1. B U L LE T I N
O F
EELLEECCTTRROOCCHHEEMMIISSTTRRYY
*Author for correspondence
Email: anahle@sharjah.ac.ae
Corrosion inhibition of (anthraquinone-2-ylmethyl) triphenyl phosphonium
bromide on mild steel in HCl solution
Ayssar Nahlé*
, Ideisan Abu-Abdoun, and Ibrahim Abdel-Rahman
Department of Chemistry, College of Arts & Sciences, University of Sharjah, Sharjah, P.O. Box: 27272, UAE
Abstract
The effect of synthesized (Anthraquinone-2-ylmethyl) triphenyl phosphonium bromide (ATPB) on the corrosion inhibition of mild
steel in 1M HCl solution was investigated electrochemically and by weight loss experiments at temperatures ranging from 303 to 343 K.
The studied inhibitor concentrations were between 1×10–7
M and 2×10–5
M. The percentage inhibition increased with the increase of the
concentration of the inhibitor. The percentage inhibition reached about 99.03 % at the concentration of 2×10–5
M at room temperature and
98.5 % at 303 K. On the other hand the percentage inhibition decreased with the increase of temperature. Using the Temkin adsorption
isotherm, the thermodynamic parameters for the adsorption of this inhibitor on the metal surface were calculated. (Anthraquinone-2-
ylmethyl) triphenyl phosphonium bromide was found to be an excellent potential corrosion inhibitor since it contained not only
phosphorus, but also three phenyl rings together with the anthraquinone constituent that contains two carbonyl groups (C O) .
Keywords: Corrosion; Inhibitor; Aromatic Phosphonium Salts; (Anthraquinone-2-ylmethyl) triphenyl phosphonium bromide; Temkin adsorption isotherm.
Introduction
Organic cations having one or more of the heteroatom
(N, P, S, and O) and containing -bonds in their structure
are the most effective corrosion inhibitors for metals due to
the interaction of the electron pair on the heteroatom and
-orbital with metal surface [1–13]. However the way of
this inhibition is not so obvious by all authors. Some of
them [1,5,6,12] have attributed this inhibition due to
chemisorption through an electron pair of the heteroatom.
Others have assumed an electrostatic interaction between
the cations and the negatively charged surface of the metal
[3,4,13].
Phosphonium salts have been found to inhibit iron
corrosion in acidic media according to the following
mechanism [6,7,12]:
(R3PR') +
+ H+
+ 2 e–
R3P + R'H (1)
Where R, R' = alkyl or aryl. If R is phenyl, the triphenyl
phosphine (PPh3) was assumed to adsorb on the metal
surface via its lone pair of electrons on the phosphorus
atom or/and through the -electrons on the phenyl rings
and consequently inhibits corrosion. The organic cation,
for example (ph)4P+
, is adsorbed on the sites of the metal
surface where the chloride anion is chemisorbed [14]:
FeCl–
+ (Ph)4P+
(FeCl–
– +
P(Ph)4)ads. (2)
The corrosion inhibition of triphenyl phosphonium salts
on various metals (Ni, Zn, and Fe) and alloys in acidic
media was studied by many authors [15–20]. In all these
studies, the phosphorus atom and the phenyl rings in the
compounds showed to be able to adsorb very well on the
metal surface and form protective layer, which in turns
increased the corrosion inhibition with the increase in the
concentration of the inhibitor, reaching in some cases 98%
and 99% inhibition [21,19].
No studies have been reported on ATPB inhibitor used
in our present work, in terms of studying both the
electrochemical effect and the temperature effect on the
corrosion inhibition of mild steel in 1M HCl solution. Mild
steel was chosen in our studies since high temperature
aggressive acids are widely used in industries in
connection to mild and low alloy steels.
In this work, ATPB was prepared via a synthesis which
produced a high percentage yield of this pure compound.
The aim of this work is to study, using potentiodynamic
and weight loss measurements, the effect of temperature
on the corrosion inhibition of mild steel in 1M HCl
solution by ATPB, and to calculate the thermodynamic
parameters. The output of this study is intended to be the
building block or the nucleus for a new family or group of
phosphonium derivatives in all studies of corrosion
inhibitors.
