A comparative electrochemical study on corrosion inhibition of iron by synthesized tetraphenyl phosphonuim iodide in acid media

Article abstract of DOI:10.13005/ojc/320428

The present work is aimed mainly to synthesize a phosphonuim salt, tetraphenylphosphonuim iodide, and evaluate inhibitive effect against carbon steel corrosion in aerated 1 M HCl solutions. Thus, tetraphenylphosphonuim iodide was synthesized by the reaction of of an equimolar quantity of a solution of phenyl iodide and triphenyl phosphine in toluene (84% yield). We have found that, the use of toluene for this reaction of triphenyl phosphine with aryl or alkyl iodide is a convenient synthesis of many phosphonium salts since the halogen as a leaving group (requires a temperature higher than boiling point of chloroform.The purity of this compound was estimated by TLC technique and microanalysis, while its structure was supported by the usual spectroscopic methods such as UV, infrared, ¹H. NMR and ¹³C. NMR. Exploration of the Corrosion measurements based on polarization resistance (Rp), potentiodynamic polarization curves indicates that tetraphenylphosphonuim iodide, in most cases act as strong inhibitors. Tetraphenylphosphonuim iodide acts as anodic type inhibitors with predominant effect on the anodic dissolution of iron. Analysis of the polarization curves indicates that charge transfer process mainly controls carbon steel corrosion in HCl solution without and with phosphonuim salt. The mechanism of corrosion inhibition by phosphonuim; salt was discussed in the light of the molecular structure of the additive. A Comparative electrochemical study with that reported in the literature revealed that the efficiency of the inhibitors follows the order: tetraphenyl phosphonum iodide >(chloromethyl) triphenyl phosphonium chloride (CTP) > tetraphenyl phosphonum chloride (TP) > triphenyl phosphine oxide (TPO) > triphenyl (phenylmethyl) phosphonium chloride (TPM).

Full text of DOI:10.13005/ojc/320428

ISSN: 0970-020 X
CODEN: OJCHEG
016, Vol. 32, No. (4):
Pg. 2015-2025
ORIENTAL JOURNAL OF CHEMISTRY
An International Open Free Access, Peer Reviewed Research Journal
2
www.orientjchem.org
A Comparative Electrochemical Study on Corrosion
Inhibition of Iron by Synthesized Tetraphenyl
Phosphonuim Iodide in Acid Media
1
1
KHALED MANSOURI , LAKHDAR SEKHRI *,
2
1
OUMELKHEIR RAHIM and AHMED TABCHOUCHE
1
Lab. Dynamic Interactions and Reactivity of Systems, Process Engineering Department,
Faculty of Applied Sciences, University Kasdi Merbah, Ouargla 30000, Algeria.
2
Electrochemical Laboratory, Chemistry Department, Faculty of mathematics and
Matter Sciences, University Kasdi Merbah, Ouargla 30000, Algeria.
*
Corresponding author: E-mail: sekhril@yahoo.fr
http://dx.doi.org/10.13005/ojc/320428
(Received: July 17, 2016; Accepted: August 20, 2016)
ABSTRACT
The present work is aimed mainly to synthesize a phosphonuim salt, tetraphenylphosphonuim
iodide, and evaluate inhibitive effect against carbon steel corrosion in aerated 1 M HCl solutions.
