Ixora coccinea extract as an efficient eco-friendly corrosion inhibitor in acidic media: Experimental and theoretical approach

The corrosion inhibition power of an eco-friendly green inhibitor Ixora coccinea extract (ICE) for mild steel in 1 M HCl and 0.5 M H 2 SO 4 was analysed using physicochemical and electrochemical techniques. 1–5 v/v% ICE was prepared and added into the corrosive acidic media for different analyses. Weight loss measurements conducted for a period of 24hrs for mild steel immersion revealed 89.38% and 77.96% inhibition capacity of ICE in 1 M HCl and 0.5 M H 2 SO 4 medium respectively. Electrochemical impedance parameters show that as concentration is increased, efficiency increases and double layer capacitance decreases. Potentiodynamic polarization techniques are also in good agreement with impedance studies and exhibit the mixed type inhibition character of ICE. Electrochemical noise spectrum also strongly supports the anti-corrosive property of ICE and the magnitude of noise signal decreases concerning the increase in ICE concentration. Ixorene, which is one of the major constituents of Ixora coccinea leaves was also studied for its corrosion inhibition nature by quantum mechanical calculations and was found to match with all other results. Adsorption studies of ICE are in accordance with Langmuir isotherm. Surface morphological studies confirm the formation of a protective barrier on mild steel surface in both media, i.e., 1 M HCl and 0.5 M H 2 SO 4 .


Introduction
In recent years, green chemistry has been gaining traction in the research world, owing to a demand for chemical technologies and commercial products that are less toxic and generate less waste. One of the promising domains, for the application of green chemistry principles, is the protection of metals from corrosion. Most of the industries work in an acidic environment which causes metal corrosion, leading to loss of metal. The best and favourable method to reduce metal degradation is the usage of corrosion inhibitors [1][2][3] . Organic compounds with heteroatoms like N, O, S, etc. in a conjugated system have been commonly used as inhibitors and they can interact with metal either by chemisorption or physisorption [4][5] . The adsorptive layer formed on the metal surface protects from the acidic solution and thereby minimizing corrosion. Due to eco-friendliness, low cost, readily available and renewable source of material, natural products like plant extracts can be used as green corrosion inhibitors [6][7][8] .
Several plant-based extracts successfully inhibited the corrosion of metals in an acidic environment. For example, Musa paradisica indicates 90% inhibition efficiency at 300 mg/L 9 , Kola nitida demonstrates 78% hindrance effectiveness at 1200 mg/L 10 , Butea monosperma demonstrates 98% restraint effectiveness at 500 mg/L 11 . Logan seed and peel extract has been utilized for corrosion inhibitor in acidic media 12 . β-sitosterol isolated from rice hulls has been reported as a good corrosion inhibitor for mild steel in acidic environments 13 . Strychnos nuxvomica, Piper longum and Mucuna pruriens seed extracts have been found as eco-friendly corrosion inhibitors for copper in nitric acid 14 . Lemongrass extract was evaluated for its anti-corrosion action in produced oilfield water 15 . In the present work, we have studied the corrosion inhibition efficiency of ethanolic extract of Ixora coccinea (Rubiaceae family) leaves for mild steel in 1 M HCl and 0.5 M H2SO4 media. The major component of Ixora coccinea leaves is ixorene which is a triterpenoid containing a hydroxyl group and unsaturated bonds, which causes anti-corrosion potential 16 . To evaluate the corrosion inhibition ability of the leaf extract in more detail, the major component ixorene was subjected to theoretical calculations.

FT-IR Spectroscopy
To identify the functional groups existing in ICE, FT-IR spectroscopy of dried powdered Ixora coccinea leaves has been examined and it is shown in Fig. 1. The peak at 3438 cm -1 indicates O-H stretching bond. The peaks at 2928 cm -1 and 2850 cm -1 are pointed alkyl C-H stretching bonds. The peaks at 1384 cm -1 and 1628 cm -1 represent C=C and aromatic C=C stretching bonds respectively. The other significant peaks indicate the presence of minor components present in ICE.

