Page | 5704

Evaluation of expired Bromocriptine drug as eco-friendly corrosion inhibitor for C-steel in

acidic media

Abd El-Aziz S. Fouda 1

, Gamila El-Sayed Abdalla Badr 1,*

1 Chemistry Department, Faculty of Science, Mansoura University, Mansoura - 35516, Egypt

*corresponding author e-mail address: gamilabadr@yahoo.com | Scopus ID 6701845516

ABSTRACT

Expired Bromocriptine drug was investigated as a possible green inhibitor for the erosion of C-steel (CS) in 1M HCl medium. Weight

loss (WL) and electrochemical techniques were utilized for studying corrosion rate. Electrochemical measurements infer that the drug

behaves as a mixed type with a predominantly anodic effect, double layer capacitance (Cdl) decreased and charge transfer resistance (Rct)

increased with raising drug doses, and hence, the inhibition efficiency (IE) increases. Atomic force spectroscopy measurements (AFM)

showed that the surface roughness of CS decreases in the presence of drug. The adsorption of the Bromocriptine on the CS surface

follows Temkin and Langmuir isotherms. The values of Gibb’s free energy suggested the electrostatic as well as chemical interaction of

Bromocriptine with CS surface. The quantum chemical parameters that are related to the corrosion effectivity of the drug have been

calculated.

Keywords: Bromocriptine drug, Corrosion, Inhibition, CS, HCl, Adsorption, AFM.

1. INTRODUCTION

CS are widely used as structural materials in many

industrial fields. The dissolution of CS in acidic environment has

an important practical value due to its widespread applications in

the manufactures of pipelines for petroleum industries and water

networks. A corrosive attack in industrial processes arises from

widely used HCl for acid pickling, descaling and cleaning.

Accordingly, some protective strategies have been utilized to

protect steel from corrosion. The highly effective way to do this is

by the addition of corrosion inhibitors [1-4]. The efficiency of

organic compounds, which can be used as inhibitors, depends on

its adsorption ability on a metal surface by replacing a water

molecule at corroding interface. Organic compounds containing

multiple bonds, heteroatoms such as N, P, S or O have been used

as excellent inhibitors for saving the metal surfaces from

dissolution either by chemical or physical adsorption [5-6]. The

factors affecting the adsorption process are the existence of

functional groups (–CHO, R-OH, -N =N, etc.), the electronic

density at the donor site, the electronic structure, the steric factor,

and the molecular size of inhibitor molecules [7-10].

Unfortunately, most of organic compounds are toxic. Researchers

attempt to find environment friendly corrosion inhibitors for

metals and alloys in different mediums [11-12]. The applicability

of pharmaceutical compounds as Eco-friendly corrosion inhibitors

for metals in acidic medium has been recognized for a long time.

Some medical drugs that were used as corrosion inhibitors are

listed in Table 1 that donates an assessment of their protection

efficiency in acidic medium [13-22].

The aim of this research is to investigate the inhibition

properties of Bromocriptine drug on the dissolution of CS in

acidic media using chemical and electrochemical methods.

Expired Bromocriptine drug has been chosen as environment

friendly corrosion inhibitor in this study owing to its structure

containing active centers (N and O atoms), high molecular weight

and being nontoxic and inexpensive. The surface morphology of

CS was investigated before and after inhibition. The

thermodynamic activation parameters, adsorption isotherms and

the quantum chemical parameters for the corrosion inhibition

processes were determined and discussed.

Table 1. % IE of some investigated drugs. Inhibitor (drug) Metal Corrosive

Medium

% protection

efficiency

References

Cloxacillin (15x10-4

M) MS HCl 81.0 [13]

Modazar (300 ppm) CS HCl 92.3 [14]

Farcolin (20% v/v) CS HCl 95,0 [15]

Ciprofloxacin (2.57x10-3

M) MS HCl 90.8 [16]

Aspirin Drug (3x10-3

M) MS H2SO4 71.8 [17]

Cefixime (8.8x10-4

M) MS HCl 91.8 [18]

Rosuvastatin MS HCl 92.0 [19]

Cialis (60x10-5

M) Zn HCl 87.2 [20]

Januvia Zn HCl 79.5 [21]

Cefuroxime Axetil Al HCl 89.9 [22]

Bromocriptine CS HCl 95.7 Our case

Volume 10, Issue 4, 2020, 5704 - 5714 ISSN 2069-5837

Open Access Journal Received: 27.02.2020 / Revised: 24.03.2020 / Accepted: 25.03.2020 / Published on-line: 29.03.2020

Original Research Article

Biointerface Research in Applied Chemistry www.BiointerfaceResearch.com

https://doi.org/10.33263/BRIAC104.704714

Evaluation of expired Bromocriptine drug as eco-friendly corrosion inhibitor for C-steel in acidic media

