JOURNAL OF RARE EARTHS, Vol. 28, No. 1, Feb. 2010, p. 109

Foundation item: Project supported by Higher Education Commission of Pakistan

Corresponding author: ZHANG Qi (E-mail: qz@buaa.edu.cn; Tel.: +86-10-82313043)

DOI: 10.1016/S1002-0721(09)60062-4

Using EIS to evaluate anti-corrosion properties of the SiCp/5A06 aluminium MMC treated by cerium conversion coatings Irfan Aziz, ZHANG Qi (张 琦), XIANG Min (项 民) (School of Materials Science and Engineering, Beijing University of Aeronautics and Astronautics, Beijing 100191, China)

Received 11 May 2009; revised 22 July 2009

Abstract: This paper evaluated the protection effect of the cerium conversion coatings on the SiCp/5A06 Al composite and the 5A06 Al alloy. Electrochemical impedance spectroscopy (EIS) was employed to examine the variation of the electrochemical variables of the samples treated and immersed in 3.5% NaCl solution at 35 °C for 1 h, which showed the enhancement of charge transfer resistance (Rt) and coating film re-sistance (Rc), i.e., the corrosion resistance of the conversion coated samples was improved. The best protection effect was found for the sam-ples treated for 60 min with 1000 μg/g CeCl3·7H2O/3.5% NaCl solution at 45 °C, followed with dried at 100 °C for 30 min. SEM/EDS ex-aminations confirmed that the enhancement in corrosion resistance properties of these materials was due to the precipitation of cerium oxides/ hydroxides over the cathodic intermetallics and SiC particulates. The XPS results revealed that the conversion coatings were composed of CeO2, Ce2O3, Ce(OH)3, Ce(OH)4, and Al2O3.

Keywords: cerium conversion coating; SiCp/5A06 Al composite; EIS; XPS; corrosion protection; rare earths

Metal matrix composites (MMCs), including Al matrix composites, have been most extensively utilized in aerospace, aircraft, automobile and electronic industries[1], owing to their high strength to weight ratio, low thermal expansion coefficient and relatively good wear resistance[2]. However, previous studies on the corrosion behaviour of Al MMCs have revealed that the corrosion sensitivity of the compos-ites enhanced with the addition of reinforcement constitu-ents[3,4].

In generally, the surface treatments used to improve the corrosion resistance of the MMCs are of the same type as those applied for Al alloys. Previously, chromate conversion coatings have been used most extensively for Al alloys[5]. Recently, the environmental friendly lanthanide compounds have successfully replaced the chromates[6–12]. Corrosion in-hibition by these lanthanide salts is generally related with the precipitation of lanthanide oxides/hydroxides over cathodic sites[6–8]. In earlier work, the effectiveness of rare earth coat-ing against corrosion was obtained by immersion in lantha-nide-based aqueous solution for rather long time. In order to make these coatings convenient for industrial applications, the treatment time has been tried to reduce by surface prepa-ration prior to conversion coating[13], addition of H2O2 oxi-dant[12] and thermal/electrochemical activation techniques[7]. During the last few decades, EIS technique has successfully been applied to study the corrosion/protection of Al alloys due to its non-destructive behaviour. Bethencourt et al.[14]

concluded that it is possible to detect the minimization of the values of those elements of the electric loop linked to the re-sponse of the cathodic intermetallics by EIS technique.

The intent of the present work was to evaluate the effect of Ce ions/NaCl concentration, solution pH, immersion time/temperature and drying temperature on the develop-ment of conversion film over the SiCp/5A06 Al MMC and 5A06 Al alloy surfaces by means of EIS. The microstructure characterisation of the Ce conversion coating was carried out by means of X-ray photoelectron spectroscopy (XPS), scan-ning electron microscopy (SEM) and energy dispersion spectroscopy (EDS).

1 Experimental

The materials used were the 5A06 Al alloy and SiCp/ 5A06 Al composite. The nominal chemical composition of the investigated matrix alloy was 5.8–6.8 Mg, 0.5–0.8 Mn, ≤0.4 Fe, ≤0.2 Zn, ≤0.1 Cu, 0.02–0.1 Ti, and balance Al. The composite was 5A06 Al added with 17.5 vol.% SiCp, which had an average size of 6.5 µm.

