Int. J. Electrochem. Sci., 8 (2013 ) 780 – 796 [602976]

Int. J. Electrochem. Sci., 8 (2013 ) 780 – 796

International Journal of
ELECTROCHEMICAL
SCIENCE
www.electrochemsci.org

Synthesis and Characterization of an Azo Dye: 4 –
(phenyldiazenyl)phenyl 2 -furoate. Electr ochemical and XPS
Study of its Adsorption and Inhibitive Properties on Corrosion
of Carbon Steel in Saline Water

Anca Moanta1, Adriana Samide1,*, Catalina Ionescu1, Bogdan Tutunaru1, Aurelian Dobritescu1,
Alain Fruchier2, Véronique Barragan -Montero3
1 University of Craiova, Faculty of Sciences, Calea Bucuresti 107 i, Craiova, Romania
2 UMR CNRS 5253 —ENSCM, 8 rue de l’Ecole Normale, 34296 Montpellier cedex 5, France
3 Equipe SyGReM, Institut des Biomolécules Max Mousseron,,IBMM, UMR 5247 , CNRS -UM1 -UM2 ,
ENSCM, 8, rue de l’Ecole Normale, 34296 Montpellier cedex 05, France
*E-mail: [anonimizat]

Received: 1 November 2012 / Accepted: 3 December 2012 / Published: 1 January 2013

The azo derivativ e namely 4 -(phenyldiazenyl)phenyl 2 -furoate (PPF), was obtained by an
esterification reaction between 4 -(phenyldiazenyl)phenol and 2 -furoyl chloride in pyridine. The dye
was characterized using elemental analysis, nuclear magnetic resonance (1H and 13C) an d mass
spectrometry with electro -spray ionization (ESI) techniques. The inhibitive properties of PPF on the
corrosion of carbon steel in saline water (SW) have been investigated using potentiodynamic
polarization, electrochemical impedance spectroscopy (EI S) and X -ray photoelectron spectroscopy
(XPS). Electrochemical measurements showed that PPF acts as corrosion inhibitor of carbon steel in
SW, by suppressing simultaneously the cathodic and anodic processes via adsorption on the carbon
steel surface. The r esults indicated that an increase in the inhibitor concentration leads to an increase in
both the charge -transfer resistance ( Rct) and inhibition efficiency ( IE) and to a decrease of the
corrosion current density ( icorr). Moreover, adsorption is spontaneou s and is best described by Temkin
isotherm. In the presence of PPF, XPS analysis confirmed the existence of a superficial layer providing
a good corrosion protection of the electrodes. Parameterized Model number 3 (PM3) method was
successfully used to pred ict the electron density distribution inside of PPF molecule.

Keywords: 4-(phenyldiazenyl)phenyl 2 -furoate; chemical synthesis; adsorption and inhibitive
properties; electrochemical measurements .

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1. INTRODUCTION
Azo colourings are the most versatile class of dyes [1,2]. Their structure has been intensely
studied and many spectral data analyses have already been reported [2,3]. The dyes have been most
widely used in fields such as dying textile fibers, biomedical studies, advanced applications in organ ic
synthesis and high technology areas like lasers, liquid crystalline displays, electro -optical devices and
ink-jet printer [2 -4]. The adsorption properties of different dyes were reported [5-7]. Organic dyes
have also been reported as effective corrosion inhibitors of mild steel in different media [8 -16].
Generally speaking, corrosion inhibitors reduce destructive attack initiated by physical, chemical and
biological factors and which affects all materials used in various fields (e.g., mining, petroleum, oil
sands and marine transportation). The c orrosion protection efficiency of organic inhibitors depends on
some physicochemical properties of the molecule (presence of O, N, S heteroatoms, functional groups,
aromaticity, steric effects, electronic density) , on the type of corrosive medium and nature of
interaction between inhibitors and the vacant d -orbital of iron [17 -21]. The inhibition performance of
organic dyes was studied using weight loss and electrochemical measurements. The results showed
that the dyes act as corrosion inhibitors by adsorption on the metal surface and that the inhibition
efficiency increases with the increasing concentration of investigated dyes [8 -16].
As part of our on -going interest on azo -derivatives and their chemical behaviour [22-26] we
have prepared a new azo -dye, namely 4 -(phenyldiazenyl)phenyl 2 -furoate (PPF). In this study, we
report the synthesis and characterization of PPF (elemental analysis, mass spectrometry, 1H and 13C
NMR), as well as the results obtained in the inv estigation of the inhibitive properties of PPF for the
corrosion of carbon steel in saline water (SW). In a previous study, we have reported the
characterization of this compound using UV –Vis, FTIR, and thermal analysis. Moreover, its stability
in differen t environments has been discussed [27].

