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Tungstate and vanadate-doped polypyrrole/aluminum flake composite coatings for the corrosion protection of aluminum 2024-T3.

Abstract Polypyrrole (PPy) doped with either tungstate or vanadate as counter anions was synthesized by chemical oxidative polymerization on the surface of aluminum (Al) flakes. This resulted in the deposition of PPy on the surface of the Al flakes leading to the formation of doped PPy/Al flake composite pigments. These composite pigments were characterized by Fourier transform infrared spectroscopy, scanning electron microscopy, energy dispersive spectroscopy, conductive-atomic force microscopy, four-point probe conductivity, and X-ray photoelectron spectroscopy. Furthermore, these composites were incorporated in an epoxy-amide binder system in order to formulate a primer for an aluminum 2024-T3 substrate. The coatings were exposed to the Prohesion test conditions and corrosion resistance properties were monitored by electrochemical impedance spectroscopy, DC polarization, galvanic coupling, and scanning electrochemical microscopy measurements. It was found that the doped PPy/Al flake coatings provided sacrificial protection to the underlying aluminum 2024-T3 substrate. Additionally, the release of dopants from PPy backbone resulted in the passivation in the defect areas improving the corrosion protection ability.

Keywords Polypyrrole, Composite coatings for corrosion protection, Electrochemical impedance spectroscopy, Galvanic coupling, Scanning electrochemical microscopy

Introduction

Corrosion is a thermodynamically favored process which primarily involves conversion of metal to its original oxide form and is often described as extractive metallurgy in reverse. (1,2) Coatings are one of the oldest form of corrosion control for metals and their alloys. (3) Barrier, inhibitors, and cathodic protection are some of the important mechanisms which are employed in coatings to combat against corrosion. (4) In modern times, the motivation behind the development of improved corrosion resistance coatings involves factors such as improved efficiency through multifunctionality, increased service life, ability to combat changing environmental conditions, and eco-friendly approaches. Well known corrosion inhibitors such as hexavalent chromates are being banned due to their toxicity,5'6 so environmental friendly and nontoxic alternatives are the focus of current research activities.

Conducting polymers (CPs) are a new class of materials which have been the subject of great interest over the last four decades. (7-10) CPs possess several attractive properties including electrical conductivity, (11) good thermal stability, (12) catalytic nature, nontoxicity, and easy synthesis by chemical and electrochemical oxidative methods. (13) CPs are employed in a variety of applications such as sensors and electrochemical devices, (14,15) actuators, (16) capacitors, (17) batteries, (18,19) light emitting diodes, (20,21) control release devices, (22,23) solar cells, (24) radar and electromagnetic interference shielding, (25,26) and corrosion protection of metals and their alloys. (27-35)

Polyaniline (PAni), polypyrrole (PPy), polythiophene (PT), and their respective derivatives are important CPs which have exhibited promise toward mitigating corrosion. (28,29,36,37) Among these, PPy is the most promising candidate for corrosion protection owing to its good conductivity, high environmental stability in its oxidized form, and low toxicity of pyrrole monomer. (38) However, PPy is insoluble in commonly used solvents in the coatings industry. (39) This insolubility coupled with its porosity, infusibility, poor mechanical properties, and insufficient adhesion leads to difficulty in processing and application in coatings. (40) In order to overcome these problems, several strategies have been employed. The first approach includes chemical structural modifications of the pyrrole monomer in order to alleviate solubility problems. (41-42) The second avenue investigated was the modification of PPy properties by incorporation of dopant counterion on its backbone. (43-44) The third strategy used was synthesizing a composite pigment of PPy and an inorganic flake, such as A1 flake or micaceous iron oxide, for overcoming the shortcomings of PPy application in the coatings. (39,45-48) In the current study, the combination of second and the third strategy is employed, which involves not only an incorporation of corrosion inhibiting dopant anions on the backbone of PPy but also the deposition of this doped PPy on the surface of A1 flake to form composite pigment.

PPy doped with tungstate anion has been previously electropolymerized on carbon steel. (49) The results indicated that the tungstate anion participated in stable oxide layer formation resulting in improvement in corrosion resistance of carbon steel. The tungstate anion participated in a passivation process thereby forming primary passive layer. (50) Tungstate-doped PPy has been also electropolymerized on the surface of aluminum 1100 with increased corrosion resistance. The improvement in corrosion protection was attributed to adsorption of tungstate anions at the defects and formation of passive film. (12) Vanadate has been shown to provide corrosion inhibition on aluminum 2024-T3 by forming adsorbed layer on its surface. (51) Vanadate adsorption, furthermore, reduced the oxygen reduction rate by blocking the sites for reduction.

Composites of CPs with other materials (montmorillonite, carbon nanotubes, titanium dioxide, flakes, zinc, zinc oxide, etc.) have been synthesized and utilized for the corrosion protection of metals and metal alloys. (52-56) The functional properties and specific morphology of these materials exercise an additional protection mechanism. For example, the platelet nature of flakes can result in the lengthening of the path for corrosive ions. The majority of the research found in literature for synthesizing doped CPs and composites have employed the electrochemical polymerization method; whereas, in the current research, a chemical oxidative polymerization is used in which large scale synthesis of composite pigment is possible along with simultaneous incorporation of corrosion inhibiting dopants. (45) This composite pigment can be handled easily on the industrial level for its application into the coatings.

Several mechanisms have been proposed for the corrosion protection by CPs for metals and their alloys. These include anodic passivation, surface ennobling, mediation of oxygen reduction, cathodic protection, barrier protection after initial galvanic coupling, and an intelligent dopant release mechanism. (28,29,57-59) The efficacy of corrosion protection depends on application conditions including substrate preparation, type of substrate (A1 or steel), methods of application, and different conductive forms of CPs. (39,60) In the present work, a corrosion inhibiting dopant release mechanism, galvanic contact of PPy with A1 flakes, and barrier protection resulting from the lamellar nature of A1 flakes are suggested to provide the corrosion inhibition.

