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Evaluation of the corrosion protection of defective polyaniline/epoxy coating by localized electrochemical impedance spectroscopy.

Abstract Defective epoxy varnish and hydrofluoric acid-doped polyaniline/epoxy (PANI-HF/EP) coatings were coated on the surface of an AZ91D magnesium alloy. The corrosion protection of the defective coatings was evaluated in 0.05 M NaCl by localized electrochemical impedance spectroscopy. Then, the surface of AZ91D magnesium alloy beneath the coating was analyzed by scanning electron microscopy-and energy dispersive X-ray spectroscopy. The results indicated that the defective epoxy varnish coating could not protect the magnesium alloy because it only served as an electrolyte barrier. By contrast, the PANI-HF/EP coating served not only as an electrolyte barrier, but also facilitated the formation of a protective layer by redox processes. Therefore, the defective PANI-HF/EP coating could protect magnesium alloy from corrosion.

Keywords Polyaniline coating. Magnesium alloy, Corrosion, LEIS

Introduction

Polyaniline (PANI) has received much attention over the past three decades because of its relatively easy preparation, excellent environmental and thermal stabilities, low cost, interesting electrical properties, and corrosion protection of metals such as stainless steel, (1,2) iron, (3,4) mild steel, (5,6) copper, (7) aluminum, (8) and aluminum alloys. (9,10) Moreover, in the authors' previous study, PANI, which was added in epoxy coating, improved the coating corrosion resistance for magnesium alloy in 3.5% NaCl solution. (11,12) To date, most of the studies on the corrosion protection performance of PANI coating have evaluated intact coating. However, some coating defects (e.g., retained solvents, physical holes, and scratches) may appear during preparation, transport, and application. When exposed to aggressive species, a coating often fails rapidly when defects appear.

Electrochemical impedance spectroscopy (EIS) has been extensively used to characterize the corrosion protection performance of coats. However, the characterization of coating performance using conventional EIS measurement only provides the average information on the coating over a large area. Consequently, the local electrochemical process at a micro-defect site, such as a pinhole in the coating, is "averaged" out. Therefore, EIS measurement may be used to characterize the general state of the coating but not on the local variation. (13,14) Some new techniques that perform local measurements have been applied to obtain a more detailed understanding of the corrosion protection performance of a defective coating. The micro-zone electrochemical techniques, which include scanning reference electrode techniques (SRET), (15,16) scanning vibrating electrode technique (SVET), (17-19) scanning Kelvin probes (SKP), (20,21) scanning electrochemical microscopy (SECM), (22,23) and localized EIS (LEIS), (24,25) have been used in corrosion studies. LEIS measurements have been extensively applied on metal substrates, (26-29) but it has only been recently applied to coated metals. (30,31) In this article, the applicability of LEIS to the electrochemical processes related to the corrosion resistance of a defective PANI coating on magnesium alloy was explored. The protective mechanism of the defective coating for magnesium alloy was also determined. [MgF.sub.2] can be easily formed on Mg by a simple chemical process, and this compound is chemically inert and can function as a barrier coating for corrosion protection. (32) Thus, hydrofluoric acid-doped PANI/epoxy (PANI-HF/EP) coating was selected.

Experimental

Materials and sample preparation

The test sample was the varnish or PANI-HF/EP-coated AZ91D die-cast magnesium alloy (Al, 9.2%; Zn, 0.65%; Mn, 0.28%; Si, 0.05%; Fe, 0.04%; Cu, 0.0025%; Ni, 0.001%; Mg balance; Shanxi Fengquan Company, China). The samples were cut into 10 mm x 10 mm x 10 mm cubes and embedded in the epoxy resin, with 10 mm x 10 mm of its surfaces exposed for testing. The test areas were ground to 800-grit finish, degreased in acetone and ethanol, and dried in air before the experiment.

PANI-HF powder was synthesized as previously described. (12) Epoxy resin (E44) and polyamide (651) were obtained from the Blue Star company (China). The PANI-HF powders were dispersed in the epoxy resin using a ball-grinding mill. The concentration of PANI-HF powders was 10 wt%. Polyamide was used as a curing agent, and a mixture of xylene and butyl alcohol was used as solvent. The PANI-HF/EP coating was cured at room temperature for 24 h and at 60[degrees]C for 24 h. The average thickness of the coating was 80 [+ or -] 5 [micro]m, as determined using a Positector 6000 Coating Thickness Gages (DeFelsko Corporation, USA). Varnish-coated AZ91D magnesium alloy panels were used as reference specimens. An artificial pinhole of 200 [micro]m in diameter was made down to magnesium alloy using a pin vise.

