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Reinforcing aluminium with cerium oxide: A new and effective technique to prevent corrosion in marine environments
Electrochemistry Communications 9 (2007) 443–448 www.elsevier.com/locate/elecom
Reinforcing aluminium with cerium oxide: A new and effective technique to prevent corrosion in marine environments P. Muhamed Ashraf b
a,1
, S.M.A. Shibli
b,*
a Central Institute of Fisheries Technology, Matsyapuri P.O., Cochin 682 029, Kerala, India Department of Chemistry, University of Kerala, Kariavattom Campus, Trivandrum 695 581, Kerala, India
Received 10 August 2006; accepted 19 September 2006 Available online 3 November 2006
Abstract Cerium oxide (CeO2) is one of the potential choices to be explored for the protection of aluminium from corrosion. However, usage of cerium ions for surface modification of aluminium would not yield any potential effect in aggressive marine environments. Metal matrix composites of aluminium can have all the merits of aluminium such as density, strength, ductility and cost. In the present study, corrosion of aluminium in saline environment was significantly suppressed by means of reinforcing the aluminium with cerium oxide. The presence of cerium oxide in the matrix did not facilitate the formation of Al2O3 which would cause potential reshift after few days due to the onset of pitting. The present results strongly lay emphasis on the potential scope of use of CeO2 for protection of aluminium in marine environments. 2006 Elsevier B.V. All rights reserved. Keywords: Aluminum; Cerium oxide; Marine corrosion; Corrosion control; Metal matrixcomposite
1. Introduction Pure aluminium undergoes different types of corrosion that proceeds with pitting in marine environments [1]. Aluminium is conventionally protected from marine corrosion by means of providing anticorrosive coatings containing chromium like zinc chromate with an antifouling top coat. In industry, aluminium and its alloys are protected from corrosion by means of anodisation or chemically converted coatings like chromating [2]. Chromate is considered as a standard corrosion inhibitor based on efficiency/cost ratio. However, chromium compounds are toxic and cause serious environmental hazards [3]. Cerium has low toxicity and is relatively abundant in nature [4,5]. Extensive studies on surface modification of aluminium alloys in aqueous solutions of cerium salts have been reported [6–9]. Cerium *
Corresponding author. Tel.: +91 471 2167 230. E-mail addresses: [email protected] (P.M. Ashraf), [email protected] (S.M.A. Shibli). 1 Tel.: +91 484 2666845. 1388-2481/$ - see front matter 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.elecom.2006.09.010
compounds are good cathodic inhibitors as they show more negative potential values [8]. Cerium has an affinity for oxygen [7] and the bonding between cerium and oxygen is unlikely to be broken under the cathodic potential region. The cathodic potential shift can be related to the formation of cerium oxide/hydroxide layer. Cerium ions form insoluble hydroxides, which enable them to act as effective cathodic inhibitors [3]. The present study has explored the use of cerium oxides (CeO2) to protect aluminium from marine corrosion. There are few reports that surface modification of aluminium by means of cerium adsorption cannot be stable effectively in aggressive marine environment [3]. Aluminium metal and alloys can be reinforced with light weight particles or fibres to produce metal matrix composites (MMCs) which can have advantages over other conventional alloys in terms of physical and mechanical properties [10–12]. Preparation of metal matrix composites of aluminium will not delimit the merits of aluminium such as density, strength, ductility and cost. Ceramic particles such as SiC and Al2O3 are extensively used for reinforcing
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P.M. Ashraf, S.M.A. Shibli / Electrochemistry Communications 9 (2007) 443–448
aluminium and its alloys. Other materials such as SiO2, ZrO2, TiB2 have also been reported as good materials for reinforcement in aluminium [13]. Cerium dioxide (CeO2), having cubic fluorite structure, has long been considered as one of the most important oxide materials because of its desirable properties such as high refractive index, good transmission, adhesion and high stability against mechanical abrasion and high temperature [14]. The present study had this background that cerium oxide can be the best choice to be investigated for protection of aluminium from corrosion. As mentioned above, usage of cerium ions for surface modification of aluminium would not be effective in aggressive marine environments. Hence the present work was designed to explore, develop and evaluate cerium oxide based MMCs of aluminium in marine environment. 2. Materials and methods Electrolytic grade pure aluminium (99.80%) ingots were used for fabrication of metal matrix composites. The aluminium ingots had the percentage composition of Cu, Fe, Mg, Mn, Ti and Zn as 0.0031, 0.1069, 0.0411, 0.0024, 0.0001 and 0.0016, respectively; the rest being aluminium. There were no chromium and nickel. The broken aluminium ingots were melted at 800 ± 10 C in a muffle furnace. Different amounts of finely powdered cerium oxide were added into the melt and stirred using a SiC rod. The melt was again kept in the muffle for another 15 min at the same temperature, stirred and poured into a red brick mould. The cast aluminium coupons were sliced with appropriate shape and polished using a series of silicon carbide papers up to 600 grits. The cerium oxide – reinforced aluminium coupons were wiped with acetone and rinsed with distilled water. The aluminium coupons reinforced with varying amounts of cerium oxide viz., 0.05, 0.10, 0.20, 0.40, 0.70 and 1.00% were designated as C1, C2, C3, C4, C5 and C6, respectively. Reinforcement with higher amount of CeO2 above 1% resulted in high rejection during casting. The unreinforced pure aluminium coupon was designated as C0. Electrochemical impedance spectroscopy, linear sweep voltammetry and other electrochemical measurements were carried out using an Autolab PGSTAT 30 plus FRA 2 corrosion measurement system. The impedance analysis was carried out using 3.5% NaCl as the electrolyte. Ag/AgCl, Pt and the coupon having 1 cm exposed area were used as reference, counter and working electrodes, respectively. Impedance analysis at the frequency range of 1 MHz to 10 Hz was carried out with reference to OCP after 30 min of exposure of the coupons in the electrolyte. The coupon having 1 cm2 exposed area was immersed in 3.5% NaCl for 1 h prior to potentiodynamic polarization studies at a scan rate of 0.005 V/s. Weight loss measurements were made after exposing the pre-weighed coupons in 3.5% NaCl for a period of 40 days as per ASTM G31-72. The coupons were cleaned using a hot mixture of 20 g potassium dichromate and 50 ml phosphoric acid in 1litre water.
