Corrosion inhibition and thermodynamic studies on carbon steel in well water by ethanolic extract of Alpinia oﬃcinarum leaves (Lesser galangal)-Zn 2+

Objectives: To examine the thermodynamics and kinetics of Alpinia oﬃc-inarum’s (AO) inhibitory response on carbon steel (CS) corrosion in well water. Weight-loss (WL), potentiodynamic polarization (PDP), electrochemical impedance spectroscopy (EIS) methods and various pHlevels are utilized to calculate the inhibition eﬃciency (IE) and corrosion rate (CR). Methods: WL and PDP and EIS techniques were performed to assess the corrosion inhibition and adsorption capacity of an ethanolic extract of AO leaves on the CS in well water. SEM and AFM techniques were analyzed to explore the creation of a safe layer onto the CS surface. Findings: The maximum IE of 93% is attained from the WL study at 303K. For the AO-Zn 2+ system, when the temperature is increased from 303K to 343K, CR is also increased and IE is decreased. Arrhenius plots show a straight line, indicating the eﬀect of temperatures on the CR. PDP ﬁnd-ings indicate that the AO-Zn 2 + system performs as a mixed-type inhibitor. The ∆ G ads values are less than -20KJ/mol, signifying that physisorption on the CS surface has occurred. The positive values of ∆ H demonstrate AO leaves are adsorb-ing endothermically onto CS surface. The high adsorption of ethanolic extract of AO leaves on the CS surface in well water is reﬂected by positive activation energy values. Langmuir model is the best ﬁt for the adsorption of ethanolic extract of AO leaves. Novelty: The nature of the adsorption process on the CS surface in well water was investigated by thermodynamic parameters ( ∆ G ads , q ads , ∆ S ads , ∆ H) and activation energy (E a ) using a combined inhibitor (AO-Zn 2+ ) system. The regression coeﬃcient (R 2 ) values were calculated using the various adsorption isotherms to determine the pattern of inhibition on the CS surface. The F-test and synergism parameters were used to establish the synergistic inhibition (SI) impact existing between ethanolic extract of AO leaves and Zn 2+ . environmentally friendly metal corrosion inhibitors. It is non-toxic, pure and readily available (14–19) . The Alpinia officinarum – Zn 2+ system has been explored for its ability to control CS corrosion in well water, corrosion inhibition of ethanolic extract of AO leaves has been investigated, and it is inferred as a good prevent from corrosion. Synergistic and Antagonistic effect of plant extract is investigated (20) . The corrosion inhibition performance of CS is discussed by green inhibitor-Zn 2+ system (19–25) .


Inroduction
Corrosion is a natural chemical process of transforming metal and alloy structures into chemically stable oxides, hydroxides and sulphides (1) . Carbon steel (CS) corrosion is a prevalent issue in the petroleum industry (2) . Steel Corrosion and its alloys have a well-studied industrial problem (3) . Natural inhibitors are earning a lot of attention in the corrosion domain due to their safety, cheap, good biocompatibility, environmentally suitability and recyclability (4) . Naturally occurring substances (Amino acids, alkaloids, polyphenols) (5)(6)(7) and plant extracts (8) are examples of widely disseminated and low-value materials such as pollutants of agricultural production activities and its wastes. Inhibition positions by material choice, utilization of defensive coatings, and cycle conditions/climate change as perhaps the most well-known method utilized for corrosion control. Natural compounds that are emphatically polar functional sets are regularly utilized corrosion inhibitors, a significant number of which depend on nitrogen, S, P and O (9,10) , considering adsorption on the CS surface, which has been seen to depend on the physicochemical properties of the functional groups and the thickness of electrons in the giver atom. In corrosion studies, the WL study is the best method to calculate the CR (3) . When the combined inhibitor is added to the blank system, the IE and CR is evaluated using different concentration of inhibitor onto metal surface (11) . PDP and EIS studies indicate that the mode of inhibition of plant extract for the corrosion resistance on the CS surface (3) . Metals exist in nature as chemical compounds, which demand the extractions of energy from their mineral form, as well as in equivalent amount of energy for chemical and electrochemical reactions during corrosion. As a result of the corrosion process, metals revert to their natural state (12) . The most prominent study in the recent times has been the hunt for environmentally safe, easily degradable, and abundantly present inhibitors. Natural elements such as fruits, leaves and flowers have been shown to effective corrosion inhibitors (13) . Plant extract has been reported as one of the most cost-effective, degradable, water soluble and environmentally friendly metal corrosion inhibitors. It is non-toxic, pure and readily available (14)(15)(16)(17)(18)(19) . The Alpinia officinarum -Zn 2+ system has been explored for its ability to control CS corrosion in well water, corrosion inhibition of ethanolic extract of AO leaves has been investigated, and it is inferred as a good prevent from corrosion. Synergistic and Antagonistic effect of plant extract is investigated (20) . The corrosion inhibition performance of CS is discussed by green inhibitor-Zn 2+ system (19)(20)(21)(22)(23)(24)(25) .

