Adsorption and corrosion inhibitive properties of piperidine derivatives on mild steel in phosphoric acid medium

Article abstract of DOI:10.1007/s11164-013-1033-3

The present study describes the inhibition effect of 4-benzyl piperidine (P1), 1,6-bis(4-benzylpiperidine-1-carboxamide)hexane (P2) and bis(4-benzylpiperidine)thiuram disulfide (P3) towards the corrosion of mild steel in 5.5 M H₃PO₄

Full text of DOI:10.1007/s11164-013-1033-3

Res Chem Intermed
DOI 10.1007/s11164-013-1033-3
Adsorption and corrosion inhibitive properties
of piperidine derivatives on mild steel in phosphoric
acid medium
•
A. Ousslim K. Bekkouch A. Chetouani
•
•
•
•
•
•
E. Abbaoui B. Hammouti A. Aouniti A. Elidrissi
F. Bentiss
Received: 24 October 2012 / Accepted: 8 January 2013
ꢀ Springer Science+Business Media Dordrecht 2013
Abstract The present study describes the inhibition effect of 4-benzyl piperidine
(P1), 1,6-bis(4-benzylpiperidine-1-carboxamide)hexane (P2) and bis(4-benzylpi-
peridine)thiuram disulfide (P3) towards the corrosion of mild steel in 5.5 M H3PO4
solution using weight loss measurements and polarization technique. The influence
of inhibitor concentration and temperature on the inhibitory behavior of P2 and P3
was investigated. Meanwhile, the inhibition efficiency (IE) was found to depend on
the molecule structure and the concentration of piperidine derivatives. The IE for
10-3 M P2 and P2 in 5.5 M H3PO4 is around 90 %. Polarization studies clearly
revealed that all the compounds used act as mixed-type inhibitors. Adsorption
isotherms were fitted by the Langmuir isotherm. Adsorption energies (DGꢀads and
DHꢀads) and kinetic parameters were evaluated. Significant correlations are obtained
between inhibition efficiency with the calculated chemical indexes, indicating that
the variation of inhibition with structure of the inhibitor may be explained in terms
of electronic properties.
A. Ousslim ꢁ A. Chetouani (&) ꢁ E. Abbaoui ꢁ B. Hammouti ꢁ A. Aouniti ꢁ A. Elidrissi
´
Laboratory of Applied Chemistry and Environment (LCAE-URAC18), Faculte des Sciences,
´
Universite Mohammed Premier, B.P. 717, 60000 Oujda, Morocco
e-mail: ahmedchetouani70@hotmail.com
A. Ousslim ꢁ K. Bekkouch
´ ´ ´
Laboratoire de Chimie Solide Minerale, Faculte des Sciences, Universite Mohammed Premier,
60000 Oujda, Morocco
A. Chetouani
Laboratoire de Chimie Physique, Centre Regionale des Metiers de l’Education et de Formation
´
(CRMEF), Region de l’Orientale, 60000 Oujda, Morocco
´
´
F. Bentiss
Laboratoire de Chimie de Coordination et d’Analytique (LCCA), Faculte des Sciences,
´
Universite Chouaib Doukkali, B.P. 20, 24000 El Jadida, Morocco
´
123

A. Ousslim et al.
Keywords Corrosion ꢁ Inhibitor ꢁ Piperidine ꢁ Mild steel ꢁ Phosphoric acid ꢁ
Adsorption ꢁ Quantum chemical
Introduction
Corrosion is natural and inevitable, but it can be minimized and delayed. One
study estimates the annual direct cost of metallic corrosion to the U.S. economy
is 3.1 % of the gross domestic product (GDP) or approximately $300 billion [1].
Beyond the huge direct outlay of funds to repair and/or replace corroded bridges,
facilities, and architectural steel, there are costs associated with accidents caused
by corroded structures, needless consumption of natural resources to constantly
repair and maintain infrastructure, and lost productivity when delays cause traffic
jams or lost opportunities. Depending on which market sector (industrial,
infrastructure, commercial, etc.) is being considered, these indirect costs may be
as high as five to ten times the direct cost. The study also states that avoidable
corrosion accounts for about one-third of the total corrosion costs and con-
cludes that these costs could be reduced by broader application of corrosion-
resistant materials and the application of the best corrosion-related technical
practices [1].
Phosphate rock is the only economical source of phosphorus for production of
phosphate fertilizers and phosphate chemicals. Most of the U.S. and world
phosphate rock resources are widely distributed marine phosphorite deposits.
Morocco has the greatest reserves, followed by South Africa and United States.
China’s reserves may be much greater than indicated and could rank as high as
fourth in the world [2, 3]. This industry frequently uses more acid, such as sulfuric
acid, which enters the prepared phosphoric acid. Effectively, phosphoric acid is the
foremost produced acid in Morocco. Two principal concentrations of H3PO4 were
obtained, of 5.5 or 11.5 M, after attack of naturally occurring phosphate rock by
H2SO4, followed by oxidation and hydration [1–3]. During the manufacturing
process, reactors, pumps and phosphoric acid storage tanks suffer from generalized
to localized corrosion phenomena according to the nature of steel material used
[4–6]. Inhibitors are used to prevent metal dissolution and minimize acid
consumption.
A corrosion inhibitor is a chemical compound which is designed to protect a
metal or alloy from corrosion. While corrosion inhibitors will not halt or completely
prevent corrosion, they can reduce the rate at which corrosion occurs, and block
early corrosion damage. A wide variety of chemicals can be used for this purpose,
although many are toxic, and must therefore be used with care to protect the
environment. One form of corrosion inhibitor is a compound added to a fluid such as
a fuel or lubricant. In this case, the corrosion inhibitor travels with the fluid,
providing protection to the systems the fluid moves through. Commonly, it forms a
thin film which prevents reactions between compounds in the fluid and systems such
as pipes. This type of corrosion inhibitor may be blended into the fluid continuously,
or added periodically to maintain a protective film.
123

