Journal of Thermal Analysis and [602550]

123
Journal of Thermal Analysis and
Calorimetry
An International Forum for Thermal
Studies

ISSN 1388-6150
Volume 118
Number 2

J Therm Anal Calorim (2014)
118:631-639
DOI 10.1007/s10973-014-3726-2Aminophylline: thermal characterization
and its inhibitory properties for the carbon
steel corrosion in acidic environment
Adriana Samide, B. Tutunaru, Catalina
Ionescu, P. Rotaru & Luminita Simoiu

123
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Aminophylline: thermal characterization and its inhibitory
properties for the carbon steel corrosion in acidic environment
Adriana Samide •B. Tutunaru •Catalina Ionescu •
P. Rotaru •Luminita Simoiu
Received: 22 September 2013 / Accepted: 23 February 2014 / Published online: 22 March 2014
/C211Akade ´miai Kiado ´, Budapest, Hungary 2014
Abstract Aminophylline (AMF) was studied as corrosion
inhibitor for carbon steel in 1.0 mol L-1HCl solution
using electrochemical measurements associated withUV–Vis spectrophotometry and optical microscopy.
Simultaneous thermogravimetry/derivative thermogravi-
metry and differential scanning calorimetry analysis wasperformed in order to determine the temperature range in
which AMF is an effective inhibitor, without the decom-
position risk that could change the inhibition mechanism.Thermal behaviour restricts AMF application as corrosion
inhibitor for carbon steel in 1.0 mol L
-1HCl solution at
temperatures B45/C176C where there are no significant mod-
ifications of the adsorption mechanism. According to the
results of electrochemical measurements, in association
with UV–Vis spectrophotometry and optical microscopytechniques, AMF is a mixed-type inhibitor for carbon steel
corrosion in 1.0 mol L
-1HCl solution, simultaneously
suppressing the anodic and cathodic processes and actingvia spontaneous physisorption on the metal surfaces.
Keywords Pharmaceutical product /C1Thermal behaviour /C1
Corrosion inhibition /C1Carbon steel /C1Electrochemical
measurementsIntroduction
The corrosion inhibition is a subject of intense research for
many scientific and industrial applications where the mass
reduction of different metals could be avoided by the
improvement of their corrosion resistance using effectivemethods to protect the underlying substrate surface.
Hydrochloric and sulphuric acid solutions are widely
used in industry, notable applications including the clean-ing and pickling of steel [ 1,2]. Because of the general
aggression effect of these solutions, the corrosive attacks
on metallic surfaces are promoted.
Mass loss measurements, voltammetric techniques and
electrochemical impedance spectroscopy have been used to
evaluate the corrosion resistance of steel in aqueous acidsolutions [ 3–5]. Since the structure life of steel is often
influenced by the surface corrosion, the composition and
thermal behaviour of the rust formed on the steel surface isof great interest. The thermal analysis method has been
applied by several workers for the study of the iron cor-
rosion products resulted in both uninhibited and inhibited
solutions [ 6–9], and to characterize some organic com-
pounds with applications in various fields [ 10–16].
The use of inhibitors is the most appropriate way to
isolate the metal from corrosive agents and thus to reduce
corrosion reactions [ 17,18]. The inhibitors are organic
molecules containing nitrogen, oxygen and/or sulphur as
electron donating heteroatoms, or heterocyclic molecules
withp-orbitals. The inhibition efficiency of these com-
pounds depends on their ability to be adsorbed on the
metallic surface. The toxic effects of most synthetic
inhibitors and the obligations to respect the norms ofhuman health and safety have lead to the research of green
alternatives, and several eco-friendly and harmless inhibi-
tors have been reported [ 18–44].A. Samide ( &)/C1B. Tutunaru /C1C. Ionescu /C1L. Simoiu
Department of Chemistry, Faculty of Mathematics and Natural
Sciences, University of Craiova, 107i Calea Bucuresti,200512 Craiova, Romaniae-mail: samide_adriana@yahoo.com
P. Rotaru
Department of Physics, Faculty of Mathematics and NaturalSciences, University of Craiova, 13 AI Cuza Street,200585 Craiova, Romania
123J Therm Anal Calorim (2014) 118:631–639
DOI 10.1007/s10973-014-3726-2
Author's personal copy

Recently, several studies have been carried out on the
inhibition of steel corrosion by the natural extracts of As-
teriscus graveolens [18],Jatropha curcas [19],Phyllanthus
amarus [20],Sesamum indicum [21],Chenopodium am-
brosioides [22],Foeniculum vulgare [23],Xylopia fer-
ruginea [24],Murraya koenigii [25],Eugenia jambolana
[26],Zenthoxylum alatum [27],Simiria tinctoria and
Guatteria ouregou [28],Oxandra asbeckii [29],Aloe vera
[30],Moringa oleifera ,Piper longum andCitrus aurantium
[31],Ginkgo [32],Haematoxylum campechianum [33],
apricot juice [ 34] and caffeic acid [ 35].
