Pineapple stem extract (Bromelain) as an environmental friendly novel corrosion inhibitor for low carbon steel in 1 M HCl

Accepted Manuscript Pineapple stem extract (Bromelain) as an environmental friendly novel corrosion inhibitor for low carbon steel in 1M HCl Mohammad Mobin, Megha Basik, Jeenat Aslam PII: DOI: Reference:

S0263-2241(18)31057-1 https://doi.org/10.1016/j.measurement.2018.11.003 MEASUR 6046

To appear in:

Measurement

Received Date: Revised Date: Accepted Date:

25 February 2017 15 September 2018 2 November 2018

Please cite this article as: M. Mobin, M. Basik, J. Aslam, Pineapple stem extract (Bromelain) as an environmental friendly novel corrosion inhibitor for low carbon steel in 1M HCl, Measurement (2018), doi: https://doi.org/10.1016/ j.measurement.2018.11.003

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Pineapple stem extract (Bromelain) as an environmental friendly novel corrosion inhibitor for low carbon steel in 1M HCl Mohammad Mobin*, Megha Basik, Jeenat Aslam Corrosion Research Laboratory, Department of Applied Chemistry, Faculty of Engineering and Technology, Aligarh Muslim University, Aligarh 202002, India

E-mail: [email protected] (Mohammad Mobin)

ABSTRACT The anticorrosion behavior of bromelain on low carbon steel (LCS) in 1 M HCl solution was studied employing weight loss, potentiodynamic polarization measurement (PDP), electrochemical impedance spectroscopy (EIS), UV-visible spectrophotometry, and surface assessment techniques like scanning electron microscopy (SEM) and energy dispersive X-ray analysis (EDAX) at 308 K - 338 K. The obtained results suggest that bromelain is an excellent corrosion inhibitor and its inhibition efficiency (%η) is both concentration and temperature dependent. %η is observed to increase with an increase in bromelain concentration and an increase in electrolyte temperature. The maximum %η of 97.6% is observed at bromelain concentration of 1000 ppm at 338 K. The inhibitor adsorption on the LCS surface is in accordance with the Langmuir adsorption isotherm. As evidenced by PDP measurements, bromelain behaves as a mixed-type inhibitor and controls both anodic and cathodic processes. Adsorption free energy of the bromelain on LCS surface together with an increase in %η with the rise in temperature is suggestive of chemical adsorption. SEM micrographs show a smoother surface for inhibited LCS specimen. Analysis of variance statistically compare the difference existing between inhibition efficiencies from gravimetric, PDP and EIS technique and suggests that they are not significantly different.

KEYWORDS: corrosion inhibitor; bromelain; low carbon steel, SEM; EIS

1. INTRODUCTION Corrosion of metallic components is an important industrial issue as it leads to the breakdown of machines and damage to the plant's infrastructure whose repair is a costly affair. The driving force that leads the metals to corrode is their temporary existence in thermodynamically less stable metallic form. Among the metallic alloys low carbon steel has been the popular choice in almost every industry due to economic reasons and adequate mechanical properties. The use of solutions of acids such as HCl and H2SO4 is quite common in a number of industrial activities such as acid pickling, acid cleaning, oil well acidizing, descaling etc, which are typically followed by substantial dissolution of metals if used uninhibited. The inhibitors added in the acid solutions obstruct the active corrosion sites by forming a physically or chemically adsorbed protective layer on the metal surfaces and reduce the corrosion to the safest limits. Both organic and inorganic compounds have been used as inhibitors to protect metal from corrosion. In general, the corrosion inhibition performance of an organic inhibitor is related to its molecular structure. Organic compounds which contain heteroatoms such as N, O, S, P and conjugated π-electrons in their aromatic ring or long carbon chain have been widely used as potential corrosion inhibitors [1–6]. Inhibitors are selected on the basis of their economic feasibility, biodegradability, efficiency and effects on the environment. Synthetic organic and inorganic inhibitors are toxic, expensive and hazardous to human beings as well as the environment and they need to be replaced by some natural environmental friendly, non-toxic and economically feasible corrosion inhibitor and hence, it is prompted to search for safe and environmentally friendly "green" corrosion inhibitors [7–11]. Plant extracts, in addition to being inexpensive, readily available and renewable are environmentally friendly and ecologically acceptable. Extracts from a number of the plant have been investigated as corrosion inhibitors in the acid medium [12–15]. Umoren et al [16] studied the corrosion inhibition of mild steel in acidic media using date palm seed extracts. The inhibitor was 86.8 % efficient at a comparatively higher concentration of 2500 ppm. In another study, Chauhan and Gunasekaran [17] reported the corrosion inhibition of mild steel by Zenthoxylum alatum plant extract in 5 and 15% aqueous HCl solution in the temperature range of 50-80 ºC employing weight loss and electrochemical techniques. Inhibition efficiency increased with increase in plant extract till 2400ppm. At the highest extract concentration (i.e. 200 ppm) the

