Corrosion behavior of 6061/Al-15 vol. pct. SiC(p) composite and the base alloy in sodium hydroxide solution

P.D. Reena Kumari; Jagannath Nayak; A. Nityananda Shetty

doi:10.1016/j.arabjc.2011.12.003

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Original article

9 (

2_suppl

); S1144-S1154

doi:

10.1016/j.arabjc.2011.12.003

Corrosion behavior of 6061/Al-15 vol. pct. SiC_(p) composite and the base alloy in sodium hydroxide solution

P.D. Reena Kumari^{^a}, Jagannath Nayak^{^b}, A. Nityananda Shetty^{^c,⁎}

a Department of Chemistry, Srinivas School of Engineering, Surathkal, Mangalore 575 025, Karnataka, India

b Department of Metallurgical and Materials Engineering, National Institute of Technology Karnataka, Surathkal, Srinivasnagar 575 025, Karnataka, India

c Department of Chemistry, National Institute of Technology Karnataka, Surathkal, Srinivasnagar 575 025, Karnataka, India

⁎Corresponding author. Tel.: +91 824 2474200; fax: +91 824 2474033. nityashreya@gmail.com (A. Nityananda Shetty)

Received: 2011-7-30, Accepted: 2011-12-10, Published: 2012-11-01

Disclaimer:
This article was originally published by Elsevier and was migrated to Scientific Scholar after the change of Publisher.

Peer review under responsibility of King Saud University.

Abstract

The corrosion behavior of 6061/Al-15 vol. pct. SiC_(p) composite and 6061 Al base alloy was investigated in a sodium hydroxide solution. The electrochemical parameters were derived from potentiodynamic polarization and electrochemical impedance spectroscopic (EIS) techniques. The results showed that the corrosion resistance of the composite was lower than that of the base alloy in selected corrosion media. The corrosion rates of both the composite and the base alloy increased with the increase in the concentration of sodium hydroxide and also with the increase in temperature. The surface morphology of the metal surface was investigated using scanning electron microscope (SEM). Activation energy was evaluated using Arrhenius equation, and enthalpy of activation and entropy of activation values were calculated using transition state equation.

Keywords

Alloys

Composite

Alkaline corrosion

Electrochemical techniques

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1 Introduction

Metal matrix composites (MMC) are important class of materials, with non metallic reinforcements incorporated in metal matrices. Al alloy matrices, mainly 2024, 5052, 6061 and 7071 have been widely used as matrix materials with silicon carbide (particles) as their major reinforcing agent (Padro et al., 2005). These materials have received increased attention due to their potentially high fracture toughness and high strength to weight ratio. SiC_p/Al-base MMCs find potential applications in high-temperature environments, especially in automobile engine parts such as drive shafts, cylinders, pistons and brake rotors, and in aerospace applications. The high thermal conductivity and low coefficients of thermal expansion of these materials have lead to a number of applications in which they are exposed to potentially corrosive environments (Candan and Biligic, 2004; Bakkar and Neubert, 2007; Griffiths and Turnbull, 1994 ). One of the main disadvantages of particulate reinforced MMCs is the influence of reinforcement on the corrosion resistance. This is of particular importance in aluminum alloy based composites where corrosion resistance is imparted by a protective oxide film. The addition of a reinforcing phase could lead to further discontinuities or flaws in the protective film, increasing the sites for corrosion initiation and rendering the composite liable to severe corrosion attack (Trowsdale et al., 1996). The study on the corrosion behavior of metal matrix composites in different aggressive environments has continued to attract considerable attention because of the several important applications of these materials (Oguzie, 2007). These composites frequently come in contact with acids or bases during the process like cleaning, pickling, de-scaling, etc. It is known that aluminum and its alloys exhibit high corrosion rate in solutions containing aggressive anions or in highly alkaline solutions (Emergul and Aksut, 2000). Therefore, studying the corrosion behavior of aluminum alloys and their composites in the alkaline medium is of prime importance. This paper deals with the corrosion behavior of 6061 Al/15 vol. pct. SiC_(p) composite and 6061 Al base alloy in aqueous sodium hydroxide solution.