In this work, tungstate or vanadate-doped PPy was synthesized on the surface of A1 flakes by employing water as a reaction medium. The composite pigments were investigated for morphology by scanning electron microscopy (SEM), for elemental composition by energy dispersive spectroscopy (EDS) and X-ray photoelectron spectroscopy (XPS), for conductivity by conductive-atomic force microscopy (CAFM) and four-point probe method, and for composite composition by Fourier transform infrared spectroscopy (FTIR). The corrosion protection properties of the formulated coatings with epoxy-amide binder system applied on aluminum 2024-T3 substrate were evaluated by electrochemical impedance spectroscopy (EIS), DC polarization technique, scanning electrochemical microcsopy (SECM), and galvanic coupling measurements with the concomitant exposure to the Prohesion test conditions (ASTM G85-A5).

Experimental work

Materials

A1 flakes (Stapa Aloxal[R] PM 2010) were kindly supplied by Eckart America. Sodium tungstate dihydrate ([Na.sub.2]W[O.sub.4] x 2[H.sub.2]O) and sodium metavanadate (NaV[O.sub.3]) were procured from MP Biomedicals, LLC, and Strem Chemicals, respectively. Pyrrole was distilled prior to use and was obtained from Alfa Aesar Co. Ammonium persulfate (APS) was obtained from EMD Chemicals Inc. Solvent used in the composite synthesis was 18.2 MQ Millipore water. Epoxy resin, EPON 828 and polyamide curing agent, EPIKURE 3175 were obtained from Momentive Specialty Chemicals Inc. Coating solvent methyl ethyl ketone (MEK) was procured from Alfa Aesar Co. Aluminum 2024-T3 (0.063" x 6" x 3") panels were obtained from Q-Panel Lab Products.

Synthesis of PPy/Al flake composite pigment

The quantities of chemicals used in the synthesis of PPy/Al flake composite pigment are outlined in Table 1. For the synthesis, water was added to the Erlenmeyer flask followed by the addition of respective amounts of dopants as indicated in Table 1. After complete dissolution of the dopant salts, Al flakes were added. Dispersion of A1 flakes in the reaction mixture followed the addition of APS. After complete dissolution of APS, pyrrole was added to the reaction mixture. The reaction was continued at ambient temperature under continuous mixing with a magnetic stirrer. After 24 h, the reaction mixture was filtered and washed with copious amounts of water. The product retained on filter paper was dried overnight in an oven at 60[degrees]C. Finally, dried product was milled with the help of mortar and pestle and sieved through the sieve #400 (having 38 [micro]m of opening diameter) and was stored in plastic vials at ambient conditions.

Acronyms were labeled for tungstate-doped (0.1 M) PPy/Al flake composite, tungstate-doped (0.01 M) PPy/Al flake composite, vanadate-doped (0.1 M) PPy/Al flake composite, vanadate-doped (0.01 M) PPy/Al flake composite as CPCCW0.1, CPCCW0.01, CPCCV0.1, and CPCCV0.01, respectively.

Coatings preparation

To prepare the substrate for coating, sandblasting with 100 [micro]m alumina grit was performed on the aluminum 2024-T3. The panels were then degreased with hexane. In order to achieve conducting primer, coatings were formulated at 20% pigment volume concentration (PVC) with epoxy resin, EPON 828 and polyamide curing agent, EPIKURE 3175. Stoichiometric ratio of 1:1 of epoxy resin to the hardener was used. Five different formulations were prepared: as-received A1 flakes, CPCCW0.1, CPCCW0.01, CPCCV0.1, and CPCCV0.01. The composite pigments were mixed with the epoxy resin followed by the hardener addition. MEK was added as a solvent. A drawdown bar with 8 mils of wet film thickness was employed for coatings application. Coatings were cured at 80[degrees]C for 2 h in oven followed by 8 days of ambient temperature curing for complete development of performance properties. The topcoat was also applied with similar application and curing conditions. For the topcoat, epoxy resin (EPON 828) was mixed with hardener (EPIKURE 3175) in the stoichiometric ratio of 1:1 and was applied with drawdown bar with 8 mils of wet film thickness. Curing was performed in oven for 2 h at 80[degrees]C with subsequent ambient temperature cure for 8 days.

Composite characterization

Morphological studies of the synthesized composites and as-received A1 flakes were performed by JEOL JSM-6490LV SEM. For the sample preparation of SEM, powdered material under investigation was sprinkled over carbon tape attached to aluminum mount. Then it was covered with gold coating sputtered with Balzers SCD 030 sputter coater. For the ease of analysis, the magnification, accelerating voltage employed, and scale bars are listed on each micrograph later in this article in the figures. NORAN System Six II system comprising a high-performance liquid-nitrogen-cooled energy dispersive X-ray detector along with controlling software on a dedicated computer was employed for EDS analysis.

FTIR characterization was performed by employing NICOLET 8700 spectrophotometer from Thermo Scientific. Prior to the analysis, all composite samples were ground fine with mortar and pestle and were mixed with KBr to make pallets for FTIR. To obtain the surface morphology and surface current density, CAFM analysis was performed by employing a Veeco Dimension 3100 atomic force microscope in contact mode and with a current sensing probe. For CAFM measurements, platinum-iridium (Pt/Ir) coated cantilevers were employed. For all the CAFM experiments, between sample and substrate 200 mV DC bias voltage was applied. Conductivity measurements were performed with a four-point probe instrument consisting of a Keithley[R] 2000 multimeter, a Keithley[R] 220 programmable current source, and Signatone[R] probes. The XPS measurements were performed on an SSX100 system (Surface Science Laboratories, Inc.) with a monochromated A1 Ka X-ray source, a hemispherical sector analyzer (HSA), and a resistive anode detector. For the measurements, the base pressure was 3.0 x [10.sup.10] Torr. For the data collection, the pressure was ca. 1.0 x [10.sup.-8] Torr. For analysis each sample was mounted separately on a sample holder using a piece of double-sized carbon tape. The X-ray spot size used for measurements was 1 x 1 [mm.sup.2], which in turn corresponded to an X-ray power of 200 W. The collection of the survey spectra was done using 12-14 scans at 150 eV pass energy and 1 eV/step. For the collection of the high resolution spectra, 50 eV pass energy and 0.1 eV/step was employed.

