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37
Studies on Corrosion Behavior of 6061 Al-15 vol. pct. SiC(p) Composite in HCl Medium by Electrochemical Techniques
K. S. Shetty, A. N. Shetty*
Department of Chemistry National Institute of Technology Karnataka, Surathkal, Srinivasnagar-575025, Karnataka, India,
e-mail: nityashreya@gmail.com
The micro-galvanic corrosion of SiC reinforced 6061 Al alloys deprives its potential application in the acidic conditions or at high temperatures. Therefore, the corrosion behavior of 6061 Al-15 vol.pct. SiC(p) composite in HCl medium at different temperatures was studied using electrochemical impedance measurements and potentiodynamic polarization curves. The studies revealed that, on increasing the ionic concentration of the medium and temperature the rate of corrosion also increases. Scanning electron microscopic imaging of polished and immersed sample in the medium showed the smooth and uneven morphology respectively. Activation parameters like energy of activation was calculated from Arrhenius plots. Entropy and enthalpy values were derived from the plots of transition state theory equation.
Keywords: 6061 Al composite, electrochemical techniques, acidic corrosion, silicon carbide reinforcement.
УДК 620.193.4:544.6
INTRODUCTION
Aluminum, being world's third most abundant element, is associated with the distinguishable properties like strength, lightness, recyclability, formability, corrosion resistance, etc. [1-3]. However, to ameliorate the performance and to boost the strength to weight ratio they are more often reinforced with ceramic SiC particulate matter and transformed into composites [4-6]. The properties of aluminum composites, such as greater stiffness, reduced coefficient of thermal expansion, etc., make it exceedingly advantageous in applications like parts of aircrafts, building materials, architectural designs and also in food and packaging industries [7-9]. Although one can witness wide array of applications of aluminum composites, the common problem encountered is the lower corrosion resistant properties exhibited by them. The main reason behind this could be attributed to the discontinuity of the surface protective oxide film after reinforcement of ceramic SiC particles [10-11]. Semi-conducting behavior of SiC particles make them to act as efficient cathodic sites which enhances micro-galvanic corrosion [12-14]. The Corrosion behavior of composites are governed by processing techniques, type of reinforcement and particle size and are specific to individual composites. Therefore it results in insufficient information on the corrosion behavior of composites as the results are not comparable. The high temperature applications of composites comprises brake rotors of vehicles, drive shafts, etc. The surface preparation of metals for electroplating techniques involve acid cleaning, scouring and so on [15-17]. The effect of such processes leads to corro-
sion, which in turn may cause undesirable effects on the functioning and applications of the material. Severe pitting attack of these composites in the presence of highly aggressive anions like chloride ion for pure aluminum and its alloys have been reported [18-19]. However, a detailed electrochemical investigation on corrosion behavior of 6061 Al-15 vol. pct. SiC(p) composite in hydrochloric acid medium in particular, seldom appears in the literature. Consequently, the present research work was carried out to make a methodical study of the corrosion behavior of 6061 Al-15 vol. pct. SiC(p) composite in varied concentrations of hydrochloric acid medium and also at different temperatures.
EXPERIMENTAL
Material
Experiments were carried out using the 6061 Al-15 vol.pct. SiC(p) composite, supplied by NIIST Trivandrum, India. The specimen was used in the extruded rod form, whose extrusion ratio is 30:1. Table 1 gives the composition of the 6061 Al alloy, as given by the supplier. The cylindrical test coupon, obtained from the rod was molded with epoxy resin, exposing a constant area of 1.102 cm2 of the sample to the electrolyte media. The test coupons polished as per the metallographic practices. The coupon was subjected to belt grinding first, followed with polishing with different grades of emery papers, ranging from rough to smooth grades, and finally the surface is mirror finished by disc polishing using legated alumina. After cleaning with double distilled water and acetone, the test coupons were dried and immediately used for electrochemical analysis.
© Shetty K.S., Shetty A.N., Электронная обработка материалов, 2015, 51(4), 68-75.
