Low cost aqueous extract of Pisum sativum peels for inhibition of mild steel corrosion

Accepted Manuscript Low cost aqueous extract of Pisum sativum peels for inhibition of mild steel corrosion

Monika Srivastava, Preeti Tiwari, S.K. Srivastava, Ashish Kumar, Gopal Ji, Rajiv Prakash PII: DOI: Reference:

S0167-7322(17)33927-2 https://doi.org/10.1016/j.molliq.2018.01.137 MOLLIQ 8591

To appear in:

Journal of Molecular Liquids

Received date: Revised date: Accepted date:

26 August 2017 30 December 2017 24 January 2018

Please cite this article as: Monika Srivastava, Preeti Tiwari, S.K. Srivastava, Ashish Kumar, Gopal Ji, Rajiv Prakash , Low cost aqueous extract of Pisum sativum peels for inhibition of mild steel corrosion. The address for the corresponding author was captured as affiliation for all authors. Please check if appropriate. Molliq(2017), https://doi.org/ 10.1016/j.molliq.2018.01.137

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ACCEPTED MANUSCRIPT Low Cost Aqueous Extract of Pisum Sativum Peels for Inhibition of Mild Steel Corrosion Monika Srivastava†, Preeti Tiwari†#, S. K. Srivastava¡, Ashish Kumar†, Gopal Ji ‡, Rajiv Prakash†* †

School of Materials Science and Technology, Indian Institute of Technology (Banaras Hindu University), Varanasi, India ‡

Department of Physics, Institute of Science, Banaras Hindu University, Varanasi, India Department of Materials and Chemistry, Research Group Electrochemical and Surface

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¡

#

First two authors are having equal contributions.

Abstract

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*Corresponding Authors Email: [email protected]

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Engineering, Vrije University of Brussels, Pleinlaan 2, 1050 Brussels, Belgium

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This work investigates the potential of Pisum Sativum (green pea) peels for the inhibition of

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mild steel corrosion in hydrochloric acid (HCl) at varied temperatures. Various techniques, like weight loss measurements, tafel polarization curves, electrochemical impedance

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spectroscopy (EIS), UV-visible and Fourier transform infrared spectroscopy (FT-IR),

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scanning electron microscopy (SEM) and atomic force microscopy (AFM), have been used to test the inhibition properties of aqueous Pisum Sativum peels extract (APSPE) in acid media.

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The results indicate that APSPE effectively inhibits the corrosion of mild steel by covering the active corrosion sites on the mild steel surface and thus lowers the electrochemical activity of the surface exposed in HCl solution. The maximum inhibition efficiencies of APSPE system at 400 mgL-1 concentration in 1M HCl are reported as: 91%, weight loss; 87% polarization curves; and 90% by EIS. The corrosion behavior of mild steel in presence of APSPE has also been investigated in the range of 1 to 4 M HCl at room temperature as well as at varied temperatures (303-333 K) in 1 M HCl. The process of mild steel corrosion inhibition in HCl by APSPE has been explained by ion chromatography analysis and DFT calculations.

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ACCEPTED MANUSCRIPT Keywords: Corrosion; Mild steel; Pisum Sativum; Peel Extract; EIS; DFT; Ion Chromatography. 1. Introduction The natural organic products like plants, vegetables and fruits are great sources of bio active molecules. The richness of the electro active molecules at a low price as well as zero toxicity

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has made the natural products as the most preferred choice of the researchers for the

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corrosion inhibition applications [1-7]. There are a number of reports on the use of natural plant products, which can be used as effective corrosion inhibitors for mild steel or metallic

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surface in acidic medium. In these reports, a common reason for high corrosion inhibition

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efficiency for the natural organic products is given as the capability of the bioactive molecules to make bonds with the metals [8-10]. Due to bond formations, surfaces of the

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metals exposed in acid media are blanketed by the inhibitor molecules; which lowers the rate of the electrochemical reactions occurring at the metal surface. Hence, the selection of a

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natural organic product as a corrosion inhibitor should be done on the basis of the content of

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bioactive molecules as well as cost and availability [11-12]. In this connection plant products

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are considered as potential corrosion inhibitors for protecting the surface of metals from acids. In fact, the first patented corrosion inhibitor employed was a natural plant product. Further, the utilization of plant extracts derived from peels or fruit seeds as corrosion inhibitor is of choice in developing countries where it is a tedious job to develop inhibitor formulations because of lack of proper resources and well established chemical industry. Mostly, the edible part of the fruits and vegetables are consumed by the individuals as well as food and agro industries. The skin or peel of these natural products are not consumed in any case and becomes a waste material. The disposal of the waste peels is a challenging task and requires proper planning for effective handling. On the other hand, there are some reports which claim that the peels of some fruits and vegetables contain more nutrients than that of edible part. In such cases, the use of peels for corrosion inhibition applications is logical and 2

ACCEPTED MANUSCRIPT economical as well as eco-friendly [13-19]. Pea is a lignocellulosic plant and belongs to the leguminosae family. Pea is grown in many parts of the world at large scale for competing with the demand of a low cost protein source for humans as well as animals (pea peels). The main phyto-chemical constituents of pea peels are lignin and cellulose. Lignins are the organic long chain biopolymers as well as aromatic and hydrophobic in nature; while

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celluloses are rich in hydroxyl ions. The literature suggests that the aromatic compounds

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containing heteroatoms in the structures have high possibility to act as good corrosion

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inhibitors [20-21]. Hence, we have selected pea peels waste for our present inhibition studies of corrosion on the ground of effective chemical constituents and low cost.

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The key purpose of the present work is set to enquire the inhibition potential of APSPE against the corrosion of mild steel surface in HCl solutions. Results of various experiments in

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this work, to serve this purpose, favor the fact that APSPE can be used as an effective

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corrosion inhibitor for the protection of mild steel surface in HCl solutions.

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2. Experimental details 2.1 Preparation of the extract

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The green pea (Pisum sativum) was bought from market in our city. The peels were removed from the pea and thoroughly rinsed with tap water. Afterwards, the peels were dried in an oven at 310 K for 48 hrs. Thus, obtained peels were powdered in a mixer. 5 gm amount of this powder was soaked in distilled water of 500 mL volume and then kept overnight on mild stirring. This solution was filtered and the residues were collected. The filtered solution was used as a main stock of the extract, and different concentrations of the extract in acid solution were prepared by using an appropriate amount of the extract from the stock solution. The residues obtained after filtration were dehumidified and weighed for knowing the exact concentration of extract. The concentration of the main stock solution was determined by

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ACCEPTED MANUSCRIPT deducting the weight of the residues from 5 gm (initial amount). The concentration of the stock solution was calculated as100 gL-1. 2.2 Preparation of test specimen The mild steel samples of 5 cm × 1 cm were cut from a big mild steel rectangular plate. The samples were first manually abraded by emery paper of grade 1/0 (1600 sianor b,

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Switzerland) for 10 minutes. Further, the samples were ground sequentially with the emery

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paper of 2/0 to 5/0 grades (each 5 minutes). At the end, the surface finishing of the samples

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was done by 6/0 grade emery paper. As prepared mild steel samples were washed with AR grade acetone and dried with tissue paper. These samples were utilized as test samples for the

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corrosion experiments. The elemental composition of used mild steel in this work was (Wt %): C-0.15, Mn-0.030, Si-0.18, S-0.024, P-0.03 and rest of Fe [22].