Survey of the literature reveals that many pharmaceu-
tical active compounds have been evaluated as effective
green corrosion inhibitors for different metals: Ampicillin
[36], Amoxycillin [ 37], Cefalexin [ 38], Cip rofloxacin [ 39],
Norfloxacin [ 40], Doxycicline [ 41], Erythromycin [ 42],
Sulfacetamide [ 43] and Streptomycin [ 44].
The aim of the present study is to investigate the thermal
behaviour of aminophylline in order to establish the tem-
perature range where its stability is observed to be recom-
mended as a corrosion inhibitor, without decomposition risk,for carbon steel in 1.0 mol L
-1HCl solution. The results of
thermal analysis were illustrated by the thermoanalytical
(TG/DTG/DSC) curves. The inhibitory and adsorptionproperties of AMF were studied using electrochemical
measurements, followed by UV–Vis spectrophotometry and
optical microscopy techniques. Aminophylline (AMF) is apharmaceutical product containing theophylline (THP) and
ethylenediamine (EDA) in ratio of 2:1 being known under
the following synonyms: (Theophylline)
2Ethylenediamine;
3,7-Dihydro-1,3-dimethyl-1H-purine-2,6-dione, compound
with 1,2-ethanediamine (2:1). Theophylline hemiethylen-
ediamine complex is a bronchodilator drug that is usuallyfound as a monohydrate or dihydrate. The molecular struc-
ture of AMF is presented in Fig. 1.
Experimental
Materials
The thermal behaviour and inhibitory properties of AMF,
a Sigma-Aldrich powder (assay C98.0 %; impurity*1 mol/mol H
2O), were investigated. The preliminary
thermal study of EDA (liquid compound) was performed
using a Sigma-Aldrich product (assay C99.5 %; vapour
density versus air of 2.07; vapour pressure of 1.3 kPa at20/C176C). The plates of carbon steel (area 1.0 cm
2) with
the following composition (mass%): C =0.1; Si =0.035;
Mn=0.4; Cr =0.3; Ni =0.3; and Fe the remainder
until 100 %, were used as working electrodes during the
electrochemical measurements. The samples were
mechanically polished with emery paper, degreased with
acetone and dried in warm air. The corrosion tests were
performed in 1.0 mol L-1HCl (AR—obtained from
Merck) blank solution and in 1.0 mol L-1HCl solutions
containing various concentrations of AMF: 0.2; 0.4 and
0.6 mmol L-1.
Methods and techniques
Thermal analysis
The thermal analysis of AMF was performed in nitrogen
atmosphere using a Diamond analyzer from Perkin-Elmer
with Pyris software. The sample (3.02 mg) was heated in
alumina crucibles in the temperature range of RT-400/C176C,
with a heating rate of 10 /C176C min-1. Moreover, ethylenedia-
mine (EDA) thermal behaviour was studied by heating of
1.7 mg in the temperature range of RT-200/C176C, with a heating
rate of 10 /C176C min-1. The dynamic conditions were achieved
by N 2gas purging with a constant flow of 150 mL min-1.
Electrochemical measurements
The potentiodynamic polarization and the measurements of
polarization resistance in time were used to determine the
corrosion current density and degree of surface coverage
(h) for carbon steel corroded in 1.0 mol L-1HCl blank
solution and in 1.0 mol L-1HCl solution containing var-
ious concentration of AMF: 0.2; 0.4 and 0.6 mmol L-1.
All the electrochemical measurements were carried outusing a VoltaLab 40 potentiostat/galvanostat, with Volta-
Master 4 software. The experiments were performed using
a glass corrosion cell with three electrodes: a platinumauxiliary electrode, a saturated Ag/AgClsat reference
electrode and the carbon steel sample as working elec-
trodes. The immersion time of the plates in the aggressivemedia was 4.0 min in open circuit, at room temperature.