Thus, tetraphenylphosphonuim iodide was synthesized by the reaction of of an equimolar quantity
of a solution of phenyl iodide and triphenyl phosphine in toluene (84% yield). We have found that,
the use of toluene for this reaction of triphenyl phosphine with aryl or alkyl iodide is a convenient
synthesis of many phosphonium salts since the halogen as a leaving group (requires a temperature
higher than boiling point of chloroform.The purity of this compound was estimated by TLC technique
and microanalysis, while its structure was supported by the usual spectroscopic methods such as UV,
1
13
infrared, H. NMR and C. NMR. Exploration of the Corrosion measurements based on polarization
resistance (Rp), potentiodynamic polarization curves indicates that tetraphenylphosphonuim iodide,
in most cases act as strong inhibitors.Tetraphenylphosphonuim iodide acts as anodic type inhibitors
with predominant effect on the anodic dissolution of iron.Analysis of the polarization curves indicates
that charge transfer process mainly controls carbon steel corrosion in HCl solution without and with
phosphonuim salt. The mechanism of corrosion inhibition by phosphonuim; salt was discussed in
the light of the molecular structure of the additive. A Comparative electrochemical study with that
reported in the literature revealed that the efficiency of the inhibitors follows the order: tetraphenyl
phosphonum iodide >(chloromethyl) triphenyl phosphonium chloride (CTP) > tetraphenyl phosphonum
chloride (TP) > triphenyl phosphine oxide (TPO) > triphenyl (phenylmethyl) phosphonium chloride
(
TPM).
Keywords: Iron; Phosphonium salt; Acid, corrosion, inhibition; potentiodynamic polarization.

2016
MANSOURI et al., Orient. J. Chem., Vol. 32(4), 2015-2025 (2016)
INTRODUCTION
report here the synthesis and characterization of
tetraphenylphosphonuim iodide acting as anodic
Phosphonium salts have gained an type inhibitors with predominant effect.
increasinginterestduetotheirsuccessfulapplications
in different fields of chemistry, such as the corrosion
inhibitors. Thus, by definition, a corrosion inhibitor
is a chemical substance that, when added in
RESULTS AND DISCUSSION
To suppor t our development of
small concentration to an environment, effectively electrochemical study for corrosion inhibition of
decreases the corrosion rate.To prevent the corrosion iron by lists of numerous chemical compounds that
of the materials, organic compounds having ð-bonds exhibit inhibitive properties such as phosphonuim
and heteroatoms such O, N and S have been widely halide derivatives in acid media, we required a
1
-4
used. The inhibitive performances of a molecule synthesis of different phosphonium salts such as
are substantially connected on its adsorption on tetraphenylphosphonuimiodide.Thus, thestrategywe
metal surface. In recent years, many experimental have adopted for this project consists of the following
methods are used to determine the corrosion steps:(i) synthesis of tetraphenylphosphonuim iodide
inhibition efficiencies and to understand the inhibition with taking considerations of cost, toxicity, availability,
mechanisms of chemical compounds such as and environmental friendliness. (ii) Employing this
5
6
weight loss, potentiodynamic polarization (PDP),
product as a chemical inhibitor to decrease the rate
of corrosion processes.
7
electrochemical impedance spectroscopy (EIS),
Fourier transform infrared spectroscopy (FTIR) and
scanning electron microscopy (SEM) in spite of Synthesis part
the fact that they are in general expensive and time Experimental
8
9
consuming too.
Materials
Triphenylphosphine (98%), iodobenzene
Inrecentyearswehavealsopublishedmany (98%), acetone (99.5%), and hexane (99%) were
papers about that phosphonium salts derivatives are purchased from Alfa Aesar; magnesium sulfate
effective against the corrosion of metals such as mild anhydrous (97%), and acetate ethyl was purchased
1
0-11
steel.
from Acros Organics; Ethanol and toluene p.a
were purchased from Biochem Chemopharma Co
Phosphonium salts appear to have a (Canada). All other reagents used were of analytical
wide range of potential applications as corrosion grade.
inhibitors As is known, iron is one of the metals
used widely used in industry and this metal may Procedure for the Synthesis
be exposed to corrosion due environmental factors. of tetraphenylphosphonuim iodide
For this reason, the synthesis and design of new
Iodobenzene (3.11g, 15.26 mmol) was
corrosion inhibitors to prevent the corrosion of iron added in small portions to well-stirred solution of
are quite important.Thus, in this opportunity we Triphenylphosphine (4g, 15.26 mmol) in toluene (20
Fig.1: UV-vis (blancEtOH); UV-vis (blanc CHCl3)

MANSOURI et al., Orient. J. Chem., Vol. 32(4), 2015-2025 (2016)
2017
mL). The resulting mixture was then heated under
refluxfor1-1.5hours.Itwasthenallowedtocooltoroom
temperature.The resulting precipitate was separated
by filtration and the crude product was recrystallized
from toluene to afford tetraphenylphosphonuim
iodide (5.98g, 84%) as white crystals, m.p. 80°C;
lmax (KBr disc) : 696.3, 742.5, 954.7, 1068.5, 1125,
06-ML).Solvents were purified according to standard
methods.