Adsorption and corrosion inhibitive properties
Corrosion inhibitors can also be sprayed or painted on to create a thin layer which
will provide protection from corrosion. Many people do this on a regular basis when
they oil locks and hinges to prevent them from rusting and to keep them moving
smoothly. The thin layer of oil acts as a corrosion inhibitor to prevent oxidation, so
that rusting cannot occur. In order to work effectively, the surface needs to be clean
when the chemical is applied, as otherwise corrosive reactions can take place
underneath the corrosion inhibitor.
Most of the efficient acid inhibitors are organic compounds that contain
mainly nitrogen, sulfur or oxygen atoms in their structure. Generally, it has been
assumed that the first stage in the mechanism of the inhibitors action in
aggressive acid media is the adsorption of the inhibitors onto the metal surface.
The processes of adsorption of inhibitors are influenced by the nature and
surface charge of the metal, the chemical structure of organic inhibitors, the
distribution of charge in the molecule and the type of aggressive electrolyte.
Chemisorptions and physisorption are the principal types of interaction between
organic inhibitors and the metal surface [7–9]; among these various classes of
inhibitors, piperidine compounds exhibit efficient inhibitory effects towards steel
in acidic media [10–13].
The selection of a suitable inhibitor depends on its chemical structure, its
concentration, the nature of the metal and the type of acid. In an acid environment,
the inhibitor acts generally by chemisorption and/or physical adsorption at the metal
surface. To be effective, an inhibitor must also displace water molecules from the
metal’s surface and interact with anodic and cathodic reaction sites to retard the
oxidation and/or reduction corrosion reaction. So, the inhibition efficiency of
organic compounds depends on the structure of inhibitor, the characteristics of the
environment in which it acts and the mode of interaction with the metal surface
[14–23]. Numerous studies in our laboratory have been devoted to the corrosion-
inhibiting effect of aqueous soluble heterocyclic organic compounds on metallic
materials. Heterocyclic organic compounds containing nitrogen and sulfur have
been proved to perform as very good inhibitors for the corrosion of metals in acidic
solutions [24–30].
In this work, three piperidine derivatives, 4-benzyl piperidine (P1), 1,6-bis(4-
benzyl piperidine-1-carboxamide)hexane (P2), and bis(4-benzyl piperidine)thiu-
ram disulfide (P3), were tested as corrosion inhibitors of mild steel in 5.5 M
H3PO4. Weight loss and potentiodynamic polarization were carried out to realise
this work. The thermodynamic parameters were calculated by studying the effect
of temperature in the range 298–333 K. To compare with the experimental
results, a theoretical study were performed using Gaussian 3 package software,
density functional theory (DFT) with the Beck’s three-parameter exchange
function, along with the Lee–Yang–Parr nonlocal correlation functional (B3LYP)
[20–22] with the 6-31G* basis set implemented in the Gaussian 03 program
package [23]. The results proved a correlation between corrosion inhibition
efficiency and a number of quantum molecular properties such as dipole moment
(l), highest occupied (EHOMO) and lowest unoccupied (ELUMO) molecular orbitals
and the difference between them (HOMO–LUMO gap), as well as some structural
parameters.
123

A. Ousslim et al.
Experimental
Materials
The 1,6-hexamethylene diisocyanate (HDI) (98 %) and 4-benzylpiperidine were
purchased from Merck. Ethanol (98 %), diethyl ether and acetone products of
Aldrich were dried before use. 4-benzylpiperidine and hexamethylene diisocyanate
were used without any further purification. Tetrahydrofuran (THF) was purified by
distillation under reduced pressure over (Na under vacuum) CaH2 and stored with
˚
molecular sieves (4 A). Dibutyltin dilaurate (DBTL 95 %) was also obtained from
Aldrich and used to catalyze the reactions. All other chemicals also purchased from
Aldrich and were of analytical grade and used without further purification.
An amount of 2.1 g (2.30 9 10-2 mol) 4-benzylpiperidine in 20 mL of THF was
placed in a three-necked round-bottomed flask equipped with thermometer,
mechanical stirrer, and a reflux condenser, . The system was kept under nitrogen
atmosphere. Then, 1.93 g (1.15 9 10-2 mol) of hexamethylene diisocyanate was
added slowly (drop by drop) when the temperature of the system reached 80 ꢁC. To
speed up the reaction, a single drop of dibutyltin dilaurate (DBTL) was used as
catalyst. The temperature was then increased to 80 ꢁC, and kept in this condition for
4 h. The resulting product was precipitated into 50 mL of methanol, filtered, and
washed with acetone to remove amine formed during the reaction and the
diisocyanate that did not react and dried at 60 ꢁC to a constant mass. The procedure
was performed twice to check the reproducibility. The synthesized product was
characterized by spectroscopy techniques and the structure was also confirmed by
the elemental analysis [13, 17]. The solid compound obtained (P2) was white in
color, and soluble in organic solvents DMF, DMSO, THF, etc. Yield: (95 %).
1H-NMR in DMSO (d6 ppm). 7.20 (10 Ha); 2.50 (4 Hb); 1.88 (8 Hc), 3.37 (8 Hd);
the Hc and Hd are coupled; 6.04 (2 He); 3.15 (4 Hf); 1.60 (4 Hg); 1.40 (4 Hh).
FTIR: max/cm-1: 1,720 (–NHCON–), urea band absorption. The chemical
structures of the inhibitors P2 are shown in Scheme 1.
The compound P3 was synthesized from commercially available P1 according to
the following procedure as described elsewhere [13, 17]. CS2 (1.8 g; 2.28 9
10-2 mol) was added to a vigorously stirred, cold (\5 ꢁC) solution of 4-benzylpi-
peridine (2.1 g; 2.30 9 10-2 mol) and triethylamine (2.3 g; 2.30 9 10-2 mol) in
N
C
O
O
C
N
CH2
+
CH2
N
H
6
P1
DBTDL
THF
80 °C
a
d
c
e
g
h
f
b
CH2
NH
NH CH2
CH2
CH2
C
O
N
CH2
N
C
O
2
2
2
P2
Scheme 1 Chemical structures of P1 and P2
123