The polarization curves were recorded with a scan rate of
1.0 mV s
-1.
UV–Vis spectrophotometry
The samples of 1.0 mol L-1HCl solution containing var-
ious concentrations of AMF above mentioned were usedH3C
CH3
H2NNH2x H2ONN
N
HNO
OH3C
CH3NN
N
HNO
O
•
Fig. 1 Molecular structure of AMF632 A. Samide et al.
123
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for spectrophotometric analysis, before and after the po-
tentiodynamic polarization. UV–Vis analysis reports were
obtained using a UV–Vis spectrophotometer, Varian-Cary
50 type, with Cary WinUV software.
Surface characterization
The surface morphologies of the carbon steel electrodes,
before and after corrosion in 1.0 mol L-1HCl solution and
in 1.0 mol L-1HCl solution containing various concen-
trations of AMF, were examined using a metallographic
microscope, type Euromex with Canon camera and inclu-ded ZoomBrowser—EOS Digital software.
Results and discussion
Thermal analysisIn order to recommend AMF as a corrosion inhibitor, its
thermal stability was studied according to the decompositionlevels which have been illustrated on the TG curve accom-
panied by the DTG/DSC curves. A preliminary study of
thermal analysis of EDA was performed, knowing that AMFcontains this compound. The results are illustrated in Fig. 2
which shows the TG/DTG (Fig. 2a) and the DSC (Fig. 2b)
curves obtained in the temperature range from RT to 180 /C176C.
It can be observed that the TG curve presents a single step of
mass loss that corresponds to one peak on DTG centred at
96.3/C176C. This is followed by one endothermic peak, reaching
a maximum at 96.3 /C176C on the DSC curve. This thermal
behaviour is attributed to the evaporation process of EDA,
starting at 40 /C176C and ending until 116 /C176C.
The thermal analysis of AMF, performed in temperature
range from RT to 400 /C176C, is presented in Fig. 3. On TG/
DTG curves (Fig. 3a) four distinctive steps of AMF
decomposition are noticed, accompanied by four endo-
thermic peaks on DSC curve (Fig. 3b) observed at the same
temperature. For a more accurate observation of thetemperature ranges where major changes of mass loss takes
place, TG/DSC curves were detailed in the range of low
temperatures up to 200 /C176C, and at high temperatures
between 200 and 400 /C176C.
As shown in Fig. 3c, at low temperatures, three mass
loss steps are observed: (i) between 40 and 117 /C176C, the
mass loss of 7.46 % is attributed to EDA evaporation; thecorresponding peak on DTG curve, and the endothermic
peaks on DSC curves (Fig. 3b, c) at 96.3 /C176C indicate a
similar process of EDA evaporation as that shown in
Fig. 2. The observed mass loss is less than that expected
(13.69 %) for the loss of 1 mole of EDA. The resultsindicate an incomplete evaporation, and suggest that a part
of EDA molecules are superficially adsorbed [ 45] on the
surface of AMF powder, and the other is intercalated, beingblocked among theophylline (THP) molecules, and/or these
are strongly linked inside of AMF particles; (ii) the second
endothermic process at 126.1 /C176C, accompanied by a mass
loss of 3.83 %, in the temperature range from 117 to
145/C176C, is also slightly smaller than the expected mass loss
(4.1 %) corresponding to 1 mole of water and (iii) between145 and 175 /C176C, the TG curve shows a sharp drop (mass
loss of 6.49 %) accompanied by the endothermic peak
which can probably be assigned to the decomposition oflinked EDA which could be fragmented as N
2/C2H4,N H 3
and H 2[46]. However, it is difficult to strictly connect the
values of the temperature ranges with the independentprocesses related to EDA evaporation, or decomposition,
and AMF dehydration. It is possible that parallel courses of
these processes take place, and the occurrence of over-lapping phenomenon cannot be excluded. It is certain that
until the temperature of 175 /C176C, the mass loss is 17.68 %,
meaning that this stable mixture of theophylline and eth-ylenediamine decomposes at low temperatures with dehy-
dration of one molecule of crystallization water and
evaporation of one molecule of EDA. This is in goodagreement with the expected (17.8 %) loss of *1.0 mol of
water and 1.0 mol of EDA. Similar results were obtained
for AMF dihydrate [ 47,48].020406080100
0 50 100 150 200
Temperature/°CTemperature/°CMass/%
–25–15–55
Derivative mass/% min–1
96.3DTGTGa
–120
–90
–60
–30
00 50 100 150 200Heat flow/mW
Endo up96.3bFig. 2 Thermoanalytical curves
of EDAThe carbon steel corrosion in acidic environment 633
123
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The TG/DSC curves of AMF obtained at high temper-
atures are presented in Fig. 3d. TG/DSC curves detailed at
high temperatures are assigned to theophylline thermal
behaviour. An inflection on the TG curve can be seen at271.8 /C176C accompanied by an endothermic peak on the DSC
curve, at the same temperature. Thus, until this tempera-
ture, other processes could occur such as elimination ofwater, and other gases resulted from the EDA decompo-
sition, and/or THP begins to sublimate [ 49], and its melting
point being at 271.8 /C176C[47,48]. Then, THP evaporates up
to 337 /C176C with minimal residue. Based on these data,
certain recommendations are necessary in order to useAMF as corrosion inhibitor for some metals in differentmedia. The possible risks which might occur in the process
of inhibition are presented in Table 1.