12
Physical measurements
1
H NMR spectra were recorded on
BRUCKER AC 300 MHz spectrometer at 0°C, and
the chemical shifts are reported in ppm relative to
-
1
1
1
434.9, 1558.4, 2341.4and 2350 cm ; H NMR:
the central line of the singlet for CDCl at 7.26 ppm.
3
+
d 7.3 (d, 8H, J = 5.13 Hz, ortho P -Ph); 7.45
Coupling constants (J values) are reported in hertz
(Hz), and spin multiplicities are indicated by the
following symbols: s (singlet), d (doublet), t (triplet),
q (quartet), m (multiplet).
+
(
1
d, 12H, J = 1.83 Hz, Meta and Para P -Ph);
3
+
C NMR: d117.9(-ve DEPT) (4C, P -Ph); 130.4
+
+
(
(
8C, Meta P -Ph); 133.6(8C, ortho P -Ph); 135.0
4C, Para P -Ph);l (blanc EtOH, CHCl ) =300 nM
+
max
3
1
3
as shown in (Fig. 1)
C NMR and DEPT were recorded on
BRUCKER AC 75 MHz spectrometer at 0°C and all
are reported in ppm relative to the central line of the
Methods
All reactions were carried out under
triplet for CDCl at 77.16 ppm.The spectra reported
3
atmospheric air conditions. Solutions were dried
are proton decoupled.
over anhydrous magnesium sulphate MgSO and
4
evaporated under reduced pressure using a rotary
evaporator (rotary evaporator (IKA Evaporator RV
IR spectra were recorded on SHIMADZU
830-FTIR spectrometer using KBr pellets. Melting
PhI
+
-
PPh3
Ph P I
4
Toluene, Reflux
Tetraphenyl
Triphenyl
Phosphine
phosphonium Iodide
Scheme 1 : Synthesis of tetraphenylphosphonuim iodide
Table 1: chemical composition of carbon steel sheet
Element
C
P
S
Si
Mn Cr
Ni
Cu
Al
Nb
V
Ti
Mo Fe
-
3
Value (×10 ) 65
2
1
245 1685 42
26
10
42
67
14
19
5
Rest
Table 2: Electrochemical parameters for carbon steel in 1 M HCl
solutions containing 10-70 ppm of tetraphenylphosphonuim iodide
HCl 1M
ppm)
Rp
IE%
Ecorr
icorr
IE%
Vcorr
E%
β ꢀ
(mV)
βc
(mV)
a
(
(ohm.cm2) (Rp)
(mV) (mA/cm2) (icorr) (mm/an) Vcorr
0
5
1
2
4
17.17
19.37
18.84
26.55
68.32
73.34
-
-482.1
-504.6
-502.9
-495.9
-499.8
-492.9
0.8861
0.6533
0.6962
0.3649
0.1968
-
10.36
7.640
8.142
4.267
2.302
-
80.0
65.3
64.3
52.4
61.4
45.9
-89.1
-82.1
-85.2
-64.6
-87.0
-60.2
11.35
8.86
35.32
74.86
76.35
26.27
21.34
58.81
77.79
26.25
21.40
58.81
77.77
0
0
0
7
0
0.1198 86.48
1.401 86.64

MANSOURI et al., Orient. J. Chem., Vol. 32(4), 2015-2025 (2016)
2018
points were recorded on a Gallenkamp melting point
apparatus, and are uncorrected.
compounds that exhibit inhibitive properties. Of
these, only very few are actually used in practice.
The presence of iodide anion in the molecule of
tetraphenyl phosphonium iodide increases the
Thin layer chromatography (TLC) was
performed on precooked 0.25 mm silica gel plates
14-17
inhibition.