Page | 5705

2. MATERIALS AND METHODS

2.1. Materials.

Bromocriptine drug that used as an inhibitor in this study

has been obtained from Amoun Pharma, Cairo Egypt. It has

chemical structure formula [(5′α)-2-Bromo-12′-hydroxy-5′-(2-

methylpropyl)-3′,6′,18-trioxo-2′-(propane-2-yl) ergotamine] which

illustrated in Figure 1. Bromocriptine stock solution (10-3 M) was

prepared by weighing the calculated weight of the drug, dissolving

in methanol then stored under refrigeration until use. The

preparation of different doses of Bromocriptine was done by

diluting the stock solution with double distilled water. The

corrosive medium (1M HCl) was prepared from analytical grade

HCl (37%) by dilution with double distilled water. The chemical

composition of CS samples used in this study is (0.2 wt.% C,

0.003 wt.% Si, 0.04 wt.% P, 0.6 wt.% Mn and balance Fe.

Figure 1. Molecular structure of the bromocriptine drug.

2.2. Weight loss (WL) method.

Coins of CS with dimension of 2 x 2 x 0.2 cm were cut for

WL tests, abraded by emery papers at different grit sizes from 180

to 1200, rinsed with double distilled water, degreased in acetone,

dried with filter paper, weighed then dipped in 100 ml of 1.0 M

HCl without and with different doses of the drug. The dipped time

30-180 minutes was employed at temperatures 25, 35, 45 and

55oC. Triplicate experiments in each case were performed to

obtain the average WL values and to check reproducibility

2.3. Electrochemical measurements.

All electrochemical experiments were carried out at 25oC

in a conventional three-electrode cell with CS specimen, platinum

and saturated calomel (SCE) electrodes as working, counter and

reference electrode, respectively. The working electrode was

pretreated as in WL experiment and dipped for thirty minutes into

a test solution until reaching a constant open circuit potential

(OCP).

Potentiodynamic polarization (PP) tests were employed by

automatic change of the electrode potential from -500 to -100 mV

against SCE at a scan rate of 1 mV/s .Electrochemical impedance

spectroscopy (EIS) tests were performed at OCP within frequency

100 kHz to 100 mHz range by AC signals at 10 mV amplitude.

The Nyquist and Bode plots are obtained from EIS diagram.

Electrochemical frequency modulation (EFM) tests were applied

at frequencies 200 and 500 mHz at 20 mV RMS amplitude. For

reproducibility, the measurements were repeated three times.

Gamry instrument PCI300/4 (Potentiostat / Galvanostat / Zra)

analyzer and DC105 software were used for PP tests, EIS300

software for EIS tests and EFM140 software for EFM tests.

Echem Analyst 5.5 carried out plotting, graphing, data fitting and

calculating.

2.4. Atomic force microscopy (AFM) analysis.

SPM 2100 AFM apparatus carried out the topographical

study of CS surface. It operates in contact mode in the air with

scan rate of images 25μm2 area and scan speed 6.68μm second. In

which the surface morphology, properties and 3D surface images

of CS were recorded by scanning the sample surface with a fine

tip. The sample was pretreated as in WL tests before immersion in

1M HCl for 12 hours at 25 oC with and without 1.12 x 10-4 M of

Bromocriptine drug. After immersion, the sample was taken out

softly, cleaned by double distilled water and acetone then

preserved in a desiccator prior investigation.

2.5. Quantum chemical calculation.

The optimization of molecular structure of Bromocriptine

molecule was done by Materials Studio DMoL3 module version 7.

A basis set of double number polarization (DNP) plus the

exchange correlation functions of Becke One Parameter (BOP)

with generalized gradient approximation (GGA) were performed.

The effect of water as solvent was treated by COSMO model

during the optimization process.

3. RESULTS

3.1. Weight loss.

The influence of drug doses, immersion time and

temperature on the %IE of CS were studied in 1M HCl with and

without different Bromocriptine drug doses. The degree of surface

coverage ( ) and the % IE of the drug was determined as in Eq.

1[23].

= (1-

) and % IE = x 100 (Eq. 1)

Where, and are the WLs for CS with and without drug

in HCl solution, respectively.

The WL curves obtained (Figure 2) showed that the % IE

increased by increasing Bromocriptine doses. The influence of

temperature on the % IE of different doses of Bromocriptine after

30 minutes dipping time was determined by WL method (Table 2).

