Optimization and Performance Evaluation of Lemon Leaves Extract as Inhibitor for Mild Steel Corrosion in a 1.0 M HCl Solution

Oyewole, Olamide; Adeoye, John Busayo; Tunbosun, Abayomi; Celestine, Chukwuma Chukwuemeka; Oyewole, Olamide; Adeoye, John Busayo; Tunbosun, Abayomi; Celestine, Chukwuma Chukwuemeka

doi:10.4152/pea.0602

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Portugaliae Electrochimica Acta

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Port. Electrochim. Acta vol.41 no.6 Coimbra Dec. 2023 Epub Dec 31, 2023

https://doi.org/10.4152/pea.0602

Research Article

Optimization and Performance Evaluation of Lemon Leaves Extract as Inhibitor for Mild Steel Corrosion in a 1.0 M HCl Solution

Olamide Oyewole¹

John Busayo Adeoye¹

Abayomi Tunbosun¹

Chukwuma Chukwuemeka Celestine¹

^¹. Sustainable Development Goal (SGD9): Industry, Innovation and Infrastructure. Department of Chemical Engineering, College of Engineering, Landmark University, Omu-Aran, Kwara State, Nigeria

Abstract

This study examined the use of LLE as an inhibitor for MS in a HCl medium, using the optimization approach. In order to determine the metabolites presence, phytochemical analyses were performed on the plant extract. Three variables factors were considered for the optimization: inhibitor C (0.2- 0.8 g/L); T (30-50 ºC); and time (2-6 h). Morphological structure was determined using SEM. Phytochemical analysis results showed the presence of saponins, alkaloids and flavonoids, which confirmed the extract as a good inhibitor. The optimal process conditions were T of 48.30 ºC, time of 2.49 h and C of 0.66 g/L, for obtaining the highest IE of 84.2 %. Meanwhile, the validated OPL gave an IE of 85.6 %. The results observed from SEM showed that a more protective film was formed on the MS surface, in the validated process level. LLE adsorption obeyed Langmuir’s isotherm, and the thermodynamic parameters were: ∆G_ads (-17.05, -18.74 and -15.35 kJ/mol^-1) and ΔH (41.73, -15.41 and -1.58 kJ/mol^-1). ∆G_ads negative values indicated LLE spontaneous adsorption, which was physisorption. It can be concluded that LLE would be recommendable as a low-cost inhibitor for MS corrosion in 1 M HCl.

Keywords: corrosion; inhibition; LLE; SEM; optimization; phytochemical analyses; RSM

Introduction

Experts believe corrosion is inevitable ¹^,¹¹. Due to the current prerequisites of MS use, its corrosion has been found to be one of the main concerns in industries, from previous studies. However, the application of an inhibitor to control metals corrosion is a suitable way of maintaining their structural strength. It was stated that the best method for corrosion protection is inhibitors, due to their low price ⁹. It has also been reported in literature that corrosion inhibition is the best value-effective mean of delaying the wear of industrial parts, and, in most cases, the two processes may enhance each other ¹²^,¹³^,²⁶^,²⁹^,⁴². The combination of corrosion and wear is known as tribocorrosion ²³^,⁴⁵.

Research has been tailored to use green inhibitors that are biodegradable, inexpensive and eco-friendly in industries, for combating corrosion. Therefore, the use of natural plant-based extracts as corrosion inhibitors is actively supported, because they are environmentally friendly, and can supplement synthetic compounds.

