Evaluation of inhibitive action of some quaternary N-heterocyclic compounds on the corrosion of Al–Cu alloy in hydrochloric acid

Abstract

The inhibitive action of some quaternary N-heterocyclic compounds namely 1-methyl-4[4′(-X)-styryl] pyridinium iodides (X: -H, -Cl and -OH)on the corrosion of Al–Cu alloy in 0.5 M HCl solutions was evaluated by potentiodynamic polarization, electrochemical impedance spectroscopy and weight loss measurements. All the studied compounds showed good inhibitive characteristics against the corrosion of Al–Cu alloy in the tested solutions and their performance increases with inhibitor concentration. Polarization data indicated that the studied compounds are cathodic inhibitors without changing the mechanism of hydrogen evolution reaction.The adsorption of all inhibitors on Al–Cu alloy obeys Langmiur adsorption isotherm. The effect of temperature (30–70 °C) on the inhibition efficiency at certain concentration of the studied compounds was investigated. The data revealed that the studied compounds have good pickling inhibitor's quality as they perform well even at relatively high temperature. The corrosion activation parameters (Ea, ΔH*, ΔS* and ΔG*) were estimated and discussed. It was found that Ea values for Al–Cu alloy corrosion in the inhibited solutions were higher than that for the uninhibited solution, indicating good inhibitor characteristics with physical adsorption mechanism. The effect of acid, s anion on the performance of the studied inhibitors was studied and discussed.

Full text

Materials Chemistry and Physics 114 (2009) 533–541
Contents lists available at ScienceDirect
Materials Chemistry and Physics
journal homepage: www.elsevier.com/locate/matchemphys
Evaluation of inhibitive action of some quaternary N-heterocyclic compounds
on the corrosion of Al–Cu alloy in hydrochloric acid
Ehteram A. Noor∗
King Abd El-Aziz university, Girls, College of Education, Chemistry Department, Al Maddares Street, Jeddah, KSA, Saudi Arabia
article info
Article history:
Received 19 June 2008
Received in revised form
19 September 2008
Accepted 29 September 2008
Keywords:
Corrosion
Inhibition
Al–Cu alloy
Hydrochloric
Pyridinium
Adsorption
abstract
The inhibitive action of some quaternary N-heterocyclic compounds namely 1-methyl-4[4(-X)-styryl]
pyridinium iodides (X: -H, -Cl and -OH)on the corrosion of Al–Cu alloy in 0.5 M HCl solutions wasevaluated
by potentiodynamic polarization, electrochemical impedance spectroscopy and weight loss measure-
ments. All the studied compounds showed good inhibitive characteristics against the corrosion of Al–Cu
alloy in the tested solutions and their performance increases with inhibitor concentration. Polarization
data indicated that the studied compounds are cathodic inhibitors without changing the mechanism of
hydrogen evolution reaction.The adsorption of all inhibitors on Al–Cu alloy obeys Langmiur adsorption
isotherm. The effect of temperature (30–70◦C) on the inhibition efficiency at certain concentration of
the studied compounds was investigated. The data revealed that the studied compounds have good pick-
ling inhibitor’s quality as they perform well even at relatively high temperature. The corrosion activation
parameters (Ea,H*, S* and G*) were estimated and discussed. It was found that Eavalues for Al–Cu
alloy corrosion in the inhibited solutions were higher than that for the uninhibited solution, indicating
good inhibitor characteristics with physical adsorption mechanism. The effect of acid,s anion on the
performance of the studied inhibitors was studied and discussed.
© 2008 Elsevier B.V. All rights reserved.
1. Introduction
Aluminium and its alloys have excellent durability and corro-
sion resistance, but, like most materials, their behaviour can be
influenced by the way in which they are used. Aluminium and its
alloys are widely used in many industries such as reaction vessels,
pipes, machinery and chemical batteries because of their advan-
tages. Hydrochloric acid solutions are used for pickling, chemical
and electrochemical etching of Al foil and lithographic panels which
substitute metallic zinc [1]. Since the metal dissolution in such
solutions is rather large, it is necessary to inhibit it by the addi-
tion of inhibitors, which should provide a good quality pickled
metal surface. Organic compounds containing polar groups such
as nitrogen, sulphur and oxygen as well as heterocyclic containing
conjugated double bonds have been reported as good inhibitors
for Al and its alloys [1,2–10]. The inhibiting of such compounds
is based on the adsorption ability of their molecules, where the
resulting adsorption film isolates the metal from the corrosion
environment. The inhibitor molecules are bonded to the metal sur-
face by chemisorption, physical adsorption or complexation, with
∗Tel.: +966 26939746.