Table 1. Composition of the base metal Al 6061 alloy
Element Cu Cr Si Mg Al
Composition (wt. %) 0.25 0.25 0.6 1.0 97.9
Medium
The electrolyte media of total five different concentrations 0.025M, 0.05M, 0.1M, 0.125M, and 0.25M were prepared by the stock dilution of 1M hydrochloric acid in double distilled water. Each of the solutions were titrated against sodium carbonate which is a primary standard to get accurate concentration using methyl orange as an indicator.
Electrochemical Measurements
The electrochemical work station, Gill AC having ACM instrument version 5 software was used to carry out the electrochemical measurements. A conventional three electrode Pyrex glass cell was used with the molded test coupon acting as a working electrode, the platinum electrode served as an auxiliary electrode, against the saturated calomel electrode (SCE) as the reference electrode. Immediately after the electrochemical impedance studies, test coupon was subjected to potentiodynamic polarization measurements without further surface treatment. The measurements were carried out in HCl media at different temperatures, ranging between 30-50oC.
Potentiodynamic Polarization Studies
The finely polished test coupons were immersed in the acid medium and was allowed to attain steady state open circuit potential (OCP). The potentiody-namic current-potential curves were recorded by polarizing the specimen to -250 mV cathodically and +250 mV anodically with respect to the open circuit potential (OCP) at a scan rate of 1 mV/s.
Electrochemical Impedance Spectroscopy (EIS) Studies
The EIS measurements were carried out by impressing a periodic 10 mV small amplitude of AC signal over a wide frequency spectrum of 100 kHz -0.01 Hz on the OCP. The impedance data were analyzed by using Nyquist plots.
All the results reported are average of three similar results.
Scanning electron microscopy (SEM) Analysis
JEOL JSM-6380LA model SEM was used to image the surface morphology of the corroded and polished specimen of 6061 Al-15 vol.pct. SiC(p) composite.
RESULTS AND DISCUSSION
Potentiodynamic Polarization Measurements
The potentiodynamic polarization curves were recorded for the corrosion of the composite in different concentrations of hydrochloric acid and at different temperatures. Potentiodynamic polarization curves for the corrosion of the composite sample in different concentrations of HCl at 40°C are shown in Fig. 1. Similar plots were obtained at other temperatures considered. The electrochemical parameters such as corrosion potential (Ecorr), the corrosion current density (icorr) and the Tafel cathodic slope pc are tabulated in the Table 2. Owing to the ill-defined anodic current plateau, the icorr values were determined by the extrapolation of linear cathodic part to the respective corrosion potentials alone [20]. The Tafel constants were calculated at 50 mV from the OCP, from slope values obtained by a tangent drawn where the cathodic region remains linear for at least one decade. The anodic region exemplifies the metal oxidation and the cathodic region interprets the evolution of hydrogen in the case of acidic conditions. The rate of corrosion (vcorr) was calculated using equation 1 [21].
K • icorr • EW P
(1)
where vcorr is the corrosion rate expressed as mm^y"1, K is the proportionality constant = 0.00327, icorr is the corrosion current density in ^A-cm"2, E.W is the equivalent weight of the composite, p is the density expressed as g-cm"3. The E.W was calculated using equation 2.
EW =
1
/ " )
(2)
where ni is the valence of the ith element of the alloy, fi is the weight fraction of the ith element in the alloy and wi is the atomic weight of the ith element in the alloy.
It may be observed that the corrosion current density and corrosion rate values increase when the concentration of HCl and the temperature are increased. As can be seen from Fig. 1 both the anodic and cathodic polarization curves are shifted towards the higher current density region when the concentration of HCl is increased. This trend implies the fact that the rate of both the anodic and cathodic reactions responsible for the corrosion of the composite sample are affected by the concentration of HCl.
In the presence of HCl, the Cl- ions are likely to percolate through the oxide film thereby retarding the self-healing ability of the oxide layer on the metal surface. This results in the formation of inter-
-400 -
-0.025 M -0.05 M -0.1 M -0.125 M -0.25 M
-900 -
-1000
U LUI II . L -UJ-UJUL--L J-LLLU11__L_LLL1H1L—l—UJUil]
IE-3
1
10
0.01 0.1
CuiTenl, mA/cm-Fig. 1. Potentiodynamic polarization curves for the corrosion of 6061 Al-15 vol. pct. SiC(p) composite in different concentrations of hydrochloric acid at 40°C.
