Corrosion protection of epoxy coating with pH sensitive microcapsules encapsulating cerium nitrate

Accepted Manuscript Title: Corrosion protection of epoxy coating with pH sensitive microcapsules encapsulating cerium nitrate Authors: Takeshi Matsuda, Kiran B. Kashi, Koji Fushimi, Victoria J. Gelling PII: DOI: Reference:

S0010-938X(18)31453-7 https://doi.org/10.1016/j.corsci.2018.12.012 CS 7806

To appear in: Received date: Revised date: Accepted date:

8 August 2018 5 November 2018 3 December 2018

Please cite this article as: Matsuda T, Kashi KB, Fushimi K, Gelling VJ, Corrosion protection of epoxy coating with pH sensitive microcapsules encapsulating cerium nitrate, Corrosion Science (2018), https://doi.org/10.1016/j.corsci.2018.12.012 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Corrosion protection of epoxy coating with pH sensitive microcapsules encapsulating cerium nitrate Takeshi Matsuda1, 2* [email protected] , Kiran B. Kashi1, Koji Fushimi3 and Victoria J. Gelling1

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

2

JFE Steel Corp., Japan

3

Faculty of Engineering, Hokkaido University, Japan

*

Corresponding author. Tel.: +81 84 945 3579.

SC RI PT

1

N

Corrosion of AA2024-T3 aluminum alloy was suppressed by existing cerium nitrate. Cerium oxides were formed on copper-enriched sites due to pH change.

A

Impedance of coated alloy with pH sensitive microcapsule (pH-MC) increased. pH-MC containing cerium nitrate suppressed blisters of coated alloy.

M

   

U

Highlights:

D

Abstract

The ability of an epoxy coating with pH sensitive microcapsules (pH-MC) to protect AA2024-T3

TE

aluminum alloy from corrosion in a NaCl solution was investigated. Immersion of the noncoated alloy in the solution revealed that addition of cerium nitrate to the solution led to a decrease in the cathode area or cathodic

EP

reactivity to form cerium oxides on copper-enriched sites. Application of the pH-MC containing cerium nitrate to the coated alloy resulted in an increase in the charge transfer resistance with time. Encapsulation of cerium ions in

CC

the pH-MC was beneficial for preventing excess elution of cerium ions and self-healing ability.

A

Key words: corrosion protection, aluminum alloy, pH sensitive microcapsule, cerium nitrate

1. Introduction Aluminum alloys are one prospective material for reducing the structural weight of transportation devices

such as car and aircraft bodies. AA2024-T3 aluminum alloy is widely used in transportation devices due to its remarkable strength and mechanical properties. However, this alloy suffers from pitting corrosion in environments containing chloride because alloying elements such as Cu, Mg and Al form intermetallic phases and initiate a local corrosion reaction, i.e., formation of pits. Hughes et al. [1] and Bethencourt et al. [2] studied the 1

role of intermetallic compounds (IMCs), especially Al2CuMg (S-phase), in the local corrosion process. It was reported that an anodic reaction occurs at the S-phase accompanied by a cathodic reaction at other IMCs [1-5] and, after the dissolution of the S-phase, copper redeposited on the IMC and/or copper was incrassated at remnants of the S-phase act as a cathode in pitting corrosion at the boundaries between IMC and aluminum grains [1, 5]. Therefore, decreasing the anodic activity of the S-phase and/or the cathodic activity of the redeposited

SC RI PT

copper on the IMC or S-phase remnants is important for preventing pitting corrosion of AA2024-T3. Furthermore, clarification of the depth profile of the corrosion products is useful for understanding the progress of corrosion. It was reported that the corrosion reaction of AA2024-T3 aluminum alloy in a 0.1 M NaCl solution progressed in 3 stages [1, 5]. Therefore, corrosion protection might be examined with the aim of reducing the reaction rate of the rate-determining stage.

In order to protect the aluminum alloy surface from corrosion, chromate coatings had been most commonly used owing to their high self-healing ability, which is derived from the oxidation reaction of

U

hexavalent chromium [6-9]. Clark el at. reported that cathodic inhibition on a surface of copper galvanically

N

coupling with aluminum in a hexavalent chromium solution was accompanied by a transient current for reduction equivalent to the formation of trivalent chromium oxyhydroxide [10]. However, strict regulations have been

A

applied to the use of certain heavy metals such as Cd, Pb, Hg and Cr(VI) due to their high toxicities, as seen, for

M

example, in the “End-of Life Vehicles Directive” enforced in Europe from 2000. Thus, alternative coating systems are required for corrosion protection of aluminum alloys as well as other materials. Various organic and/or inorganic coating systems have been proposed as alternative systems for

D

corrosion protection of aluminum alloys. Sol-gel coating consisting of silane compounds is one of these

TE

alternative systems [11-15]. The cross-linking reaction of the silane compounds and covalent bonding between hydroxyl groups and the metal substrate offer high barrier and adhesion properties which are beneficial for

EP

reducing the penetration of water molecules and chloride ions into the coating. In this system, the optimal crosslinking reaction of the silane compounds is indispensable for a high corrosion protection ability because sol-gel coatings are generally prone to cracking. On the other hand, the cerium oxide film originated from cerium

CC

compounds preferentially forms on intermetallic phases like S-phase remnants and offers a superior corrosion protection ability for aluminum alloys [16]. Therefore, a combination of cerium compounds with sol-gel coating is expected to improve coating performance and thus is regarded as one potential alternative system for corrosion

A

protection of aluminum alloys [17-25]. A composite coating consisting of bis-silane and cerium nitrate was successfully developed and applied as an alternative corrosion protection system [22, 24, 25]. Furthermore, the corrosion resistivity of alternative coatings containing cerium compounds has been discussed to clarify the corrosion protection mechanism when these coatings are applied to aluminum alloys. However, how the cerium compounds in the coatings act as a corrosion inhibitor is not clearly understood.

2

Organic coatings consisting of epoxy resin, which has a high adhesion property originating from hydroxyl groups and low cure shrinkage, are another prospective system. Anticorrosive pigments added to an epoxy coating have been developed to improve the corrosion protection ability of the epoxy coating [26-36]. However, the duration of the corrosion protection ability of the composite coating consisting of epoxy resin and anticorrosive pigments leaves to be desired. For example, the oxide resistance of an aluminum alloy coated with epoxy resin

SC RI PT

with porous vaterite calcium carbonate containing cerium nitrate was one or two orders of magnitude higher than that of the epoxy coating without a pigment at 3 hours of immersion in a 0.5 M NaCl solution [27]. However, the resistivity determined by electrochemical impedance measurements decreased gradually by three orders of

magnitude after immersion for 100 hours. Thus, it is important that the corrosion protection durability of the epoxy coating with porous vaterite calcium carbonate containing cerium nitrate is enhanced to obtain long-term corrosion protection ability. Needless to say, controlling the consumption of the inhibitor from the coating matrix is also an important issue, even in long-term use.

