Al metal matrix composite

Thin Solid Films 519 (2011) 4759–4764

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Thin Solid Films j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / t s f

Corrosion protection and formation mechanism of anodic coating on SiCp/Al metal matrix composite Chunlin He ⁎, Deyuan Lou, Jianming Wang, Qingkui Cai Institute of Surface Engineering, Shenyang University, Shenyang 110044, PR China Liaoning Key Laboratory of Advanced Materials Preparation Technology, Shenyang University, Shenyang 110044, PR China

a r t i c l e

i n f o

Available online 12 January 2011 Keywords: Al Metal matrix composite SiC Anodizing Corrosion resistance Growth mechanism

a b s t r a c t The corrosion protection from sulfuric acid anodized coatings on 2024 aluminum and SiC particle reinforced 2024 aluminum metal matrix composite (SiCp/2024Al MMC) in 3.5 wt.% NaCl aqueous solution was investigated using electrochemical methods. The results show that the anodized coating on 2024Al provides good corrosion protection to 3.5 wt.% NaCl, and the anodized coating on the SiCp/2024Al MMC provides some corrosion protection, but it is not as effective as for 2024Al because non-uniformity in thickness and cavities present are associated with the SiC particulates. Cavities above SiC particles are the reason that the anodized coating on the MMC cannot be completely sealed by hot water as with anodic Al alloy. SiC particle anodizes at a significantly reduced rate compared with the adjacent Al matrix. This gives rise to alumina film encroachment beneath the particle and occlusion of the partly anodized particle in the coating. It was found that the barrier layer of anodized Al MMC is not continuous, and it is composed primarily of the barrier layer of anodized Al matrix and a barrier-type SiO2 film on occluded SiC particles in the coating. A new formation mechanism of coating growth during anodizing of a SiCp/2024Al MMC was proposed. © 2011 Elsevier B.V. All rights reserved.

1. Introduction The reinforcement of aluminum alloys with SiC particulates leads to a new generation of engineering materials with a higher strength to weight ratio, stiffness and modulus. However, the addition of the reinforcement particles could significantly alter the corrosion behavior of these materials. Al MMCs are generally susceptible to corrosion in various environments, due to either galvanic reactions between the reinforcements and the matrix, selective corrosion at the interface because of the formation of new compounds, or to the defects at the interface, causing fissures which create pathways for corrosion [1]. Therefore, Al MMCs are generally required to have a protective coating on the surface to enhance their corrosion resistance in hostile environments. While there is a wealth of information concerning the corrosion behavior of Al alloys and methods of corrosion protection, relatively little is known about the corrosion behavior and effectiveness of corrosion protection methods of Al MMCs. Recently, reports about corrosion protection of Al MMCs have increased. Effective corrosion protection methods include anodizing [2–6], chemical passivity [3,7–9] and polymer coatings [1,5,10]. However, these methods for Al MMCs are not as effective as for Al alloys [5,6]. Of all the corrosion protection methods, anodizing has been potentially considered as one of the best methods of protecting Al MMCs [2–6]. Trzaskoma et al. [11] investigated

⁎ Corresponding author. Tel.: +86 24 62266139; fax: +86 24 62505953. E-mail address: [email protected] (C. He). 0040-6090/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2011.01.030