2 Methods

2.1

2.1 Material

In the present work, 6061 Al-15 vol. pct. SiC_(p) composite and its base alloy are under consideration. The composite is made of 6061 Al base alloy reinforced with particulate SiC (99.9% purity and 23 μm size). It was prepared by a stir casting technique at the National Institute of Interdisciplinary Science and Technology (NIIST) (Formerly RRL) Thiruvananthapuram. The experiments were performed with specimens of 6061 Al/15 vol. pct. SiC_(p) composite and 6061 Al alloy in the extruded rod form (extrusion ratio 30:1). The composition of the 6061 Al alloy is given in Table 1. Cylindrical test coupons were cut from the rods and sealed with epoxy resin in such a way that, the areas of the composite and the base alloy, exposed to the medium were 0.95 cm² and 0.785 cm², respectively. These coupons were polished as per standard metallographic practice, belt grinding followed by polishing on emery papers, and finally on polishing wheel using levigated alumina to obtain a mirror finish. It was then degreased with acetone, washed with double distilled water and dried before immersing in the corrosion medium.

Table 1 Chemical composition of the base metal.

Element	Cu	Si	Mg	Cr	Al
Composition (wt%)	0.25	0.6	1.0	0.25	Balance

2.2

2.2 Medium

Standard solutions of sodium hydroxide were prepared by dissolving analytical grade sodium hydroxide pellets in double distilled water. Experiments were carried out using a calibrated thermostat at temperatures 30 °C, 35 °C, 40 °C, 45 °C and 50 °C (±0.5 °C) in sodium hydroxide solutions of concentrations 0.05 M, 0.1 M, 0.25 M and 0.5 M.

2.3

2.3 Electrochemical measurements

Electrochemical measurements were carried out by using an electrochemical work station, Gill AC having ACM instrument Version 5 software. A three electrode compartment cell was used for the electrochemical measurements. The working electrode was made of either the composite or the base alloy as the case is. A saturated calomel electrode (SCE) and a platinum electrode were used as the reference and the counter electrode, respectively. Electrode potentials were measured with respect to saturated calomel electrode (SCE). The polarization studies were done immediately after the EIS studies on the same electrode without any further surface treatment. The experiments were carried out under unstirred and aerated conditions.

2.3.1

2.3.1 Tafel polarization studies

Finely polished composite and base alloy specimens were exposed to the corrosion medium of different concentrations of sodium hydroxide solution at different temperatures (30–50 °C) and allowed to establish a steady state open circuit potential (OCP). The potentiodynamic current-potential curves were recorded by polarizing the specimen to −250 mV cathodically and +250 mV anodically with respect to the OCP at a scan rate of 1 mV s⁻¹.

2.3.2

2.3.2 Electrochemical impedance spectroscopy (EIS) studies

In EIS technique a small amplitude ac signal of 10 mV and frequency spectrum from 100 kHz to 0.01 Hz was impressed at the OCP and impedance data were analyzed using Nyquist plots. The polarization resistance was extracted from the diameter of the semicircle in the Nyquist plot. In all the above measurements, at least three similar results were considered and their average values are reported.

2.4

2.4 Scanning electron microscopy (SEM)

The surface morphology of the composite and alloy samples immersed in sodium hydroxide media were compared with that of the un-corroded one by recording the SEM images of the surfaces using JEOL JSM-6380LA model analytical scanning electron microscope. The energy dispersive X-ray (EDX) analysis was also carried out with the same scanning electron microscope.