E-mail address: m7o7o7n@hotmail.com.
the polar groups acting as the reactive centers in the molecules
[11].
Generally, the process of inhibitor’s adsorption is influenced
by the nature and the surface charge of the metal, the chemical
structure of organic inhibitors, the distribution of charge in the
molecule, the type of aggressive electrolyte and the type of interac-
tion between the organic molecules and the metallic surface [12].
However, determination of the type corresponding to the adsorp-
tion on the metal/electrolyte phase boundary gives much valuable
information as to the adsorption process, since it makes possible
to determine such quantities as the free energy of adsorption, its
dependence on the degree of surface coverage, the character of
the adsorption layer on the metal/electrolyte phase boundary, the
magnitude and character of interaction between the molecules and
the surface atoms of the metal. Therefore, the accurate determi-
nation for the type of adsorption isotherm corresponding to the
investigated adsorption process is of primary importance.
Recently, some quaternary N-heterocyclic compounds have
been reported as good corrosion inhibitors for Al–Si alloy in HCl
[2,3] and mild steel in HCl [13] and H3PO4[14]. The present work
is an extension of the earlier works and evaluate the inhibitive
action of similar compounds on the corrosion of Al–Cu alloy in 0.5 M
HCl using electrochemical (potentiodynamic polarization, PDP, and
electrochemical impedance spectroscopy, EIS) and weight loss (WL)
measurements.
0254-0584/$ – see front matter © 2008 Elsevier B.V. All rights reserved.
doi:10.1016/j.matchemphys.2008.09.065

534 E.A. Noor / Materials Chemistry and Physics 114 (2009) 533–541
2. Experimental
2.1. Materials
The experiments wereperformed with Al–Cu alloy specimens with the following
composition: 4.320% Cu, 0.210%Mg, 0,072% Pb, 0.099% Fe, 0.223% Mn, 0.634% Si, and
Al is the remainder.
The studied inhibitors were synthesized as previously reported in the literature
[15]. The molecular formula and nomination of the studied inhibitors is shown in
Table 1.
2.2. Solutions
The used acid solutions were made from analyticalgrade hydrochloric acid (HCl),
and sulphuric acid (H2SO4). H2SO4acid was used for comparative study. Appro-
priate concentration of acid was prepared using de-ionized water in absence and
presence of certain inhibitor’s concentration. The investigated range of inhibitor’s
concentration was 1.0 ×10−5to 5.0 ×10 −4M.
2.3. Electrochemical measurements
Electrochemical measurements were carried out in standard three-electrode
round glass electrochemical cell with a platinum wire as counter electrode (CE)
and a saturated calomel electrode (SCE) as reference electrode (RE). The working
electrode (WE) was in the form of a cylindrical rod from Al–Cu alloy inserted in
glass tube of suitable diameter and fixed with araldite so that a flat surface was the
only surface exposed to the tested solutions. The electrodeworking surface area was
0.785 cm2.
Prior the electrochemical measurements, the WE was abraded with emery
papers (grade 320-400-800-1000), washed with de-ionized water and acetone,
dried at room temperature and finally immersed and left for 10min in the tested
solution to maintain the open circuit potential. EIS measurements were carried
out over the frequency range of 10 kHz to 0.1Hz, with a signal amplitude per-
turbation of 30 mV by using Potentiostate/glvanostate ACM Gill AC instrument
model 655. EIS results were analyzed using software program named ZSimDemo
3.20. After each EIS run, the instrument turns on automatically to record the PDP
curves with scan rate of 60mV min−1from potential of −750 to −500mV vs.
SCE.
2.4. Weight loss measurements
The Al–Cu alloy rod of 5 cm in length and 1 cm in diameter was abraded with
emery papers (grade 320-400-800-1000) and then washed with de-ionized water
and acetone, dried at room temperature and then weighed. After weighing accu-
rately, the specimen wasimmersed in 50 ml of 0.5 M HCl in absence and presence of
certain concentration of 1-H, 2-Cl and 3-OH. After 90 min, the specimen were taken
out, washed, dried and weighed accurately. The corrosion rate (WL,gmin
−1cm−2)
for the studied specimen is determined by using the relation:
WL =W1−W2
At∞
(1)
where W1is the weight of the specimen before corrosion, W2is the weight of the
specimen after corrosion, Ais the surface area of the specimen and t∞is the final
immersion time.
It must be pointed out that all measurements were conducted in a stagnant and
open-air solution at 30 ◦C except otherwise stated.
