic concentration of the electrolyte. The mathematical ‘rate of change’ equations[26] describe the overall oxide formation on the basis of the reaction kinetics. As shown in Figure 7, corrosion is normally a three stage/zone time-dependent reaction process for a scribe on a metal surface. The init ial fast stage-I is followed by an intermediate reaction stage-II and then a final slow stage-III. The overall oxide formation depends on the diffusion of the electrolyte through the top oxide/leachate layer on the scribe of the coated metal surface. At the same time there will be a very slow reverse process of conversion of oxide to metal which will almost be neglig ible.

4.1. Corrosion Stage-I

It is clear from Figure 7 that during stage-I the kinetics will be aggressive around the scribe-area and the oxides will

have irregular, non-uniform and loose porous morphology. In this highly energetic corrosive-stage, positively charged metal ions[27] depart the lattice-structure and leave the electrons behind in the metal. Water molecules in the electrolyte encircle the hydrated-metal ions and they diffuse freely in to the solution away from the metal surface. The negative charge caused by excess electrons on the metal surface attracts some positively charged metal ions to remain near the metal surface instead of diffusing into the bulk electrolyte. The probability of hydrated-metal ions to have direct contact with the excess surface electrons is very remote and subsequently is being reduced to metal atoms. So the prospect of the reverse process of conversion of oxide to metal will be extremely slow or almost insignificant. The dynamic of the kinetics at the negatively charged exposed scribe-bare metal surface will proceed at a reasonably fast rate for panels without any corrosion inhibitor embedded in the near coated film.

Figure 7. Schematic representation (Ref 26) of the various stages for the electrochemical reaction rates and oxide growth build-up versus exposure time for the scribe-area on coated steel panels exposed to room temperature with the EIS cell placed on the scribe film-area with an ASTM D 5894 electrolyte

4.2. Corrosion Stage-II

If corrosion inhibitor leachates are seeping into the area on and around the scribe, the corrosion reaction kinetics will be slower compared with the panels without inhibitor capsules in the intermediate reaction stage-II as a function of exposure time; the kinetic equation will be different for oxide formation. In this stage, the continuous oxide formation reaction will be sluggish due to the slow diffusion process through dense and uniform (porous morphology) layers of stage-I reaction products and oxides on the scribe. For panels (PT #X) without inhibitor capsules, fast reaction kinetics will take over and the diffusion will probably be through a severe oxide-cracked structure similar to that of fine mud-deposit particles.

4.3. Corrosion Stage-III

During this stage, the nano-size oxide particles will form slowly and will gradually diffuse with the electrolyte and the

(scribe with embedded capsules)

(scribe without embedded capsules)

Exposure Time

Oxide Buildup


result will be toward uniform oxide growth with fine pores on to the scribe. It can be clearly derived from the CPEcoat data from Figures 4A and 4B that the kinetic reached stage-III at an aggressive fast pace within the first ~10 days for five out of six replicates (PT #X) without inhibitor capsules. One out of six panels reached the stage-II intermediate stage before reaching the dormant stage-III. Since this latter stage is dominated by the diffusion controlled oxide growth process, the oxide and other reaction products probably will be more nano-structured toward a dense and uniform morphology. It is apparent from Figures 5A and 5B that fo r five out of six PCT #X panels, the kinetics were very slow for ~25 days of exposure and entered into the transitional stage-II to increase the CPEcoat magnitude for up to ~70 days of exposure. After this period, the figures reflect that the specimens entered the dormant stage-III, where fine pore nano-size oxides seem to have filled the scribe-area on the metal surface. It can also be derived from Figures 5A and 5B that in stage-III, the reaction kinetics have decelerated completely for PCT #X exposed panels due to uniform oxide deposits with fine pores that were formed with slow diffusion of liquid (water and electrolyte); the electrolyt ic solution has to diffuse deeper into the deposited oxide and ppts film on the scribe to find a fresh metal surface. The end product of oxide formation in all the three stages will be yellowish-brown precipitate/deposits on the scribe lines and the near coated surface area within the O-ring. However, the size and texture of the deposited particles will be d ifferent for different zones.