Coating characterization

Coatings prepared with CPCC were characterized by EIS measurements. For EIS, a Gamry Instruments R600 Potentiostat/Galvanostat/ZRA with Gamry Framework Version 5.58/EIS 300 software was employed with 10 mV of AC signal amplitude with 10 points/decade over 100,000 to 0.01 Hz frequency range. Potentiodynamic experiments were performed on coated panels with defect of 1 mm in width and 1.5 cm in length. EIS and potentiodynamic experiments were performed in three-electrode cell with coated substrate as a working electrode, platinum mesh as a counter electrode, and saturated calomel as a reference electrode. The electrolyte used in EIS and potentiodynamic experiments was dilute Harrison's solution (DHS, 0.35% ammonium sulfate and 0.05% sodium chloride). Galvanic coupling experiments were performed in enclosed cell with two compartments connected with salt bridge. One compartment contained aluminum 2024-T3 panel whereas the other compartment was coated sample. The working areas of samples in both compartments was 1 [cm.sup.2] resulting in area ratio of 1. Experiments were performed in DHS solution and aluminum 2024-T3 panel compartment was bubbled with air, whereas the coated panel compartment was bubbled with nitrogen to stimulate topcoated conditions. A Gamry Reference 600 Potentiostat was employed for galvanic coupling experiments in zero resistance ammeter (ZRA) mode and the coupling current and mixed potential values were measured between aluminum 2024-T3 paned and coated panel. Both primer and topcoated samples of coatings were exposed to Prohesion test conditions according to ASTM G85-A5. According to ASTM G85-A5, DHS mist is sprayed for an hour at 25[degrees]C followed by an hour of a dry stage at 35[degrees]C in the Prohesion test chamber. EIS measurements were performed in triplicate whereas potentiodynamic and galvanic coupling experiments were performed in duplicate. The data reported is for the representative sample. For SECM experiments, a CHI900B scanning electrochemical microscope (CH Instruments) was utilized with a 10-pm Pt microelectrode probe, a saturated Ag/AgCl reference electrode, and a Pt counter electrode. The electrolyte used for the experiments was a naturally aerated mediator solution of 1 mM ferrocenemethanol (FcMeOH) in 0.2 M potassium chloride (KC1).

Results and discussion

Fourier transform infrared spectroscopy (FTIR)

The FTIR spectra of CPCCW0.1, CPCCW0.01, CPCCV0.1, and CPCCV0.01 are presented in Fig. 1. Peak positions ([cm.sup.-1]) and FTIR modes of vibrations are identified in Table 2. In the composites, N-H stretching was observed around 3400 [cm.sup.-1]. (61) Due to the surrounding environment, for the tungstate-doped composite, this peak was broader due to the increased solubility of sodium tungstate dihydrate which led to probable hydrogen bonding due to water molecules adsorption. (62) The band at 3100 [cm.sup.-1] was observed due to the C-H bond aromatic stretching. The band around 1702 [cm.sup.-1] was observed due to the formation of carbonyl group either due to the nucleophillic attack by water leading to chain terminations or due to the overoxidation. (63) The peak at 1702 [cm.sup.-1] was not prominent in all of the four composite samples, suggesting minimal overoxidation and little loss in conjugation along the PPy chain. The band at 1558 [cm.sup.-1] represents ring stretching vibrations due to C-C and C=C bonds. (62) In the case of the CPCCW0.1 and CPCCW0.01; this band was slightly red shifted at 1556 and 1555 [cm.sup.-1], respectively; whereas, in the case of CPCCV0.1, and CPCCV0.01, this band was blue shifted at 1569 and 1566 [cm.sup.-1], respectively. Delocalized [pi]-electrons involving vibrations exhibit changes due to the doping and these observed shifts in this band might due to the varying degree of doping which might have been influenced by the amount of dopant and the nature of dopant. (64,65)

The combination of C-N and C=C bond stretching was observed at 1464 [cm.sup.-1]. The band at 1464 [cm.sup.-1] was not as prominent in the case of the CPCCV0.1 and CPCCV0.01 as compared to CPCCW0.1 and CPCCW0.01. An approximate conjugation length can be calculated by taking the ratio of intensities at 1465 and 1561[cm.sup.-1]. (61,62,65) The band at 1295 [cm.sup.-1] was observed due to the C-H and C-N in plane deformation. (62,66) The shift in this band was observed for the doped composite samples. Band at 1198 [cm.sup.-1] was attributed to ring breathing, as well as the band at 1090 [cm.sup.-1] was from C-C in plane vibration, and the band at 1045 [cm.sup.-1] was observed due to C-H deformation. These are characteristic band vibrations of PPy, including bands at 929 and 788 [cm.sup.-1].

Regions of 960-780 and 900-770 [cm.sup.-1] for tungstate ion band in case of CPCCW0.1 and CPCCW0.01 were observed, whereas regions of 1010-920 and 890-830 [cm.sup.-1] for vanadate ion band were observed for CPCCV0.1 and CPCCV0.01. The region, 750-490 [cm.sup.-1] was attributed for Al-O stretching vibration.67 Also, a weak tungstate ion band was observed at 490 [cm.sup.-1] for CPCCW0.1 and for CPCCW0.01. Vanadate ion band was observed in 550 and at 540 [cm.sup.-1] for CPCCV0.1, and CPCCV0.01, respectively. The results obtained from FTIR indicate the presence of tungstate in tungstate-doped PPy/Al flake composites and the presence of vanadate in vanadate-doped PPy/Al flake composites.

Scanning electron microscopy (SEM)

For the morphological investigations, SEM was performed on all CPCC samples. SEM micrographs of as-received A1 flakes, CPCCW0.1, CPCCW0.01, CPCCV0.1, and CPCCV0.01 are shown in Fig. 2. For all of the micrographs, 10000 x magnification, 15 kV accelerating voltage, and 1 [micro]m scale bar were employed and are mentioned in respective micrographs (Fig. 2). For the as-received A1 flakes (Fig. 2a), a clear and smooth morphology was exhibited. For CPCCW0.1 (Fig. 2b), spherical particles of PPy were found to be deposited on the surface of A1 flakes. The distribution of PPy formation on the surface of A1 flakes was dense. For CPCCW0.01 (Fig. 2c), spherical PPy particles and circular wire formation of PPy was observed. For CPCCV0.1 (Fig. 2d), spherical particles of PPy were formed on the surface of A1 flakes, whereas, in the case of CPCCV0.01 (Fig. 2e), along with spherical particles of PPy wires of PPy were also formed.