The coupons were rinsed with distilled water, dried and weighed. Another set of experiment was carried out in parallel and the OCP of each coupon was monitored continuously for 40 days. The exposed coupons were taken out after 40 days, washed with distilled water and subjected to EIS analysis. Impressing an anodic current of 100 mA/cm2 dissolved the surface layers of cerium oxide – reinforced aluminium coupons in sodium chloride solution. Eight batches of the coupons were subjected to such anodic stripping process with different duration in multiples of hour. The stable open circuit potential was recorded after each set of the process. Another set of pre-weighed coupons was exposed to salt spray for 100 h as per ASTM B 117-73. The coupons removed after the test were cleaned using aluminium – pickling solution as in the case of the weight loss experiment and the corrosion rates were calculated as per ASTM G1-72. The surface of fresh CeO2 – reinforced Al coupons were polished using a series of SiC papers up to 2000 grit and their SEM images were recorded using a Hitachi Scanning electron microscope. Another set of the above polished coupons were exposed in 3.5% NaCl for 40 days and their SEM images were also recorded after cleaning the coupons with distilled water and acetone. 3. Results and discussion 3.1. Physicochemical characteristics The SEM images of the aluminium coupon reinforced with 0.2% cerium oxide, revealed (Fig. 1) that the cerium oxide – reinforced aluminium was homogenous. Presence of cerium oxide was revealed as they were surrounded by aluminium solid solutions. These CeO2 particle zones could probably act as cathodic zones suppressing corrosion of the mass. Fig. 1b is the SEM images of those coupons recorded after exposure in 3.5% NaCl solution for a period of 40 days. The image revealed that the presence of CeO2 as in dark light patches was intact even after continuous exposure in aggressive sodium chloride solution. The images of other coupons having very low or very high concentration revealed that they were not metallurgically improved as in the case of the former. 3.2. Electrochemical evaluation 3.2.1. Open circuit potential decay The variations in open circuit potential (OCP) of the cerium oxide – reinforced aluminium coupons, when exposed in 3.5% NaCl was monitored continuously for a period of 40 days. The average OCP was found to be varying in between 0.593 ± 0.286 and 0.824 ± 0.033 V (Fig. 2). Significantly, higher values of standard deviation were noted in the case of the un-reinforced aluminium (control), and those correspond to 0.05% and 0.1% CeO2 reinforcement. This was due to the low stability of the
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445
formed oxide, which caused the anodic shift, had the stability only up to 17 days. Similar observations were also noted in the case of C1 and C2, but only after 17 and 30 days, respectively. This could be due to the formation of metastable pits, which initiate and grow for a short period below the critical pitting potential and then repassivate [1]. The aluminium reinforced with 0.4% cerium oxide showed a trend of irregular variation of OCP throughout the period, revealing that the composite could not form a protective and stable oxide scale on its surface. The OCP of C3, C5 and C6 were found comparatively stable throughout the exposure period and it varied from 0.714 to 0.802 V, 0.708 to 0.858 V, and 0.760 to 0.863 V, respectively. These data clearly revealed that a gradual increase in the stability of OCP could be achieved with increased amounts of cerium oxide reinforcement. The presence of CeO2 in aluminium not only improved the electrochemical characteristics of the reinforced aluminium but also formed stable protective oxide film on the surface.
Fig. 1. The SEM images of 0.2% CeO2 cerium oxide – reinforced aluminium (C3) (a) fresh (b) after exposure in 3.5% NaCl.
Fig. 2. The average OCP with standard deviation of the cerium oxide – reinforced aluminium coupons exposed in 3.5% NaCl for 40 days.