For the 5A06 Al alloy samples (20 mm×15 mm×2 mm), after polishing with 1200# abrasive paper and degreased with acetone for 5 min, the conversion coating was carried out as follows: (1) 3.5% NaCl aqueous solution (pH 6) with addi-tion of CeCl3·7H2O (500, 1000, 2000, and 5000 μg/g respec-tively) at 45 °C for 60 min, followed by drying at 100 °C for 30 min; (2) 1000 μg/g CeCl3·7H2O solution (pH 6) at 45 °C for 60 min, followed by drying at 100 °C for 30 min; (3) 3.5% NaCl solution added 1000 μg/g CeCl3·7H2O with pH of 5, 5.5, 6 and 7 respectively, followed by drying at 100 °C for 30 min; (4) immersed for (60 and 120 min respectively) in 3.5% NaCl solution with 1000 μg/g CeCl3·7H2O (at pH 5.5),

110 JOURNAL OF RARE EARTHS, Vol. 28, No. 1, Feb. 2010

followed by drying at 100 °C for 30 min; (5) Immersed for 60 min at 45 and 70 °C, respectively in 3.5% NaCl solution (pH 5.5) with addition of 1000 μg/g CeCl3·7H2O, followed by drying at 100 °C for 30 min; (6) immersion for 60 min at 45 °C in 3.5% NaCl solution with 1000 μg/g CeCl3·7H2O (pH 5.5), followed by drying at 50 and 100 °C for 30 min.

While, for the SiCp/5A06 Al MMC samples, after polish-ing with 1200# abrasive paper and degreased with acetone for 5 min, the conversion coating was carried out by immer-sion in 3.5% NaCl solution (pH 5.5) with addition of 1000 μg/g CeCl3·7H2O, at 45 °C for 60 min, followed by drying at 100 °C for 30 min.

After a 1 h immersion period in 3.5% NaCl solution at 35 °C, the coating effectiveness was evaluated by CHI750C elec-trochemical work station using a three electrode cell, with a flat working electrode of 1 cm2 exposed surface areas. Im-pedance measurements were recorded at open circuit poten-tial (OCP) in a frequency range of 10 kHz to 10 mHz and a sinusoidal potential perturbation of 10 mV amplitude. The impedance data were analyzed by the ZsimpWin 3.10 EChem Software (Michigan, USA).

The surface morphology and chemical composition analy-

sis were made by optical microscope (Olympus, BX51M), SEM (Hitachi, S-530) equipped with an EDS spectrometer (Oxford, INCA) and XPS (AXIS-ultra instrument from Kra-tos analytical using monochromatic Al Kα radiation (225 W, 15 mA, 15 kV)), respectively.

2 Results and discussion

2.1 Surface analysis after conversion coatings

The SEM images of the 5A06 Al alloy and SiCp/5A06 Al MMC corresponding to the samples treated in 1000 μg/g CeCl3·7H2O accompanied with 3.5% NaCl aqueous solution at 45 °C for 60 min, followed by dried at 100 °C for 30 min are shown in Figs. 1(a) and 2(a), respectively.

The analysis of EDS revealed that cerium oxide/hydroxide precipitated over very few regions, preferentially on the ca-thodic intermetallics Al(Fe, Mn)/Al(Fe, Mn, Si) and on SiCp. While, no deposition of Ce oxide/ hydroxide was noted on anodic intermetallics (Al(Si, Mg)) (Fig. 1(c)). Further, SEM image revealed the existence of pores and crevices at the matrix/reinforcement interfaces (shown as point C and D, respectively in Fig. 2(a)) on the composite surface.

Fig. 1 SEM image of Ce treated 5A06 Al alloy (a) and EDS spectra recorded on points A and B, respectively (b, c)

Fig. 2 SEM image of Ce treated SiCp/5A06 Al MMC (a) and EDS spectra recorded on A and B, respectively (b, c)

Irfan Aziz et al., Using EIS to evaluate anti-corrosion properties of the SiCp/5A06 aluminium MMC treated by cerium… 111

On the bases of EDS results, it can be assumed that the

mechanism of formation of conversion film on specimens is based on two simultaneous steps. Firstly, anodic (metal dis-solution) and cathodic (oxygen reduction) reactions occur immediately after immersion in solution containing NaCl accompanied with CeCl3[15]. In the second step, the Ce3+ and Al3+ cations react with OH¯ ions and form Ce(OH)3/Ce(OH)4 on cathodic sites and aluminium hydroxide on the rest of metal matrix, which transform to CeO2/Ce2O3 and Al2O3 af-ter drying.