It has been reported that the final surface morphology, mechanical, and electrical properties are directly influenced by parameters employed in the synthesis of PPy. (68-70) The presence of dopants in the synthesis of PPy influences the morphology of resultant product. (64) The nature of dopants also influences the conductivity, mechanical properties, and morphology of PPy. (71, 72) Polyhydroxyl sulfonate has been used as a dopant for the synthesis of PPy. It was found that as the concentration of the dopant was increased in the synthesis, the size of resultant PPy decreased. (73) Morphologies such as rings, frames, and platelets were obtained for PPy in the presence of different flurosurfactants and [beta]-naphthalenesulfonic acid. (74) Dodecylbenzenesulfonate-doped PPy exhibited compact and globular morphology. (64) Also, the presence of dopant influences the rate of pyrrole oxidation resulting in different morphology. (75) Lowering the concentration of dopant from 0.1 to 0.01 M in both tungstate and vanadate-doped PPy/Al flake composite resulted in the formation of wires of PPy along with spherical particles on the surface of A1 flakes. It is hypothesized that the effect of reduced concentration of dopant influences the rate of pyrrole polymerization and pyrrole nucleation for the polymerization resulting in different morphology.

Energy dispersive spectroscopy (EDS)

For elemental composition, EDS analysis was performed at two different points on the same sample. According to the product data sheet of as-received A1 flakes, it has a composition of 55-80% aluminum, 24-45% 1-methoxy-2-propanol, and up to 1% of proprietary phosphonic acid with undisclosed additive. There is a presence of aluminum oxide layer on the surface of flakes. As observed in Fig. 3, the as-received A1 flakes showed a presence of carbon from l-methoxy-2-propanol and oxygen from aluminum oxide and aluminum. Along with carbon, oxygen, and aluminum, nitrogen, sulfur, and tungsten were observed for CPCCW0.1 (Fig. 4). These two areas were specifically selected with one having highly dense PPy and another area with less dense PPy. As observed in Fig. 4, area 2 (less dense PPy area) exhibited a lower amount of nitrogen and tungsten whereas it showed a higher amount of aluminum. Even though PPy is not observed in this area (Fig. 4) it is present if the image is reformed at higher magnifications. The presence of nitrogen in area 2 also proves the formation of PPy.

For CPCCW0.01 (Fig. 5), area 1 is on PPy wire and area 2 is on a flake. In this case, a lower amount of tungsten was observed as compared to CPCCW0.1. This may be due to the lower concentration of tungsten dopant employed in the synthesis, resulting in lower doping. In this case, on the wire PPy (area 1), a significant amount of nitrogen was also observed, confirming the wires made up of PPy.

For CPCCV0.1 (Fig. 6), both areas 1 (less dense PPy) and 2 (more dense PPy) exhibited the presence of vanadium, confirming doping of PPy along with nitrogen, oxygen, carbon, and aluminum. For CPCCV0.01 (Fig. 7), both areas 1 (less dense PPy) and 2 (more dense PPy) exhibited the presence of vanadium but doping was lower than that in CPCCV0.1. In this case of CPCCV0.01, sulfur doping was also observed along with vanadium. A lower concentration of vanadium dopant has to compete with sulfur from oxidant APS in the reaction.

Conductivity

Topography, deflection, and current images were obtained from CAFM experiments. Pellets of the composite sample and as-received A1 flakes were made in pellet presser from International Crystal Laboratories and were glued to an aluminum mount; the silver epoxy was used for CAFM studies. As-received Al flakes did not show any current density on the current image, as observed in Fig. 8. The lack of conductivity in the case of as-received Al flakes could be attributed to the presence of aluminum oxide layer on the surface of Al flakes. Topography image for as-received Al flake (Fig. 8) also exhibited similar neat and pristine morphology, as shown in SEM micrographs (Fig. 2a). All of the CPCC samples, namely CPCCW0.1, CPCCW0.01, CPCCV0.1, and CPCCV0.01, displayed current density on current image, as seen in Figs. 9,10, 11, and 12, respectively.

The four-point probe conductivity method was employed for the quantitative measurement of the conductivity. Pellets prepared for CAFM experiments were used in four-point probe conductivity measurements. The conductivity values were calculated by using equation (1)

[sigma] = (ln 2/[pi]t) (I/V) (1)

Here t, I, and V are thickness, current, and voltage, respectively. Conductivity results obtained from the four-point probe method are displayed in Fig. 13. Both tungstate-doped composites (0.1 M and 0.01 M) have comparable conductivities, whereas CPCCV0.01 showed an order of higher magnitude in conductivity than CPCCV0.1. A similar observation was found in the CAFM results as well. The current density observed in the case of the CPCCV0.01 (Fig. 12) was higher than that of the CPCCV0.1 (Fig. 11). The higher conductivity can be attributed to the doping due to both sulfur and vanadate, in case of CPCCV0.01 (EDS result in Fig. 7) as opposed to only vanadate in the case of CPCCV0.1 (EDS results in Fig. 6). As observed in Fig. 13, the conductivity of the vanadate-doped composites was lower than that of tungstate-doped composite. Polarons and bipolarons produced for the charge compensation of incorporated dopant anions results into the higher conductivities for the doped PPy. (76-78) There is a possibility of formation of varied amounts of polarons and bipolarons in the case of the different CPCC. CPCC (tungstate and vanadatedoped PPy composites) are the combination of inorganic Al flakes and PPy. The presence of inorganic flake could be also influencing the conductivity of the composite. It has been found that the presence of inorganic pigment titanium dioxide (Ti[O.sub.2]) resulted in a decrease in conductivity after 20% by weight concentration in dodecyl bezenesulfonate-doped PPy-Ti[O.sub.2] composite. (79) So, the presence of second component apart from CPs results in the changes in conductivity of CPCCs.

X-ray photoelectron spectroscopy (XPS)

The doping level and elemental composition for the synthesized composites was obtained by XPS measurements. XPS measures composition at the surface with a depth of approximately 10 nm. As observed in Fig. 14, both tungstate and vanadate-doped composites were doped with tungstate and vanadate, respectively. It was also found that dopant level was less in the case of 0.01 M concentration of dopant as compared to 0.1 M concentration of dopant in the synthesis of both the composites, as observed in Fig. 14.