3 Results and discussion

3.1

3.1 Scanning electron microscopic study

The SEM photographs of the surfaces of the composite and the base alloy before and after immersion in 0.5 M sodium hydroxide are given in Fig. 1. The surface of the composite (Fig. 1a) shows a uniform distribution of SiC particles on the alloy matrix. The surface morphology of the freshly polished base alloy (Fig. 1c) shows homogenous single phase with scratches of mechanical polishing. The faceting seen in the Fig. 1b and d indicates severe corrosion of both samples. Sodium hydroxide being a highly corrosive medium for aluminum, both the base alloy and the composite samples are attacked more or less uniformly. However, the composite sample appears to have undergone more severe corrosion than the base alloy. This may be attributed to the corrosion due to the galvanic action, with the SiC particles acting as cathodic sites. This is well supported by the EDX analysis of the surface. Fig. 2 portrays the EDX spectrum of the corroded sample of the composite in sodium hydroxide solution. The spectrum shows peaks for aluminum and oxygen suggesting the presence of aluminum oxide/hydroxide. The presence of the peaks of silicon and carbon demonstrates the existence of SiC on the surface of the composite. Fig. 3 depicts the EDX spectrum for the corroded sample of the base alloy in the sodium hydroxide solution, with peaks corresponding to aluminum and oxygen, indicating the presence of only aluminum and aluminum oxide/hydroxide.

SEM image of (a) freshly polished surface of the composite (b) after immersion in 0.5 M NaOH. (c) Freshly polished surface of the base alloy (d) after immersion in 0.5 M NaOH.

EDX spectrum of the corroded sample of the composite.

EDX spectrum of the corroded sample of the base alloy.

3.2

3.2 Tafel polarization measurements

Figs. 4 and 5 show the potentiodynamic polarization curves recorded for Al/SiC_(p) composite and Al 6061 base alloy, respectively, in sodium hydroxide solutions of different concentrations at 30 °C. Similar results were obtained at other four temperatures also. The electrochemical parameters such as corrosion potential (E_corr), corrosion current density (i_corr) , anodic Tafel slope (b_a), the cathodic Tafel slope (b_c) and the corrosion rate (υ_corr) at different concentrations and different temperatures of the sodium hydroxide medium for both the composite and the base alloy are summarized in Tables 2 and 3, respectively.

The Tafel plots for the corrosion of Al/SiC(p) in different concentrations of NaOH at 30 °C.

The Tafel plots for the corrosion of Al 6061 base alloy in different concentration of NaOH at 30 °C.

Table 2 Results of Tafel polarization and electrochemical impedance studies for the corrosion of composite.

Medium	Tafel extrapolation						AC impedance, R_p (Ω cm²)
Medium	T (°C)	E_corr (mV)	b_a (mV dec⁻¹)	−b_c (mV dec⁻¹)	i_corr (mA cm⁻²)	CR (mm y⁻¹)	AC impedance, R_p (Ω cm²)
0.05 M NaOH	30	−1385	461	353	1.076	11.55	14.73
	35	−1399	409	381	1.228	13.58	13.53
	40	−1398	499	384	1.419	15.70	12.38
	45	−1355	512	461	1.619	17.91	10.73
	50	−1383	583	525	1.887	20.87	9.80

0.1 M NaOH	30	−1406	403	335	1.962	21.01	7.07
	35	−1421	358	304	2.047	21.93	6.00
	40	−1428	378	325	2.206	23.63	5.08
	45	−1418	388	380	2.590	27.73	4.65
	50	−1410	394	365	2.903	31.09	3.29

0.25 M NaOH	30	−1455	221	211	3.442	38.07	2.88
	35	−1439	236	218	3.697	40.89	2.66
	40	−1447	225	219	3.823	42.28	2.42
	45	−1442	222	204	4.071	45.03	2.05
	50	−1446	237	223	4.316	47.74	1.60

0.5 M NaOH	30	−1462	460	405	9.946	106.81	1.70
	35	−1466	490	359	10.603	113.87	1.58
	40	−1470	425	419	10.859	116.62	1.50
	45	−1476	447	420	11.433	122.76	1.28
	50	−1471	474	419	11.807	126.46	1.15

Table 3 Results of Tafel polarization and the electrochemical impedance studies for the corrosion of base alloy.