Adsorption and corrosion inhibitive properties
+
CS2
ET3N +
CH2
N
H
+
I2
P1
Ethanol
a
c
d
b
S
S
CH2
N
C
C
N
CH2
O
O
P3
Scheme 2 Chemical structures of P1 and P3
ethanol (20 mL) and the stirring and cooling continued for 1 h. To oxidize the
dithiocarbamate thus formed to the corresponding thiuram disulfide, I2 (2.8 g) was
then added in portions, complete discharge of the color being awaited after each
addition. Finally, methanolic solution I2 was added dropwise until a yellow color
persisted. The salt, dithiocarbamate, was oxidized to the corresponding thiuram
disulfide. The excess of iodine was neutralized with a solution of Na2S2O3 (10 % by
weight).The mixture was then cooled and the thiuram disulfide precipitate was
filtered and allowed to dry. The solid compound obtained (P3) was yellow in color,
and soluble in organic solvents such as CH2Cl2 and CHCl3. Yield: 3.70 g (80 %).
1H-NMR in CDCl3 (d ppm). 7.20 (10 Ha); 2.50 (4 Hb); 1.88 (8 Hc), 3.37 (8 Hd); the
Hc and Hd are coupled. IR: max/cm-1: 1,280–1,190 (NCSS). The chemical
structure of the inhibitor P3 is shown in Scheme 2.
The studied compounds, P2 and P3, were synthesised using commercially
available P1 as previously reported [13]. The molecular structures of the inhibitors
P1, P2, and P3 are shown in Fig. 1. The choice of such products has been driven by
very satisfactory results of their application as inhibitors of iron corrosion in NaCl
(3 %) 31, HCl (0.5 M) [32–34], copper in various acid media [35–40], bronze in
NaCl (3 %) [41], and of mild carbon steel in HCl medium [42, 43] and H2SO4
solution [13, 17, 44].
The material used in this study was mild steel with a chemical composition
(in wt%) of 0.09 % P; 0.38 % Si; 0.01 % Al; 0.05 % Mn; 0.21 % C; 0.05 % S; and
the remainder iron (Fe). The mild steel samples were pre-treated prior to the
experiments by grinding with emery paper SiC (120, 600, and 1,200), then rinsed
with distilled water, degreased in acetone in an ultrasonic bath immersion for 5 min,
washed again with bidistilled water, and then dried at room temperature before use.
The acid solutions (5.5 M H3PO4) were prepared by dilution of an analytical
reagent grade 85 % H3PO4 with double-distilled water. The concentration range of
inhibitor was 10-5 to 10-3 M.
Corrosion tests
The gravimetric measurements were carried out at the definite time interval of 4 h at
room temperature using an analytical balance (precision 0.1 mg). The mild steel
123

A. Ousslim et al.
CH2
N
H
4-benzyl piperidine (P1)
CH2
N
C
O
NH
CH2
NH
C
O
N
CH2
6
1,6-di(4-benzyl piperidine-1-carboxamide) hexane (P2)
CH2
N
C
S
S
C
S
N
CH2
S
bis(4-benzyl piperidine) thiuram disulfide P3
Fig. 1 Molecular structures of P1, P2, and P3
specimens used had a rectangular form (length = 2.5 cm, width = 2 cm, thick-
ness = 0.05 cm). Gravimetric experiments were carried out in a double glass cell
equipped with a thermostated cooling condenser containing 100 mL of non-de-
aerated test solution. After the immersion period, the mild steel specimens were
withdrawn, carefully rinsed with bidistilled water, ultrasonically cleaned in acetone,
dried at room temperature, and then weighed. Duplicate experiments were
performed in each case and the mean value of the weight loss calculated.
Electrochemical measurements were carried out in a conventional three-electrode
cylindrical Pyrex glass cell. All electrochemical tests have been performed at 308 K
in non-de-aerated solutions. The working electrode (WE) in the form of a disc cut
from mild steel had a geometric area of 1 cm2 and was embedded in polytetra-
fluoroethylene (PTFE). A saturated calomel electrode (SCE) and a platinum
electrode were used as reference and auxiliary electrodes, respectively. A fine
Luggin capillary was placed close to the working electrode to minimize ohmic
resistance. The working electrode was immersed in the test solution during 30 min
until a steady-state open circuit potential (Eocp) was obtained. Potentiodynamic
polarization experiments were conducted using an electrochemical measurement
system Tacussel Radiometer PGZ 301 controlled by a PC supported by Voltamaster
4 Software. The polarization curve was recorded by polarization from -800 to
-100 mV with a scan rate of 1 mV s-1 under air atmosphere.
Computational method
All theoretical calculations were performed using Gaussian 3 package software ,
DFT (density functional theory) with the Beck’s three-parameter exchange
123

