Abstract

Environmental green compounds have emerged as powerful inhibitors
for corrosion of metals and alloys. So, this article attempts to show that
barbituric (BA) and thiobarbituric Acids (TBA) play as good green corrosion
inhibitors for copper in presence of 3.5% or 0.6mol/L NaCl. A combination of
quantitative and qualitative tools were used in this investigation such as
electrochemical impedance spectroscopy(EIS), potentiodynamic polarization,
FT-IR spectroscopy, scanning electron microscopy (SEM) and energy dispersive
X-ray spectrometer (EDX). Polarization measurements indicate that these
compounds can function as a mixed type of inhibitor. It was found that, the
adsorption isotherm of these inhibitors on the copper surface obeyed
Flory-Huggins isotherm. Also, the effect of temperature in presence and absence
of inhibitors was done according to Arrhenius isotherm. Some thermodynamic
functions of dissolution and adsorption processes was calculated such as, Ea,
?H*, ?Go and ?S*. The barbaturic and
thiobarbaturic acids recorded high inhibition efficiency of copper metal in
0.6mol/L NaCl at concentrations 5×10-3mol/L and 1×10-3mol/L
respectively.

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Keywords

Barbituric,
Thiobarbituric, Potentiodynamic Polarization, Electrochemical impedance
spectroscopy (EIS), Flory-Huggins isotherm.

 

* Corresponding author [email protected]

*Address: National Research Centre, 33 El Bohouth St.( El-Tahrir St.
former) Dokki- Giza- P.O.12622

 

1. Introduction

Corrosion
is a continuous and complicated problem dealing with the deterioration of metal
as a result of chemical attack or reaction with the environments 1,2. Actually,
corrosion cannot be eliminated completely but can be retarded or prevented to
some extent by using inhibitors. Recently, the world give more attention
towards the protection of environment, by using of non-toxic green inhibitors
is the main concern of many researchers 3-6.

The
significant effect of corrosion inhibitors depends on the complexation with
metal and the extent of dissolving them in water. However, the formation of
stable and insoluble complex between inhibitor molecules and hydrated metal
ions lead up to decrease the dissolution of metal, thus having an effect of
retarding the corrosion. Clearly, the corrosion inhibition efficiency is
controlled by the existence of heteroatoms (e.g. O, N, S) of high electron
density 7-9, and aromatic rings with polar groups and ?-electrons in the
inhibitor molecule 6, 10,11. The electron pairs of mercapto 12 and amino
group 13 can support the adsorption of inhibitor molecules to the metal
surface. Naturally, these compounds can be adjusted to progress the strength of
their adsorption bonds to the conversed metals. Also, these can be done by
retarding the cathodic and/or anodic processes or adsorbing on metallic surface
by forming adsorbed layer which act as compact barrier film 14.

Pharmaceutically
active substances such as barbituric acid and its derivative such as
thiobarbituric acid used as corrosion inhibitors for mild steel protection in
different media 14-17. The adsorption of TBA on mild steel surface was found
to take place through mainly the electrostatic interaction 3.
Udhayakala et.al 17 used Fukui function (which provide information to relate to the atoms in a molecule that
have a higher tendency to either loose or accept an electron or pair of
electrons) to
show the nucleophilic and electrophilic attacking sites in the inhibitors.

Copper
is suitable for making a wide range of application in submarine engineering
18, seawater desalination, pipelines and heat exchanger 19. So, the inhibiting
copper corrosion is a continuing concern in industries. Films formed on the
copper surface not only prevent the surface from corrosion but also improve the
morphology of the surface 20.

Recently,
investigators have examined the effect of some N-heterocyclic compounds on
copper with changing the placement or the number of substituent groups and also
by inserting heterocyclic compounds such as triazole, tetrazole, pyrazole and
imidazole 21-24.

