Experimental, theoretical and simulation studies of extracted crab waste protein as a green polymer inhibitor for carbon steel corrosion in 2 M H3PO4

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Surfaces and Interfaces

journal homepage:www.elsevier.com/locate/surﬁn

Experimental, theoretical and simulation studies of extracted crab waste protein as a green polymer inhibitor for carbon steel corrosion in 2 M H

₃

PO

₄

Amr S. Ismail

^a,⁎

, Ahmed A. Farag

^b

aPetrochemicals Department, Egyptian Petroleum Research Institute, 11727 Nasr City, Cairo, Egypt

bPetroleum Applications Department, Egyptian Petroleum Research Institute, 11727 Nasr City, Cairo, Egypt

A R T I C L E I N F O

Keywords:

Crab waste protein Carbon steel Green inhibitor H3PO4

Quantum chemical calculations Molecular dynamics simulations

A B S T R A C T

Crab waste protein (CWP) has been extracted, characterized and examined as a green polymer inhibitor for carbon steel in 2 M H3PO4media by weight loss and electrochemical techniques. The inhibition of CWP ac- celerates by increasing the inhibitor content, but the temperature has hardly aﬀected the inhibition eﬃciency.

Thermodynamic data clearly show that the adsorption mechanism of CWP on the carbon steel surface in 2 M H3PO4solution is mainly physical adsorption. Moreover, the adsorption of the CWP molecules was found to follow a Langmuir adsorption isotherm. Results of potentiodynamic polarization measurements revealed that the CWP acts as a mixed-type eco-friendly inhibitor. The electrochemical impedance spectroscopy (EIS) technique was employed to pattern the inhibition process through proper equivalent circuit pattern. Theoretical parameters derived from quantum chemical calculations as well as binding energy derived from molecular dynamics simulations experiments adequately corroborate the trend of experimental inhibition eﬃciencies of the tested inhibitors.

1. Introduction

Various industrial attempts require the providing of corrosion inhibitors to thefluids procedures to control the corrosion rate of the metallic structures [1–3]. The inhibitors are particularly imperative regarding the industrial acidic actions that associated with carbon steel surfaces such as pickling, cleaning, etching, oxide film removal and surface cleaning [4]. Phosphoric acid is utilized in the planning of fertilizers preparation, passivation and phosphating process. However, phosphoric acid is a strong acid medium, but it considers strong cor- rosiveness on steel surface. There are lower considerations with the corrosion inhibition in the phosphoric corrosive medium by correlation with sulfuric and hydrochloric acids [5,6]. Regularly, effective organic inhibitors including in its structure either N, S, or O atoms or electro- negative practical groups andπ-electrons in triple or conjugated double bonds[7]. The inhibiting effect of these organic composites depends on the adsorptive collaborations between the inhibitor molecules and the metal surface[8]. In such way, the proposed inhibition mechanism may be through van der Waals electrostatic force between the charged metal and the charged inhibitor particles, empty d-orbitals of iron in the metal connecting with lone pair electrons of electrons and/orπ-electrons of the inhibitor molecules, or maybe blend of these methods[9]. Various searches were led to assess some naturally extracted substances as

corrosion green inhibitors for various metals in various conditions [10–20]. The extracted amino acid protein from several marine life forms wastes could be utilized as a green corrosion inhibitor[21]. Huge amounts of crab are gathered from the sea annually. The remaining waste of crab displays some ecological and economic challenges[22].

Crab by-product consists of whole shells, chitin, protein concentrates, meat and the viscera[23]. Management of crab waste is an excellent source of crude protein polymer (about 45%). The extracted protein consists of several types of amino acids such as alanine, glycine, and proline[24]. A protein is a linear biopolymer in which the monomer units are the amino acids. The amino acid monomers joined together through the linking between amino group (NH2) of one amino acid with a carboxylic acid group (COOH) of another amino acid to form an amide bond (peptide bond). Consequently, the polypeptides consist of many amino acids linked together plus the side chains contain “R” groups were reported[25–27]. In general, an amino acid is containing functional groups as an amine group (–NH2) and carboxyl group (–COOH), attached with various side chains speciﬁc to each amino acid.

Therefore, the structure of amino acids included carbon (C), hydrogen (H), oxygen (O), and nitrogen (N) elements, besides other elements are found in the side chains[28]. In this way, the amino acids extracted from marine living beings could be a candidate to use as corrosion inhibitors. Besides, marine waste products are eco-friendly, low-cost,

https://doi.org/10.1016/j.surﬁn.2020.100483

Received 1 November 2019; Received in revised form 1 January 2020; Accepted 19 February 2020

⁎Corresponding author.

E-mail address:[email protected](A.S. Ismail).

Surfaces and Interfaces 19 (2020) 100483

Available online 21 February 2020

T

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promptly accessible and renewable materials. The aim of the present study is to survey the possibility of using the amino acid protein extracted from crab waste (CWP) as an inexpensively and green inhibitor for carbon steel corrosion in 2 M H3PO4, utilizing weight loss, potentiodynamic polarization curves and electrochemical impedance spectroscopy (EIS). The impact of temperature and thermodynamic parameters on the corrosion rate was determined and examined.