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2.3 Weight loss measurements

For weight loss measurements, the size of the test samples was selected as 5 cm × 1 cm. To

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measure the weight loss due to corrosion, the samples were dipped in 1 M HCl having

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different concentrations of the extract for 3 hrs at room temperature (298 ±2 Kelvin) and

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weighed by an electronic balance (Mettler Toledo, least count ±0.1 mg) before and after immersion. Weight loss inhibition efficiency (ηwl) and surface coverage (θ) were calculated by using formula as: 𝜂𝑤𝑙 % = 𝜃=

𝑊0 −𝑊𝑖

𝑊0 −𝑊𝑖 𝑊0

𝑊0

× 100

(1) (2)

Where Wi and W0 indicated weight loss in acid solution with and without inhibitor, respectively. Accordingly, the corrosion rates (Cr) could be extracted by weight loss measurements by the following equation: 𝐶𝑟 (𝑚𝑚𝑝𝑦) =

87.6𝑊 𝐴𝑡𝑑

(3)

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ACCEPTED MANUSCRIPT Where, Cr is the rate of corrosion in milli meter per year (mmpy), W/A was used for weight loss of test samples per unit area (mg/cm2) in time t (hours), while d could be taken as 7.85 g cm-3 (density of mild steel). 2.4 Electrochemical measurements The electrochemical measurements on the test samples (1 cm × 1 cm, one side) were

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performed in a three necked glass cell (volume 100 mL) with the help of a CH 7041C

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electrochemical workstation, made in the USA. A three electrode setup: Ag/ AgCl, reference

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electrode; mild steel test samples as working electrodes and platinum foil as a counter electrode; was established for the electrochemical measurements. The test samples were

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immersed in the glass cell having 1 M HCl at different concentrations of the extract, and

behavior of the mild steel electrodes.

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experiments were conducted after 15 minutes immersion to compensate the non stationary

To record Tafel curves, the mild steel electrode was polarized with an excitation signal of 5

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mV amplitude in the potential range of ±250 mV vs. Ag /AgCl at 0.5 mVs-1 scan rate with

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respect to open circuit potential (OCP). For EIS curves, a 5 mV AC signal was applied to the

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electrochemical system at OCP in the range of 10 MHz-100 kHz. The curves obtained by Tafel polarization and EIS test was fitted with CHI 7041C software and ZSimpWin 3.21 software. The different electrochemical corrosion inhibition efficiencies, viz., Polarization curves ηp, polarization resistance ηPR and charge transfer resistance ηRt), were determined from the following equations [11]: 𝜂𝑃 (%) =

0 𝑖 𝑖𝑐𝑜𝑟𝑟 −𝑖𝑐𝑜𝑟𝑟

𝜂𝑃𝑅 (%) = 𝜂𝑅𝑡 (%) =

0 𝑖𝑐𝑜𝑟𝑟

𝑃𝑅 𝑖 −𝑃𝑅 0 𝑃𝑅 𝑖 𝑅𝑡𝑖 −𝑅𝑡0 𝑅𝑡𝑖

𝑋 100

(4)

𝑋 100

(5)

𝑋 100

(6)

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ACCEPTED MANUSCRIPT where iocorr and iicorr indicated corrosion current densities in 1M HCl without and with APSPE. Similarly, PR0 and R0t were used for the polarization resistance and charge transfer resistance without the extract in acid solution; while PRi and Rit indicated the resistances in presence of APSPE extract in 1 M HCl. 2.5 Characterization of the extract and surface analysis

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UV-visible spectrum of the extract was recorded with a Biotek spectrophotometer (Epoch2,

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USA). An amount of 25 µL of APSPE was soaked in 2 mL double distilled water and thus

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prepared solution was analyzed from 200 - 900 nm wavelength range. FT-IR spectrum of APSPE was obtained by a Nicolet 6700, Thermo scientific instrument using special scanning

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head for analyzing liquid samples. The pure extract was scanned from 450 - 4000 cm-1 wave

using the available standard database.

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numbers; and different peaks of the spectrum were assigned to a particular functional group

The surface images of polished, inhibited and uninhibited mild steel samples were captured

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by Carl Zeiss microscope (model SUPRA 40, made in Germany). AFM analysis on the mild

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steel samples was done with the help of NT-MDT multimode microscope (made in Russia)

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in semi contact mode. The mild steel samples of 1 cm2 surface area were dipped in 1 M HCl without and with 400 mg L-1 APSPE for 3hr. Afterwards, the samples were taken out from the acid solutions and washed with distilled water, then dried with tissue paper. The samples were put in an oven at 303 K for 5 minutes to remove the moisture completely. Thus prepared test samples were used for SEM and AFM study. 2.6 Ion chromatography (IC) As a result of anodic corrosion of mild steel, iron could go in the solution either in ionic state or in the form of corrosion products. In the case, inhibition of corrosion could lower the amount of iron (ion concentration) in the test solution. Hence, it was logical to investigate the amount of iron in inhibited and uninhibited acid solutions. The concentration of iron was determined by Compact IC flex 930 Metrohm (made in Switzerland) equipped with UV 6

ACCEPTED MANUSCRIPT visible detector and nucleosil 5SA (Metrohm) column. Prior to run the experiments, the whole system was calibrated with the standard solution of ammonium iron (II) sulphate. The mild steel samples were dipped in 100 mL of 1 M HCl in absence and presence of the extract (400 mg L-1). After 5 hrs, the samples were drawn out from the solution. This solution was investigated for Fe2+ ions concentration alone and after the addition of various concentration

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of 1.0 M ascorbic acid in the solution. The purpose of ascorbic acid addition in the solution

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was to convert Fe3+ ions to Fe2+ ions present in solution.