3. Results and Discussion
3.1. Potentiodynamic polarization (PDP)
Anodic and cathodic polarization curves of Al–Cu alloy in
0.5 M HCl in absence and presence of various concentrations of
1-H, 2-Cl and 3-OH at 30 ◦C are shown in Fig. 1. An analysis
of the polarization curves indicates that at low over potential,
the Tafel relationship is followed showing that both anodic and
cathodic reactions are activation controlled [16]. In general, the
presence of increasing amount of the studied compounds leads
to dramatically decrease in the cathodic current density associ-
ated with limited increase in the anodic current density. However
the inhibited systems were shifted towards cathodic potentials,
emphasizing that the studied compounds act predominately as
cathodic inhibitors. This result is in good agreement with that
obtained in previous works [2,3] when similar compounds were
studied as inhibitors for the corrosion of Al–Si alloy in 0.25M
HCl.
The electrochemical parameters such as corrosion potential
(Ecorr), corrosion current density (icorr), anodic (ba) and cathodic
(bc) Tafel slopes are estimated by using Tafel ruler. The IEi% can be
given by the following equation:
IEi%=1−icorr
io
corr ×100 (2)
According to the electrochemical theory, polarization resistance
(Rp) is inversely proportional to the corrosion current density and
can be estimated from Stern–Geary equation [17]:
Rp=B
icorr
(3)
where ˇ=(babc)/(2.303(ba+bc)).
Table 2 represents the electrochemical parameters (ba,bc,Ecorr,
icorr,Rp and IEi%) obtained from PDP measurements for Al–Cu alloy
in 0.5 M HCl in absence and presence of various concentrations of
the studied compounds at 30 ◦C.
Table 1
Names, structures and abbreviations of the studied inhibitors.
Abbreviation Structure Inhibitor Nomination
1-H 1-Methyl-4[4(-H)-styryl] pyridinium iodide
2-Cl 1-Methyl-4[4(-Cl)-styryl] yridinium iodide
3-OH 1-Methyl-4[4(-OH)-styryl] pyridinium iodide

E.A. Noor / Materials Chemistry and Physics 114 (2009) 533–541 535
Table 2
Electrochemical parameters obtained from PDP measurements of Al–Cu alloy in 0.5M HCl in absence and presence of various concentrations of 1-H, 2-Cl and 3-OH.
Cinh (M) ba(mV dec−1)−bc(mV dec−1)−Ecorr (mV) icorr (mA cm−2)Rp(cm2)IEi%
1-H
0.0 89.75 96.49 590.33 6.70 3.01 –
1.0 ×10−575.74 98.32 593.24 3.66 5.08 45.37
5.0 ×10−543.39 107.82 603.64 2.14 6.28 68.06
1.0 ×10−442.79 92.15 610.49 0.67 18.94 90.00
2.5 ×10−446.52 98.28 615.45 0.32 42.84 95.22
2-Cl
0.0 89.75 96.49 590.33 6.70 3.01 –
1.0 ×10−561.25 99.31 592.89 3.07 5.36 54.18
5.0 ×10−549.64 94.48 599.88 1.84 7.68 72.54
1.0 ×10−436.25 93.90 605.15 0.39 29.12 94.18
2.5 ×10−437.89 90.10 630.60 0.16 72.39 97.61
3-OH
0.0 89.75 96.49 590.33 6.70 3.01 –
1.0 ×10−538.34 98.42 591.02 2.85 4.20 57.46
5.0 ×10−547.41 93.08 600.15 1.48 9.22 77.91
1.0 ×10−436.32 90.23 611.01 0.37 30.39 94.48
2.5 ×10−443.38 90.80 635.49 0.13 98.05 98.06
The data in Table 2 can be interpreted as follows:
•For the inhibited systems, the values of bcwere somewhat greater
than the values of ba, once again the studied inhibited systems are
under cathodic control.
•The cathodic Tafel slopes (bc) remain almost unchanged for the
uninhibited and inhibited systems, indicted that the inhibitive
action of the studied compounds is due to adsorption of
inhibitor cations on the cathodic active sites. In this case
the hydrogen evolution reaction (cathodic reaction) was sup-
pressed without changing its mechanism (i.e. simple blocking
mechanism).
•The anodic Tafel slopes remain almost unchanged for the inhib-
ited systems but with values lesser than that for the uninhibited
system. It was known that the dissolution of Al in inhibitor-free
HCl solution may be occurred by a simple mechanism with the
formation of AlCl3[18]:
Al +1/2O2⇔AL:O
ads (4-a)
Al:O
ads +Cl−⇔Al : OCl−
ads (4-b)
Al : OCl−
ads
r.d.s.