Table 3 demonstrates the different ratio of dopants to nitrogen. Dopant/nitrogen ratio signifies the level of doping in the composite of PPy and A1 flake. For CPCCW0.1, WIN ratio was higher than SIN, whereas in the case of CPCCW0.01, SIN ratio was higher than WIN. This signified a greater amount of tungstate incorporation as compared to sulfur in the case of CPCCW0.1 and a higher amount of sulfur incorporation in the case of CPCCW0.01. With lower concentration of tungstate in the synthesis of CPCCW0.01, the tungstate anion encountered possible competition with the sulfur anion to be incorporated into the PPy backbone. Similarly, VIN ratio was greater than SIN for CPCCV0.1 and the SIN ratio was greater than V/N ratio for CPCCV0.01 for the same reason as cited for the tungstate-doped composite.

Electrochemical impedance spectroscopy (EIS)

EIS has been employed for the quantitative assessment of coatings against the corrosion. (80,81) EIS measurements were performed on primers for initial, 6 h, 7 days, 15 days, and 35 days of exposure to the Prohesion test conditions. Bode plots for as-received A1 Hake coating, CPCCW0.1 coating, CPCCW0.01 coating, CPCCV0.1 coating, and CPCCV0.01 coating are presented in Figs. 15, 16, and 17, respectively. As observed in Fig. 15, as-received A1 flake coating exhibited high impedance at low frequency and capacitive behavior with phase angle close to -90. This is typical EIS behavior observed for barrier type coating indicating that no electrolyte had yet penetrated through the coatings. (39) There was a little drop in the low frequency impedance of the as-received A1 flake coating was observed as the exposure duration in Prohesion test chamber was increased from the initial time to the 35 days.

For the CPCCW0.1 coating and CPCCW0.01 coating, as observed in Fig. 16, lower impedance values were observed for the PPy containing A1 flakes coatings. A similar observation was noted for CPCCV0.1 coating and CPCCV0.01 coating, as shown in Fig. 17. The low frequency impedance further decreased as the exposure to the Prohesion test conditions was increased from initial to 35 days. Impedance was almost constant at the 35th day of Prohesion exposure of all CPCCW0.1 coating, CPCCW0.01 coating, CPCCV0.1 coating, and CPCCV0.01 coating.

Lower impedance values in the PPy containing composite coatings can be attributed to the conducting nature of PPy. There is possibility of passing of alternating current (employed in EIS) through the conducting pathways created by PPy instead of the dielectric pathways of epoxy binder. (39-45) Lower impedance values in the case of PPy-doped composite coatings does not mean that these coatings are not protective in nature, as will be evidenced by exposure tests mentioned later in this paper. For PPy containing coatings, salt spray test results exhibited better corrosion protection even though lower impedance values were observed in EIS measurements for same coatings. (82) This behavior was attributed to the electroactive nature of PPy. A similar behavior is observed in this paper for CPCCW0.1 coating, CPCCW0.01 coating, CPCCV0.1 coating, and CPCCV0.01 coating as observed in Prohesion test results (discussed later), the samples exhibited minimal corrosion. The active nature of PPy induces other forms of corrosion protection mechanism such as charge transfer mechanism for the corrosion protection instead of barrier type corrosion protection mechanism. (83) Another reason for lower impedance values observed for PPy containing composite can be the porosity of PPy resulting in extended percolation networks. (84) The active nature of PPy along with its combination with A1 flakes were further investigated for the corrosion protection mechanism in more controlled experiments of galvanic coupling in the following sections.

Equivalent electric circuit modeling and equivalent fitting

Equivalent electric circuit modeling and the obtained EIS data was performed by using ZView2 software from Scribner[R] Associates Inc. The results of data fitting are presented in Table 4. The circuit models used for the fitting are shown in Fig. 18. In Fig. 18, [R.sub.1] is solution resistance, [R.sub.2] is coatings resistance, [R.sub.3] is polarization resistance, [C.sub.c] is coatings capacitance, [C.sub.dl] is double layer capacitance, and CPE is constant phase element which represents pseudo capacitance. The impedance of CPE can be obtained by equation (2)

[Z.sub.(CPE)] = 1/[(T)[(j[omega]).sup.P]], (2)

where T is capacitance, j is an imaginary component, [omega] is the angular frequency ([omega] = 2[pi]f, f is the frequency), P is the power (0 [less than or equal to] n [less than or equal to] 1), and [Z.sub.(CPE)] is the impedance of CPE. P value close to 1 represents capacitor behavior.

Only one time constant was evident for as-received A1 flake coating up to the 7th day of exposure. This data was modeled by using circuit model as represented in Fig. 18a, whereas after that, two time constants were evident and that data was modeled by using circuit model as represented in Fig. 18b. In the case of CPCCW0.1 coating, CPCCW0.01 coating, CPCCV0.1 coating, and CPCCV0.01 coating, two time constants were evident from the start of the exposure. A second time constant implied the reaction happening at interface of PPy and A1 flake and not the reaction at the interface of coating and surface as it was just the beginning of the exposure and no corrosion reaction had even started. This data was modeled by employing the circuit model represented in Fig. 18b. For the CPCCW0.1 coating, CPCCW0.01 coating, CPCCV0.1 coating, and CPCCV0.01 coating in the second time constant, initially were capacitive and [CPE.sub.dl]--T started increasing as the exposure to the Prohesion test conditions increased but still the value of P remained close to one indicating capacitive behavior. Again, for these coatings the value of [R.sub.ct] did not decrease below [10.sup.-3] [OMEGA] after 35 days of exposure and it stayed at the constant value. These results of fit and modeling were in agreement with the results obtained from EIS.

Prohesion test exposure

Panels were exposed to Prohesion conditions according to ASTM G85-A5. Photographs of panels (without topcoat) exposed for 35 days and panels with topcoat exposed for 65 days are shown in Fig. 19. As-received A1 flake coating without topcoat exhibited corrosion product in the scribe and delamination along the scribe. Lesser quantities of white corrosion product of A1 alloy most probably aluminum oxide were observed in the scribe for CPCCW0.1 coating, CPCCW0.01 coating CPCCV0.1 coating, and CPCCV0.01 coating but no delamination along the scribe was observed. For the topcoated as-received A1 flake coating severe delamination and corrosion was observed along the scribe, whereas for CPCCW0.1 coating, CPCCW0.01 coating, CPCCV0.1 coating, and CPCCV0.01 coating, less delamination and less corrosion were observed. Minimum corrosion was observed for topcoated CPCCW0.1 coating possibly due to the high conductivity of the composite.