the corrosion resistance of SiC/Al MMC after anodizing in sulphuric acid electrolyte, and found that the anodized porous hardcoat significantly improved the corrosion resistance of the MMC. Mansfeld and Jeanjaquet [5] evaluated the corrosion resistance of anodized SiC/Al and graphite/Al MMCs during exposure to NaCl solution using electrochemical impedance spectroscopy. Visual observation of the samples after the test did not show corrosion damage on the surface. Chen and Mansfeld [2] evaluated the protection efficiency of coatings on SiC/Al 6092 MMC formed by Ce–Mo process and anodizing, and the anodized surfaces were sealed by immersion in hot water, cerium nitrate and two different dichromate solutions. They found significant improvements in corrosion resistance only for the anodized surface with a dichromate seal. So far, however, only limited success has been achieved with anodization because of multiple problems. Very little is known about the effect of SiC particles on the growth mechanism of coating during anodizing of SiC/Al MMCs in sulfuric acid. Lin et al. [6] suggested the possibility of cessation of growth of pores when they met SiC particles in the Al matrix of MMC. However, this is not in agreement with the cross sectional morphology of anodic coating on the Al MMCs observed by others [4,12]. Based on SEM and TEM observation, they proposed that only part of the pores above SiC particles are terminated while the others are deflected and branched, growing around the particles [4,12]. This leads to the encroachment of the porous anodic alumina beneath the particle and eventual occlusion of the particle in the anodic film. However, the effect of oxidization of SiC particles on growth mechanisms of porous coating was not taken into account [4,12]. It is suggested that the barrier layer of anodized SiC/Al MMC is discontinuous [6,12,13], but neither proof nor

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explanation could be given. Currently, much work is reported about anodizing of SiC compound in HF acid in order to form a porous material [14,15]. To our knowledge, however, no work has been reported on the effect of both SiC particles and their anodizing on the growth mechanism and corrosion resistance of anodic coatings formed on Al MMCs in sulfuric acid. The aim of the present study was to evaluate the corrosion resistance and to propose the formation mechanism of anodized coating on a 17% SiCp/2024 MMC in terms of oxidation of SiC particles. 2. Materials and methods The materials studied were alloy 2024 and SiCp/2024 MMC as rolled sheet prepared by a powder metallurgy technique, and the details on sample preparation were given elsewhere [16]. The final sheet was approximately 1.2 mm in thickness. The composition of the 2024Al alloy used was with 4.2% Cu, 1.5% Mg, 0.7% Mn, and 0.2% Fe, bal. Al. The MMC contained 17% volume fraction of 3.5 μm SiC particulates. Anodized samples were prepared by a sulfuric acid anodizing process. Prior to anodizing, the surfaces were abraded on 800# emery paper, and were then immersed in an alkaline solution at 60–70 °C for 10 min, deoxidized at 50–60 °C for 5 min in a deoxidizing solution, and cleaned thoroughly. The anodizations were carried out at 22 °C and fixed current density from 5 mA/cm2 to 20 mA/cm2 for 40 min in 16 wt.% H2SO4 aqueous solution. The anodized samples were sealed in hot water at 95–100 °C for 30 min. The untreated specimens were mounted in epoxy, mechanically ground followed by polishing with 3.5 μm diamond suspensions, rinsed with distilled water and acetone, air dried, and stored in a desiccator until used. The exposed areas of the untreated and anodized samples were about 2.0 cm2 and 2.5 cm2, respectively, and the edges at the epoxy mold were masked with paraffin wax to prevent crevice corrosion. Solutions were prepared with analytical grade NaCl and distilled water. During the measurement, the solutions were quiescent and open to the air. Electrochemical measurements were made at 25 °C in a threeelectrode cell containing 3.5% NaCl aqueous solution. The instruments used were a Solartron 1250 frequency response analyser and an EG&G Model 273 potentiostat. A saturated calomel electrode (SCE) was used as the reference electrode, and the counter electrode was graphite. Before electrochemical measurements, the samples were immersed in solution for approximately 2 h until a steady open circuit potential was recorded. Potentiodynamic polarization tests were carried out at a scanning rate of 0.166 mV/s. Electrochemical impedance was measured by perturbing the open circuit potential of the specimens with a 10 mV ac signal with a frequency decreasing from 105 Hz to 5 mHz. For studies of the cross-section of the anodized specimen using scanning electron microscopy (SEM), the sample was embedded in epoxy, polished to 0.5 μm diamond suspensions and lightly coated with carbon. 3. Results and discussion 3.1. Morphology of anodized coating Fig. 1 shows the cross-section of the anodized 17% SiCp/2024Al MMC formed at current densities of 5 mA/cm2 and 25 mA/cm2, respectively. The light regions within the anodic coating in Fig. 1 are believed to be SiC particles, EDS analysis revealed silicon, oxygen and carbon [12]. This indicates that the SiC particles in the anodic coating are oxidized. The relatively dark regions within the cross-section of the anodic coating above SiC particles represent cavities which are believed to be generated as a result of much slower rate of anodization of SiC than Al alloys in sulfuric acid and electric field assisted dissolution of the porous layer of alumina at the pore base above the SiC particle. Similarity to anodizing of Si [17], anodic films on SiC may be of limited