Preparation of carbon steel (CS) specimens:
CS specimens were cut into 3.5cm x 1.5cm x 0.2cm for examinations. These CS were polished using 1/0 to 6/0 grade emery sheets. To remove contaminants, they were degreased through acetone, washed in double distilled (DD) water and dried and kept in a desiccator before weighing and dipping. All the weighing was done through digital balance (Shimadzu AY62) by a sensitivity of 0.1mg in 200g range. The CS specimens were serially numbered for identification. For inserting the hook, a small smooth hole of 0.2mm was drilled nearby the top edge centre of the specimen, the surface of CS specimens were carefully coated with epoxy resin (araldite) leaving 1cm 2 exposed surface area.

Preparation of ethanolic extract of Alpinia officinarum leaves
The AO plant leaves were cut into little bits and dried in an air oven at 70 • C for 2h. They were finely pulverized into powder. The sample (10g) was dissolved in 200ml ethanol and then refluxed for 3h in a 250ml RB flask. The refluxed solution was then thoroughly filtered through Whatmann filter paper and filtrate was heated on a water bath to completely vaporize the moisture content, yielding the dry compound. The dried compound (1g) was dissolved in a 100ml standard measuring flask filled to the mark with DD water. Corrosion test solutions ranging from 100ppm to 500ppm were made from the stock solution using a specific quantity of ethanolic extract of plant leaves in well water and using it for the corrosion investigations.
https://www.indjst.org/ It reduces postprandial blood glucose increases by inhibiting carbohydrate metabolism. The recovery of insulin-discharging ß-cells in the pancreas can be aided by eating bits of galangal.

Mass spectrum (MS) study
The structure of the bioactive component (Beta-caryophyllen) found in ethanolic extract of AO leaves is presented below and, Figure 2 represents the mass spectrum of the key active constituent found in the ethanolic extract of AO leaves. https://www.indjst.org/

Weight-loss method (WL)
WL study was operated to calculate the CR and percentage of IE (26) . CS specimens were polished to mirror finish by 1/0 to 6/0 grade emery sheets, degreased through acetone, washed with DD water, dried with an air drier and kept in a desiccator for 15minutes. Then the CS specimens were weighed accurately using digital balance (Shimadzu AY62).

Determination of IE and CR
The corrosion inhibition of the ethanolic extract of AO leaves can be determined using weight-loss study. CS was immersed in well water with various concentrations of AO leaves at 303K. After one day of immersion, CS samples were collected, washed with tap water, dried and gauged to determine the IE (27,28) and CR (28)(29)(30) using the following equations: Where, W B and W I is the weight-loss for Blank and inhibitor system.
Where, mmpy is millimetres per year, WL is weight-loss, D is Density (7.85g / cm 2 ), A is surface area (cm 2 ) and T is immersion period (h).

Calculation of surface coverage (θ )
Surface coverage values can be calculated using the WL study (31) . These values are used to identify the nature of the adsorption for combined inhibitor molecules (AO-Zn 2+ ) and a suitable isotherm model to the adsorption process. When the IE value is 100%, the surface coverage value should be unity (28) . https://www.indjst.org/

Electrochemical techniques
EIS and PDP methods were directed in three-cathode electrochemical cell gathering by utilizing the model:760D, CHI (electrochemical analyzer). A reference electrode operates as a saturated calomel electrode. The Counter electrode serves as a platinum (Pt) sheet. CS specimen behaves as a working electrode. IE was calculated using the following formulas (32) .
Where, R ct is charge transfer resistance of blank and AO-Zn 2+ system.
Where, 'I corrosion ' is corrosion potential of the AO-Zn 2+ and blank system.

Atomic force microscopy (AFM)
AFM is one of the most powerful techniques for understanding the surface morphology (33,34) . It has become a highly reliable tool for roughness investigation of surfaces (35) . Two dimensional (2D) AFM images were obtained using the Veeco dinnova instrument with a scan rate of 0.6 Hz/Seconds. AFM image analyses were performed to obtain the root mean square roughness (R rms ), average roughness (R a ) and maximum peak to valley height (P-V) values (36) .

Scanning electron microscopy (SEM)
SEM study was carried out in JEOL MODEL 6390-SEM. SEM images of pure CS specimen (uncorroded), CS submerged in well water without inhibitor for one day duration (corroded) and CS specimens submerged in well water with an optimum concentration of inhibitor for one day duration (protected) were taken. These SEM images were analyzed to compare the smoothness on the surface of CS and to conclude the adsorption film formation by the inhibitor on the CS specimens (37)(38)(39) .