coupons exposed in aggressive corrosion medium of NaCl. The aluminium reinforced with 0.2% CeO2 (0.775 ± 0.014 V) exhibited the lowest standard deviation revealing the highest stability of these coupons in the aggressive environment. The open circuit potential of the un-reinforced aluminium was found to be shifted to its minimum value within 11days of the exposure and it reshifted to its normal equilibrium value after 17th day. This revealed that the initially
3.2.2. Linear sweep polarization analysis The linear polarization curves of all the different cerium oxide – reinforced aluminium coupons including the control-coupon were recorded. The corrosion potential (Ecorr) values were found to be varying in between 0.681 and 0.807 V (Table 1). Ecorr of all the cerium oxide – reinforced aluminium coupons were found to be significantly low when compared to the control. The lowest Ecorr value was noted in the case of aluminium reinforced with 0.2% CeO2. The anodic current in the E vs. log i plot of 0.2% cerium oxide – reinforced aluminium was found to be extremely stable during the initial stage of anodic polarization, revealing good stability of the matrix. The corrosion current density (Icorr) varied in between 2.27 · 10 7 and 4.86 · 10 6 A/cm2. The polarization resistance, Rp, was found to vary in between 1070 and 48,800 X. The presence of cerium oxide in aluminium was the cause for the displacement of cathodic branch towards lower current densities as well as a decrease in the corrosion potential of the system. 3.2.3. Impedance analysis Impedance spectra of the different cerium oxide – reinforced aluminium are presented in Fig. 3. Pure aluminium exhibited an impedance pattern with a wide arc at high freTable 1 The linear sweep voltammetric parameters of cerium oxide – reinforced aluminium in 3.5% NaCl CeO2 reinforcement (%) 0 0.05 0.10 0.20 0.40 0.70 1.00
Ecorr (V) 0.681 0.698 0.714 0.807 0.747 0.731 0.769
Icorr (A/cm2) 1.13 · 10 4.86 · 10 1.28 · 10 7.89 · 10 2.60 · 10 2.27 · 10 3.26 · 10
6 6 6 7 6 7 7
Rp (X) 8.19 · 103 3.35 · 103 1.01 · 104 1.25 · 104 4.61 · 103 2.66 · 104 6.69 · 104
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P.M. Ashraf, S.M.A. Shibli / Electrochemistry Communications 9 (2007) 443–448 Table 2 The impedance data of cerium oxide reinforced aluminium in fresh coupons and after 40 days of exposure in 3.5% NaCl
7 6
- Z' ' / o h m
5
Details
CeO2 reinforcement (%)
Rp (X)
CPE (F)
Fresh coupons
0 0.05 0.10 0.20 0.40 0.70 1.00
8.94 8.44 80.58 85.15 81.91 82.60 83.59
3.68 · 10 3.07 · 10 1.09 · 10 1.01 · 10 1.01 · 10 9.77 · 10 1.02 · 10
8
0 0.05 0.10 0.20 0.40 0.70 1.00
8.622 78.91 84.24 79.44 81.57 82.57 83.50
2.96 · 10 1.12 · 10 9.55 · 10 1.24 · 10 1.12 · 10 1.07 · 10 1.08 · 10
8
4 3 2 1
C0NTROL C1
0 -3
-2
-1
0
1 Z' / ohm
2
3
4
90
After 40 days of exposure
80 70
-Z'' / ohm
60 50 40
Impedance parameters of high frequency region n 8 8 8 8 9 8
8 9 8 8 8 8
1.164 1.193 1.199 1.195 1.188 1.179 1.198 1.275 1.1957 1.159 1.214 1.1947 1.1819 1.1923
30 20 10 C2
0 -80
-70
-60
-50
40
-30 Z' / ohm
-20
-10
0
10
175 150
-Z' ' / oh m
125 100 C3 0.20%
75 50 25 0 -100
-75
-50
-25
0
25
50
75
Z' / ohm
Fig. 3. The Nyquisit plots of the control and different cerium oxide – reinforced aluminium coupons.
quency region and a small one at low frequency region. A similar trend was observed in the case of 0.05% and 0.10% cerium oxide – reinforced aluminium also. Their Rp values were found to be significantly low compared to other treatments (Table 2). The impedance spectra of the coupons C3–C6 showed reverse characteristics, i.e., a small arc at high frequency region and wide one at low frequency region. The polarization resistance, CPE and n values of the semi circle of high frequency region varied from 8.94 to 85.15 X, 9.77 · 10 9 to 3.68 · 10 8 F and 1.164 to 1.199, respectively. There was no marked variation in ‘n’ though the values were found to be comparatively low in the case of the un-reinforced aluminium. These results revealed that the outer layer formed in the case of the control and 0.05% CeO2 – reinforced aluminium could be attributed to the formation of aluminium oxide. The presence of low amount of cerium oxide
in C1 had not made any significant influence on polarization resistance. These results revealed that the cerium oxide reinforcement in aluminium had strengthened both the weak outer layer of aluminium oxide and also the inner cerium oxide-aluminium layer. The inner layer had the leading role in corrosion protection character of the metal matrix [15]. Similarly, the constant phase element and capacitance of the high frequency semi circle were found to be significantly low in the case of C3–C6, reiterating the role of cerium oxide reinforcement. The impedance spectra of C4, C5, and C6 are found to be very similar to that of C3 and hence not shown here. The impedance data revealed that 0.2% cerium oxide – reinforcement could yield better performance compared to the aluminium reinforced with higher amount of cerium oxide. There was no proportional improvement in the performance with increase in CeO2 content, as evidenced based on the impedance parameters. According to Yu et al. [16] rare earth metal oxide based surface modification of aluminium alloy creates a barrier to the supply of oxygen and the supply of electron from the aluminium alloy surface to the corrosion medium. In the present case, a similar mechanism can probably exist in between the aluminium CeO2 composite surface and the corrosion medium. Hence the necessary reactions of corrosion would have been suppressed, thereby reducing the driving force of corrosion. 3.3. Evaluation of corrosion rate 3.3.1. Weight loss tests The coupons were immersed in 3.5% NaCl solution for a period of 40 days under laboratory conditions. The corrosion rate (Table 3) and the corrosion inhibition efficiency were determined to be in the range of 1.814 · 10 4 to 8.007 · 10 3 mpy and 61.91 to 97.73%, respectively. The corrosion inhibition efficiency was found to be decreasing