Fig. 1. SEM micrographs of cross-section of anodized MMC formed at different current densities, showing thinner film occurring at the positions with more SiC and thicker film at those with fewer SiC particles. (a) 5 mA/cm2; (b) 25 mA/cm2.

thickness and do not appear to be porous. As shown in Fig. 1, the cavities observed above the partially oxidized SiC particles have sizes and shapes dependent on the morphology of the upper surface of particle (see the A point in Fig. 1b), current density applied (Fig. 1), and aggregation of particles (see the B region in Fig. 1b). It is found that larger current densities tend to cause lager cavities. The aggregation of particles will also influence the morphology of cavities and the quality of the oxide layers, due to possible overlapping of particles' zone of influence (see the B region in Fig. 1b). FratilaApachitei et al. [18] also found the similar phenomenon when they studied the morphology of anodic film formed on AlSi(Cu) alloys. Cavities observed above the partially oxidized SiC particles were present throughout the anodic oxide film thickness, reflecting the distribution of SiC particles in the alloy (Fig. 1). Clearly, fine SiC particles were readily occluded in the oxide film, whereas coarse particles required increased anodic oxidation of the adjacent aluminum matrix, i.e. prolonged anodizing times. Generally, with increased anodizing time more defects are evident due to partially oxidized SiC and its associated cavity. Either for SiC particles of complex shape or for aggregation of particles (see the B region in Fig. 1b), metallic aluminum was occasionally revealed beneath the particles or aggregation, but within the anodic alumina film, indicating significant masking of the underlying substrate by the particles. Others have reported similar results [17]. Fig. 1 clearly shows the presence of SiC particles at the oxide/alloy interface and the effect of SiC particles on the thickness of the coating. The oxidation of individual particles has proceeded at a significantly reduced rate compared with the aluminum matrix because of more difficulty in anodizing SiC and lower conductivity of SiC. The more rapid oxidation of the matrix leads to the encroachment of the porous anodic alumina beneath the particle and eventual occlusion of the particle in the anodic film. As a consequence, the oxide/substrate interface became locally scalloped, and the anodized coating formed on the MMC was non-uniform in thickness. Generally, thicker coating

C. He et al. / Thin Solid Films 519 (2011) 4759–4764

grows in areas of low particle density (for example, the direction of the arrow D pointing in Fig. 1b), whereas thinner coating grows in areas of high particle density (for example, the direction of the arrow C pointing in Fig. 1b) because resistance is offered to the growth of porous layer by the particles. A similar phenomenon was found when AlSi(Cu) alloys were anodized [18]. Fig. 1 also shows that the coating formed at lower current density for the same length of time is thin and non-uniform in thickness, but the surface is compact, the size of the cavity above the SiC particle is small, and the density of the cavity is low. The microstructural feature of the anodized coating formed at lower current density makes the coating exhibit better corrosion resistance compared with that at higher current density, as shown in Fig. 2. This figure shows that the corrosion current density of anodic film formed at 5 mA/cm2, 2.9 × 10−9 A cm−2, is obviously lower than that formed at 25 mA/cm2, 1.9 × 10−8 A cm−2, and the corrosion potential for the former anodic film is also lower than that for the latter. The lower corrosion rate is associated with the difference in cathodic behavior and not the anodic behavior, reflecting the protection of the anodic film as shown in Fig. 2. 3.2. Formation mechanism of anodized coating The principal electrochemical reactions occurring during anodization of aluminum have been studied in detail [19–21]. Al3+ ions form at the metal/oxide interface Al→Al