200 300 400 500 600 Z'. ohm ■cm-
Fig. 2. Nyquist plots for the corrosion of 6061 Al-15 vol. pct SiC(p) composite in different concentrations of hydrochloric acid at 30°C.
Table 2. Potentiodynamic polarization data for the corrosion of 6061 Al-15 vol. pct. SiC(p) composite in hydrochloric acid.
Concentration (M) Temp (0C) Ecorr (mV) -Pc (mV-dec-1) icorr (^A-cm-2) Vcorr (mm-y-1)
0.025 30 -696.15 215.82 54.63 0.59
35 -676.98 192.23 102.63 1.10
40 -693.97 216.08 134.45 1.45
45 -690.68 225.49 161.74 1.74
50 -714.11 224.30 180.88 1.95
0.05 30 -666.43 209.23 122.59 1.32
35 -683.48 207.85 125.68 1.35
40 -676.98 204.29 191.69 2.06
45 -686.10 198.35 288.82 3.11
50 -693.68 217.36 385.91 4.15
0.1 30 -664.80 166.40 150.88 1.62
35 -659.46 199.00 269.59 2.90
40 -665.21 188.60 497.58 5.36
45 -691.22 204.50 726.33 7.82
50 -702.65 223.52 874.56 9.41
0.125 30 -660.51 270.02 643.35 6.92
35 -667.24 290.42 679.18 7.31
40 -667.35 247.53 791.76 8.52
45 -678.65 321.56 857.07 9.22
50 -656.99 220.43 978.53 10.53
0.25 30 -658.21 293.79 898.34 9.67
35 -664.10 224.44 1239.40 13.34
40 -658.86 308.04 1807.00 19.45
45 -684.72 277.1 1878.30 20.22
50 -667.12 246.89 2135.90 22.99
mediate soluble complex as given in the equation 3 below.
Al(s)+ nCr^AlC^ 3e -. (3)
The complex thus formed enhances the dissolution of the aluminum ions from the lattice into the solution and leads to local thinning of the passive layer making it pervious for further attack by chloride ions leading to severe pitting attack [16].
The equations (4) and (5) represent the anodic and cathodic reaction in acidic condition. The overall reaction involved in the corrosion reaction of the composite is given in equation (6).
( Al3++ 3e")x 2, (4)
(2H ++ 2eH2 t)x 3, (5)
2AL ) + 6H + ^ 2Al3+ + 3H21. (6)
Electrochemical Impedance Spectroscopy Measurements
The impedance plots for the corrosion of the composite in different concentrations of hydrochloric acid at 30oC are shown in the Fig. 2. The Nyquist plots presented one high frequency capacitive loop and a low frequency inductive loop. From the semicircles in the plots it can be inferred that the process of corrosion is largely charge-transfer controlled [22]. A large capacitive loop at high frequency (HF) region and a small inductive loop at lower frequency (LF) region observed in the impedance spectra are consistent with the plots reported in the literature for the corrosion of pure aluminum or aluminum alloys in medium like HCl, H2SO4, etc. [23]. The HF capacitive loop indicates the bulk relaxation process of the oxide layer that is present on the surface of aluminum [24]. Additional clarification of the presence of HF capacitive loop is imputed to the oxidation of aluminum at the electrode-electrolyte interface in precise [25]. The LF inductive loop is ascribed to the adsorption and inclusion of the Cl- and H+ ions into the oxide film [26]. These data are reliable with the mechanism that proposed the chemisorption of Cl-ions into the oxide surface forming oxide-chloride complex. Aluminum oxide film is regarded to be a parallel circuit of resistor due to the ionic conduction in the oxide film and a capacitor due to its dielectric properties [22]. For better interpretation of the EIS data, an equivalent circuit displayed as an inset in the Fig. 3, comprising of five circuit elements was derived from the simulation plot shown in the Fig. 3. The equivalent circuit consists of solution resistance (Rs), a constant phase element (Q) that is parallel to the charge transfer resistance (Rct) and inductive resistance (Rl). The Rl is in series with the inductor L. The constant phase element (Q) replaces an ideal capacitor (C) as the nature of semi-circle in the Nyquist plot appears to be depressed. This depression in the plot may be attributed to the inhomogene-ity of the solid surface of the composite during reinforcement with SiC particle. Equation 7 gives the expression using which the double layer capacitance may be calculated.