U

Inclusion of microcapsules [37-40], nanofibers [41] or hollows [42] which contain an inhibitor in the

N

coating matrix is effective for controlling the inhibitor consumption and has been studied for an advanced alternative coating system. Encapsulation of the inhibitor in microcapsules is one prospective method because

A

synthesis of microcapsules is a common process and emulsion polymerization fully protects the core inhibitors

M

with shell micelles. Although self-healing ability cannot continue once the microcapsules are ruptured, it is possible that a superior corrosion resistance can be obtained by encapsulating the inhibitor which forms the passive layer. One of benefits of encapsulation is the application of inhibitors which cannot generally be added to

D

the coating matrix because the shell component isolates the inhibitor and the coating matrix. Conventional studies

TE

of microcapsules which are incorporated in the coating matrix for a self-healing system have focused on the encapsulation of an organic resin and cross-linking agent [43]. Encapsulation of inhibitors has also been studied

EP

as a means of protecting metal substrates from corrosion [37, 44, 45]. With these microcapsules, external stress applied by scratching or abrasion is necessary to rupture the microcapsules so as to repair microcracks in the coating and/or to prevent corrosion of the metal substrate. However, there is a limit to protection of metals from

CC

corrosion by this type of composite coating because only external stress triggers the rupture of microcapsules, but the coating is also deteriorated by chemical and/or photic effects. Therefore, microcapsules containing an inhibitor in a composite coating that can release inhibitors not only in response to external stress but also other physical

A

and/or chemical effects are suitable as an advanced alternative coating system which is effective for long-term corrosion protection in various environments. Corrosion in aqueous environments causes changes in the acidity of the solution at reacting sites on metal substrates. The solution in the vicinity of the cathode becomes rather basic due to the reduction reaction of oxygen and/or protons. Therefore, providing microcapsules with pH sensitivity to the local solution is expected to be beneficial for controlling the corrosion rate and achieving long-term protection of metal substrates [46-50]. For 3

example, pH sensitive microcapsules (pH-MC) which can be ruptured in an alkaline environment protect the core materials, such as a corrosion inhibitor, until the reduction reaction of oxygen and/or protons commences. The pH-MC can release the inhibitors not only in response to external stress but also in response to pH change, and once the reduction reaction commences, the corrosion inhibitor can be released even without physical stress. A pH-MC with a microcapsule shell composed of a cross-linked polyester was designed to release the inhibitors in

SC RI PT

response to a hydrolysis reaction [51, 52]. Underfilm corrosion of a steel sheet was suppressed by adding pH-MC containing cerium nitrate to the coating matrix, and release of the cerium ion was confirmed on a corroding steel surface [52]. As mentioned above, cerium nitrate is an effective corrosion inhibitor for aluminum alloys.

However, the combination of pH-MC with cerium nitrate as the corrosion prevention container in an alternative coating system for aluminum alloys and the corrosion protection mechanism of aluminum alloys coated with the alternative coating containing cerium compounds has not been studied sufficiently.

In this study, the open circuit potential (OCP) of a bare AA2024-T3 aluminum alloy in a NaCl solution

U

containing cerium nitrate was monitored, followed by SEM-EDX analysis of the corrosion products on the alloy

N

surface. The ability and mechanism of epoxy coating with pH-MC containing cerium nitrate in corrosion protection of the aluminum alloy were investigated by electrochemical impedance spectroscopy (EIS). A salt

A

spray test (SST) of coated alloy samples with a cross-cut was also conducted to investigate whether the epoxy

M

coating with pH-MC containing cerium nitrate prevents excess blister formation on the alloy as an alternative coating system.

D

2. Experimental

TE

2.1. Materials

Aluminum alloy AA2024-T3 plates with a thickness of 1.2 mm were obtained from Q Panel Company or

EP

Chuo Kozai Co., Ltd. Their chemical composition is shown in Table 1. Sorbitane trioleate (SpanTM 85), pentaerythritol-tetra (3-mercaptopropionate) PETMP and 30 wt.% hydrogen peroxide were purchased from TCI. Fresh hydrogen peroxide was used in each experiment to avoid the deterioration of a reagent. Polyoxyethylene-

CC

20-sorbitane monolaurate (TweenTM 20) was purchased from Research Organics Inc. Mineral oil and hydrogen peroxide were purchased from Avantor Performance Materials. Cerium nitrate hexahydrate was purchased from Johnson Matthey Company. Epoxy resin (BeckopoxTM VEP 2381W/55WA), an amine hardener (BeckopoxTM EH

A

623W/80WA) and butylated urea-formaldehyde BUF resin (CymelTM U-80) were supplied by Allnex. Deionized water was used. All solvents and reagents were used without further purification.

2.2. Sample preparation The pH-MC were synthesized by water-in-oil emulsion polymerization as reported previously [52]. 12 g of sorbitane trioleate and 3 g of polyoxyethylene-20-sorbitane monolaurate were added to 200 g of mineral oil in 4

a 500 mL resin kettle as surfactants. 4 g of BUF, 4 g of PETMP, 10 g of acetone, 2 g of water, 5 g of hydrogen peroxide and 1 g of cerium nitrate hexahydrate were mixed in another beaker and then poured into the surfactants for emulsification. The solution was continuously stirred by an impeller for 30 min at 2,000 rpm. After stirring, the emulsion was sonicated for 10 min and then polymerized for 3 h at 50°C and 700 hPa. The microcapsules containing cerium nitrate were purified by filtration and centrifugation and are called Ce-MC. Blank

SC RI PT

microcapsules were also prepared, wherein water was included during the formulation instead of cerium nitrate, and are called BL-MC. The particle sizes of these microcapsules were analyzed by a scanning electron

microscope (SEM: JEOL JSM-6490LV). For the calculation of particle size, 20 microcapsules were randomly chosen from each pH-MC and image processing was performed. An inductively coupled plasma optical emission spectrometer (ICP-OES: Varian 715-ES) was used to determine the quantity of encapsulated cerium ions. For the ICP-OES measurement, a certain amount of Ce-MC was crushed by a mortar, added to a 0.1 M H2SO4 solution and sonicated for 30 min, and the solution was filtrated.

