AA6063 Corrosion

Sanaulla Pathapalya Fakrudeen, Lokesh H. B, Ananda Murthy H. C, Bheema Raju V. B /
International Journal of Engineering Research and Applications (IJERA) ISSN: 2248-9622
www.ijera.com Vol. 2, Issue 5, September- October 2012, pp.2049-2061
Electrochemical Behaviour of AA6063 Alloy in Hydrochloric Acid
using Schiff Base Compounds as Corrosion Inhibitors.

Sanaulla Pathapalya Fakrudeen*, Lokesh H. B**, Ananda Murthy H. C***,
Bheema Raju V. B****.
*
Department of Engineering Chemistry, HKBK College of Engineering, Nagawara, Bangalore Karnataka, India
**
Department of Engineering Chemistry, Vivekananad Institute of Technology, , Bangalore-Karnataka, India.
***
Department of Engineering Chemistry, R N Shetty Institute of Technology,
Bangalore, Karnataka, India.
****
Department of Engineering Chemistry, Dr. Ambedkar Institute of Technology,, Bangalore , Karnataka, India.

ABSTRACT
The electrochemical behavior of however, are reactive materials and are prone to
aluminium alloy AA6063 were investigated corrosion. A strong adherent and continuous
using Schiff base compounds namely N, N’-bis passive oxide film is developed on Al upon
(Salicylidene)-1, 4-Diaminobutane (SDB) and exposure to aqueous solutions. This surface film is
N, N’-bis (3-Methoxy Salicylidene)-1, 4 amphoteric and dissolves when the metal is
Diaminobutane (MSDB) as corrosion inhibitors exposed to high concentrations of acids or bases
in presence of 1M Hydrochloric Acid by weight [1]. Hydrochloric acid solutions are used for acid
loss, Potentiodynamic polarization(PDP), cleaning, acid de-scaling, chemical and
electrochemical impedance spectroscopy(EIS) electrochemical etching in many chemical process
and scanning electron microcopy (SEM). industries where in aluminium alloys are used. It is
Potentiodynamic polarization study revealed very important to add corrosion inhibitors to
that the two Schiff bases acted as mixed type prevent metal dissolution and minimize acid
inhibitors. The change in EIS parameters is consumption [2]. Most of the efficient acid
indicative of adsorption of Schiff bases on inhibitors are organic compounds that contain
aluminum alloys surface leading to formation of mainly nitrogen, sulfur or oxygen atoms in their
protective layer. The weight loss study showed structure. The choice of inhibitor is based on two
that the inhibition efficiency of these compounds considerations: first it could be synthesized
increases with increase in concentration and conveniently from relatively cheap raw materials;
vary with solution temperature and immersion second, it contains the electron cloud on the
time. The various thermodynamic parameters aromatic ring or electronegative atoms such as
were also calculated to investigate the nitrogen, oxygen in relatively long-chain
mechanism of corrosion inhibition. The effect of compounds. Numerous organic substances
methoxy group on corrosion efficiency was containing polar functions with nitrogen, oxygen,
observed from the results obtained between and sulphur atoms and aromatic rings in a
SDB and MSDB. The effectiveness of these conjugated system have been reported as effective
inhibitors were in the order of MSDB>SDB. The corrosion inhibitors for aluminium alloys [3-5].
adsorption of Schiff bases on AA6063 alloy Some Schiff bases have been reported earlier as
surface in acid obeyed Langmuir adsorption corrosion inhibitors for aluminum alloys [6-8], iron
isotherm. The surface characteristics of [9-10] and copper [11-12]. Compounds with π-
inhibited and uninhibited alloy samples were bonds also generally exhibit good inhibitive
investigated by scanning electron microscopy properties due to interaction of π orbital with metal
(SEM). surface. Schiff bases with RC = NR′ as general
formula have both the features combined with their
Keywords: Electrochemical techniques, structure which may then give rise to particularly
Corrosion, Schiff base, Adsorption, Aluminum potential inhibitors.
alloy.
The present work is aimed at investigating
1. Introduction: the inhibitive ability of two Schiff base molecules
Corrosion of aluminum and its alloys has on corrosion of AA6063 in 1M Hydrochloric acid
been a subject of numerous studies due to their medium. The weight loss, potentiodynamic
high technological value and wide range of polarization and electrochemical impedance
industrial applications especially in aerospace and techniques were employed to study inhibitive effect
house-hold industries. Aluminum and its alloys, of two Schiff base molecules at different
concentrations. The effect of temperature,