Medium	Tafel extrapolation						AC impedance, R_p (Ω cm²)
Medium	T (°C)	E_corr (mV)	b_a (mV dec⁻¹)	−b_c (mV dec⁻¹)	i_corr (mA cm⁻²)	CR (mm y⁻¹)	AC impedance, R_p (Ω cm²)
0.05 M NaOH	30	−1476	265	233	0.676	7.41	16.66
	35	−1472	303	259	0.704	7.72	14.09
	40	−1460	490	382	1.265	13.87	12.71
	45	−1459	533	422	1.521	16.69	10.03
	50	−1468	510	390	1.629	17.89	8.80

0.1 M NaOH	30	−1475	237	226	1.254	13.75	7.48
	35	−1478	276	269	1.524	16.72	5.81
	40	−1479	267	268	1.83	20.03	5.19
	45	−1468	344	339	2.27	24.93	4.81
	50	−1467	361	363	2.66	29.18	3.73

0.25 M NaOH	30	−1486	223	220	2.34	25.69	4.60
	35	−1497	189	185	2.56	28.08	3.93
	40	−1506	211	199	2.95	32.31	3.63
	45	−1498	220	244	3.67	40.25	3.15
	50	−1486	196	204	4.09	44.91	2.87

0.5 M NaOH	30	−1501	313	289	5.57	61.15	2.86
	35	−1492	323	293	6.10	66.89	2.24
	40	−1496	308	281	6.63	72.71	2.04
	45	−1481	262	265	7.04	77.20	1.75
	50	−1496	320	359	8.98	98.55	1.45

It is seen from the data in the tables that the corrosion current density (i_corr) increases with the increase in the sodium hydroxide concentration for both the composite and the base alloy. It is also evident from the (Figs. 4 and 5) and the data in the tables that the E_corr values are shifted in the negative potential direction with the increase in the concentration of sodium hydroxide. The trend is common with both the samples.

The decrease in corrosion potential (E_corr) value indicates the loss of passivity of the specimen due to thinning of primary oxide layer by the chemical dissolution action of hydroxide ions (OH^-) on increasing the concentration of the alkali (Pyun et al., 1999). Further, it is seen that the shift in E_corr is more in the case of composite than in the case of the base alloy. This indicates that thinning of primary oxide layer by hydroxide ion (OH^-) attack is more on the composite than on the base alloy. This could also be considered to be because of the discontinuities in the oxide film due to the presence of SiC particles on the surface of the composite, as seen from the SEM images (Figs. 1 and 2).

Fig. 6 shows the Tafel plots obtained for the composite and the base alloy in 0.5 M sodium hydroxide solution at 30 °C. The corrosion current density values indicate that the composite undergoes more corrosion than the base alloy.

The Tafel plots for the corrosion of Al/SiC(p) composite and Al 6061 base alloy in 1 M NaOH at 30 °C.

The corrosion of aluminum and aluminum alloys in the alkaline medium can be explained on the basis that the surface of these materials are covered by a passive film of alumina formed by the reaction of aluminum with air (Emergul and Aksut, 2000; Dhayabaran et al., 2004). When aluminum metal or alloy is immersed in alkaline solutions, the alumina layer rapidly dissolves due to the chemical dissolution (Lee and Kim, 2001). The cathodic reaction on the film covered electrode surface is mainly the reduction of water (Emergul and Aksut, 2000):

(1)

2 H_{2} O + 2 e \to H_{2} + 2 O H^{-}

The hydroxide ions formed increase the pH at the film/solution interface considerably, where as the pH of the bulk solution does not change. As hydrogen overvoltage on aluminum is very low, liberation of hydrogen takes place readily on the metal surface (Pyun et al., 1999; Dhayabaran et al., 2004). The increased local pH on the base metal surface accelerates the corrosion reaction, and damages the passive film.

It has been reported that anodic dissolution of aluminum in the alkaline medium takes place through a stepwise addition of surface hydroxyl species, culminating in the chemical dissolution of Al(OH)₃ in the presence of surface oxide film (Kyung and Kim, 2001). The overall anodic reaction taking place in the corrosion of aluminum in the alkaline solution is represented as under (Yan et al., 2007; Awad et al., 1979; Mohammed et al., 2008; Wang et al., 2007; Shao et al., 2001; Al-Kharafi and Badawy, 1998 ):

(2)

{Al}_{(s)} + 4 {OH}^{-} \to Al {(OH)}_{4}^{-} + 3 e^{-}

The overall main reaction taking place at the electrode surface can be represented by the following equation:

(3)

2 Al + 2 {OH}^{-} + 6 H_{2} O \to 2 Al {(OH)}_{4}^{-} + 3 H_{2}

The lower corrosion resistance of the composite can be attributed to its microstructure which consists of a fine dispersion of SiC particles in an Al-alloy based matrix. Therefore, it may be assumed that SiC reinforcement affects the corrosion behavior, through the modification of microstructures of the matrix alloy by altering the size and distribution of intermetallic phases between the reinforcement and the matrix (Padro et al., 2005).