Adsorption and corrosion inhibitive properties
functional along with the Lee–Yang–Parr nonlocal correlation functional (B3LYP)
[20–22] with 6-31G* basis set implemented in the Gaussian 03 program package
[23]. This approach has been shown to yield favorable geometries for a wide variety
of systems. This basis set gives good geometry optimizations. The geometry
structure of piperidine derivatives was optimized under no constraint. The following
quantum chemical parameters were calculated from the obtained optimized
molecular structure: the energy of the highest occupied molecular orbital (EHOMO),
the energy of the lowest unoccupied molecular orbital (ELUMO), the energy band gap
(DEgap = EHOMO - ELUMO), the dipole moment (l), and the total energy (ET).
Results and discussion
Effect of immersion time
We conducted this study to avoid confusion between the rate of inhibition of
inhibitors and passivation phenomenon that could occur if the immersion time is
very long. In this study, we were interested in compounds P2 and P3 at 10-3
M
during time intervals between 30 min and 6 h. Figure 2 represents the variation of
E% with time of mild steel immersion in H3PO4 in the absence and presence
10-3 M of these inhibitors. Analysis of these curves shows that the weight loss
(g cm-3) increases with time during a period of 6 h (immersion time) both in acid
and inhibited acid. This phenomenon is interpreted by the presence of oxonium ions
as well as air which are called the natural motors of corrosion.
For every concentration, the mean value of the corrosion rate CR (mg cm-2 h-1
)
was determined and the inhibitor efficiency, EWL (%), was calculated using Eqs. 1
and 2, respectively [34]
100
90
80
70
10-3M P2
10-3M P3
60
50
40
0
1
2
3
4
5
6
Time (h)
Fig. 2 Effect of time on E% of mild steel in 5.5 M H3PO4 with and without P2 and P3 at 10-3 M at
298 K
123

A. Ousslim et al.
Wb ꢂ Wa
CR ¼
ð1Þ
ð2Þ
At
ꢀ
ꢁ
wi
w0
EWLð%Þ ¼ 1 ꢂ
ꢃ 100
where Wb and Wa are the specimen weights before and after immersion in the tested
solution, w0 and wi are the values of corrosion weight losses of mild steel in
uninhibited and inhibited solutions, respectively, A the area of the mild steel
specimen (cm2), and t is the exposure time (h).
The corrosion rate in an acid medium in the absence of inhibitor is relatively
stable after 4 h of immersion whereas in the presence of inhibitors the corrosion rate
is almost constant from the 3rd hour of immersion. Figure 2 shows that mass loss
increases with the immersion time in either the absence or presence of the inhibitor.
The shape of the curves is almost a straight line; this implies that the metal surface is
devoid of insoluble products.
Furthermore, the inhibition efficiency (Fig. 2) of the tested compounds increased
for the first 4 h of immersion before it stabilized on its maximum value. Finally, we
can conclude that the net decrease of weight loss in the presence of P2 and P3 is an
important indicator that these compounds play the role of efficient inhibitors of mild
steel in test solutions.
Effect of inhibitor concentration
Corrosion rate was thereafter determined at different concentrations of 4-benzylpi-
peridine (P1), 1,6-di(4-benzyl piperidine-1-carboxamide)hexane (P2) and bis(4-
benzyl piperidine)thiuram disulfide (P3) in 5.5 M H3PO4 after 4 h at 298 K. The
inhibition efficiencies (EWL) were estimated from Eq. 2.
Table 1 gives the values of the corrosion rate (mg cm-2 h-1), inhibition
efficiency (E%) and surface coverage (h) obtained by E%/100 for mid steel
corrosion in 5.5 M H3PO4 and in the presence of inhibitors tested at different
concentrations. The results show that corrosion rates decrease with the increase of
P1, P3, and P3 (Fig. 3). In the presence of P2 and P3 at 10-3 M, the corrosion rate
is observed to reduce significantly. The compounds P2 and P3 have the best
inhibitory efficiencies and attain 93 and 91 %, respectively, at 10-3 M.
Effect of temperature
The effect of temperature (298–333 K) on the corrosion behavior of mild steel in the
presence and absence of bis(4-benzyl piperidine)thiuram disulfide (P2) and 1,6-di(4-
benzyl piperidine-1-carboxamide)hexane (P3) at 10-3 M in 5.5 M H3PO4 was studied
using the weight loss method after 4 h immersion period (Table 2). Both in the absence
and the presenceofdifferent inhibitors, the corrosion rateofmildsteel increases with the
riseintemperature. TheinhibitionefficiencyofP2and P3 decreaseswithtemperatureto
reach a value of 26 and 28 % at 333 K, respectively. The activation energies for the
corrosion process were calculated from the Arrhenius equation:
123

Adsorption and corrosion inhibitive properties
Table 1 Corrosion parameters for mild steel in 5.5 M H3PO4 in the absence and presence of different
concentrations of P1, P2, and P3 from weight loss measurements at 298 K
Inhibitor
Cinh (M)
CR (mg cm-2 h-1
)
EWL (%)
h
Blank
–
5.8
5.3
4.9
4.6
2.8
1.9
4.8
2.6
1.8
1.4
0.5
3.0
2.0
0.9
0.6
0.4
–
8
P1
1 9 10-5
5 9 10-5
1 9 10-4
5 9 10-4
1 9 10-3
1 9 10-5
5 9 10-5
1 9 10-4
5 9 10-4
1 9 10-3
1 9 10-5
5 9 10-5
1 9 10-4
5 9 10-4
1 9 10-3
0.08
0.15
0.20
0.52
0.67
0.17
0.55
0.69
0.76
0.91
0.48
0.65
0.84
0.89
0.93
15
20
52
67
17
55
69
76
91
48
65
84
89
93
P2
P3
100
80
60
40
20
P1
P2
P3
0,0000 0,0002 0,0004 0,0006 0,0008 0,0010
C(mol/L)
Fig. 3 Variation of inhibition efficiency with inhibitor concentration for mild steel in 5.5 M H3PO4 with
different concentrations of P1, P2, and P3
ꢀ
ꢁ
Ea
CR ¼ k exp ꢂ
ð3Þ
RT
where Ea is the apparent activation corrosion energy, R is the universal gas constant,
k is the Arrhenius pre-exponential factor, and T is the absolute temperature.
Table 2 shows that the inhibition efficiency of inhibitors P2 and P3 in the acid
medium decreases slightly in the temperature range 298–313 K. A similar effect has
been reported by other authors [27–47]. Between 318 and 333 K, the efficiency
suffered a very pronounced drop.
123