Galvanic coupling measurements

Galvanic coupling measurements were performed on the samples without a topcoat in an oxygen-free local environment. In order to simulate the local environment with limited oxygen as under the topcoat, the coated sample compartment was purged with nitrogen. In these experiments, the cathode for bare aluminum 2024-T3 was signified by positive current. The results of coupling current are presented in Fig. 20. All of the composite coatings exhibited positive coupling current suggesting that PPy/Al flake coatings were acting as anode and sacrificially protecting aluminum 2024-T3 (Fig. 20). For CPCCW0.1 coating, coupling current increased rapidly. A similar behavior was also observed for CPCCV0.01 coating. The corrosion protection was also maximum for both of these coatings as observed in the Prohesion test results (Fig. 19).

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It has been observed that, in contact with aluminum alloy, PPy tends to be reduced as it is coupled with aluminum alloy. (57) The reduction of PPy induces the anodic dissolution of aluminum; in this case activating A1 flakes which further protects sacrificially the underlying aluminum 2024-T3 substrate. The dissolution maintained high coupling current levels. (39) Reduced and neutral form of PPy has been also found to provide sacrificial protection to aluminum 2024-T3. (85) The reduced PPy also serves as oxygen scavenger reducing corrosion rate and in the process itself gets oxidized. PPy which is oxidized again keeps A1 flake dissolving and keeps galvanic current high until all A1 flakes are consumed. As soon as corrosion begins, the anodic reaction at metal surface results in the liberation of electrons which are utilized in CP reduction leading to the release of corrosion inhibiting dopant anion as shown in Scheme 1. Oxygen reduction at CPs surface causes replenishing of the depleted charge. (86-88) As mentioned earlier, this reduction of oxygen leads to the oxygen scavenging by CPs resulting in reduced corrosion rates. The replenishment of the charge also results in the stabilization of the potential in the passive region on the metal surface. (86) In the case of the composite pigment of PPy and A1 flakes, the galvanic contact of PPy with A1 flakes results in the reduction of PPy and, thereby, the release of corrosion inhibiting dopant anions. (39) There is the possibility of combination of released anions with cations of the substrate forming an impervious layer resulting in reduction of corrosion rate.

Potentiodynamic scans

As observed in the potentiodynamic scans (Fig. 21), the corrosion potential values were shifted by approximately 300 mV in a more noble direction for CPCCW0.1 coating, CPCCW0.01 coating, CPCCV0.1 coating, and CPCCV0.01 coating as compared to the as-received A1 flake coating. This positive shift may be attributed to the release of the dopant (tungstate or vanadate) into the defect area due to reduction of PPy in contact with A1 flakes and underlying aluminum substrate. The dopant (tungstate or vanadate) may combine with aluminum cations forming impervious layer of oxide leading to passivation. (49)

As observed in Fig. 21, the corrosion current was also less in the case of CPCCW0.1 coating, CPCCW0.01, and CPCCV0.01 than that of as-received A1 flake coating. Corrosion current did not decrease in the case of CPCCV0.1 but passivation was still achieved as observed in potentiodynamic scans. This might be due to the lower conductivity of CPCCV0.1 as compared to other composites, resulting in slow release of dopant. The corrosion protection exhibited by PPy/Al flake composite pigment was due to the combined effect of active nature of PPy, galvanic coupling, and dopant release due to passivation.

Scanning electrochemical microscopy (SECM)

Samples dimensions for SECM analysis were squares of approximately 2 [cm.sup.2]. Prior to immersion in the SECM cell, for the analysis a deep scribe was made manually with a razor blade. SECM immersion experiments were performed in open to air conditions. While performing immersion experiments over several days, the sample cell was covered in parafilm to prevent evaporation whenever the experiment was not running. Each morning fresh mediator solution was added to the cell. LabVIEW software was used for the determination of height of the probe from the substrate. (89) At the desired height above the substrate, the probe was positioned and probe scan curves were obtained by laterally moving the probe over the scribe (at a rate of 17 [micro]m/s), simultaneously probe current was also measured. For reduction of dissolved [O.sub.2], the probe potential was -0.7 V. Prior to each scan a quiet time of 200-300 s was determined which was necessary for establishing a stable baseline probe current.

As observed in Fig. 22, day 1 showed a relatively large decrease in the [O.sub.2] reduction current as the SECM probe passed over the scribe. Such a feature is indicative of significant corrosion occurring at the exposed scribe as [O.sub.2] reduction in the corrosion cell leads to less [O.sub.2] available at the probe electrode. Both the CPCCW0.1 and CPCCV0.1 coatings exhibited a much lower level of [O.sub.2] depletion, consistent with corrosion inhibition by these coatings. After several days of immersion, 02 depletion in the as-received A1 flake coating is greatly diminished, and a single spike was visible. SEM examination of the scribed area reveals significant buildup of corrosion product. (90) After several days of immersion, 02 depletion in the CPCCW0.1 and CPCCV0.1 coatings appeared to be slightly enhanced. SEM images of the scribed areas on the vanadate-containing coating showed that it was mostly free of corrosion product, however, some areas of corrosion product buildup were evident. The tungstate-containing coating, however, showed no evidence of corrosion product.

Conclusions

Tungstate and vanadate-doped PPy/Al flake composites were synthesized by chemical oxidative polymerization. The synthesized composites were conductive in nature as observed in CAFM and four-point probe measurements. Doped PPy/Al flake composite coatings exhibited improved corrosion protection in the Prohesion cabinet exposure test. The galvanic coupling current measurements demonstrated sacrificial corrosion protection mechanism provided by doped PPy/Al flake composite coatings to the underlying aluminum 2024-T3 substrate. Potentiodynamic scans suggested dopant release from PPy, resulting in passivation in the defect thereby enhancing the corrosion protection. In this way, sacrificial protection was combined with active dopant release mechanism in the formed doped PPy/Al flake composite coatings resulting in increased corrosion protection.