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3. Results and Discussion

3.1 Theoretical Investigation of Corrosion Inhibition Process

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As per literature survey, lignin and cellulose are reported as the key phyto-chemical constituents of the pea peels [23]. Hence, it is logical to assume that inhibition potential of

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APSPE will depend on the inhibition performance of these two compounds. The theoretical investigation of the corrosion inhibitors is a very useful and highly favoured method now-a-

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days because it takes less time and provides accurate information about the inhibition

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characteristics of the inhibitors. Hence, before performing any experiment, the potential of

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APSPE was checked by density functional theory (DFT) calculations. First, the structure of possible phyto-chemical constituents was optimized by B3LYP using Basis Set 6-31 G (d, p). Then, various quantum chemical parameters, viz., EHOMO, ELUMO and dipole moment (μ), were determined by B3LYP / 6-31 G (d, p) level of theory in water in order to get band gaps theoretically. Molecular orbital (MO) picture is produced by generating the MOs by population analysis with 0.02 isovalue using Gauss View. Further, highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) were together named as frontier molecular orbital because they are forming the outermost boundary of the electrons of the present system. Further, the values of HOMO and LUMO were obtained by using Gaussian 09 programme where as HOMO, LUMO were plotted by Gauss view. The optimized structure of the compound for HOMO and LUMO are displayed in Figure1. 7

ACCEPTED MANUSCRIPT The ionization potential (I) and electron affinity (A) of the inhibitor molecules (lignin and cellulose) could be related with EHOMO and ELUMO by Koopmans theorem [24]. The A and I could be given as -ELUMO and -EHOMO, respectively. The other parameters, like absolute hardness (η), softness (σ) and absolute electronegativity (χ), was determined by the following

𝐼−𝐴 2 1 𝜂

(7)

(8)

(9)

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𝜎=

2

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𝜂=

𝐼+𝐴

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𝜒=

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equations:

Figure 1. Optimized structure, HOMO and LUMO of (a-c) lignin and (d-f) cellulose in water, where grey, white and red balls show C, H and O atoms respectively. (For colored references in the figure, reader is referred to the web version of the article) 8

ACCEPTED MANUSCRIPT Table 1. Quantum chemical parameters for inhibitor calculated from DFT calculations Inhibitor EHOMO(eV) Molecules Lignin -4.993

ELUMO(eV) ΔE (eV) μ (D)

η (eV)

σ (eV-1) χ (eV)

-1.139

3.854

6.518

1.927

0.518

3.066

Cellulose

0.971

7.647

10.187 3.825

0.261

2.852

-6.676

According to the quantum chemical theory, the adsorption behaviour of the inhibitor

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molecules on the surface of mild steel occurs because of donor-acceptor electronic

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interactions between them. An ability of a good electron donor and acceptor is characterized

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by the EHOMO and ELUMO values. High EHOMO values correspond to a good electron donor and low ELUMO values indicate a good electron acceptor. However, the inhibition efficiency of an

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inhibitor cannot be investigated on the basis of EHOMO and ELUMO values only. The difference between HOMO and LUMO energy levels (ΔE) is also an important factor to judge the

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potential of an inhibitor. It is believed that low ΔE values are associated with chemical reactivity of the molecules and hence low ΔE values are preferred for higher inhibition

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efficiencies. Dipole moment (μ) is a structural parameter and shows the polarization ability of

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the molecules. As per the reports, the high value of μ leads to the high inhibition efficiency

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because greater adsorption of inhibitor molecules on the metal surface can block more active surface sites. The reactivity and stability of the inhibitor molecules are known by the parameters such as, η, σ and χ. The softness is related to the chemical reactivity of a molecule. High σ values (low η value) correspond to low ΔE values and indicate high reactivity of a molecule as the soft molecule can easily make bonds with the metal surface. High electro negativity helps in adsorption of the molecules on metal surface. Overall, it can be stated that low hardness, high softness, high electro negativity, high dipole moment and low ΔE value is required for high inhibition efficiency. Table 1 lists the quantum chemical parameters obtained by DFT and mathematical calculations. A careful analysis of Table 1 revealed that lignin shows lesser ΔE values, lower hardness, greater softness and higher electro negativity than cellulose. This fact suggests that 9

ACCEPTED MANUSCRIPT lignin will contribute more than cellulose in inhibition process of mild steel by APSPE in 1 M

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HCl.

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Figure 2. Schematic illustration of mild steel-Inhibitors (lignin and cellulose) interactions. Prior to reaching any final conclusion, it was necessary to check the interactions between

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mild steel and lignin/cellulose. As explained earlier, HOMO and LUMO represents the electron donation and electron acceptance property of the molecule respectively. Hence,

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differences between EHOMO of lignin/cellulose and ELUMO of iron (E1) and vice versa (E2)

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were determined and shown in Figure 2. A fast analysis of Figure 2 revealed that E2 was

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greater than E1 for both lignin and cellulose, which indicated that both lignin and cellulose were better electron donors than electron acceptors. Hence, corrosion inhibition ability of lignin and cellulose could be compared on the basis of electron donating ability. Further analysis of Figure 2 disclosed that E1lignin-Iron was lesser than E1cellulose-Iron; and E2Iron-cellulose was greater than E2Iron-lignin. Both the facts suggested that the inhibition ability of APSPE will be primarily controlled by lignin. The bonds between APSPE and iron can be formed via sharing of the electrons donated by lignin and cellulose; however, lignin will play a vital role in that. The bond formation through back donation (electron donated by iron) is also possible; especially in lignin as E1 is very close to E2. Overall, the preliminary examination of APSPE by DFT calculations has proposed that APSPE can act as corrosion inhibitor for mild steel. 10

ACCEPTED MANUSCRIPT The observed inhibition ability of APSPE will depend on the electro activity of lignin molecules. However, the real situation may change slightly because the calculations based on DFT have been executed with the molecules in their neutral states. 3.2 Characterization of the prepared extract The FT-IR spectrum of the aqueous extract of the Pisum Sativum peels is shown in Figure 3.

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The composition details of the extract could be obtained through the different peaks (3350

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cm-1, 1075cm-1, 1638 cm-1, 2361 cm-1) in the IR spectrum as shown in Figure 3. From the

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literature survey, it is known that lignin and cellulose are the major constituents of the pea peels [23]. Accordingly, the presence of the constituents could be identified from FT-IR

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spectra. Confirmation of lignin in the extract could be detected from an intense and wide absorption peak at 3350 cm-1, which could be received for the –OH stretching vibrations of

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the hydroxyls groups in guaiacyl or syringyl monomeric unit of lignin [25-27]. The peak at 1638 cm-1could be assigned to –C=O stretching vibration of alpha-keto carbonyl of cellulosic

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structure. The absorption band at 1075 cm-1could be ascribed to –C-OH stretching vibration

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of the cellulosic backbone [28-30]. The spectral band at 1152 cm-1could corresponds to C-O-

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C anti symmetric bridge stretching vibration found in cellulose and hemicelluloses.

Figure 3. FT-IR spectrum of APSPE

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ACCEPTED MANUSCRIPT In the UV-visible spectrum, APSPE exhibited the major absorption bands at 269 nm, 304 nm and 362 nm as shown in Figure 4. The peak at 269 nm could be attributed to n-σ* electronic transitions, which could indicate the presence of heteroatom’s (oxygen) in the lignin cellulosic backbone. The absorption band at 304 nm could be correlated with the n-π* transition; while the band at 362 nm was detected due to π-π* transition. By the investigation

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of FT-IR and UV-visible spectroscopic analysis, it could be predicted that the main phyto-

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chemical constituents of the extract were lignin and cellulose, which was in accordance with

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the other reports and show similar properties to the efficient and proved inhibitors. XRD

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analysis of the extract showed crysatalline nature of the extract molecules.