3þ

þ 3e

−

ð1Þ

and migrate into the oxide layer. At the oxide/electrolyte interface the water-splitting reaction þ

2−

3H2 O→6H þ 3O

ð2Þ

occurs and is rate-determining [22]. The O2− ions migrate, due to the electric field, within the oxide from the oxide/solution interface toward the metal/oxide interface, to form Al2O3: 3þ

2Al

2−

þ 3O

→Al2 O3

ð3Þ

The protons in the electrolyte can locally dissolve the oxide at the oxide/electrolyte interface: þ

3þ

Al2 O3 þ 6H →2Al

þ 3H2 O:

ð4Þ

Since the microstructure of across-section of the anodized layer, as determined by SEM, and the structure of the anodized layer, as determined by impedance spectra for the SiCp/2024Al MMC(as shown in Figs. 4 and 7 below), are different from that for the 2024Al, the

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mechanism for the formation of anodic coating must be different due to SiC particulates. Based on growth mechanisms for anodic coatings on SiC/Al MMCs proposed in the literature [4,6,12], a new mechanism is assumed to consist of the following steps. During anodizing of matrix, the barrier layer is formed on the Al metal surface according to reactions (1) through (3) with reaction (2) as the predominating step in acidic solution, and pores form in the outer part of the barrier layer due to the dissolution of Al2O3 in the acid solution according to reaction (4). This starts the growth of the porous layer. The pores grow perpendicular to the surface with the barrier layer being continually converted into the porous layer and the new barrier layer being formed at the metal/coating interface. With further anodizing, the alloy/coating interface recedes and the SiC particles are encountered. The aluminum matrix adjacent to the particle will recede more rapidly, as shown in Fig. 1. Further, the SiC particles oxidize beneath an overlying alumina film as a result of O2− ingress under the high field. The growth of anodic SiO2 proceeds with oxygen generation, which was observed during anodizing, presumably as a result of the semi-conducting nature of the Si–O bond [23]. As a consequence, oxygen gas-filled voids develop above the oxidizing particles. The anodic SiO2 film and the presence of oxygen gas-filled voids represent pathways of high effective resistance for continued film growth. In adjacent matrix regions, comparatively rapid film growth proceeds with the result that the SiC particles, their filmed surface and the enveloping oxygen gas-filled voids are occluded in the porous anodic alumina film. With further anodizing, an increasing volume fraction of occluded, partially anodized SiC particles is present in the film. In areas remote from SiC particles, regular porous anodic coating develops [12,17,18]. Because the SiC particles are distributed randomly and the number of the SiC particles is large in the matrix, the film thickness is non-uniform. Now it can be inferred that the structure of anodized coating formed on the MMC is distinctly different from that of anodized Al and Al alloys, its barrier layer is not continuous, and is mainly composed of the barrier layer of the anodized Al matrix and a barrier-type SiO2 film on occluded SiC particle in the coating. Thus the SiC particles in the SiCp/2024Al MMC not only impede the process of anodizing, but also introduce defects and do great damage to the integrality and uniformity of the film. No doubt, this can make the corrosion resistance of the anodized coating bad. 3.3. Corrosion resistance of the anodized coatings The typical anodic polarization curves for anodized 2024Al and SiCp/2024Al MMC at current density of 5 mA/cm2 and blank samples in 3.5 wt.% NaCl are shown in Fig. 3. Table 1 is the corrosion current densities obtained from polarization curves in Fig. 3. It is clearly shown from Table 1 that the anodized coatings provide good

1.0

0.8 0.6

5mA cm-2 25mA cm-2

0.5

E/V

E/V

0.2 0.0

1

1.Anodized 2024Al 2.Anodized MMC 3.Untreated 2024Al 4.Untreated MMC

0.4

2

0.0 -0.2 -0.4

-0.5

-0.6 1 -1.0 10-10

-0.8 10-9

10-8

10-7

10-6

10-5

10-4

10-3

10-2

I /A cm-2

-15

10

-14

10

-13

10

-12

10

-11

10

-10

10

10

-9

-8

10

I/A Fig. 2. Potentiodynamic polarization curves of the anodized MMCs as a function of current densities applied.