Some of the extracts used for the green inhibitors include elephant grass, Ocimum gratissimum, barley grass, bitter Kola leaf ²^,³^-⁶^,⁷, Pimenta dioica leaf ⁸, Chenopodium ambrosioides¹⁰, Gongronema latifolium¹⁴, Sidaacuta stem ¹⁵, Ananas comosus¹⁶, Napoleonaea imperalis¹⁷, Picralima nitida¹⁸, Geissospermum leavea¹⁹, Citrus aurantium²⁰, peppermint oil ²¹, Morinda tinctoria²², Vernonia amygdalina²⁴^,²⁸, Lavandula and Ricinus communis oil ²⁵^,³², Psidium guajava³⁰, rubber leaf ³³, groundnut leaves, Katemfe seed ³⁴^,³⁵, Sapum ellipticum³⁷, Pterolobium hexapetalum and Celosia argentea³⁸, Azadirachta indica⁴⁰, Aloe vera gel ⁴¹, Mansoa alliacea⁴³Mauclea latifola⁴⁴ and orange zest ⁴⁶. In addition, ²⁷ revealed that it is important to achieve a fresh class of low toxicity, eco-friendly and high reliability inhibitors. Compounds containing N, S or O are recognized as strong acid corrosion inhibitors ⁴. Non-conventional materials used as inhibitors have already spurred studies on environmentally sustainable inhibitors that use agricultural waste, because they are readily available, biodegradable and antioxidant ⁶.

Many researchers have used statistical methods, like RSM, for optimizing independent variable factors, such as inhibition C, T, time and corrosion, in order to secure optimal expected responses, such as WL, CR and IE ⁴^,²⁴.

The aim of this study was to optimize LLE inhibitive properties against MS corrosion, in a HCl media, using RSM.

Materials and methods

Materials

MS (from the Mechanical Engineering Department at Landmark University in Omu-Aran, Kwara State, Nigeria) was mechanically cut into coupons with the dimensions of 2.5 x 2.2 cm. Inside the coupons, a 0.1 mm hole was drilled. Then, the coupons were degreased with acetone, dried, and stored in a moisture-free desiccator.

Methods

LLE preparation

Lemon leaves were collected in the Nigerian city of Omu-Aran, Kwara State. They were washed, drained and air-dried, before being pulverized with a blender. Soxhlet extraction was used to collect the oil from the lemon leaves, and ethanol was used as solvent. Ethanol was removed with a rotary evaporator, and LLE was stored in bottles, for later use.

Phytochemical analysis

LLE phytochemical analysis was carried out, in order to determine the existence of metabolites, such as alkaloids, saponins, tannins, and flavonoids, which would render an extract as a reactive inhibitor.

Experimental design

In order to determine the optimal LLE process parameters for MS in 1 M HCl, three variables were considered: X1, time; X2, T; and X3, inhibitor C, at three levels. CCD of 20 experimental runs was developed using the 3 operating variables. Table 1 is for variables and levels, while variables interactions are shown in Table 2.

Table 1 Experimental range of LLE on MS in a 1 M HCL solution of the independent variables, with factor levels for the inhibition.

Table 2 CCD factors and levels.

For the three variables, the matrix was varied at 3 levels (-1, 0 and +1). This methodology was adapted from ³⁵^-³⁷. Software used to analyze the data was Design Expert 6.0.8. The mathematical empirical model is defined as Eq. 1.

***

where Y is the response or dependent variable, X₁ and X₂ are the independent variables and B₀, B₁, B₂, B₁₁, B₂₂ and B₁₂ are R². RSM theory and applications are highlighted in literature ³⁹.

Gravimetric measurements

WL measurements were conducted via the predicted variable interactions. WL (average weight), CR and IE were calculated using Eqs. 2, 3 and 4.

***

where W₁ and W₂ are the weight before and after immersion, respectively.

***

where r is CR in g/cm²/h, A is the surface area in cm², and t is the time in h.

***

Surface characterization

MS corrosion inhibitor morphology, with maximum IE and optimal process variables, was examined using SEM. MS surface morphology and elemental analysis were characterized by JEOL JSM-7600F.

Results and discussion

Phytochemical analysis

Phytochemical analysis results (Table 3) showed that organic compounds, such as alkaloids, flavonoids and saponins, were present.

Table 3 LLE phytochemical constituents.

These organic bioactive compounds have anti-inflammatory and anti-oxidant characteristics, which explained LLE corrosion IE. The findings have been verified by ³⁵^-³⁷.