maximum inhibition efficiency in 5% HCl was 95% whereas in 15% HCl the inhibitor was only 91% efficient. Schinopsis lorentzii extract was evaluated as a green corrosion inhibitor for low carbon steel in 1 M HCl solution using Tafel polarization, linear polarization and EIS [18]. The extract acted as a slightly cathodic inhibitor and exhibited a modest inhibition efficiency of 66% at 2000ppm. In a recent publication leaves extracts of Aquilaria subintergra (non inoculated, physically inoculated, chemically inoculated) was investigated as a sustainable mild steel corrosion inhibitor in 1M HCl [19]. The inhibitor was 93.2% efficient at 1500ppm. The performance of plant extract as a corrosion inhibitor is normally imputed to the presence of complex organic species in their composition. Polar functions with N, S, or O atoms, conjugated double/triple bonds as well as aromatic rings in the molecular structures of these organic compounds are the major centers responsible for adsorption on the metallic surfaces. In the recent past, a number of other compounds derived from natural products have been studied as effective inhibitors for low carbon steel corrosion in acid solutions. Natural polymer pectin was investigated as a corrosion inhibitor for mild steel in HCl solution at varying temperature [20]. At 298K pectin was 90.3% efficient at an inhibitor concentration of 2000ppm which increased to 94.2% at 318K. Bromelain, which is extracted from the pineapple stem, is a proteolytic enzyme belonging to the cysteine endopeptidase family [21]. It contains various distinct cysteine proteinases having identical but different amino acid sequences as well as differences in proteolytic specificity and sensitivity [22]. Studies had shown that stem bromelain also contains a small amount of highly branched carbohydrate moiety [23]. Stem bromelain finds its application over a wide range of pH hence it finds applications in various important industries. The various applications include food supplementation, meat tenderization and baking processes [24]. It also finds application in reducing inflammation (especially for sinuses), ulcerative colitis and osteoarthritis [25]. In this work we are introducing a novel environmental friendly compound "Bromelain" as an effective corrosion inhibitor for LCS in 1 M HCl solution at the temperature 308-338 K. Techniques employed to evaluate the performance of bromelain comprises: weight loss, electrochemical

impedance

spectroscopy,

potentiodynamic

polarization,

UV

spectroscopy, scanning electron microscopy, thermodynamic and activation parameters.

2. EXPERIMENTAL PROCEDURE

visible

2.1. Sample Preparation LCS coupons of the rectangular shape of size 2.5 x 2.0 x 0.1 cm with surface area 10.9 cm2 were used for weight loss measurements. Circular coupons with exposed surface area 1.0 cm2 are used for electrochemical measurements. Chemical composition (weight %) of LCS, as analyzed by spark optical emission spectrometer, is as follows: 0.068 C, 0.039 Mn, 0.001 S, 0.022 P, 0.045 Cr, 0.067 Mo, 0.015 Al, 0.033 V and remaining Fe. Coupons of LCS were polished successively using different grades of SiC papers (grade 320-1200). Polished coupons of LCS were cleaned with acetone followed by double distilled water and then dried in warm air.

2.2. Test Solution Preparation 37% analytical grade HCl solution was employed to prepare a 1 M HCl corrosive solution. A stock solution of 1000 ppm of bromelain (procured commercially) is prepared in 1 M HCl. Desired concentrations i.e., 100, 200, 300, 500, 700 ppm were obtained by diluting the stock HCl solution of 1000 ppm. Double distilled water was used for dilution of the solutions. 200mL of test solution was used for weight loss experiments whereas electrochemical experiments were performed in the 1L test solution.

2.3. Weight Loss Experiments The coupons were fully immersed in the acid solutions with different concentrations of bromelain at temperatures 308, 318, 328 and 338 K, respectively for a period of 6 h. After completion of the immersion, the coupons were taken out and thoroughly rinsed with distilled water. The corrosion product formed on the surface of the coupons was mechanically removed by gently scrubbing with a bristle brush. Coupons were then successively washed with distilled water and acetone and dried to a constant weight. The experiments were performed on triplicate specimens in order to ensure the reproducibility of results and average corrosion rate was calculated. The calculation of the corrosion rate (mg cm-2 h-1) and %η was done using the relationships reported elsewhere [26].