3

2

-1.0

-7

10 cm-2

-6

10

10

4 -5

10-4 10-3 10-2 10-1

Fig. 3. Potentiodynamic polarization curves for the materials in 3.5% NaCl.

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Table 1 The corrosion current densities obtained from polarization curves in Fig. 3. Materials

2024Al

MMC

Anodized 2024Al

Anodized MMC

Icorr/A cm−2

1.7 × 10−6

6.6 × 10−6

6.8 × 10−10

2.9 × 10−9

corrosion protection to 3.5% NaCl for both 2024Al and the MMC, and their corrosion current densities are all 3 orders of magnitude lower than those of the corresponding untreated samples. Like the different corrosion resistance provided by the untreated MMC and the matrix shown from Table 1, the anodized coating on the MMC is also not as effective as for 2024Al formed under the same anodizing condition. For the untreated MMC, the presence of SiC particles is beneficial to increase the number of CuAl2 particles, and this can result in a decrease in corrosion resistance of the MMC because of galvanic corrosion between CuAl2 phase and the Al matrix [1,24]. Moreover, galvanic corrosion between the SiC and the matrix can also appear [1,24]. As to the anodized MMC, the lower corrosion resistance compared with that for anodized 2024Al alloy results from both the cavities present above the SiC particles and the non-uniformity in coating thickness due to oxidation of SiC particles (Fig. 1). Electrochemical impedance spectroscopy is a very sensitive technique for evaluation of corrosion resistance of anodized coatings and polymer coatings on aluminum surfaces [2,25,26]. Fig. 4 shows the impedance spectra obtained during exposure to 3.5% NaCl for 67 days at 25 °C for the anodized 2024Al at current density of 5 mA/ cm2. The spectra show the capacitance Cp of the porous layer in the high frequency region, the capacitance Cb of the barrier layer in the low frequency region, and the resistive component Rp of the porous layer in the frequency range between 1 Hz and 50 Hz. Similar impedance data have been reported for anodized 6061Al [6]. The

(a)

108 2h 8d 49d 56d 61d 67d

106 105

10-4

10-9

2

0

10

20

10-2

10-1

100

101

102

103

104

105

40

50

60

70

80

50

60

70

80

(b) 10-4

-90 -80

MMC 2024Al

-70

Cp/F cm-2

Phase Angle /degree

30

t/d

f / Hz

(b)

10-6

10-8

103

101 10-3

MMC 2024Al

10-7

104

10

10-3

10-5

107

|Z|/Ω cm2

impedance values are almost unchanged with increasing immersion time before 49 days, but there is a second phase peak at the lowest frequencies after 56 days immersion indicating that pitting occurs, and the impedance at the lowest frequencies greatly decreases (Fig. 4). However, only one to three small pits are easily seen by the naked eye on the anodized sample before 56 days. The number of pits observed by the naked eye increases to 4 to 7 when the coating is immersed in NaCl solution for 61 days. More pits are observed with optical microscopy. These results demonstrate that anodized coatings produced in sulfuric acid followed by hot water sealing provide good corrosion resistance for 2024Al. After 56 days, the spectra show capacitive behavior in wide range frequency. Because of the double structure of the anodic coating formed on Al alloys, the impedance data could be analyzed using the equivalent circuit in Fig. 5 [13]. Here Rs and Rb are the resistances of the solution and the barrier layer, respectively. Calculations utilizing the model

Cp/F cm-2

(a)

Fig. 5. The equivalent circuit for anodized aluminum alloys.

-60 -50 -40 2h 8d 49d 56d 61d 67d

-30 -20 -10 0 10-3

10-5

10-2

10-1

100

101

102

103

104

105

f/ Hz Fig. 4. Bode plots for anodized and hot water sealed 2024Al as a function of exposure time to 3.5% NaCl.