The maximum % IE obtained for Bromocriptine was 95.74

% for 1.12 x 10−4 M of drug at 55oC. Also, the effect of dipping

time on the % IE of 1.12 x 10−4 M Bromocriptine drug on the

dissolution of CS in 1 M HCl at different temperatures was

investigated (Table 3). The % IE of the drug was found to increase

by increasing the time of immersion, the maximum % IE was

reached 98.2 % for 1.12 x 10−4 M of drug at 55oC after dipping

time of 180 minutes.

Figure 2. WL - time curve of CS in 1M HCl solution as blank and with

various doses of Bromocriptine at 25oC.

Abd El-Aziz S. Fouda, Gamila El-Sayed Abdalla Badr

Page | 5706

All results obtained from WL tests displayed that the %IE

of Bromocriptine drug increases by increasing doses, immersion

time and temperature. Therefore, this action can be referred to

strong adsorption on the CS surface that leads to more surface area

of CS covered by Bromocriptine molecules [24].

Table 2. % IE of different doses of Bromocriptine at temperatures (25-

55°C) after 30 min dipping.

2.3 x 10- 5

4.6x10-5

6.9x10-5

9.2x10-5

1.12x10-4

Conc., M

Temp.,o

C % IE

38.36 58.68 67.75 76.0 82.6 25

40.89 60.89 70.24 77.3 88.4 35

42.85 63.42 72.08 79.5 92.1 45

45.89 64.24 73.04 81.4 95.7 55

Table 3. Effect of dipping time on the % IE of 1.12 x 10−4 M

Bromocriptine drug in 1M HCl of CS dissolution at different

temperatures.

Temp., o C

Time, min.

25oC 35

oC 45

oC 55

oC

% IE

30 82.56 88.39 92.11 95.74

60 86.73 89.29 92.90 96.18

90 90.39 92.13 93.37 97.21

120 93.01 94.48 95.30 97.67

180 94.86 95.32 96.29 98.22

Thermodynamic parameters such as activation energy

( activation entropy (

) and activation enthalpy

( are a paramount and a valuable procedure for illustrating

the adsorption behaviour of an inhibitor. The activation

parameters for the corrosion process were calculated by Arrhenius

and transition state equations (Eq. 2 and Eq. 3) [25].

(Eq. 2)

(

) (

) (Eq. 3)

Where, stands for corrosion rate, A for Arrhenius

factor, N for Avogadro’s number and h for Planck’s constant.

The activation parameters were calculated (Table 4) from the

slope and intercept of Figure 3 ( vs.

) and Figure 4

(

vs.

), exhibit that the inhibited solutions reduce the

apparent activation energy than uninhibited solution, followed by

a tedious lowering by raising inhibitor dose.

Table 4. Activation parameters for CS dissolution in blank and in

different doses of Bromocriptine in 1M HCl.

Conc., M -

(J mol-1

K-1

)

(J mol-1

K- 1

)

,

(J mol-1

K- 1

)

,

(J mol-1

K- 1

)

Blank 129.2 44.5 47.2

2.3 x 10-5

144.8 41.1 43.8

4.6 x 10-5

150.3 40.4 43.1

6.9 x 10-5

155.2 39.6 42.3

9.2 x 10-5

164.3 37.6 40.3

1.12 x 10-4

241.6 35.4 38.1

This may be referred to the chemical adsorption of

Bromocriptine drug on the surface by sharing or transferring

charge from it to the metal surface, forming a film that makes a

barrier between the CS surface and the environment, and prevents

more dissolution of CS at higher temperatures in acidic media

[26]. In addition, the decrease in the values of and the

increase in % IE with the increase in temperature by using WL

method confirmed the chemisorption mechanism [27].

Figure 3. Log CR vs. 1000/T curves for CS in 1M HCl with and without

different Bromocriptine doses.

Figure 4. Log CR/T vs. 1000/T curves for CS in 1M HCl with and without

different Bromocriptine doses.

The positive values of shows that the nature of CS

dissolution process with and without inhibitors is endothermic

[28]. The negative values of activation entropy of suggest

that the rate-determining step for the activated complex represents

an association rather than dissociation [29].

3.2. Adsorption isotherms.

The mode of adsorption of Bromocriptine drug on the

surface of CS takes place by either physisorption, chemisorption

or comprehensive adsorption. To find the suitable adsorption

isotherm for the adsorption of Bromocriptine on the CS surface in

1M HCl, several adsorption isotherms like Temkin, Langmuir,

kinetic-thermodynamic model, Freundlich, and Flory- Huggins

were tested. The correlation coefficients obtained from it is R2=

0.99678, 0.99642, 0.98525, 0.9796 and 0.96546 respectively. The

values of surface coverage θ were evaluated from Equation 1. The

value of (R2) was used to determine the best-fit isotherms for the

adsorption of Bromocriptine on the CS surface in 1M HCl that

found obeys Temkin and Langmuir adsorption isotherm.