2049 | P a g e

immersion time on corrosion behaviour of samples were cut into cylindrical test specimens
aluminium alloy was also studied in absence and and moulded in cold setting Acrylic resin exposing
presence of inhibitor at various concentrations. For a surface area of 1.0 cm2 for electrochemical
further confirmation aluminum alloy samples were measurements. For weight loss experiments the
analyzed by scanning electron microscopic (SEM) cylindrical alloy rods were cut into 24 mm dia x
technique. 2mm length -circular cylindrical disc specimens
The inhibition effect of Schiff base compounds are using an abrasive cutting wheel and a 2mm
reported on steel [13-14], Copper [15], and pure mounting hole at the centre of the specimen was
aluminium and its alloys [16-19], in acidic drilled. Before each experiment, the electrodes
medium. However, no substantial work has been were abraded with a sequence of emery papers of
carried out on corrosion inhibition of aluminium different grades (600, 800, and 1200), washed with
alloys in acidic medium by Schiff bases. Thus, it double distilled water, degreased with acetone and
was thought worthwhile to study the corrosion dried at 353 K for 30 min in a thermostated electric
inhibition effect of Schiff base compounds oven and stored in a moisture-free desiccator prior
namely N, Nʹ-bis (Salicylidene)-1, 4- to use. The corrosive medium selected for this
Diaminobutane (SDB) and N, Nʹ-bis (3-Methoxy study was 1M hydrochloric acid, which was
Salicylidene)-1, 4 Diaminobutane (MSDB) on prepared from analytical grade 37 percent acid
AA6063 Alloy in 1M Hydrochloric acid medium. concentrated (Merck ) in double distilled water.

2. Experimental
2.1 Materials
The alloy samples were procured from
M/S. Fenfe Metallurgical, Bangalore, India. The
typical chemical composition of AA6063 alloy in
weight percentage is shown in Table 1. The alloy

Table-1. Typical Chemical Composition of AA 6063

Element Cu Mg Si Fe Mn Ti Cr Zn Others Al
Wt.% 0.10 0.45 - 0.2 - 0.35 0.10 0.10 0.10 0.10 0.15 Reminder
0.90 0.6 Max Max Max Max Max Max

2.2 Inhibitor. melting points, Fourier transform infrared
The Schiff Bases were prepared by the spectroscopy (FT-IR) and Proton Nuclear Magnetic
condensation of respective aromatic aldehydes with Resonance (1H NMR). The structure, molecular
each of diamines as per the reported procedure formula, molecular mass, melting points are shown
[20]. All reagents used were of analytical grade in Table-2.
procured from Sigma Aldrich. N,N'-
bis(Salicylidene)-1,4-Diaminobutane (SDB) was N,N'-bis(Salicylidene)-1,4-Diaminobutane
prepared by slow addition of Salicylaldehyde (2 IR (KBr cm-1): 3400(OH), 3054(=C-H), 2903(-
mmol) in 30 mL methanol over a solution of 1,4- CH), 1628(C=N).
1
diaminobutane (1mmol) in 30 mL methanol and HNMR (CDCl3): δ 1.79-1.82(t, 4H, -CH2CH2-), δ
N,N'-bis(3-MethoxySalicylidene)-1,4-
3.62-3.65(t, 4H, -CH2-N) 6.84–7.31 (m, 8H, ArH).
Diaminophenelyne (MSDB) by slow addition of
Methoxysalicylaldehyde (2 mmol) in 30 mL 8.34(s, 2H, N=CH), 13.49 (s, 1H, OH),
methanol over a solution of 1,4-diaminobutane (1
N,N'-bis(3-Methoxy Salicylidene)-1,4-
mmol) in 30 mL methanol taken in a 250 mL
condensation flask. In each case, 2-3drops of acetic Diaminobutane
acid was added to the mixture of aldehyde and
IR (KBr cm-1): 3429(OH), 2996(=C-H), 2932(-
diamine with stirring at constant temperature 25ºC
for 1 hour. Further the mixture was refluxed for 4-5 CH), 1628(C=N). 1253(-OCH3).
hours on a water bath, heating occasionally to 1
HNMR (CDCl3): δ 1.80-1.83(t, 4H, -CH2CH2-), δ
improve the yield of the product. The reaction
mixture was cooled to room temperature overnight 3.63-3.66(t, 4H, -CH2-N) δ 3.90(s, 6H, -OCH3),
and the colored compound was filtered off and
6.77–7.2 (m, 6H, ArH). 8.32(s, 2H, N=CH), 14.00
dried. The compounds were recrystallised with
ethanol. The product identity was confirmed via (s, 1H, OH),