2. EXPERIMENTAL
2.1. Synthesis
4-(phenyldiazenyl)phenol, pyridine and 2 -furoyl chloride used in the synthesis were Aldrich
products. 0.85 g (6.51 mmol) of 2 -furoyl chloride were added to a solution formed by 1.3 g (6.56
mmol) of 4 -(phenyldiazenyl)phenol in 30 mL of pyridine. The obtained solution was stirred for 60
minutes at room temperature and then left to stand overnight, after which it was poured over 100 mL
of distilled water. 2 mL of concentrated HCl solutio n were then added and the mixture was filtered on
a G 3 filter, the precipitate was washed with water and dried in the drying oven at 95 C. The product
was crystallized from ethanol to give PPF as a crystalline yellow powder, insoluble in water and stable
in air at room temperature. Yield: 83%. R f: 0.65 (toluene:acetone 9:1 V/V). The melting point was
determined using a Boetius apparatus without correction: m.p.: 102 C.

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2.2. Characterization
2.2.1. Spectral studies
Elemental analysis of carbon, hydrogen and nitrogen has been performed using a Carlo -Erba
O/EA 1108 analyzer. The mass spectrum obtained using electrospray ionization method (ESI) was
realized on a Waters Micromass ZQ spectrometer using methanol as the solvent.
The NMR spectra were recorded at 20° C on a Bruker DRX -400 spectrometer working at
400.13 MHz for 1H and 100.62 MHz for 13C. The chemical shifts (δ) of 1H and 13C spectra are reported
in ppm/TMS, with the 1H CHCl 3 and 13C CDCl 3 signals at 7.26 and 77.00 ppm respectively. The
coupling constants (J) are reported in Hz. 1D spectra (1H and 13C-APT) and 2D spectra of homonuclear
(COSYGP) and inverse heteronuclear (HMQCGP) correlations were recorded with the standard
BRUKER sequences.

2.3. Corrosion tests
2.3.1. Materials
The used carbon steel had the following composition (% weight): C=0.1; Si=0.035; Mn=0.4;
Cr=0.3; Ni=0.3, with t he balance in Fe. The samples were mechanically polished with different grades
of emery paper (down to 600), degreased with acetone and dried. All of the tests were performed in
saline water (SW) containing 0.15 mol L-1 NaCl and 0.001 mol L-1 HCl (pH = 3) without PPF and
with various concentrations of the dye.

2.3.2. Electrochemical measurements
Electrochemical measurements were performed using a Voltalab 40 model PGZ301
potentiostat/galvanostat driven by a personal computer with VoltaMaster 4 software. A typical three
electrodes cell with a working electrode made of carbon steel with an active surface of 1 cm2 was used.
The auxiliary electrode was a platinum plate (1 cm2) and the reference electrode was represented by a
saturated Ag/AgCl electrode. Potenti odynamic polarization curves were obtained with the scan rate of
1 mV s-1, in a potential range from -800 mV to -100 mV. The immersion time of the plates in the SW
blank and in SW containing various concentrations of PPF was 30 minutes in open circuit at r oom
temperature.
Electrochemical impedance spectroscopy (EIS) measurements were carried out after 30
minutes immersion time of the carbon steel plates in corrosive media, at the corrosion potential ( Ecorr),
in a frequency range from 105 Hz to 10-1 Hz by a parturition signal of 10 mV amplitude peak to peak,
at room temperature.