4.4. Corrosion Reactions, in General

Corrosion of iron is represented by the following basic electrochemical equations.

Anodic half-reaction Fe0 Fe+2 + 2e- (metal surface) Cathodic half-reaction 2H+ + 2e- H2 (orig inating

from the electrolyte) Overall corrosion reaction Fe0 + 2H+ Fe+2 + H2 The probable net half-reactions taking place[24] on the

exposed scribe-area of the coated steel surface are • Anodic reaction mechanism at the scribe

Fe0 + H2O Fe (OH)ads + H+ + e- Fe (OH)ads Fe(OH)+

sol + e- Fe (OH)+

sol Fe2+sol + H2O

• Cathodic reaction mechanism of steel involv ing oxygen reduction

O2 + 2H2O + 4e- 4OH-

So the overall total corrosion reactions occurring on the exposed scribed metal surface are

4 Fe0 + 3O2 + 2H2O 4FeOOH (light yellow ppts) 2FeOOH Fe2O3.H2O (deep yellow-brown ppts) The yellow-brown color precipitates are clearly visib le

on the scribe of the coated steel surface in Figures 2A and 2B without/with the attached EIS cell respectively, and thus supporting the probable reaction mechanism presented for the formation of the yellow-brown precip itates.

5. Summary and Conclusions The equivalent electrical circuit-analog model was

employed to analyze the EIS experimental data for evaluating the performance of embedded corrosion-inhibitor capsules in to the epoxy primer and a polyurethane topcoat layer on Q-steel panels exposed to an ASTM D 5894 electrolyte in a laboratory environment. For scribed/ damaged coating on a metal structure, high frequency coating capacitance and phase angle area in a Bode plot are extremely useful parameters for corrosion degradation evaluation. They may provide interpretable information about the coating properties (porosity, diffusion etc.) on and near a scribe-area on a coated metal surface exposed to acid ic media.

High frequency CPEcoat and low frequency Rct data compiled for the scribe film-area o f the coated panels with a 7-element circuit model have demonstrated the effectiveness and superior perfo rmance of the self-healing corrosion - inhibit ing microcapsules. Oxidation kinetics reaction velocity around the scribe film-area diminished several fold with exposure time as compared with control specimens without embedded microcapsules in the p rimer. In particu lar, it has demonstrated the need to use corrosion-inhibitor micro -capsulate technology for extending the useful life of the naturally corroding metal infrastructure system.

An accidental scratch on a coated metal surface with corrosion-inhibitor microcapsules embedded in the dried primed film will rupture them with this action. The results have shown that when damaged or ruptured, the active corrosion inhibit ing agents encased in embedded microcapsules are released in the presence of a fluid as leachates into the inflicted damage area to d iminish the metal oxidation reaction kinetics and thus help extend the functional life of a would-be corroding system.

Seeping of inhibitor leachates forming a hydrophobic film on the scribe-area decreases the attack of acidic electrolytes and determines the corrosion inhib ition performance of the entire system as compared with the non-capsules primed panels. The leachate-precipitate-residue film does not act as a barrier coating since this film is permeable to the acidic electrolyte. However, as oxidation process kinetic initiates, continuous deposit of the embedded capsule leachates and oxides on to the scribe-area reduces the reaction kinetics. Visualizing the corrosion degradation situation of an unintentional inflicted damage/scratch on to a painted metal infrastructure exposed in real-world corrosive environmental exposure conditions, the work has concluded that EIS electrical circuit-analog modeling of the frequency dispersion experimental data is a useful method and may be extended to study the corrosion-inhibition performance of a damaged coated film on a metal substrate. It has demonstrated and provided an impetus for the need to embed self-healing corrosion-inhibitors into the paint system before painting a metal infrastructure for protection from corrosion degradation.


ACKNOWLEDGEMENTS The research was supported by a grant

(W9132T-06-C-0021) to the author (UPR-Mayaguez) from the U. S. Army Corps of Engineers ERDC-CERL, Champaign, IL. The author (ret ired research professor) is grateful to Dr. Ashok Kumar and Dr. Larry Stephenson for providing the opportunity and funding support for the work.

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