A. Ousslim et al.
Table 2 Inhibition efficiency obtained from the corrosion rate at 10-3 M of different inhibitors in 5.5 M
H3PO4 at different temperatures after 4 h immersion period
Temperature (K) Blank
R (mg cm-2 h-1
P2
R (mg cm-2 h-1
P3
R (mg cm-2 h-1
C
)
C
)
EWL (%)
C
)
EWL (%)
298
303
308
313
318
323
333
5.8
0.5
91
90
86
82
77
45
28
0.4
93
89
79
69
64
43
26
8.1
10.0
13.8
19.9
21.2
49.8
0.8
1.4
0.9
2.1
4.3
7.1
2.4
4.5
11.7
35.9
12
37
The graphical representation of these results may be explicated by Fig. 4, which
reveals that increasing the temperature in the range from 298 to 333 K causes an
increase in corrosion rates of steel in an acid medium in the absence and presence of
inhibitors. This increase was less severe in the presence and absence of the
inhibitors; however, the inhibition efficiency decreases with the increase in
temperature.
Arrhenius plots for the corrosion rate of mild steel are given in Fig. 5. Values of
Ea for mild steel in 5.5 M H3PO4 in the absence and presence of various
concentrations of bis(4-benzyl piperidine) thiuram disulfide (P2) and 1,6-di(4-
benzyl piperidine-1-carboxamide)hexane (P3) were determined from the slopes of
ln(CR) versus 1/T plots [18, 22, 23] (Fig. 6).
Enthalpy (DHaꢀ) and entropy (DSꢀa) activation energies may be evaluated by the
alternative formulation (Eq. 3)
ꢀ
ꢁ
ꢀ
ꢁ
RT
Nh
DSa
DHa
RT
CR ¼
exp
exp ꢂ
ð4Þ
R
where h is Plank’s constant, N is Avogadro’s number, DSa is the entropy of acti-
vation, and DHa is the enthalpy of activation.
100
10 -3M P2
10 -3M P3
75
50
25
300
310
320
330
Temperature (K)
Fig. 4 Variation of the inhibition efficiency as function of the temperature of P2 and P3 at 10-3
M
123

Adsorption and corrosion inhibitive properties
4
3
2
1
Acid
0
+ P2
+ P3
-1
0,0030
0,0031
0,0032
0,0033
0,0034
1 / T (K)
Fig. 5 Arrhenius plots for mild steel corrosion rates (CR) in 5.5 M H3PO4 inthe absence and presence of
10-3 M of P2 and P3
-2
-3
-4
Blank
-5
-6
-7
10-3 P2
10-3 P3
3,0
3,1
3,2
3,3
3,4
1000 / T (K)
Fig. 6 Transition-state plots for mild steel corrosion rates (CR) in 5.5 M H3PO4 in the absence and
presence of 10-3 M of P2 and P3
Table 3 Activation parameters for mild steel in 5.5 M H3PO4 in the absence and presence of P2 and P3
Inhibitor
R2
Ea (kJ mol-1
)
DHa (kJ mol-1
)
DSa (J mol-1 K-1
)
Blank
P2
0.990
0.994
0.998
48.33
103.25
103.88
45.72
100.6
103.9
-77.04
85.0
P3
98.3
From the plots of ln(W/T) versus 1/T, the straight lines are obtained with a slope
of (-DHaꢀ/R) and an intercept of (ln(R/Nh) ? DSaꢀ/R). The calculated values of Ea,
DHaꢀ and DSꢀa are collected in Table 3.
The addition of these compounds to the corrosive solution is accompanied by an
increase in activation energy; this could be attributed to the presence of an energy
barrier for the corrosion reaction due to the existence of the inhibitor to inter-phase
metal/electrolyte, which shows a change in the mechanism of transition metal in
solution.
123

A. Ousslim et al.
It was clear from this study that the values of Ea in the presence of different
inhibitors are higher than those for the uninhibited acid solution. Previous studies
have shown that, while chemisorptions of an inhibitor lead to a decrease in
activation energy, physisorption is associated with an increase [19–21].
The positive sign of the activation enthalpy (DHaꢀ) reflects the endothermic nature
of the steel dissolution process (Table 3) and suggesting that the dissolution reaction
will be more blocked sites from the metal surface, which the increase in entropy
(DSꢀa) in the presence of an inhibitor implies. This result indicates that the corrosion
changes from a more ordered system to a disordered system. The compounds
studied and the activated complex in the rate-determining step represents
association rather than dissociation, indicating that an increase in disorder takes
place on going from reactants to the activated complex. These data reveal that the
inhibition of the corrosion reactions is affected without the mechanism changing.
The entropy of activation in the presence and absence of the inhibitors is large and
negative. This implies that the activation complex in the rate-determining step
represents association rather than dissociation, indicating that a decrease in disorder
takes place on going from reactants to the activated complex [17, 18].
Potentiodynamic polarisation study
The electrochemical behavior of C38 steel in 5.5 M H3PO4 in the absence and
presence of bis(4-benzyl piperidine)thiuram disulfide (P2) and 1,6-di(4-benzyl
piperidine-1-carboxamide)hexane (P3) was investigated. Typical curves in the
absence and presence of various concentrations of P3 and P3 at 298 K are shown in
Fig. 7.
Examination of curves indicate that, in general, the presence of increasing
amounts of P3 and P3 leads to a decrease in both the cathodic and anodic current
densities, but with a more pronounced decrease in the cathodic domain at elevated
concentrations. Table 4 presents the various electrochemical parameters of P1 and
P3 at 10-3 M, such as corrosion potential (Ecorr), corrosion current density (icorr),
100
10
100
10
Blank
5.5M H3PO4
+ 10-5 M P2
+ 5 10-5M P2
+ 10-4 M P2
+ 5 10-4 M P2
+ 10-3 M P2
+ 10-5M P3
+ 5 10-5M P3
+ 10-4M P3
+ 5 10-4M P3
+ 10-3M P3
1
1
0.1
0.1
0.01
0.01
-800
-600
-400
-200
0
-800 -700 -600 -500 -400 -300 -200 -100
E ( m V / S C E )
E ( m V / S C E )
Fig. 7 Potentiodynamic polarization curves of mild steel in 5.5 M H3PO4 for various concentrations of
P2 and P3 at 298 K
123