⇒Al : OClcomp +e (4-c)
Al : OClcomp +2Cl−+2H+fast
⇒AlCl3+H2O (4-d)
In the present study the inhibited solutions contain both I−(from
inhibitor dissociation) and Cl−ions (from the aggressive acid
solution), hence the dissolution process (anodic process) may be
proceeded by the formation of both AlCl3and AlI3leading to the
observed decrease in bavalue.
•The values of Ecorr were shifted to the cathodic direction with the
addition of increasing amount of the studied inhibitors, indicating
cathodic control mechanism.
•The value of icorr decreases while the value of Rpincreases with
the increase of inhibitor concentration which associated with an
increase in the corresponding IEi% value up to 95.22%, 97.61% and
98.06% for 1-H, 2-Cl and 3-OH, respectively.
•According to icorr,Rpand IEi% data, The inhibitive proper-
ties of the studied compounds can be given by the following
order:
1-H <2-Cl <3-OH
3.2. Electrochemical impedance spectroscopy (EIS)
The impedance spectrum obtained on Al–Cu alloy in 0.5 M HCl
at the open circuit potential is presented in Fig. 2 as Nyquist plot
at 30 ◦C. Fig. 2 shows a large semicircular capacitive loop at high
frequency which is then followed by a large inductive loop at low
frequencies. According to Bessone et al. [19] and de Wit et al. [20],
the high frequency capacitive loop could be attributed to the oxide
layer on Al. On the other hand, Lenderink et al. [21] have been
attributed the low frequency inductive loop to the relaxation of
adsorbed species like H+
ads. Inductive behaviour is also observed for
the pitted active state and attributed to the surfacearea modulation
or salt film property modulation [22].
The influence of various concentrations of 1-H compound on
the impedance spectra of Al–Cu alloy in 0.5 M HCl at the open cir-
cuit potential is shown in Fig. 3. Similar spectra were obtained for
2-Cl and 3-OH but are not shown. The increase of both the capac-
itive loop and the charge transfer resistance with the increase of
inhibitor concentration can be observed clearly in the recorded
Nyquist plots. As in the previous case, the inductive part of the
impedance was mainly determined by the relaxation process of H+,
Cl−adsorption and Al-dissolution, while the high frequency capac-
itive loop can be correlated with dielectric properties of a surface
layer; i.e. [metal-oxide–hydroxide-inhibitor]ads complex [23]. The
appearance of the inductive loop for all inhibited systems indicates
that the studied inhibitors do not affect the anodic process and this
result is in good agreement with that obtained from polarization
measurements.
However, the obtained semicircles either in absence or in pres-
ence of inhibitor were depressed. Deviations of this kind are
referred to as frequency dispersion, have been attributed to inho-
mogeneties of solid surfaces as the Al surface is always [24].A
practical way to represent distributed processes such as corrosion
of a rough and inhomogeneous electrode is with an element that
follows its distribution. The constant phase element (CPE) meets
that requirement. The impedance of CPE can be given in the form
[23]:
ZCPE =(A(iω)n)−1(5)
where the coefficient Ais a combination of properties related to
both the surface and the electroactive species. The exponent nhas
values between −1 and 1. A value of −1 is characteristic of an induc-
tance, a value of 1 corresponds to a capacitor, a value of 0.5 can be
assigned to diffusion phenomena.

536 E.A. Noor / Materials Chemistry and Physics 114 (2009) 533–541
Fig. 1. Potentiodynamic polarization curves for Al–Cu alloy corrosion in 0.5M HCl
in absence (1) and in presence (2) 1.0×10 −5M, (3) 5.0 ×10−5M, (4) 1.0 ×10−4M and
(5) 2.5 ×10 −4M of 1-H.
An equivalent circuit of five elements depicted in Fig. 4awas
used to simulate the measured impedance data as shown in Fig. 4b.
This consists of CPE in parallel to the parallel resistors Rtand RL, and
the later is in series with the inductor L. When an inductive loop is
present, the polarization resistance Rpcan be calculated from [6]:
Rp=RL×Rt
RL+Rt
(6)
Table 3 gives the numerical values of Rpand CPE of Al–Cu alloy in
0.5 M HCl solution in absence and presence of the studied inhibitors
Fig. 2. Nyquist plot for Al–Cu alloy in 0.5M HCl.
Fig. 3. Nyquist plot for Al–Cu alloy in 0.5M HCl in presence (1) 1.0 ×10−5M, (2)
5.0 ×10−5M, (3) 1.0 ×10−4M, (4) 2.5×10 −4M and (5) 5.0 ×10−4M of 1-H.
