EIS equivalent circuit model: electrolytic damage assessment of scribed coated steel surface embedded with inhibitor capsule
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https://www.eduzhai.net American Journal of M aterials 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, M ayagüez, 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 o xidation 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 o xide film on the scribe-area of a damaged coated film on the metal substrate. Keywords Steel, EIS, Equivalent Circuit Modeling, Po ly mer Coatings, Corrosion Inhibition, Self-healing Inhibitor Micro cap s u les 1. Introduction Surface treat ment 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. M icro-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 chro miu m and cadmiu m which is toxic to hu mans performed well and protected the metal surface, but have been eliminated under EPA guidelines because they are to xic to humans. An example is the utilization of non-toxic metal-based epoxy primer pre-treat ment systems[8, 9], zinc-titaniu m-silica based nanostructured coatings and mu ltifunctional micro capsulated corrosion-inhibitor system to protect aluminiu m alloys and steel surface fro m corrosion. Sev eral early p ion eers [11-14] h av e d ev elo ped an d discussed the science of ut ilizing EIS fo r the po ly mer degradat ion p rocess. Instru ment manu facturers and * Corresponding author: email@example.com (Narinder K. Mehta) Published online at https://www.eduzhai.net Copyright © 2013 Scientific & Academic Publishing. All Rights Reserved others have developed software programs to model the experimental data to electrical circuit-analog models. Poly mer 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. The technique has also been used to study the system fo r environ mental degradation of polymer-graphite co mposite and also when attached to a 7075-T6 alu minu m panel with a titaniu m bolt and exposed to salt water at roo m and elevated temperatures. Recent work using the EIS technique 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 to study corrosion reaction mechanism on the growth of iron oxide film as part of the reaction products on and around the s cribe 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. M ehta: EIS Equivalent Circuit M odel: 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. 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 ce ll 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 e xposure study. Figures 2A and 2B de monstrate 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 2-component mixed BLUE polyurethane top-coat brush application; (~5-12 mils dry film) Silvery/white sandblasted bare steel surface Microcapsules (~12 % wt) mixed into white non-chromate based 2-component epoxy primer for 15-mils metal applicatorapplication; (~12-13 mils dry film) Identification Panel Description (PT #X) Primer + topcoat (PCT #X) Capsules mixed Primer + topcoat # is the identification number of a test panel 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 Figure 1. Primed and top coated Q-steeltest 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 M aterials Science 2013, 3(5): 149-161 151 2.1.2. Instrumentation for EIS Study The Potentiostat/Galvanostat/ZRA EIS 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 cla mped 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 calo mel 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 fro m 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 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 (Oh m’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. Ho wever, for co mplex systems, we use impedance, Z a frequency dependent parameter and a measure of a circuit ’s tendency to impede the flo w 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 e xactly 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 (Hert z). These three parameters will be used to show Bode plot for the EIS e xpe rimental data. 