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Table-2. The structure, molecular formula, molecular mass, melting points

Structure and Name Molecular Formula Molecular Melting
Mass Point

CH N (CH2)4 N CH
C18H20N2O2 296.36 89°C
OH HO
N,N'-bis(Salicylidene)-1,4-Diaminobutane (SDB)

CH N (CH2)4 N CH

O OH HO O
H3C CH3
C20H24N2O4 356.42 152°C
N,N'-bis(3-Methoxy Salicylidene)-1,4-Diaminobutane
(MSDB)

2.3 Weight loss measurements experiments were measured after immersion of alloy
Weight loss measurements were specimens for 30 minutes to establish a steady state
performed on aluminium alloys as per ASTM open circuit potential in absence and presence of
Method [21]. The test specimens were immersed inhibitors at 303 K.
in 100mL 1M hydrochloric acid solution in absence
and presence of different concentrations (25,50,75 Tafel plots were obtained using
and 100 ppm ) of SDB and MSDB at different conventional three electrode Pyrex glass cell with
temperature ranges (303, 313, 323 and 333 K) in alloy specimen (1cm2) as working electrode (WE),
thermostated water bath. The difference in weight platinum electrode (Pt) as an auxiliary electrode and
for exposed period of 2, 4, 6 and 8 hours was taken standard calomel electrode (SCE) as reference
as the total weight loss. The weight loss electrode. All the values of potential were referred to
experiments were carried out in triplicate and SCE..Tafel plots were obtained by polarizing the
average values were recorded. The corrosion rate electrode potential automatically from – 250 to +
was evaluated as per ASTM Method [21]. The 250 mV with respect to open circuit potential (OCP)
percentage of inhibition efficiency (µWL%) and at a scan rate 1mV s –1. The linear Tafel segments of
the degree of surface coverage (θ) were calculated anodic and cathodic curves were extrapolated to
using equations (1) and (2): corrosion potential (Ecorr) to obtain corrosion current
densities (Icorr). The inhibition efficiency was
evaluated from the Icorr values using the following
Wo – Wi (1) relationship (3):
µWL% = x 100
Wo
i0corr – icorr (3)
µp% = x 100
i0corr
Wo – Wi (2)
θ =
1
Where, i0corr and icorr are values of corrosion current
Wo densities in absence and presence of inhibitor
respectively.
Where Wi and Wo are the weight loss
values of aluminium alloy sample in the presence EIS measurements were carried out in a
and absence of the inhibitor and θ is the degree of frequency range from 100 kHz to 0.01 Hz with
surface coverage of the inhibitor. small amplitude of 10mV peak -to-peak, using AC
signal at OCP. The impedance data was analyzed
2.4 Electrochemical measurements. using Nyquist plot and Echem software ZSimpWin
Potentiodynamic polarization (PDP) and version 3.21 was used for data fitting. The
electrochemical impedance spectroscopy (EIS) inhibition efficiency (Rct %) was calculated from
measurements were performed using CH660c the charge transfer resistance (Rct) values using
electrochemical work station. All electrochemical following equation (4):