Experimental
Synthesis of (anthraquinone-2-ylmethyl) triphenyl
phosphonium bromide
1.5 g of 2-bromomethylanthraquinone (5 ×10–3
mole),
1.5 g of triphenylphosphine (5.73×10–3
mole) were
dissolved in 25 ml of toluene in 100 ml round bottomed
flask equipped with a Teflon coated magnetic stirring bar
Bulletin of Electrochemistry 23 (2007) 201–209

2. Ayssar Nahlé et al. / Bulletin of Electrochemistry 23 (2007) 201–209202
and a reflux condenser. The reaction mixture was refluxed
for 4 hours. After that time the mixture was cooled to room
temperature. The salt was filtered and washed with toluene
and dried under vacuum (Scheme 1). Yield 90%, M.P.
312–315K. IR spectrum (KBr) revealed the CO stretching
at 1674 cm–1
. 1
H NMR (CDCl3) spectrum displayed CH2
signal at 5.8 (2H, , J=13.0 Hz). UV spectrum has the
following absorption maxima max and extinction
coefficients max (written in parenthesis) in
dichloromethane: 328 nm (5.3×104
), 259 nm (8.7×104
),
and 233 nm (7.66×104
).
Electrode preparation (Electrochemistry)
A 5-mm diameter piece cut from a mild steel rod (IS
226 containing 0.18 % C, 0.6 % Mn, and 0.35 % Si)
supplied by Reliable Steel Traders, Sharjah, UAE, formed
the working electrode and was mounted using Araldite
epoxy resin in a glass tube that fits in the electrochemical
cell. Prior to each experiment, the working mild steel
electrode was abraded using a series of carborundum
papers starting with 600 grades and ending with 1200
grades. The electrode surface was then polished with 0.3-
m alumina on cloth, washed with deionized distilled
water, and rinsed with pure ethanol before being
transferred to the electrochemical cell that contained
deaerated fresh electrolyte.
Instrumentation (Electrochemistry)
In this work, the same electrochemical cell (made in
Southampton University, England) used in previous works
[21,23–26] and described in Figure 1 was employed. This
electrochemical cell consisted of mild steel working
electrode (WE), a saturated calomel electrode (SCE) as a
reference electrode (RE), and platinum gauze counter
electrode (CE). Prior to each experiment, the electrolyte
was deaerated by nitrogen bubbling. The cell was designed
in a way that the nitrogen was allowed to escape into the
solution, precluding its collection at the electrode surface.
In order to protect the working electrode from any
substance that may be produced at the counter electrode
during the electrochemical reactions, the counter-electrode
compartment was separated from the working-electrode
compartment with a glass frit. The following
electrochemical equipment and chemicals were used:
A PC controlled Sycopel AEW2-1000 electrochemical
workstation (supplied from Sycopel Scientific Limited,
England) capable of driving currents up to 1 A with an
output potential across the cell of up to 10 V.
Fig. 1. The electrochemical cell. 1) gas bubbler; 2) B 12 glass socket;
3) Platinum gauze (counter electrode); 4) glass frit; 5) inlet for nitrogen
gas; 6) luggin capillary (reference electrode); 7) iron rod (working
electrode) and 8) epoxy.
Analytical-grade hydrochloric acid (Ajax), 2-
bromomethylanthraquinone, triphenylphosphine, dichloro-
methane, and toluene were obtained fro Aldrich Chemical
Company and used without further purification.
Measuring procedure (Electrochemistry)
Electrochemical corrosion measurements (Tafel plots)
were carried out on the mild steel electrode, prepared as
described before, in 1.0 M HCl and in 1.0 M HCl
containing various concentrations of the prepared ATPB
inhibitor. The concentration of the inhibitor ranged from
1.0×10–7
M to 2.0×10–5
M. Due to the restricted solubility
of ATPB inhibitor, higher concentrations could not be
prepared.