3.2.1. Temkin Adsorption Isotherm.

Temkin supposes the breakdown in the heat of adsorption

of molecules in the layer decreases linearly with coverage due to

adsorbent–adsorbate interactions, and the adsorption characterized

by a uniform distribution of the binding energies up to some

maximal [30]. Temkin isotherm is applicable for the

chemisorption of species to form a monolayer on the surface [31].

Figure 5 shows the Temkin isotherm curve that deduces from Eq.

4 [32].

θf = log Kads + log C (Eq. 4)

Evaluation of expired Bromocriptine drug as eco-friendly corrosion inhibitor for C-steel in acidic media

Page | 5707

where f = -2a and represent the heterogeneous factor of the metal

surface and the molecular interactions in the adsorption layer. The

mutual repulsion of molecules occurs if f > 0, and the attraction

takes place if f < 0, where (a) is the molecular interaction

parameter can have positive value when the attraction forces occur

between the adsorbed molecules and have a negative values when

the repulsive forces occur between the adsorbed molecules [33].

Table 5 shows that the values of f > 0 and the value of (a)

in all cases are negative, suggested the existence of repulsion in

the adsorption layer and the inhibitor molecules will form a

monolayer on the CS surface [33].

Figure 5. Temkin adsorption isotherm curve of Bromocriptine drug for

the dissolution of CS in 1M HCl at 25oC.

Table 5. Equilibrium constant ( ), adsorption free energy ( ),

heterogeneous factor (f) and molecular interaction parameter (a) for the

adsorption of inhibitors on CS in 1M HCl at different temperatures.

Temkin Adsorption Isotherm

Temp., oC x 10 5 M-1 -

kJ mol-1 f a

25 1.79 39.9 1.59 -0.79

35 1.81 41.3 1.53 -0.76

45 1.87 42.7 1.50 -0.75

55 1.99 44.3 1.49 -0.74

The surface of the metal would have all manner of heat

adsorption at different types of sites. The value of

covering (θ) at the beginning of adsorption is small and the

adsorbed molecules would bind to more attracting sites with

greatest bonding power, thus would be highly negative [34].

Generally, when values of are negative and around or more

than -40 kJ mol-1 this indicate that the adsorption involves charge

sharing or transferring from the inhibitor molecules to the surface

of metal to create a coordinate bond by chemisorption

mechanism, while those around −20 kJ mol−1 or lower indicates

physical adsorption mechanism [35].

From the results obtained, the values of Kads increased with

increasing temperature and the values of are negative for

all cases. The negative sign of demonstrates the

spontaneous adsorption of inhibitors on the surface of metal and

the stability of the adsorbed layer. This agreed well with our

finding average value of (39.9- 24.60 kJ.mol-1)

and suggested the mixed mechanism of the Bromocriptine on the

CS surface.

3.2.2. Langmuir adsorption isotherm.

The Langmuir hypothesis that the energy of adsorption is

independent of the surface coverage (θ) and there is no interaction

between the adsorbed molecules [36]. The Langmuir equation (Eq.

5) shows that one molecule only can be adsorbed on one site of

adsorption formed saturated monolayer.

=

(Eq. 5)

Where C = drug dose, θ = coverage degree, and Kads = equilibrium

constant of the adsorption process given from the intercept of the

lines. The standard free energy of adsorption was

calculated from Eq. 6.

=

exp (

) (Eq. 6)

Where 55.5 = water molar dose in the solution. The Langmuir

adsorption isotherm curve of Bromocriptine molecule on the CS

surface in 1M HCl is shown in Figure 6. Moreover, the essential

characteristic Langmuir isotherm is the dimensionless separation

factor which describes the type of isotherm and given by Eq. 7

[37].

=

(Eq. 7)

The smaller value indicates highly favorable adsorption. If

>1 it is unfavorable, =1 it is linear, 0< <1 it is favorable,

and if =0 it is irreversible [38]. Table 6 gives the estimated

values of , and for Bromocriptine at different doses

and different temperatures. All values were found less than one

supported that the adsorption processes are favorable, and the

fixation capacity grows rapidly with dose in equilibrium in liquid

phase. In addition, the values obtained around -40 kJ mol-1

supported that the process of inhibition is a mixed one.

Figure 6. Langmuir isotherm for the adsorption of Bromocriptine

molecule on CS surface in 1M HCl at 25oC.

Table 6. Equilibrium constant ( ), adsorption free energy ( ) and

dimensionless separation factor, for the adsorption of inhibitor on CS

at different temperatures in 1 M HCl.