The
aims of this work are to determine the efficiency of BA and TBA acids for using
as green corrosion inhibitors of copper metal in 0.6mol/L NaCl solution by
using various electrochemical techniques, also to illustrate the difference
between the ability of BA and TBA to performance as corrosion inhibitors. The
chemical structures of BA and TBA are given in Fig. (1).

                                       (a)
                              (b)

Fig.(1)
The chemical structures of (a) barbaturic acid and (b) thiobarbaturic
acid.  

 

2. Experimental details

2.1. Electrochemical measurements

The
working electrode is made of copper containing 99.5 wt % Cu, 0.002 wt% Ni,
0.018 wt% Al, 0.005wt% Mn and 0.115 wt% Si. The copper electrode was polished
with a series of emery paper then washed with distilled water. The corrosion
potential of the working electrode was measured against Ag/AgCl as a reference
electrode and pure Pt-wire was used as counter electrode. The concentration of
sodium chloride which used as the aggressive medium in this work was 3.5% or
0.6mol/L NaCl with different concentrations of the inhibitors (1×10-2,
5×10-3, 2.5×10-3 and 1×10-3mol/L). The
electrochemical measurements such as potentiodynamic polarization and impedance
studies were carried out using Autolabpotentiostat/galvanostat PGSTAT302N. Potentiodynamic
polarization curves were investigated by changing the electrode potential from
0.2V to 0.8V with potential scan rate of 1 mVs?1. For studying the
temperature effect, the same procedure of polishing the working electrode was
happened and it was immersed in 0.6mol/L NaCl with and without selected
inhibitor concentration at different temperature. The corrosion parameters i.e.
corrosion current density icorr and corrosion potential Ecorr
were evaluated from the intersection of the linear anodic and cathodic branches
of the Tafel plots. The values of inhibition efficiency of BA and TBA were
calculated according to the following equation:

Where

 are the uninhibited and inhibited corrosion
current densities, respectively.

The
electrochemical impedance spectroscopy (EIS) of the electrode surface after immersion
in 0.6mol/L NaCl with and without inhibitor has been carried out with Ac
voltage amplitude of 10 mV using an electrochemical impedance system. The
frequency range used in the study was 0.02–105Hz. All the electrochemical
impedance measurements were carried out at open circuit potential.

2.2. Surface Morphology

The
surface morphology of the copper specimens after 3 days of  immersion in 0.6mol/L NaCl with and without  the ideal concentration of the inhibitor of BA
 (5×10-3 mol/L) and (1×10-3mol/L)
of TBA was performed on scanning electron microscope JEOL-JSM-5600 equipped
with an energy dispersive X-ray spectrometer OXFORD Link-ISIS-300.

2.3. FTIR Study

IR
spectra were recorded for the powder of the inhibitors and the film of the
inhibitors adsorbed on the copper metal with the aid of JASCO 4600 model FTIR
spectrometer.

 

3. Results and Discussion

3.1 Polarization Study

Cathodic
and anodic polarization curves of copper in 0.6mol/L NaCl with and without
different concentrations of BA and TBA inhibitors were shown in Figs. (2, 3)
The corrosion parameters predicted from Tafel polarization curves recorded for
the copper metal in 0.6mol/L NaCl solution with different concentrations of BA
and TBA were listed in Table (1). It was observed that the corrosion rate of
copper in 0.6mol/L NaCl decreased in presence of the inhibitors, the decrease
in the anodic and cathodic current density may be due to formation of barrier
film on the metal surface which is probably related to the adsorption of
inhibitor molecules on the electrode surface, beside elevating hydrogen
evolution reaction or oxygen-reduction reaction which preventing the formation
of soluble CuCl2 25. In other words at the first region of anodic
polarization the current density increases may be due to formation of soluble
CuCl2 which after some times the current density decreased
attributed to formation of mainly CuCl film and then increased again because of
the dissolution of the film to produce of Cu(II) ions26.