Table 3
Electrochemical parametersobtained from EIS measurements of Al–Cu alloy in 0.5 M
HCl in absence and presence of various concentrations of 1-H, 2-Cl and 3-OH.
Cinh (M) Rp(cm2) CPE ×103(F cm−2)IER%
1-H
0.0 2.88 3.37 –
1.0 ×10−54.56 1.60 36.84
5.0 ×10−57.83 0.98 63.22
1.0 ×10−427.20 0.20 89.41
2.5 ×10−452.08 0.11 94.47
5.0 ×10−485.46 0.10 96.63
2-Cl
0.0 2.88 3.37 –
1.0 ×10−55.44 1.56 47.06
5.0 ×10−59.11 0.91 68.38
1.0 ×10−443.24 0.19 93.34
2.5 ×10−472.18 0.06 96.01
5.0 ×10−4106.67 0.05 97.30
3-OH
0.0 2.88 3.37 –
1.0 ×10−55.86 1.29 50.85
5.0 ×10−511.28 0.77 74.47
1.0 ×10−445.14 0.08 93.62
2.5 ×10−4125.76 0.06 97.71
5.0 ×10−4152.38 0.04 98.11

E.A. Noor / Materials Chemistry and Physics 114 (2009) 533–541 537
Fig. 4. The equivalent circuit model used to fit the experimental data.
at 30 ◦C. It must be noted that all the studied systems give n values
approximately of unity, indicating the predominance of capacitive
behaviour. The IER% values can be calculated from Rpdata as fol-
lows:
IER%=1−R−1
p
Ro−1
p×100 (7)
where Ro
pand Rpare the polarization resistances in absence and
presence of inhibitor, respectively. The calculated IER% values were
also recorded in Table 3.
It was observed that the value of CPE decreases while the value
of Rpincreases with increasing inhibitor concentration, indicated
that the studied compounds inhibit the corrosion of Al–Cu alloy in
0.5 M HCl solution by the adsorption mechanism [25] and the thick-
ness of the adsorbed layer increases with the increase of inhibitor
concentration. However, Good consistency between the values of
Rpand IE% obtained from both PDP and EIS measurements was
observed.
Inspection of the impedance diagrams in Fig. 3 and the
corresponding values of Rpin Table 3, it was observed that
with an increase in inhibitor concentration the capacitive loop
increases more strongly than Rpvalue, indicating that the adsorbed
inhibitor’s cations on the electrode surface have no influence on
the rate of the anodic process [23]. This result agrees well with the
suggested cathodic control mechanism.
3.3. Weight loss measurements
3.3.1. Effect of inhibitor concentration
With the calculated corrosion rate (Eq. (1)) in absence and
presence of certain inhibitor’s concentration, the percentage of
inhibition efficiency (IE%) can be estimated as follows:
IEWL%=1−WL
o
WL ×100 (8)
where o
WL and WL are the corrosion rates in absence and presence
of inhibitor.
The values of WL and IEWL% for Al–Cu alloy in 0.5 M HCl in
absence and presence of various concentrations of 1-H, 2-Cl and
3-OH at 30 ◦C are given in Table 4. The obtained data revealed
that with increasing the concentration of the studied compounds,
a considerable decrease in the corrosion rate was observed which
normally associated with an increase in the corresponding values
of IEWL%. The performance of the studied compounds as corrosion
inhibitors can be written in the following increasing order:
1-H <2-Cl <3-OH
The above order is in good agreement with that obtained from elec-
trochemical measurements (PDP & EIS). It is also in the same order
of the electron donor property of the substituent type [26] except
that for chloro substituent. The unexpected behaviour of chloro
substituent was reported by some authors [27,28].
3.3.2. Adsorption isotherms
The primary step in the action of inhibitors in acid solutions
is generally agreed to be adsorption on to the metal surface. This
involves the assumption that the corrosion reactions are prevented
from occurring over the area (or the active sites) of the metal sur-
face covered by adsorbed inhibitor species, whereasthese corrosion
reactions occurred normally on the inhibitor-free area [29]. Accord-
ingly, the fraction of the surface covered with inhibitor species
(=IEWL%/100) can be followed as a function of inhibitor con-
centration and solution temperature. When the fraction of the
surface covered is determined as a function of the concentration
at a constant temperature, adsorption isotherm could be evaluated
at equilibrium conditions. The variation of surface coverage with
concentration of the studied compounds is shown in Fig. 5. These
curves have S-shaped adsorption isotherms that are characterized
by an initial increase in value with inhibitor concentration up to
2.5 ×10−4M, after which the value of does not change appreciably
Table 4
Corrosion parameters obtained from WL measurements of Al–Cu alloy in 0.5M HCl
in absence and presence of various concentrations of 1-H, 2-Cl and 3-OH.