AMF inhibitory propertiesPotentiodynamic polarization
The potentiodynamic polarization was performed with a
scan rate of 1.0 mV s
-1in 1.0 mol L-1HCl blank solution
and in 1.0 mol L-1HCl solution containing various con-
centrations of AMF: 0.2; 0.4 and 0.6 mmol L-1, after the
carbon steel electrode prepolarization in open circuit for10 min at room temperature. The UV–VIS scans were020406080100
0 100 200 300 400 500
Temperature/°C Temperature/°C
Temperature/°C Temperature/°CMass/%
–20–15–10–505
Derivative mass/% min–1
96.3
154.6
271.8
336.8TG
DTGa–20
–10
0
10
0 100 200 300 400 500Heat flow/mW
Endo up96.3154.5271.8
336.8b
80859095100
25 85 145 205Mass/%–20
–15
–10
–5
0
Heat flow/mW
Endo up96.3
126.1154.57.46 %3.83 %
6.31 %TG
DSCc
0306090
200 250 300 350 400Mass/%–20
–15
–10
–5
0
5
10
Heat flow/mW
Endo up271.8
336.8TG
DSCdFig. 3 Thermoanalytical curves
of AMF detailed as aTG/DTG
curves, bDSC curve, cTG/DSC
curves of decomposition steps at
low temperatures and dTG/
DSC curves decomposition
steps at high temperatures
Table 1 Ranges of temperatures, media and potential risks related to the use AMF as corrosion inhibitor
Temp./ /C176C Medium Composition THP; EDA/mole AMF Risks
THP EDA
\45 Aqueous 2.0 mol 1.0 mol No risk
45–75 Aqueous 2.0 mol \1.0 mol Gradual and uncontrolled release of EDA
75–100 Aqueous 2.0 mol Far less than 1.0 mol Change of inhibition mechanism; this should
prevailing discussed in relation to THP
100–250 Organic heat-transfer
agents\2.0 mol – Change of inhibition mechanism; gradual and
uncontrolled release of THP
[250 – – – Not suitable634 A. Samide et al.
123
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recorded for inhibited 1.0 mol L-1HCl solution with dif-
ferent AMF concentrations, before and after corrosion, in
order to discuss the AMF electrochemical stability. Fig-
ure4presents the potentiodynamic curves (Fig. 4a) and
UV–Vis scans (Fig. 4b) obtained in given laboratory
conditions.
From Fig. 4a, it can be observed that the presence of
AMF in 1.0 mol L-1HCl solution causes the following
transformations on the polarization curves:
– the corrosion potential ( Ecorr) was shifted to higher
values, while the polarization curves have been shifted
to lower current regions. These phenomena certify theinhibition tendency of this compound that is more
obvious with increasing of its concentration in hydro-
chloric acid solution;
– both cathodic (hydrogen evolution) and anodic (carbon
steel dissolution) reactions were simultaneously inhib-
ited in the same manner;
– the cathodic curves give approximately parallel lines,
suggesting that the hydrogen discharge reaction lowers,
its activation being controlled [ 50,51]b yA M F
addition in aggressive medium; and
– in the vicinity of E
corr, an appreciable decrease in the
current density is observed starting with the concen-tration of 0.2 mmol L
-1AMF, suggesting the forma-
tion of an anodic protective layer on the carbon steel
surface [ 51].