Further, XPS study was carried out to evaluate the chemi-cal states and compositions of elements that appeared in the cerium bearing conversion coatings. The peaks in the survey spectra revealed the presence of Ce, O, C, and Al in the conversion films (Fig. 3(a)). Tjong and Huo[16] reported that the existence of C (at B.E. 285–290 eV) may be the con-tamination from air during transportation and insertion of specimens into XPS spectrometer.

Fig. 3(d) shows the XPS Ce 3d spectrum of the thermal conversion coatings. According to Refs. [7,11,17], it can be concluded that Ce4+ is the dominant oxidation state in case of conversion coating on SiCp/5A06 Al MMC due to the exis-tence of characteristic peaks at binding energies of 882.7, 898.5, 900.8, 907.5 and 916.7 eV. However, the existence of another peak at 886.1 eV reveals that Ce3+ also exist in the coating. While, the main peaks at binding energies of 885.9 and 904.3 eV accompanied with less dominate peaks of binding energies 882.3, 901.0 and 916.6 eV show the major contribution of Ce3+ species in case of 5A06 Al alloy con-version film.

Due to the presence of two non-overlapping Al 2p peaks at 74.6 and 71.4 eV for 5A06 Al alloy and single peak at 74.6 eV for SiCp/5A06 Al MMC (Fig. 3(b)), it can be con-firmed that Al existed in the form of Al2O3 in the coatings. The appearance of Ce and Al in the form of CeO2/Ce2O3, Ce(OH)3/Ce(OH)4 and Al2O3 can further be verified from the O 1s spectrum of conversion film.

The O 1s spectrum (Fig. 3(c)) reveals the two binding en-ergy peaks at 529.8 and 531.6 eV for SiCp/5A06 Al MMC and one broad peak with a full width half maximum (FWHM) of more than 2 eV centered at 531.9 eV for matrix alloy. The peak located at 529.8 eV is assigned as oxygen in the form of CeO2

[11,17]. The other one is centered at 531.6/531.9 eV, which is reported as the coexistence of Ce in the form of Ce2O3/Ce(OH)3/Ce(OH)4 and Al as Al2O3

[11,16,17]. On the bases of XPS analysis, it can be sum-marized that conversion film consists of CeO2, Ce2O3, Ce(OH)3, Ce(OH)4 and Al2O3.

2.2 Electrochemical impedance spectroscopy (EIS) studies

Previously, an EIS study to characterise the corrosion be-haviour of SiCp/5A06 Al MMC and 5A06 Al alloy revealed the existence of two capacitive and one inductive time con-stants for the composite and two capacitive time constants for the matrix alloy[18]. It was concluded that capacitive loop in the high frequencies was coupled with the formation of oxide layer. The second capacitive time constant for the ma-trix alloy in the low and medium frequency ranges for the composite was assigned to the combined effect of double layer capacitance-charge transfer resistance.

Fig. 3 XPS spectra of the Ce conversion coating on the SiCp/5A06 Al MMC and 5A06 Al alloy

(a) Survey spectrum; (b) Al 2p spectrum; (c) O 1s spectrum; (d) Ce 3d spectrum

112 JOURNAL OF RARE EARTHS, Vol. 28, No. 1, Feb. 2010

Fig. 4 EIS spectra acquired from the conversion treated 5A06 Al alloy as a function of CeCl3·7H2O concentration (with and without 3.5% NaCl)

(a) Nyquist plots; (b) Bode plots

In order to evaluate the corrosion inhibition effect of Ce coatings, EIS studies were done for the 5A06 Al alloy sam-ples treated in conversion solutions (pH 6) with different concentrations of CeCl3·7H2O, and with/without NaCl. The EIS spectra (Nyquist and Bode plots) after 1 h immersion at 35 °C in 3.5% NaCl solution, with/without conversion treatments are presented in Fig. 4. From Nyquist curves (Fig. 4(a)), it can be seen that the diameter of the curves obtained from the treated samples is larger than that obtained from the untreated, and the largest one is attained from the sample treated in solution of 1000 μg/g CeCl3·7H2O with addition of 3.5% NaCl. The similar trend can also be noted from an in-crease in |Z|, particularly in low frequency region, for all Ce treated samples in impedance vs. frequency curves (Fig. 4(b)). Further, a greater flattening of the maximum in the θ-lg(f) curves after coatings, also provides an evidence about the effectiveness of the conversion treatments.