60F254purchased from Merck.
Thecomparativestudyofthisphosphonium,
tetraphenyl phosphonium iodide with phosphonium
chloride salts reported by K.F. Khaled revealed that
Interpretation
We have published several papers
18
describing the general route for the synthesis of aryl
or alkyl triphenyl phosphonium halide such as methyl
triphenyl phosphonium iodide by using triphenyl
this phosphonium salt is the convenient inhibitor.
Corrosion behaviour study
Experimental details
1
0,11
phosphine and aryl or alkyl halide in CHCl3.
The procedure was adopted for the synthesis
of tetraphenyl phosphonium iodide except that
toluene was used instead of chloroform as shown
in scheme-1.
Chemicals and solutions
Working electrodes were cut from a carbon
steel sheet, sample of the following chemical
composition (as percentage):
We have found that, the use of toluene
for this reaction of triphenyl phosphine with aryl
or alkyl iodide is a convenient synthesis of many
phosphonium salts since the halogen as a leaving
group (requires a temperature higher than boiling
point of chloroform).
This material (XC70) is utilised widely in
petroleum and gas industry. Distilled and deionizer
water was used for solution preparations. Stock
solutions of 1 M HCl were employed as the blank,
i.e., tetraphenylphosphonuim iodide-free. For the
experiments containing tetraphenylphosphonuim
iodide, the appropriate weight was added to blank
solutions to reach final concentrations of 5, 10, 20,
40 or 70 ppm.
The general procedure involves dropwise
addition of an equimolar quantity of a solution of
phenyl iodide in toluene to a stirred solution of
triphenyl phosphine in toluene at 0°C. After the
addition was complete, the reaction mixture was
refluxed for 1-1.5 hours. Work-up of the reaction
product gave the corresponding phosphonium
salt, Tetraphenyl phosphonium iodide. Tetraphenyl
phosphonium iodide was either commercially
available or prepared from triphenyl phosphine and
phenyl iodide as described earlier.
Cell, electrodes and coupons
A double-walled glass three-electrode cell
was used to carry out the corrosion tests.The working
electrode was a disc of carbon steel with a geometric
2
area of 1 cm .Prior to each polarization experiment,
the electrode surface was polished with 1200-4000
emery paper, degreased with acetone, rinsed with
deionizer water and air-dried.The auxiliary electrode
was a platinum rod and the reference electrode was a
saturated calomel electrode (SCE) connected to the
cell by a bridge. All potentials in the text are quoted
versus this reference electrode.
Tetraphenyl phosphonium iodide will be
served as a chemical inhibitor to decrease the rate
of corrosion processes. In the oil extraction and
processing industries, inhibitors have always been
considered to be the first line of defense against
corrosion. A great number of scientific studies have
been devoted to the subject of corrosion inhibitors.
However, most of what is known has grown from
trial and error experiments, both in the laboratories
and in the field. Rules, equations, and theories to
Apparatus
Experiments were carried out under static
conditions at 30°C on an potentiostat/galvanostat
Applied research, potentiodynamic polarization
curves were obtained using a PGZ301 potentiostat
withVoltamaster 4 version 7.08 soft ware (radiometer
analytical SAS).
13
guide inhibitor development or use are very limited.
The scientific and technical corrosion literature
has descriptions and lists of numerous chemical

2
019
MANSOURI et al., Orient. J. Chem., Vol. 32(4), 2015-2025 (2016)
carbon steel electrode in 1M HCl as a function of
Procedures
The open-circuit potential (OCP) versus the tetraphenylphosphonuim iodide concentration
time and potentiodynamic polarization curves is shown in Table 1.
were measured for 0.5 h. before starting the
potentiodynamic polarization experiments.The initial
The OCP values were obtained after 30
potential sweep was always negative to the OCP min immersion of the electrode in the test solutions
previously measured. The initial and final potentials under non-polarized conditions, while the Ecorr values
were fixed according to the technique employed were determined by extrapolation of the Tafel lines,
(
Tafel, polarization resistance, etc.).