Cdl _ Y0 (2n/max )
(7)
Table 3 comprises of the results of EIS, from which it can be interpreted that as the concentration of the acid increases, the value of Rp decreases, which means that the resistance for the process of corrosion is decreased there by enhancing the degree of corrosion. Similar trend was witnessed by the results of potentiodynamic polarization measurements discussed above. EIS results of all other temperatures considered were similar to the one mentioned.
Effect of Temperature
The effect of temperature on the corrosion rate of the composite was assessed by running the EIS and potentiodynamic polarization curves at five different temperatures, starting from 30oC with an increment of 5oC up to 50oC.
The Fig. 4 and Fig. 5 depict the EIS and potentiodynamic polarization plots for the corrosion of the composite at different temperatures in 0.05M HCl and 0.25M HCl, respectively. From Fig. 4 one can infer that as the temperature rises the diameters of the semi-circles, which contribute to the polarization resistance in the impedance plot tend to diminish, indicating the increase in the corrosion rate [27]. Likewise, as seen in Fig. 5 the polarization curves shift to the higher icorr values as the temperature is increased, indicating an increase in the corrosion rate with the increase in temperature. These observations could be attributed to the fact that as the temperature increases the rate of the corrosion reaction increases just as any other chemical or electrochemical reactions. Arrhenius equation 9 was used to calculate the energy of activation (Ea) for the corrosion reaction.
Vcorr _
(9)
where A in the equation is a constant and R is the ideal gas constant. The Arrhenius plots for the corrosion of the composite in HCl medium of different concentrations are presented in Fig. 6. The activation energies were calculated using the slope obtained from linear fit of the plot [28].
The enthalpy of activation (AH#) and entropy of activation (AS#) were calculated from the transition state theory equation 10.
The exponent n in the equation 3 is the deviation parameter from the ideal capacitor behavior, fmax is the frequency at which the imaginary component of the impedance is maximum and Y0 is the amplitude.
In the presence of inductive loop, the polarization resistance (Rp) was calculated using the formula mentioned in equation 8.
v _ RT „AS*/R„-AH*/RT
corr~ Nh
(10)
R _ Rl * Rt
p Rl + RCI
(8)
where N is the Avogadro number, T is temperature in Kelvin and h is Planck's constant. The plots of ln (ycorr/T) versus 1/T for the corrosion of the composite in HCl medium of different concentrations are given in the Fig. 7. The enthalpy of activation and entropy of activation were then calculated from the slope and intercept values of the linear plot [29].
Fig. 3. Simulation plot and equivalent circuit (inset) for the corrosion of 6061 Al-15 vol. pct. SiC(p composite in 0.1M HCl at 40°C. Table 3. EIS data for the corrosion of 6061 Al-15 vol. pct. SiC(p) composite in hydrochloric acid
Concentration (M) Temp (oC) Rp (Q-cm2) Y„ (^F-cm-2) n
0.025 30 308.7 55.0 0.78
35 119.0 119.6 0.80
40 64.6 180.4 0.80
45 51.2 251.9 0.80
50 48.6 260.2 0.75
0.05 30 228.6 67.5 0.80
35 113.5 129.8 0.80
40 57.1 181.0 0.80
45 45.6 280.4 0.80
50 42.3 291.1 0.76
0.1 30 213.7 83.3 0.80
35 100.2 136.5 0.80
40 55.4 201.3 0.80
45 41.3 343.2 0.80
50 21.1 565.1 0.78
0.125 30 173.4 89.3 0.80
35 71.8 153.6 0.80
40 53.3 210.4 0.80
45 33.9 351.2 0.80
50 17.2 601.1 0.80
0.25 30 126.3 107.8 0.80
35 71.6 177.8 0.80
40 51.4 231.4 0.80
45 21.8 413.8 0.80
50 11.7 509.3 0.80
350 300 250
Ы, 200 с
Ц
É 150
50 0 •50
0 50 100 150 200 250 300 350 Z', oh m cm-
Fig. 4. Nyquist plots for the corrosion of 6061 Al-15 vol. pct. SiC(p) composite in 0.05M HCl at different temperatures.