By analysing the EIS spectra, it can be concluded that all the curves exhibited two capacitive loops, one corresponding to the frequencies range of spectrum between 10 kHz to 1 Hz, and another in the range between 1 Hz to 10 mHz. Therefore, it can be assumed that overall response of the system is as-sociated with (Rc–Qc) loop owing to the formation of film on the alloy surface and (Rt–Cdl) loop due to the double layer formed at metal-solution interface. Mishra et al.[19] reported that the 1st loop (Rc–Qc) encompasses all the information re-lated with the surface layer and the possible defects that may be present within it. The EIS results were analysed with ZSimpWin program using the equivalent circuit Rs(Qc(Rc(CdlRt))) shown in Fig. 5, and the values of parame-ters obtained are tabulated in Table 1.

From Table 1, it can be observed that the solution resis-tance (Rs) remained constant, with/without conversion treatments, and an enhancement in impedance was recorded for all considered cases of treatment. These increases of impedance values were strongly linked by the value of Rc and Rt. Further, the results showed a slight deviation from typical capacitive behaviour at high frequencies, possibly due to uneven oxide film formation on cathodic intermetal-lics and the rest of metal matrix. However, by comparison between the samples treated with different concentrations of

CeCl3, it was found that the best protection effect can be at-tained by the treatment with solution having 1000 μg/g CeCl3·7H2O with 3.5% NaCl. It means that coatings under these conditions are favourable to completely block the ca-thodic sites due to the precipitation of cerium ox-ide/hydroxide and to hinder micro-cell reaction on the sam-ples.

Alternatively, it can be observed that at 500 μg/g CeCl3 addition, both film resistance (Rc) and charge transfer resis-tance (Rt) values were significantly lower than the samples treated in 1000 μg/g cerium chloride solution. This behav-iour was assumed to be due to the short supply of available Ce3+ cations to completely cover the cathodic sites. In con-trast, the drop in Rc and Rt values for the samples treated with solution having higher concentration of CeCl3 were most probably due to the aggressive nature of Cl¯ anions that may cause to increase the system’s activity, and hence coun-teract, to some extent, the effectiveness of the Ce3+ cations as inhibitors.

On the other hand, remarkable increase of the impedance value was observed for the samples treated in 1000 μg/g CeCl3·7H2O accompanied with 3.5% NaCl solution as com-pared to absence of NaCl from conversion solution. It means that Cl¯ in the coating solution could trigger and accelerate the conversion coatings by activating cathodic sites where oxygen reduction occurs, resulting in pH increase locally. As a consequence of this process, a mixed film would form on the cathodic areas and the rest of the metal matrix.

Fig. 5 Equivalent electrical circuit used to fit the corrosion behav-

iour of cerium treated and blank 5A06 Al alloy

Irfan Aziz et al., Using EIS to evaluate anti-corrosion properties of the SiCp/5A06 aluminium MMC treated by cerium… 113

Table 1 Parameters obtained by modeling of experimental impedance spectra for CeCl3·7H2O conversion treated SiCp/5A06 Al com-

posite and 5A06 Al alloy to the Rs(Qc(Rc(CdlRt))) model

Variables Rs/

(Ω·cm2)

Qc

(Ω–1 sn/cm2)

n Rc/ (kΩ·cm2)

Cdl/ (μF·cm–2)

Rt/ (kΩ·cm2)

Δ (Rc+Rt)

5A06 Al alloy Blank sample CeCl3·7H2O(μg/g)+3.5%NaCl 500 1000 2000 5000 1000 (Without NaCl) pH 5 5.5 6 7 Immersion temperature 45 °C 70 °C Immersion time 60 min 120 min Drying temperature 100 °C 50 °C