2. Experimental & techniques

2.1. Materials

The seafood solid wastes of freshwater crab selected for the current study were gathered from theﬁsh processing unit at El-Obour central market, Egypt. Known amounts of the wastes were washed cautiously with tap water, dried and put away under 0 °C until use. The composition proportion by weight of the utilized carbon steel samples was as follows: 0.37% C, 0.22% Si, 1.27% Mn, 0.18% Ni, 0.13% Mo, 0.017%

Cr, 0.015% P, 0.01% Cu, 0.007% S, and the rest Fe. The corrosive medium (2 M H3PO4) was prepared by dilution of an analytical reagent grade 98% H3PO4with distilled water.

2.2. Techniques

The molecular weight of CWP measured by gel permeation chro- matography (GPC) utilizing a Supremamax 3000 column (Polymer Standard Service, Mainz, Germany) with 2% CH3COOH/0.2 M buffer (CH3COONa) as eluent (1 ml/min). An FT-IR spectra analysis was obtained using the ATI Mattson model Genesis Series (USA) infrared spectrophotometer adopting the KBr technique. X-ray diffraction (XRD) patterns were recorded in the range 2θ= 4–80°, using Philips Powder Diffractometer with Cu Kα1 radiation.

2.3. Weight loss measurements

The weight loss values were examined dependent on the standard Fig. 1.Chart of (a) X-ray diﬀraction, (b) FTIR, and (c) GPC for CWP molecules.

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techniques[29]. Firstly, the samples (73 mm × 22 mm × 3 mm) were weighed and immersed in 2 M H3PO4solution without and with various concentrations inhibitors at a known time interval of 12 h at 298 K.

After the immersion time frame, the steel samples were pulled back, carefullyﬂushed with distilled water, ultrasonically cleaned in acetone, dried and weighed once more. The tests were operated in triplicate and the mean value of weight loss was recorded.

2.4. Electrochemical measurements

The electrochemical experiments were applied using a conventional three-electrode cylindrical Pyrex glass cell. The carbon steel electrode with a 1 cm²exposed surface area was utilized as a working electrode.

The platinum electrode and saturated calomel electrode (SCE) were utilized as an auxiliary electrode and the reference electrode, individually. Electrochemical measurements were employed on the Volta- Lab PGZ402 potentiostat controlled by Volta-Master 4 programming.

Prior to performing the electrochemical measurements, the working electrode was immersed in 2 M H3PO4solution without and with different concentrations of the inhibitor for 30 min. to establish a steady- state open circuit potential (Eocp). The potentiodynamic polarization curves were gained from−900 mV to−200 mV (vs. SCE) with a scan rate of 1 mV s⁻¹. Moreover, the electrochemical impedance spectroscopy (EIS) tests were performed in between the highest frequency limit of 100 kHz and lowest frequency limit of 0.03 Hz at open circuit potential with an amplitude of 10 mV using AC signal. The impedance data were analyzed and ﬁtted with the simulation ZSimpWin 3.60, equivalent circuit software. All the experiments were employed in triplicate manner and average values were obtained.

2.5. Extraction & characterization

0.5 kg from solid crab waste wasﬂooded in 1000 ml of tap water in a separate glass vessel at a temperature of 323 K for 2 h with consistent mechanical stirring utilized for the high viscosity solution or the solution which contain enormous solid segments. Hydrolyzing agents of 30% acetic acid and 0.5 M sodium carbonates were added step by step to the mixture at temperature 353 K for a time of 4 h. At that point, the acquired mixture was separated to hold the wastes (solid phase).

Through the drying procedure over a more extended time frame at ambient conditions, the water proportion of protein was vanished step by step and the solid protein phase of CWP obtained as shown inFig. 1.

The supernatant (liquid phase) is a solution which mainly based on protein hydrolysate containing high protein proportion of discharging peptides and amino acids[30]. The fundamental amino acid percentage composition of snow crab protein separated in alkaline (2% aqueous KOH) was recorded as listed inTable 1 [31]. The structure of isolated CWP molecules was affirmed by XRD, FTIR and GPC analysis as shown in Fig. 2. The XRD diagram was displayed in Fig. 2a. Mostly, the polypeptide chain of the protein tends to overlay into a three-dimen- sional structure[32]. Wherefore, it is predicating that protein will have no known structure. Fig. 2a shows a sharp and high intensity (32.4, 100.0, and 32.6%) diffraction lines at d-distances 1.97 A° for protein source of the CWP. These considerable lines are the fundamental lines describing proteins and polymers as per standard ASTMD3906-03. The FTIR diagram of the CWP is appeared inFig. 2b. It very well observed that there are absorption bands of C]O groups at 1648 cm⁻¹because of stretch weakly combined with CeN stretch and NeH bending. In addition, there are absorption bands of CeN groups at 1570 cm⁻¹ because of stretch strongly combined with NeH bending. The absorption band at 1433 cm⁻¹identified with NeH in-plane bending combined with CeN stretching and also incorporates CH and NH distortion vibrations. The broad absorption band of OH groups corresponded to NH groups showed up at 3433 cm⁻¹. The basic absorption band appeared at 2923 cm⁻¹because of the rich CHeand CH]groups on the main structure of the protein [33,34]. The molecular weight of the CWP

was gained using GPC as appeared inFig. 2c. It was obtained that an intense peak was available at high MWvalue (994 g mole^–1) for amino acid protein-polymer chains of CWP. The polydispersity index of CWP chains was found 1.025, which revealed that the polymer chains approach uniform chain length[35].