 

 

 In this study we notice that the open circuit
potential changed rapidly to less negative values and then become stable
corresponding to the free corrosion potential, Ecorr, of the metal.
The change in Ecorr is about 65mV by at least 85mV that is an
evidence of acting the inhibitors as mixed type 27. The preferential
adsorption of these compounds plays a role in blocking the active sites in both
anodic and cathodic reactions which leading to inactivation of part of the
surface to the aggressive attack 28. This adsorption results in the formation
of protective complex with copper ions and can be described as the following
mechanism 29

Cu(S)
+ BA(aq)

Cu:BA(ads)
+ H+(aq)

 

 

 

Noticeably, the
values of inhibition efficiency for using TBA as inhibitor are higher than that
of BA. This may be related to the molecular structure of the inhibitor which
gives or accepts electrons to or from the metal surface leading to form of new
bonds 30. The direction of inhibitor molecule is substantial where it gives a
chance to construct stronger bond, moreover due to the interaction of
?-electrons of the ring with empty d orbitals of copper metal  resulted in parallel orientation of inhibitor
molecule 31. 

3.2 Impedance studies

The Nyquist plots for copper in 0.6mol/L solution of NaCl in
presence and absence of different concentrations of TBA and BA was examined. All
the experimental results investigated by using Randles equivalent circuit with
one time constant shows in Fig.(4). 
Where Rs  is the
resistance of the electrolyte, Qdl is the constant phase element
displaying the double layer and the Rct the charge–transfer
resistance representing the most suitable factors to observe the protective
properties of the film. The measured EIS parameters for adsorbed BA on copper
surface are deduced and outlined in Table (2), from it we can noticed that the
Rct values display increasing tendency with increasing the
concentration of BA. Furthermore, the values of Qdl which
corresponding to the double layer capacitance Cdl decreased with an
increase in the concentrations of BA. This result can be related to the
adsorption of the inhibitor molecules at copper/solution interface 32. The
values of Cdl are estimated according to the following equation: Cdl
= Yo(w)n-1 , where Yo is modulus of CPE refers
to the capacitance  of the formed film, w
is Warburg impedance and n is the phase shift. The presence of Warburg impedance
in the electrochemical circuit pointed to the porous nature of the modulated
surface 33,34.

Fig.(5) For BA exhibits one capacitive loop conforming one time
constant in Bode plots presented in Fig.(6). It is apparent that the diameter
of the Nyquist plots is approximately amounting to the value of the
charge-transfer resistance (Rct) during the corrosion process. In
other words, the capacitive loop does not show ideal behavior increasing or
decreasing with addition of various concentrations of the inhibitors (BA and TBA)
this result suggested that impedance dispersion take place. This result can be
interpreted that there is more than one electrochemical process occurred, also
may be due to the rudeness of the electrode surface. Obvious that the values of
the phase angle registered much higher values nearly 63o in the
presence of the inhibitors (BA and TBA) indicated that the surface protected by
formation of CuBA or CuTBA films. The deviations of the phase shift than the
ideal value (unity) in this case could be possible as a result of
irregularities and inhomogeneity of the surface 35. Thus the same situation
happened with using TBA under the same conditions which shown in Figs. (7,8)
where their results listed in Table (3), except that the barbituric acid
recorded high inhibition efficiency of copper surface at concentration 5×10-3mol/L
but TBA reported to some extent the same ratio of inhibition efficiency about
94% at concentration 1×10-3mol/L. The difference in concentration
may be attributed to the more solubility of TBA than BA due to the replacement
of oxygen atom by sulfur atom in TBA 36. All the impedance parameters and
also the polarization measurements of the concentration 1×10-2mol/L were
not matching with the other concentrations, this may be due to the dissolution
of the formed film. This result cleared from the value of the inhibition
efficiency at this concentration

 

3.3 Adsorption Isotherm

Generally, the investigation of
adsorption processes are determined by adsorption isotherms, which provide
structural information about the linkage between the additive inhibitor and the
surface of the metal which resulted in retardation of the corrosion rate by
weakness the diffusion of corrosive species or increasing the resistance of the
metal surface. Flory-Huggins isotherm was the best suitable model that fitted
the experimental data which used to describe the adsorption characteristics of
BA and TBA on copper surface in 0.6mol/L NaCl solution.