Local electrochemical impedance measurements

LEIS was performed according to the study of Lillard et al. (33-35) The principles of LEIS were similar to those employed in conventional EIS, i.e., a small sinusoidal voltage perturbation is applied to a working electrode sample, and the resulting current is measured to calculate impedance. Contrary to conventional EIS, LEIS enables the investigation of the local zones of a working electrode. (36) The sketch map of an LEIS apparatus is shown in Fig. 1. LEIS measurements were performed through a scanning electrochemical workstation, which was composed of an SCV370 scanning control unit, an M273A and a PG580R potentiostat, an M5210 lock-in amplifier (frequency range, 12.5 kHz to 0.5 Hz), and a video camera system.

The five-electrode arrangement consisted of a typical three-electrode arrangement [working electrode (WE), counter electrode (CE), and reference electrode (RE)], while two micro "reference" electrodes were used to detect the local potential gradient in the solution. The counter electrode and two micro "reference" electrodes formed the probe of the LEIS apparatus (Fig. 2). The two micro "reference" electrodes consisted of Teflon-coated platinum wires, which were positioned on and in a conical plastic holder. One of these electrodes protruded from the tip of the cone, and the other was a ring placed around the cone 3 mm from the tip. The local impedance [Z.sub.local] was calculated as follows:

[Z.sub.local] = [DELTA][V.sub.applied]/[i.sub.local] (1)

The applied voltage [DELTA][V.sub.applied] is the potential difference between the working and reference electrodes and equal to the sinusoidal voltage perturbation. In the LEIS measurements, [DELTA][V.sub.applied] was applied by the M273A potentiostat. The local AC current density ([i.sub.local]) was calculated using Ohm's law:

[i.sub.local] = [DELTA][V.sub.local]/d x [kappa], (2)

where k is the conductivity of the electrolyte, d is the distance between the two micro "reference" electrodes, and [DELTA][V.sub.local] is the local potential gradient between the two micro "reference" electrodes. In the LEIS measurements, [DELTA][V.sub.local] was measured by a PG580R potentiostat.

The microprobe was stepped over a designated area on the electrode surface. The distance between the probe-tip and the sample surface was approximately 60 [micro]m, which was adjusted and monitored through a video camera system. The scanning took the form of a raster in the x-y plane. The step size during LEIS scanning was controlled to obtain a plot of 16 lines x 12 lines with a scanning area of 2 mm x 1.5 mm. The resultant resolution was 125 [micro]m pixel size. The scanning area and step size were measured by the SCV370 scanning control unit. The AC disturbance signal was 20 mV, and the excitation frequency for impedance measurements was fixed at 50 Hz. The LEIS was measured at the open circuit potential. The corrosive medium was a 0.05 M NaCl solution, and the conductivity of the electrolyte solution (A:) was 4.72 mS [cm.sup.-1], which was measured by an HI 8633 Conductivity Meter (HANNA, Italy).

SEM and EDS

After LEIS measurement, the coatings on the AZ91D magnesium alloy surface were removed, and the surface was observed by SEM and EDS (JSM-6480, Japan).

Results

LEIS measurements

A uniform color value was applied to all figures to determine the difference between the epoxy varnish and PANI-HF/EP coatings. Impedance increased from 0 [OMEGA] to 20 k[OMEGA], which corresponds to the change in color from navy blue to red. The change in the impedance of the epoxy varnish coating with a 200-[micro]m defect under different immersion times in 0.05 M NaCl solution is described in Fig. 3. A two stage process was observed: that is, an initial decrease in the earliest stage of observation (immersion for 12 h) and a gradual increase from 32 h to the end of immersion for 164 h. The LEIS map in Fig. 3a exhibited blue at all measured areas. The diameter of the defect area increased. These results indicated that corrosion had spread outwards from the defect area. The impedance value at the defect area was approximately 500 [OMEGA]. The impedance value at this area did not change after 12 h, but that of the adjacent area decreased (Fig. 3b). After 32 h immersion, impedance value at the adjacent area of the defect increased. The LEIS map showed a large green area surrounding the defect area and some red islets at the brim of the scanned area when the defective epoxy varnish coating was immersed for 164 h (Fig. 3d). This result indicated that the impedance value was increased by more than 9 k[OMEGA]. However, the impedance value at the defect area just increased to 700 [OMEGA].