2.6. Quantum chemical studies

The theoretical calculations were carried out using MP2 functional with a 3–21 G (d,p) basis set for all atoms. The parameters such as high occupied molecular orbital energies (EHOMO) and low unoccupied molecular orbital energies (ELUMO), an energy gap (ΔE) between LUMO and HOMO, dipole moment (μ) and the number of the transferred electron (ΔN) were determined.

Table 1

Essential amino acid composition percentage in CWP.

Name Structure Percentage (%)

Arginine 6.66

Histidine 3.58

Isoleucine 2.67

Leucine 5.14

Lysine 2.51

Methionine 1.93

Phenylalanine 5.98

Threonine 4.74

Tryptophan 0.78

Valine 7.07

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2.7. Molecular dynamics (MD) simulations

MD simulation studies were performed using Material Studio 2017 software from BIOVIA Accelrys Inc. Fe (1 1 0) surface is the most densely packed and also the most stable[36]. Therefore, the Fe (1 1 0) was ﬁrst cleaved from pure Fe crystal and relaxed by minimizing its energy using molecular mechanics with a slab thickness of 5 Å. The Fe (110) plane was enlarged to a (10 × 10) supercell to provide a large surface for the interaction with the inhibitors. After that, a vacuum slab with 30 Å thicknesses was built above the Fe (110) plane. The interaction energy (Eint) of molecules with Fe surface was obtained using the following equation:

= − +

E_int E_total (E_{Fe surface} E_molecule) (1)

whereEtotalis the total energy of the molecules and the metal surface system; Esurface is deﬁned as the energy of metal surface without adsorption of molecules andEmoleculeis the energy of isolated molecules.

The binding energy is the negative of the interaction energy and is given as:

= −

E_bin E_int (2)

3. Results and discussion

3.1. Gravimetric study

3.1.1. Eﬀect of cwp concentration

The eﬀect of the addition of CWP at various concentrations on the corrosion of carbon steel was studied by the weight loss method at 298 K after 12 h of immersion period in 2 M H3PO4media. The corrosion rate (ν) and inhibition eﬃciencyηWL(%) were calculated according to the following equations[37]:

=

ν W

St Δ

(3)

= ⎡

⎣

− ⎤

⎦

∘ ×

∘

η ν ν

(%) ν 100

WL (4)

whereΔWis the average weight loss,Sis the total surface area (cm²) of the specimen,tis the immersion time (h),ν○andνare the values of corrosion rate (mg cm⁻²h⁻¹) in uninhibited and inhibited solutions, individually. The acquired values ofνandηWLwere recorded inTable 2.

It is clear from these outcomes that the CWP inhibits the corrosion of carbon steel at all concentrations utilized in this work.Fig. 3a shows the variety ofνandηWLwith the CWP contents. It very well exhibited that theνof carbon steel diminishes whileηWLincreases as the CWP concentration increases. It was discovered that the maximumηWLof 94.5%

is accomplished at 5 × 10^–3M of CWP inhibitor and a further increment in proportion did not create any obvious change in the inhibition performance. The corrosion inhibition can be identiﬁed with the adsorption of the amino acid molecules of CWP at the steel/phosphoric acid solution interface[38]. As a matter of fact, the adsorption of the CWP on the carbon steel could happen because of the formation of links between the d-orbital of iron atoms (including the removal of H2O molecules from the metal surface), and the lone sp² electron pairs present on the heteroatoms as N atoms and other adsorption centers in the amino acid molecules of CWP.

Fig. 2.(a) Variation of corrosion rate and inhibition eﬃciency, (b) Frumkin, (c) Langmuir and (d) Temkin isotherm adsorption models of CWP on the carbon steel surface in 2 M H₃PO₄at 298 K.

Table 2

The weight loss parameters obtained for carbon steel in 2 M H3PO4solution without and with various concentrations of CWP at 298 K.

Conc. (M) ν(mg cm⁻²h⁻¹) θ η(%)

Blank 2.4631 – –

1 × 10⁻⁴ 0.6306 0.744 74.4

5 × 10⁻⁴ 0.4039 0.836 83.6

1 × 10⁻³ 0.2376 0.904 90.4

5 × 10⁻³ 0.1365 0.945 94.5

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3.1.2. Adsorption isotherm

Fundamental information on the adsorption of inhibitor on the metal surface can be described by adsorption isotherm. Adsorption interaction occurs between the organic molecules and the metal surface is higher than that between the water molecule and the metal surface.