The electrochemical cell was filled with 60 milliliters of
the electrolyte. After that, the solution was deaerated with
nitrogen gas; then the working electrode equilibrium
potential was monitored and recorded vs. SCE until it
reached a steady state. At this stage, the electrode potential
was scanned from –700 mV to –300 mV vs. SCE at a
sweep rate of 1 mV.s–1
. The computer driving the
electrochemical workstation presented the resulting data as
a plot of logarithm of the absolute value of the current
(log , mA) against the electrode potential (E, mV) vs.
SCE. The experiment was repeated for at least three times
with a newly polished electrode and fresh electrolyte,
while extreme experimental precautions were taken, in
order to ensure the reproducibility of the results. Once
reproducible plots were obtained, the corrosion currents
were then extrapolated from the log (logarithm of
current, mA) vs. electrode potential plots (Tafel).
Results and Discussion
Figure 2 shows the anodic and cathodic polarization
curves (Tafel plot) of the mild steel electrode in deaerated

3. Ayssar Nahlé et al. / Bulletin of Electrochemistry 23 (2007) 201–209 203
1.0 M HCl solution with and without the addition of
various concentrations of (Anthraquinone-2-ylmethyl)
triphenyl phosphonium bromide. The presence of the
inhibitor affected both the anodic and cathodic branches of
the curve as it can be clearly seen; this shows that ATPB
acted as a mixed inhibitor. The corrosion current of mild
steel electrode in each solution was determined by locating
the intersections of extrapolated tangents of the anodic and
cathodic curves at the corrosion potential (Erest) using the
Tafel plot. As a result, the corrosion current was found to
decrease with the increase of the concentration of the
inhibitor as shown in Table I. In the absence of inhibitor
(in 1.0 M HCl), the corrosion current was found to be 0.76
mA; which in turn dropped down to 0.0084 mA when the
concentration of ATPB in 1.0 M HCl reached 2.0×10–5
M
(Table I).
-4
-3
-2
-1
0
1
2
-0.70 -0.60 -0.50 -0.40 -0.30
Potential, E / V vs. SCE
LogI,mA
Fig. 2. Anodic and cathodic polarization curves of mild steel in an
uninhibited 1.0 M HCl solution and in 1.0 M HCl containing various
concentrations of (Anthraquinone-2-ylmethyl) triphenyl phosphonium
bromide. 1) 1.0 M HCl; 2) 1.0 M HCl + 1.0×10–7
M inhibitor; 3) 1.0 M
HCl + 1.0×10–6
M inhibitor; 4) 1.0 M HCl + 1.0×10–5
M inhibitor and 5)
1.0 M HCl + 2.0×10–5
M inhibitor.
The percentage inhibition values of ATPB at various
concentrations in 1.0 M HCl were calculated according to
the following equation and the results are shown in Table
I:
Percentage Inhibition
. .. .
. .
100
Corr CorrUninh Inh
Corr Uninh
I I
I
(3)
where: . .Corr Uninh
I corrosion current in the uninhibited
solution, and . .Corr Inh
I corrosion current in inhibited
solution.
Figure 3 shows the plot of the percentage inhibition
versus the concentration of ATPB. Figure 3 shows that the
percentage inhibition has steeply increased from 75.79%
(with 1×10–7
M ATPB inhibitor) to 92.76% with 1×10–6
M
ATPB inhibitor. After that, the percentage inhibition
slightly increased further to reach 99.03% in the solution
containing 2×10–5
M ATPB inhibitor.
Table I
Tafel corrosion currents and percent inhibitions of
(Anthraquinone-2-ylmethyl) triphenyl phosphonium bromide at
various concentrations in 1.0 M HCl at room temperature.