3.3

3.3 Electrochemical impedance measurements

Nyquist plots for the corrosion of the composite and the base alloy in sodium hydroxide solutions of different concentrations at 30 °C are given in Figs. 7 and 8, respectively. Similar results were obtained at other four temperatures also. As can be seen from the figures, the impedance diagrams show semicircles, indicating that the corrosion process is mainly charge transfer controlled (Trowsdale et al., 1996; Montecelli et al., 1997).

Nyquist plots for the corrosion of Al/SiC(p) composite in different concentrations of sodium hydroxide at 30 °C.

Nyquist plots for the corrosion of Al 6061 base alloy in different concentrations of sodium hydroxide at 30 °C.

The general shapes of the curves are identical for the base alloy in NaOH solutions of different concentrations, with a large capacitive loop at higher frequencies (HF) and an inductive loop at intermediate frequencies (IF), followed by a second capacitive loop at lower frequency (LF) values. Similar results have been reported in the literature for the corrosion of pure aluminum in alkaline solutions (Shao et al., 2003; Abdel-Gaber et al., 2008). The impedance spectra of 6061Al-SiC composite consist of two capacitive loops, a bigger one at higher frequencies (HF) and the other smaller one at lower frequencies (LF). The high frequency capacitive loop could be assigned to the charge transfer of the corrosion process and to the formation of oxide layer (Mansfeld et al., 1987). The oxide film is considered to be a parallel circuit of a resistor due to the ionic conduction in the oxide film, and a capacitor due to its dielectric properties (Mansfeld et al., 1988). According to Brett (Brett, 1990, 1992) the capacitive loop is corresponding to the interfacial reactions, particularly, the reaction of aluminum oxidation at the metal/oxide/electrolyte interface. The process includes the formation of Al⁺ ions at the metal/oxide interface, and their migration through the oxide/ solution interface where they are oxidized to Al³⁺. At the oxide/solution interface, OH^- or O^2- ions are also formed. The fact that all the three processes are represented by only one loop could be attributed either to the overlapping of the loops of processes, or to the assumption that one process dominates and, therefore, excludes the other processes (Wit and Lenderink, 1996).

The inductive loop at intermediate frequencies may be due to the relaxation process in the oxide layer, present on the metal surface by the adsorbed intermediate species as ${OH}_{ads}^{-}$ (Lorenz and Mansfeld, 1981). The second capacitive loop observed at low frequencies could be assigned to the metal dissolution (Rehim et al., 2002). The impedance data were analyzed using an equivalent circuit (EC) that tentatively models the physical processes occurring at the metal-electrolyte interface. The EC, consisting of six elements as shown in Fig. 9, was used to simulate the measured impedance data for the corrosion of the composite in the sodium hydroxide solution. In this equivalent circuit, R_s is the solution resistance with the value at a high frequency intercept on the real impedance axis and L represents an inductive element. This also consists of two R₁-Q₁ and R₂-Q₂ terms in a series with L and R_s. The EC consisting of seven elements depicted in Fig. 10 was used to simulate the measured impedance data on the base alloy. R_ox is the resistance of the native oxide layer; C_dl describing the capacitance of the oxide film. This also consists of L in parallel to R₂ but in series with R₁, resulting from the interruption of anodic dissolution of Al by the surface charge buildup (Lee and Kim, 2001). According to the reported mechanism Al dissolves into the solution in the form of Al³⁺ through the generation of Al⁺ or Al²⁺ intermediate species (Shao et al., 2001; Al-Kharafi and Badawy, 1998). Therefore, the polarization resistance, R_p, might be represented by the sum of R₁ and R₂ in the equivalent circuit.