Several isotherms including Frumkin, Langmuir, and Temkin, isotherms are conducted to ﬁt the experimental data. The adsorption isotherms can be expressed in the following equations[39]:

−θ − =

θ aθ K C

1 exp( 2 ) ads (Frumkin isotherm)

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= +

C

θ K1 C

(Langmuir isotherm)

ads (6)

− aθ = K C

exp( 2 ) ads (Temkin isotherm) (7)

whereθis the degree of surface coverage from the weight loss measurements (Table 2),Cis the inhibitor concentration,Kadsis the equilibrium constant andais the molecular interaction parameter. Based on the values of the correlation coefficient (R²) as shown inFigs. 3(b–c), it is evident that the Langmuir isotherm (Fig. 3c) was the bestfit model for the adsorption of CWP molecules on the carbon steel surface. The equilibrium constant Kads can be calculated from the intercept of Fig. 3c. The higher value ofKads(20,000 mol⁻¹) indicated more effi- cient adsorption of CWP molecules on the carbon steel surface. The dimensionless separation factor,RL[40], can be calculated byEq. (6), indicates a highly favorable adsorption process, in the case of smaller value (0<RL<1).

= +

R K C

1

L 1

ads (8)

The calculatedRLfor all concentrations of CWP was found in the range 0.33–0.01, conﬁrming highly eﬃcient adsorption [41,42].

Meanwhile, according to the relationship between the equilibrium constantKadsand the standard free energy of adsorptionΔG_ads^o can be derived as follows[43]:

⎜ ⎟

= ⎛

⎝

− ⎞

⎠

K G

RT 1

55.5exp Δ

ads adso

(9) whereTis the absolute temperature in K. The negative value ofΔG_ads^o indicates that the adsorption of the CWP molecules on the carbon steel surface is a spontaneous process[44]. Generally, the adsorption type is regarded as physisorption if the absolute value ofΔG_ads^o was of the order of 20 kJ mol^–1or lower. The inhibition behavior is due to the electrostatic interactions between the inhibitor molecules and iron atoms.

When the absolute value ofΔG_ads^o is of the order of 40 kJ mol^–1 or higher, the adsorption could be seen as chemisorption. In this process, the covalent bond is formed by the charge sharing or transferring from the inhibitor molecules to the metal surface[45]. The calculatedΔG_ads^o value in this study indicate that the adsorption mechanism of CWP on carbon steel in 2 M H3PO4solution at 298 K is mainly physisorption [46]. Based on the literature the steel surface in acidic solution is

negatively charged at the corrosion potential [47]. Therefore, CWP cationic forms (CWPH⁺) are preferentially adsorbed through physisorption (electrostatic interactions) mechanism between positively charged nitrogen atoms in the( NH )− ⁺₃ groups of amino acids in CWP and negatively charged carbon steel surface. Actually, amino acids molecules can be adsorbed chemically via donor-acceptor interactions between theπ-electrons of the aromatic systems (as phenylalanine and tryptophan amino acid derivatives) and unshared electrons pairs of the heteroatoms (as nitrogen and sulfur atoms in amino acid derivatives) to form a bond with the vacant 3dorbitals of the metal surface, which act as a Lewis acid, leading to the forming a protective chemisorbed in- hibitorﬁlm.

3.1.3. Eﬀect of temperature

Temperature is an important kinetic factor that affects the corrosion rate of metal and modifies the adsorption of the organic molecules on the metal surface. To illustrate the effect of temperature on the corrosion inhibition of CWP, gravimetric experiments were carried out at the temperature range of 298–333 K, without and with 5 × 10^–3M of CWP for 12 h of immersion, and the obtained data, listed inTable 3. It is obvious that the corrosion rate (ν) increased with the rise of temperature and it is more pronounced for the blank solution (2 M H3PO4).

Therefore, the inhibition eﬃciency depends on the temperature, indicating that higher temperature dissolution of carbon steel pre- dominates on the adsorption of CWP at the metal surface. This can be demonstrated by the decrease of the strength of the adsorption between CWP and the metal surface at high temperature and can suggest a physisorption mechanism [48]. The activation thermodynamic parameters of the corrosion process can be calculated by ArrheniusEq. (8) and transition stateEq. (9)as following[49]:

=− +

ν E

RT A

ln( ) ^a ln( )

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⎜ ⎟

⎛

⎝

⎞

⎠=− + ⎡

⎣⎢ ⎛

⎝

⎞

⎠ + ⎛

⎝

⎞

⎠

⎤

⎦⎥ ν

T

H RT

R N h

S

ln Δ R

ln Δ

a

A

a

(11) whereEais the apparent activation corrosion energy,Ris the universal gas constant, T is absolute temperature, A is the Arrhenius pre- Fig. 3.(a) Arrhenius and (b) Transition-state plots for carbon steel corrosion rates in 2 M H₃PO₄in absence and in presence of 5 × 10^–3M of CWP.

Table 3

The weight loss parameters obtained for carbon steel in 2 M H3PO4solution without and with (5 × 10⁻³M) CWP at diﬀerent temperatures.