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2.3.3. X -ray photoelectron spectroscopy (XPS)
X-ray photoelectron spectroscopy (XPS) spectra were recorded in a VG ESCA 3 Mk II –
EUROSCAN spectrometer with a Mg K_ X -ray source ( 1486.7 eV photons energy) operated at 300 W
(accelerating voltage 12.5 kV, emission current 24 mA). The pressure in the analysis chamber did not
exceed the value of 2 -3·10-8 torr during all the period of spectra acquisitions. In order to perform the
surfac e charge compensation, a FG40 flood gun device (Specs GmbH – Germany) has been used, with
a 0.1 mA electronic current at a 2 eV energy. The samples were measured in an “as received”
condition with no other surface cleaning treatment (chemical etching or Ar + ion beam bombardment).
Survey spectra were recorded with a window of 1200 eV and 100 eV pass energy. The Gaussian
profile lines for curve fitting procedure was used. Binding energy calibration was done by linking the
reference to C(1s) line, the binding of C-C or C -H located at 285 eV.

3. RESULTS AND DISCUSSION
3.1. Synthesis and characterization
PPF has been obtained using the reaction of 4 -(phenyldiazenyl)phenol with 2 -furoyl chloride in
pyridine (Scheme 1). The composition and purity of the synthes ized azo -ester was confirmed by
elemental analysis. The obtained elemental analyses values for the compound C 17H12N2O3 are the
following: calculated: C, 69.86; H, 4.11; N, 9.59; found: C, 69.93; H, 4.16; N, 9.55.

Scheme 1. 4-(phenyldiazenyl)phenyl 2 -furoate (PPF) synthesis

The peaks that appear in the ESI+ MS spectra of the investigated azoderivative (Fig. 1) are the
corresponding adduct ion with sodium [M+Na]+ at m/z 315, with a relative intensity of 100%, the
protonated molecular ion [M+H]+ at m/z 2 93 (RI=5.38%, Fig. 1, detail a), and the cluster ion
[2M+Na]+ at m/z 607 (RI=1.26, Fig. 1, detail b).

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Figure 1. ESI mass spectrum of PPF. The description of the NMR spectra of the new dye is realized
below.

Figure 2. (a) 1H NMR spectrum of PPF; (b ) 1H – 1H correlation spectrum of PPF.

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785
Fig. 2a shows that the signals of the four protons of the disubstituted aromatic ring form an
AA’XX’ system. The 2D COSY spectrum (Fig. 2b) shows that the two parts of this system are at 7.39
ppm and 8.01 ppm. Assumin g that the substituent effects [28] of the two groups (R -N=N – and O -C(O) –
R') are additive, signals at 7.39 and 8.01 ppm may be assigned to Haa' and Hxx' respectively. The
signals of the monosubstituted aromatic ring protons are displayed as an AA'MM'P syst em at 7.92 ppm
(H4 and H 4’), 7.54 ppm (H 5, H 5’), and 7.48 ppm (H 6). On the other hand, H 9, H 10 and H 11 form double
doublets at 7.43 ppm, 6.62 ppm and 7.70 ppm respectively.
The signals in 13C NMR spectrum (Fig. 3a) have been assigned with the help of the 2D HMQC
spectrum (Fig. 3b) and the substituent additive effects [29]. The results of these assignments are
gathered in Table 1.

Figure 3. (a) 13C NMR spectrum of PPF; (b) 1H – 13C correlation spectrum of PPF.

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Table 1. 1H and 13C NMR parameters of PPF

1H 13C
Atom δ (ppm) and coupling constants Atom δ (ppm)
– – C1 152.07 *
– – C2 150.30 *
– – C3 152.45 *
H4 and H 4’ 7.92 C4 and C 4’ 122.83
H5 and H 5’ 7.54 C5 and C 5’ 129.05
H6 7.48 C6 131.07
– – C7 156.48
– – C8 143.64
H9 7.43 (3JH9-H10=3.5 Hz; 3JH9-H11=0.8
Hz) C9 119.79
H10 6.62 (3JH10-H9=3.5 Hz; 3JH10-H11=1.7
Hz) C10 112.26
H11 7.70 (3JH11-H10=1.7 Hz; 3JH11-H9=0.8
Hz) C11 147.35
Ha and H a' 7.39 Ca and C a' 122.20
Hx and H x' 8.01 Cx and C x' 124.09
* These assignments may be interchanged.