1.0 M
HCl
1.0 M HCl
+
1.0×10–7
M
(Anthraquinone
-2-ylmethyl)
triphenyl
phosphonium
bromide
1.0 M HCl
+
1.0×10–6
M
(Anthraquinon
e-2-ylmethyl)
triphenyl
phosphonium
bromide
1.0 M HCl
+
1.0×10–5
M
(Anthraquinon
e-2-ylmethyl)
triphenyl
phosphonium
bromide
1.0 M HCl
+
2.0×10–5
M
(Anthraquinon
e-2-ylmethyl)
triphenyl
phosphonium
bromide
Icorrosion
(mA)
0.76 0.184 0.055 0.0285 0.0074
%
Inhibition
– 75.79 92.76 96.25 99.03
The high percentage inhibition could be attributed to the
orientation of the three-phenyl groups and the benzyl
group in ATPB, which seemed to lay flat on the electrode
surface while the phosphorus atom adsorbed on the
electrode surface. This possible explanation could justify
the increase in the inhibition at 2.0×10–5
M ATPB
inhibitor. When the electrode was pulled out from the 1.0
M HCl containing 2.0×10–5
M ATPB inhibitor (at the end
of the experiment), the mild steel surface was extremely
clean (as it was initially polished before the immersion)
and showed the absence of any oxide layer as the one
encountered in the experiments where lower inhibitor
concentrations in 1.0 M HCl were used.
0
10
20
30
40
50
60
70
80
90
100
1.0E-07 1.0E-06 1.0E-05 1.0E-04
Inhibitor Concentration, M
%Inhibition
Fig. 3. Percentage inhibition of different concentrations of
(Anthraquinone-2-ylmethyl) triphenyl phosphonium bromide on mild
steel surface in 1.0 M HCl solution.
The inhibition efficiency of organic compounds depends
on molecular size, molecular structure, and mode of
interaction with the metal surface. (Anthraquinone-2-
ylmethyl) triphenyl phosphonium bromide is a potential
corrosion inhibitor since it contains not only phosphorus,

4. Ayssar Nahlé et al. / Bulletin of Electrochemistry 23 (2007) 201–209204
but also three phenyl rings together with an anthraquinone
group that contains two carbonyl bonds (C O). The high
inhibition efficiency may be attributed to the preferred flat
orientation of this compound on the metal surface. The
interaction occurs between the delocalized -electrons of
the three phenyl rings, the two-carbonyl bonds (C O), and
the lone pair of electrons on P and O atoms with the
positively charged metal surface.
Specimen preparation (Effect of Temperature)
Rectangular specimens (1 cm × 2.3 cm × 0.3 cm) cut
from large sheet of 3 mm thick mild steel (IS 226
containing 0.18 % C, 0.6 % Mn, and 0.35 % Si) supplied
by Reliable Steel Traders, Sharjah, UAE were used for
weight loss measurements. A 2-mm diameter hole was
drilled close to the upper edge of the specimen and served
to be hooked with a glass rod for immersion purposes.
Prior to each experiment, the specimens were polished
with 600-grade emery paper, rinsed with distilled water,
degreased with acetone, dried, and finally weighed
precisely on an accurate analytical balance.
Instrumentation (Effect of Temperature)
The experimental set-up consisted of a 250-ml round
bottom flask fitted with a reflux condenser and a long glass
rod on which the specimen was hooked and in turn
immersed in a thermally controlled water bath [27,28].
Analytical-grade hydrochloric acid (Ajax) and ATPB used
as prepared without farther purification.
Measuring procedure (Effect of Temperature)
The flask was filled with 100 ml of 1M HCl solution
either with or without ATPB of various concentrations,
then placed in water bath. As soon as the required working
temperature was reached, the precisely weighed mild steel
specimen was immersed in the solution, and left there for
exactly six hours, after which the sample was removed,
rinsed with distilled deionized water, degreased with
acetone, dried, and finally weighed precisely on an
accurate analytical balance. This procedure was repeated
with all the samples with a variety of inhibitor
concentrations ranging from 1×10–7
M up to 2×10–5
M; and
at temperatures ranging from 303 K to 343 K.
Weight loss corrosion tests were carried out on the mild
steel in 1M HCl in the absence or presence of ATPB over a
period of 6 hours. Table II represents the corrosion rate
[mg.cm–2
.h–1
], and the percentage efficiency for ATPB
inhibitor with concentrations varying from 1×10–7
M to
2×10–5
M at 303, 313, 323, 333, and 343 K, respectively.
The percentage efficiency was calculated according to the
following equation:
% Inhibition
. .
.