Based on these observations it can be concluded that
AMF acts as a mixed-type inhibitor by forming a protectivelayer, as an effective barrier which interposes to electrode/
medium interface, suppressing the metal ionization. In
previous studies [ 51–53], a similar behaviour for other
pharmaceutical products was reported. All the phenomena
previously described indicate that the corrosion current
(i
corr) values decrease with the increase of AMF concen-
tration. The corrosion current ( icorr) was calculated by
extrapolation of anodic and cathodic Tafel lines to corro-
sion potential ( Ecorr) using VoltaMaster 4 software.The electrochemical parameters such as the corrosion
potential ( Ecorr), corrosion current density ( icorr), anodic
and cathodic Tafel slopes ( baandbc), as well as the inhi-
bition efficiency (IE), as a function of AMF concentration(C-AMF) are given in Table 2. The inhibition efficiency
percentage (IE) of AMF was determined from polarization
measurements according to Eq. 1[51–53]:
IE¼io
corr/C0icorr
io
corr/C2100 ð1Ț
where io
corrandicorrare the corrosion current densities of
carbon steel in 1.0 mol L-1HCl solution without and with
AMF, respectively.
From Table 2, it can be observed that (i) with the
increase in AMF concentration, icorr values gradually
decrease; (ii) the anodic ( ba) and cathodic ( bc) Tafel slopes
are slightly changed after the inhibitor addition and (iii)
this indicates that AMF influences the anodic and cathodic
processes, and consequently, the inhibition efficiency (IE)increases with AMF concentration, reaching a maximum
value of 87.3 % at 0.6 mmol L
-1AMF in 1.0 mol L-1
HCl solution.
To study the electrochemical stability of AMF, as well
as its adsorption capacity on carbon steel surface, UV–Vis
spectra of studied solutions, before and after corrosionprocesses, were performed. Figure 4b illustrates the UV–
Vis scans obtained for the studied AMF concentrations–2–10123
–1.1 –0.9 –0.7 –0.5 –0.3 –0.1 0.1E/V vs.Ag/AgCllogi/mA cm–2 1
2
3
41. 1.0 M HCl blank
2. 0.2 mM AMF3. 0.4 mM AMF4. 0.6 mM AMFa
0123456
200 250 300 350Wavelength/nmAbsorbance 1. 0.2 mM AMF
2. 0.4 mM AMF3. 0.6 mM AMF123
a
ba. Before corrosion
b. After corrosion
a
b a
bbFig. 4 Polarization anodic and
cathodic curves obtained forcarbon steel corroded in
1.0 mol L
-1HCl blank solution
and in 1.0 mol L-1HCl
solution containing various
concentrations of AMF, at room
temperature ( a); UV–Vis scans
obtained for various AMFconcentrations in 1.0 mol L
-1
HCl solution, before and after
potentiodynamic polarization ( b)
Table 2 Electrochemical parameters and inhibition efficiency (IE)
obtained from Tafel polarization for carbon steel corroded in
1.0 mol L-1HCl solution in the absence and in the presence of
various concentrations of AMF, at room temperature
C-AMF/
mmol L-1Ecorr/V vs.
Ag/AgClicorr/
mA cm-2ba/
mV dec-1bc/
mV dec-1IE/
%
0 -0.55 2.83 103 135 0
0.2 -0.542 1.17 86 130 58.6
0.4 -0.522 0.63 81 141 77.8
0.6 -0.512 0.36 87 145 87.3The carbon steel corrosion in acidic environment 635
123
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before and after potentiodynamic polarization. From
Fig. 4b, it can be observed that the UV–Vis spectra of the
AMF show one intense band at 270 nm. A similar result
was obtained for AMF in water [ 54], the difference con-
sisting in a slightly alteration of wavelength from 271 [ 54]
to 270 nm. After corrosion, it can be observed that AMF
absorbance slightly decreases without altering the wave-
length of adsorption maximum, and consequently theinhibitor concentration in corrosive medium follows the
same trend. A very small decrease in the AMF concen-
tration suggests that an adsorption process of the organiccompound from aqueous phase on the electrode surface
may occur.
AMF action mechanism
In order to discuss the AMF film stability, the variation
of polarization resistance ( R
p) in time was recorded.
Moreover, the degree of surface coverage ( h) values were
calculated using Eq. 2, and consequently the inhibition
efficiency ( IE) will be determined as % IE=100/C1h
[50,51,53,54].
h¼Rp/C0Ro
p
Rp/C18/C19
ð2Ț
where RpandRo
prepresent the polarization resistance at a
given time in the presence and in the absence of AMF,
respectively.