Effect of concentration of inhibitor
The weight loss measurements of mild steel in 1 M HCl and 0.5 M H2SO4 solutions at room temperature with and without various concentrations (1-5 v/v %) of ICE for 24 hours are recorded in Table 1. It is evident from the data that as the concentration of inhibitor is increased; the percentage inhibition efficiency also increases in both the acidic media. The results show that ICE is an efficient inhibitor in 1 M HCl attaining maximum inhibition efficiency at 5% as 89.38%. This is due to the number of adsorbed organic molecules of inhibitor ICE on mild steel is more in HCl than H2SO4 medium.   Any prominent shift in the position of the absorbance maximum of the medium measured before and after the immersion of mild steel with inhibitor points to an interaction between Iron and inhibitor molecules. [17][18][19] . From Fig. 2, it's clear that in both HCl and H2SO4 medium, there is a lowering of absorbance of absorption maxima after 24hrs of immersion of mild steel, even though both spectra have a similar shape. So, this lowering of absorbance after the immersion of mild steel can be attributed to the adsorption of some molecules of ICE from the solution on the mild steel surface.

Electrochemical studies 2.3.1 Electrochemical Impedance Spectroscopy
EIS studies are alternate current methods mainly used to analyse the corrosion behaviour at metal/solution interface 20 . The equivalent circuit employed in this study is Randle's circuit (Fig. 3). It includes solution resistance Rs, charge transfer resistance Rct and double layer capacitance Cdl. The deformities on the metal surface cause deviations from the ideal dielectric property of the metal. So, a constant phase element (CPE) is preferred to Cdl 21 .
where Q is the measure of CPE, n is the power of the magnitude of CPE, ω is the angular frequency and j is the imaginary unit. The Nyquist and Bode plots of mild steel without and with the inhibitor ICE using various concentrations (1-5v/v %) in 1 M HCl and 0.5 M H2SO4 at room temperature are shown in Fig. 4 and Fig. 5. In Nyquist plots, it is seen that the low-frequency region is plotted to a straight line and the high-frequency region is plotted to a semi-circle. The slight fall in the semi-circular plot is ascribed to the heterogeneous and roughness of the mild steel. The straight-line portion represents the diffusion from solution 22 . As the concentration of ICE is raised, the size of the loop also increases. This is can be ascribed to the increase in the impedance of inhibited mild steel. The efficiency also increases in the process. The impedance data obtained from the above equivalent circuit for both media are given in Table 2.

Fig. 3. Randles circuit used for electrochemical measurements
As the concentration of inhibitor increases, double-layer capacitance (Cdl) decreases which indicates that the thickness of the electrical double layer increases with respect to the concentration. This is due to the adsorption of inhibitor molecules on the mild steel surface. The maximum inhibition efficiency of 91.55% is seen at a 5% concentration of ICE in 1 M HCl.

Potentiodynamic polarization studies
The potentiodynamic polarization data of mild steel in 1 M HCl and 0.5 M H2SO4 with and without ICE are given in Table 3 and corresponding Tafel plots are depicted in Fig. 6 and linear polarization curves are pictured in Fig. 7.  The potentiodynamic polarization data reveals that the greater the concentration of ICE, the smaller is the corrosion current density. The inhibition efficiency is also seen to increase. The inhibition efficiency of ICE for mild steel reached an upper limit of 93.67% in 1 M HCl solution and 87.02% in 0.5 M H2SO4 solution at higher concentration. From the Tafel plots in Fig. 6, it is seen that both cathodic and anodic curves are influenced by the addition of various concentrations of ICE which shows the mixed type inhibition character of ICE. . It is seen that if the concentration of ICE is increased, the protecting power of ICE is also increased. For inhibitor molecules, current and potential noise is lower than the uninhibited system. Higher magnitude of potential noise signal of uninhibited acid media indicates appreciable localised metallic corrosion on metal surface 23 .