The changes in the LEIS maps of PANI-HF/EP-coated magnesium alloy around the defect area over a period of 164 h immersion are shown in Fig. 4. The LEIS maps of the PANI-HF/EP coating distinctly differed from those measured from the epoxy varnish coating. The sharp color gradient from blue to red between the defect and adjacent areas for 3 h exposure in 0.05 M NaCl solution, as shown in Fig. 4a, indicated that the impedance value of the PANI-HF/EP coating gradually increased from the defect area outwards to the adjacent area. The impedance value at the defect area was approximately 1.5 k[OMEGA], which was higher than that of the epoxy varnish coating. When immersion time was increased to 32 h, the impedance value of the PANI-HF/EP coating gradually decreased and the defect area slightly increased. When immersion was further prolonged to 164 h, the LEIS map depicted in Fig. 4d (insert) was basically red. In order to better understand the result, the range of the values of representative color legends is expanded in Fig. 4d. The result showed that the impedance value was about 50 k[OMEGA] for the entire scanning region. After 164 h immersion, the impedance value of the PANI-HF/EP coating at the defect area was approximately 26 times higher than that at 3 h immersion. However, the LEIS results of the epoxy varnish coating at the defect increased slightly after 164 h immersion.

Surface morphology analysis

The morphologies of the AZ91D magnesium alloy beneath the epoxy varnish and PANI-HF/EP coatings after LEIS measurement in 0.05 M NaCl solution are shown in Fig. 5. Compared with that beneath the PANI-HF/EP coating, the surface beneath the epoxy varnish coating showed more severe corrosion, with numerous white corrosion products around the defect area (Fig. 5 a). Furthermore, many corrosion spots were also observed on the specimen surface away from the defect. Conversely, white corrosion product was not observed on the surface of the AZ91D magnesium alloy beneath the PANI-HF/EP coating (Fig. 5b), but the surface was clearly masked by a gray film. The defect was not evidently different from the surrounding metal.

The corrosion morphology of the defect area of the AZ91D magnesium alloy beneath the coatings after LEIS measurement was further investigated by SEM (Fig. 6). Very serious corrosion was observed on the surface of the AZ91D magnesium alloy beneath the epoxy varnish coating, in which a lot of corrosion products and a large corroded region were formed. The defect was not distinct from the corroded region. In the case of PANI-HF/EP coatings, the signs of corrosion were less evident except at the defect region (Fig. 6c). From the inspection of amplificatory multiple of SEM (Fig. 6d) after LEIS measurement, the diameter of the defect was found to slightly increase.

Surface component analysis

EDS point analyses were performed to investigate the film composition. Figure 7 shows the SEM images corresponding to EDS spectra of regions of AZ91D magnesium alloy substrate beneath the epoxy varnish and PANI-HF/EP coatings. EDS elemental analytical results are provided in Table 1. For the epoxy coating, point A corresponds to the corrosion region that mainly comprised oxygen and magnesium. At a distance from the corroded region, much magnesium and non-remarkable oxygen was detected (point B), indicating the absence of corrosion on the magnesium alloy substrate beneath the epoxy varnish coating. For the PANI-HF/ EP coating, at the corrosion region (point A), a large amount of oxygen and magnesium was present, which was similar to the observation with the epoxy varnish coating. However, the composition of point B for PANIHF/EP was different from that of the epoxy varnish coatings. Fluorine was also detected on the surface of the magnesium alloy beneath the PANI-HF/EP coating.

Discussion

Corrosion protection of the defective epoxy varnish coating

The corrosion reactions of magnesium in a neutral media can generally be described as follows:

Anodic reaction:

Mg [right arrow] [Mg.sup.2+] + 2[e.sup.-] (3)

Cathodic reactions:

2[H.sub.2]O + 2[e.sup.-] [right arrow] [H.sub.2] + 2O[H.sup.-] (4)

Total reactions:

[Mg.sup.2+] + 2O[H.sup.-] [right arrow] Mg[(OH).sub.2] (5)

Epoxy varnish coating only acts as an electrolyte barrier. Therefore, when a defect appears in the epoxy varnish coating, the underlying magnesium alloy substrates could directly come into contact with the electrolyte, and electrochemical reactions could take place at the defect and adjacent areas. Thus, the anodic dissolution of magnesium alloy substrates [reaction (3)] occurs. Simultaneously, reaction (4) occurs when electrons released from reaction (3) arrive at cathodic sites. A build-up of O[H.sup.-] under the coatings results in alkalization, which in turn weakens the adhesion between the coatings and magnesium substrates. A previous study had indicated that the adhesion of epoxy varnish coating was lower and reduced rapidly during immersion. (12) The electrochemical reactions would therefore expand easily from the defect below the organic coating to the surrounding area. The LEIS results also indicated that the defective epoxy varnish coating provided lower protection to magnesium alloy than the PAN1-HF/EP coating. Moreover, the corrosion rate was high and the corroded area extended around the defect when the electrolyte came into contact with the magnesium alloy surface via the defect. This result was confirmed by the reduced impedance value of the epoxy varnish coating from 3 h to 12 h immersion (Figs. 3a and 3b). The increased impedance value for 164 h immersion (Fig. 3c) was the result of the accumulation of corrosion products. This finding was confirmed by the severe corrosion and large corroded region beneath the epoxy varnish coating (Fig. 6a). The corrosion process of the defective epoxy varnish coating is presented in Fig. 8.