Medium Temp. ν(mg cm⁻²h⁻¹) ηg(%)

Blank 298 2.4631 ‒

313 5.0215 ‒

323 8.1418 ‒

333 15.5613 ‒

CWP 298 0.1365 94.5

313 0.7055 86.0

323 1.8283 77.5

333 8.4573 45.7

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exponential factor,his Plank's constant,Nis Avogadro's number,ΔHa, ΔSais the enthalpy and entropy of activation, respectively. Arrhenius plots for the corrosion rate of carbon steel are given inFig. 4a. Values of apparent activation energy of corrosion (Ea) reported inTable 4which determined from the slope of ln (ν) vs. 1/T plots (Fig. 4a). The linear regression coeﬃcient was close to unity, indicating that the carbon steel

corrosion in phosphoric acid can be demonstrated using the kinetic model. The calculated value ofEain the blank 2 M, the H3PO4solution was 43 kJ mol^–1) which in agreement with previously described[50].

The obtainedEavalue in the presence CWP is 95 kJ mol^–1. The increase value of theEain presthe ence of CWP may be interpreted as physisorption mode[51].Szaueret al. reported that the increase inEacan be due to a decrease in the adsorption of the inhibitor on the steel surface with ian ncrease in temperature [52]. As adsorption decreases more desorption of inhibitor molecules occurs due to these two opposite processes are in equilibrium. Therefore, at higher temperatures, more desorption of inhibitor molecules occurs resulting in larger surface area of steel becomes in contact with aggressive environment leading to increased corrosion rates with increase in temperature[53]. To calcu- late the values ofΔHaandΔSathe curve ofln( )^ν

T against

T

1 was plotted Fig. 4.(a) OCP and (b) Polarization plots of carbon steel electrode obtained in 2 M H3PO4solution and containing various concentrations of CWP at 298 K.

Table 4

Thermodynamic parameters obtained for carbon steel in 2 M H3PO4solution without and with (5 × 10⁻³M) CWP.

Medium Ea(kJ mol^–1) ΔHa(kJ mol^–1) ΔSa(kJ mol^–1) Ea–ΔHa(kJ mol^–1)

Blank 43 40 −104 2.62

CWP 95 92 46 2.62

Fig. 5.(a) Nyquist plots, (b) Bode plots and (c) electrochemical circuit model of carbon steel electrode obtained in 2 M H₃PO₄solution and containing various concentrations of CWP at 298 K.

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as shown inFig. 4b. A straight line is obtained with a slope of (⁻ ^H

R Δ a) and an intercept of[ln( ^R )+( )]

N h S R Δ A

a from which the values ofΔHaand ΔSaare calculated and are listed inTable 4. It was found that theΔHa

value for dissolution reaction of carbon steel in the presence of CWP (92 kJ mol^–1) is higher than that in its absence (40 kJ mol^–1). The positively ofΔHaindicate the endothermic nature of the carbon steel dissolution process and suggesting that its dissolution is slow in the presence of CWP. The values of Ea andΔHavary in the same way permitting to verify the known thermodynamic relation (ΔHa=Ea−RT) as shown inTable 4. The positive value ofΔSain the presence of CWP, and the negative value of uninhibited solution (Table 4) is generally demonstrated as an increase in disorder as the reactants are converted to the activated complexes[54]. Furthermore, this behavior can be described as a result of the replacement process of H2O molecules through adsorption of CWP on the steel surface[55].

3.2. OCP and potentiodynamic polarization testing

The behavior of carbon steel corrosion and the surface layer por- osity can be obtained by the open circuit potential (OCP) measurements [56]. The time-dependent variation of OCP of carbon steel immersed in 2 M H3PO4solution without and with different concentrations of CWP inhibitor is presented inFig. 5a. It was observed that the OCP values were shifted in a positive direction. This indicates that the kinetics of the anodic reaction of carbon steel in 2 M H3PO4solution was influ- enced more strongly in the presence of the CWP inhibitor. This behavior is suggesting the formation of a protecting layer due to the adsorption of the inhibitor molecules on the metal surface decreasing the active sites of the carbon steel surface. However, the shifted in OCP curves between the absence and presence of the inhibitor is less than 85 mV, which indicates that the investigated inhibitor is a mixed-type inhibitor as reported by Riggs [57]. The potentiodynamic polarization curves obtained for carbon steel after 30 min of immersion in 2 M H3PO4so- lution without and with CWP are shown inFig. 5b. The values of corrosion potential (Ecorr), corrosion current density (Icorr), inhibition efficiency (η) cathodic (βc) and anodic (βa) Tafel slope were listed in Table 5. Based on the corrosion current density values the values of inhibition efficiency (η) were calculated as per the following relationship[58]:

= −

×

η I I

(%) I 100

Tafel

corr corr i corr

( )