U

As samples, the aluminum alloy plates were cut to a size of 150 mm x 70 mm and blasted with sand of

N

220 mesh grit at 2.8 kg cm-2 until uniform grey surfaces were obtained. After sand blasting, the aluminum alloy plates were degreased with hexane to remove any black residue. The synthesized microcapsules were added to an

A

epoxy resin with a stoichiometric amount of a polyamide hardener. The solid component ratio of the

M

microcapsules was adjusted to 20 wt.%, and the total amount of solid in the paint was adjusted by adding water so that a consistent dry film was achieved. A drawdown bar of 200 μm was used for application of the coating to the sandblasted aluminum alloy plates so as to control the dry film thickness to 30 ± 3 µm. The film thickness was

D

measured with an electromagnetic film thickness meter (DeFelsko PosiTector 6000). The coated plates containing

TE

Ce-MC and BL-MC were allowed to cure for at least 7 days and are called Ce-coating and BL-coating, respectively. A clear coating from which the microcapsules were excluded was also applied on a plate as a

EP

reference sample and is called Ep-coating. The details of the coating and the coated aluminum alloy plates are provided in Table 2.

CC

2.3. Characterization

A specimen aluminum alloy plate with or without coating was used as the working electrode in a three-

electrode electrochemical cell consisting of the working electrode, a platinum counter electrode and a silver/silver

A

chloride reference electrode. The specimen was immersed in a 5 wt.% NaCl solution with or without 1 mM cerium nitrate, and OCP was measured continuously. EIS was also performed at the same potential as OCP by superimposing a sinusoidal voltage ±10 mV in a frequency range from 105 to 10–2 Hz. All electrochemical measurements were carried out at ambient temperature.

5

SEM and EDX analyses of the surface and cross section of a specimen bare aluminum alloy plate immersed in NaCl solution were conducted. The acceleration voltage was adjusted to 15 kV for the surface or 5 kV for the cross section. Ion milling with Ar+ ions was used for preparation of the cross-sectional sample. A cross-cut (cross-shaped) artificial defect was applied by scratching the coated specimen surface with an engraver, and an SST was conducted by spraying the specimen with a 5 wt.% NaCl solution and holding the

SC RI PT

specimen in air of 95% RH at 35°C in accordance with ASTM B117. Angle of the coated specimens was kept at 20° during SST. Visual assessments were performed after periodic intervals.

3. Results and discussion 3.1. Microcapsule synthesis

If the particle size of microcapsules is larger than the thickness of the dry film, it is thought that the

microcapsules will form coating defects and paths where water molecules and/or chloride ion approach the alloy

U

surface. Therefore, it is important to control the particle size of the microcapsules so as to be smaller than the dry

N

film thickness. Figure 1 shows SEM images of the synthesized pH-MCs. The contrast of the Ce-MC is sharper than that of the BL-MC, probably because the Ce-MC microcapsules contain Ce, which can generate secondary

A

electrons effectively. Both pH-MCs have a spherical shape with diameters ranged from 2 to 10 μm, which is

M

obviously smaller than the dry film thickness of 30 μm, regardless of containing cerium nitrates. The amount of C or S in both pH-MCs was evaluated by EDX analysis, while the amount of Ce in the CeMC was measured by ICP-OES after crushing the Ce-MC followed by soaking in sulfuric acid. The ratio of C to

D

S is associated with the amount of cross-linking agent in a shell matrix. The C/S ratios of both Ce-MC and BL-

TE

MC were 4.2 to 4.3, suggesting that the shell components of the Ce-MC and BL-MC are equivalent. The weight ratio of the encapsulated cerium nitrate in the Ce-MC was 6.8 wt.%, clearly indicating that cerium nitrate was

EP

successfully encapsulated in the shell matrix consisting of BUF and PETMP. The amount of encapsulated cerium nitrate was calculated to be 79 % of the amount added in the polymerization process. This demonstrates that most of the cerium nitrate used for microcapsule synthesis was encapsulated, whereas the remainder might have been

CC

rinsed out by water during the purification process.

3.2. Immersion test of bare aluminum alloy

A

Figure 2 shows the typical time variation of the OCP of the bare AA2024-T3 aluminum alloy plate in 5

wt.% NaCl solutions with or without 1 mM cerium nitrate. In the initial stage, the values of OCP are ca. –0.55 VSSE in both solutions. Although the change of ambient temperature in a day period might induce daily variation, the OCPs in both solutions tend to shift in a less noble direction day by day. The OCP in the cerium nitrate-free NaCl solution shows a noisy response and reaches –0.65 VSSE after 9 days, while the OCP in the cerium nitratecontaining NaCl solution is relatively stable and reaches a lower potential (–0.75 VSSE) after 9 days. The noisy 6

response of OCP in the NaCl solution suggests that the formation of metastable pitting occurs with continuously repeated depassivation and repassivation. The values of OCP in the initial stage are nearly equal to the OCP (= – 0.51 VSSE) of AA2024-T3 in a 0.6 wt.% NaCl solution reported by Boag et al. [5] The less noble shift of the OCP observed in the NaCl solution is attributed to acceleration of an anodic reaction of the aluminum alloy because a large amount of aluminum oxides were formed on the specimen surface, as will be described later. The difference

anodic reactions or cathodic reactions on the aluminum alloy surface.

SC RI PT

of OCP after immersion for 9 days suggests that the cerium ion in the solution can play a role of decelerating the

Figure 3 shows photographs of the surfaces of the aluminum alloy specimens immersed in the NaCl solutions for 9 days. The whole surface of the specimen immersed in the NaCl solution is covered with products (Fig. 3a), while the surface of the specimen immersed in the solution containing cerium nitrate (Fig. 3b) is only partially covered with products and over 80 % of the surface remained as an intact surface. The product-forming surface of the specimen immersed in the NaCl solution can be roughly divided into two parts. One forms pits and

U

a white product (assigned to A in Fig. 3a) and the other is covered with a slightly green product (assigned to B in

N

Fig. 3a). On the specimen immersed in the cerium nitrate-containing solution, pale yellow products (assigned to C in Fig. 3b) were confirmed in addition to the intact surface (assigned to D in Fig. 3b) after the removal from the

A

solution. The color of pale yellow indicates that trivalent cerium ions turned to tetravalent ions due to the natural