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Rict – R0ct
µ Rct % = x 100 (4) -0.5
Rict
-1.0
0
Where, R ct and Rict
are the charge transfer -1.5
resistance in
absence and presence of inhibitor, respectively. -2.0

-2
log i / mA / cm
-2.5
2.5 Scanning electron microscopy (SEM).
The surface morphology of the corroded surface in -3.0
the presence and absence of inhibitors were studied
-3.5 Balnk63
using scanning electron microscope (SEM) [Model
No JSM-840A-JEOL]. To understand the SDB25ppm
-4.0 SDB50ppm
morphology of the aluminium alloy surface in the
SDB75ppm
absence and presence of inhibitors, the following -4.5
SDB100ppm
cases were examined.
-5.0
(i) Polished aluminium alloy specimen. -0.5 -0.6 -0.7 -0.8 -0.9 -1.0 -1.1
(ii) Aluminium alloy specimen dipped in 1M HCl.
E / mV vs. SCE
(iii) Aluminium alloy specimen dipped in 1M HCl 1(a)
containing 100 ppm of Schiff base. )

3. Results and Discussion
3.1 Potentiodynamic polarisation (PDP) -0.5
The polarization measurements of -1.0
AA6063 alloy specimens were carried out in 1M
-1.5
Hydrochloric acid, in the absence and in the
presence of different concentrations (25 -100 ppm) -2.0
of SDB and MSDB at 303K in order to study the
-2
log i / mA cm

-2.5
anodic and cathodic reactions. The Fig.1(a) and (b).
represents potentiodynamic polarisation curves -3.0
(Tafel plots) of AA6063 alloy in 1M Hydrochloric -3.5
acid in absence and presence of various Balnk63
-4.0
concentrations of SDB and MSDB at 303K MSDB25ppm
respectively. The electrochemical parameters i.e. -4.5 MSDB50ppm
corrosion potential (Ecorr), corrosion current density MSDB75ppm
-5.0 MSDB100ppm
(icorr), cathodic and anodic Tafel slopes (ba and bc)
associated with the polarization measurements of -5.5
-0.5 -0.6 -0.7 -0.8 -0.9 -1.0 -1.1
SDB and MSDB are listed in Table.3. The
inhibition efficiency (µp %) of inhibitors at E / mV vs. SCE
different concentrations was calculated from the 1(b)
equation (4). It is observed from the PDP results
Figure. 1. potentiodynamic polarisation curves
that, in presence of inhibitors, the curves are shifted
(Tafel plots) of AA6063 alloy in 1M
to lower current density (icorr) regions and Tafel
Hydrochloric acid in absence and presence of
slopes ba and bc values increased with increase in
various concentrations of (a) SDB and (b)
concentration of inhibitors showing the inhibition
MSDB at 303K
tendency of SDB and MSDB. The corrosion
potential (Ecorr) values do not show any appreciable
shift, which suggest that both inhibitors acted as
mixed type but predominantly cathodic inhibitors
[22-23]. This can probably be due to the adsorption
of protonated Schiff base molecules on the cathodic
and anodic sites

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Table.3. Potentiodynamic polarisation parameters of AA6063 alloy in 1M Hydrochloric acid in absence and
presence of various concentrations of SDB and MSDB at 303K

3.2 Electrochemical impedance spectroscopy.
The effect of the inhibitor concentration on the
impedance behavior of AA6063 alloy in 1M 60 Blank63
SDB25ppm
Hydrochloric acid was studied and Nyquist plots of 50 SDB50ppm
AA6063 in absence and presence of various SDB75ppm
40 SDB100ppm
concentrations of Schiff bases are given in Fig 2 (a)
and (b). 30
2
-Z i /  cm