3.2. Study on corrosion inhibition of carbon steel in saline water in presence of PPF
3.2.1. Potentiodynamic polarization
Potentiodynamic anodic and cathodic polarization scans were carried out in SW blank and in
SW containing different concentrations of PPF: 0.04 mmol L-1; 0.06 mmol L-1; 0.08 mmol L-1; 0.1
mmol L-1 (Fig. 4a). From Fig. 4a it can be seen that, in the presence of PPF, the curves are shifted to
lower current regions and to higher anodic potential areas , showing the inhibition property of this dy e.
Iron oxidation is suppressed by PPF more strongly than the cathodic process is stimulated and
the corrosion potential ( Ecorr) becomes higher, with an increase in PPF concentration (Fig. 4a). This
result suggests that the addition of this dye in SW distu rbs the cathodic reaction s and reduces the
anodic dissolution of iron . Moreover, Fig. 4a show s that cathodic polarization curves give rise to Tafel
lines, indicating that the reactions of all cathodically reduced components , such as hydrogen evolution
and oxygen reduction, are activation -controlled [30]. The anodic curves show th at the inhibition
process depends upon electrode potential. Indeed, for an overall voltage higher than -215 mV vs.
Ag/AgCl sat, the presence of PPF does not modify the E- logi curves , which indicate s that the
dissolution process dominates the inhibition [30]. Therefore, in the vicinity of corrosion potential
(Ecorr), a decrease in the cu rrent density is observed starting with the PPF concentration of 0.04 mmol
L-1. This phenomenon ref lects the formation of an anodic protective film on the electrode surface [30],
via PPF adsorption on the substrate . The above mentioned changes are more significant with the
increase of PPF concentration. These modifications are associated with the decrea se of corrosion

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current, and consequently , with inhibition efficiency ( IE) increase . Based on these results, we
concluded that : (i) PPF acts as a corrosion inhibitor in SW by suppressing simultaneously the cathodic
and anodic processes with anodic predomin ance; (ii) the organ ic film formed o n electrode surface is
stable until the potential value of -215 mV; (iii) above this potential , PPF partial desorption is possible.
-5-3-113
-800 -600 -400 -200 0
E (mV vs. Ag/AgCl)log I (mA / cm 2)
SW blank solution
SW / 0.04 mM PPF
SW / 0.06 mM PPF
SW / 0.08 mM PPF
SW / 0.1 mM PPF

5'b
1'2'4 5 3
12
3'4'1. y = 81.91x – 428.3
1'. y = -111x – 556.423. y = 83.969x – 304.79
3'. y = -97.35x – 551.972. y = 84.62x – 354.47
2'. y = -97.748x – 569.45
4. y = 85.37x – 318.67
4'. y = -92.897x – 599.9
5. y = 84.563x – 299.92
5'. y = -90.167x – 610.86
-800-700-600-500-400-300
-1.5 -1 -0.5 0 0.5 1 1.5
log i (mA / cm2)E(mV vs. Ag/AgCl)
HCl blank solution HCl / 0.04 mM PPF HCl / 0.06 mM PPF
HCl / 0.08 mM PPF HCl /0.1 mM PPF

Figure 4. Potentiodinamic curves (a) and Tafel diagram (b) of carbon steel corroded in saline water
(SW) without and with various concentrations of PPF, at room temperature.