M

oxidation. Considering the lower OCP after immersion for 9 days, it is suggested that inhibition of the cathodic reaction suppressed the formation of products with cerium nitrate. In order to investigate the products formed during immersion in the solutions, surface and cross-sectional

D

SEM and EDX analyses were conducted. The elements Na and Cl from the electrolyte were detected at 1.0 and

TE

2.7 keV, respectively, with very weak intensities in the EDX spectra. Figure 4a and 4b show surface SEM images of the aluminum alloy immersed in the NaCl solution for 9 days. In area A in Fig. 3 (Fig. 4a), it is clear that a pit

EP

with a diameter of ca. 100 µm has formed (assigned to A1) and the residual surface (assigned to A2) is relatively flat and is covered with a number of particles. The surface of B in Fig. 3 (Fig. 4b) also has a flat morphology with a smaller number of particles (assigned to B1). Figures 4c-4e show the EDX spectra measured at A1, A2 and B1,

CC

respectively. It is apparent that Cu is present at the bottom of the pit, indicating that the anodic reaction site is strongly related to the incrassation of Cu. On the other hand, both Figs. 4d and 4e show the presence of Al and O on the surfaces, suggesting that the surface except for the pit is covered with aluminum oxides which are pitting

A

corrosion products and/or native oxide. A number of the particles on the flat surfaces of A2 and B1 are composed of not only Al and O but also Na and Cl, which were formed during sample preparation. Figure 5 show cross-sectional SEM images and EDX mappings of Al, O, Cu and Mg at A1 and A2 in Fig. 4a. In Fig. 5a, the pit seems to be composed of two large and at least two small interconnected spaces. The spaces are mainly occupied by aluminum hydroxide and/or oxide, but the large spaces are not fully occupied. It is interesting that the cavity of the upper large space is smaller than that of the lower one. Furthermore, it is clear 7

that the pit is surrounded by sub-µm particles. EDX mapping shows that Cu is present at the walls of the spaces and at particles, while Mg is observed only at the particles, indicating that the particles are remains of an IMC, such as the S-phase. It is suggested that most parts of the aluminum matrix and/or the IMC exposed to the solution were dissolved and formed a pit with aluminum hydroxide and/or oxide. Dissolution of the IMC might be selective, in that Mg and Al are easily dissolved compared with Cu or might be accompanied by redeposition of

SC RI PT

Cu, resulting in the enrichment at the walls of the spaces. Aluminum hydroxide and/or oxide form as precipitates on the Cu layer in the pit due to the change in solution acidity. On the other hand, Fig. 5b shows that the white product on the surface around the pits is composed of inner and outer layers. The inner layer is relatively uniform and has a thickness of ca. 1.2 µm, while the outer layer is nonuniform and is thicker than the inner layer. Although both layers are mainly composed of Al and O, Cu is depleted in the outer layer and Mg is concentrated at an interlayer between the inner and outer layers. It is reasonable to think that the inner layer is aluminum oxide which formed just after specimen preparation and/or immersion in the solution, and the outer layer is the

U

precipitated product formed during the immersion test. The precipitation seems to be caused by a two-step

N

reaction at other sites. The Mg interlayer formed during the first step, which might be deposition as magnesium hydroxide following the dissolution of Mg-containing phase, and the Cu-depleted aluminum hydroxide and/or

A

oxide outer layer formed during the following step, which includes alkalization of the surface. If an anodic

M

reaction occurs on an IMC such as the S-phase rather than at the aluminum matrix, the selective dissolution of the IMC can provide magnesium and aluminum ions to other surfaces and form the interlayer and outer layer, respectively. The order of precipitation might be determined by the solubility of the ions. Since the solubility

D

products of Mg(OH)2 and Al(OH)3 are 5.7x10–12 [53] and 1.1x10–33 [54], respectively, while the values of pKa of

TE

Mg(OH)2 and Al(OH)3 are 11.4 [55] and 5.9 [56], respectively, magnesium ions can diffuse further than aluminum ions and form depositions of Mg(OH)2 even at sites far from their origin. Considering the source of Mg

EP

is only the S-phase in the AA2024-T3 aluminum alloy, magnesium ions are dissolved in an early stage of immersion, followed by the deposition of Mg(OH)2 as an interlayer. After the formation of the interlayer, the deposition of aluminum hydroxide and/or oxide occurs by alkalization of aluminum ions, which are also

CC

generated from the S-phase. In any case, the cross-sectional analyses revealed the presence of a Cu layer on the pit wall and the formation of a Mg interlayer and Cu-depleted outer layer on the surface around the pit. The pale yellow product and intact area on the surface were also analyzed by the same procedure. Figure

A

6a shows an SEM image of the surface of C in Fig. 3b. It is clear that the product is composed of some particles (assigned as C1) besides the intact parts (assigned as D1). Formation of pits was also observed on the alloy surface of C1. Figures 6b and 6c are the EDX spectra at C1 and D1, respectively. It is obvious that the products at C1 contain aluminum oxide and/or cerium oxide, while D1 contains few oxides. Figures 7a and 7b show crosssectional SEM images and EDX mappings of the pit and products, respectively, observed at C1. Although the pit diameter is ca. 5 µm, which is one-tenth the size of the pit shown in Fig. 5a, the pit is covered with a lid with a 8

thickness of ca. 400 nm and filled by products. The other surface except for the pit (Fig. 6b) is also covered with a product layer whose thickness is several 100 nm, and this layer has a few cracks. EDX mapping confirmed that both the products of the pit lid and the surface layer are composed of cerium hydroxide and/or oxide. Although the pit lid is disconnected from the product layer, this seems to be due to the lack of a fragile oxide layer during sample preparation. In the pit, Cu and Ce are concentrated with Al and O in the lower and upper spaces,

SC RI PT

respectively, and Mg is detected at the interlayer between the lower and upper spaces. Cu and Ce are mutually inconsistent, and Cu seems to be present as a metal while the other substances are hydroxides and/or oxides. In any case, it is found that the presence of a significant amount of cerium oxide on the surface might be the reason why the product appears pale yellow, and the aluminum alloy in the NaCl solution containing cerium nitrate shows a less noble OCP than that in the solution without cerium nitrate. This finding follows from the fact that Ce could be precipitated on cathodic sites such as S-phase remnants, on which Al and Mg rapidly dissolved and the cathodic reaction was suppressed [16]. The formation of incrassated Cu acting as a cathodic site and of cerium

U

oxide over the pits on the aluminum oxide seems to be important for decreasing the corrosion rate of the

N

aluminum alloy. A thin Mg layer, which is probably magnesium hydroxide on incrassated Cu, might form due to

A

the reduction of water and dissolved oxygen.