Temp., oC

Langmuir isotherm

x 10 3 M-1 -

kJ mol-1

25 24.96 35.1 0.99

35 25.86 36.3 0.99

45 27.05 37.6 0.99

55 28.18 38.9 0.99

3.3. Potentiodynamic polarization.

Figure 7 shows PP curves (Tafel plot) of CS electrode in 1

M HCl solution and in the presence of different doses of

Bromocriptine drug. Corrosion potentials (Ecorr), corrosion current

densities (icorr), cathodic and anodic Tafel slopes (βc, βa) and % IE

were obtained by Tafel extrapolation method are tabulated in

Table 7.

Abd El-Aziz S. Fouda, Gamila El-Sayed Abdalla Badr

Page | 5708

The results display that the icorr and CR decreases while the

IE increases up to values of 88.79% at 25oC with increasing

Bromocriptine dose. This can be referred to the interaction

between the electron donating groups (O, N, π-electrons on

aromatic ring) of Bromocriptine with the CS surface. Therefore,

the active sites of the CS surface are blocked leading to a decrease

of the surface area available for H+ ions and protecting it from Cl-

ions attack whereas the reaction mechanism is not affected [39].

If the shift in Ecorr is more than 85 mV in an inhibited

system with respect to uninhibited, the inhibitor could be

recognized as cathodic or anodic type whereas if the shift in Ecorr

is less than 85 mV, it could be recognized as a mixed type of

inhibitor [40]. The results obtained reveal that Ecorr shifted in the

positive direction reinforcing the possibility of formation of iron

II- Bromocriptine complex. In addition, the largest displacement

of Ecorr in the Tafel plots from the blank solution was about 44 mV

which is less than 85 mV and approved that Bromocriptine acted

as a mixed inhibitor but predominantly anodic effect [41].

Moreover, the shift in the anodic Tafel slope βa in

comparison with the blank solution may be due to a change in the

charge transfer coefficient αa for the anodic dissolution of iron

[42]. The values of anodic Tafel slope βa were found slightly

changing by adding inhibitor that may be due to the adsorption of

chloride ions/or inhibitor molecules on the CS surface [43]. The

cathodic Tafel slope βc changes by the addition of inhibitor

because the presence of the oxide film can markedly influence the

surface reduction process, by affecting the energetics of the

reaction at the double layer, or by magnification of a barrier to

change transfer through the oxide film or both.

Figure 7. Potentiodynamic polarization curves of CS in 1M HCl with and

without different Bromocriptine doses at 25oC.

Table 7. Electrochemical parameters obtained from polarization curves of

CS in 1 M HCl in the absence and presence of different Bromocriptine

doses at 25oC.

Conc

x105M

-Ecorr

mV vs

SCE

icorr

mA cm-2

βa

mV dec-1

βc

mV dec-1

CR

mmy-1

%IE

Blank 289 3.55 99 202 41.33

2.3 245 2.34 46 194 27.41 34.0

4.6 251 1.41 44 172 16.45 60.4

6.9 265 1.13 53 109 13.17 68.3

9.2 266 0.74 51 66 8.68 79.1

1.12 261 0.40 41 46 4.65 88.8

3.4. EIS.

EIS is a powerful and accurate method that successfully

applied to study the corrosion inhibition processes. The impedance

diagrams do not show perfect semicircle in most cases due to the

frequency dispersion arising from heterogeneity and roughness of

the electrode surface [44]. The EIS diagram displays one single

capacitive loop, which demonstrates that the dissolution of CS

mainly controlled by the charge transfer process [45].

Figure 8 shows the Nyquist plots for CS in 1M HCl with

and without different doses of Bromocriptine drug. The

characteristic semicircles appear in the impedance diagram for the

examined drug are indicating that the process occurs under

activation control. The size of semicircles increases with an

increase of the drug dose, confirming that the impedance increases

with increasing the drug doses.

Table 8 gives the EIS parameters using the equivalent

circuit model shown in Figure 10. EIS data showed that the Rct

values increases due to the increasing thickness of the electrical

double layer and Cdl values decreases due to a decrease in a local

dielectric constant and/or an increase in the thickness of the

electrical double layer by increasing the adsorption of the drug at

the metal solution/interface [46].

Figure 8. Nyquist plots for carbon steel corrosion in 1M HCl solution

with and without different concentrations of Bromocriptine at 25ºC.

Table 8. Impedance parameters and %IE for CS dissolution in1 M HCl

solution in the absence and presence of different doses of Bromocriptine

drug at 25oC.

Conc., x105 M Rct, ohm cm

2 Cdl, μF cm

-2 n θ %IE

Blank 17.49 99.20 0.81 ----- -----

2.30 26.95 88.20 0.86 0.351 35.1

4.60 39.69 78.11 0.83 0.559 55.9

6.90 54.58 62.60 0.92 0.680 68.0

9.20 80.85 53.00 0.93 0.784 78.4

1.12 117.3 42.30 0.93 0.851 85.1

Figure 9. Bode plots for carbon steel corrosion in 1M HCl solution in the

absence and presence of different concentrations of Bromocriptine at

25ºC.