Weight-loss method for AO-Zn 2+ system
Weight-loss study was carried out for CS specimens immersed in well water for one day in the absence and presence of different concentrations of Zn 2+ ions (10, 20 and 30ppm) and ethanolic extract of AO leaves. The IE and CR are calculated and given in Table 2. IE and different concentrations of AO leaves (100, 200, 300, 400 and 500ppm) in absence and presence of Zn 2+ is plotted (IE Vs ppm) and displayed in Figure 3. An ethanolic extract of AO leaves (500ppm) alone offered 63% IE and dropped the CR from 0.1809 to 0.0669, while 30ppm of Zn 2+ alone offered 29% IE and dropped the CR from 0.1809 to 0.1284 ( Table 2). The IE of AO and Zn 2+ alone do not have maximum. However, the combination of AO (500ppm) and Zn 2+ (30ppm) has an IE of 93% and CR of 0.0127, making it the maximum IE for AO-Zn 2+ system. Therefore, the IE is increased and CR is also reduced, when the concentration of combined inhibitor (AO-Zn 2+ ) system is increased (30,40) . https://www.indjst.org/ Anodic reaction (28,41) : Fe → Fe 2+ + 2e − Cathodic reaction (41) :

Effect of pH on AO -Zn 2+ System
The IE and CR are computed using different pH levels. At pH 8, AO (500 ppm) and Zn 2+ (30ppm) provide the highest IE of 93%. Table 3 provides the data on the influence of pH on AO -Zn 2+ system. The maximum CR was found from an ethanolic extract of AO leaves at pH 3.0; when the pH was lowered to 3 by adding dilute HCl, the IE was reduced. This is because, at pH 8, the protective film of the ethanolic extract of AO leaves is damaged by the robust attack of H + ions when the acid is added. The CR has been reduced from 0.0496 to 0.0127 for AO-Zn 2+ system. When pH of an ethanolic extract of AO leaves is raised to 12 by adding NaOH, the CR increases. This could be related to may be the fact that at high pH, the amount of Zn 2+ accessible for transport towards the CS surface is reduced, as Zn 2+ in a bulk solution precipitate as Zn(OH) 2 (42,43) . Graph for effect of pH on CR for AO-Zn 2+ system is given in Figure 4 .

Effect of Immersion Period on AO-Zn 2+ System:
The weight-loss investigation for the CS in well water containing various concentrations of AO leaves with additive of Zn 2+ ions was used to investigate the effect of immersion period at 1, 3, 5, and 7 days. Table 4 summarizes these findings. Figure 5 shows the IE and CR of combined inhibitor (500ppm of ethanolic extract of AO leaves: 30ppm of Zn 2+ ) system as well as the effect of immersion period (1, 3, 5 and 7days). The maximum IE of 93% is recorded from a 500ppm of AO and 30ppm of Zn 2+ system in one day immersion period at 303K. The percentage of IE is reduced for AO leaves extract, when contact period is increased from 1 to 7 (days) (44) . The effect of immersion period on an ethanolic extract of AO leaves proved that protective layer formed on the CS surface deteriorates and dissolves in solution by the corrosion media and accelerates the corrosion process (42,(45)(46)(47)(48) .

Effect of temperature on CR and IE
The CR and IE were computed for the CS immersed in well water containing the ethanolic extract of AO leaves at various temperatures (313K, 323K, 333K and 343K) using weight-loss study. Table 5 summarizes the observed CR and IE for AO-Zn 2+ system. When the temperature is increased from 303K to 343K, the CR is increased and also IE is decreased. Due to enhance desorption process on the CS surface at higher temperature (12) and solubility of the protective film formed rapidly on the CS surface (49) . This finding suggested that the components of the ethanolic extract of AO leaves are physisorbed on the CS surface (49)(50)(51)(52) . Figure 6 depicts the effect of temperature on IE using various concentration of AO extract. https://www.indjst.org/

Kinetic Studies on Temperatures
The effect of temperature on the CR is indicated by Arrhenius plots, which show a straight line for blank and an optimum concentration of ethanolic extract of AO leaves on the CS in well water. The Arrhenius plots are drawn between CR and 1/T for 303K, 313K, 323K, and 333K ( Figure 7). Arrhenius equation (53) :

Kinetic studies on corrosion process
Activation energy (E a ) and enthalpy change data were computed to identify the adsorption process and type of reaction on the CS surface using ethanolic extract of AO leaves-Zn 2+ . When the concentration of ethanolic extract of AO leaves is raised, E a and ∆H values are increased. E a and ∆H values are positive in sign. The positive values of E a reveal that a highly adsorption on the CS surface (54) . The values of ∆H indicate that adsorption of AO extract is endothermically on the CS surface (31,53,(55)(56)(57) .
The results are given in Table 6 . The activation energy (27,30) and enthalpy change (55) values are calculated using the following equations.