0.74 4.02 2.90 1.05 2.71 2.15 2.83 2.35 2.91 5.60 2.35 2.30 2.35 2.85 2.35 2.60

10.8×10–6

13.9×10–6

9.8×10–6

10.1×10–6

10.2×10–6

9.6×10–6

13.1×10–6

8.4×10–6

9.8×10–6

11.4×10–6

8.4×10–6

6.8×10–6

8.4×10–6

9.5×10–6

8.4×10–6

8.7×10–6

0.89 0.87 0.89 0.88 0.87 0.83 0.88 0.92 0.89 0.85 0.92 0.92 0.92 0.89 0.92 0.89

37 106 349 319 117 3 81 370 349 172 370 214 370 220 370 323

34.9 69.9 33.5 32.1 17.5 0.9 144 5.4 33.5 32.6 5.4 32 5.4 15.7 5.4 6.1

41.6 63.6 229 158 115 262 40 504 229 122 504 90 504 184 504 396

1 2.16 7.35 6.06 2.95 3.37 1.54 11.12 7.35 3.74 11.12 3.86 11.12 5.14 11.12 9.15

SiCp/5A06 Al MMC Blank sample Treated under optimum conditions

3.21 3.39

17.4×10–6

14.8×10–6

0.88 0.79

1.63 56

2.4 60

15.8 98.6

1 8.86

Nyquist curves acquired from the 5A06 Al alloy samples

treated with 1000 μg/g CeCl3·7H2O/3.5% NaCl solutions with different pH, are presented in Fig. 6. The solution pH was adjusted with diluted NaOH and HCl. The EIS results were analysed by using the equivalent circuit model, as pre-sented in Fig. 5. From the data, as tabulated in Table 1, a variation of impedance values with pH was recorded. This variation is assumed to be linked with the availability of OH– anions and Ce3+ cations. It was noticed that a maximum im-pedance (Rc+Rt) value appeared at pH 5.5. A reduction of the constant phase element (Qc) and double layer capacitance (Cdl) values to their minimum, indicated that these conditions are favourable for the growth of a perfect film on the sample surface. Presently, the difference between the Cdl values ob-tained from the samples cerium coated under different pH values is quite evident (as shown in Table 1). Mansfeld[20]

reported that Cdl values are associated with the real corroding

Fig. 6 Nyquist plots obtained from conversion treated 5A06 Al al-

loy as a function of pH

areas of the sample surface, and a lower values of Cdl is re-lated to a smaller real corroding area. Further, the smooth-ness of the film on the sample surface can be confirmed from the value of n, which was very close to 1 (n>0.9). Ochao and Ganesca[21] concluded that n is a measure of surface rough-ness and when it approaches to 1, which indicates that the surface is smooth.

In addition, it can also be noted from Table 1 that the pro-tection effect was not as good as pH 5.5 when the bulk solu-tion pH value was more than or less than 5.5. It is most probably due to the insufficient availability of OH– or Ce3+ ions for forming Ce oxide/hydroxide islands on cathodic in-termetallics. Mishra and Balasubramaniam[19] observed that charge transfer resistance of the electrical double layer after treatment is related with the resistance offered by the pre-cipitation of lanthanide oxide/hydroxide on the cathodic in-termetallics. Pardo et al.[7] reported that coating efficiency declines at pH > 6 due to the deposition of lanthanide com-pound in solution instead of over the sample surface. Ac-cording to Pourbaix[22], the solubility of Ce3+ varies with pH as the following equilibrium relation: 2Ce3++3H2O=Ce2O3+6H+; lg[Ce3+]=22.15–3pH (1)

Further, a comparison regarding the variations of coating solution temperature, immersion time and drying tempera-ture is presented in Fig. 7. As shown in Table 1, when the immersion time was longer than 60 min or the solution tem-perature was relatively high (70 °C), the decrease of the film resistance and charge transfer resistance was noted. However, rather obvious variation of Rt values revealed that this is pos-sibly due to corrosive behaviour of conversion solution to Al alloys. While, a relatively greater flattening of maximum in

114 JOURNAL OF RARE EARTHS, Vol. 28, No. 1, Feb. 2010

Bode plots (Fig. 7(b)) and the largest diameter of Nyquist curve (Fig. 7(a)) attained after conversion treatment in solu-tion for 60 min at 45 °C and post-drying at 100 °C for 30 min, confirmed the best coating conditions for the 5A06 A alloy. On the bases of XPS analyses, it can be assumed that the improvement of the corrosion protection after drying re-sulted from the oxidation of some remnant of Ce3+ to Ce4+[7].