Evaluation of expired Bromocriptine drug as eco-friendly corrosion inhibitor for C-steel in acidic media

Page | 5709

Figure 10. Equivalent circuit model used to fit experimental EIS data.

These lead to an increase of θ and % IE. The Bode plots

(Figure 9) show only one well-resolved peak and one capacitive

time constant referred to the charge transfer process pointing to a

depressed semicircle in Nyquist plots suggesting the dissolution of

metal is governed by charge transfer process [47]. The plot shows

that the phase angle increases with an increase in the dose of

Bromocriptine up to an optimal level, which may be due to a

decrease in the capacitive behavior at the metal surface arising

from the decrease in the metal dissolution and impedance

behavior.

3.5. EFM.

EFM technique provides a new procedure for

electrochemical erosion controlling [48]. Figure 11 (a, b) shows

the EFM spectrum obtained for CS in 1 M HCl at 25 oC without

and with 1.12 x 10-4 M of Bromocriptine, respectively. The

corrosion kinetic parameters were determined and listed in Table

9. It is obvious that the (icorr) and the CR decrease while the %IE

increases by increasing of Bromocriptine dose, which suggests

that the rate of the electrochemical reaction is reduced due to the

formation of a barrier layer over the CS surface by the drug. The

values of βc and βa do not show any appreciable change indicating

that the studied drug is a mixed type inhibitor [49].

The incredible quality of the EFM is the causality factor,

which serves as an interior check on the authenticity of the EFM

measurement [42]. The standard values for CF-2 and CF-3 are

two and three, respectively. The deviation of causality factors

from their ideal values might be due to that, the perturbation

amplitude was too small or that the resolution of the frequency

spectrum is not high enough. It is worth mentioning that the

results obtained from the various techniques utilized in this study

are in good agreement and follow almost the same trends.

3.6. AFM.

AFM is a powerful imaging tool for studying the

morphology of metal surface from micro to Nano scale and the

effect of inhibitor on progress and development of the dissolution

at the metal/solution interface [50].

Images investigation permitted evaluation of surface

roughness over an area scale of 8 μm. Figure 12 (a-c) shows 3D

AFM pictures of CS surface in the absence and presence of drug.

The roughness of the CS surface in 1 M HCl reaches 392.6 nm

(Figure 12b), while in the presence of drug it decreases to 91.3 nm

(Figure 12c). From the images obtained we found that the CS

surface in the presence of drug (Figure 12 c) is more compact and

uniform in comparison with its absence (Figure 12 a and b).

Figure 11. EFM recorded for CS corrosion in 1M HCl solution in the

absence (a) and in the presence (b) of 1.12 x 10-4 M expired

Bromocriptine drug at 25oC.

Therefore, the drug can efficiently protect the CS surface

from corrosion. Table 10 shows the values of root-mean-square

roughness and maximum peak-to-peak height obtained from AFM

images that decreased in the presence of inhibitor and confirm that

the inhibited surface is smoother than the uninhibited one. This is

due to the formation of a protective film on the metal surface by

the adsorption of Bromocriptine on the CS surface, which

improves and retards the dissolution reaction.

3.7. Quantum chemical calculations of inhibitor molecule.

Quantum chemical computation was used to explain and

correlate the experimental results to give an indication about the

inhibition effect on the metal surface. The frontier molecular

orbitals including Energy of the Highest Occupied Molecular

Orbital (EHOMO), Energy of the Lowest Unoccupied Molecular

Orbital (ELUMO), energy gap (ΔE = ELUMO - EHOMO) and dipole

moment (μ) which are related to the corrosion effectiveness of the

compound are studied. Using these parameters, a set of intrinsic

molecular properties [51-53] like ionization potential (I), electron

affinity (A), hardness (η), softness (σ), electronegativity (χ),

electrophilicity index (ꞷ), nucleophilicity index (ε) and the

fraction of transferred electrons (ΔN) values between the metal

surface and Bromocriptine drug were calculated and recorded in

Table 11.

Table 9. EFM spectra of CS corrosion in 1 M HCl solution in the absence and presence of different doses of Bromocriptine at 25oC.