M

3.3. Electrochemical measurements

Aluminum alloys coated with an epoxy coating containing pH-MC were immersed in a 5 wt.% NaCl solution for 20 days. Figure 8 shows the typical time variation of the OCP of the coated alloys. Although the OCP

D

value of the Ep-coating is relatively stable (–0.88±0.01 VSSE), the OCPs of the Ce-coating and BL-coating shift

TE

with the passage of time. The OCP of the Ce-coating shifts positively from –0.88 to –0.83 VSSE for 20 days, while the OCP of the BL-coating shifts negatively from –0.72 to –0.84 VSSE for 10 days and then shifts positively to –

EP

0.81 VSSE after an additional 10 days. At all times, the OCP of the BL-coating is noble compared with that of the Ce-coating, indicating the lower reactivity for the anodic reaction or the higher reactivity for the cathodic reaction on the BL-coating. From the same viewpoint, it is suggested that the less noble OCP of the Ep-coating compared

CC

with the OCPs of the Ce-coating and BL-coating is due to the higher reactivity for the anodic reaction or the lower reactivity for the cathodic reaction. The complicated OCP shifts of the Ce-coating and BL-coating also

A

suggest that the pH-MC in the epoxy coating varied during immersion in the solution and/or pH change. EIS measurement was also performed to investigate the interfacial structure of the coated aluminum alloy

in the solution. Figure 9 shows the Bode line plots of the coated specimens measured after immersion in the NaCl solution for 2, 10 and 20 days. Although three minima depending on frequency were confirmed in each of the phase angle curves, suggesting the presence of three time-constants, the impedance at lower frequency is related to the sum of two resistance elements and a Warburg impedance element. The impedances of the Ce-coating and BL-coating at 0.01 Hz are 2-4 x 105 Ω cm2 after 2 days, and these values are one order of magnitude smaller than 9

that of the Ep-coating. The impedances of the Ce-coating and Ep-coating are constant or increase slightly for 20 days, while that of the BL-coating decreases to 2.7 x 105 Ω cm2 at 20 days. The difference of these impedances indicates that the pH-MC contained in the coating leads to the formation of defects or pinholes in the coating, probably due to the agglomeration of pH-MC in the coating matrix, even though the particle size of the pH-MC is smaller than the dry film thickness of the epoxy coating. The slight increase in the impedance of the Ce-coating

SC RI PT

(from 1.8 x 105 Ω cm2 to 3.9 x 105 Ω cm2) suggests that the relatively lower resistance of the coating allows the corrosion to change the solution acidity, resulting in release of the contents of Ce ions from the pH-MC, which is attributed to corrosion resistance. Although the distribution of microcapsules in the coating and the pH gradient across the coating are important to verify the corrosion protection mechanism of pH sensitive microcapsules, the local pH around the S-phase remnants becomes higher than 9.5 [57], and this alkaline environment enables cleavage of the pH-MC.

The appearance of the three time-constants is maintained in the Bode line plots, although the shapes of the

U

phase angle curves change over immersion time. The equivalent circuit model shown in Fig. 10, which has

N

commonly been applied to coated aluminum alloys [58-60], was used for curve fitting of Fig. 9. Assuming that solution resistance Rs has a constant value of 30 Ω cm2, as mentioned in a previous report [52], the resistances and

A

capacitances are summarized in Fig. 11. Constant phase elements (CPEs) Qc and Qdl were used as substitutes of

M

coating capacitance and double-layer capacitance for the equivalent circuit to fit the impedance behavior more accurately. The CPE behavior is defined as follows: Z (Q) = Y0-1 (jω)-n

D

where Z is the impedance of Q, where Y0 is the CPE constant, j is the imaginary component, ω is the angular

TE

frequency and n is a CPE exponent that shows the heterogeneity or roughness of the surface [61]. CPE can represent resistance (n = 0, Y0 = R), capacitance (n = 1, Y0 = C), inductance (n = –1, Y0 = L) or Warburg

EP

impedance (n = 0.5, Y0 = W) [62]. The exponent n of Qc of Ep-coating, Ce-coating or BL-coating was over 0.9 while that of Qdl of Ep-coating, Ce-coating or BL-coating were over 0.6, reflecting surface roughness of sandblasted aluminum alloy. The exponent n of Q of Ep-coating, Ce-coating or BL-coating at low frequency was

CC

approximately 0.6, indicating that the RQ parallel circuit becomes equivalent to a simple Q element with a W element. The pore resistance Rpo of the Ce-coating and BL-coating are one or two orders of magnitude smaller than that of the Ep-coating, and the coating capacitance Qc of the Ce-coating and BL-coating are one or two

A

orders of magnitude smaller than that of the Ep-coating. The large difference in Rpo suggests that the pH-MC creates fine paths that allow penetration of the electrolyte solution in the epoxy resin matrix, even in the absence of obvious defects such as cross cuts. Since Qc is related to penetration of the electrolyte [63], the small Qc of the Ce-coating and BL-coating means that the barrier property of these coating is originally inferior. In the pull-off adhesion test of coated steel sheet instead of coated aluminum alloy sheet, there were no differences between the clear epoxy coating and the epoxy coating containing the microcapsules. It is considered that the adhesion of 10

capsules/coating is strong enough. Therefore, the epoxy coating of Ce-coating and BL-coating is stable physically, but is imperfect chemically. The Qc of the Ce-coating is larger than that of the BL-coating at 2 days, and the Qc of the Ce-coating decreases with immersion time, while that of the BL-coating increases. It is indicated that the diffusion processes which decrease cerium and nitrate ions from the Ce-MC and increase electrolytes such as sodium and chloride ions into the BL-MC result in changes in Rpo and Qc. It is also considered that the increase

SC RI PT

in film thickness of Ce-coating with immersion time due to the formation of cerium oxide and/or the swelling of epoxy coating results in decrease of Qc. The exchange of the electrolyte through the MCs in the coating might suppress or accelerate corrosion of the substrate alloy. Although the BL-MC enables continuous corrosion of the aluminum alloy, the Ce-MC forms cerium oxide following corrosion and thereby protects the alloy from further corrosion, being considered a self-healing behavior of cerium ions.