2.4. Electrochemical Measurements

Both PDP and EIS experiments were performed using Potentiostat/galvanostat from Autolab, model 128N. Experiments were performed in AUTOLAB 1L corrosion cell in which LCS coupon embedded in specimen holder acted as a working electrode, platinum (Pt) wire of large area as a counter electrode and silver/silver chloride electrode (saturated KCl) as a reference electrode. A Luggin Haber capillary was inserted in the set up in such a manner that the capillary's tip was close and parallel to working electrode so that IR drop can be minimized. Before commencement of the experiments, the working electrode was immersed in the test solution and open circuit potential (OCP) was continuously monitored until it was stabilized and a steady state potential was reached. 60 min immersion, in general, was adequate to attain steady OCP. Each experiment was done at 308 K under the unstirred condition and repeated at least thrice to check the reproducibility and acceptable data was obtained. Potentiodynamic polarization curves were recorded by scanning the potential between -250 to 250 mV at a rate of 0.001 V/s with respected to steady-state potential. Polarization curves were analyzed using NOVA 1.11 software and parameters such as corrosion potential (E corr), corrosion current densities (Icorr), and anodic and cathodic Tafel slopes (ßa, ßc) were obtained. EIS measurements were done at OCP within the frequency range of 10-2 Hz to 105 Hz with 10 mV perturbation. The charge transfer resistance (Rct) and the double layer capacitance (Cdl) were computed from Nyquist plots. The calculated Icorr and Rct values were used to measure %η by following equations given elsewhere [26].

2.5. SEM Analysis Surface morphology of low LCS coupons in inhibited and uninhibited solutions was undertaken employing SEM (model: JEOL JSM-6510LV). Abraded LCS coupons were suspended in uninhibited and bromelain inhibited (optimum concentration) acid solution, retrieved after 6 h, washed with distilled water, dried and subjected to SEM analysis.

2.6. UV-Visible Spectrophotometric Measurements This study was undertaken for 1M HCl solution containing 1000 ppm bromelain prior to and after immersion of steel coupons at a temperature of 308 K for 6 hours. Perkin Elmer spectrophotometer, Lambda 25 was used to record the spectra.

3. RESULTS AND DISCUSSION 3.1. Potentiodynamic Polarization Study The anodic and cathodic polarization graphs obtained for LCS in 1 M HCl solution in absence and presence of varying concentration of bromelain are shown in Figure 1. The parameters such as Ecorr, Icorr, ßa and ßc, and % η obtained from polarization plots are depicted in Table 1. It is clearly seen from the Figure 1 that in presence of bromelain the current density of the cathodic and anodic branch of polarization curves is displaced towards lower values than those recorded in the free acid solution [27]. The displacement in I corr is more pronounced with the increase in bromelain concentration. % inhibition is calculated by Icorr value. Also, it can be seen that the %η increases with a rise in bromelain concentration. This behavior shows that bromelain acts as an effective corrosion inhibitor for the LCS in 1 M HCl medium. The data in Table 1 show that in presence of bromelain the values of Ecorr tend to shift towards the positive (anodic) direction, with the extent of shift being less than 85mV with respect to blank LCS sample. This indicates that bromelain in HCl solution acts as mixed-type corrosion inhibitor with preponderantly anodic effect i.e. anodic Fe dissolution reaction is more inhibited than cathodic hydrogen evolution reaction [28]. Values of ßa and ßc changes with inhibitor concentration, however, the shift are not recognized to follow a definite trend. Slight variation in ßa and ßc values in presence of different concentrations of bromelain indicates that both the anodic metal dissolution and cathodic hydrogen evolution processes are inhibited. Considering the variation in polarization resistance (Rp) values, there is an increase in the Rp with the addition of bromelain in the acid medium. Increasing the value of Rp with inhibitor concentration retards the rate of corrosion and suggests effective inhibition performance in presence of bromelain [29]. Increasing the Rp value also suggested that the bromelain molecule effectively retard the polarization process at the metal-acid interface and hence inhibition acknowledged [30].

3.2. EIS Measurements EIS measurement was conducted to study the corrosion processes that occur at the LCS surface as well as at the interface of electrode/electrolyte without and with inhibitor. Nyquist plots and Bode diagrams for LCS electrode immersed in uninhibited and bromelain inhibited HCl solution at 308 K at the OCP is presented in Figure 2. The impedance spectra (Figure 2 A)

obtained without and with bromelain exhibit a single capacitive loop, which suggest that a single charge transfer process occurred during the dissolution of LCS in 1M HCl in absence and presence of bromelain [31]. Further, an increase in the diameter of the capacitive loop was noticed with increasing the bromelain concentration. This reflects the effectualness of bromelain at higher concentrations [32]. The continuous increase in the diameter of the loop with increasing concentration of bromelain suggests that the impedance of LCS increases due to increased coverage by the bromelain molecules adsorbed on the electrode surface [33]. Bromelain molecules adsorbed on the surface of LCS forming an inhibitive film covering the reaction sites and hence increases the impedance. The shape of the impedance diagrams in presence of bromelain is identical with that of blank solution which suggests that there is no change in the corrosion mechanism on the addition of inhibitor [34]. Capacitive loops are not exact semi circles i.e., it is depressed to some extent. This is ascribed to the frequency dispersion effect of interfacial impedance due to roughness and inhomogeneity of the electrode surface and adsorption of bromelain [35]. From the above facts, the equivalent circuit with one time constant is fitted as presented in Figure 3. The circuit comprises of a parallel connection between the Rct and Cdl and these impedance elements are connected in series with solution resistance (Rs). A constant phase element (CPE) is introduced in place of Cdl to obtain more accurate fit as the Cdl does not behave as an ideal capacitor [36]. The impedance of a CPE is acknowledged by the following expression:

Z CPE 

1 Y0 ( j ) n

(1)

Where Y0 is the magnitude of CPE, j is the imaginary number and its value is equal to the square root of -1 i.e.,

, ω is the angular frequency in rad s-1 (ω = 2πfmax) and n corresponds to the

phase shift. The value of 'n' lies between 0 and 1 and it represents the magnitude of deviation from ideal behavior. For n=0, ZCPE represents a resistance with R= Q -1, for n=1 a capacitance with C= Q and for n= -1 an inductance with L= Q-1. The circuit for Cdl can be calculated by employing the following equation as:

Cdl  Y0 ( max ) n1

(2)

Where ωmax is 2πfmax (fmax denotes the imaginary component of impedance at a maximum frequency) [37]. Values of Rct were obtained from the equivalent circuit and listed in Table 2. This table also contains values of ZCPE and Cdl. The value of Rct increases whereas the Cdl value decreases in inhibited media. Increase in Rct is due to an increase in surface coverage by the inhibitor. The decrease in Cdl with an increase in the thickness of the adsorbed layer is due to adsorption of bromelain at the metal/solution interface which in turn decrease in local dielectric constant [38]. The χ² values between 10-3 and 10-5 are suggestive of ideal fit [39]. The obtained χ² values (Table 2) are well within this range and hence support fairly good quality of the equivalent circuit used. Nyquist plots demonstrated that the inhibition effect of bromelain is increased by increasing concentration, which is further confirmed by Bode diagrams. The Bode plots imply only one phase maxima revealing that the process of corrosion is occurring through one step corresponding to one time constant. In Bode modulus diagram, the impedance modulus was observed to increase with increasing bromelain concentration for the entire frequency range which indicates reduced corrosion rates in inhibited acid solutions. The phase angle at high frequencies gave a general idea of the anticorrosion behavior of inhibitor. More negative value of phase angle tells the capacitive nature of the electrochemical process. A more negative value of the phase angle at high frequency was observed with increasing bromelain concentration which indicates excellent inhibitive behavior at higher bromelain concentration [40].

3.3. Weight Loss Measurements Table 3 lists the values of corrosion rates for LCS in 1M HCl solution at 308, 318, 328 and 338 K without and with different concentration bromelain. The corresponding % are also given in Table 3. It is evident from the table that the rate of LCS dissolution decreases, while % increases with increase in inhibitor concentration. This can be justified as the heteroatom's present in the bromelain molecule helps in getting adsorbed on the LCS surface, which increases the surface coverage area and hence suppressed corrosion rate of the reaction [41]. It is indicated that % of bromelain increases with the increase in bromelain concentration and electrolyte temperature. % reaches the maximum value of 97.6% at a bromelain concentration of 1000 ppm at 338 K. However, no remarkable corrosion inhibition was recognized beyond bromelain concentration of 1000 ppm, which may be attributed to the withdrawal of bromelain molecules back in to the bulk solution when concentration of bromelain closed to or beyond critical

concentration [42]. An increase in the electrolyte temperature increases the % at all the studied concentrations, indicating chemical adsorption. The strong chemical bond forces were responsible for such type of adsorption, which involves charge sharing or transfers from the inhibitor molecules to the LCS surface to form a coordinate type of linkage [43]. In order to establish the superiority of studied inhibitor the inhibition efficiency of bromelain at optimum concentration was compared with a number of other studied inhibitors derived from natural products and the results are enlisted in Table 4. Bromelain exhibited higher inhibition efficiency at comparatively much lower or comparable concentration making it more practical and economically viable inhibitor.

3.3.1. Effect of Immersion Time To demonstrate the stability of bromelain over a period of time the effect of immersion time on the %η of the bromelain at 1000 ppm was studied at 308 K. Bromelain retains a very high %η uptill 168 h as presented in Figure 4. This confirms that the adsorbed film of bromelain in acid solution lasts long.