10-6

0

10

20

30

40

t /d Fig. 6. Time dependence of Cp (a) and Cb (b) for the anodized materials during exposure to 3.5% NaCl.

C. He et al. / Thin Solid Films 519 (2011) 4759–4764

(Fig. 5) show that both Cp and Cb are almost unchanged after immersion in 3.5% NaCl solution for 49 days as shown in Fig. 6. This is because both the porous layer and the barrier layer possess a compact structure after the coating was sealed in hot water at 95–100 °C for 30 min, and it is more difficult for NaCl solution to penetrate the porous and the barrier layers. The increase in interfacial area between the porous layer (or barrier layer) and the solution, which is responsible for increasing Cp (or Cb) according to the model of a parallel-plate capacitor with a dielectric between its plates, is not significant within 49 days, explaining the almost changeless trends in Cp (or Cb) during this period. Moreover, the unchanged values of Cp and Cb along with the almost unchanged surfaces of anodic coatings observed during exposure to 3.5% NaCl solution indicate that the coating thickness is thinning at an obviously slow rate before the pits are observed by the naked eye on the coating surface. Also because of the compact structure of the anodic coating, the decrease in Cp at the initial stages of immersion, which may be due to the slow hydration of Al2O3 to form Al(OH)3 and thus sealing or blocking some of the pores in the porous alumina film, does not seem to be obvious as shown in Fig. 6. When the exposure times in 3.5% NaCl exceed 56 days, both the Cp and the Cb greatly increase, indicating that pitting takes place on the coating surface. The appearance of small pits is an indication of increasing the interfacial area between the porous film and the solution, and this can cause an increase in Cp. Fig. 7 shows the impedance spectra obtained during exposure of the anodized MMC, formed at current density of 5 mA/cm2, to 3.5% NaCl for 74 days at 25 °C. The shape of these spectra is similar to that for anodized SiCp/6061Al [6], but differs from that for anodized 2024Al (Fig. 4). The impedance is distinctly smaller than that of anodized 2024Al in the whole frequency region (Figs. 4 and 7). This indicates that the corrosion resistance of the anodized MMC may be

(a)

107 2h 6d 14d 23d 29d 49d 56d 74d

106

|Z| / Ω cm2

105 104 103 102 101 100 10-3

10-2

10-1

100

101

102

103

104

105

102

103

104

105

f / Hz

(b) -80 Phase Angle /degree

-60 -40 -20

2h 6d 14d 23d 29d 49d 56d 74d

0 20 40 60 10-3

10-2

10-1

100

101

f / Hz Fig. 7. Bode plots for anodized and hot water sealed SiCp/2024Al MMC as a function of exposure time to 3.5% NaCl.