It is clear from the figure that the 20
impedance diagrams obtained yield a semicircle
10
shape. This indicates that the corrosion process is
mainly controlled by charge transfer. The general 0

shape of the Nyquist plots is similar for all samples -10
of AA 6063 alloy, with a large capacitive loop at
-20
higher frequencies and inductive loop at lower
frequencies. The similar impedance plots have been 0 10 20 30 40 50 602 70 80 90 100 110
2(a)
Zr /  cm
reported for the corrosion aluminum and its alloys
in hydrochloric acid [24-30].
Blank63
The Nyquist plot with a depressed semicircle with 80
MSDB25ppm
the center under the real axis is characteristic 70 MSDB50ppm
MSDB75ppm
property of solid electrode and this kind of 60
MSDB100ppm
phenomenon is known as the dispersing effect [31- 50

32] An 40
2

equivalent circuit fitting of five elements was used
-Z i /  cm

30

to simulate the measured impedance data of 20

AA6063 alloy is depicted in Fig.3. 10

0

-10

-20

-30
2(b)
0 10 20 30 40 50 60 70
2
80 90 100 110 120
Zr /  cm

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Figure. 2. Nyquist plot for AA6063 alloy in 1M [35]. The double layer capacitance (Cdl) can be
Hydrochloric acid in absence and presence of calculated from the equation.(6) [36].
various concentrations of (a) SDB and (b) MSDB (6)
at 303K Cdl = Y0 (ωmax) n – 1
Where Cdl is the double layer capacitance and ωmax
is the angular frequency at which – Z″ reaches
maximum and n is the CPE exponent.

The electrochemical impedance parameters Rs, Rct,
ω, CPE, n and Cdl are listed in Table-4.

The inhibition efficiency was evaluated by
Rct and Cdl values of the impedance data, it is
shown from Table.(4) that charge transfer
resistance ( Rct ) of inhibited system increased and
double layer capacitance (Cdl) decreased with
increase in inhibitor concentration. This was due to
adsorption of Schiff base molecule on the metal
surface, the adsorbed inhibitor blocks either
cathodic or anodic reaction or both formation of
physical barrier, which reduces metal reactivity.
The effect of inhibitor may be due to changes in
electric double layer at the interface of solution and
metal electrode. The decrease in double layer
Figure..3. The equivalent circuit model used to fit capacitance (Cdl) can be caused by decrease in local
the experimental impedance data. dielectric constant and /or increase in the thickness
of electric double layer, this suggest that the Schiff
The equivalent circuit includes solution base molecules inhibit the aluminium alloy by
resistance Rs, charge transfer resistance Rct, adsorption at the metal – acid interface. It is
inductive elements RL and L. The circuit also evident that the inhibition efficiency increases with
consists of constant phase element, CPE (Q) in increase in inhibitor concentration which is in good
parallel to the parallel resistors Rct and RL, and RL agreement with the Potentiodynamic polarization
is in series with the inductor L. The impedance results.
spectra for the aluminium alloy in absence and
presence of the inhibitors are depressed. The Table-4. Electrochemical impedance parameters of
deviation of this kind is referred as frequency AA6063 alloy in 1M Hydrochloric acid in absence
dispersion, and has been attributed to and presence of various concentrations of (a) SDB
inhomogeneous of solid surface of aluminum alloy. and (b) MSDB at 303K
Assumption of a simple Rct –Cdl is usually a poor
approximation especially for systems showing
depressed semicircle behavior due to non – ideal
capacitive behaviour of solid electrodes [33]. The
capacitor in the equivalent circuit can be replaced
by a constant phase element (CPE), which is a
frequency dependent element and related to surface
roughness. CPE is substituted for the respective
capacitor of Cdl in order to give a more accurate fit.
The impedance function of a CPE is defined in
impedance [34] representation as (5).