3.3.2. Effect of Temperature Effect of temperature in the range of 308-338 K without and with various concentrations of bromelain in 1 M HCl after 6 hours of immersion has been evaluated and shown in Table 3. With an increase in temperature from 308-338 K the %η increases at all the studied concentrations. Activation parameters of the corrosion process were computed from the Arrhenius equation as:

 E  CR  A exp   a   RT 

(3)

where CR is the corrosion rate, Ea is activation energy, A is a pre-exponential factor, R is universal gas constant and T is absolute temperature. The Arrhenius plots for the CR of LCS are presented in Figure 5. Values of Ea and A for LCS in 1 M HCl were computed by linear regression of log CR vs 1/T and listed in Table 5. Values of Ea for an inhibited acid solution, which ranged from 65.61 to 44.21 kJ/mol, was lesser in comparison to the Ea value of uninhibited acid solution i.e., 72.92 kJ/mol. The decrease in Ea value with a rise in bromelain

concentration in the acid solution is ascribed to the increased adsorption of bromelain on the LCS surface with the increase in electrolyte temperature. Due to the increased adsorption of bromelain molecules at higher temperatures, the LCS surface was in lesser contact with corrosive acid solution thus exhibiting lower corrosion rates with increasing temperature. A smaller Ea for inhibited acid solution combined with increased inhibition efficiency with rise in temperature is suggestive of a specific interaction between the LCS surface and bromelain molecules along with the formation of the bromelain/Fe2+ complex. Further, both Ea and A influence the corrosion process at a certain temperature. However, if the value of A is extremely high as compared to Ea then A acts as a dominating factor in determining LCS corrosion. In the present study, the increasing bromelain concentration reduced the Ea which in turn accelerate the corrosion rate of LCS. However, with an increase in bromelain concentration the value of A was significantly decreased. The decrease in the values of A resulted in the reduction of corrosion rates of LCS [50]. So, it is clear that in this case, the reduction of A was a deciding factor, which affected the corrosion rate of LCS in 1M HCl. The other activation parameters such as enthalpy (ΔH) and entropy (ΔS) of adsorption may be computed from temperature effect. Alternative formula of Arrhenius equation for calculating ΔH and ΔS activation parameter is as follows:

CR 

RT  S   H  exp   exp    Nh  R   RT 

(4)

Where h is Planck's constant (6.6X10-34 joule-seconds), N is the Avogadro's number (6.023X1023 mol-1), ΔS is the entropy of activation, ΔH is the enthalpy of activation, R the molar gas constant and T the absolute temperature. Plots of log CR/ T versus 1/T for blank and various concentration of inhibitor are depicted in Figure 6. The straight lines were obtained with slope of

H    R   S     from which ΔH and ΔS were calculated and   and intercept log  Nh   2.303R   2.303R  given in Table 5. The positive sign of ΔH proposes that the process of dissolution of LCS is endothermic in nature and the process is very slow [51]. The values of ΔS are large and negative in both blank and inhibited solution. This indicates that the activated complex in the rate determining step represents association rather than dissociation, which indicates that ongoing from the reactant to the activated complex a decrease in disorder takes place [52].

3.4. Adsorption Isotherms The information pertaining to the adsorption of bromelain molecules on LCS surface in 1 M HCl solution can be investigated by an adsorption isotherm. The calculated surface coverage (θ) values were made to fit various isotherms to select the isotherm with the best fit. The best fit was found for Langmuir isotherm given as.

C





1 C K ads

(5)

where C is bromelain concentration, Kads is equilibrium constant of adsorption-desorption. The plot of C versus C/θ in 1 M HCl at various concentration of bromelain at temperature range of 308-338 K, respectively gave a straight line. The linear fit evaluates good correlation coefficient (R2>0.999), which indicates the best fitting of the Langmuir model at all studied temperatures [53]. The slight departure of the slope from unity specifies the interaction between the adsorbate species on LCS surface. The value of Kads obtained from the intercept of plots increases with an increase in temperature which specifies that with an increase in the temperature the adsorption of bromelain molecules on LCS surface increases. Langmuir isotherms for bromelain are shown in Figure 7. Standard free energy change of adsorption, ΔGads was obtained from Kads value by using the following equation:

Gads   RT ln(55.5 K ads )

(6)

where R is universal gas constant, T is absolute temperature and 55.5 is the molar concentration of water. Factor 55.5 mol L-1 was substituted with 1000 g L-1 to meet the units of Kads [54]. Values of thermodynamic parameters are listed in Table 6. The negative value of ΔGads reveals that the process is spontaneous and adsorbed layer on the LCS surface is stable [55]. The value of ΔGads decreases (become more negative) with increasing temperature, which indicates that the process is endothermic in nature [56]. Value of Kads also increases with rise in temperature, which indicates that chemisorption is taking place. Value of ΔGads close to -20KJ/mol or less suggests that the process is electrostatic in nature i.e., physical adsorption occurs in which

charged organic molecules and charged metal surface is electrostatically attracted. The value of ΔGads close to -40 KJ/mol or more indicates chemisorptions, which involves sharing of charge or transfer from inhibitor molecules to the surface of metal through forming a coordinate bond. The values of ΔGads obtained in the present investigation are more than -20 KJ/mol but less than - 40 KJ/mol, which suggests both physisorption and chemisorption (mixed adsorption) [57].