% IE CF-3 CF-2 CR

mpy

-βc

mVdec-1

βa mVdec-

1

icorr μA

cm-2

Conc,

M

----- 3.05 2.02 296.40 178 98 648.80 Blank

38.2 3.02 1.94 182.57 169 95 400.67 2.30 x 10-5

58.2 2.95 2.19 122.80 175 91 271.00 4.60 x 10-5

67.6 2.69 2.02 95.66 169 81 210.32 6.90 x 10-5

79.0 2.74 1.92 62.00 175 90 136.10 9.20 x 10-5

85.3 2.86 1.91 42.78 166 79 95.39 1.12 x 10-4

Abd El-Aziz S. Fouda, Gamila El-Sayed Abdalla Badr

Page | 5710

Figure 12. AFM 3D pictures of CS (a) free specimen (b) in 1M HCl for 12 hours (c) in 1M HCl with 1.12 x 10-4 M of expired Bromocriptine drug for

12 hours at 25 oC.

Table 10. AFM parameters for CS: (a) free specimen (b) in 1M HCl for 12 hrs. (c) in 1M HCl with 1.12 x 10-4 M expired Bromocriptine drug at 25 oC

for 12 hrs. immersion.

c

c

b a Parameters

91.3 392.6 14.1 The roughness average (Sa) in nm

-9.4 -17.3 -18.0 The mean value (Sm ) in nm

115.8 493.2 18.1 The root mean square (Sq) in nm

-285.9 -3141.9 -58.3 The valley depth (Sv) in nm

803.2 1557.9 40.8 The peak height (Sp) in nm

1089.1 4699.8 99.1 The peak-valley height (Sy) in nm

The optimized molecular structures, HOMO and LUMO

for neutral Bromocriptine molecule using DmoL3/ GGA/BOP are

shown in Figure 13. The Figure shows a planer arrangement of

Bromocriptine molecule, the HOMO, and the LUMO are not

distributed over the whole molecule and are condensed on

localized positions on one side of molecule. The HOMO of the

molecule includes phenyl ring, heterocyclic amine rings and its

substituent like CH3-group, O-atom on carbonyl group and Br-

substituent on heterocyclic ring suggesting that the Br- substituent

at this position has some electron donating effect. There is a slight

extension of the LUMO to the O-atom attached to the C- atom that

present in the heterocyclic ring with two heteroatoms (O and N).

The C-atom of the heterocyclic ring that is directly attached

to the Br-substituent is involved in the LUMO of molecule,

indicating the inductive electron-withdrawing effect of the Br-

atom on this carbon. A higher value of the EHOMO indicates the

capability of a molecule to give electrons to a proper acceptor

molecule that has vacant molecular orbital while a lower value of

ELUMO indicates the tendency towards the ability of molecule to

accept electrons. The EHOMO and the ELUMO values are correlated

with the ionization potential (I = -EHOMO) and electron affinity (A=

-ELUMO) values of molecules respectively. The ability of inhibitor

molecule to be adsorbed on the metal surface increases with a

higher EHOMO value and a lower ELUMO value and consequently

better corrosion inhibition effectiveness. The smaller value of

energy gap (ΔE= ELUMO - EHOMO) of the inhibitor molecule

indicates a stronger interaction between it and metal surface.

The dipole moment (μ) is a significant electronic parameter

arising from random distribution of charges on the atoms in the

molecule. The higher value of μ most likely facilitates the dipole-

diploe interactions and increases the adsorption of inhibitor on the

metal surface. The electronegativity χ = (I+A)/2 of inhibitor gives

an information around the occurrence of coordinated covalent

bond among the metal and the molecule. In this work, the

corrosion inhibition activity of the inhibitor molecule has been

designed as inhibitor on iron metal. The electronegativity value

obtained (χ = 2.83) is lower than the experimental χ value of the

iron metal. Therefore, the iron metal will take electrons from the

inhibitor molecule to form coordinate bonds. The fraction of

electron transferred (ΔN) between the molecule and CS surface

was calculated by Pearson method [54]. According to this method,

when system of different electronegativity is in contact with each

other, the electrons pass from lower towards higher

electronegativity system, this flow of electrons continues until the

chemical potential becomes equal. Eq. 8 was used to calculate the

ΔN value.

ΔN =

𝜂 𝜂 (Eq. 8)

Where, φmet stand for the work function values for the iron surface

planes Fe (100), Fe (110) and Fe (111) that equal to 3.91, 4.82 and

3.88 eV, respectively. χinh stand for electronegativity of inhibitor,

while ηmet and ηinh stand for the hardness of iron and inhibitor

molecule, respectively. ηmetal = 0 and φmet = 4.82 eV for Fe (110)

surface has been put into consideration because of its packed

surface and higher stabilization energy. When ΔN > 0 the

electrons will transfer from inhibitor to metal surface atom and

vice versa [55]. In addition, the inhibitor molecule will be able to

donate electrons when ΔN < 3.6 [56]. The calculated value of

Bromocriptine (ΔN = 0.66) is positive and less than 3.6. Suggested

the ability of molecule to donate electrons to the vacant d-orbital

of metal atoms. Other important parameters like global hardness η

= (I – A)/2 and its reciprocal (global softness σ = 1/ η ) provide

information about the activity, reactivity and stability of the

inhibitor. HASB principle, considered that the inhibitor molecule

acts as a soft base and metal as soft acid [57]. Soft molecules have

small energy gap than hard ones due to their higher reactivity.