WL measurements

MS WL at 1.0 M HCl was calculated at different time intervals, from 2 to 6 h, with of LLE at a C of 0.2, 0.5 and 0.8 g/L, over T of 303, 313 and 323 K. The obtained values were used to calculate WL and IE, using equations 2 and 3. Table 4 shows WL, CR and IE results. IE increased with higher LLE C, due to the presence of active metabolites.

Table 4 CCD factor levels of independent variables with response

Evaluation of a corrosion regression model

CCD was used for assessing the correlation between the variables of the experimental process and CR. A polynomial quadratic regression reaction with CR (Y) and process variables, such as time (A), T (B) and the inhibitor C, is shown in Eqs. 5 and 6.

***

The final response reaction in terms of CCD is as in Eq. 6.

***

Statistical analysis result

In addition, the variance analysis (ANOVA) is presented in Table 5.

Table 5 Analysis of variance (ANOVA)

ANOVA results indicated that the quadratic model was appropriate for analyzing the experimental data. R₂ was 0.8778, which proved to be a good fit between the forecast and the experimental.

Surface response plots for MS with LLE

3-D plots (Figs. 1 -3) show the relationship between the variables affecting the corrosion process validity and LLE. Fig. 1 shows that CR increased with higher T, and decreased significantly over time.

Figure 1 LLE variation of time and T effect on MS CR.

Fig. 2 shows that CR decreased with inhibitor C, as shown in Fig. 3.

Figure 2 Variation of time and LLE C effect on MS CR.

Figure 3 Variation of T and LLE C on MS CR.

This in agreement with what was reported by ³³. Moreover, for verifying the model prediction, the optimal condition values were applied to three independent replicates, and IE was 84.2%. LLE usefulness to protect MS in HCl was established.

Results on surface analysis

Figs. 4a and 5a show SEM results of blank and coated MS, respectively. The micrographs revealed that, in the inhibitor absence (Fig. 4), the surface was severely corroded.

Figure 4 Micrograph of blank MS and corresponding EDX.

In the inhibitor presence (Fig. 5), at OPL (validated), the exposed MS surface morphology showed the development of a passive film layer (white patches) by the LLE active constituents, which displayed considerable corrosion resistance, in agreement with ³⁵. Figs. 4 and 5 show EDX results of the blank and coated MS samples, respectively. The coating formed on MS was attributed to the presence of LLE phytochemical constituents, which underwent the adsorption mechanism. The results showed that MS in Fig. 5 has more heteroatoms than in Fig. 4, which indicates that the inhibitor (Fig. 5) prevented the corrosion process.

Figure 5 Micrograph of coated MS and corresponding EDX MS OPL (validated).

Corrosion mechanism

The blank MS coupon clearly showed that corrosion activities occurred, which caused its surface to be porous. The MS coupon surface with inhibitor was smooth, whereas white patches appeared on it, mainly due to the protective oxide, at optimum C. The adsorption mechanism has been commonly used to describe the corrosion inhibition effect of organic molecules on the metal/acidic solution interface. These molecules influence the inhibitors chemical, structural and electronic characteristics. Thermodynamic studies of the adsorption process in the study suggested that LLE physisorption onto the MS surface was lower than -20 kJ/mol^{-1 (}³¹, and ΔH was -15.41, -36.67 and -61.49 kJ/mol^-1. The results are shown in Table 7. Therefore, the reaction was spontaneous, because ∆G_ads was negative, and LLE inhibition was an exothermic process. ∆S_ads values, with inhibitor C of 0.2, 0.5 and 0.8 g/L, and at T of 303, 313 and 323 K, are shown in Table 8. ∆S_ads decreased as T increased with each higher C, and this showed a more ordered behavior that led to enhanced IE.

Experimental validation

As shown in Table 6, the optimum process level variables values observed were: time of 2.49 h; T of 48.30 ºC; and C of 0.65 g/L. At this ideal conditions, the experiment was conducted for validating the predicted optimum values. IE of 85.63% was observed, which was in close agreement with 84.2% obtained from the regression model.