100Uninh Inh
Uninh
W W
W
(4)
where WUninh = corrosion rate without inhibitor; and WInh =
corrosion rate with inhibitor. Figures 4 and 5 show the
plots of the corrosion rate of the mild steel in 1M HCl as a
function of ATPB concentration at 303, 313, 323, 333, and
343 K. At 303 K (Figure 4), the corrosion rate dropped
from 0.961 mg.cm–2
.h–1
(1M HCl in the absence of ATPB
inhibitor) to 0.259 mg.cm–2
.h–1
(73% inhibition) when
1×10–7
M of ATPB was present in the 1M HCl solutions.
0
0.2
0.4
0.6
0.8
1
1.2
1.4
1.6
1.8
2
1.0E-07 1.0E-06 1.0E-05 1.0E-04
Concentration, M
CorrosionRate,mg.cm
-2
.h
-1
Fig. 4. Effect of concentration of (Anthraquinone-2-ylmethyl)
triphenyl phosphonium bromide on the corrosion rate (mg.cm–2
.h–1
) of
mild steel in 1M HCl at various temperatures. = 303 K; = 313 K and
= 323 K.
0
5
10
15
20
25
1.0E-07 1.0E-06 1.0E-05 1.0E-04
Concentration, M
CorrosionRate,mg.cm
-2
.h
-1
Fig. 5. Effect of concentration of (Anthraquinone-2-ylmethyl)
triphenyl phosphonium bromide on the corrosion rate (mg.cm–2
.h–1
) of
mild steel in 1M HCl at various temperatures. = 333 K and = 343 K.
This corrosion rate continued to decrease rapidly to
reach 0.076 mg.cm–2
.h–1
(92.1 % inhibition) at a
concentration of 1×10–6
M, followed by a slight decrease
reaching 0.014 mg.cm–2
.h–1
(98.5% inhibition) when the
inhibitor’s concentration was 2×10–5
M.

5. Ayssar Nahlé et al. / Bulletin of Electrochemistry 23 (2007) 201–209 205
Table II
Effect of concentration of (Anthraquinone-2-ylmethyl) triphenyl phosphonium bromide on the corrosion rate (mg.cm–2
.h–1
) and
percentage efficiency of mild steel in 1M HCl at various temperatures.
Temperature (K)
343333323313303
%
Efficiency
Corrosion
Rate
%
Efficiency
Corrosion
Rate
%
Efficiency
Corrosion
Rate
%
Efficiency
Corrosion
Rate
%
Efficiency
Corrosion
Rate
Concentration
of
Inhibitor
—26.280—12.225—4.671—1.394—0.9611M HCl
22.320.40835.67.87860.91.82767.10.459730.2591M HCl + 1×10–7
M
52.612.46168.33.87388.50.539900.13992.10.0761M HCl + 1×10–6
M
65.19.16178.92.57693.70.29594.50.07795.90.0391M HCl + 1×10–5
M
727.36184.21.92795.40.2196.30.05198.50.0141M HCl + 2×10–5
M
At 313 K (Figure 4), the curve showed to have similar
shape as that obtained at 303 K, the corrosion rate
decreased rapidly from 0.456 mg.cm–2
.h–1
(1×10–7
M of
ATPB inhibitor with a percentage of inhibition equal to
67.1%) to 0.139 mg.cm–2
.h–1
(96.3 %) at 1×10–6
M of
ATPB. In the same manner, at 323 K (Figure 4), the
corrosion rate steeply decreased from 1.827 mg.cm–2
.h–1
(1×10–7
M of ATPB inhibitor with a percentage of
inhibition equal to 60.9%) to 0.539 mg.cm–2
.h–1
(88.5%
inhibition) with 1×10–6
M.
After that the corrosion rate smoothly decreased to
breach a value 0.215 mg.cm–2
.h–1
(95.4% inhibition) with
2×10–5
M. At 333 K (Figure 5), the corrosion rate also
decreased largely from 7.877 mg.cm–2
.h–1
(35.6%
inhibition) with 1×10–7
M down to 3.873 mg.cm–2
.h–1
(68.3% inhibition) with 1×10–6
M, then decreased slightly
until a corrosion rate of 1.927 mg.cm–2
.h–1
(84.2%
inhibition) with 2×10–5
M of ATPB inhibitor.