P.M. Ashraf, S.M.A. Shibli / Electrochemistry Communications 9 (2007) 443–448 Table 3 The corrosion rate values (mpy) of cerium oxide – reinforced aluminium exposed in 3.5% NaCl at room temperature (40 days) and that in salt spray chamber (100 h)
-0.75
CeO2 reinforcement (%)
Corrosion rate (mpy)
-0.81
0.00 0.05 0.10 0.20 0.40 0.70 1.0
8.007 · 10 3.048 · 10 9.991 · 10 1.814 · 10 2.931 · 10 9.653 · 10 5.011 · 10
3 3 4 4 3 4 4
Salt spray 2.070 · 10 3.285 · 10 1.613 · 10 1.265 · 10 2.056 · 10 1.289 · 10 1.276 · 10
3
-0.77 -0.79
O CP V
Weight loss studies
447
-0.83 -0.85
3 3 3 3 3
-0.87 -0.89 -0.91 C0
C1
in the order of 0.20% > 1.00% > 0.70% CeO2 reinforced aluminium. This observation revealed that the reinforcement of aluminium with cerium oxide of these concentrations had improved the metallurgical characteristics of aluminium and enhanced the formation of effective protective film on its surface. This trend was found to be in accordance with that of the results based on OCP decay, LSV and impedance analysis. 3.3.2. Salt spray test Few aluminium coupons were exposed in a salt spray chamber for a period of 100 h continuously and the corrosion rates were calculated based on the weight loss data. The corrosion rate was determined to be in the range from 1.265 · 10 3 mpy to 3.285 · 10 3 mpy. The aluminium coupon reinforced with 0.05% CeO2 exhibited high corrosion rate. Lowest corrosion rate was noted in the case of 0.20% CeO2 – reinforced aluminium (Table 3). In the aggressive salt spray environment, the 0.2% cerium oxide – reinforced aluminium showed 39% more corrosion inhibition efficiency than the control. Thus the optimum concentration of CeO2 reinforcement in aluminium was confirmed to be 0.2% since reinforcement of more quantity of CeO2 did not shown the equivalent corrosion resistance. Hence further studies of structural analysis were carried out with this composition of C3. 3.4. Structural analysis 3.4.1. Anodic stripping The coupons were subjected to anodic stripping by applying an anodic current of 100 mA/cm2. After every multiples of 1-h intervals of current interruption, the OCP of the freshly exposed surface was monitored. The process was continued for eight batches (8 h). The un-reinforced coupons exhibited significantly higher OCP than the reinforced coupon (Fig. 4), due to the influence of the freshly formed surface – oxide layer. The control-coupon showed the highest standard deviation while the C5 showed the lowest. All the CeO2 – reinforced aluminium exhibited lower OCP with low standard deviation. This was attributed to the slow and steady formation of CeO2aluminium oxide layer. These results revealed that there
C2
C3
C4
C5
C6
TREATMENTS
3
Fig. 4. The average OCP with standard deviation of the cerium oxide – reinforced aluminium coupons, measured after subjected to anodic stripping.
was uniformity in the composite reinforcement, improving the metallurgical characteristics that led to high stability of aluminium in the saline environment. 3.4.2. Impedance characteristics after marine exposure The electrochemical impedance spectra of the coupons retrieved, after exposing in 3.5% NaCl for 40 days, was recorded. The Nyquisite plots of impedance analysis were found to be similar to the corresponding spectra shown in Fig. 3. The Rp values varied in between 8.622 and 83.498 in the semicircle at high frequency region (Table 2). The coupon C1 was the exceptional case, where an increase in Rp values from 8 to 79 X at the high frequency region was recorded. These results reiterated the role of cerium oxide on the stability of aluminium in the aggressive marine environment. All these results clearly indicate the effective corrosion inhibition characteristics of cerium oxide reinforced aluminium in saline environments. 4. Conclusion The corrosion of aluminium, in marine environments, can be suppressed significantly by means of reinforcing aluminium with cerium oxide. The results of the present physicochemical, structural and electrochemical evaluation studies clearly revealed the effective role of cerium oxide reinforcement in aluminium exploring its use in marine environments. The best aluminium metal matrix composite prepared with optimum concentration of 0.20% CeO2 performed excellently. Even though increase in cerium oxide content caused improved corrosion protection during some of the tests, the improvement was not in proportional to the amount of CeO2 incorporated. The present results lay strong emphasise on the potential scope of use of CeO2 for protection of aluminium in marine environments. Acknowledgements The authors sincerely thank Dr. K. Devadasan, Director and Dr. B. Meenakumari Head, Fishing Technology
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Division of Central Institute of Fishing Technology, Kochi, for providing facilities to carry out the present work. References [1] Z. Szklarska-Smialowska, Corr. Sci. 41 (1999) 1743–1767. [2] L.E.M. Palomino, J.F.W. de Castro, I.V. Aoki, H.G. de Melo, J. Braz. Chem. Soc. 14 (2003) 651–659. [3] M. Bethencourt, F.J. Botana, J.J. Calvino, M. Marcos, M.A. Rodriguez-Chacon, Corros. Sci. 40 (1998) 1803–1819. [4] M.A. Arenas, M. Bethencourt, F.J. Botana, J. de Damborenea, M. Marcos, Corros. Sci. 43 (2001) 157–170. [5] M.A. Arenas, A. Conde, J.J. de Damborenea, Corros. Sci. 44 (2002) 511–520. [6] F. Mansfeld, S. Lin, S. Kim, H. Shih, J. Electrochem. Soc. 137 (1990) 78–82.