Figure 4. UV-visible spectrum of APAPE 3.3 Weight loss measurements

Before performing the electrochemical experiments, the inhibition potential of the APSPE was first checked by weight loss measurements at different concentrations of inhibitor in 1 M HCl. The results, as shown in Figure 5, disclosed that the rate of corrosion for mild steel was greatly affected by the presence of the extracts in acid solutions. The corrosion rates (Cr) were retarded in accordance with the increase in APSPE concentration in the solution. A 10 fold decrease in Cr value was acknowledged at the maximum inhibitor concentration used in this study. The corrosion rate at 0 mgL-1 extract concentration was as 31.9 mmpy, while it 12

ACCEPTED MANUSCRIPT was 3.16 mmpy at 400 mg L-1 of APSPE. The effectiveness of the extract could also be seen in the values of inhibition efficiency at different inhibitor concentrations. Near about 91 % inhibition efficiency was achieved using 400 mg L-1of APSPE in 1M HCl. The increase in inhibition efficiency and the decrease in corrosion rates with the increase in concentration of inhibitor could be described on account of surface coverage values. Inhibitor molecules could

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be adsorbed on the surface of mild steel and thus covered a certain area of the exposed

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electrode in HCl. By increasing the inhibitor concentration, the surface coverage was greatly

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increased due to the availability of more inhibitor molecules to be adsorbed on its surface. Hence, higher corrosion inhibition was acknowledged with increasing the concentration of

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APSPE.

Figure 5. The corrosion rates and inhibition efficiencies at various concentrations of APSPE. It was clear through the discussions made above that the effectiveness of the extract corresponded to the adsorption of the inhibitor molecules on the mild steel surface. Hence, it was

necessary to study the adsorption behavior of APSPE in 1 M HCl and quantify the

degree of adsorption. To find an equilibrium adsorption-desorption coefficient (Kads) and adsorption mechanism, the surface coverage values were fitted according to the three popular isotherm models, viz., Langmuir, Frumkin, and Temkin. However, the best fit was achieved using the Langmuir isotherm model (Figure 6). The linear regression coefficient of the fitting 13

ACCEPTED MANUSCRIPT was close to 1 (higher than Frumkin and Temkin), which clearly showed the quality of fitting and linear adsorption behavior of the inhibitor molecules. However, the slope value was considerably less than one; which was the artifact of either a strong interaction between adsorbate molecules at adjacent sites, or adsorption of inhibitor molecules at more than one sites. The value of Kads was obtained using the relationship among the inhibitor concentration

𝐶𝑖𝑛ℎ

=𝐾

1

𝑎𝑑𝑠

+ 𝐶𝑖𝑛ℎ

(10)

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𝜃

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Cinh, surface coverage θ and Kads as given below [31, 1]:

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Figure 6. The results of Langmuir isotherm fitting of APSPE adsorption on mild steel in 1 M HCl. 3.3.1 Effect of Acid Concentration The inhibition behavior of APSPE (400 mg L-1) with the changes in HCl concentration (1-4 M) was determined by the weight loss measurements technique at room temperature. A quick analysis of Figure 7 disclosed that inhibition efficiency of APSPE diminished with the rise in acid concentration. The linear regression coefficient (R2) was calculated as 0.9937, which suggested that corrosion inhibition efficiency of the inhibitor molecules decreased linearly with the acid concentration. There might be two cases for the decrease in inhibition efficiency; first, the adsorbed molecules/metal-inhibitor complexes were not stable at high

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ACCEPTED MANUSCRIPT acid concentration and hence ran off from the surface of mild steel; and second, the adsorption of molecules on the metallic surface was hindered by the excess chloride ions. To investigate the inhibitive results of APSPE on metallic corrosion with the alteration in acid concentration, a kinetic model was proposed by Mathur and Vasudevan [32]. This model relates the corrosion rates with various concentration of acid in the following manner: (11)

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ln Cr= ln k + BC

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where k denotes a constant and measure the rate of metals destruction occurred by acid and B

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is a reaction constant, which is used to quantify the alteration in corrosion behaviour of

Figure 7. APSPE.

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metallic surfaces with acid concentration.

Inhibition efficiency at different concentration of HCl with 400 mg L-1

According to the above mentioned kinetic model, a graph is plotted between ln Cr and acid concentration for both HCl and Inhibitor. The technical parameters obtained from the graph are shown in Figure 8 and Table 2. The linear regression coefficient (R2) of shown curves were close to 1, which suggested that the corrosion attributes of the mild steel electrode in presence and absence of inhibitor in different acid concentration could be explained by the kinetic model. Table 1 indicated that the k value for blank acid solutions was very high; however, a much lower value of k was achieved with the addition of 400 mg L-1 APSPE in acid solutions. This information meant that the destruction of mild steel surface caused by 15

ACCEPTED MANUSCRIPT acid was significantly retarded by APSPE. In addition, it was observed via investigation of Table 2 that the B value was higher for the acid solutions having the extract in comparison to that for blank acid solutions. This signified that the corrosion behaviour of mild steel was significantly enhanced in presence of inhibitor. Thus, both k and B values suggested that APSPE was effective in lowering of mild steel corrosion at higher acid concentration also;

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however, the inhibition ability linearly degraded with the rise in acid concentration.

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Figure 8. Straight line fitting of ln Cr versus acid concentration at RT with and without 400 mg L-1 APSPE. Table2. Technical parameters obtained from the linear fitting of the curve drawn between ln Cr and Cinh. Acid 1MHCl

Cinh (mg L-1) 0 400

k(mg L-1) 22.41 1.36

B(M-1) 0.402 1.007

3.3.2 Effect of temperature rise on inhibition potential In various industries, the corrosion inhibitors are used at higher temperatures than room temperature. Raising the experimental temperature can speed up the corrosion process and increase the corrosion rate significantly. Accordingly, the corrosion performance of the inhibitors can be affected with the increase in temperature. Hence, the corrosion inhibition behaviour of APSPE in 1 M HCl was analyzed by weight loss measurement technique at 16

ACCEPTED MANUSCRIPT various temperatures (30˚, 40˚, 50˚ and 60˚ C). The Arrhenius equation was applied to quantify the effect of temperature on corrosion rate in terms of activation energy and Arrhenius Factor (A) for the corrosion reactions [33]. The equation could be written as given below: ln 𝐶𝑟 = ln 𝐴 −

𝐸𝑎 𝑅𝑇

(12)

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In where Cr denotes the rate of metallic corrosion in acid, Ea indicates apparent activation

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energy at temperature T (Kelvin) and R is described as the universal gas constant (8.314 J K-1

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mol-1).