Adsorption and corrosion inhibitive properties
and the calculated Ei (%). The inhibition efficiencies, EI (%), are calculated from
Icorr values using the following equation
Icꢀorr ꢂ Icorr
EIð%Þ ¼
ꢃ 100
ð5Þ
Icꢀorr
where Icꢀorr and Icorr are the corrosion current densities for mild steel electrode in the
uninhibited and inhibited solutions, respectively, determined by extrapolation of the
cathodic Tafel lines to the corrosion potential.
Regarding Table 4 and the curves in Fig. 7, we conclude that the cathodic Tafel
relationship is followed, showing that cathodic reactions are activation controlled.
Different concentrations of inhibitors tested (P3 and P3 in H3PO4) decrease the
values of the Tafel slope (bc), in comparison with that obtained in the case of the
acid alone. This decrease is more marked as the inhibitor concentration becomes
higher. This may suggest that the addition of inhibitors to the electrolyte has an
effect on the reduction mechanism of hydrogen and indicates that the inhibitive
action of the studied compounds is due to adsorption of the inhibitor on the cathodic
active sites. The hydrogen reduction (cathodic reaction) is then retarded with the
changing of its mechanism.
The lower value of icorr is obtained at 10-3 M, so the inhibition efficiency reaches
the maximum of 66, 95, and 96 % for P2, P3, and P3, respectively. The inhibited
interface keeps almost the same Ecorr, emphasizing that bis(4-benzyl piperidine)
thiuram disulfide (P2) and 1,6-di(4-benzyl piperidine-1-carboxamide)hexane (P3)
act as mixed-type inhibitors (Table 4; Fig. 7. The results obtained by this
electrochemical method are very similar to those obtained by the gravimetric
method. The agreement between the two methods is very satisfactory. The order of
inhibition efficiency of the tested compounds decreases in the following sense.
P2 [ P3:
Table 4 Potentiodynamic polarisation parameters and corresponding inhibition efficiency for the cor-
rosion of the mild steel in 5.5 M H3PO4 without and with the addition of various concentrations of P2 and
P3 at 298 K
Inhibitor
Cinh (M)
Ecorr (mV/ECS)
-bc (mV dec-1
)
Icorr (lA cm-2
)
EI (%)
Blank
–
-476
-507
-494
-486
-464
-439
-478
-480
-484
-481
-478
211
215
216
201
190
173
224
220
170
165
144
2,183
1,621
920
–
28
59
71
81
96
52
58
83
89
95
P2
1 9 10-5
5 9 10-5
1 9 10-4
5 9 10-4
1 9 10-3
1 9 10-5
5 9 10-5
1 9 10-4
5 9 10-4
1 9 10-3
651
423
96
P3
1,077
949
370
246
115
123

A. Ousslim et al.
Quantum chemical parameters and adsorption isotherm of inhibitors
Quantum chemical parameters
The theoretical calculation of the partial and the total charge density of heteroatoms
in the molecular structure of P2 and P3 are calculated by the program Parameter
Estimation for the Treatment of Reactivity Applications (PETRA) which allows us
to show the positively and negatively charged atoms (Scheme 3; Table 5). Thus, the
atoms nitrogen, oxygen and sulfur, having total partial densities of negative charge,
are probably the active sites of adsorption of these compounds to the metal surface
by forming coordination bonds with donor–acceptor orbitals vacant ‘‘d’’ of iron.
Thus, it is important to note that the nitrogen atoms and negatively charged oxygen
atoms are the anchor points of these compounds for the formation of complexes
with iron. The theoretical calculation of the total partial charge density of
heterocompounds P2 and P3 can reinforce the suggestion that the inhibitory effect
contributed by the piperazine group is greater than that provided by the piperidine
group.
The structure of compounds P3 is related to the (C=O) group which creates more
electron delocalization and therefore causes the stability of the structure 61. This
results in an increase in electron density around nitrogen atoms, which gives an
improvement in the coverage of the surface of the steel. The molecular structure of
the optimized inhibitor P3 is given in Scheme 3.
Moreover, the presence of the disulfide in the organic structure leads to the
formation of a bond dp–dp resulting from overlapping of 3d electrons of the iron
atom to empty 3d orbitals of the sulfur atom 67. The high efficiency of the inhibitor
is attributed to the size of the molecule covering the surface and thereby inhibiting
the corrosion of the metal. In the compound P3, the presence of C2S4 group
constituting the rigid center of the molecule and its adsorption to the surface of the
mild steel remains horizontal. There is thus, in this case, a maximum reactive
surface. The optimized molecular structures of inhibitors are given in Scheme 4.
To confirm the results, quantum chemical calculations were performed for the
compounds studied to find a link between inhibition efficiency and molecular
O
1
H
N
1
N
S
S
3
N
H
3
S
4
S
N
7
6
26
N
13
10
19
N
O
12
P3
P2
Scheme 3 Molecular structures of P2 and P3
123