i.e., with the electrode polarized. Although the
nature of the experiments is different, comparisons
between the OCP and Ecorr values are important to
Open-circuit potential and corrosion potential
The variation in the open-circuit potential understanding the primary action of the corrosion
OCP) and the corrosion potential (Ecorr) of the inhibitor.The change in the OCP and Ecorr values with
(
Fig. 2: Potentiodynamic polarization curves for carbon steel in 1 M HCl and solutions
containing (a: 0 ppm), (b: 5 ppm), (c: 10 ppm), (d: 20 ppm), (e: 40 ppm) and (f: 70 ppm) of
tetraphenylphosphonuim iodide
90
-480
80
-485
70
6
0
-490
50
-495
40
E% (R p)
E% (Vcorr)
30
-500
20
10
-505
0
-10
0
10
20
30
40
50
60
70
80
0
10
20
30
40
50
60
70
80
C(ppm)
C(ppm)
Fig. 3:Variation of the corrosion potential with
the concentration of tetraphenylphosphonuim
iodide in 1M HCl solution
Fig. 4:Variation of the inhibition efficiency (as
calculated from Rp and Vcorr results) with the
concentration of tetraphenylphosphonuim
iodide in HCl 1M solution

MANSOURI et al., Orient. J. Chem., Vol. 32(4), 2015-2025 (2016)
2020
the addition of the inhibitor is often a useful indication
of which reaction is more affected: cathodic or
anodic. The OCPs observed for carbon steel in 1M
HCl solutions containing 5, 10, 20, 40 and 70 ppm
tetraphenylphosphonuim iodide were slightly more
negative than that in the absence of the organic
phosphonuim inhibitor.
On the other hand, Ecorr was shifted by
20 mV towards more negative potentials under
1
00
-
0
.9
.8
.7
0.6
.5
.4
.3
.2
the same conditions. These results suggest
that tetraphenylphosphonuim iodide can retard
both reactions under open-circuit conditions: the
oxidation of the oxide-free iron and the discharge
of the hydrogen ions to produce hydrogen gas on
the surface steel, since only a slight change in the
OCP values was observed. However, when the
working electrode is polarized, the data suggest that
tetraphenylphosphonuim iodide acts preferentially on
the cathodic sites, delaying the hydrogen evolution
0
8
0
0
6
0
0
4
0
0
0
2
0
0
0
10
20
30
40
50
60
70
80
C(ppm)
reaction, and the displacement of the E was more
well-defined.
corr
19
Fig. 5:Variation of the inhibition efficiency
as calculated from icorr and θ surface
coverage results) with the concentration of
tetraphenylphosphonuim iodide in HCl 1M
solution
(
Adsorption is the mechanism generally
accepted to explain the inhibitory action of organic
2
0
corrosion inhibitors. Adsorption of inhibitors can
affect the corrosion rate in two ways:
-
1.2
1.3
1.4
1.5
1.6
1.7
1.8
1.9
2.0
90
-
temkin
80
-
70
L angmuir
-
60
-
50
40
30
20
10
-
E quation
y = a + b*x
0.99911
E quation
y = a + b*x
0.93881
Adj. R -S quare
-
Adj. R -S quare
Value
S tandard E rror
0.67763
Value
S tandard E rror
0.09517
a
Intercept
S lope
14.42467
0.94597
-
a
Intercept
S lope
-0.99565
-0.98168
B
0.01629
b
0.14315
-
0
10
20
30
40
50
60
70
80
0.2
0.3
0.4
0.5
0.6
A (θ(icorr))
0.7
0.8
0.9
A (C ppm)
-
1.0
1.1
1.2
1.3
1.4
1.5
-
F r umk i n
Linear Fit of B
-
-
-
E quation
y
= a + b*x
Adj. R -S quare
0.92571
-
Value
-1.71171
0.83343
S tandard E rror
0.08884
a
Intercept
S lope
-1.6
b
0.13452
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
A (θ(icorr))
Fig. 6: Isotherms for the adsorption of tetraphenylphosphonuim
iodide on the surface of carbon steel in 1 M HCl

2021
MANSOURI et al., Orient. J. Chem., Vol. 32(4), 2015-2025 (2016)
i) By decreasing the available reaction Regarding the anodic region of the
(
area;the so-called geometric blocking effect;and (ii)
by modifying the activation energy of the cathodic
and/or anodic reactions occurring in the inhibitor-
free metal in the course of the inhibited corrosion
process. It is a difficult task to determine which
aspects of the inhibiting effect are connected to the
geometric blocking action and which are connected
to the energy effect.