Figure 9. Arrhenius plots for mild steel in 1 M HCl using various concentrations of APSPE. Weight loss measurements were executed at different temperatures of the interest and the corrosion rates were calculated by the equation 3. Arrhenius plots for mild steel surface in absence and presence of different inhibitor concentrations were prepared by plotting ln Cr against 1000 /T as shown in Figure 9. The different technical parameters obtained by linear fitting of the Arrhenius curves are entered in Table 3. A careful analysis of Figure 9 disclosed that the addition of inhibitor in hydrochloric acid caused an increase in the slope and intercept values of the straight lines, which suggested that Ea and A values became higher than that in alone HCl [34]. This fact meant that the energy level of the corrosion process was raised by 17

ACCEPTED MANUSCRIPT APSPE molecules via molecular adsorption and hence corrosion on mild steel surface in 1 M HCl was slowed down. Furthermore, Table 3 revealed that the apparent activation energy values were positive for both inhibited and uninhibited solutions; and higher in the acid solutions having APSPE as compared to blank acid solutions. This information suggested that

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APSPE molecules were primarily bound to the mild steel surface via physisorption.

Ea (kJ mol-1) 33.50 56.20 67.26

400

2.57×1014

78.48

Δ H* (kJ mol-1) 30.92 53.62 64.67

Δ S* Ea - Δ H -1 -1 (J mol K ) (kJ mol-1) -108.33 2.58 -41.73 2.58 -9.56 2.59

75.90

29.76

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A (mmpy) 3.66×107 1.08×1011 5.10×1012

2.58

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1 M HCl

Conc. (mg L-1) Blank 200 300

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Acid

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Table 3. Activation and thermodynamic parameters of mild steel in 1 M HCl without and with various APSPE concentrations.

Figure 10.Transition state plots for mild steel in 1 M HCl with various APSPE concentrations. To have insights into the global interaction of APSPE molecules with the surface of mild steel at elevated temperatures, transition state plots were prepared by straight line fitting between Cr/T and 1000/T and shown in Figure 10. The fitting equation could be described as given below:

𝑙𝑛

𝐶𝑟 𝑇

𝑅

= (𝑙𝑛 𝑁ℎ +

∆𝑆 ∗ 𝑅

)−

∆𝐻 ∗ 𝑅𝑇

(13) 18

ACCEPTED MANUSCRIPT Where ∆𝐻∗ denotes change in enthalpy of activation, ∆𝑆 ∗indicates entropy of activation, N is Avogadro’s number and h is Planck’s constant. A quick analysis of Figure 10 evidence that the slope and intercept values were increasing with increase in inhibition concentration, which was similar to the facts revealed by Arrhenius plots. The higher values of ∆𝐻 ∗and ∆𝑆 ∗for inhibited solutions in comparison to the blank HCl solution indicated that the

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energy level of the corrosion reactions was raised by the inhibitor molecules. In other words,

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it could be said that the Fermi energy level of the mild steel surface became higher due to

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adsorbed inhibitor molecules. Hence, lower corrosion rates were acknowledged for the mild steel electrodes that were immersed in acid solutions containing APSPE molecules. The

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lower dissolution of metal in inhibited solutions, evidenced by positive and higher ∆𝐻∗ values, could also be one of the prime reasons for the acknowledgement of lower corrosion rates in

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case of inhibited electrodes. Furthermore, Table 3 confirmed that the relation between Ea and ∆𝐻 ∗ values was in accordance with the literature, i.e., approximately constant differences

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between them for both inhibited and blank acid solutions. This information showed the high

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reliability of the weight loss measurements conducted at different temperatures.

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Further analysis of the thermodynamic parameters provided the fact that the ∆𝑆 ∗ values became more positive with the amount of APSPE in 1 M HCl. These facts indicated that randomness of metal-inhibitor interface enhanced in presence of APSPE. In blank acid solution, the exposed surface of mild steel was freely available to the corrosive molecules. However, the area was significantly decreased due to adsorbed inhibitor molecules; which restricted the ion exchange and hydrogen reduction at the metal inhibitor interface. Due to the changed surface conditions, the system was forced to undergo from a more ordered to a random arrangement. Therefore, the entropy of the system was increased with the inhibitor concentration. The randomness of the system could also be increased due to the transformation of the reactants into activated complexes and specific adsorption of these 19

ACCEPTED MANUSCRIPT complexes on the mild steel surface. The free activation energy ΔG* was calculated by the following equation: ΔG* = ΔH* - TΔS*

(14)

Table 4. The free activation energy Δ G* and the free energy of adsorption Δ G0 obtained at

Δ G*(kJ mol-1)

Conc. (mg L-1)

313K 64.82 66.68 67.66

323K 65.91 67.09 67.75

400

66.88

66.58

66.28

65.98

333

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Blank 200 300

303K 63.74 66.26 67.56

-19.71

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1 M HCl

333K 66.99 67.51 67.85

Δ G0 (kJ mol ) T (K) Value 303 -21.92 313 -20.68 323 -20.08 -1

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Acid

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different temperatures.

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The ΔG* values were determined at each experimental temperature and listed in Table 4. The ΔG* values were positive and increased with the temperature rise for both blank acid

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solutions and inhibited solutions. This information proposed that the activated complexes lost

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their stability with the rise in temperature and probability of their formation was also partly inhibited. On the other hand, ΔG* values for the inhibited solutions at a particular

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temperature indicated that the stability of activated corrosion complexes went lesser with the increase in inhibitor concentration. The standard adsorption energy ΔG0 at different temperatures were calculated by the equation 12 and listed in Table 4.

𝐾𝑎𝑑𝑠 =

𝑒𝑥𝑝

∆𝐺0 (− ) 𝑅𝑇

𝐶𝑠𝑜𝑙𝑣𝑒𝑛𝑡

(15)

Where Csolvent is used for the concentration of water in solution (1×106 mg L-1), R stands for a gas constant (8.314 JK-1mol-1) and T is the absolute temperature at which the experiment was conducted. The Kads values were acquired by fitting the weight loss data at various temperatures and shown in supporting information (Figure S2). The Kads values decreased 20

ACCEPTED MANUSCRIPT with the rise in experimental temperature, which indicated either absorption rate of inhibitor molecules lowered or desorption rate of inhibitor molecules from the surface accelerated with the temperature rise. The ΔG0 value varied between -20 kJ mol-1 and -40 kJ mol-1but close to 20 kJ mol-1, which advocated that APSPE molecules were adsorbed on mid steel mainly via

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physisorption and partially by chemisorption.

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3.4 Tafel polarization curves

Figure 11.Tafel polarization curves for mid steel at various concentrations of APSPE in

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1 M HCl at RT.