[7] Y.C. Lu, M.B. Ives, Corros. Sci. 34 (1993) 1773–1781. [8] Y.C. Lu, M.B. Ives, Corros. Sci. 37 (1995) 145–155. [9] W.G. Fahrenholtz, M.J. O’Keefe, H. Zhou, J.T. Grant, Surf. Coat. Technol. 155 (2002) 208–213. [10] K.T. Kashyap, C. Ramachandra, C. Datta, B. Chatterjee, Bull. Mater. Sci. 23 (2000) 47–49. [11] J.A. Hooker, P.J. Doorbar, Mater. Sci. Technol. 16 (2000) 725– 731. [12] J. Cadek, K. Kucharova, K. Milicka, S. Zhu, Metall. Mater. 37 (1999) 213–245. [13] J.M. Torralba, C.E. da Costab, F. Velasco, J. Mater. Process. Technol. 133 (2003) 203–206. [14] W. Xiao, Q. Guo, E.G. Wang, Chem. Phys. Lett. 368 (2003) 527– 531. [15] X. Yu, C. Cao, Thin Solid Film 423 (2003) 252–256. [16] X. Yu, C. Cao, Z. Yao, D. Zhou, Z. Yin, Corros. Sci. 43 (2001) 1283– 1294.
Reinforcing aluminium with cerium oxide: A new and effective technique to prevent corrosion in marine environments P. Muhamed Ashraf b
a,1
, S.M.A. Shibli
b,*
a Central Institute of Fisheries Technology, Matsyapuri P.O., Cochin 682 029, Kerala, India Department of Chemistry, University of Kerala, Kariavattom Campus, Trivandrum 695 581, Kerala, India
Received 10 August 2006; accepted 19 September 2006 Available online 3 November 2006
Abstract Cerium oxide (CeO2) is one of the potential choices to be explored for the protection of aluminium from corrosion. However, usage of cerium ions for surface modification of aluminium would not yield any potential effect in aggressive marine environments. Metal matrix composites of aluminium can have all the merits of aluminium such as density, strength, ductility and cost. In the present study, corrosion of aluminium in saline environment was significantly suppressed by means of reinforcing the aluminium with cerium oxide. The presence of cerium oxide in the matrix did not facilitate the formation of Al2O3 which would cause potential reshift after few days due to the onset of pitting. The present results strongly lay emphasis on the potential scope of use of CeO2 for protection of aluminium in marine environments. 2006 Elsevier B.V. All rights reserved. Keywords: Aluminum; Cerium oxide; Marine corrosion; Corrosion control; Metal matrixcomposite
1. Introduction Pure aluminium undergoes different types of corrosion that proceeds with pitting in marine environments [1]. Aluminium is conventionally protected from marine corrosion by means of providing anticorrosive coatings containing chromium like zinc chromate with an antifouling top coat. In industry, aluminium and its alloys are protected from corrosion by means of anodisation or chemically converted coatings like chromating [2]. Chromate is considered as a standard corrosion inhibitor based on efficiency/cost ratio. However, chromium compounds are toxic and cause serious environmental hazards [3]. Cerium has low toxicity and is relatively abundant in nature [4,5]. Extensive studies on surface modification of aluminium alloys in aqueous solutions of cerium salts have been reported [6–9]. Cerium *
Corresponding author. Tel.: +91 471 2167 230. E-mail addresses: [email protected] (P.M. Ashraf), [email protected] (S.M.A. Shibli). 1 Tel.: +91 484 2666845. 1388-2481/$ - see front matter 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.elecom.2006.09.010
compounds are good cathodic inhibitors as they show more negative potential values [8]. Cerium has an affinity for oxygen [7] and the bonding between cerium and oxygen is unlikely to be broken under the cathodic potential region. The cathodic potential shift can be related to the formation of cerium oxide/hydroxide layer. Cerium ions form insoluble hydroxides, which enable them to act as effective cathodic inhibitors [3]. The present study has explored the use of cerium oxides (CeO2) to protect aluminium from marine corrosion. There are few reports that surface modification of aluminium by means of cerium adsorption cannot be stable effectively in aggressive marine environment [3]. Aluminium metal and alloys can be reinforced with light weight particles or fibres to produce metal matrix composites (MMCs) which can have advantages over other conventional alloys in terms of physical and mechanical properties [10–12]. Preparation of metal matrix composites of aluminium will not delimit the merits of aluminium such as density, strength, ductility and cost. Ceramic particles such as SiC and Al2O3 are extensively used for reinforcing
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P.M. Ashraf, S.M.A. Shibli / Electrochemistry Communications 9 (2007) 443–448
aluminium and its alloys. Other materials such as SiO2, ZrO2, TiB2 have also been reported as good materials for reinforcement in aluminium [13]. Cerium dioxide (CeO2), having cubic fluorite structure, has long been considered as one of the most important oxide materials because of its desirable properties such as high refractive index, good transmission, adhesion and high stability against mechanical abrasion and high temperature [14]. The present study had this background that cerium oxide can be the best choice to be investigated for protection of aluminium from corrosion. As mentioned above, usage of cerium ions for surface modification of aluminium would not be effective in aggressive marine environments. Hence the present work was designed to explore, develop and evaluate cerium oxide based MMCs of aluminium in marine environment. 