The corrosion current density ( icorr) was calculated at intercept of the anodic and cathodic Tafel
lines to corrosion potential, using VoltaMaster 4 software. The T afel diagram is presented in Fig. 4b.
The characteristic equations of anodic and cathodic Tafel lines are inserted in the graph from Fig.4b.
The electrochemical parameters such as : corrosion potential ( Ecorr), corrosion current density ( icorr),
anodic and cathodic Tafel slopes ( ba & bc) derived from polarization curves and corresponding
inhibition efficiency ( EI) values at different PPF concentrations are given in Table 2. The inhibition
efficiency percentage ( IE) of PPF was determined from polarization mea surements according to the
following equation, Eq. 1 [31,32] :

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788
100
0
corrcorrcorr
ii0i
IE
(1)

where i0
corr and icorr are the corrosion current densities of carbon steel in SW without and with PPF,
respectively.
The r esults showed that the inhibition efficiency increases with increasing PPF concentration,
reaching a maximum value of 92.6 % at 0.1 mmol L-1 of PPF. Moreover, the slopes of the anodic and
cathodic Tafel lines ( ba & bc) were slightly altered by increasing t he tested compound concentration
(Table 2). The small change may be due to surface blockage by PPF. This could be attributed to the
fact that the anodic carbon steel dissolution and cathodic reaction s were both inhibited by this dye
through merely blocking the reaction sites of carbon steel surface , without affecting the anodic and
cathodic reaction mechanism [33].

3.2.2. Adsorption isotherm
To express th e adsorption quantitatively, different adsorpt ion isotherms may be applied. These
isotherms character ize the met al/inhibitor/environment system and fit the degree of surface coverage
(θ) values . In our study , the best fit was found to obey Temkin adsorption isotherm whi ch may be
expressed by Eq. 2 [34 ]:
KCθ)(f exp
(2)

Equation 2 may be written as the following expres sion, Eq.3:

logCf2.303logKf2.303θ 
(3)

where C is the concentration (mol.L-1) of the inhibitor in the bulk electrolyte, θ is the degree of
surface coverage (θ = IE/100) , K is the adsorption equilibrium constant and f is the number of surface
active sites occupied by one inhibitor molecule.
The plot of θ against logC for the PPF is given in Fig. 5. The straight line relationship was
obtained suggesting the validity of Temkin model for the adsorption of PPF on carbon steel s urface,
under the simulated laboratory conditions . The equation and deviation from linearity (R2) are inserted
in the graph presented in Fig. 5. The R2 value is very close to unity, which indicates a str ong
adherence of the assumption of Temkin adsorption isotherm to experimental data. The slope of this
line equals 2.303/f and the intercept is [(2.303/f)·log K], from which the value of K was calculated. It
can be observed that K has a value of 513105 L mol-1 and f = 4.24. It is noticed that the value of “ f” is
more than unity. This means that PPF molecules will form a monolayer on the steel surface [35].
The equilibrium constant of adsorption K obtained from the intercepts of Temkin adsorption
isotherm is related to the free energy of adsorption ( ΔG ads0) as follows, Eq.4 [35]:

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789




 TRG
exp55.51Kads0Δ
(4)

where R is the universal gas constant, T is the temperature ( K) and 55.5 is the molar
concentration of water in the solution. The negative value ( -42.5 kJ mol-1) obtained for ΔG ads0 suggests
that the inhibitor’s molecules are adsorbed on carbon steel surface. Moreover, this value indicates a
spontaneous adsorption of PPF molecules and usually characterizes their strong interaction with the
metal surface [34-37]. The value of ΔGads0 of (-40 kJ mol-1) is usually accepted as a threshold value
between chemo sorption and physical adsorption [37]. The value of ΔG ads0 obtained in our study ( -42.5
kJ mol-1) indicates a chemical adsorption mechanism.

y = 0.5428x + 3.0995
R2 = 0.9958
0.70.750.80.850.90.95
-4.5 -4.4 -4.3 -4.2 -4.1 -4 -3.9
log C-PPF/mol L-1θ

Figure 5. Temkin adsorption plots of carbon steel corroded in saline water (SW) without and with
various concentrations of PPF, at room temperature.