Cinh (M) WL ×105(g cm−2min−1)IEWL %
1-H 2-Cl 3-OH 1-H 2-Cl 3-OH
0 5.512 –
1.0 ×10−53.529 2.929 2.817 35.98 46.86 48.89
5.0 ×10−52.084 2.025 1.994 62.2 63.26 63.82
1.0 ×10−41.455 1.401 1.146 73.61 74.58 79.21
2.5 ×10−40.333 0.275 0.275 94.02 95.01 95.01
5.0 ×10−40.104 0.099 0.097 98.12 98.2 98.24

538 E.A. Noor / Materials Chemistry and Physics 114 (2009) 533–541
Fig. 5. Variation of surface coverage () for Al–Cu alloywith inhibitor concentration.
with increasing inhibitor concentration, suggesting a formation of a
complete monolayer adsorbate film on the Al surface. Accordingly,
by far the results were best fitted by Langmuir adsorption isotherm
(Fig. 6), which is given by [30]:
Cinh−1=1
Kads
+Cinh (9)
where Kads (M−1) is the Langmuir constant which is defined as the
equilibrium adsorption constant and related to the free energy of
adsorption by the following equation:
log Kads =−log CH2O−Gads
2.303RT (10)
Table 5
Langmuir,s adsorption parameters for 1-H, 2-Cl and 3-OH in 0.5M HCl on Al–Cu
alloy.
Inhibitor Slope Kads ×10−4(M−1)−Gads (kJ mol−1)r2
1-H 0.96 3.08 36.14 0.998
2-Cl 0.97 4.03 36.81 0.998
3-OH 0.98 5.08 37.4 0.999
where CH2Ois the concentration of water in solution expressed
in M, Ris the universal gas constant and Tis the absolute tem-
perature. Table 5 represents the estimated Langmuir’s adsorption
parameters and the correlation coefficients (r2) of the given straight
lines. The data in Table 5 can be interpreted as follows:
•The slope of Langmuir’s straight lines was closed to unity meaning
that each inhibitor molecule occupies one active site on the metal
surface.
•The studied inhibitors showed high values for the equilibrium
constant of adsorption indicating that they adsorbed strongly
onto Al–Cu alloy surface and their adsorption ability can be given
in the following increasing order:
1-H <2-Cl <3-OH
•The high negative values of Gads also indicate strong and spon-
taneous adsorption of the studied compounds onto Al–Cu alloy
surface.
Moreover, the essential characteristic of Langmuir isotherm can
be expressed in terms of a dimensionless separation factor, KL[31]
which describe the type of isotherm and is defined by
KL=1
1+KadsCinh
(11)
Fig. 6. Langmuir,s adsorption isotherm for 1-H, 2-Cl and 3-OH in 0.5 M HCl on Al–Cu alloy.

E.A. Noor / Materials Chemistry and Physics 114 (2009) 533–541 539
Table 6
The values of dimensionless separation factor, KL, for 1-H, 2-Cl and 3-OH at various
concentrations.
Cinh (M) KL
1-H 2-Cl 3-OH
1.0 ×10 −50.7645 0.7128 0.6631
5.0 ×10−50.3937 0.3317 0.2825
1.0 ×10 −40.2451 0.1988 0.1645
2.5 ×10−40.1149 0.0903 0.073
5.0 ×10−40.061 0.0473 0.0379
Mean value 0.3158 0.2762 0.2442
If
KL>1 unfavourable;
KL=1 linear
0<K
L<1 favourable;
KL=0 irreversible;
Table 6 gives the estimated KLvalues for 1-H, 2-Cl and 3-OH
at different concentrations. It was found that all KLvalues are less
than unity confirming that the adsorption process is favourable.
However, according to the mean value of KLfor each inhibitor, the
inhibitory action of the studied compounds can be written in the
following order:
1-H <2-Cl <3-OH
3.3.3. Effect of temperature
The effect of temperature on the corrosion rate (WL) of Al–Cu
alloy in 0.5 M HCl in absence and presence of 5.0×10 −4M of all the
investigated inhibitors wasdetermined in the temperature range of
30–70 ◦C and illustrated in Fig. 7. It was observed that the corrosion
rate increases with temperature for all the studied systems and its
extent was more pronounced in the uninhibited system, indicating
the good inhibitive properties of the studied compounds under the
studied conditions. Table 7 givesthe variation of inhibitor efficiency
(IEWL%) with temperature. Table 7 shows good pickling inhibitors,
qualities as the studies compounds retain their protective proper-
ties even at relatively high temperature (70 ◦C).