2. Materials and methods Electrolytic grade pure aluminium (99.80%) ingots were used for fabrication of metal matrix composites. The aluminium ingots had the percentage composition of Cu, Fe, Mg, Mn, Ti and Zn as 0.0031, 0.1069, 0.0411, 0.0024, 0.0001 and 0.0016, respectively; the rest being aluminium. There were no chromium and nickel. The broken aluminium ingots were melted at 800 ± 10 C in a muffle furnace. Different amounts of finely powdered cerium oxide were added into the melt and stirred using a SiC rod. The melt was again kept in the muffle for another 15 min at the same temperature, stirred and poured into a red brick mould. The cast aluminium coupons were sliced with appropriate shape and polished using a series of silicon carbide papers up to 600 grits. The cerium oxide – reinforced aluminium coupons were wiped with acetone and rinsed with distilled water. The aluminium coupons reinforced with varying amounts of cerium oxide viz., 0.05, 0.10, 0.20, 0.40, 0.70 and 1.00% were designated as C1, C2, C3, C4, C5 and C6, respectively. Reinforcement with higher amount of CeO2 above 1% resulted in high rejection during casting. The unreinforced pure aluminium coupon was designated as C0. Electrochemical impedance spectroscopy, linear sweep voltammetry and other electrochemical measurements were carried out using an Autolab PGSTAT 30 plus FRA 2 corrosion measurement system. The impedance analysis was carried out using 3.5% NaCl as the electrolyte. Ag/AgCl, Pt and the coupon having 1 cm exposed area were used as reference, counter and working electrodes, respectively. Impedance analysis at the frequency range of 1 MHz to 10 Hz was carried out with reference to OCP after 30 min of exposure of the coupons in the electrolyte. The coupon having 1 cm2 exposed area was immersed in 3.5% NaCl for 1 h prior to potentiodynamic polarization studies at a scan rate of 0.005 V/s. Weight loss measurements were made after exposing the pre-weighed coupons in 3.5% NaCl for a period of 40 days as per ASTM G31-72. The coupons were cleaned using a hot mixture of 20 g potassium dichromate and 50 ml phosphoric acid in 1litre water.
The coupons were rinsed with distilled water, dried and weighed. Another set of experiment was carried out in parallel and the OCP of each coupon was monitored continuously for 40 days. The exposed coupons were taken out after 40 days, washed with distilled water and subjected to EIS analysis. Impressing an anodic current of 100 mA/cm2 dissolved the surface layers of cerium oxide – reinforced aluminium coupons in sodium chloride solution. Eight batches of the coupons were subjected to such anodic stripping process with different duration in multiples of hour. The stable open circuit potential was recorded after each set of the process. Another set of pre-weighed coupons was exposed to salt spray for 100 h as per ASTM B 117-73. The coupons removed after the test were cleaned using aluminium – pickling solution as in the case of the weight loss experiment and the corrosion rates were calculated as per ASTM G1-72. The surface of fresh CeO2 – reinforced Al coupons were polished using a series of SiC papers up to 2000 grit and their SEM images were recorded using a Hitachi Scanning electron microscope. Another set of the above polished coupons were exposed in 3.5% NaCl for 40 days and their SEM images were also recorded after cleaning the coupons with distilled water and acetone. 3. Results and discussion 3.1. Physicochemical characteristics The SEM images of the aluminium coupon reinforced with 0.2% cerium oxide, revealed (Fig. 1) that the cerium oxide – reinforced aluminium was homogenous. Presence of cerium oxide was revealed as they were surrounded by aluminium solid solutions. These CeO2 particle zones could probably act as cathodic zones suppressing corrosion of the mass. Fig. 1b is the SEM images of those coupons recorded after exposure in 3.5% NaCl solution for a period of 40 days. The image revealed that the presence of CeO2 as in dark light patches was intact even after continuous exposure in aggressive sodium chloride solution. The images of other coupons having very low or very high concentration revealed that they were not metallurgically improved as in the case of the former. 3.2. Electrochemical evaluation 3.2.1. Open circuit potential decay The variations in open circuit potential (OCP) of the cerium oxide – reinforced aluminium coupons, when exposed in 3.5% NaCl was monitored continuously for a period of 40 days. The average OCP was found to be varying in between 0.593 ± 0.286 and 0.824 ± 0.033 V (Fig. 2). Significantly, higher values of standard deviation were noted in the case of the un-reinforced aluminium (control), and those correspond to 0.05% and 0.1% CeO2 reinforcement. This was due to the low stability of the
P.M. Ashraf, S.M.A. Shibli / Electrochemistry Communications 9 (2007) 443–448
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formed oxide, which caused the anodic shift, had the stability only up to 17 days. Similar observations were also noted in the case of C1 and C2, but only after 17 and 30 days, respectively. This could be due to the formation of metastable pits, which initiate and grow for a short period below the critical pitting potential and then repassivate [1]. The aluminium reinforced with 0.4% cerium oxide showed a trend of irregular variation of OCP throughout the period, revealing that the composite could not form a protective and stable oxide scale on its surface. The OCP of C3, C5 and C6 were found comparatively stable throughout the exposure period and it varied from 0.714 to 0.802 V, 0.708 to 0.858 V, and 0.760 to 0.863 V, respectively. These data clearly revealed that a gradual increase in the stability of OCP could be achieved with increased amounts of cerium oxide reinforcement. The presence of CeO2 in aluminium not only improved the electrochemical characteristics of the reinforced aluminium but also formed stable protective oxide film on the surface.
Fig. 1. The SEM images of 0.2% CeO2 cerium oxide – reinforced aluminium (C3) (a) fresh (b) after exposure in 3.5% NaCl.