The equivalent circuit model used to fit the experimental data of the composite.

The equivalent circuit model used to fit the experimental data of the base alloy.

The measured values of polarization resistance decrease with the increase in the concentration of sodium hydroxide, indicating an increase in the corrosion rate for the base metal and the composite with the increase in the concentration of sodium hydroxide. This is in accordance with the observations obtained from potentiodynamic measurements.

It is seen from the impedance spectra that the frequency range over which the samples display capacitive behavior is narrower in the case of composites than in the case of the base alloy. The narrower frequency range observed indicates that the natural oxide layer formed on the composite surface is less protective. The addition of SiC as a reinforcing phase could have led to discontinuities in the protective film, thereby increasing the number of sites where corrosion can be initiated and causing higher corrosion on the composite (Abdel-Gaber et al., 2008). Also, the reinforcement of silicon carbide is highly cathodic to the matrix and causes the galvanic effect at the interfaces between the matrix and the reinforcement. These results are consistent with the potentiodynamic polarization data obtained.

3.4

3.4 Effect of temperature

The effect of temperature on the corrosion rate of the base alloy and the composite was studied by measuring the corrosion rate at different temperatures between 30-50 ^°C. Figs. 11 and 12 represent the potentiodynamic polarization curves at different temperatures for the corrosion of the composite and the base alloy, respectively, in 0.5 M sodium hydroxide solution. Figs. 13 and 14 represent the Nyquist plots for the same. It is clear from the plots that the corrosion rate increases with the increase in temperature for both the base alloy and the composite in the entire concentration range of sodium hydroxide studied. This is attributable to the enhanced chemical film dissolution due to enriched OH^- ions and depleted $Al {(OH)}_{4}^{-}$ ions at the film/solution interface. Thus with the increasing temperature the oxide passive film becomes thin, porous and less protective as a result of dissolution of the film by the electrolyte (Foley and Nguyen, 1982).

The Tafel plots for the corrosion of Al/SiC(p) at different temperatures in 0.5 M sodium hydroxide solution.

The Tafel plots for the corrosion of base alloy at different temperatures in 0.5 M sodium hydroxide solution.

Nyquists plots for the corrosion of Al/SiC(p) composite in 0.5 M sodium hydroxide at different temperatures.

Nyquists plots for the corrosion of Al 6061 base alloy in 0.5 M sodium hydroxide at different temperatures.

The electrochemical parameters, i_corr , anodic Tafel slope b_a, cathodic Tafel slope b_c and polarization resistance R_p that is associated with the polarization and impedance measurements in the sodium hydroxide medium of different concentrations and at different temperatures for the corrosion of both the composite and the base alloy are summarized in Tables 2 and 3, respectively.

The energy of activation for the corrosion of the base alloy and the composite were calculated using Arrhenius law,

(4)

ln (υ_{corr}) = - E_{a} / RT + A

where E_a is the apparent effective activation energy, T is the absolute temperature, R is the universal gas constant, and A is Arrhenius pre-exponential factor and υ_corr is the corrosion rate.

An alternative formulation of the Arrhenius equation is the transition state equation (Salih and Juaid, 2007; Satpati and Ravindran, 2008):

(5)

ln (υ_{corr}) = (RT / Nh) \exp Δ S^{#} / R \exp - Δ H^{#} / RT

where N is the Avogadro’s number, h the Planck’s constant, ΔH^# the enthalpy of activation and ΔS^# is the entropy of activation.

Figs. 15 and 16 show the Arrhenius plots for the corrosion of the composite and the base alloy, respectively, in sodium hydroxide solutions of different concentrations, using the data obtained from potentiodynamic polarization method. Figs. 17 and 18 show the plots of ln(υ_corr/T) versus 1/T for the corrosion of the composite and the base alloy, respectively, in sodium hydroxide solutions of different concentrations. As seen from the figures, the plots give straight lines from which the activation parameters were determined from their slopes and intercepts. The calculated values of E_a, ΔH^# and ΔS^# for the two samples, are given in Table 4. The data given in Table 4 show that, values of E_a of the corrosion of Al/SiC_(p) composite are lower than those of the Al 6061 base alloy in all the concentrations of sodium hydroxide, confirming to the suggestion that the corrosion resistance of the base alloy is higher than that of the composite. The negative sign of ΔS^# indicates that the activation complex formation step involves association rather than dissociation (Abdel-Gaber et al., 2008).