 

Figure (9) offer the linear
representation of the function

 against

 37 according to Flory-Huggins equation,
which is expressed as:

Where, n is the number of ions occupying
adsorption sites; KFH is the equilibrium constant (L mol-1),
Co is the equilibrium concentration and

 is the degree
of surface coverage. The values
of the regression coefficient (R2) are nearly close to one proved
that the adsorption of BA and TBA on copper surface obeys Flory-Huggins
isotherm. In addition, the values of the
equilibrium constant KFH obtained from this isotherm and the
change of standard free energy (DGo) of adsorption can be
specified according to the following equation and listed in Table (4).

Where, R is the universal gas
constant (8.314mol-1K-1) and T is the absolute
temperature. The
adsorption processes are always classified into chemisorption or physicosorption
related to the values of DGo 38. The chemisorption case is considered
when DGo is equal to
or more negative than (-40kJmol-1) where charge shared or transferred from the inhibitor molecules to the
copper surface forming coordinate covalent bond. While the physic-sorption type is regarded if DGo is equal to or less negative than (-20kJmol-1)
in which an electrostatic interaction between charged molecules and charged
metal surface is considered.

According to Table (4), the values
of DGo are in between -20 and -40kJmol-1, which
indicates that the adsorption of the inhibitor molecules include a mixed-type physicosorption
and chemisorption mechanism 39.

 

 

3.4. Effect of
Temperature

Activation variables for the adsorption features of the studied
compounds on copper in 0.6mol/L NaCl were calculated by supposing a direct
relationship between log icorr (the rate of constant for the
corrosion reaction) and 1/T according to the Arrhenius pattern as
the equation of the form:

Where A is the frequency factor, R is universal gas
constant and T is absolute temperature. The relationship between log
icorr and 1/T presented straight lines with good degree
of linearity as shown in Fig.(10).

 

The activation energies concluded from the plots of copper immersed
in 0.6mol/L NaCl without and with optimum concentration of BA and TBA are
listed in Table (5). The values of Ea for inhibited copper
are higher than that for the uninhibited due to the adsorption of inhibitor
molecules on the active sites on copper surface which increases the energy
barrier connected with corrosion reaction. Also, the increase in activation
energy may be explained as a result of the physical adsorption that happen at
the first step which is proved by decreasing the inhibition efficiency with
increasing the temperature 40,41.

 

Fig. (11) presents the plots of log icorr/T versus
1/T depending on the transition state equation:

Where N is Avogadro’s number and h is plank’s
constant. This relationship gives straight lines with slope and intercept equal
to

and

 
respectively.

 

The values of ?S* and DH* are listed in
Table (6). The positive sign of enthalpy ?H* in presence of TBA and
BA as inhibitor points to the nature of the reaction suggesting  endothermic one, also indicates that the dissolution
of copper is slow 42. The change in entropy DS* is negative for the adsorption of 
BA and for the blank medium, proposing that the adsorption process is
dominated by associative interaction between the copper and the inhibitor
molecules rather than the disorder that taking place between the metal and the
water molecules, while the positive value of entropy have the opposite behavior
43.

 

 

 

 

Table
(6): Thermodynamic parameters for the adsorption of optimum concentration of
inhibitor molecules on copper surface in 0.6mol/L NaCl at different
temperatures.

 

 

 

 

 

 

 

 

 

 

 

3.5.
FTIR Spectral Study

   Different functional groups for
corrosion product on the copper in presence of BA and TBA were realized from of
FTIR spectra in Fig. (12) which clarify difference between the spectra of the
adsorbed inhibitors and that of pure ones. That was noticeable from the
shifting of the wavenumber from higher to lower values.