DOI 10.1007/sl1998-014-9633-4

N. Jadhav ([mail]), V. Gelling

Department of Coatings and Polymeric Materials, North Dakota State University, Fargo, ND 58102, USA

e-mail: niteen.jadhav@my.ndsu.edu; n3.uict@gmail.com

M. B. Jensen

Department of Chemistry, Concordia College, Moorhead, MN 56562, USA

Acknowledgments The authors gratefully acknowledge the support of this research by US Army Research Laboratory under Grant Nos. W911NF-09-2-0014, W911NF-10-2-0082, and W911NF-11-2-0027.

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Table 1: Synthesis reactions for PPy/AI flake composite pigment

                 Tungstate-doped (0.1 M)   Tungstate-doped (0.01 M)
                 PPy/AI flake composite     PPy/AI flake composite
                       (CPCCW0.1)                (CPCCW0.01)

Water (mL)               500                       500
Sodium                   9.15 g                    0.915 g
tungstate
dihydrate
sodium                   --                        --
metavanadate
Al flakes (g)            15                        15
APS (g)                  11.4                      11.4
Pyrrole (mL)             6.95                      6.95

                 Vanadate-doped (0.1 M)   Vanadate-doped (0.01 M)
                 PPy/AI flake composite   PPy/AI flake composite
                       (CPCCV0.1)               (CPCCV0.01)

Water (mL)               500                      500
Sodium                   --                       --
tungstate
dihydrate
sodium                   6.05 g                   0.605 g
metavanadate
Al flakes (g)            15                       15
APS (g)                  11.4                     11.4
Pyrrole (mL)             6.95                     6.95

CPCC conducting polymer containing composite

Table 2: Peak positions ([cm.sup.-1]) and FTIR modes of vibrations
for CPCCW0.1, CPCCW0.01, CPCCV0.1, and CPCCV0.01

Wavenumbers ([cm.sup.-1]) and modes of      CPCCW0.1      CPCCW0.01
vibration                                 (wavenumber,   (wavenumber,
                                          [cm.sup.-1])   [cm.sup.-1])

3400, N-H stretching                      3398           3408
3100, C-H aromatic stretching             3118           3120
1702, carbonyl group                      1700           1700
1558, C-C and C=C ring stretching         1556           1555
1464, C-N and C=C stretching              1470           1472
1295, C-H and C-N in plane deformation    1318           1311
1198, ring breathing                      1197           1202
1090, C-C in plane deformation            1100           1102
1045, C-H deformation                     1050           1049
929 and 788 PPy characteristic peak,      928, 801,      926, 789,
  960-780 and 900-770 tungstate ion         700, 680       675, 613
  band, 1010-920 and 890-830 vanadate
  ion band, 750-490 Al-O stretching
  vibration
490 weak tungstate ion band               490            490, 437
540-490 vanadate ion band                 --             --

Wavenumbers ([cm.sup.-1]) and modes of      CPCCV0.1      CPCCV0.01
vibration                                 (wavenumber,   (wavenumber,
                                          [cm.sup.-1])   [cm.sup.-1])

3400, N-H stretching                      3421           3422
3100, C-H aromatic stretching             3100           3120
1702, carbonyl group                      1698           1720
1558, C-C and C=C ring stretching         1569           1566
1464, C-N and C=C stretching              1463           1470
1295, C-H and C-N in plane deformation    1311           1311
1198, ring breathing                      1209           1203
1090, C-C in plane deformation            1099           1099
1045, C-H deformation                     1055           1050
929 and 788 PPy characteristic peak,      429, 789,      926, 789,
  960-780 and 900-770 tungstate ion         681, 600       672, 610
  band, 1010-920 and 890-830 vanadate
  ion band, 750-490 Al-O stretching
  vibration
490 weak tungstate ion band               --             --
540-490 vanadate ion band                 550            540

Table 3: Dopant ratios obtained by XPS

Dopant ratio    SIN     WIN     V/N

CPCCW0.1       0.056   0.556   --
CPCCW0.01      0.148   0.007   --
CPCCV0.1       0.065   --      0.150
CPCCV0.01      0.137   --      0.011

Table 4: Circuit elements obtained through EIS data

                                 Time          [R.sub.c]
                                          ([OMEGA] [cm.sup.2])

As-received Al flake coating    Initial    3.50 x [10.sup.12]
                                6 h        5.31 x [10.sup.11]
                                7 days     2.21 x [10.sup.11]
                                15 days    2.25 x [10.sup.8]
                                35 days    2.36 x [10.sup.7]
CPCCW0.1 coating                Initial    3.88 x [10.sup.5]
                                6 h        3.66 x [10.sup.4]
                                7 days     1.36 x [10.sup.3]
                                15 days    2.75 x [10.sup.3]
                                35 days    1.16 x [10.sup.3]
CPCCW0.01 coating               Initial    2.05 x [10.sup.6]
                                6 h        2.36 x [10.sup.s]
                                7 days     1.09 x [10.sup.3]
                                15 days    1.68 x [10.sup.3]
                                35 days    7.74 x [10.sup.2]
CPCCV0.1 coating                Initial    6.32 x [10.sup.5]
                                6 h        4.77 x [10.sup.5]
                                7 days     5.94 x [10.sup.2]
                                15 days    6.34 x [10.sup.2]
                                35 days    5.32 x [10.sup.2]
CPCCV0.01 coating               Initial    5.87 x [10.sup.2]
                                6 h        3.05 x [10.sup.2]
                                7 days     5.01 x [10.sup.2]
                                15 days    6.69 x [10.sup.2]
                                35 days    6.20 x [10.sup.2]

                                 Time            [CPE.sub.c]