1
ZCPE =
( Y0jω)n (5)

Where, Y0 magnitude of CPE, n is
exponent of CPE, and are frequency independent,
and ω is the angular frequency for which – Z″
reaches its maximum value, n is dependent on the
surface morphology : – 1 ≤ n ≤ 1. Y0 and n can be
calculated by the equation proved by Mansfeld et al

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3.3 Weight loss measurements 90
The experimental data of weight loss (∆w),
SDB
percentage of inhibition efficiency (µWL%), MSDB
Corrosion Rate (C.R.) in mmpy and degree of 80
Surface Coverage (θ) for AA6063 in 1M
Hydrochloric acid in absence and presence of
various concentration of SDB and MSDB Schiff 70

% IE
bases at 2 hours of exposure time and different
temperature are shown in Table. 5.
60
3.3.1 Effect of inhibitor concentration
The variation of inhibition efficiency (µWL%) with
inhibitor concentration is shown in Fig..4(a). 50
Increase in inhibition efficiency at higher 0 2 4 6 8 10

concentration of inhibitor may be attributed to Time / h

larger coverage of metal surface with inhibitor 4(b)
molecules. The maximum inhibition efficiency was
achieved at 100 ppm and a further increase in 90

inhibitor concentration caused no appreciable SDB
MSDB
change in performance
3.3.2 Effect of immersion time 80

The effect of immersion time on inhibition
efficiency is shown in Fig..4(b). All the tested
Schiff bases show a decrease in inhibition 70
% IE

efficiency with increase in immersion time from 2
to 8 hours. This indicates desorption of the Schiff
Base over a longer test period and may be 60

attributed to various other factors such as formation
of less persistent film layer on the metal surface,
and increase in cathodic reaction or increase in 50
303 313 323 333
ferrous ion concentration [37]. 4(c) Temp / K
3.3.3 Effect of temperature
The influence of temperature on inhibition
efficiency of two Schiff bases compounds is shown Figure. 4. Variation of inhibition efficiency with (a)
in Fig.4(c). The inhibition efficiency for the two Inhibitor concentration (b) Exposure time (c)
Schiff base compounds decreases with increase in Temperature in 1M Hydrochloric acid for SDB and
temperature from 303 to 333K. The decrease in MSDB.
inhibition efficiency with rise in temperature may
be attributed to desorption of the inhibitor
molecules from metal surface at higher
temperatures and higher dissolution rates of
aluminium at elevated temperatures.

90

SDB
85
MSDB

80
% IE

75

70

65

60
0 25 50 75 100 125

C / ppm
4(a)

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Table- 5. Weight loss parameters for AA6063 in 1M hydrochloric acid in the absence and presence of various
concentrations of SDB and MSDB at 2 hours of exposure time and different temperature.

3.3.4 Thermodynamic activation parameters 3.0
Thermodynamic activation parameters are Blank-63
important to study the inhibition mechanism. The SDB25ppm
2.5 SDB50ppm
activation energy (Ea) is calculated from the SDB75ppm
logarithm of the corrosion rate in acidic solution is 2.0
SDB100ppm
-1

a linear function of (1/T) -Arrhenius equation (7):
Log (CR) / mmy

log (CR) = – Ea / 2.303RT + A 1.5
(7)
1.0
Where, Ea is the apparent effective activation
energy, R is the universal gas constant and A is the 0.5
Arrhenius pre exponential factor. Plots of logarithm
of corrosion rate obtained by weight loss 0.0
measurement versus 1/T gave straight lines and 2.9 3.0 3.1 3.2 3.3
3 -1
slope equal to (– Ea/2.303R) as shown in Figs. 5(a) (1 / T)10 / K 5(a)
and 5(b) for SDB and MSDB respectively. The Ea
values calculated are listed in Table.6.