The charge transfer resistance Rct of the Ce-coating increases with immersion time and achieves a steady value larger than that of the BL-coating after 10 days, whereas the values of the BL-coating and Ep-coating are

U

relatively stable. This corresponds to the result obtained in Fig. 8, in which OCP becomes less noble over

N

immersion time. Therefore, the formation of cerium oxide on the Ce-coating alloy seems to decrease the reactivity of the cathodic reaction for aluminum alloy corrosion rather than increase the anodic reactivity. The tendency of

A

decreasing cathodic reactivity also corresponds to the OCP result for the bare aluminum alloy, in which the

M

cathodic reactivity is suppressed by the precipitation of cerium oxide on incrassated Cu.

3.4. SST

D

Figure 12 shows photographs of the aluminum alloys coated with an epoxy coating with or without pH-

TE

MC having an artificial cross-cut scratch before and after the SST for 29 and 87 days. On the Ep-coating, most of the scratched parts were seriously corroded and were covered with white products within 29 days, whereas no

EP

corrosion products and blisters were observed on the residual unscratched parts even after 87 days, indicating that both anodic and cathodic reactions intensively occurred at the scratched parts. On the Ce-coating and BL-coating, on the other hand, some scratched parts were corroded and blisters formed on unscratched parts at 29 days.

CC

Although the amounts of corrosion and blisters increased with SST time, in comparison with the BL-coating, the Ce-coating could suppress the formations of corrosion products on the scratched part and of blisters on the unscratched parts. It is clear that encapsulating cerium nitrate in the coating is effective for inhibiting corrosion of

A

the alloy. These findings are in agreement with the EIS results. The Rct of the Ce-coating increase with time, while the Rc of the Ep-coating were larger than those of the Ce-coating and BL-coating. Although the 87-day SST is a relatively severe corrosion test, the SST obviously shows that the Ce-coating is an effective system for corrosion protection of the aluminum alloy.

4. Conclusions 11

An epoxy coating with pH-MC containing cerium nitrate was developed and applied to AA2014-T3 aluminum alloy plates. Addition of cerium nitrate with a concentration of 1 mM to a 5 wt.% NaCl solution shifted the OCP of the bare aluminum alloy in a less noble direction and changed the corrosion behavior of the alloy. This change was due to the formation of cerium oxide on intermetallic phases enriched with Cu reacting as the cathode. EIS of the alloy coated with the epoxy coating including Ce-MC in the NaCl solution showed coating

SC RI PT

resistance and capacitance one or two orders of magnitude smaller than those of the alloy coated with a clear coating and increases in the charge transfer resistance in long-term immersion. SST revealed that the alloy coated with the Ce-coating shows a better corrosion protective ability, reducing the number of blisters formed over the long term. It was concluded that the Ce-coating is an effective corrosion protection system of the aluminum alloy surface.

Acknowledgments

U

The authors would like to acknowledge the support of this work by the Army Research Laboratories

N

(W911NF-11-2-0027), which generously provided funding.

A

CC

EP

TE

D

M

A

References [1] A. E. Hughes, A. Boag, A. M. Glenn, D. McCulloch, T. H. Muster, C. Ryan, C. Luo, X. Zhou, G. E. Thompson, Corr. Sci. 53 (2011) 27-39. [2] M. Bethencourt, F. J. Botana, M. J. Cano, M. Marcos, J. M. Sánchez-Amaya, L. González-Rovira, Corr. Sci. 51 (2009) 518-524. [3] A. Boag, A. E. Hughes, A. M. Glenn, T. H. Muster, D. McCulloch, Corr. Sci. 53 (2011) 17-26. [4] A. M. Glenn, T. H. Muster, C. Luo, X. Zhou, G. E. Thompson, A. Boag, A. E. Hughes, Corr. Sci. 53 (2011) 40-50. [5] A. Boag, R. J. Taylor, T. H. Muster, N. Goodman, D. McCulloch, C. Ryan, B. Rout, D. Jamieson, A. E. Hughes, Corr. Sci. 52 (2010) 90-103. [6] T. Kvackaj, R. Bidulsky´, Aluminium Alloys, Theory and Applications, InTech, Rijeka, 2011. [7] J. G. Kaufman, Aluminum Alloy Database, in: Knovel portal, updated 2009. [8] I. J. Polmear, Cast aluminium alloys, in: Light Alloys (Fourth Edition), Butterworth-Heinemann, Oxford, 2005, pp. 205-235. [9] G. M. Scamans, N. Birbilis, R. G. Buchheit, J.A.R. Editor-in-Chief: Tony (Eds.), Shreir’s Corrosion, Elsevier, Oxford, 2010, pp. 1974-2010. [10] W. J. Clark, J. D. Ramsey, R. L. McCreery, G. S. Frankel, J. Electrochem. Soc., 149 (2002) B179−B185. [11] J. S. Gandhi, W. J. van Ooij, J. Mater. Eng. Perform. 13 (2004) 475-480. [12] T. L. Metroke, J. S. Gandhi, A. Apblett, Prog. Org. Coat. 50 (2004) 231-246. [13] D. Zhu, W. J. van Ooij, Corr. Sci. 45 (2003) 2177-2197. [14] V. Palanivel, D. Zhu, W. J. van Ooij, Prog. Org. Coat. 47 (2003) 384-392. [15] M. L. Zheludkevich, R. Serra, M. F. Montemor, I. M. M. Salvado, M. G. S. Ferreira, Surf. Coat. Technol. 200 (2006) 3084-3094. [16] L. Paussa, F. Andreatta, D. De Felicis, E. Bemporad, L. Fedrizzi, Corr. Sci. 78 (2014) 215-222. [17] K.A. Yasakau, J. Carneiro, M. L. Zheludkevicha, M. G. S. Ferreira, Surf. Coat. Technol. 246 (2014) 6-16. [18] R.V. Lakshmi, G. Yoganandan, K. T. Kavya, B. J. Basu, Prog. Org. Coat. 76 (2013) 367-374. [19] K. A. Yasakau, M. L. Zheludkevich, O. V. Karavai, M. G. S. Ferreira, Prog. Org. Coat. 63 (2008) 352-361. [20] N. C. Rosero-Navarro, S. A. Pellice, A. Dura´n, M. Aparicio, Corr. Sci. 50 (2008) 1283-1291.