Adsorption and corrosion inhibitive properties
Table 5 Theoretical calculation of the total partial charge density of the heteroatoms of P2 and P3
Compound
Total partial charge density
P2
N (3), N (10)
-0.1191
N (13), N (26)
-0.1057
O (1), O (12)
-0.5269
P3
N (7), N (19)
-0.1364
S (1), S (6)
-0.2012
S (3), S (4)
-0.017
P2 (a)
P2 (b)
P3 (a)
P3 (b)
Scheme 4 Horizontal (a) and vertical (b) molecular structures for P2 and P3
structure and an explanation based on quantum parameters of the inhibitors as
highest occupied molecular orbital HOMO, lowest unoccupied molecular orbital
LUMO, and gap energies. Higher HOMO energy (EHOMO) of the molecule means a
higher electron donating ability and low LUMO energy (ELUMO) indicates that the
acceptor accepts electrons easily.
123

A. Ousslim et al.
Table 6 Quantum parameters of P2 and P3
Compound
ET (kcal mol-1
)
l (Debye)
EHOMO (eV)
ELUMO (eV)
DEgap (eV)
P2
P3
-102.2 9 104
-163.8 9 104
6.36
1.23
-8.49
-2.80
2.59
7.12
11.08
9.92
The quantum chemical indices and representations of ELUMO, EHOMO, dipole
moment (l), and the total energy of P2 and P3 are reported in Table 6 and Scheme 5,
respectively. From Tables 1 and 6, we note that the inhibition efficiencies of these
molecules increase when DEgap decreases. Effectively, the compounds P2 and P3
have the best inhibitory efficiencies and attain 93 and 91 %, respectively, at 10-3M.
These results showed that the inhibition mechanism is based primarily on an
adsorption process performed by charge transfer from the inhibitor to the metal. The
increase in inhibition efficiency can be attributed to the difference, DE. The lower the
energy band (in absolute value), the higher the inhibition efficiency.
In the studied inhibitors, the results obtained in Table 6 show that P3 has both the
highest HOMO and the lowest gap energies. The bis(4-benzylpiperidine) thiuram
disulfide (P2) transfers electrons easily in comparison to P2, and then P3 is the best
inhibitor. Moreover, from the negative sign of the coefficient of EHOMO, it can be
concluded that the adsorption of these inhibitors on the steel surface has importance.
A correlation between the inhibition efficiency of P3 with lower values of DE and l
dipole moment was found which was in a good agreement with the experimental
findings [31].
The main difference between the molecular structure of these compounds is due
to changes of the part (–CO–N–(CH2)3)2 and (–CS–S)2 between bis (4-benzylpi-
peridine), while values of energy and ELUMO and EHOMO calculated for P2 and P3
demonstrate a significant difference in calculated values. The main difference
between these two molecules, P2 and P3, is the density distribution of the HOMO is
localized on thiocarbonyl and thioamide. This distribution thus ensures strong
adsorption onto the metal surface (Schemes 6, 7). The inhibitory action of these
inhibitors depends on the concentration and the position of the nitrogen atom in the
piperazine ring.
The evaluation of inhibitory capacity revealed that P3 is the most effective
inhibitor in the conditions described above. Therefore, the main difference is the
geometry of these molecules which can be explained by the number of oxygen,
sulfur and nitrogen atoms. These atoms contribute to the chemisorptions of these
molecules by way of links, ‘‘donor–acceptor’’, between the nonbonding electrons of
the oxygen atoms, sulfur and nitrogen and p electron orbitals of the phenyl and ‘‘d’’
unsaturated metal.
We concluded that the adsorption of studied compounds on the metal surface
occurs on the basis of donor–acceptor interactions between the p-electrons of the
sulfur or nitrogen atoms and the vacant d-orbital of iron surface atoms. The
molecule may be adsorbed on the metal surface by horizontal or vertical formd,
involving the displacement of water molecules from the metal surface.
123

Adsorption and corrosion inhibitive properties
Scheme 5 Distribution density of HOMO and LUMO for P2 and P3
Scheme 6 Representation of molecule adsorption P2 at horizontal mode (a) or vertical mode (b)
Scheme 7 Representation of molecule adsorption of P3 at horizontal mode (a) or vertical mode (b)
123