The behaviour of mild steel against polarization of the surface in 1 M HCl with different concentrations of APSPE is shown in Figure 11. The trend of polarization curves suggested that addition of inhibitor in low amount in hydrochloric acid solution shifted the curves towards the lower current region in anodic potential direction, with respect to the blank acid solution. However, the higher concentration of inhibitor in HCl caused shifting of the curves towards more cathodic potential direction and lowered the corrosion current significantly. Thus APSPE influenced both cathodic and anodic reactions, which could also be visualized in Figure 11 via changes in the shape of Tafel polarization curves. However, a careful analysis of the curves disclosed that the inhibitor worked more prominently on cathodic reactions in comparison to anodic reactions. Hence APSPE 21

ACCEPTED MANUSCRIPT provided mixed type inhibition to mild steel electrode in HCl and retarded corrosion rate majorly by inhibiting cathodic corrosion reactions. The various technical parameters related to the kinetics of mild steel corrosion, viz. Ecorr, icorr, slopes of anodic and cathodic polarization curves (βa and βc) and corrosion inhibition efficiency ηp, were obtained by the fitting of Tafel polarization curves with CHI 7041C

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software and listed in Table 5. The trend of the changes in Ecorr values favored the facts

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disclosed by the preliminary analysis of the polarization curves. First, Ecorr of mild steel

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moved in an anodic potential direction at lower inhibitor concentration and then shifted in a cathodic potential direction at higher inhibitor concentration with the respect to the Ecorr of

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the uninhibited electrode. The range of the potential change was within 85 mV of the potential of bare mild steel electrode (ΔEcorr=47 mV), which confirmed that APSPE worked

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as mixed type inhibitor for corrosion on mild steel in 1 M HCl [35, 36]. Furthermore, Table 5 revealed that βc values were higher than βa values at all inhibitor concentrations. In addition,

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the change in βc values with the inhibitor concentration was greater than the changes in βa

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values. Both the facts mentioned the same information that APSPE was more effective in

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inhibition of cathodic reactions (hydrogen reduction) than anodic reactions (mild steel dissolution).

The prediction of corrosion rates can be done only on the basis of corrosion currents. The higher corrosion current will give the higher corrosion rate and vice versa. A close investigation of Table 5 suggested that the corrosion current values were significantly dropped in presence of APSPE in acid solution. The trend of decrease in icorr values was in accordance with the increase in concentration of inhibitor. The maximum inhibition of mild steel corrosion was achieved with 400 mg L-1of APSPE in 1 M HCl. Further, the corrosion current could also be related to the intensity of the corrosion reactions occurring on the electrode surface. Thus, decrease in icorr could correspond to the suppression of cathodic and anodic reactions. This behaviour was manifested in the polarization resistance values. The PR 22

ACCEPTED MANUSCRIPT values of mild steel were higher in 1 M HCl in presence of inhibitor than its absence. This fact suggested that adsorption of inhibitor molecules on the active sites of mild steel surface were restricted the cathodic and anodic corrosion reactions. The extra barrier formed by APSPE molecules increased the resistance of the mild steel surface, which influenced the polarization resistance and corrosion current values. The inhibition efficiencies, i.e., ηp and

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ηPR, were close to each other and followed the same trend, which showed that the

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interpretation of mild steel corrosion behavior on the basis of icorr and PR could be considered

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valid and reliable.

Table 5. Corrosion parameters obtained from the polarization curves of mild steel at

βa (mV dec1 ) 96 85 84 79 86

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-448 -440 - 443 -466 - 495

icorr (µA cm2 ) 237 145 102 61 30

-βc (mV dec1 ) 196 222 161 163 125

ηp

PR (Ω cm2)

% 119 39 185 57 235 74 385 87 738

ηPR % 35 49 69 84

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Blank 100 200 300 400

Ecorr.(mV, Ag/AgCl)

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Conc. (mg L-1)

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different concentration of APSPE in 1 M HCl at RT.

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3.5 Impedance analysis by EIS

Figure 12 shows the response of mild steel electrode at open circuit potential in 1 M HCl in absence and presence of APSPE at room temperature. A quick analysis of the impedance curves disclosed that response of the mild steel-electrolyte system was originated from the resistive and capacitive elements of the system. The Nyquist plots illustrated that the response of the system was received in the form of a single semi-circle in absence as well as in presence of APSPE in 1 M HCl. This fact indicated that the corrosion as well as corrosion inhibition process was regulated by charge transfer reactions only (single time constant). Further analysis of Figure 12 revealed that the diameter of the semicircle; which corresponds to charge transfer resistance Rct in case of single time constant, was increasing with the amount of APSPE in acid solution. In addition, the response of the system obtained was not a perfect semi-circle (centre depressed under x axis in present case). This 23

ACCEPTED MANUSCRIPT behaviour of the mild steel electrode could correspond to the irregular current distribution on the mild steel surface, which made the system to behave as a constant phase element. This pseudo capacitive behaviour of the system could be originated from the irregularity of the exposed surface,

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arising from adsorption of APSPE molecules and corrosive molecules on the surface of mild steel.

Figure 12.Nyquist plots for mild steel with different concentration of APSPE in 1 M HCl at RT. Inset- Equivalent electrical circuit (EEC) used for fitting the impedance

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curves.

To mimic the corrosion behaviour of mild steel in acid solution at different concentration of

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APSPE, the impedance curves were fitted according to the EEC shown in inset of Figure 12. In this EEC, Rs is solution resistance and belong to a high frequency region. The behaviour of mild steel electrode at low frequency is represented by parallel combination of a constant phase element (CPE) and charge transfer resistance (Rct). Assuming normal distribution at the surface, the pseudo capacitance of CPE could be found out from equation [37]: 𝐶𝐶𝑃𝐸 = (𝑄 ∗ 𝑅1−𝛼 )1/𝛼

(16)

Where Q represents magnitude of CPE, R is resistance associated with the CPE and α measure the deviation of an electrode from ideal capacitive behaviour due to surface irregularities. The values

24

ACCEPTED MANUSCRIPT of various elements of EEC were determined by fitting the impedance curves. The values as well as relative model error (modulus of residual/experimental modulus) are listed in Table 6.

Table 6. The values of various element of EEC obtained by fitting the impedance plots for mild steel in 1 M HCl at various APSPE concentrations at RT.

300

ηRct %

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0.7195 (1.02%) 0.7559 (0.7%) 0.7914 (0.52%) 0.7785 (0.45%) 0.7769 (0.26%)

Q CCPE (10-6Ω-1sαcm-2) (µF cm-2) 386 63.56 (0.63%) 193 56.20 (0.36%) 121 41.80 (3.23%) 160 56.09 (2.59%) 151 58.80 (1.44%)

χ2

-

3.63 × 10-3

79.6

8.19 × 10-4

84.2

3.79 × 10-4

87.8

4.43 × 10-4

90.1

3.31 × 10-4

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400

α

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200

Rct (Ω cm2) 23.13 (0.96%) 113.73 (0.56%) 147.05 (1.23%) 186.89 (1.16%) 233.53 (0.67%)

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100

RS (Ωcm2) 2.23 (3.29%) 5.25 (1.91%) 2.07 (1.25%) 2.42 (1.03%) 3.40 (0.54%)

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Conc. (mgL-1) Blank

Figure 13. Bode modulus and bode phase plots for mild steel with different concentration of APSPE 1 M HCl at RT. The trends in Rct values revealed that the charge transfer resistance of mild steel electrode were increasing by the addition of APSPE in HCl in a successive manner, which could also be observed in Nyquist plots and bode modulus plots (Figure 12 and Figure 13) without data modelling. Such an increase in R values could correspond to higher potential drop across the 25

ACCEPTED MANUSCRIPT mild steel-HCl interface in presence of an extra protective layer of adsorbed inhibitor molecules originated at the interface. In other words, the corrosion reactions occurring at active sites of the surface were retarded by adsorbate; accordingly, the current originating from anodic and cathodic corrosion reactions was significantly dropped in presence of inhibitor in comparison to its absence. The decrease in current or increase in R ct values was

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continued up to the addition of 400 mg L-1 APSPE in 1 M HCl. Further analysis of Table 6

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disclosed that the capacitance values did not follow any trend. The CCPE values decreased up

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to the addition of 200 mg L-1 APSPE in HCl but slightly increased afterwards; however, the values were considerably lower in presence of inhibitor than its absence, in any case. So, it

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can be concluded that mild steel surface became either more compact or smoother with inhibitor molecules than in blank acid solution. An increase in capacitance of the electrode at

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higher inhibitor concentration could correspond to the irregularities of the surface, which was also documented by α and phase angle values (Figure 13).