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0.5
3.0
Blank-63
Blank-63 MSDB25ppm
MSDB25ppm 0.0 MSDB50ppm
2.5 MSDB50ppm MSDB75ppm
MSDB75ppm MSDB100ppm

-1
Log CR / T / mmy K
MSDB100ppm -0.5

-1
2.0
-1
Log (CR) / mmy

-1.0
1.5

1.0 -1.5

0.5 -2.0

2.9 3.0 3.1 3.2 3.3
0.0 3 -1
(1 / T)10 / K
2.9 3.0 3.1 3.2 3.3 6(b)
(1 / T)10 / K
3 -1
5(b)
Figure..6. Arrhenius plot of log (CR/T) versus 1/T
Figure.5. Arrhenius plot of log CR versus 1/T in in absence and presence of (a) SDB and (b) MSDB
absence and presence of (a) SDB and (b)MSDB
Table-6. Thermodynamic parameters of activation
A plot of log (CR/T) versus 1/T gave a of AA6063 in 1M HCl in presence and absence of
straight line, Figs. 6(a) and (b) with a slope of (– different concentrations of SDB and MSDB
ΔH/2.303 R) and an intercept of [(log (R/Nh) +
(ΔS/2.303 R)], from which the values of ΔS* and Table-6. Thermodynamic parameters of activation
ΔH* were calculated. The straight lines were of AA6063 in 1M HCl in presence and absence of
obtained according to transition state equation (8): different concentrations of SDB and MSDB

C R= RT/ N h exp(–ΔH*/ RT) exp (ΔS*/ R)
(8)

Where, h is the Plank constant, N is the Avogadro
number, ΔS* is entropy of activation and ΔH* is the
enthalpy of activation. The ΔS* and ΔH* values
calculated are listed in Table. 6

0.5
Blank-63
SDB25ppm
0.0 SDB50ppm
SDB75ppm
SDB100ppm
-1
Log CR / T / mmy K

-0.5
-1

-1.0

-1.5

-2.0

2.9 3.0 3.1 3.2 3.3
3 -1
(1 / T)10 / K

6(a)

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The Ea values of aluminium alloy in 1M Where C(inh) is inhibitor concentration and K(ads) is
Hydrochloric acid in the presence of Schiff base an equilibrium constant for adsorption and
compounds are higher than those in the absence of desorption.
Schiff bases. The increase in the Ea values, with The K(ads) was calculated from the intercepts of the
increasing inhibitor concentration is attributed to straight lines on the C(inh) / θ axis Fig.7(a) and
physical adsorption of inhibitor molecules on the standard free energy of adsorption of inhibitor
metal surface [38]. In other words, the adsorption ΔG0ads was calculated using the relation (10);.
of inhibitor on the electrode surface leads to
formation of a physical barrier that reduces the (10)
ΔG0ads = – RT ln (55.5 Kads)
metal dissolution in electrochemical reactions [39].
The inhibition efficiency decreases with increase in
temperature which indicates desorption of inhibitor To calculate heat of adsorption (ΔH0ads) and
molecules as the temperature increases [40]. entropy of adsorption (ΔS0ads) ln K(ads) vs. 1/T was
The values of enthalpy of activation (ΔH*) are plotted as shown in Fig.7(b). The straight lines
positive; this indicates that the corrosion process is were obtained with a sloe equal to (– ΔH0ads / R)
endothermic. The values of entropy of activation and intercept equal to (ΔS0ads / R + ln 1/55.5). The
(ΔS*) are higher in the presence of inhibitor than values of equilibrium constant (K(ads)), Standard
those in the absence of inhibitor, The increase in free energy of adsorption(ΔG0ads ), enthalpy of
values of ΔS* reveals that an increase in adsorption (ΔH0ads) and entropy of adsorption
randomness occurred on going from reactants to (ΔS0ads) are listed in Table.7.
the activated complex [41-43].
The negative values of standard free of
3.3.5 Adsorption isotherms adsorption indicated spontaneous adsorption of
It is generally assumed that the adsorption of the Schiff bases on aluminium alloy surface. The
inhibitor at the interface of metal and solution is the calculated standard free energy of adsorption
first step in the mechanism of inhibition aggressive values for the Schiff bases are closer to – 40 kJ
media. It is also widely mole–1 and it can be concluded that the adsorption
of Schiff bases on the aluminium surface is more
acknowledged that adsorption isotherms provide chemical than physical one [45]. The sign of
useful insights into the mechanism of corrosion enthalpy and entropy of adsorption are positive and
inhibition. The investigated compounds inhibit the is related to substitutional adsorption can be
corrosion by adsorption at the metal surface. attributed to the increase in the solvent entropy and
Theoretically, the adsorption process has been to a more positive water desorption enthalpy. The
regarded as a simple substitution adsorption increase in entropy is the driving force for the
process, in which an organic molecule in the adsorption of the Schiff bases on the aluminium
aqueous phase substitutes the water molecules alloy surface.
adsorbed on the metal surface [44]. The surface The adsorption of Schiff base on the aluminium
coverage (θ) value calculated from weight loss data alloy surface can be attributed to adsorption of the
for different concentrations of Schiff bases was organic compounds via phenolic and iminic groups
used to explain the best adsorption isotherm. The in both cases. Among these two Schiff bases, the
value of surface coverage (θ) was tested chelate effect of MSDB is greater than that of SDB.
graphically for fitting a suitable adsorption This is due to the presence of two electron donating
isotherm. Attempts were made to fit surface groups of –OCH3 in MSDB structure than SDB.
coverage (θ) values of various isotherms including The more efficient adsorption of MSDB is the
Langmuir, Freundlich and Temkin isotherms. result of electronegative oxygen atoms present in
Among three adsorption isotherms obtained, the the MSDB compared to SDB Structure.
best fitted isotherm was the Langmuir adsorption
isotherm (C(inh) / θ vs. C(inh) ) Fig.7(a) with the
linear regression coefficient values (R2) in the
range of 0.9994 - 0.9996. The Langmuir adsorption
isotherm can be expressed by following equation
(9):