-400 -500
> -600 £
1 -700 H
£ -800 -900 -1000
0.01 0.1 ! 10 Current, m A/cm-
Fig. 5. Potentiodynamic polarization plots for the corrosion of 6061 Al-15 vol. pct. ofSiC(p) composite in 0.25M HCl at different temperatures.
3.0
2.4
>1 1.8
Ë
£
"h 1.2
p
0.6
-0.0
-0.6
■ 0.025 M
• 0.05 M
A 0.1 M
V 0.125 M
A 0.25 M
0.00310 0,00315 0,00320 0,00325 0.00330 1/T, K 1
Fig. 6. Arrhenius plots for the corrosion of 6061 Al-15 vol. pet. of SiC(p) composite.
The Table 4 enlists activation parameters for the corrosion of 6061 Al-15 vol.pct. SiC(P) composite in HCl medium. It is evident from the results presented in the Table that value of activation energy, Ea decreases with the increase in the concentration of the acid, which is confirmatory with the increase in the rate of corrosion. The entropy of activation (AS*) is negative, indicating an association process in the formation of the intermediate activated compound [30-35].
Table 4. Activation parameters for the corrosion of 6061 Al-15 vol. pct. SiC(p) composite
Concentration Ea AH# AS#
(M) (kJ/mol) (kJ/mol) (J/K/mol)
0.025 46.69 44.09 -102.45
0.05 50.82 48.22 -84.77
0.1 77.37 74.77 -26.88
0.125 17.45 14.85 -180.16
0.25 35.18 32.58 -117.96
■ 0.025 M • 0.05 M *> 0.1 M »0.125 M - 0.25 M
_6'4 0.00310 0.00315 0.00320 0.00325 0.00330 1/T, K'!
Fig. 7. Plots of ln ( vcoJT) versus 1/T for the corrosion of 6061 Al-15 vol. pct. of SiC(p) composite.
Scanning Electron Microscopic Study
The SEM images of the freshly polished sample and the sample immersed in 0.25M HCl for one hour at room temperature are shown in Fig. 8 and Fig. 9, respectively. Figure 8, shows a smooth and even surface and also ceramic SiC reinforced particle whereas Fig. 9 shows rough morphology and the metal dissolution around the reinforced particles is evident. The corrosion appears to be severe around the SiC particles due to the formation of minute electrolytic cell within the restricted area, SiC ceramic acting as cathode and metal acting as anode [22]. The galvanic action triggers the corrosion process.
Fig. 8. SEM image of freshly polished surface of 6061 Al-15 vol. pct SiC(p) composite.
Fig. 9. SEM image of the corroded surface of Al-15 vol. pct. SiC(p) composite.
CONCLUSIONS
A detailed investigation on the corrosion behavior of 6061 Al-15 vol. pct. SiC(p) composite in hydrochloric acid medium of different concentrations and temperatures enabled to draw the following conclusions:
• The corrosion process of the composite is markedly influenced by the environmental parameters like temperature and the HCl concentration.
• Rate of the corrosion increases with the increase in the temperature and acid concentration.
• The kinetics of the corrosion process complies with Arrhenius law.
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Received 14.07.14 Accepted 21.11.14
Реферат
Микро-гальваническая коррозия алюминиевого сплава 6061 упрочняемого SiC лишает возможности его использования в кислых средах или при высоких температурах. Поэтому коррозионное поведение композита алюминиевый сплав 6061 с 15%об. SiC в среде
HCl при различных температурах изучалась методами измерения электрохимического импеданса и регистрации потенциодинамических поляризационных кривых. Исследования показали, что при увеличении ионной концентрации среды и температуры скорость коррозии увеличивается. Изображения поверхности полированного образца и после его выдержки в коррозионной среде, полученные методом сканирующей электронной микроскопии, показали гладкую и неровную морфологии, соответственно. Параметры активации такие, как энергии активации рассчитывали по кривым Аррениуса. Значения энтропии и энтальпии были получены на основании кривых уравнения теории переходного состояния.
Ключевые слова: 6061 алюминиевый композит, электрохимические методы, кислотная коррозия, упрочнение карбидом кремния.