Fig. 7. Variation of corrosion rate, WL, for Al–Cu alloy in 0.5M HCl in absence and
presence 5.0 ×10 −4M of 1-H, 2-Cl and 3-OH compounds at different temperatures.
Table 7
The values of IEWL% for 5.0 ×10 −4M of 1-H, 2-Cl and 3-OH in 0.5 M HCl at different
temperatures.
T(◦C) IEWL%
1-H 2-Cl 3-OH
30 98.12 98.2 98.24
40 97.06 97.61 97.28
50 92.59 95.18 92.64
60 86.4 95.05 89.29
70 82.88 94.02 82.76
The corrosion reaction can be regarded as an Arrhenius-type
process. The activation parameters for the studied systems were
calculated from Arrhenius equation (Eq. (12)) and transition state
equation (Eq. (13)) as follows:
log WL =log A−Ea
2.303RT (12)
log WL
T=log R
hN +S∗
2.303R−H ∗
2.303RT (13)
where Ais Arrhenius factor, Eais the apparent activation corrosion
energy, Nis the Avogadro’s number, h is the Plank’s constant and,
S* and H*are the entropy and the enthalpy changes of activa-
tion corrosion energies for the transition state complex. Arrhenius
and transition state plots for the corrosion rates (WL) of Al–Cu
alloy in absence and presence of 5.0 ×10−4M for each inhibitor are
given in Fig. 8 and the corresponding activation parameters (Ea,
H* and S*) for the corrosion process were estimated and listed
in Table 8. The change in the activation free energy (G*) of the cor-
rosion process can be calculated at each experimental temperature
by applying the famous equation:
G∗=H∗−TS∗(14)
The obtained G*values was also listed in Table 8. According to
the data recorded in Table8 the following discussion can be written:
Fig. 8. Arrhenius plots (solid lines) and transition-state plots(dashe d lines) of corro-
sion rate, WL, for Al–Cu alloy in 0.5 M HCl in the absence and presence 5.0 ×10−4M
of 1-H, 2-Cl and 3-OH.

540 E.A. Noor / Materials Chemistry and Physics 114 (2009) 533–541
Table 8
Corrosion activation parameters for Al–Cu alloy in 0.5M of HCl in absence and presence 5.0 ×10−4M of 1-H, 2-Cl and 3-OH.
Inhibitor Ea(kJ mol−1)H* (kJ mol−1)S*(Jmol
−1K−1)G* (kJ mol−1)
30 ◦C40
◦C50
◦C60
◦C70
◦C
Free acid 49.59 46.91 −171.18 98.78 100.49 102.2 103.91 105.62
1-H 101.1 98.42 −34.46 108.86 109.21 109.55 109.9 110.24
2-Cl 76.97 74.29 −113.93 108.81 109.95 111.09 112.23 113.37
3-OH 101.09 98.42 −35.42 109.15 109.51 109.86 110.21 110.57
•The results showed positive sign for both Eaand H*, reflecting
the endothermic nature of corrosion process. It is obviously seen
that the activation energy strongly increases in the presence
of inhibitor. Some authors [10,32,33] attributed this result to
that the inhibitor species are physically adsorbed on the metal
surface. In this respect the comparison of the inhibiting action of
the investigated compounds in HCl and H2SO4will be of definite
interest.
As observed, the trend of Eafor the studied inhibitors is not
the same with that obtained from inhibition efficiency. The
lower activation energy for compound 2-Cl as compared to that
of compound 1-H may be explained according to Riggs and
Hurd [34], as they stated that at higher level of surface coverage
the corrosion process may proceed on the adsorbed layer of
inhibitor and not on the metal surface leading to a decrease in
the apparent activation energy and in some cases becomes less
than that obtained in the absence of inhibitor.
•The negative values of S* pointed to a greater order produced
during the process of activation. This can be achieved by the
formation of activated complex represents association or fixation
with consequent loss in the degrees of freedom of the system
during the process [35].
•The values of G* were positiveand showed limited increase with
rise in temperature, indicating that the activated complex was not
stable and the probability of its formation decreased somewhat
with rise in temperature. So, the increase in the rate of corrosion
with rise in temperature (Fig. 7) can be attributed to large num-
ber of corrosion species passing into an activated state with a less
stable configuration [35]. However, G* values for the inhibited
systems were more positive than that for the uninhibited systems
revealing that in cores of inhibitor addition the activated corro-
sion complex becomes less stable as compared to its absence.