Table 6 Optimum variables for the corrosion inhibition process

Results on the adsorption mechanism

LLE inhibitor prevented MS dissolution by being adsorbed onto the metal/acidic solution interface, forming a layer that protected it. Langmuir’s adsorption isotherm (Fig. 6) was estimated using Equation 7 ¹⁵.

***

where θ is the surface layer.

Figure 6 Langmuir’s adsorption isotherm.

The graph of C θ versus C, in Fig. 7, can be used to obtain K_ads, at different T, and R² is shown in Table 7.

The results showed that, at a T of 40 ºC, Langmuir´s adsorption isotherm was best fitted. On the contrary, at 50 ºC, linear plot slopes were larger than unity. The result further showed that, as T increased, K_ads values decreased.

In the equation below, K_ads is related to ΔG_ads.

***

The calculated K_ads in Table 7 was used to estimate ∆G_ads at different T.

Figure 7 Freundlich’s adsorption isotherm.

Table 7 Parameters of the various adsorption isotherms for LLE adsorption onto the MS surface.

∆G_ads negative values indicated LLE components spontaneous adsorption onto the MS surface, with strong interactions between them ¹². ∆G_ads value from the study was lower than -20 kJ/mol^-1, which indicated physisorption, as shown in Table 8 ³¹.

Table 8 Thermodynamic parameters of LLE corrosion inhibition.

Therefore, the reaction was spontaneous, because ∆G_ads was negative, and LLE inhibition was an exothermic process. Moreover, Figs. 8 and 9 are the graphs for the Temkin’s and Flory-Huggin’s adsorption isotherms, respectively.

Figure 8 Temkin’s adsorption isotherm.

Figure 9 Flory-Huggins’ adsorption isotherm.

Fig. 10 is the graph of 𝐼𝑜𝑔 𝜃 1−𝜃 against 1000 𝑇 , at various C, respectively.

Figure 10 Graph of 𝐼𝑜𝑔 𝜃 1−𝜃 against 1000 𝑇 , at various C.

Conclusion

LLE showed OPL with time of 2.49 h, T of 48.3 ºC and an inhibitor C of 0.66g/L, with an IE of 85.6%. The extract IE improved with an increase in C. The experimental value obtained as an IE of 85.6% agreed with the result obtained from the regression model (84.2%). SEM images clearly indicated that the MS surface was protected against dissolution by LLE, which acted as a corrosion inhibitor. It is clear that MS corrosion inhibition induced by LLE was due to physisorption. The extract can be used as environmentally-friendly corrosion inhibitor, in industries. It can be concluded that LLE, which is agro-waste, has been converted into a sustainable product as an effective inhibitor against MS corrosion.

Authors’ contributions

Olamide Oyewole: wrote the paper; made the experimental design and analysis; interpreted the results. John Busayo Adeyoye: made the experimental analysis; made adsorption experiments; interpreted results. Abayomi Tunbosun: obtained the extracts; made the experiments. Chukwuemeka Celestine Chukwuma: made the experimental analysis.

Abbreviations

C: concentration

CCD: central composite design

Cor Total: amount of variation around the mean of observations. The model explains part of it, the residual explains the rest.

CR: corrosion rate

DF: degree of freedom

EDX: energy dispersive X-ray

IE: inhibition efficiency

K_ads: adsorption equilibrium constant

LLE: lemon leaves extract

MS: mild steel

OPL: optimal process level

R²: determination coefficient

RSM: response surface methodology

SEM: scanning electron microscopy

T: temperature

WL: weight loss

Symbols definition

∆G_ads: standard free energy of adsorption

ΔH: change in enthalpy

ΔH_ads: enthalpy of adsorption

∆S_ads: entropy of adsorption

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Received: January 25, 2022; Accepted: May 25, 2022

Corresponding author: olawale.olamide@lmu.edu.ng

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