Fig. 2. The average OCP with standard deviation of the cerium oxide – reinforced aluminium coupons exposed in 3.5% NaCl for 40 days.
coupons exposed in aggressive corrosion medium of NaCl. The aluminium reinforced with 0.2% CeO2 (0.775 ± 0.014 V) exhibited the lowest standard deviation revealing the highest stability of these coupons in the aggressive environment. The open circuit potential of the un-reinforced aluminium was found to be shifted to its minimum value within 11days of the exposure and it reshifted to its normal equilibrium value after 17th day. This revealed that the initially
3.2.2. Linear sweep polarization analysis The linear polarization curves of all the different cerium oxide – reinforced aluminium coupons including the control-coupon were recorded. The corrosion potential (Ecorr) values were found to be varying in between 0.681 and 0.807 V (Table 1). Ecorr of all the cerium oxide – reinforced aluminium coupons were found to be significantly low when compared to the control. The lowest Ecorr value was noted in the case of aluminium reinforced with 0.2% CeO2. The anodic current in the E vs. log i plot of 0.2% cerium oxide – reinforced aluminium was found to be extremely stable during the initial stage of anodic polarization, revealing good stability of the matrix. The corrosion current density (Icorr) varied in between 2.27 · 10 7 and 4.86 · 10 6 A/cm2. The polarization resistance, Rp, was found to vary in between 1070 and 48,800 X. The presence of cerium oxide in aluminium was the cause for the displacement of cathodic branch towards lower current densities as well as a decrease in the corrosion potential of the system. 3.2.3. Impedance analysis Impedance spectra of the different cerium oxide – reinforced aluminium are presented in Fig. 3. Pure aluminium exhibited an impedance pattern with a wide arc at high freTable 1 The linear sweep voltammetric parameters of cerium oxide – reinforced aluminium in 3.5% NaCl CeO2 reinforcement (%) 0 0.05 0.10 0.20 0.40 0.70 1.00
Ecorr (V) 0.681 0.698 0.714 0.807 0.747 0.731 0.769
Icorr (A/cm2) 1.13 · 10 4.86 · 10 1.28 · 10 7.89 · 10 2.60 · 10 2.27 · 10 3.26 · 10
6 6 6 7 6 7 7
Rp (X) 8.19 · 103 3.35 · 103 1.01 · 104 1.25 · 104 4.61 · 103 2.66 · 104 6.69 · 104
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P.M. Ashraf, S.M.A. Shibli / Electrochemistry Communications 9 (2007) 443–448 Table 2 The impedance data of cerium oxide reinforced aluminium in fresh coupons and after 40 days of exposure in 3.5% NaCl
7 6
- Z' ' / o h m
5
Details
CeO2 reinforcement (%)
Rp (X)
CPE (F)
Fresh coupons
0 0.05 0.10 0.20 0.40 0.70 1.00
8.94 8.44 80.58 85.15 81.91 82.60 83.59
3.68 · 10 3.07 · 10 1.09 · 10 1.01 · 10 1.01 · 10 9.77 · 10 1.02 · 10
8
0 0.05 0.10 0.20 0.40 0.70 1.00
8.622 78.91 84.24 79.44 81.57 82.57 83.50
2.96 · 10 1.12 · 10 9.55 · 10 1.24 · 10 1.12 · 10 1.07 · 10 1.08 · 10
8
4 3 2 1
C0NTROL C1
0 -3
-2
-1
0
1 Z' / ohm
2
3
4
90
After 40 days of exposure
80 70
-Z'' / ohm
60 50 40
Impedance parameters of high frequency region n 8 8 8 8 9 8
8 9 8 8 8 8
1.164 1.193 1.199 1.195 1.188 1.179 1.198 1.275 1.1957 1.159 1.214 1.1947 1.1819 1.1923
30 20 10 C2
0 -80
-70
-60
-50
40
-30 Z' / ohm
-20
-10
0
10
175 150
-Z' ' / oh m
125 100 C3 0.20%
75 50 25 0 -100
-75
-50
-25
0
25
50
75
Z' / ohm
Fig. 3. The Nyquisit plots of the control and different cerium oxide – reinforced aluminium coupons.