Fig. 9 represents the variation of H* with S*. This correla-
tion can be treated as the isokinetic relationship, where the slope
␤represents the isokinetic temperature [36]. The slope of the
straight line is 428 K, which is much higher than the experimen-
tal temperature. This indicates that the corrosion reaction is under
activation-control, where the addition of the studied inhibitors
plays an important role in reducing the corrosion rate but without
changing the corrosion mechanism as indicated by the observed
parallelism between H* and S* values.
3.3.4. Effect of the acid’s anion
Table 9 gives the values of corrosion rates (WL) for Al–Cu alloy
in 0.5 M of HCl and H2SO4in the absence and presence 1.0 ×10−4M
Fig. 9. H*vs.S* for Al–Cu alloy in 0.5 M HC in absence and presence 5.0 ×10−4M
of 1-H, 2-Cl and 3-OH.
of 1-H, 2-Cl and 3-OH. The calculated IEWL% values were also listed
in Table 9. It was found that in spitethe corrosion rate of the studied
alloy in HCl solution is higher than that in H2SO4solution, the inves-
tigated inhibitors are drastically more effective in the former than
in the latter. This result can be interpreted on the basis of three fac-
tors, these are the metal charge density, the size of the acid’s anion
and the chemical structure of the inhibitor.
As known the surface charge of the metal is due to the electri-
cal field which emerges at the interface on the immersion in the
electrolyte. It can be determined according to Antropov [37] by
comparing the potential of zero charge (PZC) and the rest potential
of the metal in the corresponding medium. An aluminium surface in
an aerobic environment always is covered with aluminium oxide.
It was stated that the pH of zero charge for aluminium oxide is
9–9.1 [38]. Below the isoelectric point (pH < 9) the Al surface has
a positive charge [38] that leads to electrostatic attraction of the
negatively charged species (Cl−or SO2−
4). The studied inhibitors are
organic compounds in the salt form in which the organic part being
the cation while the inorganic part being the anion (I−ions). So, in
view of the above the adsorption mechanism may occur as follows:
•Firstly, the acid’s anions (Cl−or SO2−
4) adsorb physically on the
positively charged metal surface, giving rise for a net negative
charge on the metal surface.
Table 9
Corrosion rate and inhibition efficiency for Al–Cu alloy in 0.5M of HCl and H2SO4in the absence and presence 1.0 ×10−4M of 1-H, 2-Cl and 3-OH.
The medium WL ×10−5(g cm−2min−1)aWL ×10 −5(g cm−2min−1)bIEWL%b
1-H 2-Cl 3-OH 1-H 2-Cl 3-OH
HCl 5.512 1.455 1.401 1.146 73.6 74.58 79.21
H2SO40.072 0.058 0.067 0.044 19.44 6.94 38.8
a0.0 M.
b1.0 ×10 −4M.

E.A. Noor / Materials Chemistry and Physics 114 (2009) 533–541 541
•Secondly, the organic cations are physically attracted to the
anions layer which is formed on the metal surface.
According to the ionic volume of the acid’s anions the small-
est anion (Cl−) attracted more faster to the metal surface than the
biggest one (SO2−
4) leading to good inhibitor performance in HCl
solution (Table 9).
4. Conclusion
The main conclusions are as follows:
•All the studied inhibitors show good inhibitive action against the
corrosion of Al–Cu alloy in 0.5 M HCl solution.
•The value of IE% increases with increasing inhibitor concentration
and after certain concentration it does not change appreciably
with inhibitor concentration.
•PDP measurements revealed that the studied inhibitors can be
classified as cathodic inhibitors without changing the cathodic
reaction mechanism.
•Good agreement between the data obtained from weight loss and
electrochemical measurements.
•The adsorption of all inhibitors on Al–Cu alloy obeys Langmiur,s
adsorption isotherm.
•The values of both Kads and Gads indicated that all the studied
inhibitors are strongly adsorbed on the Al–Cu alloy in 0.5M HCl.
•The IE% of the studied compounds decreases slightly with temper-
ature increase, showing good protective properties even at 70◦C
(82.9%, 94.0% and 82.2% for 1-H, 2-Cl and 3-OH, respectively).
•The Eavalues for Al–Cu alloy corrosion in the inhibited solutions
are higher than that for the uninhibited solutions indicating good
inhibitor characteristics associating with physical mechanism.
•The studied inhibitors (1.0 ×10 −4M) show inhibitive properties
in HCl more than in H2SO4, emphasizing the predominance of
physical adsorption mechanism.
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