(12) whereIcorrandIcorr(i)are the corrosion current densities for carbon steel electrode in the uninhibited and inhibited solutions, respectively. It is clear that the addition of CWP inhibitor molecules 2 M H3PO4solution causes a remarkable decrease in corrosion current density (Icorr) at all inhibitor released concentrations compared to the uninhibited blank solution. Moreover, the corrosion potential was slightly shifted to positive values for inhibited solutions. This behavior indicating that the carbon steel became passive by a geometric blocking eﬀect of the adsorbed inhibitive species that forming a protective layer from CWP molecules on the carbon steel surface capable to provide corrosion protection[59]. However, the displacement in the corrosion potential

between in the absence and presence of the inhibitor is not enough to decide the anodic type inhibitor, therefore, the inhibitor behaves as a mixed-type inhibitor, that agrees with open circuit potential measurements[60].Fig. 5b shows the same behavior for inhibited and uninhibited Tafel lines, suggesting that the inhibitor blocked the active site on the carbon steel surface without aﬀecting the corrosion mechanism.

This demonstrates that the inhibitive action of CWP due to the adsorption and formation of a protective layer on the carbon steel surface.

3.3. Electrochemical Impedance Spectroscopy (EIS)

To examine the inhibition mechanism in more detail, the EIS technique was utilized.Fig. 6a shows the acquired Nyquist charts of carbon steel in 2 M H3PO4 medium in the absence and presence of various proportions of CWP. The Nyquist diagram, of carbon steel without and with the effect of CWP, is portrayed by one capacitive complex at high frequency (HF) with little inductive one at low frequency (LF) values [61]. The capacitive loop at high-frequency region can be identified with the charge move, while the inductive loop at the low-frequency district is attributed to the relaxation procedure of the adsorbed inter- mediates that controlling the anodic mechanism[62]. All things con- sidered, the inductive conduct is possibly identified with the con- sequence of corrosion reaction byproducts layer that balanced out on the electrode surface (such as, iron phosphates) beside the inhibitor molecules and their reactive products[63]. Clearly, the diameters of capacitive loops (Fig. 6a) increment with the expansion in CWP proportion, demonstrates the increase of charge move opposition and up- grade of the inhibition effect on carbon steel corrosion. It is obvious from these plots (Fig. 6a) that the forms of the impedance plots for the inhibited and uninhibited electrodes are not basically different. Its proof that the expansion of CWP to 2 M H3PO4solutions expands the charge transfer impedance by formation of protection layer on the carbon steel surface, without change different aspects of the corrosion performance. Similar outcomes utilizing the polarization recorded that reported CWP addition does not change the electrochemical reactions mechanism. The Nyquist impedance plots of carbon steel in 2 M H3PO4

containing CWP were characterized byﬁtting the experimental data to develop the appropriate physical models and extract the parameters which characterize the corrosion process. The comparable circuit model used toﬁt the experimental impedance data was [Rs(QRct)], which in- clude solution resistance (Rs), constant phase element (Q), and charge transfer resistance (Rct). The values ofRct can be calculated by the following equation[64]:

= −

R_ct Z_{r At low f}( ) Zr At high f( ) (13)

Table 5

Potentiodynamic electrochemical parameters for the corrosion of carbon steel in 2 M H3PO4solution without and with various concentrations of CWP at 298 K.

Conc. (M) –E(mV) Icorr(mA/cm²) βa(mV) –βc(mV) η(%)

Blank 503.5 4.8956 151.6 187.9 –

1 × 10⁻⁴ 496.3 1.1979 139.1 186.7 75.5

5 × 10⁻⁴ 492.4 0.7859 109.7 170.6 83.9

1 × 10⁻³ 492.0 0.6241 68.7 156.7 87.3

5 × 10⁻³ 491.5 0.4334 76.8 130.8 91.1

Fig. 6.The frontier molecular orbital of represented amino acids of CWP.

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quency dispersion, which attributed to roughness and inhomogeneities of the solid surface[65]. The impedance of theCPEis expressed as:

= Z Y jω

1

CPE ( )

o n (14)

whereYois the magnitude of CPE (inΩ⁻¹sⁿcm⁻²),jthe imaginary root, ω the sine wave modulation angular frequency (in rad s⁻¹), j² = –1 is the imaginary root and n is an empirical exponent (0≤n≤1) which measures the deviation from the ideal capacitive behavior. From the values of Rct, the related inhibition eﬃciency,ηZ

(%), was determined and listed inTable 6, according to the following equation:

= −

×

η R R

(%) R 100

Z

ct i ct

ct i ( )

( ) (15)

whereRctandRct(i) are the charge-transfer resistance values without and with inhibitor, respectively. It's found thatηZ(%) increases with the CWP concentration, reaching its maximum value at 5 × 10⁻³M (ηZ

(%) = 92.3). On the other hand, it is apparent from Bode plots that the addition of CWP to 2 M H3PO4solution results in an increase in the impedance modulus which further increases with rising released inhibitor concentration (Fig. 6b). This can be attributed to the formation of a protectivefilm of the inhibitor molecules on the carbon steel surface which inhibits the dissolution of iron. As shown inFig. 6b, there is only a single time constant for the corrosion process at the metal-solution interface in the phase-angle plots. The increase in the peak heights with rising proportions of CWP implies a more capacitive re- sponse at the metal-solution interface because of the absorption of inhibitor molecules and hence more inhibition influence. EIS data also confirm the inhibiting performance of CWP, which expose the same trend as those gained from weight loss and the potentiodynamic polarization methods.