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As per findings of the Tafel polarization curves, APSPE inhibited cathodic reactions more

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efficiently as compared to anodic reactions. Accordingly, an increase in inhibition efficiency

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with inhibitor concentration could correspond to the enlargement of blocked surface area; which restricted the hydrogen reduction on the surface. Also, an increase in Rct values depicted that the inhibitor molecules provided good barrier properties to the mild steel surface and retarded metal dissolution rate in 1 M HCl. Thus, the corrosion inhibition of mild steel in hydrochloric acid was achieved with APSPE. The errors associated with the determination of the element’s values were less than 5%, which showed the high reliability of the interpretation of the corrosion behavior of mild steel in acidic condition (1M HCl) in absence and presence of APSPE. 3.6 Surface analysis Figure 14 illustrates the changes observed in polished mild steel surface morphology after 3 hours immersion in 1 M HCl in absence and presence of 400 mg L-1 APSPE . Figure 14a 26

ACCEPTED MANUSCRIPT showed that surface of polished mild steel was not smooth everywhere and having small cracks on the surface that might act as a center of attraction for corrosion reactions to begin. During immersion of the electrode in blank HCl solution, the fast and aggressive corrosion reactions damaged the metal badly at several places; which could be documented by the highly rough surface as shown in Figure 14 b. However, the presence of inhibitor affected the

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corrosion of mild steel in 1 M HCl and slowed down the rate of corrosion reactions. As a

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result, less damage of mild steel surface was acknowledged in presence of inhibitor than its

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absence.

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a

Figure 14. Surface images of mild steel (a) polished (b) corroded in 1 M HCl and (c) inhibited by 400 mg L-1 of APSPE in 1 M HCl at RT. To quantify the damage in 1 M HCl in absence and presence of APSPE, the surface roughness of mild steel specimen was determined by AFM study. Figure 15 shows 2-

27

ACCEPTED MANUSCRIPT dimensional and 3-dimesional AFM images of mild steel and mild steel in absence and

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presence of inhibitor.

Figure 15. 2-dimensional and 3-dimensional topography images of mild steel surface (a) polished (b) corroded in 1 M HCl and (c) inhibited by 400 mg L -1 of APSPE in 1 M HCl at RT. Preliminary investigation of 2D AFM images revealed that the maximum surface roughness was in the order of 150 nm, 1 µm and 400 nm for polished, corroded and inhibited mild steel sample. This information clearly indicated that APSPE significantly mitigated the damage of mild steel surface in 1 M HCl, which was also portrayed by 3D AFM images. Furthermore, the height-profile diagram (Figure S3, supporting information) was obtained for AFM images 28

ACCEPTED MANUSCRIPT and corresponding surface roughness was determined (Table 7). The roughness of polished mild steel surface was increased to 17 (Sq) and 25 (Sa) times due to violent acid attack. However, the smoothness of the surface was considerably increased in presence of APSPE and the surface roughness was just 6 (Sq) or 8 (Sa) times of bare mild steel. The maximum peak to valley height (maximum roughness) also supported the facts revealed by RMS and

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average roughness.

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Table 7: Different parameters related to Surface roughness of mild steel sample obtained by height-profile diagram Sample RMS Average Peak-to-valley roughness roughness maximum Sq(nm) Sa (nm) Height St(nm) polished surface 9.32 5.16 174.22 in 1M HCl 157.36 127.87 1019.69 with inhibitor in 1M HCl 50.51 38.95 431.10

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3.7 Semi-quantitative analysis of Fe ions in solutions

In a process of steel corrosion in aqueous solutions, iron ions (anodic reaction) are dissolved in

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the solutions accompanied by the reduction of hydrogen (cathodic reaction) at the metal-acid

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interface. As per the results of Tafel test, APSPE lowers the rate of as well as (iron dissolution). Hence, it seems logical to check the quantity of either iron ions or hydrogen in the corrosive

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solutions. We have investigated the difference in the quantity of Fe ions without and with APSPE in 1 M HCl after 5 hours by Ion Chromatography (IC) technique. In general, the dissolution of iron in aqueous hydrochloric solution involves the transfer of two electrons. The transfer of electrons may take place via two different paths and it can be described by the chemical reactions (a-f) as written below. However, it is believed that iron dissolves through both paths simultaneously. [38-40]. (First path)In aqueous solutions: OH- ion controlled reaction Fe + H2O ↔ [FeOH]ads. + H+ + e-

(a)

[FeOH]ads.↔ [FeOH]+ads. + e-

(b)

[FeOH]

+

+

ads.+H

2+

↔Fe + H2O

(c) 29

ACCEPTED MANUSCRIPT (Second path)In aqueous solutions: Cl- ion controlled reaction Fe + H2O + Cl-↔ [FeClOH]-ads.+ H+ + e-

(d)

[FeClOH]-ads.↔ [FeClOH]ads. + e-

(e)

+

2+

-

[FeClOH]+ H ↔ Fe + Cl +H2O

(f)

Apart from these reactions, Fe2+can also be converted into Fe3+via oxygenation (equation g) with dissolved oxygen in presence of acidic media. The rate of this conversion as well as the

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stability of newly formed Fe3+ depends on the anions present in the solution [41, 42]. The

4Fe2+ + O2 + 4H+ ↔ 4Fe3+ + 2H2O

(g)

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concentration of Fe3+ ions in the acidic solutions by IC.

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conversion of Fe2+ into Fe3+is a very slow process; accordingly it is very difficult to detect the

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In order to quantify the effect of APSPE on iron dissolution, the quantity of Fe2+ions in 1 M HCl with and without 400 mg L-1 APSPE was calculated by IC technique. Figure S4

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(supporting information) illustrated that iron dissolution was significantly dropped in presence of inhibitor in acid solution in comparison to its absence. Near 71% reduction in the

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dissolution of iron (1.8gmL-1 to 0.52gmL-1) was acknowledged. The drop in Fe2+ ions could be

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attributed to the adsorption of inhibitor molecules, which restricted the formation of [FeOH]+

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and [FeClOH]- [43-45]. Moreover, no change in Fe2+concentrations was observed even after the addition of ascorbic acid at the either case of study. It meant that dissolved iron in Fe3+ form was either absent, or present in a very low concentration at given conditions (after 5 hrs of exposure).