Adsorption studies
The mechanism of corrosion inhibition between inhibitor molecules and mild steel metal was illustrated using adsorption isotherms. Several adsorption isotherms like Langmuir, El-Awady, Frumkin, Temkin, Freundlich, and Flory-Huggins were considered for the study. The best suitable isotherm was detected using the value of correlation coefficient (R 2 ).
In this study, the metal-ICE interaction was observed to be obeying Langmuir adsorption isotherm. According to Langmuir adsorption isotherm, there are a large number of similar adsorption sites and little chance for lateral interaction. Equation (2) shown below explain Langmuir adsorption isotherm as 25 where Cinh, Kads and θ represent the concentration of inhibitor, equilibrium constant and the surface coverage respectively. Standard free energy of adsorption ∆G 0 ads can be derived from the given value of Kads using Eq. (3) 26 .
where 55.5 is the molar concentration of water, R is the universal gas constant and T is the temperature in K. The value of ∆G 0 ads determines whether the interaction between the inhibitor molecules and the metal surface is physisorption or chemisorption. If it is up to -20 kJ/mol, the nature of the interaction is physisorption, whereas if it is more negative than -40 kJ/mol, interaction is chemisorption. From the values of R 2 in Fig. 10, it can be seen that Langmuir adsorption isotherm is the best suitable one for the adsorption of ICE on mild steel. The calculated ∆G 0 ads for ICE are -32.88 and -29.58 kJ/mol in 1 M HCl and 0.5 M H2SO4 respectively shows that an electrostatic and chemical interaction exists between the inhibitor and the charged metal surface. ICE is composed of various organic compounds in it. The major component of Ixora coccinea leaves is ixorene. The adsorption of all the components of ICE on the mild steel surface may be attributed to the inhibitive action of the extract. Fig. 11 represents the predominant interaction of the major component ixorene with the mild steel surface. By transferring electrons from the Oxygen atom present in the -OH group and the interaction through the double bonds, ixorene molecules can be adsorbed onto the mild steel surface.

Suface Morphological Studies
To strengthen the understanding of the mechanism of ICE on the surface of mild steel, morphological studies were performed by taking the SEM images of metal coupons. Fig. 12a shows the SEM picture of smoothening mild steel metal. Figs. 12b, c, d, e shows the surface of mild steel metal after immersion in 1 M HCl and 0.5 M H2SO4 respectively without and with ICE. The SEM 1 (2) images clearly exhibit that the surface is severely damaged in the absence of the inhibitor ICE. It could be seen that the surface corrosion is getting reduced in HCl solution with ICE and the surface is more smooth and perfect in it than H2SO4 solution. So it can be confirmed that ICE acts as an excellent green corrosion inhibitor in acidic media.

Quantum mechanical calculations
Quantum mechanical parameters like HOMO, LUMO, Δ , Ionisation energy (I), Electron affinity (A), Chemical potential, electronegativity ( ), hardness ( ) and the number of transferred electrons (Δ ) of ixorene are tabulated in Table 4. The HOMO and LUMO pictures of ixorene are depicted in Fig. 13. The Δ value is low for ixorene, which indicates that ICE has remarkable inhibition efficiency. This low Δ value 4.884 eV of ixorene attributes the low energy requirement for the transfer of electrons from HOMO of ixorene to the vacant orbitals of Fe. The Δ values provide information regarding the interaction between donor-acceptor molecules 27 . This in turn proves agreement between quantum mechanical calculations and experimental results.

Conclusions
 Ixora coccinea leaf extract (ICE) acts as an efficient green inhibitor for corrosion of mild steel in HCl and H2SO4 medium. As the concentration of the inhibitor increases, the protecting power also increases.  On comparing, ICE in HCl medium shows higher efficiency than H2SO4 medium.  UV-Visible spectrum of extract solutions suggests the possibility of adsorption of constituents present in ICE and also proves the inhibition capacity of ICE.  Electrochemical impedance analysis exhibits that charge transfer resistance increases and double layer capacitance decreases according to the concentration of ICE.  Potentiodynamic polarisation measurements exhibit a mixed type character of inhibition for ICE  Quantum mechanical calculations of ixorene -a major component present in ICE supports the inhibition power of ICE.  The adsorption studies of ICE shows that it obeys Langmiur adsorption isotherm.  The protecting power of ICE is also confirmed by surface morphological studies.

Inhibitor
The Ixora coccinea leaves were washed with distilled water; shade dried for two weeks and made fine-grain using a grinder. 5 g of the granulated leaves was refluxed with 100 ml ethanol for 4 hr. It is then filtered after keeping overnight and concentrated into 50 ml. The corrosion inhibition action of the Ixora coccinea extract (ICE) was analysed by preparing 1-5 v/v% solutions in 1 M HCl and 0.5 M H2SO4.