potentiodynamic polarization curves, there is no
evidence of passive film formation on the electrode
surface either in the presence or in the absence of
the inhibitor. Also, the currents remained almost the
same for the blank and the 5 and 70 ppm tetraphenyl
phosphonuim iodide in the active dissolution region of
the metal, but increased for higher concentrations of
the organic phosphonuim compound.This behavior
could be related to a change in the anodic reaction
mechanism (iron dissolution), corroborated by a
decrease in the anodic Tafel slope with increasing
concentration of tetraphenylphosphonuim iodide
(Fig.3).
Polarization measurements
The kinetics of the anodic and cathodic
reactions occurring on carbon steel electrodes in 1 M
HCl solutions with different tetraphenylphosphonuim
iodide concentrations (5–70 ppm) was studied
through the polarization measurements.
This behavior could originate from the
adsorption of the inhibitor at low concentrations.
In addition to the energy effect, the pronounced
lowering of the anodic Tafel slope could be partly
due to electrochemical desorption of the inhibitor at
higher concentrations from the metal surface during
The complete potentiodynamic polarization
curves are shown in Fig. 2. The electrochemical
parameters, i.e. corrosion current density (icorr),
anodic (b ) and cathodic (b ) Tafel constants and
a
c
21
polarization resistance (R ), shown in Table 2, were
dissolution.
p
collected from Tafel plots and polarization resistance
experiments carried out separately.
Summarizing, the lowering of the anodic
Tafel slope can be interpreted as being due to (i)
the adsorption of tetraphenylphosphonuim iodide at
lower concentrations (energy effect), (ii) a change
in the mechanism reaction, and (iii) the desorption
of tetraphenylphosphonuim iodide at higher
concentrations, all in agreement with the mechanism
proposed above.
Regarding the potentiodynamic polarization
curves, as previously observed, it can be clearly seen
that the Ecorr values shifted to more negative potentials
with an increase in the tetraphenylphosphonuim
iodide concentration. This effect may be related
to the adsorption of the organic phosphonuim
compound at the active sites of the electrode surface,
retarding the corrosion reaction. The increase in
the tetraphenylphosphonuim iodide concentration
caused a clear decrease in the cathodic current
density, but did not change significantly the cathodic
Tafel slopes (Table 2), indicating that the hydrogen
evolution reaction is diminished exclusively by the
surface blocking effect of tetraphenylphosphonuim
The data shown in the Table 1 clearly
show that the icorr values decreased and the Rp
values, as expected, increased in the presence
of tetraphenylphosphonuim iodide, indicating that
the corrosion process was suppressed with the
increase in inhibitor concentration. The inhibition
efficiency (IE%) was computed for mild steel in 1 M
HCl solutions containing 5, 10, 20, 40 and 70 ppm of
tetraphenylphosphonuim iodide from the Tafel plots
2
0
iodide.
Table 3: Isotherm adsorptions of tetraphenylphosphonuim
iodide on the surface of carbon steel in 1 M HCl
isotherme
Equation linear
R2
K(M-1)
g or a
∆Gads(kj/mol)
Langmuir
Temkin
Frumkin
(C/θ)=0.946C+14.425
Log(/C)= -0.982 -0.996
Log(/(1-) C)=0.833 - 1.712 0.926
0.999
0.939
32325.407
1262458.56
2583282.966
0.946
0.982
0.833
-35.677
-44.757
-46.531

MANSOURI et al., Orient. J. Chem., Vol. 32(4), 2015-2025 (2016)
and polarization resistance measurements.The R%,
2022
Langmuir:
IE% values were obtained from icorr and R data using
p
the equations below:
Rearrangement gives
.