3.4. Computation studies

3.4.1. Quantum chemical calculations

The optimized molecular structures and the corresponding HOMO and LUMO electron density surfaces of valine, arginine, and phenylalanine as represented examples of CWP amino acid molecule using PM3 model chemistry are illustrated inFigs. 7. The electron distribution of HOMO shows data about the centers or the sites that are well on the way to give the electrons to the comparing orbital of the acceptor molecule, while the electron dissemination of the LUMO gives data about the centers of the molecule that are bound to acknowledge the electron from an appropriate donor molecule. Fig. 7 shows that the electron density of the HOMO mainly localized only over the amine groups in the amino acid structure of valine (Fig. 7a) and arginine (Fig. 7c), while for phenylalanine the electron density restricted over the amine group as well as phenyl rings (Fig. 7e). The carboxylic group of the amino acid makes small contributions to the LUMO, as the electron density mainly localized over amino groups and phenyl rings.

The quantum chemical parameters,EHOMO, ELUMO,ΔE, and μ(dipole moment) that directly influence electronic interaction between inhibitor molecule and metal surface have been calculated and listed in Table 7. On the basis of earlier reports available in the literature it can be concluded that the value ofEHOMOof a molecule is a measure of the tendency to donate its HOMO electrons to the identical acceptor molecule[66], while, theELUMOis a parameter that defines the affinity of molecule to accept electrons into its LUMO from appropriate donor molecule[67]. The energy gapΔE(ELUMO–EHOMO) is another very important parameter which can be used to predict the reactivity of molecules. Generally, a molecule with a low value ofΔEassociated with high chemical reactivity and therefore high inhibition efficiency[68].

The patterns of theΔEacquired in the present work for characterized inhibitors are in good agreement with the order of experimental inhibition eﬃciencies. The number of transferred electron (ΔN) calcula- tion based on the quantum chemical method using the following equation[69]:

= −

N χ +χ

γ γ

Δ ( )

2( )

Fe Inh

Fe Inh (16)

Fig. 7.Equilibrium adsorption conﬁgurations of represented amino acids of CWP on the Fe (1 1 0) surface at 298 K obtained by MD simulations: (a and b) valine, (c and d) arginine, and (e and f) phenylalanine.

Table 7

Calculated quantum chemical parameters of presented amino acids.

Parameters Valine Arginine Phenylalanine

E(HOMO) −8.814 −8.418 −8.294

E(LUMO) −2.075 −1.835 −1.235

ΔE(L-H) 6.739 6.583 7.059

I 8.814 8.418 8.294

A 2.075 1.835 1.235

χ(inh) 5.445 5.127 4.765

γ(inh) 3.370 3.292 3.530

χ(Fe) 7.000 7.000 7.000

γ(Fe) 0.000 0.000 0.000

ΔN 0.539 0.563 0.492

μ 3.510 3.820 4.370

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whereχFeandχInhare the absolute electronegativity of iron and inhibitor, andγFeandγInhare the absolute hardness of iron and inhibitor, respectively. These terms are related to an electron aﬃnity (A) and ionization potential (I) as presented inEqs. (17)and(18):

= + χ I A

2 (17)

= − γ I A

2 (18)

where according toKoopman'stheorem, theIandAare related to the frontier orbital energies according to theEqs. (19)and(20) [70]:

= −

I EHOMO (19)

= −

A ELUMO (20)

The values ofχandηwere calculated using the values ofIandA obtained from quantum chemical calculations. The theoreticalχvalue is 7.0 eV mol⁻¹andγvalue is 0 eV mol⁻¹for iron[71]. Generally, the maximum fraction of electron transfer indicating that the most absorb eﬀectively on the carbon steel surface. The entire quantum chemical calculations show that corrosion inhibition in the present study occurs via electron transfer from high electron density of the inhibitors to the surface Fe atoms.

3.4.2. Molecular dynamics simulations

As of late, MD simulation has featured as a fundamental tool in the characterization of the adsorption performance of the inhibitor molecules on the metallic surface[72]. MD simulations can sensibly give data about the most ideal arrangement of adsorbed inhibitor molecules on the metal surface and have become a entrenched tool in computational chemistry. In our present article, MD simulations were employed to discuss the adsorption properties of three represented amino acids on the Fe (110) surface. The equilibrium conﬁgurations for valine, arginine, and phenylalanine are represented in Fig. 8(a–f), respectively.