Mild steel coupon preparation
Mild steel composition %:-C-12.37% and Fe-87.63% were the specimens used. For weight loss measurements, the metal specimens were cut into 1 cm 2 coupon. For electrochemical analysis, the metal samples had a 1 cm 2 surface area exposed to 100 ml of the acidic medium. The metal coupons were physically abraded with 100, 220, 440, 800, 1000, 1500 and 2000 grade of sandpaper, washed with distilled water and acetone.

Acid media preparation
The electrolytic solution 1 M HCl and 0.5 M H2SO4 were made by dilution of analytical pure HCl and H2SO4 (Merck) using distilled water.

UV-Visible Spectroscopy
UV-Visible absorption spectrum of Ixora coccinea leaf extract (ICE) in 1 M HCl and 0.5 M H2SO4 separately was measured with UV-Visible spectrophotometer Shimadzu UV-1800, before and after 24hrs of immersion of mild steel.

IR Spectroscopy
IR spectrum of dried powdered samples of Ixora coccinea leaves was recorded in Shimadzu IR Affinity -1 model FT-IR spectrophotometer using KBr pellet method.

Weight loss measurement
It is a gravimetric method to measure the corrosion rate. The dimensions of the polished metal coupons were measured, which were then pre-weighed and immersed in 50 ml acidic solutions of 1 M HCl and 0.5 M H2SO4 separately, together with different inhibitor concentrations (1-5 v/v%) for 24 hours at room temperature. After the immersion period, the metal coupons were washed with distilled water, dried and then weighed. The corrosion rate (ʋ) was calculated using equation (4) 28 : where K =87600, W-average weight loss of coupon (g), S-total area of the coupon (cm 2 ), D-density of metal (g cm -3 ), t-period of immersion (h). The percentage of inhibition efficiency was deduced from the values of ʋ, using equation (5) 29 .
where ʋ0 and ʋ indicate the rate of corrosion of metal coupons immersed in acid solutions in the absence and presence of the inhibitor ICE, respectively.

Electrochemical Measurement
Electrochemical investigations were done in a three-electrode cell using Ivium CompactStat electrochemical work station and IviumSoft software. A platinum electrode was employed as the counter electrode, and a saturated calomel electrode (SCE) was used as the reference electrode. Mild steel samples, 1cm 2 surface area exposed to 100ml acidic solutions, were made as a working electrode. The measurements were conducted at room temperature using 1 M HCl and 0.5 M H2SO4 with various concentrations (1-5 v/v %) of ICE. The EIS measurements were conducted across a frequency range of 1 KHz to 100 mHz using a signal amplitude of 10 mV. Using Rct values from Nyquist plots, corrosion inhibition efficiency of ICE was determined using equation (6) 30 .
where R'ct and Rct are the charge transfer resistance of the working electrode in the presence and absence of inhibitor ICE, respectively. Potentiodynamic polarisation measurements were done under open circuit potential in the potential range of -250 to +250 mV at a scan rate of 1mV/sec. From the polarisation curves, polarisation resistance values were obtained. From which percentage of inhibition efficiency, ɳ% was found by Eq. (7) 31 .
where RP 0 and Rp are the polarization resistance values in the absence and presence of inhibitor ICE respectively.
Electrochemical noise (ECN) analysis was performed in a three-electrode system, which includes two mild steel electrodes having an exposed surface area of 1 cm 2 and SCE as a reference electrode. Here one mild steel electrode acts as a working electrode and the other as a counter electrode. ECN analyses were performed for a period of 1200 s.

Scanning Electron Microscopy
The surface morphological analysis of the inhibited and uninhibited mild steel surfaces in 1 M HCl and 0.5 M H2SO4 was performed by scanning electron microscope (JEOL Model JSM-6390LV). The polished metal coupons of surface area 1 cm 2 were dipped in acid solutions in the absence and presence of ICE. After keeping the systems for 24 hours at room temperature, they were removed and dried, which are then subjected to SEM analysis.

Quantum mechanical calculations
The corrosion inhibition power of inhibitors can be associated with the frontier molecular orbital energies 32 . There is a donor-acceptor interaction between Fe vacant orbitals and the inhibitor filled molecular orbitals. This interaction can cause the prevention mechanism of metal corrosion 33 . As the ΔE ( LUMO − HOMO) value of inhibitors lowers, greater is the tendency to bind on the metal surface strongly 34 . Quantum mechanical calculations for the major component ixorene were carried out using DFT method by GAMMES software.