4. Conclusions

The inhibition potential of Pisum Sativum (green pea) peels against mild steel corrosion was investigated by theoretical as well as experimental techniques. The preliminary examination of the inhibitor by DFT calculations suggested that the APSPE could be adsorbed on the surface mild steel via donor-acceptor interactions. Thus, APSPE could retard the mild steel corrosion. The similar facts were disclosed by the findings of weight loss measurements as well as electrochemical techniques, i.e., APSPE molecules could block the active cathodic and anodic 30

ACCEPTED MANUSCRIPT sites by being adsorbed on the surface of mild steel. The inhibitor was the most active at 400 mg L-1 in 1 M HCl with 91 % inhibition efficiency. Similar inhibition efficiency was reported by EIS (90%) and polarization curves (87%). The performance of inhibitor was also checked at different temperatures and in different acid concentration, which also revealed that APSPE could effectively retard the mild steel corrosion in HCl. However, the inhibition efficiency of

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APSPE was lowered with acid concentration and temperature rise. Adsorption of inhibitor

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molecules followed Langmuir adsorption theorem (R2>0.99) at all the studied temperatures.

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The morphological analysis confirmed that the inhibitor retarded the surface damage, significantly. The chromatography analysis revealed that APSPE could lower the mild steel

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dissolution in HCl. The free adsorption energy informed that APSPE adsorption on mid steel at different temperature was primarily controlled by physisorption process. The APSPE is

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extremely cheap, environmentally safe, easily synthesized and effective in mitigation of mild steel corrosion in HCl. Hence APSPE has great potential for being utilized as a potential

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Acknowledgements

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corrosion inhibitor for mild steel in the chloride environment.

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One of the author (MS) wants to acknowledge DST, New Delhi for providing support and CIFC, IIT (BHU) Varanasi for characterization of sample. First two authors are having equal contributions and carried out major part of experimental work. SKS is responsible for theoretical part and calculations (DFT) and AK is responsible for IC and analytical work. GJ is taken care for overall work and writing of the manuscript. Supporting Information Figure S1, XRD pattern of the extract; Figure S2, Kads values obtained at different temperatures; Figure S3, Height Profile diagram obtained from AFM for mild steel surface; Figure S4, Ion chromatography of the acid solutions alone and with inhibitor; Figure S5, OCPTime curves; and Table S1, the slope, regression coefficient and intercept values for Langmuir, Frumkin and Temkin isotherms. 31

ACCEPTED MANUSCRIPT 5. References

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1. G. Ji, S. Anjum, S. Sundaram, R. Prakash. Musa paradisica peel extract as green corrosion inhibitor for mild steel in HCl solution, Corros. Sci. 90 (2015) 107-117. 2. G. Ji, P. Dwivedi, S. Sundaram, R. Prakash. Inhibitive Effect of Chlorophytum borivilianum Root Extract on Mild Steel Corrosion in HCl and H2SO4 Solutions, Ind. Eng. Chem. Res. 52 (2013) 10673-10681. 3. A. Y. El-Etre, Inhibition of aluminum corrosion using Opuntia extract, Corros. Sci. 45 (2003) 2485-2495. 4. S. K. Shukla, M. A. Quraishi, R. Prakash, A self-doped conducting polymer polyanthranilic acid : An efficient corrosion inhibitor for mild steel in acidic solution, Corros. Sci. 50 (2008) 2867-2872. 5. G. Ji, S.K. Shukla, E. E. Ebenso. R. Prakash, Argemone mexicana Leaf Extract for Inhibition of Mild Steel Corrosion in Sulfuric Acid Solutions, Int. J. Electrochem. Sci. 8 (2013) 10878-10889. 6. S. A. Umoren, Z. M. Gasem, I. B. Obot, Electrochemical investigation of Irbesartan drug molecules as an inhibitor of mild steel corrosion in 1 M HCl and 0.5 M H2SO4 solutions, Ind. Eng. Chem. Res. 52 (2013) 14855-14865. 7. G. Ji, S.K. Shukla, P. Dwivedi, S. Sundaram, E. E. Ebenso, R. Prakash, Green Capsicum annuum Fruit Extract for Inhibition of Mild Steel Corrosion in Hydrochloric acid solution, Int. J. Electrochem.Sci. 7 (2012) 12146-12158. 8. D. K. Yadav, D.S. Chauhan, I. Ahamad, M.A. Quireshi, Electrochemical behavior of steel/acid interface: adsorption and inhibition effect of oligomeric aniline, RSC Adv. 3 (2013) 632-646. 9. K. S. Parikh, K .J. Joshi, Natural compounds onion, garlic and bitter gourd as corrosion inhibitors for mild steel in hydrochloric acid, Trans. SAEST 39 (2004) 29-35. 10. M. Lashgari, A.M. Malek, Fundamental studies of aluminium corrosion in acidic and basic environments: theoretical predictions and experimental observations, Electrochem. Acta. 55 (2010) 5253-5257. 11. E. E. Oguzie, K. L. Oguzie, C. O. Akalezi, I. O. Udeze, J. N. Ogbulie, V. O. Njoku. Natural products for materials protection: Corrosion and microbial growth inhibition using Capsicum frutescens biomass extracts, ACS Sustainable Chem. Eng. 1 (2), (2013) 214225. 12. O. K. Abiola, J.O.E. Otaigbe, O.J. Kio. Gossipium hirsutum L. extracts as green corrosion inhibitor for aluminum in NaOH solution, Corros. Sci. 51 (2009) 1879-1881. 13. Pereira, S.S.A.A.; Pegas, M.M.; Fernandez, T.L.; Magalhães, M.; Schontag, T.G.; Lago, D.C.; de Senna, L.F.; D’Elia, E. Inhibitory action of aqueous garlic peels extract on the corrosion of carbon steel in HCl solution.Corros. Sci. 2012, 65, 360-366. 14. Rafaela González-Montelongo, M. Gloria Lobo, Mónica González, Antioxidant activity in banana peel extracts: Testing extraction conditions and related bioactive compounds, Food Chem., 119 (2010) 1030-1039. 15. Shinichi Someya, Yumiko Yoshiki, Kazuyoshi Okubo, Antioxidant compounds from bananas (Musa Cavendish) Food Chem., 79 (2002) 351-354.

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ACCEPTED MANUSCRIPT Highlights

Waste Pisum Sativum peels are utilized in inhibition of mild steel corrosion loss.

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Mild steel loss can be inhibited with 91% efficiency by 400 mgL-1 of the extract.

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Effects of temperature rise and acid concentration are also monitored.

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Ion chromatography is used to support the electrochemical results.

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Pro inhibition behavior of the extract molecules is explained with DFT

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calculations.

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