Careful inspection ofFig. 8shows that during the MD simulation process, the three amino acids moved gradually near the Fe (110) surface with almostﬂat orientation. Therefore, it has been concluded that investigated inhibitors can be adsorbed on the carbon steel surface through the amine groups of three amino acids, beside the phenyl ring in phenylalanine. Furthermore, it has been reported that emptyd-orbitals of the surface Fe atoms can facilitate the adsorption process by accepting the electrons from the inhibitors molecules. In the studied amino acids, the nonbonding electrons of the N atoms andπ- electrons of the phenyl moieties provide suﬃcient electronic density to transfer in the emptyd-orbitals of the surface Fe atoms in order to form a stable coordinate bond. Generally, the adsorption energy is the energy released (or needed) when the relaxed adsorbate component is adsorbed on the substrate. The values of interaction energy (Einteraction) and binding energy (Ebinding) between studied inhibitors and Fe (110) surface were evaluated when systems reach equilibrium. It was found that

theEbinding values for valine, arginine, and phenylalanine were 35.7, 63.4 and 30.4, respectively. Therefore, the interaction energy (–Ebinding) is negative for all studied inhibitors which suggest that adsorption of the amino acids on Fe (110) surface can take place spontaneously[73].

3.5. Scanning electron microscopy (SEM)

Fig. 8a and b show the SEM images and EDX spectra of the carbon steel surface afterflooded in 2 M H3PO4, for a time of 12 h, in absence and presence of 5 × 10^–3M of CWP inhibitor, respectively. The SEM micrographs indicated extreme corrosion of the carbon steel is seen after immersion in the acidic medium without inhibitor (Fig. 8a). There is the formation of corrosion products that apparently covers the whole surface of the steel. These corrosion products then form a permeable layer that does not permit the protection of carbon steel, hence a extreme attack is found.Fig. 8a additionally shows the EDX spectra for the carbon steel after exposure to 2 M H3PO4without inhibitor. It is very well see that the EDX spectrum in 2 M H3PO4medium displays the characteristic peaks of certain elements contained in the chemical composition of the surface of the electrode. Moreover, the addition of the CWP inhibitor to the aggressive medium has visibly decreased the corrosive attack of the carbon steel as present in the SEM image (Fig. 8b). Hence, this prevents the formation of corrosion products that causes the deterioration of the steel surface. Additionally, EDX graph (Fig. 8b) proclaims that the existence of CWP inhibitor leads to ap- pearance of the characteristic peak of oxygen and carbon, which are exist in the chemical composition of the CWP inhibitor. Furthermore, the Fe peaks are significantly suppressed relative to uninhibited steel surface sample. These Fe lines occur because of the overlying inhibitor film on the steel surface[9].

3.6. Explanation for inhibition

The extracted polymer of crab waste protein (CWP) is composed of several naturally amino acids. Likewise, the inhibition mechanism of CWP could be revealed to the adsorption of its molecules on the metal surface.

•

Firstly, the protonated molecules of amino acids were physically adsorbed on the negatively charged carbon steel surface in the acidic solution of phosphoric acid (Fig. 9a).

•

The physical adsorption can also, happen indirectly between the protonated molecules of amino acids and the already adsorbed an- ions in the solution at the positively charged metal surface (Fig. 9b).

•

Furthermore, the amino acid molecules could adsorb chemically through the unshared electron pairs of heteroatoms (i.e. O, N, and S) and/or theπ-electrons of the aromatic ring of amino acid to the vacant d-orbital of surface metal atoms (Fig. 9c).

•

On the other side, the functional groups of amino acids may form an insoluble complex on the metal surface (Fig. 9d), this mode has been

Fig. 8.SEM/EDX image of the carbon steel after 12 h of immersion in 2 M H3PO4(a) without and (b) with CWP inhibitor.

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reported for cysteine on copper [74,75].

4. Conclusions

CWP shows high inhibition performance for the corrosion of carbon steel in 2 M H3PO4at 298 K, and the inhibition efficiency,η(%), in- crements with an expansion in the inhibitor content. Theη(%) of CWP is found to diminish relatively with rising temperature (298–333 K) and its expansion to 2 M H3PO4promots an increment of evident activation energy (Ea) of the corrosion mechanism. The corrosion procedure is stifled by the adsorbed molecules of CWP on the carbon steel surface and Langmuir isotherm model was accomplished the morefit isotherm.

The obtained value ofΔG_ads^o reveals that the inhibitor molecules of CWP adsorbed on the carbon steel surface in 2 M H3PO4 medium funda- mentally by electrostatic-adsorption. As indicated by the potentiodynamic polarization results, CWP can be classiﬁed as a mixed inhibitor.

The EIS spectra are well described by the proposed structural models.

The weight loss, polarization, and EIS procedure are in appropriate manner. The MD simulations reveal that three amino acids molecules adsorb on the carbon steel surface in the planar orientation with higher negative interaction energy, which is in accordance with the experi- mentally observed inhibition eﬃciency.

Declaration of Competing Interest

All authors have participated in (a) conception and design, or analysis and interpretation of the data; (b) drafting the article or re- vising it critically for important intellectual content; and (c) approval of theﬁnal version.

This manuscript has been submitted, is under review, another journal or other publishing venue.

The authors have no aﬃliation with any organization with a direct or indirect ﬁnancial interest in the subject matter discussed in the manuscript.

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