Corrosion protection of the defective PANI-HF/ EP coating

PANI has three basic structures, namely a fully reduced leucoemeraldine base (LEB), a fully oxidized pernigraniline base (PNB), and a half-oxidized/half-reduced emeraldine base (EB) state. EB is the most stable state. The protonated or doped EB form has high conductance, while LEB and PNB are mainly insulators. PANI can be interconverted between EB and LEB by a redox process. (37,38)

[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] (6)

The PANI-HF/EP coating not only serves as an electrolyte barrier, but it also forms an oxide film by redox processes [reaction (6)]. In the authors' former study, (12) the protective mechanism of the PANI-HF/EP coating for magnesium alloy is not only the formation of an oxide film because of the redox reaction of PANI but also the secondary reaction of the formation of a dopant magnesium complex. Concomitant to the redox reaction with EB-based PANI salt (PANI-HF) is the fluorine anions released from PANI-HF. These fluorine anions formed Mg[F.sub.2] with magnesium alloy, which is difficult to dissolve in water. Moreover, the PANI-HF/EP coating maintained a higher adhesion than the epoxy varnish coating during immersion. (12) The delamination of the PANI-HF/EP coating was restrained. The electrolyte hardly extended around the defect. Therefore, the PANI-HF/EP coating could provide better corrosion resistance on AZ91D magnesium alloy than the epoxy varnish coating.

The higher impedance value of the PANI-HF/EP coating at 3 h immersion (Fig. 4a) indicated that the PANI-HF/EP coating serves as a stronger electrolyte barrier than the epoxy varnish coating. After 32 h immersion, a decreasing impedance value of the PANI-HF/EP coating in the scanned area was observed (Fig. 4b). This condition originated from the penetration of ions or water into the coating and/or the slight corrosion of the magnesium alloy surface. The increased impedance value of the PANI-HF/EP coating for 164 h immersion was due to the formation of a gray protective layer (Fig. 5b). A large amount of oxygen (Table 1) detected on point B of the magnesium alloy surface beneath the PANI-HF/EP coating indicated that the redox reaction of PANI-HF occurred on the magnesium alloy surface. Furthermore, EDS analytical results showed that the protection layer was composed of an oxidation layer caused by the redox reaction with PANI-HF and the insoluble Mg[F.sub.2] because of the fluorine anions released from PANI-HF on the surface of the AZ91D magnesium alloy. Therefore, defective PANI-HF/EP coating could inhibit the corrosion of magnesium alloy. The corrosion process of the defective PANI-HF/EP coating is illustrated in Fig. 9.

Conclusions

The corrosion protections of defective epoxy varnish and PANI-HF/EP coatings were evaluated by LEIS in a 0.05 M NaCl solution. The results indicated that the defective epoxy varnish coating provided lower protection to magnesium alloy than the PANI-HF/EP coating. The defective PANI-HF/EP coating could protect magnesium alloy from corrosion by forming a protective layer.

DOI 10.1007/s11998-015-9679-y

Y. Zhang, P. Li

College of Shipbuilding Engineering, Harbin Engineering University, Nantong ST 145, Harbin 150001, China

Y. Zhang, Y. Shao ([mail]), G. Meng, T. Zhang, F. Wang

Corrosion and Protection Laboratory, College of Materials Science & Chemical Engineering, Harbin Engineering University, Nantong ST 145, Harbin 150001, China e-mail: shaoyawei@hrbeu.edu.cn

Y. Shao, G. Meng, T. Zhang. F. Wang

State Key Laboratory for Corrosion and Protection, Institute of Metal Research, Chinese Academy of Sciences, Wencui RD 62, Shenyang 110016, China

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Table 1: Elemental analysis on the surface of AZ91D
alloy beneath epoxy varnish coating and PANI-HF/EP
coating from EDS results in Fig. 7

                                      Element

Coating           C       N       0        Mg      F      Al

Epoxy varnish
A                3.03    8.88   65.53    22.56    --     --
B               15.23    3.28    5.38    71.89    --     4.23

PANI-HF/EP
A                7.39    8.77   61.17    21.80    --     0.87
B               17.17    6.96   27.76    46.43    0.23   1.44

The concentrations are in At.%


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Author:Zhang, Yingjun; Shao, Yawei; Meng, Guozhe; Zhang, Tao; Li, Ping; Wang, Fuhui
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Date:Jul 1, 2015
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