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 do main is that the measured dispersion data can be described analytically emp loying an equivalent electrica l circuit-analog as a model. The e le ments of the circuit represent the microscopic processes involved in the transport of mass and charge. If a constant phase element (CPE) is present, a mo re 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 ad justs all equivalent circuit parameters simultaneously, thus obtaining the optimu m fit to the measured EIS dispersion data. The program uses a circu it description code, wh ich allo ws the use of various equivalent circuits for simu lation and analysis. The results presented here are derived fro m 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 fro m ideal behavior. CPE may be regarded as an imperfect capacitor where surface in-ho mogeneity may cause deviation fro m 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 environ ment. A typical examp le 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 dela mination 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 and will significantly increase the dielectric coefficient of a coating. However, in this work, leaching out of the inhibitor fro m 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 co mpact deposits on the scribe; in other words, it will make them hydrophobic. As a function of exposure time, mo re 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. Eval uation of an Inflicted Damaged Area For evaluating a scribe (damage) area on a coated metal 152 Narinder K. M ehta: EIS Equivalent Circuit M odel: Electrolytic Damage Assessment of the Scribed Coated Steel Surface Embedded with Corrosion Inhibitor Capsules 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) poly mer coating on a metal surface, CPEcoat is of the order of >10-9 farads/cm2 and impedance is of the order of >109 oh ms/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 co mpounds 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 dela minate 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. Equi valent 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 o xide and reaction products including leachates as a film with cracks and defects; a deviation fro m ideal behavior. Hence, the utilization of CPE will beco me a paramount factor for providing a better fit to the model for fitting the EIS data. Simp le 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 simp le 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 s pectrum 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 e xperimental 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 aug mented 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, 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 fro m ~0-1. If the EIS experimental data has a lot of hash or the s oftware program cannot model the data precis ely, then it results in n > 1; in other words the program has reached its limit to precisely model the e xperimental 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 accu mulat 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 ro mpted 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 co mparing ‘goodness of fit’ of different circuit models with the same set of data. The excellent fit obtained with the 7-element model with a mod ified sub-circuit co mponent did emphasize and suggests the existence of an o xide/leachate layer with d ifferent properties. The magnitude of other important sub-circuit parameters (CPE oxide/capsule ppt and CPEdl) was approximate ly constant and thus validated the credibility of the model used in the work. 3.4.1. 7-element Model CPEcoat and its Corresponding n Co mponent Data Figures 4A and 5A a re 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-a reas 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 co mponent 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 American Journal of M aterials Science 2013, 3(5): 149-161 153 penetration of electrolyte/water since CPEcoat reached a component value also decreased from ~1 in itially to ~0.3 for value of ~10-4 farads.cm2 after 27 days of exposure; the n the same exposure time. 104 -40 PCT32X RT 39D.DTA FitResult -30 |Z| theta 103 -20 -10 102 0 10-1 100 101 102 103 104 105 Frequency (Hz) Rs CPEdl Rct Element Rs CPEdl-T CPEdl-P Rct Freedom Free(±) Free(±) Free(±) Free(±) Chi-Squared: Weighted Sum of Squares: Data File: Circuit Model File: Mode: Maximum Iterations: Optimization Iterations: Type of Fitting: Type of Weighting: Value 348.8 0.0007309 0.54501 23994 Error 3.0585 2.9865E-05 0.015109 23030 Error % 0.87686 4.0861 2.7722 95.982 0.03005 3.4256 C:\06 Army RT EIS data for coated panels\PCT32X RT 39D.DTA C:\SAI\ZModels\simple Randle model.mdl Run Fitting / Freq. Range (0.1 - 100000) 100 0 Complex Calc-Modulus (A) 154 Narinder K. M ehta: EIS Equivalent Circuit M odel: Electrolytic Damage Assessment of the