The variations of Rpandhin time are both shown in
Fig. 5.
From Fig. 5a, it is observed that the increase of polari-
zation resistance in time becomes much more obvious athigh AMF concentrations. This suggests that AMF acts by
adsorption on carbon steel surface and that the mechanism
of inhibitor film formation is tightly linked to the AMF
concentration. Moreover, it can be estimated that the
adsorption process of AMF molecules on carbon steelsurface prevails over the desorption phenomenon. Thesame trend is also observed for the degree of surface
coverage ( h), excepting the first moments which mark a
relatively sharp decrease of its values (Fig. 5b). This could
be explained by the presence of some anodic areas on the
carbon steel surface, where corrosion processes are rela-tively intense, inducing instability and permeability to
inhibitor film. At the concentration of 0.2 mmol L
-1AMF,
the fluctuations of hvalues persist, which means that the
inhibitor concentration is too low to ensure the formation
of a continuous and somewhat compact layer to protect the
metal surface. For the concentrations higher than0.2 mmol L
-1, it is observed that hvalues are stable
reaching an average value of 0.65 at an AMF concentration
of 0.4 mmol L-1and 0.8 for 0.6 mmol L-1AMF,
respectively. Consequently, IEreaches the values of 65,
and 80 %, respectively, and these being slightly different
from those obtained from potentiodynamic polarization.This difference is predictable because in the given condi-
tions, the inhibitor film carries out several cycles of
adsorption–desorption of AMF molecules from carbon thesteel surface until concretization of its texture. Based on
Freundlich adsorption isotherm [ 55], Eq. 3could be used,
as a possible relationship between hand AMF concentra-
tion, in the range where hhas a constant value:
h¼K/C1C
n; ð3Ț
where Ki s the equilibrium constant of adsorption–
desorption ( K),Cis the AMF concentration, nis a heter-
ogeneous factor of metal surface and 1/ nrepresenting the
number of surface active sites occupied by one inhibitor
molecule.
Equation 3can be also written in the following form,
Eq.4:
lnh¼lnKțnlnC ð4Ț
Thus, taking into consideration these observations, the
following system of equations can be used to determine K
andn:060120180240
0 200 400 600
Time/sRp/Ωcm21.0 M HCl blank
0.2 mM AMF
0.4 mM AMF
0.6 mM AMFa
0.40.60.81
0 200 400 600
Time/sθ0.2 mM AMF
0.4 mM AMF
0.6 mM AMFbFig. 5 The variation of
polarization resistance in timefor carbon steel corroded in
1.0 mol L
-1HCl blank solution
and in 1.0 mol L-1HCl
solution containing AMF ( a);
degrees of surface coverage over
time, involving the formation ofAMF inhibitor film ( b)636 A. Samide et al.
123
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lnh1¼lnKțnlnC1 ð5Ț
lnh2¼lnKțnlnC2 ð6Ț
where h1=0.65,h2=0.8, C1=4910-4mol L-1and
C2=6910-4mol L-1.
The equilibrium constant of adsorption–desorption ( K)
process was used to calculate the free energy of adsorption(DG
o
ads) using Eq. 7[50–52,55]:
DGo
ads¼R/C1Tln 1 =55:5 ðȚ /C0 lnK ½/C138 ð 7Ț
where R is the universal constant of gases
(8.31 J mol-1K-1),Tis the temperature (298 K) and 55.5
is the value of the molar concentration of water in the
solution, ln K=3.58.
The calculated value for DGo
adsaround of -19.0 kJ mol-1
(consequently, binding energy value of AMF molecule at
carbon steel surface of 0.19 eV) is consistent with the elec-trostatic interaction between the charged molecules and the
charged metal surface and/or different H–, Cl–, bridges or
Van der Waals bonds (physical adsorption) [ 56,57]. This
result also shows that the adsorption of AMF molecules on
the metal surface is a spontaneous process. To note that, the
DG
o
adsvalue was calculated taking into account two con-
centrations of AMF inhibitor where constant values for h
were obtained; considering that, in these cases only, ade-
quate amounts of inhibitor there are to form the stable anduniform layers and consequently, the adsorption process
prevails on the desorption one. Although, it could be appre-
ciated as a relative value, not so reliable, -19.0 kJ mol
-1is
indicative for the adsorption of AMF molecules, this being
higher than -40.0 kJ mol-1that is usually accepted as athreshold value between chemical and physical adsorption
[50,51,55].