                                           T (F/[cm.sup.2])      P

As-received Al flake coating    Initial   1.54 x [10.sup.-10]   0.97
                                6 h       1.56 x [10.sup.-10]   0.98
                                7 days    2.73 x [10.sup.-10]   0.94
                                15 days   1.70 x [10.sup.-10]   0.97
                                35 days   1.67 x [10.sup.-10]   0.97
CPCCW0.1 coating                Initial   8.86 x [10.sup.-9]    0.80
                                6 h       3.32 x [10.sup.-8]    0.69
                                7 days    3.69 x [10.sup.-6]    0.42
                                15 days   6.91 x [10.sup.-6]    0.38
                                35 days   6.09 x [10.sup.-6]    0.35
CPCCW0.01 coating               Initial   2.79 x [10.sup.-9]    0.86
                                6 h       6.48 x [10.sup.-9]    0.81
                                7 days    2.69 x [10.sup.-6]    0.47
                                15 days   1.05 x [10.sup.-5]    0.37
                                35 days   1.84 x [10.sup.-5]    0.27
CPCCV0.1 coating                Initial   6.15 x [10.sup.-9]    0.82
                                6 h       1.29 x [10.sup.-8]    0.77
                                7 days    6.24 x [10.sup.-7]    0.46
                                15 days   1.14 x [10.sup.-5]    0.35
                                35 days   1.80 x [10.sup.-5]    0.25
CPCCV0.01 coating               Initial   8.01 x [10.sup.-7]    0.55
                                6 h       1.40 x [10.sup.-7]    0.64
                                7 days    1.68 x [10.sup.-6]    0.45
                                15 days   5.24 x [10.sup.-6]    0.41
                                35 days   1.18 x [10.sup.-4]    0.33

                                 Time          [R.sub.c]
                                          ([OMEGA] [cm.sup.2])

As-received Al flake coating    Initial    --
                                6 h        --
                                7 days     --
                                15 days    2.71 x [10.sup.11]
                                35 days    3.94 x [10.sup.11]
CPCCW0.1 coating                Initial    1.79 x [10.sup.6]
                                6 h        4.67 x [10.sup.5]
                                7 days     8.38 x [10.sup.4]
                                15 days    8.24 x [10.sup.4]
                                35 days    5.26 x [10.sup.4]
CPCCW0.01 coating               Initial    7.17 x [10.sup.6]
                                6 h        2.95 x [10.sup.6]
                                7 days     9.09 x [10.sup.4]
                                15 days    3.93 x [10.sup.4]
                                35 days    1.19 x [10.sup.4]
CPCCV0.1 coating                Initial    8.31 x [10.sup.5]
                                6 h        9.30 x [10.sup.5]
                                7 days     4.12 x [10.sup.4]
                                15 days    4.05 x [10.sup.4]
                                35 days    1.58 x [10.sup.4]
CPCCV0.01 coating               Initial    7.17 x [10.sup.5]
                                6 h        2.37 x [10.sup.5]
                                7 days     4.05 x [10.sup.4]
                                15 days    4.83 x [10.sup.4]
                                35 days    2.41 x [10.sup.4]

                                 Time            [CPE.sub.dl]

                                           T (F/[cm.sup.2])      P

As-received Al flake coating    Initial   --                    --
                                6 h       --                    --
                                7 days    --                    --
                                15 days   1.06 x [10.sup.-10]   0.79
                                35 days   1.45 x [10.sup.-10]   0.93
CPCCW0.1 coating                Initial   7.41 x [10.sup.-8]    0.63
                                6 h       8.88 x [10.sup.-7]    0.79
                                7 days    1.19 x [10.sup.-5]    0.66
                                15 days   1.79 x [10.sup.-5]    0.74
                                35 days   6.78 x [10.sup.-5]    0.75
CPCCW0.01 coating               Initial   1.27 x [10.sup.-8]    0.77
                                6 h       1.34 x [10.sup.-7]    0.87
                                7 days    8.69 x [10.sup.-6]    0.73
                                15 days   2.29 x [10.sup.-5]    0.72
                                35 days   1.97 x [10.sup.-5]    0.78
CPCCV0.1 coating                Initial   7.46 x [10.sup.-8]    0.84
                                6 h       2.95 x [10.sup.-7]    0.80
                                7 days    2.61 x [10.sup.-5]    0.68
                                15 days   7.26 x [10.sup.-5]    0.66
                                35 days   2.15 x [10.sup.-4]    0.81
CPCCV0.01 coating               Initial   4.03 x [10.sup.-8]    0.88
                                6 h       1.64 x [10.sup.-6]    0.62
                                7 days    3.08 x [10.sup.-5]    0.61
                                15 days   3.66 x [10.sup.-5]    0.66
                                35 days   1.17 x [10.sup.-4]    0.76

                                 Time     [chi square]

As-received Al flake coating    Initial      0.003
                                6 h          0.006
                                7 days       0.007
                                15 days      0.001
                                35 days      0.002
CPCCW0.1 coating                Initial      0.017
                                6 h          0.009
                                7 days       0.020
                                15 days      0.009
                                35 days      0.006
CPCCW0.01 coating               Initial      0.005
                                6 h          0.006
                                7 days       0.031
                                15 days      0.018
                                35 days      0.014
CPCCV0.1 coating                Initial      0.010
                                6 h          0.005
                                7 days       0.010
                                15 days      0.030
                                35 days      0.009
CPCCV0.01 coating               Initial      0.004
                                6 h          0.008
                                7 days       0.021
                                15 days      0.035
                                35 days      0.015

Fig. 3: EDS of as-received Al flakes

          C-K     O-K    Al-K

Area 1   33.36   19.68   46.96
Area 2   31.66   21.56   46.78

Fig. 4: EDS of CPCCW0.1

          C-K     N-K     O-K    Al-K    S-K    Cu-K   W-K

Area 1   61.33   10.68   11.19   15.42   0.20          1.18
Area 2   59.80   7.62    10.24   21.10          0.29   0.95

Fig. 5: EDS of CPCCW0.01

          C-K     N-K     O-K    Al-K    S-K    W-K

Area 1   58.53   11.20   16.40   13.37   0.37   0.15
Area 2   56.02   8.72    14.56   20.62          0.09

Fig. 6: EDS of CPCCV0.1

          C-K    N-K     O-K    Al-K    V-K

Area 1   55.79   9.15   10.30   24.42   0.34
Area 2   58.90   9.84   8.89    22.10   0.27

Fig. 7: EDS of CPCCV0.01

          C-K    N-K     O-K    Al-K    S-K    V-K

Area 1   51.54   8.02   23.99   15.47   0.75   0.23
Area 2   56.44   8.25   22.26   12.14   0.71   0.20


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Please note: Some tables or figures were omitted from this article.
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Article Details
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Author:Jadhav, Niteen; Jensen, Mark B.; Gelling, Victoria
Publication:Journal of Coatings Technology and Research
Geographic Code:1USA
Date:Mar 1, 2015
Words:10338
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