A. Ousslim et al.
Adsorption isotherm
Attempts have been made to correlate h values to the most appropriate adsorption
isotherm: the best known are the Langmuir, Temkin, Frumkin and Freundlich
isotherms. We therefore conclude that P2 exercises its effects through physisorption
on the surface. It retards corrosion at lower temperature but its inhibition effect falls
at elevated temperatures. This assumption may be reinforced by the determination
of the heat of adsorption, while the Langmuir isotherm supposes monolayer physical
adsorption of the formed complex. The equation leads to DHads [17]:
ꢀ
ꢁ
h
DHads
RT
¼ AC exp ꢂ
ð7Þ
1 ꢂ h
where A is a temperature independent constant, C the inhibitor concentration, and
DHads is the adsorption heat.
The nature of the adsorption process can be clarified further if the heat of
adsorption DHads value is known. It has been shown that for DHads \ 10 kJ mol-1
the adsorption is most probably physical in character [23–25], while for
DHads [ 10 kJ mol-1, chemisorptions probably occur. Figure 8 shows the plot of
the logarithm of (h/1 - h) as a function of the reciprocal of the temperature. The
slope determined leads to the calculation of heat adsorption. The deduced DHads
gives 107.1 kJ mol-1 indicating that the adsorption process of P3 is exothermical in
nature and there is chemisorption on the mild steel surface from 5.5 M H3PO4
solutions. Chemical adsorption processes on the mild steel surface were reported for
various S- and N-containing organic inhibitors which have been characterized with
cations formation in acid media [26, 28, 30].
Attempts have been made to correlate h values to the most appropriate adsorption
isotherm. The best correlation coefficient (R2) value was obtained by the Langmuir
adsorptionisotherm. Thesurfacecoveragehisrelatedtotheequilibriumadsorptioncon-
stant kads and the concentration of inhibitor C by the rearranged Langmuir equation [14]
C
1
¼
þ C
ð8Þ
h
kads
The standard adsorption free energy (DGꢀads) was obtained by the relationship:
3,0
2,5
P3
P2
2,5
2,0
2,0
1,5
1,5
1,0
1,0
0,5
0,0
0,5
0,0
-0,5
-1,0
-1,5
-0,5
-1,0
0,00295 0,00300 0,00305 0,00310 0,00315 0,00320 0,00325 0,00330 0,00335 0,00340
0,00295 0,00300 0,00305 0,00310 0,00315 0,00320 0,00325 0,00330 0,00335 0,00340
1 / T
1 / T
Fig. 8 Variation of lnh/(1 - h) versus T-1 of P2 and P3 on mild steel in 5.5 M H3PO4 solution
123

Adsorption and corrosion inhibitive properties
ꢀ
ꢁ
1
DGꢀads
RT
kads
¼
exp ꢂ
ð9Þ
55:5
where 55.5 is the concentration of water in solution expressed in mol l-1
.
We remark that the best fit was obtained with the Langmuir isotherm (Fig. 9).
The slope is close to unity (1.018) with R2 = 0.9997. The deduced values of DGaꢀds
obtained are negative and mean that the adsorption of P3 on the steel surface is a
spontaneous process. Furthermore, the negative values of DGꢀads also show the
strong interaction of the inhibitor molecule onto the steel surface [9, 13, 15, 16].
It is well known that DGꢀads values around -20 kJ mol-1 or lower are consistent
with the electrostatic interaction between the charged molecules and the charged
metal (physisorption). Those more negative than -40 kJ mol-1 involve charge
sharing or transfer from the inhibitor molecules to the metal surface to form a
coordinate bond (chemisorption) [17, 18]. The calculated DGꢀads value in our case
(Table 7) is therefore consistent with deductions based on the corrosion rates, that
the adsorption mechanism of the P2 and P3 on mild steel in 5.5 M H3PO4 solution
was one of chemisorption without excluding the physical interactions.
The mechanism of the inhibitors action is their adsorption onto the metal surface
to block the interface reactivity and then to retard or stop the corrosion process. P2,
the best inhibitor, is a heterocyclic compound containing several heteroatoms (four
nitrogen and four sulfur). These heteroatoms and aromatic rings, named centers of
adsorption, interact easily with the metal surface. A representation of a proposed
0.0012
0.0010
0.0008
0.0006
0.0004
P2
0.0002
P3
0.0000
0.0000 0.0002 0.0004 0.0006 0.0008 0.0010
C ( M )
inh
Fig. 9 Langmuir adsorption isotherm of P2 and P3 on mild steel in 5.5 M H3PO4 at 298 K
Table 7 Adsorption data of P2
and P3 on mild steel in 5.5 M
H3PO4
Inhibitor
R2
Kads (M-1
)
DGꢀads (kJ mol-1
)
P2
P3
0.9957
0.9999
31,895.3
63,557.8
-35.67
-37.37
123

A. Ousslim et al.
mode of adsorption of the studied compounds P2 and P3 is shown in Schemes 6 and
7. The adsorption of the piperidine on the metal surface occurs on the basis of
donor–acceptor interactions between the p-electrons of the sulfur or nitrogen atoms
and the vacant d-orbital of the iron surface atoms. The molecule may be adsorbed on
the metal surface by horizontal or vertical forms, involving the displacement of
water molecules from the metal surface.
Conclusion
The results of the present work can lead to the following conclusions:
•
•
•
The organics tested reveal excellent protective qualities for mild steel against
corrosion in H3PO4 medium (5.5 M). This results in an increase in inhibition
efficiency. This increase is particularly marked when the concentration is high
and attains around 92 %.
The cathodic curves exhibit a wide linear range indicating that the Tafel law is
well verified in the field. The discharge of the proton is then in a pure activation
kinetics, without modification of the reduction mechanism of the proton at the
surface of the steel in H3PO4 (5.5 M).
The study of the influence of temperature in the range between 298 and 333 K
showed that the inhibition rate decreases with temperature and the activation
energy is modified by the presence of inhibitors. The change could be attributed
to the chemisorptions of the inhibitor to the surface of the steel.
The electrochemical impedance results show that the Nyquist diagrams obtained
have a single capacitive loop indicating that the corrosion of mild steel is mainly
controlled by the charge transfer process.
•
•
This study has also suggested that the inhibitors act by adsorption studied by
forming a protective layer on the metal surface and the piperidine tested
adsorbed according to the Langmuir isotherm. Determination of heat of
adsorption reinforced the chemisorptions postulated.
•
•
The inhibitory action of these inhibitors depends on the concentration and the
position of the nitrogen atom in the piperidine ring.
Negative values of the free energy of adsorption imply that these compounds are
strongly adsorbed in a spontaneous way to the surface of the steel. The
calculation of quantum energies agrees the higher inhibition of P3 in comparison
with that of P2.
•
A good accordance between the experimental methods.
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