quency region and a small one at low frequency region. A similar trend was observed in the case of 0.05% and 0.10% cerium oxide – reinforced aluminium also. Their Rp values were found to be significantly low compared to other treatments (Table 2). The impedance spectra of the coupons C3–C6 showed reverse characteristics, i.e., a small arc at high frequency region and wide one at low frequency region. The polarization resistance, CPE and n values of the semi circle of high frequency region varied from 8.94 to 85.15 X, 9.77 · 10 9 to 3.68 · 10 8 F and 1.164 to 1.199, respectively. There was no marked variation in ‘n’ though the values were found to be comparatively low in the case of the un-reinforced aluminium. These results revealed that the outer layer formed in the case of the control and 0.05% CeO2 – reinforced aluminium could be attributed to the formation of aluminium oxide. The presence of low amount of cerium oxide
in C1 had not made any significant influence on polarization resistance. These results revealed that the cerium oxide reinforcement in aluminium had strengthened both the weak outer layer of aluminium oxide and also the inner cerium oxide-aluminium layer. The inner layer had the leading role in corrosion protection character of the metal matrix [15]. Similarly, the constant phase element and capacitance of the high frequency semi circle were found to be significantly low in the case of C3–C6, reiterating the role of cerium oxide reinforcement. The impedance spectra of C4, C5, and C6 are found to be very similar to that of C3 and hence not shown here. The impedance data revealed that 0.2% cerium oxide – reinforcement could yield better performance compared to the aluminium reinforced with higher amount of cerium oxide. There was no proportional improvement in the performance with increase in CeO2 content, as evidenced based on the impedance parameters. According to Yu et al. [16] rare earth metal oxide based surface modification of aluminium alloy creates a barrier to the supply of oxygen and the supply of electron from the aluminium alloy surface to the corrosion medium. In the present case, a similar mechanism can probably exist in between the aluminium CeO2 composite surface and the corrosion medium. Hence the necessary reactions of corrosion would have been suppressed, thereby reducing the driving force of corrosion. 3.3. Evaluation of corrosion rate 3.3.1. Weight loss tests The coupons were immersed in 3.5% NaCl solution for a period of 40 days under laboratory conditions. The corrosion rate (Table 3) and the corrosion inhibition efficiency were determined to be in the range of 1.814 · 10 4 to 8.007 · 10 3 mpy and 61.91 to 97.73%, respectively. The corrosion inhibition efficiency was found to be decreasing
P.M. Ashraf, S.M.A. Shibli / Electrochemistry Communications 9 (2007) 443–448 Table 3 The corrosion rate values (mpy) of cerium oxide – reinforced aluminium exposed in 3.5% NaCl at room temperature (40 days) and that in salt spray chamber (100 h)
-0.75
CeO2 reinforcement (%)
Corrosion rate (mpy)
-0.81
0.00 0.05 0.10 0.20 0.40 0.70 1.0
8.007 · 10 3.048 · 10 9.991 · 10 1.814 · 10 2.931 · 10 9.653 · 10 5.011 · 10
3 3 4 4 3 4 4
Salt spray 2.070 · 10 3.285 · 10 1.613 · 10 1.265 · 10 2.056 · 10 1.289 · 10 1.276 · 10
3
-0.77 -0.79
O CP V
Weight loss studies
447
-0.83 -0.85
3 3 3 3 3
-0.87 -0.89 -0.91 C0
C1
in the order of 0.20% > 1.00% > 0.70% CeO2 reinforced aluminium. This observation revealed that the reinforcement of aluminium with cerium oxide of these concentrations had improved the metallurgical characteristics of aluminium and enhanced the formation of effective protective film on its surface. This trend was found to be in accordance with that of the results based on OCP decay, LSV and impedance analysis. 3.3.2. Salt spray test Few aluminium coupons were exposed in a salt spray chamber for a period of 100 h continuously and the corrosion rates were calculated based on the weight loss data. The corrosion rate was determined to be in the range from 1.265 · 10 3 mpy to 3.285 · 10 3 mpy. The aluminium coupon reinforced with 0.05% CeO2 exhibited high corrosion rate. Lowest corrosion rate was noted in the case of 0.20% CeO2 – reinforced aluminium (Table 3). In the aggressive salt spray environment, the 0.2% cerium oxide – reinforced aluminium showed 39% more corrosion inhibition efficiency than the control. Thus the optimum concentration of CeO2 reinforcement in aluminium was confirmed to be 0.2% since reinforcement of more quantity of CeO2 did not shown the equivalent corrosion resistance. Hence further studies of structural analysis were carried out with this composition of C3. 3.4. Structural analysis 3.4.1. Anodic stripping The coupons were subjected to anodic stripping by applying an anodic current of 100 mA/cm2. After every multiples of 1-h intervals of current interruption, the OCP of the freshly exposed surface was monitored. The process was continued for eight batches (8 h). The un-reinforced coupons exhibited significantly higher OCP than the reinforced coupon (Fig. 4), due to the influence of the freshly formed surface – oxide layer. The control-coupon showed the highest standard deviation while the C5 showed the lowest. All the CeO2 – reinforced aluminium exhibited lower OCP with low standard deviation. This was attributed to the slow and steady formation of CeO2aluminium oxide layer. These results revealed that there
C2
C3
C4
C5
C6
TREATMENTS
3
Fig. 4. The average OCP with standard deviation of the cerium oxide – reinforced aluminium coupons, measured after subjected to anodic stripping.
was uniformity in the composite reinforcement, improving the metallurgical characteristics that led to high stability of aluminium in the saline environment. 3.4.2. Impedance characteristics after marine exposure The electrochemical impedance spectra of the coupons retrieved, after exposing in 3.5% NaCl for 40 days, was recorded. The Nyquisite plots of impedance analysis were found to be similar to the corresponding spectra shown in Fig. 3. The Rp values varied in between 8.622 and 83.498 in the semicircle at high frequency region (Table 2). The coupon C1 was the exceptional case, where an increase in Rp values from 8 to 79 X at the high frequency region was recorded. These results reiterated the role of cerium oxide on the stability of aluminium in the aggressive marine environment. All these results clearly indicate the effective corrosion inhibition characteristics of cerium oxide reinforced aluminium in saline environments. 4. Conclusion The corrosion of aluminium, in marine environments, can be suppressed significantly by means of reinforcing aluminium with cerium oxide. The results of the present physicochemical, structural and electrochemical evaluation studies clearly revealed the effective role of cerium oxide reinforcement in aluminium exploring its use in marine environments. The best aluminium metal matrix composite prepared with optimum concentration of 0.20% CeO2 performed excellently. Even though increase in cerium oxide content caused improved corrosion protection during some of the tests, the improvement was not in proportional to the amount of CeO2 incorporated. The present results lay strong emphasise on the potential scope of use of CeO2 for protection of aluminium in marine environments. Acknowledgements The authors sincerely thank Dr. K. Devadasan, Director and Dr. B. Meenakumari Head, Fishing Technology
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