3.5. SEM and EDAX Analysis Surface morphology of corroded and uncorroded metals have widely been recorded in order to confirm the interaction of the inhibitor with the LCS surface and formation of a protective film [58]. The results are presented in Figure 8. The surface morphology of LCS before immersion in HCl solution (Figure 8a) shows a defect-free surface with parallel features, which are associated with polishing scratches. Figure 8(b) display the severely corroded surface of LCS due to the attack of aggressive 1 M HCl without inhibitor. LCS Figure 8(c) shows surface morphology of LCS coupon immersed in 1.0 M HCl solution containing 1000 ppm of inhibitor. An even surface with less pits and cracks was observed which reveals the formation of a protective film by the bromelain on LCS surface [59]. The protective film formed on a LCS surface was examined using an EDAX analysis. Figure 9 (a,b,c) show the EDAX spectra of polished, blank and inhibited sample of LCS. Figure 9 (a) reveals that the polished surface of LCS before immersion in 1 M HCl solution contains Fe, Mn, V, C, Mo, and Al. The steel surface after immersion in an uninhibited HCl solution (Figure 9 b) shows the peaks of O and Cl in addition to the characteristic peaks constituting steel. This suggests that the corrosion of Fe took place through the formation of iron chlorides and/or iron oxides. Figure 9(c) shows the elements found in the protective layer formed on the LCS surface in presence of bromelain. The peaks of O, N and S appear in EDAX, which confirmed the presence of an adsorbed layer of bromelain molecules containing hetero-atoms. These molecules adsorbed on LCS surface thus prevent the corrosion.

3.6. UV-Visible Spectrophotometric Investigation UV- visible spectroscopic measurements were taken in order to get an insight of the possibility of the occurrence of complex formation between LCS and the inhibitor molecules. It has been shown that a complex formation between two species in solution takes place when there

is a change in the value of absorbance or change in the position of the absorption maximum [60]. UV–visible spectra for the 1 M HCl solution with bromelain before and after LCS immersion for 6 h at 308 K are presented in Figure 10. The spectrum of the 1 M HCl solution containing bromelain before the immersion of LCS shows a band of wavelength around 273.48 nm which is ascribed to π-π* or n-π* electronic transition. The absorption spectrum obtained after the 6 h immersion of LCS for the bromelain inhibited HCl solution showed a shift of the absorption band from 273.48 to 333.49 nm (bathochromic shift). The shift in the absorption band confirms the formation of a complex between bromelain and Fe+2 ions released during corrosion reaction.

3.7. Statistical Analysis The statistical test was employed for comparing the inhibition efficiency of bromelain obtained by using different techniques such as weight loss, potentiodynamic polarization and electrochemical impedance spectroscopy. The test was performed at a confidence level of 95% with a significance level of α = 0.05 by ANOVA and the obtained results were displayed in Table 7. If the obtained p-value is greater than the significance value, then the means are not statistically significant. The p-value for bromelain is 0.78 which is greater than 0.05. Thus, the obtained inhibition efficiencies using different techniques are not significantly different.

3.8. Mechanism of Corrosion Inhibition The inhibition process of LCS in the studied environment can be explained by the adsorption of bromelain on the LCS surface. Adsorption of bromelain on LCS surface decreases corrosion. The bromelain constituents contain O, N and S atoms as mentioned in the EDAX analysis. Thus, these compounds could be protonated in the acid solution. The charge on low carbon steel surface can be determined by using the following relation as Ecorr-EZPC= Փ, where Փ is correlative scale, EZPC is the potential of zero charge. The value of EZPC for low carbon steel in 1 M HCl is reported as - 498 mV vs. Ag/AgCl [61]. The obtained value of E corr for low carbon steel in 1 M HCl is - 488 mV vs. Ag/AgCl. Since Ecorr-EZPC>0, so the steel surface bears positive charge [62] and favors the adsorption of Cl- ions present in the corrosive solution. This turns the charge on steel surface from positive to negative causing the protonated bromelain constituents to electrostatically interact (physisorption) with the LCS surface. The bromelain molecules may be adsorbed on the LCS surface via the chemisorption mechanism, involving the displacement of

water molecules from the LCS surface and sharing of electrons between the heteroatoms and LCS. Hydrogen bonding with N-atom also assisted the adsorption of inhibitor molecule on the LCS surface. Partial transference of p-electrons and a lone pair of electrons located on nitrogen, oxygen and sulfur atoms also plays a role in the adsorption of bromelain on the LCS surface [63].

4. CONCLUSION 

Bromelain acts as an efficient green inhibitor for LCS 1 M HCl solution at temperature 308-338 K. Inhibition efficiency of bromelain increases with an increase in the concentration of the inhibitor and with the rise in electrolyte's temperature attaining maximum inhibition efficiency of 97.6% at 1000 ppm and 338 K.