12

A

CC

EP

TE

D

M

A

N

U

SC RI PT

[21] M. L. Zheludkevich, R. Serra, M. F. Montemor, K. A. Yasakau, I. M. Miranda Salvado, M. G. S. Ferreira, Electrochim. Acta 51 (2005) 208-217. [22] V. Palanivel, Y. Huang, W. J. van Ooij, Prog. Org. Coat. 53 (2005) 153-168. [23] S. Kozhukharov, V. Kozhukharov, M. Schem, M. Aslan, M. Wittmar, A. Wittmar, M. Veith, Prog. Org. Coat. 73 (2012) 95-103. [24] L. M. Palomino, P. H. Suegama, I. V. Aoki, M. F. Montemor, H. G. De Melo, Corr. Sci. 51 (2009) 12381250. [25] L. M. Palomino, P. H. Suegama, I. V. Aoki, M. F. Montemor, H. G. De Melo, Corr. Sci. 50 (2008) 12581266. [26] S. J. García, H. R. Fischer, P. A. White, J. Mardel, Y. González-García, J. M. C. Mol, A. E. Hughes, Prog. Org. Coat. 70 (2011) 142-149. [27] D. Snihirova, Sviatlana V. Lamaka, M. F. Montemor, Electrochim. Acta 83 (2012) 439-447. [28] A. C. Balaskas, I. A. Kartsonakis, L. A. Tziveleka, G. C. Kordas, Prog. Org. Coat. 74 (2012) 418-426. [29] T. Hu, H. Shi, S. Fan, F. Liu, E. Han, Prog. Org. Coat. 105 (2017) 123-131. [30] M. L. Zheludkevich, S. K. Poznyak, L. M. Rodrigues, D. Raps, T. Hack, L. F. Dick, T. Nunes, M. G. S. Ferreira, Corr. Sci. 52 (2010) 602-611. [31] X. Yuan, Z. F. Yue, Z. Q. Liu, S. F. Wen, L. Li, T. Feng, Prog. Org. Coat. 90 (2016) 101-113. [32] M. Yan, C. A. Vetter, V. J. Gelling, Corr. Sci. 70 (2013) 37-45. [33] A. C. Balaskas, I. A. Kartsonakis, D. Snihirova, M. F. Montemor, G. Kordas, Prog. Org. Coat. 72 (2011) 653-662. [34] M. Jiang, L. Wu, J. Hu, J. Zhang, Corr. Sci. 92 (2015) 118-126. [35] I. A. Kartsonakis, E. Athanasopoulou, D. Snihirova, B. Martins, M. A. Koklioti, M. F. Montemor, G. Kordas, C. A. Charitidis, Corr. Sci. 85 (2014) 147-159. [36] I. A. Kartsonakis, E. P. Koumoulos, A. C. Balaskas, G. S. Pappas, C. A. Charitidis, G. C. Kordas, Corr. Sci. 57 (2012) 56-66. [37] M. Kope´c, K. Szczepanowicz, G. Mordarski, K. Podgórna, R. P. Socha, P. Nowak, P. Warszy´nski, T. Hack, Prog. Org. Coat. 84 (2015) 97-106. [38] D. Raps, T. Hack, M. Kolb, M. L. Zheludkevich, O. Nuyken, in: J. Baghdachi, T. Provder (Eds.), Smart Coatings III – ACS Symposium Series, American Chemical Society, Washington, DC, 2010, pp. 165-189. [39] A. Latnikova, D. Grigoriev, M. Schenderlein, H. Mohwald, D. Shchukin, Soft Matter 8 (2012) 1083710844. [40] E. Koh, S. Y. Baek, N. K. Kim, S. Lee, J. Shin, Y. W. Kim, New J. Chem. 38 (2014) 4409-4419. [41] A. Yabuki, T. Shiraiwa, I. W. Fathona, Corrosion Science 103 (2016) 117-123. [42] Y. C. Feng, Y. F. Cheng, Corros. Eng. Sci. Technology, 51 (2016) 489-497. [43] R.L. White, N.R. Sottos, P.H. Geubelle, J.S. Moore, M.R. Kessler, S.R. Sriram, E.N.Brown, S. Viswanathan, Nature 409 (2001) 794-797. [44] H. Wang, R. Akid, Corr. Sci. 50 (2008) 1142-1148. [45] S. H. Boura, M. Peikari, A. Ashrafi, M. Samadzadeh, Progress in Organic Coatings 75 (2012) 292-300. [46] N. P. Tavandashti, M. Ghorbania, A. Shojaeia, J. M. C. Mold, H. Terrynd, K. Baert, Y. Gonzalez-Garcia, Corr. Sci. 112 (2016) 138-149. [47] Z. Zheng, M. Schenderlein, X. Huang, N. J. Brownbill, F. Blanc, D. Shchukin, ACS Appl. Mater. Interfaces 7 (2015) 22756-22766. [48] M. L. Zheludkevich, J. Tedim, M. G. S. Ferreira, Electrochim. Acta 82 (2012) 314-323. [49] T. Chen, J. J. Fu, Nanotechnology 23 (2012) 1-12. [50] H. Choi, K.Y. Kim, J. M. Park, Prog. Org. Coat. 76 (2013) 1316-1324. [51] W. Li and L.M. Calle, Proc. 1st. Int. Conf. on Self Healing Materials (2007) 1-10. [52] T. Matsuda, N.Jadhav, K.B. Kashi, M. Jensen, A. Suryawanshi and V.J. Gelling, Prog. Org. Coat. 90 (2016) 425-430. [53] R. C. Weast, M. J. Astle, W. H. Beyer, CRC Handbook of Chemistry and Physics 66th Edition, CRC Press Inc, Boca Raton, Florida, B-222 (1985) [54] K. H. Gayerl, L. C. Thompson, O. T. Zajieck, Can. J. Chern. 36 (1958) 1268. 13

SC RI PT

[55] I. Bertini, H. B. Gray, E. I. Stiefel, J. S. Valentine, Biological Inorganic Chemistry Structure and Reactivity 1St Edition, University Science Books, Sausalito, California, 176 (2006) [56] M. E. Essington, Soil and Water Chemistry 2nd Edition, CRC Press Inc, Boca Raton, Florida, 223 (2015) [57] R. Oltra, B. Vuillemin, F. Rechou, C. Henon, Electrochem. Solid-State Lett. 12 (2009) C29–C31. [58] M. Yan, C. A. Vetter, V. J. Gelling, Corr. Sci. 70 (2013) 37-45. [59] M. Yan, C. A. Vetter, V. J. Gelling, Electrochim. Acta 55 (2010) 5576-5583. [60] N. Jadhav, C. A. Vetter, V. J. Gelling, Electrochim. Acta 102 (2013) 28-43. [61] D. A. Lopez, S.N. Simison, S.R. de Sanchez, Electrochim. Acta 48 (2003) 845-854. [62] M. S. Morad, Corros. Sci. 42 (2000) 1307-1326. [63] X. Liu, J. Xiong, Y. Lv, Y. Zuo, Prog. Org. Coat. 64 (2009) 497-503.