C(inh) 1
= + C(inh) (9)
θ K(ads)

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140

2
SDB-R =0.9996
120 2
MSDB-R =0.9994

100
C//ppm

80

60

40

7(a)
20
0 25 50 75 100 125

C / ppm

10.0

SDB
MSDB

9.5
-1
ln Kads/M

9.0

8.5

7(b)
8.0
2.9 3.0 3.1 3.2 3.3 Table-7. Thermodynamic parameters for the adsorption of
3
(1 / T)10 / K
-1 inhibitor in 1M HCl on AA6063 alloy at different temperatures

Figure.7. (a) Langmuir adsorption isotherm plot
and (b) Heat of adsorption isotherm plot for SDB
and MSDB

3.4 Scanning electron microscope (SEM)
Scanning electron microscopy of the
AA6063 sample of inhibited and uninhibited metal
samples is presented in Fig. 8. The SEM study
shows that the inhibited alloy surface is found
smoother than the uninhibited surface.

2059 | P a g e


a b

c d
Figure. 8. Scanning electron micrographs of (a) Polished AA6063 alloy, (b) After immersion in 1M HCl for
2h, (c) After immersion in 1M HCl for 2h in presence of 100 ppm SDB and (d) After immersion in 1M HCl for
2h in presence of 100 ppm MSDB.

4. Conclusions
1. The investigated Schiff bases are good 7. Scanning Electron Microscopy (SEM)
inhibitors for aluminium alloy 6063 in 1M shows a smoother surface for inhibited alloy
Hydrochloric acid solution. samples than uninhibited samples due to
2. In weight loss studies, the inhibition formation of protective barrier film.
efficiency (µWL%) of the Schiff bases
increase with increase in inhibitor References:
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increase in immersion time and temperature. Mater. Chem.Phys., 109(2008)492-499.
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demonstrate the Schiff bases under T.V.Valand
investigation act as mixed type but Mater.Chem.Phys.,29(1984)579-585.
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in accordance to the order: MSDB > SDB M.Kabbasakaloglu, Mater.Chem.Phys.,
for AA6063. 89(2005)176.
6. The adsorptions of Schiff bases on alloy 8. Hahner, G., Woll, Ch., Buck, M., Grunze,
surface in 1M HCl solution are governed by M., 1993. Langmuir 9 (1), 955.
Langmuir adsorption isotherm.

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