3.2.3. Electrochemical impedance spectroscopy (EIS)
Figs. 6a and 6b show the Nyquist and Bode plots for carbon steel in SW without and with PPF;
it can be seen that the impedance response of carbon s teel in SW shows a significant change after PPF
addition (Fig. 6b). This indicates that the carbon steel impedance increases with increasing the
inhibitor concentration and, consequently, the inhi bition efficiency increases. From Fig.6a it can be
seen that Nyquist curves are co nsisted in one capacitive loop, corresponding to one phase angle
maximum in Bode diagram ( Fig. 6b).
The equivalent circuit which fits well the experimental data, includes the following elements:
the solution resistance (Rs) of the bulk electrolyte, the double layer capacitance (Cdl) of the electr olyte
at the metal surface and the charge -transfer resistance (Rct) of the metal [30], which is placed in
parallel with Cdl. The int ersection of the capacitive loop with the real axis represents the charge
transfer resistance, Rct, at very low frequencies and the electrolytic resistance, Rs, at very high

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frequencies which enclosed between the working electrode and the reference one [30]. The double
layer capacitance, Cdl, was derived from frequency, at which the imaginary component of the
impedance (-Zimax) was maximal [35,38] using the relationship, Eq. 5 [35,38 ]:

ct dliRCZf21) (max
(5)

a
060120180240300
0 100 200 300 400
Zr (Ω cm2)- Zi (Ω cm2)SW blank solution
SW / 0.04 mM PPF
SW / 0.06 mM PPF
SW / 0.08mM PPF
SW / 0.1 mM PPF

b
00.511.522.53
-1 1 3 5
log Freq (Hz)log Z (Ω cm2)-80
-60
-40
-20
0
phase (degree)SW blank solution SW / 0.04 mM PPF
SW / 0.06 mM PPF SW / 0.08 mM PPF
SW / 0.1 mM PPF

Figure 6. (a) Nyquist plots and (b) Bode plots for carbon steel corroded in saline water (SW) without
and with various concentrations of PPF after immersion time of 30 minutes, at room
temperature.

More pronounced frequency arcs were obtained for the sample s which were immersed in SW
containing various concentrations of PPF. This behaviour is usually assigned to changes in density and
composition of the substrate layer. It is clear that PPF presence produced a higher charge -transfer
resistance ( Rct) value, w hich is interpreted in terms of formation of an effective protective layer that
diminishes the corrosion processes.
The impedance parameters derived from EIS measurements Rs, Rct, Cdl were calculated using
VoltaMaster 4 software with an error of ±1 %, and are listed in Table 2.

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Table 2. Electrochemical parameters and inhibition efficiency ( IE) obtained from potentiodynamic
polarization and EIS for carbon steel corroded in SW without and with different concentrations
of PPF, at room temperature

Electroche mical results
C-PPF/
mmol L-1 From potentiodynamic polarization From EIS
Ecorr/mV
vs.Ag/AgCl icorr/
mA cm-2 ba/
mV dec-1 bc/
mV dec-1 IE/
% Rs/
Ω cm2 Cdl/
μF cm-2 Rct/
Ω cm2 IE/
%
0 -482 0.23 81.9 111 0 1.16 259.7 34.6 0
0.04 -474 0.066 84.6 97.7 71.3 1.01 135.8 118.3 70.7
0.06 -469 0.043 83.9 97.3 81.3 0.85 91.8 176.3 80.4
0.08 -458 0.026 85.3 92.9 88.7 0.4 69.6 237.8 85.4
0.1 -449 0.017 84.5 90.1 92.6 0.37 49.9 318.8 89.2

EIS results show that Rs and Cdl decrease and Rct increases suggesting t hat the amount of
adsorbed inhibitor molecules increases. This decrease in Cdl could be attributed to the decrease in local
dielectric constant and/or an increase in the thickness of the electrical double layer [39,40] , signifying
that PPF acts by adsorpti on at the interface of metal/solution. The inhibition efficiency ( IE) was
determined using t he following relationship, Eq. 6 [31,35]:

1000




ctct ct
RRRIE
(6)

where Rct and Rct0 represent the charge -transfer resistances in the prese nce and absence of PPF.
Inspection of the data in Table 2 reveals that the addition of PPS in saline water increases Rct
and decreases Cdl, and consequently enhances IE reaching a value of 89.2% at 0.1 mmol L-1 PPF. It
can be seen that the values of inhibi tion efficiency obtained from potentiodynamic polarization and
EIS measurements are in good agreement.