A

CC

EP

TE

D

M

A

N

U

Figure captions Fig. 1. SEM images of pH-MC encapsulated (a) with cerium nitrate (Ce-MC) and (b) without inhibitor (BL-MC). Reproduced with permission from Prog. Org. Coat. 90 (2016) 425-430. Copyright © 2016, Elsevier. Fig. 2. Time-variation in OCP of aluminum alloy immersed in NaCl solution with or without cerium nitrate. Fig. 3. Photographs of aluminum alloy surfaces after OCP measurement for 9 days in (a) 5 wt.% NaCl solution and (b) the NaCl solution with 1 mM cerium nitrate. Positions of A-D are specified pit and white product, green product, pale yellow product, and intact part, respectively. Fig. 4. SEM images of aluminum alloy surface at (a) A and (b) B specified in Fig. 3a. EDX spectra measured at (c) A1, (d) A2 and (e) B1. Fig. 5. Cross sectional SEM images of corroded aluminum alloy at (a) A1 in Fig. 4a and (b) B1 in Fig. 4b. Distributions of Al, O, Cu, and Mg obtained by EDX were also mapped. Fig. 6. (a) SEM image of aluminum alloy surface at yellow product of Fig. 3b. EDX spectra measured at (b) C1 and (c) D1 in Fig. 6a. Fig. 7. Cross sectional SEM images of corroded aluminum alloy at (a) C1 and (b) D1. Distribution of Al, O, Cu, and Mg obtained by EDX was also mapped. Fig. 8. Time-variation in OCP of coated specimens; △: Ep-coating, ×: BL-coating, ○: Ce-coating immersed in 5 wt.% NaCl solution. Fig. 9. Bode line plots of coated specimens; △: Ep-coating, ×: BL-coating, ○: Ce-coating after immersion in 5 wt.% NaCl solution for (a) 2, (b) 10, and (c) 20 days. Fig. 10. Equivalent circuit model consisting of solution resistance Rs, pore resistance Rpo, charge transfer resistance Rct, coating capacitance (Cc), double-layer capacitance (Cdl) and Warburg impedance Ws. Constant phase elements Qc and Qdl are substitutes of Cc and Cdl. Fig. 11. Time-variations of (a) Rpo, (b) Rct, (c) Qc and (f) Qdl of coated specimens; △: Ep-coating, ×: BL-coating, ○: Ce-coating. Fig. 12. Photographs of specimens coated (a) Ep-coating, (b) BL-coating and (c) Ce-coating after SST for 29 and 87 days. Scale bars indicate the length of 50 mm.

14

Figures

U

SC RI PT

Figure 1. SEM images of pH-MC encapsulated (a) with cerium nitrate (Ce-MC) and (b) without inhibitor (BLMC). Reproduced with permission from Prog. Org. Coat. 90 (2016) 425-430. Copyright © 2016, Elsevier.

EP

TE

D

M

A

N

Figure 2. Time-variation in OCP of aluminum alloy immersed in NaCl solution with or without cerium nitrate

A

CC

Figure 3. Photographs of aluminum alloy surfaces after OCP measurement for 9 days in (a) 5 wt.% NaCl solution and (b) the NaCl solution with 1 mM cerium nitrate. Positions of A-D are specified pit and white product, green product, pale yellow product, and intact part, respectively.

15

SC RI PT

CC

EP

TE

D

M

A

N

U

Figure 4. SEM images of aluminum alloy surface at (a) A and (b) B specified in Fig. 3a. EDX spectra measured at (c) A1, (d) A2 and (e) B1.

A

Figure 5. Cross sectional SEM images of corroded aluminum alloy at (a) A1 in Fig. 4a and (b) B1 in Fig. 4b. Distributions of Al, O, Cu, and Mg obtained by EDX were also mapped.

16

SC RI PT

EP

TE

D

M

A

N

U

Figure 6. (a) SEM image of aluminum alloy surface at yellow product of Fig. 3b. EDX spectra measured at (b) C1 and (c) D1 in Fig. 6a.

A

CC

Figure 7. Cross sectional SEM images of corroded aluminum alloy at (a) C1 and (b) D1. Distribution of Al, O, Cu, and Mg obtained by EDX was also mapped.

17

SC RI PT

TE

D

M

A

N

U

Figure 8. Time-variation in OCP of coated specimens; △: Ep-coating, ×: BL-coating, ○: Ce-coating immersed in 5 wt.% NaCl solution.

A

CC

EP

Figure 9. Bode line plots of coated specimens; △: Ep-coating, ×: BL-coating, ○: Ce-coating after immersion in 5 wt.% NaCl solution for (a) 2, (b) 10, and (c) 20 days.

18

SC RI PT U

EP

TE

D

M

A

N

Figure 10. Equivalent circuit model consisting of solution resistance Rs, pore resistance Rpo, charge transfer resistance Rct, coating capacitance (Cc), double-layer capacitance (Cdl) and Warburg impedance Ws. Constant phase elements Qc and Qdl are substitutes of Cc and Cdl.

A

CC

Figure 11. Time-variations of (a) Rpo, (b) Rct, (c) Qc and (f) Qdl of coated specimens; △: Ep-coating, ×: BL-coating, ○: Ce-coating.

19

SC RI PT U N

A

CC

EP

TE

D

M

A

Figure 12. Photographs of specimens coated (a) Ep-coating, (b) BL-coating and (c) Ce-coating after SST for 29 and 87 days. Scale bars indicate the length of 50 mm.

20

Tables Table 1. Composition of aluminum alloy (2024 T3). content / wt.% Fe

Mg

Mn

Si

Zn

4.4

0.5

1.5

0.6

0.5

0.25

Ti

Cr

Al

SC RI PT

Cu

0.15

Supplier

balance

Q Panel Company

0.2

1.5

0.5

0.1

0.02

0.02

Chuo Kozai co., ltd

U

4.6

Ep-coating

A

N

Table 2. Coating composition of specimen.

Ce-coating

BL-coating

62

62

18

18

78

Polyamide hardener / wt.%

22

Content in MC

-

Ce(NO3)3

H2O

0

20

20

32

33

D TE

/ wt.%

29

A

CC

EP

Dry film thickness / µm

M

Epoxy resin / wt.%

21

balance