The result obtained for 1/ n(around of 2.0 value) indi-
cates that a single molecule of AMF disables several active
sites from the metal surface [ 50].
The inhibition mechanism of AMF requires knowledge
of inhibitor action at the electrode–solution interface. In
acid solution, both THP and EDA can be in the protonatedforms. The surface charge of carbon steel in HCl solution
is known to be positive [ 56]. Thus, the Cl ions are
adsorbed on metal surface generating excess negativecharge towards the solution. This indicated that the elec-
trostatic interactions of the protonated inhibitor molecules
on the negatively charged metal surface are favoured. Theadsorption of the AMF molecules can also occur on the
mild steel surface by direct interaction of the lone pairs of
electrons on nitrogen atoms and d-orbital of Fe [ 56]. The
lone pairs of electrons of nitrogen atoms from EDA or
imidazole ring of theophylline could also form complexes
with Fe
2?ions occurred from the anodic dissolution of
carbon steel. Both EDA-metal as well as THP-metal
complexes are known [ 58,59]. Ethylenediamine forms a
large number of complexes with many metal ions,including iron [ 58]. EDA usually acts as a monodentate or
bidentate ligand, but this can also form the protonated
complexes [ 58]. These complexes can adsorb onto the
steel surface through Van der Waals bonds, and this is
consistent with the binding energy value of 0.19 eV,
attesting that the anchoring of these complexes at thecarbon steel surface enhances the substrate protection
against corrosion [ 59].
Fig. 6 The microscopic images
of carbon steel surfaces abefore
corrosion, bafter corrosion in
1.0 mol L-1HCl blank
solution, cafter corrosion in
1.0 mol L-1HCl solution
containing 0.4 mol L-1AMF
anddafter corrosion in
1.0 mol L-1HCl solution
containing 0.6 mol L-1AMFThe carbon steel corrosion in acidic environment 637
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Surface characterization
The microscopic images of carbon steel surfaces before
and after the electrochemical measurements obtained in
1.0 mol L-1HCl blank solution and in 1.0 mol L-1HCl
solution containing various concentrations of AMF are
shown in Fig. 6. The characteristic morphology of the
carbon steel surface before the corrosion process isobserved in Fig. 6a. After the electrochemical measure-
ments, carbon steel surface was coated with large corrosion
spots which change its texture (Fig. 6b). This process is
quite different compared to those obtained in the presence
of AMF (Fig. 6c, d). The feature of the texture, which
could be attributed to a protective layer on carbon steelsurface, is nuanced, highlighting that the intensity of the
corrosion spots decreases with increase in AMF concen-
tration (Fig. 6c, d).
Conclusions
Thermal analysis showed that at low temperatures, the
following events occurred: (i) between 40 and 117 /C176C, the
mass loss of 7.46 % was ascribed to EDA evaporation; (ii)
40 and 175 /C176C the mass loss of 10.22 % was assigned to
the overlapped processes of EDA evaporation/decomposi-tion and the elimination of hydration water, which simul-
taneously take place. Thus, AMF thermal instability
restricts its application as corrosion inhibitor at high tem-peratures, indicating a confidence interval that does not
exceed 45 /C176C.
The potentiodynamic polarization has indicated that
AMF acts as a mixed-type inhibitor in 1.0 mol L
-1HCl
solution, simultaneously suppressing anodic and cathodic
processes by formation of an adsorbed protective layer oncarbon steel surface. The value around of -19.0 kJ mol
-1
obtained for DGo
adsis consistent with the physical adsorp-
tion mechanism, and this also showing that the adsorption
of AMF molecules on metal surface is a spontaneous
process. A sinergic physical adsorption mechanism
involving H–, Cl–, bridges and Van der Waals bonds via
AMF complexes, which can provide the anchoring of
inhibitor at carbon steel surface is supported by bindingenergy value of 0.19 eV obtained in this study.
The UV–Vis scans recorded for 1.0 mol L
-1HCl
solution containing the studied AMF concentrations indi-cated its high electrochemical stability by a slight decrease
of absorbance after corrosion associated with adsorption
process of inhibitor molecules from aqueous phase toelectrode surface.
The optical microscopy, by comparative images, has
indicated that the feature of protective layer on the carbonsteel surface is nuanced.References
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