In the same behavior, at the temperature of 343 K a very
steep decrease of the corrosion rate between 1×10–7
M and
1×10–6
M ATPB inhibitor, and corresponding to 20.408
mg.cm–2
.h–1
(22.3% inhibition) and 12.461 mg.cm–2
.h–1
(52.6% inhibition) respectively; reaching 7.361 mg.cm–2
.h–
1
(72% inhibition) when the concentration of ATPB was
2×10–5
M.
Figures 4 and 5 showed that as the temperature
increased, the effect of the concentration of the inhibitor on
the decrease of the corrosion rate was more significant.
Figure 6 shows the plots of the percent inhibition versus
the concentration of the inhibitor at temperatures of 303,
313, 323, 333, and 343 K, respectively. This figure showed
that the percent inhibition was affected by the increase of
temperature from 303 to 343K, at all ATPB inhibitor
concentrations (1×10–7
to 2×10–5
M).
The data obtained from the weight loss measurements
(Table III) were plotted in accordance to Arrhenius
equation:
ln .
aE
rate const
RT
(5)
where Ea = activation energy [kcal.mol–1
], R = gas constant
[kcal.mol–1
], T = absolute temperature [K], and const. =
constant
0
10
20
30
40
50
60
70
80
90
100
1.0E-07 1.0E-06 1.0E-05 1.0E-04
Inhibitor Concentration, M
%Inhibition
Fig. 6. Effect of concentration of (Anthraquinone-2-ylmethyl)
triphenyl phosphonium bromide on the percent inhibition of mild steel in
1M HCl at various temperatures. = 303 K; = 313 K; = 323 K;
= 333 K and = 343 K.
The Arrhenius plots of the corrosion of mild steel in 1M
HCl solution (ln corrosion rate as a function of 1/T) with
or without the presence of (Anthraquinone-2-ylmethyl)
triphenyl phosphonium bromide at concentrations ranging
from 1×10–7
M to 2×10–5
M are plotted in Figure 7. From
this figure, the slope (–Ea/R) of each line was determined
and used to calculate the activation energy according to
equation 5, with R = 1.987×10–3
kcal.mol–1
(Table IV).

6. Ayssar Nahlé et al. / Bulletin of Electrochemistry 23 (2007) 201–209206
Table III
The data obtained from the weight loss measurements for Arrhenius equation: (I/T) against ln(corrosion rate).
Ln Corrosion Rate (mg.cm–2
.h–1
)
(1/T) ×103
K–1
1M HCl 1M HCl + ×10–7
M 1M HCl + 1×10–6
M 1M HCl + 1×10–5
M 1M HCl + 2×10–5
M
3.30 –0.03978 –1.35093 –2.57702 –3.24419 –4.2687
3.19 0.332177 –0.77871 –1.97328 –2.56395 –2.97593
3.10 1.541373 0.602675 –0.61804 –1.22078 –1.53712
3.00 2.503483 2.063947 1.354029 0.946238 0.655964
2.92 3.268808 3.015927 2.522604 2.215501 1.996196
Table IV
The activation energy (Ea) for the corrosion of mild steel in 1M HCl with and without (Anthraquinone-2-ylmethyl) triphenyl
phosphonium bromide inhibitor at various concentrations.
Activation Energy, Ea (kcal.mol–1
)
System
2×10–5
M 1×10–5
M 1×10–6
M 1×10–7
M
1M HCl 18.27 18.27 18.27 18.27
1M HCl + (Anthraquinone-2-ylmethyl) triphenyl
phosphonium bromide
33.69 29.98 28.11 24.08
Table V
Effect of concentration of (Anthraquinone-2-ylmethyl) triphenyl phosphonium bromide on surface coverage for mild steel in 1M HCl at
various temperatures.