Arrehnius plots for the corrosion of Al/SiC(p) composite.

Arrehnius plots for the corrosion of base alloy.

Plots of ln(CR/T) vs 1/T for the corrosion of composite at different concentrations.

Plots of ln(CR/T) vs 1/T for the corrosion of base alloy at different concentrations.

Table 4 Activation parameters for the corrosion of composite and base alloy.

NaOH (M)	E_a (kJ mol⁻¹)		ΔH^# (kJ mol⁻¹)		−ΔS^# (J mol⁻¹ K⁻¹)
NaOH (M)	Composite	Base alloy	Composite	Base alloy	Composite	Base alloy
0.05	23.75	41.24	21.15	38.64	154.9	101.25
0.1	16.51	31.00	13.91	28.40	174.18	129.59
0.25	8.93	23.99	6.33	21.39	193.89	147.78
0.5	6.73	17.79	4.13	15.19	192.55	161.00

4 Conclusions

Based on the systematic study of the corrosion behavior of 6061 aluminum alloy and its composite 6061 Al-15 vol. pct. SiC_(p) composite in different concentrations of sodium hydroxide at different temperatures by electrochemical methods the following conclusions are made.

The potentiodynamic polarization and impedance studies of the corrosion behavior of the composite 6061 Al-15 vol. Pct. SiC(p) and the base alloy in sodium hydroxide solutions showed that the corrosion rate of the composite is higher than that of the base alloy.
The corrosion rates of both the base alloy and composite increase with the increase in the concentration of sodium hydroxide.
The corrosion rates of both the base alloy and the composite increase with the increase in temperature.
The experimental results show that the composite as well as the base alloy do not form the passivation layer in the chosen corrosion media.

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Wang J.B., Wang J.M., Shao H.B., Zhang J.Q., Cao Chu.-Nan., . The corrosion and electrochemical behavior of pure aluminum in alkaline methanol solutions. J. Appl. Electrochem.. 2007;37:753-758.
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Wit J.H., Lenderink H.J.W., . Electrochemical impedance spectroscopy as a tool to obtain mechanistic information on the passive behavior of aluminum. Electrochim. Acta.. 1996;41:1111-1119.
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Yan Z., Yu-su S., Yuan-sheng W., . Synergetic effect of stannate with o-aminophenol on inhibiting H₂ evolution of Al anode in strong alkaline media. J. Shanghai Univ.. 2007;11:426-431.
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Introduction

Methods

Material
Medium
Electrochemical measurements
Scanning electron microscopy (SEM)

Results and discussion

Scanning electron microscopic study
Tafel polarization measurements
Electrochemical impedance measurements
Effect of temperature

Conclusions

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[Google Scholar]

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[Google Scholar]

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[Google Scholar]

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[Google Scholar]

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[Google Scholar]

[26] Shao H.B., Wang J.M., Zhang Z., Zhang J.Q., Cao C.N., . The cooperative effect of calcium ions and tartarate ions on the corrosion inhibition of pure aluminum in an alkaline solution. Mat. Chem. Phys.. 2003;77:305-309.
[Google Scholar]

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[Google Scholar]

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[Google Scholar]

[29] Wit J.H., Lenderink H.J.W., . Electrochemical impedance spectroscopy as a tool to obtain mechanistic information on the passive behavior of aluminum. Electrochim. Acta.. 1996;41:1111-1119.
[Google Scholar]

[30] Yan Z., Yu-su S., Yuan-sheng W., . Synergetic effect of stannate with o-aminophenol on inhibiting H₂ evolution of Al anode in strong alkaline media. J. Shanghai Univ.. 2007;11:426-431.
[Google Scholar]