This facilitate the transfer of electrons and support good

inhibition efficiency. Thus, the molecules that have higher

softness value will be easily adsorbed on the metallic surfaces.

The obtained softness value in this study (σ = 0.67) for

Bromocriptine is good enough in comparison with some published

molecules before [58]. The back donation of charges (ΔE back-

Evaluation of expired Bromocriptine drug as eco-friendly corrosion inhibitor for C-steel in acidic media

Page | 5711

donation = -η/4) for the inhibitor governs the interaction between the

inhibitor molecule and the metal surface [59]. The ∆E back-donation

indicates that when η > 0 and ∆E back-donation < 0 the charge transfer

to a molecule, followed by a back-donation from the molecule, is

strongly favoured. Herein, the ∆E back-donation = -0.38 eV that denote

that back donation, is preferred for the inhibitor. The

electrophilicity index (ꞷ= Pi2/2 η ) is a measure of the energy

depletion resulting from the maximum electron flow between the

transmitter and the receiver and indicates the ability of molecule to

receive electrons. The nucleophilicity index (ε = Pi. η ) is a

molecular structure identifier and displays the capability of

molecule to give electrons [60], thus the inhibition efficiency

increases by decreasing ꞷ value or increasing ε value. Based on

these criterions, it is apparent from obtained results that

Bromocriptine drug has a higher affinity to give electrons to metal

surface i.e. act as electron donating molecule and its adsorption

occurs via forward donation.

3.8. Mechanism of inhibition.

The dissolution inhibition of the Bromocriptine molecule is

essentially due to its molecular structure containing electron donor

groups (O and N) besides π-electrons on an aromatic ring. The

interactions among the π-electrons on the ring (an electron donor)

and the empty d-orbitals of CS surface atoms (an electron

acceptor) facilitate the adsorption of organic molecule by

chemisorption bond [61]. The organic inhibitor can form an

insoluble complex by the reaction with the metal and make a

protective film on the surface of CS [62]. The explanation of

mechanism of adsorption and inhibition influence of

Bromocriptine in HCl solution is as follow:

For Physisorption mechanism, Bromocriptine is a

heterocyclic compound having N atoms, which can be readily

protonated in acidic media to give cationic form that may be

adsorbed on the cathodic sites of CS and reduce the evolution of

hydrogen as in Eq. 9.

Bromocriptine + xH+ ↔ [Bromocriptinex]x+ (Eq. 9)

The protonated inhibitor may be adsorbed via the

electrostatic interactions between positive charge on the

compound and the formed negative charges on the CS surface

(due to the adsorption of Cl- ions on the positive CS surface, after

that, the CS surface acquires negative change), i.e. there might be

a synergism among Cl- and protonated inhibitor [63]. While for

chemisorption mechanism, the electrons may be transferred from

the inhibitor molecule to the unoccupied d- orbital of iron.

Optimized Molecular Structures

HOMO

LUMO

Figure 13. The optimized molecular structures, HOMO and LUMO of

Bromocriptine drug using DmoL3/ GGA/BOP

Table 11. The calculated quantum chemical parameters for Bromocriptine drug using DMoL3/GGA/BOP.

ΔN110

ΔEback-

donation

(eV)

ε (eV)

2

ꞷ (eV)

σ

(eV)-1

η

(eV)

Pi

(eV)

χ (eV) μ

Debye

ΔE (eV) ELUMO

(eV) EHOMO

(eV)

0.66 -0.38 -4.25 2.66 0.67 1.50 -2.83 2.83 6.052 3.006 -1.326 -4.332

4. CONCLUSIONS

The results obtained from this study for all measurements

confirmed that the Bromocriptine drug acts as effective inhibitor

for the dissolution of CS in 1 M HCl. The maximum inhibition

efficiency obtained is 98.22 % for a 1.12x10-4M dose at 55˚C, and

180 minutes. The inhibition efficiency rises by increasing the dose

of Bromocriptine, temperature, and immersion time. The

Bromocriptine drug is a mixed type inhibitor with a predominantly

anodic effect. The adsorption mechanism tested occurs via the

chemisorption and physisorption mechanism. The adsorption of

the Bromocriptine molecule on the CS surface follows Temkin

and Langmuir adsorption isotherms.

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