..(4)
..(5)
...(1)
...(2)
...(3)
Temkin:
.
Frumkin:
0
corr
0
p
Where i and R are the corrosion current
or it s linear forme:
density and the polarization resistance, respectively,
measured in solutions without inhibitor and icorr and
R are the same parameters determined in solutions
p
containing inhibitor. The results obtained are given
in Table1.
(
6)
Where is the surface coverage, K the
adsorption–desorption equilibrium constant, C
the inhibitor concentration and g the adsorbate
interaction parameter. Considering that the double-
layer capacitance is proportional to the surface not
covered by the inhibitor, the fraction of the surface
covered by adsorbed molecules was determined
As can be seen, the IE% increased with
increasing concentration of tetraphenylphosphonuim
iodide, but the IE% values obtained from the Tafel
experiments (icorr) were higher than those obtained
through the polarization resistance (R ).At a glance,
p
this behavior suggests that the inhibitor action is
dependent on the potential applied and time of
25
according to the ratio.
polarization, since in the R experiments only ±23
p
mV around Ecorr was applied to the working electrode
and the duration of the experiment was shorter.
However, the data clearly show that the carbon
steel electrochemical corrosion rate decreased in
the presence of tetraphenylphosphonuim iodide.
The phosphonuim salt was thus effectively adsorbed
on the mild steel surface impeding the corrosion
process supporting the mechanism discussed, which
proposes that tetraphenylphosphonuim iodide acts
by blocking the cathodic sites without a change in
the mechanism of the hydrogen evolution reaction
and by modifying the activation energy of the anodic
reaction occurring in the inhibitor-free metal in the
course of the inhibited corrosion process.
.
..(7)
0
Where icorr and i
are the corrosion
current densities in the presence and absence of
the inhibitor, respectively
corr
The increasing steel surface coverage
as a function of tetraphenylphosphonuim iodide
concentration is shown in Fig.6. To determine which
adsorption isotherm best fits the surface coverage
and to calculate the free-energy of adsorption, the
respective plots were obtained.The three isotherms
tested fitted well the data obtained, as can be
The constant K is related to the standard
free energy of adsorption (DGads) by the equation:
Adsorption isotherms
In order to evaluate the adsorption process
of tetraphenylphosphonuim iodide on the carbon
steel surface, Langmuir, Temkin and Frumkin
adsorption isotherms were obtained according to
.
..(8)
2
2-24
Values of the constant K and the standard
the following equations.
free energy of Adsorption by different isotherms for

2023
MANSOURI et al., Orient. J. Chem., Vol. 32(4), 2015-2025 (2016)
18
tetraphenyl phosphonuim iodide molecules in the
table 3.
(EIS). Moreover, the experimental results provides
further evidence that CTP, TP, TPO, and TPM act as
inhibitors for iron in acid environments, while TPP
is an accelerator. These compounds are mixed-
type inhibitors and the inhibition efficiency is due
to the presence of chloride ion which increased
with increasing concentrations. By comparison our
results with those obtained by Khaled, tetraphenyl
phosphonium iodide derivative show good cathodic
inhibition performance.
Where K is the equilibrium constant of
the adsorption process. Fig. 6. shows plot of C/
against C where straight lines are obtained. For
the inhibitor, the slope of the straight line is equal
to unity while the straight line obtained in case of
tetraphenylphosphonuim iodide has a slope >1.
So, it could be concluded that Phosphonuim salt is
adsorbed at the steel surface following Langmuir
isotherm without interaction between the adsorbed
molecules. In case of tetraphenylphosphonuim
iodide, deviation of the slope from unity can be
explained in terms of repulsion or attraction of
the adsorbed molecules adjacent to each other,
a fact which was ignored during the derivation
of Langmuir isotherm. The interaction between
tetraphenylphosphonuim iodide molecules can be
ascribed to the steric hindrance exerted by the tetra
phenyl groups.