Ecorr values in presence of bromelain tend to shift towards the anodic direction, with the extent of shift being less than 85mV with respect to blank LCS sample. This indicates that bromelain in HCl solution acts as mixed-type corrosion inhibitor with predominantly anodic effect



Electrochemical impedance studies reveal that the introduction of bromelain into acid solution decreases the double layer capacitance (Cdl) and increases the charge transfer resistance (Rct). The decrease in the Cdl and increase in the Rct values in the presence of bromelain is associated with the adsorption of bromelain molecules over the LCS surface, which interns slows down the corrosion process.



The obtained values of regression coefficient (R2) for Langmuir adsorption isotherm is > 0.999, which indicates that Langmuir isotherm is best fitted in the current study.



The adsorption free energy of the bromelain on LCS surface in the range of -25.68 to 26.86 KJ/mol suggests mixed adsorption mechanism.



UV- visible spectroscopic measurements confirmed the existence of a complex between bromelain and Fe+2 ions.



A less corroded surface morphology for inhibited steel compared to uninhibited steel confirmed the formation of protective film of bromelain molecules on the LCS surface.



ANOVA statistical test suggests that obtained inhibition efficiencies using different

techniques are not significantly different.

ACKNOWLEDGMENT One of the authors Megha Basik acknowledges DST-PURSE for providing financial assistance.

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Figure Captions: Figure 1 Tafel plots of low-carbon steel in 1 M HCl in the absence and presence of different concentrations of bromelain at 308 K Figure 2 (A) Nyquist plots for low-carbon steel in 1 M HCl in the absence and presence of different concentrations of bromelain at 308 K. (B) Bode Modulus and Bode Phase plots for lowcarbon steel in 1 M HCl in the absence and presence of different concentrations of bromelain Figure 3 Equivalent circuit Figure 4 Variation of % η with time Figure 5 Log CR vs 1/T plots in absence and presence of bromelain Figure 6 Log CR/T vs 1/T plots in absence and presence of bromelain Figure 7 Langmuir adsorption isotherm for bromelain adsorbed on the LCS surface in 1 M HCl at different temperatures Figure 8 Surface characterization by SEM for low carbon steel in 1M HCl (a) Polished surface (b) in the absence of inhibitor for 6 h (c) in the presence of 1000 ppm concentration of inhibitor for 6 h Figure 9 EDAX spectra of (a) Polished LCS surface, (b) LCS exposed in 1 M HCl and (c) LCS exposed in 1 M HCl in the presence of 1000 ppm concentration of bromelain Figure 10 UV–visible spectra of (a) 1 M HCl with 1000 ppm bromelain; (b) 1 M HCl with 1000 ppm bromelain after LCS immersion for 6 h at 308 K

Figure 1

(A)

(B) Figure 2

Figure 3

Figure 4

Figure 5

Figure 6

Figure 7

(a)

(b)

(c)

Figure 8

(a)

(b)

(c) Figure 9

Bromelain +2

Bromelain+Fe

Absorbance

2

(b)

(a) 0

300

400

500

Wave length (nm)

Figure 10

600

700

Table 1 Potentiodynamic polarization results for low carbon steel in 1 M HCl without and with different concentrations of the bromelain at 308 K. Inhibitor concentration (ppm)

Ecorr (mv)

Icorr (A/cm2)

βa/ (mV/dec)

βc/

CR (mmpy)

%

(mV/dec) -

Blank

- 488

1.01x10-3

117.1

- 73.9

12.43

100

- 473

3.62x10-4

127.3

- 80.2

4.20

66.19

200

- 464

8.86x10-5

120.3

- 116.5

1.03

91.72

300

- 459

8.33x10-5

118.7

- 93.7

0.97

92.22

500

- 447

7.85x10-5

105.8

- 89.0

0.91

92.66

700

- 455

6.66x10-5

137.6

- 101.4

0.77

93.78

1000

- 444

4.83x10-5

102.8

- 74.4

0.56

95.49

Table 2

Electrochemical parameters of impedance for low carbon steel in 1 M HCl without and with different concentrations of bromelain at 308 K.

Rs

Rct

Cdl x10-4

(Ωcm2)

(Ωcm2)

(Fcm-2)

Blank

0.85

14.68

6.35

69.6

6.45x10-4

0.99

-

100

0.97

42.85

5.72

84.6

5.82x10-4

0.99

65.75

200

2.22

117.3

2.04

201.8

2.07x10-4

0.99

87.49

300

2.76

213.6

1.33

370.1

1.34x10-4

0.99

93.13

500

2.53

275.8

1.03

229.6

1.04x10-4

0.99

94.68

700

5.28

376.9

0.98

291.3

9.93x10-5

0.99

96.11

1000

5.37

443.1

0.72

155.7

7.32x10-5

0.99

96.69

Inhibitor concentration

χ2 x 10-3

%

CPE (F) Y0

n

(ppm)

Table 3 Corrosion parameters for carbon steel in 1 M HCl in absence and presence of the bromelain from weight loss measurements at different temperatures.