3.2.4. Surfaces characterization. XPS analysis
The samples corroded in SW blank and SW containing 0.1 mmol L-1 PPF, after
potentiodynamic polarization, were also examined using XPS surface analysis.
Survey spectrum (recorded in a 1200 eV window) of carbon steel surface corroded in SW
containing 0.1 mmol L-1 PPF shows peaks at 285 eV, 532 eV and 710 eV binding energy
corresponding to C(1s) (attributed to adventitious C characteristic line and C from PPF molecule),
O(1s) and Fe(2p3/2) lines. The peaks at 400 eV, 199.5 eV and 1072.3 eV correspond to N(1s) (from
PPF molecule), Cl(2p) and Na(1s) lines, respectively (contamination which resulted from the sampl e
being exposed to electrolyte).
Fig. 7a shows the high resolution of XPS spectrum for the Fe(2p3/2) region of carbon steel
corroded in SW containing 0.1 mmol L-1. A similar spectrum was obtained for carbon steel corroded in

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SW blank solution. The binding energies for the peaks have been referenced to C -C bond at 285 eV.
Metallic iron appeared at 706.6 eV and Fe3+(2p3/2) appeared at 710.6 eV and 712.3 eV. The positions
and energy values are very close to those observed either for α, γ -FeO(OH) and for Fe 2O3 structures,
respectively [ 41]. In order to differentiate between FeO(OH) and Fe 2O3, we have also monitored the
O(1s) region (Figs. 7b and 7c).

Figure 7. XPS spectra of carbon steel corroded in saline water (SW): a- iron spectrum for carbon steel
corro ded in SW containing 0.1 mmol L-1 PPF; b- O(1s) spectrum for carbon steel corroded in
SW blank solution; c- O(1s) spectrum for carbon steel corroded in SW containing 0.1 mmol L-1
PPF; d- N(1s) spectrum for carbon steel corroded in SW containing 0.1 mmol L-1 PPF.

For the oxygen peak of both samples: corroded in SW blank solution and in SW containing 0.1
mmol L-1 PPF, three well resolved peaks were observed in each case, at: 529.9, 531.4, 532.6 eV (Fig.
7b) and 529.9, 531.3, 532.1 eV (Fig. 7c) relate d to met al oxide, hydroxides [ 42] and C＝O bonds [43]
or other adsorbed speci es such as oxygen from water [41]. For α – FeO(OH) or/and γ – FeO(OH) two
well resolved peaks were observed at 529.9 eV (O-2) and 531.4/531.1 eV (OH-). The O(1s) peak for
Fe2O3 was found at 530.1 eV and 529.9 eV, respec tively. It can be noticed that, in presence of PPF
(Fig. 7c), t he adsorbed species amount is larger than in absence of PPF (Fig. 7b), related to –O-CO-
group and oxygen atom from furanic ring. Oxide -hydroxides of iron may occur in anhydrous