Scribed Coated Steel Surface Embedded with Corrosion Inhibitor Capsules 104 -40 PCT32X RT 39D.DTA FitResult -30 |Z| theta 103 -20 -10 102 0 10-1 100 101 102 103 104 105 Frequency (Hz) Rs CPEcoat Rcoat CPEdl Rct Element Rs CPEcoat-T CPEcoat-P Rcoat CPEdl-T CPEdl-P Rct Freedom Free(±) Free(±) Free(±) Free(±) Free(±) Free(±) Free(±) Chi-Squared: Weighted Sum of Squares: Data File: Circuit Model File: Mode: Maximum Iterations: Optimization Iterations: Type of Fitting: Type of Weighting: Value -1890 5.9414E-05 0.029838 2731 0.00044272 0.72787 6049 Error 4885.4 0.00022492 0.018986 4955.2 0.00026172 0.0080183 10384 Error % 258.49 378.56 63.63 181.44 59.116 1.1016 171.66 0.00059133 0.065638 C:\06 Army RT EIS data for coated panels\PCT32X RT 39D.DTA C:\SAI\ZModels\AppendixC Coated Metal.mdl Run Fitting / Freq. Range (0.1 - 100000) 100 0 Complex Calc-Modulus (B) American Journal of M aterials Science 2013, 3(5): 149-161 155 104 -40 PCT32X RT 39D.DTA FitResult -30 |Z| theta 103 -20 -10 102 10-1 Rs 100 101 102 103 Frequency (Hz) CPEcoat Rcoat CPEoxide /caps ule ppt Roxide /caps ule ppt CPEdl Rct 104 0 105 Element Freedom Rs Free(±) CPEcoat-T Free(±) CPEcoat-P Free(±) Rcoat Free(±) CPEoxide/capsuleFrpepet(-±T) CPEoxide/capsuleFrpepet(-±P) Roxide/capsule ppFtree(±) CPEdl-T Free(±) CPEdl-P Free(±) Rct Free(±) Value -290 1.5863E-05 0.20431 694 0.00022656 0.7454 142.9 0.00034819 0.74539 3416 Error 826.75 2.892E-05 0.060452 838.28 0.00013786 0.10916 70.963 0.00014463 0.050411 259.98 Error % 285.09 182.31 29.588 120.79 60.849 14.644 49.659 41.538 6.763 7.6107 Chi-Squared: Weighted Sum of Squares: 0.00018925 0.020439 Data File: Circuit Model File: Mode: Maximum Iterations: Optimization Iterations: Type of Fitting: Type of Weighting: C:\06 Army RT EIS data for coated panels\PCT32X RT 39D.DTA C:\SAI\ZModels\AppendixC Coated Metal with Oxide 2.mdl Run Fitting / Freq. Range (0.1 - 100000) 100 0 Complex Calc-Modulus (C) Figure 3. (A) 3-element, (B) 5-element and (C) 7-element circuit model and the corresponding fitted graphsto the EIS experimental frequency dispersion data for PCT 32X specimen exposed for 39-day at room temperature 156 Narinder K. M ehta: EIS Equivalent Circuit M odel: Electrolytic Damage Assessment of the Scribed Coated Steel Surface Embedded with Corrosion Inhibitor Capsules CPEcoat / farads.cm2 1e-2 1e-3 1e-4 1e-5 1e-6 1e-7 1e-8 1e-9 1e-10 1e-11 1e-12 1.8 1.6 1.4 1.2 1.0 0.8 0.6 0.4 0.2 0.0 PT 2X PT 3X PT 4X PT 5X PT 8X PT 10X 0 10 20 30 40 Exposure Time in Days (A) PT 2X PT 3X PT 4X PT 5X PT 8X PT 10X n component (CPEcoat-P) 0 10 20 30 40 Exposure Time in Days (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 American Journal of M aterials Science 2013, 3(5): 149-161 157 for o xide-aug mentation 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 co mponent 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 fro m 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. 1e-3 1e-4 1e-5 CPEcoat / farads.cm2 1e-6 1e-7 1e-8 1e-9 1e-10 PCT 32X PCT 27X PCT 25X PCT 23X PCT 21X PCT 20X 1e-11 1.8 0 20 40 60 80 100 Exposure Time in days (A) n component (CPE coat-P) 1.6 PCT 32X PCT 27X 1.4 PCT 25X PCT 23 X 1.2 PCT 21X PCT 20X 1.0 0.8 0.6 0.4 0.2 0.0 0 20 40 60 80 100 Exposure Time in Days (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 wit h no inhibitor microcapsules in the primer as a funct ion of exposure t ime 158 Narinder K. M ehta: EIS Equivalent Circuit M odel: Electrolytic Damage Assessment of the Scribed Coated Steel Surface Embedded with Corrosion Inhibitor Capsules CPEoxide/capsules ppts. and CPEdl magnitude as obtained from the model basically re ma ined 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 oh ms.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 oh ms.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 oh ms.cm2 after appro ximately 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. 1e+5 1e+4 Rct / ohms.cm2 1e+3 1e+2 1e+1 1e+5 1e+4 PT 10x PT 8x PT 5x PT 4x PT 3x PT 2x 0 10 20 30 40 Continuous Exposure, Days (A) PCT 32x PCT 27x PCT 25x PCT 23x PCT 21x PCT 20x Rct / ohms.cm2 1e+3 1e+2 0 20 40 60 80 100 Continuous Exposure, Days (B) Figure 6. Polarization resistance, Rct data for a 7-element modelfor the exposed coated panels as a function of exposure time. (A) Six PT #Xpanels without embedded inhibitor microcapsules in primer and (B) six RCT #X panels with imbedded inhibitor microcapsules in the primer
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