Temperature 308 K

318 K

Inhibitor CR concentration (mgcm-2 h-1) (ppm)

θ

%

CR (mgcm-2 h-1)

Blank

0.97±0.0558

-

-

100

0.37±0.0245

0.62

200

0.15±0.0014

300

328 K

338 K

%

CR (mgcm-2 h-1)

θ

%

CR (mgcm-2h-1)

θ

%

1.49±0.0526 -

-

5.39±0.0328

-

-

10.55±0.0661

-

-

62.1

0.52±0.0032 0.65

65.4

1.75±0.0151

0.67

67.4

3.08±0.0241

0.70

70.8

0.84

84.0

0.20±0.0009 0.86

86.2

0.57±0.0033

0.89

89.3

0.86±0.0348

0.91

91.8

0.13±0.0031

0.86

86.5

0.16±0.0059 0.89

89.3

0.41±0.0157

0.92

92.4

0.62±0.0149

0.94

94.1

500

0.10±0.0011

0.89

89.4

0.13±0.0018 0.91

91.5

0.33±0.0058

0.93

93.7

0.47±0.0074

0.95

95.5

700

0.08±0.0010

0.91

91.5

0.09±0.0028 0.93

93.3

0.29±0.0059

0.94

94.6

0.36±0.0089

0.96

96.5

1000

0.06±0.0037

0.93

93.1

0.07±0.0023 0.94

94.7

0.24±0.0053

0.95

95.5

0.25±0.0059

0.97

97.6

θ

Table 4 Comparison of the Inhibition Efficiency of tested Bromelain with other reported Plant extract obtained for LCS in acidic medium.

Natural extract Bromelain Date Palm Seed Extracts Zenthoxylum alatum plant extract Schinopsis lorentzii Extract Pectin iota-carrageenan Aquilaria subintergra leaves extracts Gum Acacia

Medium HCl HCl HCl HCl HCl HCl HCl H2SO4

Temp 308 K 303 K 303 K 303 K 308 K 283 K 303 K 303 K

Optimum concentration 1000 ppm 2500 ppm 2400 ppm 2000 ppm 2000 ppm 1600 ppm 1500 ppm 1500 ppm

Inhibition Efficiency 93.1 86.8 % 95% 66 % 92 % 63.3% 93.2% 83.09%

Technique Used

Reference

Weight loss Electrochemical Impedance spectroscopy Electrochemical Impedance spectroscopy Potentiodynamic polarization Weight loss Weight loss Weight loss Weight loss

Present study [16] [17] [18] [20] [44] [19] [45]

Rollinia occidentalis extract Thymus algeriensis extract Acalypha torta Leaf Extract xanthan gum

HCl HCl HCl HCl

298 K 298 K 303 K 303 K

1000 ppm 1000 ppm 1000 ppm 1000 ppm

85.7% 76.6% 89% 74.24%

Potentiodynamic polarization Weight loss Weight loss Weight loss

[46] [47] [48] [49]

Table 5 Activation parameters for low carbon steel in 1 M HCl in the absence and presence of different concentrations of the bromelain.

Inhibitor concentration (ppm)

Pre-exponential factor (A)

ΔH

Ea

ΔS

(kJ mol-1)

(kJ mol-1)

(J mol-1 K-1)

(g m-2 h-1) Blank

2.16 x 105

72.92

70.24

-18.72

100

4.11 x 104

65.61

62.92

-50.52

200

3.54 x 103

53.28

50.59

-97.44

300

1.45 x 103

48.50

45.82

-114.50

500

1.26 x 103

48.23

45.55

-117.29

700

1.08 x 103

47.89

45.21

-120.19

1000

5.33 x 102

44.21

41.52

-133.71

Table 6 The thermodynamic parameters of adsorption of the inhibitor at different concentrations for low carbon steel in 1 M HCl solution. Temp. (K)

Slope

308 318 328 338

1.03 1.02 1.01 0.99

Intercept

0.044 0.038 0.030 0.027

R2

0.999 0.999 0.999 0.999

Kads (Lg -1)

Gads

22.68 26.25 33.11 35.97

-25.68 -26.06 -26.65 -26.86

(kJ mol-1)

Table 7 Analysis of variance (ANOVA) for inhibition efficiency of CQ and CQ-ZrAc inhibitor using different techniques in 1M HCl.

Degree of Freedom

Sum of squares

Mean square F-value

P-value

Between Techniques

2

77.44

38.72

0.78

Residual

15

1970.59

131.37

Total

17

2048.04

Bromelain 0.29



Bromelain, an environmental-friendly compound was evaluated as corrosion inhibitor.



The % η of bromelain reached 97.6% at 1000 ppm in 1 M HCl.



Bromelain behaved as mixed type of inhibitor.



The adsorption of bromelain on low carbon steel surface obeys Langmuir adsorption model.