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[FeO(OH)] or hy drated [FeO(OH)·nH 2O] forms. The monohydrate [FeO(OH)·H 2O] might otherwise
be described as iron (III) hydroxide [Fe(OH) 3], and is also known as hydrated iron oxide or yellow iron
oxide .
Further relevant information about the surface chemistry of the carbo n steel corroded in SW
containing 0.1 mmol L-1 PPF is obtained from the XPS spectral analysis of N(1s) photo -peak (Fig. 7d).
The spectral simulation of the N(1s) photo -peak (Fig. 7d) for carbon steel corroded in presence of PPF
shows organic nitrogen speci es at 400.1 eV such as amine, amide, imine, etc [4 4,45]. Indeed, this
binding energy corresponds to C -N bonds from PPF molecule.
Taking these data into account, we may conclude that: (i) XPS demonstrates the presence of a
superficial layer on the corroded carbon steel surface in SW blank solution, confirming the formation
of a product, as a result of corrosion action. At this stage, it was showed that the main product of
corrosion is formed of Fe3+ species which are similar to those shown by Fe3+ oxide/oxyh ydroxide
consisting of Fe 2O3 and -FeOOH and/or -FeOOH, -FeOOH/-FeOOH and Fe(OH) 3, where
oxyhydroxides are the main phase .; (ii) in the presence of PPF , the protective layer consists of a thin
organic film and an amount of the iron compounds already des cribed above; (iii) the survey spectrum
shows that this layer can incorporate Cl- and Na+ ions; (iv) t he presence of nitrogen and oxygen atoms,
as active centres in PPF molecule can enhance the binding between carbon steel surface and this azo
ester, which consequently results in the formation of a compact and impermeable layer. We assume
that PPF molecules are adsorbed on the metal surface through interactions between the metal and
the lone pairs of electrons of: (a) nitrogen from the azo group ( -N=N -) and (b) oxygen from –O-
CO- group and, more probably, from the furanic ring.

3.2.5. Adsorption mechanism

Theoretical chemistry has been used to explain the mechanism of corrosion inhibition.
Parameterized Model number 3 (PM3) semiempirical method was success fully used to predict the
electron density distribution inside of PPF molecule. The optimized geometry (Fig.8a) and the maps
(2D and 3D) of electron density distribution of the studied azo dye (PPF) are given in Fig.8.
Figs.8b and 8c reveal that the highes t levels of electron density are found in the vicinity of the
oxygen atoms attached to carbon atom from –O-CO- group, in vicinity of the oxygen atom from the
furanic ring and in vicinity of nitrogen atoms from the azo group. Moreover, lowest levels of elec tron
density are found in the inside of aromatic and furanic rings. Therefore, our prognosis is that this azo
dye can be adsorbed on the carbon steel surface using these active centers: (i) nitrogen atoms from the
azo group ( -N=N -); (ii) oxygen from furani c ring; (iii) oxygen atoms attached to carbon from –O-CO-
group. This hypothesis is in good agreement with XPS results.

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Figure 8. The optimized geometry (a); total electron density surface: 2D map (b) and 3D map (c) for
PPF (the electron rich regions a re violet and the electron poor regions are green).

4. CONCLUSIONS
4-(phenyldiazenyl)phenyl 2 -furoate is a novel dye obtained by reacting 4 –
(phenyldiazenyl)phenol with 2 -furoyl chloride in pyridine. The structure of this new compound has
been assigned us ing 1H and 13C NMR, mass spectrometry and elemental analysis.
PPF behaves as a corrosion inhibitor of carbon steel in saline water. In SW, at 0.1 mmol L-1, it
has an inhibition efficiency of 91 ± 1 % obtained from potentiodynamic polarization and
electroch emical impedance spectroscopy. PPF acts as adsorption inhibitor on carbon steel surface;
adsorption is spontaneous and is best described by Temkin isotherm.
In the presence of PPF, XPS analysis confirmed the existence of a superficial layer providing a
good corrosion protection of the electrodes. XPS analysis confirms the adsorption of PPF on carbon
steel surface ; at this stage, the main product of corrosion is a non-stoichiometric Fe3+
oxide/oxyhydrox ide, consisting of a mixture of , -FeOOH, Fe(OH) 3 and Fe2O3, where
oxyhydroxides represent the main phase.
PM3 method was successfully used to predict the electron density distribution inside of
PPF molecule. Thus, PPF can be adsorbed on the carbon steel surface through interactions
between the metal and the lone pairs of electrons of nitrogen atoms from the azo group ( -N=N -),
oxygen from –O-CO- group and oxygen atom from the furanic ring. The adsorption layer functions as a
barrier isolating the metal from the corrosion.

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