Once attained the optimum conditions for 5A06 Al alloy, the same were applied to composite samples with similar matrix composition. Fig. 8 presents the EIS spectra obtained from the SiCp/5A06 Al composite samples immersed in 3.5% NaCl solution for 1 h at 35 °C, which were untreated and cerium treated (degreased with acetone, immersion in 1000 μg/g CeCl3·7H2O+3.5%NaCl solution for 60 min at 45 °C, and followed with drying at 100 °C for 30 min) prior to the immersion. The data above the real axis were analysed by the application of equivalent circuit model (as shown in Fig. 5), and the values of parameters are tabulated in Table 1. It can be seen that total resistance (Rc+Rt) increased about nine times after conversion treatment. These results showed that the present coating conditions are favourable to improve the pitting corrosion resistance of SiCp/5A06 Al composite. XPS studies proved that Ce4+ is the dominant oxidation state in the case of SiCp/5A06 Al MMC bearing Ce coating, which pro-

vides protection to the substrate similar to chromate conver-sion coating[7].

From Table 1, by comparing the Rc+Rt values acquired from the composite with that from the monolithic alloy, both were conversion treated under similar conditions, it can be noted that the treatment improved corrosion resistance of the matrix Al alloy more than that of the SiCp/5A06 Al compos-ite. On the bases of SEM image (Fig. 2(a)), it can be con-cluded that relatively less protection effect for composite material was most probably due to the formation of non-integral film as a result of existence of pores and crev-ices at the SiCp/matrix interface.

In order to evaluate the corrosion behaviour of the Ce treated samples, a comparison between the treated (under optimum conditions) and untreated samples was made by immersion in sea water environment for one week. The OM images (Fig. 9) of SiCp/5A06 Al MMC specimens after 7 d of immersion in 3.5% NaCl at 35 °C showed a smooth sur-face layer for Ce treated samples, whereas a higher number of pits appeared on blank samples. This indicated that ce-rium treatments could inhibit the pitting in sea water envi-ronment for longer time, probably due to the precipitation of Ce oxide/hydroxide on the cathodic intermetallics, which reduced the localized alkaline corrosion.

Fig. 7 EIS spectra acquired from the 5A06 Al alloy treated with different immersion time, immersion and drying temperature

(a) Nyquist plots; (b) Bode plots (1) Immersion at 45 °C, 60 min, dry at 50 °C, 30 min, (2) Immersion at 70 °C, 60 min, dry at 100 °C, 30 min, (3) Immersion at 45 °C, 120 min, dry at 100 °C, 30 min, (4) Immersion at 45 °C, 60 min, dry at 100 °C, 30 min

Fig. 8 Comparison of EIS spectra acquired from Ce conversion treated and untreated SiCP/5A06 Al MMC samples

(a) Nyquist plots; (b) Bode plots

Irfan Aziz et al., Using EIS to evaluate anti-corrosion properties of the SiCp/5A06 aluminium MMC treated by cerium… 115

Fig. 9 OM images of SiCp/5A06 Al composite after 7 d of immersion in 3.5% NaCl at 35 °C

(a) Blank sample; (b) Ce treated sample

3 Conclusions

(1) EIS studies showed that the conversion treatment brought the samples a surface layer which increased the overall resistance (charge transfer resistance and film resis-tance) significantly, and hence reduced the overall corrosion rate.

(2) The SEM and EDS analyses revealed that an im-provement in corrosion resistance of the materials was due to the formation of Ce oxide/hydroxide islands on the cathodic intermetallics/SiCP, and an Al2O3 film on the metal matrix.

(3) XPS results indicated that cerium was incorporated as Ce3+ as well as Ce4+ species in the conversion films.

(4) The optimal effect was attained by treating samples with 1000 μg/g CeCl3·7H2O+3.5% NaCl solutions (pH 5.5), and the time and temperature of immersion should not ex-ceed 60 min and 45 °C, respectively due to aggressiveness of the conversion solutions on the 5A06 Al alloy.

(5) The improved resistance offered by higher drying tem-perature (100 °C) was most probably due to the oxidation of Ce3+ to Ce4+.

(6) The conversion treatment offered higher film resis-tance (Rc) and more smooth (n>0.9) surface to the 5A06 Al alloy than it did to the SiCp/5A06 Al, indicating that there were higher density of discontinuities in the film on the composite than that on the 5A06 Al alloy, which might be due to the existence of pores and crevices at the particu-late/matrix interface within the composite.

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