Temperature (K)
343333323313303
Surface
Coverage
( )
Surface
Coverage
( )
Surface
Coverage
( )
Surface
Coverage
( )
Surface
Coverage
( )
Concentration
of
Inhibitor
0.2230.3560.6090.6710.7301M HCl + 1×10–7
M
0.5260.6830.8850.9000.9211M HCl + 1×10–6
M
0.6510.7890.9370.9450.9591M HCl + 1×10–5
M
0.7200.8420.9540.9630.9851M HCl + 2×10–5
M
The increase of concentration of ATPB (from 1×10–7
M
to 2×10–5
M), increased the activation energies for the
corrosion of mild steel in 1M HCl (initially 18.27
kcal.mol–1
) (Table IV).
Table V shows the surface coverage of various
concentrations of ATPB (from 1×10–7
M to 2×10–5
M) on
mild steel surface as a function of temperature. These
values were extracted from the corresponding percent
efficiency values reported earlier in Table II. The plot of
surface coverage, , against the natural logarithm of the
concentration of the inhibitor, ln C, for mild steel at
various inhibitor temperatures is shown in Figure 8.
After examining these data and adjusting them to
different theoretical adsorption isotherms, it was concluded
that the inhibitor was adsorbed on the mild steel surface
according to Temkin isotherm:
– 2a = lnK C (6)
where a = molecular interaction constant, = degree of
coverage, K = equilibrium constant for the adsorption
reaction, and C = concentration of the inhibitor.
The equilibrium constant for the adsorption reaction, K,
is related to the standard free energy of adsorption via
equation 7 [29]:
1
exp
55.5
G
K
RT
(7)

7. Ayssar Nahlé et al. / Bulletin of Electrochemistry 23 (2007) 201–209 207
where K = equilibrium constant for the adsorption
reaction, 55.5 = concentration of water [mol.L–1
], G =
standard free energy [kcal.mol–1
], R = gas constant
[kcal.mol–1
], and T = absolute temperature [K].
-6
-5
-4
-3
-2
-1
0
1
2
3
4
2.8 3 3.2 3.4
1/T x 103
, K-1
LnCorrosionRate,mg.cm
-2
.h
-1
Fig. 7. Effect of temperature on the corrosion rate of mild steel in 1M
HCl solution with and without the presence of various concentrations of
(Anthraquinone-2-ylmethyl) triphenyl phosphonium bromide. = 1 M
HCl; = 1×10–7
M; = 1×10–6
M; = 1×10–5
M and = 2×10–5
M.
0.2
0.4
0.6
0.8
1
-18 -14 -10
Ln Concentration, M
SurfaceCoverage
Fig. 8. Effect of concentration of (Anthraquinone-2-ylmethyl)
triphenyl phosphonium bromide on the surface coverage of mild steel in
1M HCl at various temperatures. = 303 K; = 313 K; = 323 K;
= 333 K and = 343 K.
According to equation 6, the straight lines shown in
Figure 8 will have the following slopes and intercepts:
Slope =
1
2a
(8)
Intercept =
1
ln
2
K
a
(9)
Combination of equations (8) and (9) leads to the
following relationships:
Intercept = (Slope) (ln K) (10)
Intercept
Slope
K e (11)
Using equation (11), the equilibrium constant for the
adsorption reaction, K, was calculated.
The free energy of adsorption of the inhibitor, G, was
calculated using equation 7 at various temperatures (303 K
to 343 K) as shown in Table VI.
The enthalpy of adsorption, H, for the inhibitor was
calculated from the following equation (12) and shown in
Table VII:
H = Ea – RT (12)
The entropy, S, was calculated at various temperatures
for the inhibitor using the following equation (13) and
shown in Table VIII:
G = H – T S (13)
The results in Table IV show that the activation energy
(Ea) for the corrosion of mild steel in the presence of the
inhibitor at all concentrations (1×10–7
M to 2×10–5
M) are
higher compared to the activation energy in the absence of
ATPB inhibitor (between 24.08 to 33.69 vs. about 18.27
kcal.mol–1
). This can be attributed to the fact that higher
values of Ea in the presence of inhibitor compared to its
absence are generally consistent with a physisorption,
while unchanged or lower values of Ea in inhibited solution
suggest charge sharing or transfer from the organic
inhibitor to the metal surface to form coordinate covalent
bonds (chemisorption).