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much lower. The impedance behavior for the anodized MMC with phase angle close to 45° is similar to the transmission line impedance found for porous electrodes [27]. This result suggests that the anodized surface has a porous structure as a result of the presence of SiC particles which are not covered by a continuous oxide [13]. Because the anodic coating was sealed in hot water as with anodic Al alloy, the porous structure is believed to result from a much slower rate of oxidation of SiC than Al alloys in sulfuric acid [12], and the anodized coating cannot be completely sealed by hot water as with anodic Al alloy. Maybe just because of the porous structure which is mainly associated with oxidation of SiC particles (Fig. 1), the anodized coating on the MMC exhibits lower corrosion resistance than that of anodic Al alloy (Figs. 4 and 7). Furthermore, for the MMC, phase angle tends toward zero at low frequencies indicating that the resistance of the barrier layer is being approached. However, for the anodized 2024 alloy, this seems to appear only after 56 days, i.e. when pit corrosion has taken place (Fig. 4). This further indicates that the barrier layer of anodized MMC may be discontinuous or less effective. The appearance of a second phase angle in the low frequency region after 14 days indicates that pitting has occurred (Fig. 7b). However, pits were not visible by naked eye observation of the surface at this time. The pits gradually grew with exposure time, they were easily visible after 23 days, but there were only one to two small pits. Even after immersion exposure for 39 days, there were still very few pits. However, after exposure for 49 days, many pits appeared, and after 56 days the surface of the anodic coating began to crumple and some regions flaked, which indicates that both the porous layer and barrier layer are thoroughly damaged. The small changes of impedance spectra with exposure time from 14 days to 46 days (not shown in Fig. 7) show that the anodized SiCp/2024Al sealed with hot water provides some corrosion protection, but it is not as effective as for 2024Al, as shown in Fig. 4. Applying the results of the experimental impedance data to the model in Fig. 5 shows that the capacitance Cp of the porous layer and the capacitance Cb of the porous layer change with exposure times in 3.5 wt.% NaCl solution. For the anodized coating on the MMC, both Cp and Cb slightly decrease in the first 2 days (Fig. 6). This may be due to the slow hydration of Al2O3 to form Al(OH)3, thus sealing or blocking some of the pores in the porous alumina film during exposure to NaCl solution. The Cp tends to be stable between 2 and 29 days, and then it continuously increases with exposure time. This is due to the porous structure that is associated with the oxidation of SiC particles. The cavities present above the SiC particles (Fig. 1) can act as preferred pathways for penetration of Cl− ions. However, because of the presence of SiC particles, this pathway may not be straight; before Cl− ions reach the barrier layer they will travel from one cavity to another nearby. Moreover, these cavities become saturated with the NaCl solution in a short time. As a consequence, the increase in interfacial area between the porous layer and the solution, which is responsible for increasing Cp, is not significant within a limited period, explaining the almost unchanging trends in Cp between 2 and 29 days. With increasing exposure time, some shorter pathways without passing through the cavities above the SiC particles can be developed for Cl− ions ingress because the porous alumina was locally attacked, thus causing the increase in interfacial area between the porous film and the solution. Then Cp continually increases over 29 days as shown in Fig. 6a. However, it is much more difficult for the corrosion products due to corrosion of the alumina film and the Al matrix to migrate outwards because of the obstruction of SiC particles and tortuous pathways. This can cause relatively slow development of pitting. Meanwhile, if the volume of corrosion products is great enough to produce great stress, the anodic coating may begin to crumple and flake away as observed after immersion beyond 56 days. However, for the anodized 2024Al alloy, this phenomenon was not observed even after immersion in 3.5% NaCl solution for 75 days and appearance of severe pitting. Fig. 6b clearly shows that the increase in Cb appears after immersion in 3.5% NaCl solution over 6 days. This is because the

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barrier layer is dissolving and thinning. Because the thickness of the barrier layer is only several nanometers, this thinning effect is significant for the barrier layer whereas it can be neglected for the porous layer with thickness of several microns. Over 49 days the rapid increase in Cb is also because part of the barrier layer flaked off from the Al matrix, which indicates that the barrier layer has been thoroughly destroyed. 4. Conclusions (1) The anodized coating on the SiCp/2024Al provides some corrosion protection, but it is not as effective as for 2024Al because of non-uniformity in thickness and the presence of cavities associated with the SiC particulates. Cavities above the SiC particles prevent complete sealing of the anodized coating on the MMC in the way that anodic Al alloy is sealed. (2) SiC particle anodizes at a significantly reduced rate compared with the adjacent Al matrix, causing alumina film encroachment beneath the particle and occlusion of the partly anodized particle in the coating. As a consequence, the oxide/substrate interface becomes locally scalloped, and the anodized coating is non-uniform in thickness. In addition, oxidation of SiC, as well as electric field assisted dissolution of the porous layer of the alumina at the pore base above the SiC particle, appears to be associated with voids in the coating. (3) The cavities observed above the partially oxidized SiC particles have sizes and shapes dependent on the morphology of the upper surface of particles, current densities applied, and aggregation of particles. (4) The barrier layer of anodized Al MMC is not continuous. It is mainly composed of the barrier layer of the anodized Al matrix and a barrier-type SiO2 film on occluded SiC particle in the coating.

Acknowledgements This work was supported by the Natural Science Fund (No.20052003) of the Liaoning Province, P. R. China. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10]

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