The spectrum of pure BA and its adsorbed inhibitor display that the
broad peaks corresponding to ?(OH) and ?(NH) in pure BA appeared as sharp
nature with CuBA associating with shifting from its position to lower value for
?(OH) group (3500-3310cm-1) and higher value for ?(NH) group (3244-
3277cm-1).  

 

The stretching of CH group in BA appeared at 2900 cm-1
exhibits lower wave number 2844 cm-1 for BA adsorbed on copper
surface Also, the peaks around 1780 and 1678 cm-1 which are assigned
to ?(C=O) undergoes shift to lower wavenumber after adsorption on the metal
surface to become at 1733 and 1644 cm-1. This may be due to the
covalent bonding of the carbonyl group to empty hybrid orbital of copper atom
44.This description confirmed the ability of the inhibitor molecules to
coordinate with Cu2+ ions.

Also, the same shifting appears in FTIR spectrum of TBA. The
occurrence of peaks with sharp nature at wavenumber 955, 877, 833 and 778 cm-1
pointed to the chemisorption of BA and TBA on copper surface 45.

   The characteristic band of (C=O)
group of TBA, adsorbed on copper surface, appear at the same wavenumber of pure
TBA at 1706 and 1666 cm-1 but have a shape of medium peaks comparing
with weak peaks for pure TBA.

   The observed weak peak at
1350 cm-1in the TBA spectrum assigned to ?(C=S) + ?(NH) which
shifted to higher wavenumber 1392 cm-1, that can be related to
specific molecular interactions thus leads to formation of coordination bond
between sulfur hetero atom and unfilled d-orbital of copper atom46.

3.6. Surface Morphology Investigation (SEM/EDX)

Scanning electron microscopy and X-ray spectroscopy is one of the
most ordinarily techniques for assessment and analyzing the surface and
elemental constituents of corrosion products attached samples. SEM images
appeared the interaction between metals and the corrosive media, so the
morphology of the metal surface and collection of corrosion products can be
determined by SEM analysis .Fig. (13) exhibits the SEM micrograph of copper
specimens  after 4 hours of immersion in
0.6mol/L NaCl in presence and absence of the studied inhibitors.

The surface of copper sample returned from the solution without
inhibitor is strongly deteriorated, offering deep cavern which is related to
direct attack of copper by aggressive ions. While the SEM images of copper
specimens with the best concentration of TBA and BA offer relatively smoother
surfaces, which is due to the adsorption of the inhibitor molecules on metallic
surface. The protection of metallic surface was occurred by isolating surface
from the corrosive medium leading to less damaged and smoother surface.

The EDX spectra of copper surfaces are shown in Fig. (14), it is
obvious that the intensity of oxygen in the EDX spectrum of blank copper sample
is referred to the formation of oxide film by oxidizing copper surface. By
comparing the intensity of oxygen in the EDX spectra of copper surfaces in
presence and absence of inhibitors, increasing the intensity in presence of
inhibitors is noticeable which indicates the adsorption of inhibitor molecules
on the copper surface. The presence of nitrogen and sulfur in the EDX spectra
of copper come from the inhibitor containing solutions definitely approve the
adsorption of inhibitors on the metallic surface.

 

Conclusions

Barbaturic (BA) and thiobarbaturic (TBA) acids act as mixed-type
inhibitors impede both the anodic and cathodic reactions on the corrosion of
copper in 0.6mol/L NaCl solution, the inhibition efficiencies for BA and TBA
have maximum values at 5×10-3,    1×10-3mol/L respectively.
According to electrochemical impedance spectroscopy (EIS) technique, morphology
analysis and potentiodynamics polarization, the mechanism of adsorption of
inhibitor molecules on copper surface suggested to be physic-sorption and
chemisorption modes. 

 

 

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