The increase in the activation energies for the corrosion
is attributed to a decrease in the adsorption of the inhibitor
on the metal surface as the temperature increased; and
subsequently, an increase in the corrosion rate will result
due to the greater exposed area of the metal surface to the
acid.
Tables 6 to 8 show the thermodynamic data obtained in
the presence of the inhibitor at various temperatures. These
thermodynamic quantities represent the algebraic sum of

8. Ayssar Nahlé et al. / Bulletin of Electrochemistry 23 (2007) 201–209208
the values for adsorption and desorption. The negative
value of G indicates the spontaneous adsorption of
inhibitor on the surface of the mild steel. The free energy,
G, varies from –22.15 kcal.mol–1
at 303 K to –15.64
kcal.mol–1
at 343 K. The adsorption process is believed to
be exothermic and associated with a decrease in entropy
S) of solute, while the opposite is true for the solvent
[30]. The gain in entropy, which accompanies the
substitutional adsorption process, is attributed to the
increase in the solvent entropy (Table VIII). This agrees
with the general suggestion that the values of G increase
with the increase of inhibition efficiency [31–33] as the
adsorption of organic compound is accompanied by
desorption of water molecules off the surface.
Table VI
The Free energy of adsorption ( Gads) for mild steel in 1M HCl
in the presence of (Anthraquinone-2-ylmethyl) triphenyl
phosphonium bromide inhibitor at various temperatures (303 K –
343 K).
G (kcal.mol–1
)
303 K 313 K 323 K 333 K 343 K
–22.15 –20.86 –19.66 –16.31 –15.64
Table VII
The enthalpy of adsorption ( H) for mild steel in 1M HCl
in the presence of 2×10–5
M (Anthraquinone-2-ylmethyl)
triphenyl phosphonium bromide inhibitor at various
temperatures (303 K – 343 K).
H (kcal.mol–1
)
303 K 313 K 323 K 333 K 343 K
33.09 33.07 33.05 33.03 33.01
Table VIII
The change in entropy ( S) for mild steel in 1M HCl in the
presence of (Anthraquinone-2-ylmethyl) triphenyl phosphonium
bromide inhibitor at various temperatures (303 K - 343 K).
S, kcal. K–1
.mol–1
303 K 313 K 323 K 333 K 343 K
0.182 0.172 0.163 0.148 0.142
The high inhibition efficiency may be attributed to the
presence of three phenyl rings and the anthraquinone group
in the structure of this inhibitor. The interaction occurs
between the delocalized -electrons of the three rings, the
anthraquinone group, the two-carbonyl bonds (C O), and
the lone pair of electrons on P and O atoms with the
positively charged metal surface.
These results agree with Fouda et al. [33,34] who
suggested that the inhibition efficiency of organic
compounds depends on many factors including their
charge density, number of adsorption sites, heat of
hydrogenation, mode of interaction with the metal surface,
and formation of metallic complexes.
Conclusions
(Anthraquinone-2-ylmethyl) triphenyl phosphonium
bromide was found to be a highly efficient inhibitor for
mild steel in 1.0 M HCl solution, reaching about 99.03 %
at 2.0×10–5
M and room temperature, a concentration
considered to be very moderate. Even an inhibitor with
half of this concentration, 1.0×10–5
M, did provide a very
high and acceptable inhibition of 96.25 % at room
temperature.
(Anthraquinone-2-ylmethyl) triphenyl phosphonium
bromide is a potential corrosion inhibitor since it contains
not only phosphorus atom, but also phenyl and
anthraquinone groups together with two carbonyl groups
(C O). It was apparent from the molecular structure that
this compound would be adsorbed onto the metal surface
through the lone pair of electron of phosphorus and oxygen
and pi electrons of the aromatic rings (phenyl and
anthraquinone) and the carbonyl groups.
The percentage of inhibition in the presence of this
inhibitor was decreased with temperature, which indicates
that physical adsorption was the predominant inhibition
mechanism because the quantity of adsorbed inhibitor
decreases with increasing temperature.
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