A Comparative electrochemical study
with that reported in the literature revealed that
the efficiency of the inhibitors follows the order:
tetraphenyl phosphonum iodide >(chloromethyl)
triphenyl phosphonium chloride (CTP) > tetraphenyl
phosphonum chloride (TP) > triphenyl phosphine
oxide (TPO) > triphenyl (phenylmethyl) phosphonium
2
6-
chloride (TPM).It was also reported in the literature
30
that the efficiency of halide inhibitors follows the
-
-
-
-
order: I > Br >Cl >F .
Adsorption of tetraphenylphosphonuim
iodide molecules was found to obey Frumkin’s
model which is given by linear equation (Log(/(1-)
C)=0.833 - 1.712) and Fig.5 where g is the parameter
of interaction between molecules adsorbed on
the metal surface. The characteristic parameters
of adsorption of tetraphenylphosphonuim iodide
were found to obey Temkin’s isotherm: g = -0.982,
K = 0.996. The negative value of g indicates the
presence of repulsive forces between the adsorbed
species of tetraphenylphosphonuim iodide. For
tetraphenylphosphonuim iodide, values of K
and DGads by different isotherm were found to be
CONCLUSION
Tetraphenyl phosphonum iodide has been
successfully synthesized in high yield. Exploration
of electrochemical studies showed that the results
obtained from tetraphenylphosphonuim iodide are a
excellent corrosion inhibitor for carbon steel under
acidic conditions.The maximum inhibition efficiency
was 86.48% for 70ppm. Excellent agreement
between the inhibition efficiencies calculated using
polarization techniques was obtained.
The adsorption of the phosphonuim
salt inhibitor onto the carbon steel surface was
characterized by the decrease in (i) the cathodic
and anodic current densities observed in the
potentiodynamic polarization curves carried out in
the presence of tetraphenylphosphonuim iodide.
-
1
[
32325.407, 1262458.56 and 2583282.966] M
-
1
and [-35.677, -44.757 and -46.531] kJ mol while
the high negative value of DGads indicates that
tetraphenylphosphonuim iodide is strongly adsorbed
(
chemisorptions) on the steel surface.
These results are analogous to those
The organic phosphonuim molecule
acts by blocking the cathodic surface sites and
by modifying the activation energy of the anodic
reaction.The former effect was demonstrated by the
decrease in the cathodic current densities observed
in the potentiodynamic polarization curves without
changes in the respective Tafel slope, while the
later was observed through the changes in the Tafel
slopes of the anodic polarization curves.
reported by K.F. Khaled which involves inhibiting
action of (chloromethyl) triphenyl phosphonium
chloride (CTP), tetraphenyl phosphonum chloride
TP), triphenyl phosphine oxide (TPO), triphenyl
phenylmethyl) phosphonium chloride (TPM) and
triphenyl phosphine (TPP) on the corrosion of iron in
M HCl solution using potentiodynamic polarization
and electrochemical impedance spectroscopy
(
(
1

MANSOURI et al., Orient. J. Chem., Vol. 32(4), 2015-2025 (2016)
The chimisorption aspect of the adsorption ACKNOWLEDGMENT
2024
0
process was well-established by the value of ÄG
[-35.677, -44.757 and -46.531] kJ mol calculated
ads
-
1
=
The authors wish to express their sincere
thanks to Algerian Ministry of Higher Education
and Scientific Research (MHESR) for their support
and providing the necessary facilities to carry out
this research. Sincere thanks are due to Dr. Ali
Lounes, the Dean of the faculty of mathematics and
Matter sciences, Kasdi Merbah-Ouargla University,
Algeria for his assistance and providing a laboratory
for carrying out this research.
from the slope of the Langmuir, Temkin and Frumkin
isotherms which best fitted the experimental data of
the relationship between tetraphenylphosphonuim
iodide concentration and surface coverage;R =0.999,
.939 and 0.926.The lower activation energy of the
corrosion process in the presence of the corrosion
inhibitor, when compared to that in its absence,
also demonstrated the chemisorptive nature of the
adsorption.
2
0
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