Thermodynamic studies on adsorption process
Thermodynamic parameters are computed to detemine the type of adsorption on the surface of CS. Adsorption of ethanolic extract of AO leaves has a spontaneous process (40) onto CS surface with physisorption (57) , as evidenced by the free energy of adsorption (∆G ads ) and heat of adsorption (q ads ) negative values (27,53) . ∆G ads have negative values of less than -20KJmol -1 , implying that the nature of adsorption is physisorption (30,53,(56)(57)(58)(59) for the investigative inhibitor system. When compared to the other three temperatures, ∆G ads has the highest negative value of -18.4KJmol -1 at 303K and the lowest negative value of -16.4KJmol -1 at 333K. The negative values of entropy of adsorption (∆S ads ) show that an ethanolic extract of AO leaves form a protective layer on the CS surface (53,54,(56)(57)(58)60) . Result of thermodynamic parameters are given in Table 7 . The ∆G ads (53,61) , q ads (27) and ∆S ads (4,28,58) values are calculated using the following equations.

Adsorption isotherms
Adsorption models are analyzed to identify the mode of corrosion inhibition, the nature of interaction between the adsorbed inhibitor and CS surface (62,63) . In present work, four adsorption isotherm models are studied for regression co-efficient (R 2 ) values obtained. From R 2 values of four adsorption isotherm (Langmuir, Flory-Huggins, EL-Awady and Temkin) are almost equal to one (59,(62)(63)(64) . R 2 value of Langmuir isotherm model is much higher than other three adsorption model (Table 8 ). Therefore, the adsorption behavior of the inhibitor has followed a Langmuir model. All the isotherms are represented by the common form (31,65) .

Synergism parameter (SI) :
The synergism parameters are utilized to determine the synergistic effect of AO-Zn 2+ system (68) . Synergism parameter (S I ) is a measure of synergistic inhibition of corrosion (S I ) (69) . The synergism parameter (S I ) is computed by using the formula (67) : ; ' ′ θ 1 ' and ' θ 2 ' are surface coverage for AO − Zn 2+ ; · θ ′ 1+2 is cumulative surface coverage to AO-Zn 2+ . The data for Synergism Parameters are given in Table 10 .

F-study
F-test was analyzed to examine the effect of Zn 2+ on the IE of ethanolic extract of AO leaves as statistically significant or not onto CS surface (70,71) . Data of F study are shown in Table 11. The obtained F-values for 10ppm and 20ppm of Zn 2+ are 1.38 and 1.77 for AO system. These F-values are smaller amount than the critical F-value 5.32. Hence, they are not statistically significant (47) . The calculated F-value for 30ppm of Zn 2+ is 6.78 for the concentration of AO system. Because this value is higher than the critical F-value 5.32, this value is statistically significant (47) . https://www.indjst.org/

Potentiodynamic polarization study
PDP investigation yielded the data of Corrosion current (72) , corrosion potential (73) , anodic and cathodic slope values (74) and linear polarization resistance (75) obtained from Tafel curves. When the CS is submerged in blank system, corrosion potential is -580mV. The AO-Zn 2+ (500ppm-30ppm) system shifted into -620mV from -580mV in a binary system. It has been reported (76) that if the corrosion potential are greater than 85mV in well water, the type of inhibition mechanism is designated as either oxidation or reduction reaction. In this investigation, highest distortion shows through AO is < 41mV, from which it could be attributed to the mixed mode of inhibition. Similar observations are observed and reported the inhibitive nature of AO extract to be mixed-type inhibitor (75) . LPR and I corr value for well water is 963Ωcm 2 and 5.5431µA/cm 2 for AO-Zn 2+ (500ppm-30ppm) systems, the LPR is raised from 963 to 3249Ωcm 2 , as well as I corr is decreased to 0.9479 from 5.543µA/cm 2 for AO and Zn 2+ system, Therefore, from the above result, the LPR values are increased but I corr values are decreased (77) , when added the concentration of inhibitor to blank (well water) system. These results confirm the thin layer is developed on the CS surface (78) .

AC Impedance Spectra
The EIS investigation was utilized to determine the safe film formed on the CS surface. During a thick layer formation on the CS surface using AO-Zn 2+ system, this incorporated with increases R ct and reduces C dl value (79) . The semicircle in this study implies that R ct may be involved in the inhibition of corrosion process (80) . A semicircle is formed over a CS sheet by using AC impedance analysis to account for roughness and the interfacial phenomenon, a semicircle found to be deviated instead of ideal semicircle (81) . Table 13 shows the Nyquist plots' results. The CS immersed in well water has a R ct of 391Ωcm 2 and a C dl of 1.3137µF/cm 2 . For the AO-Zn 2+ system, R ct value has been increased from 391 to 2610Ωcm 2 , while C dl value has been dropped from 1.3137 to 0.02975µF/cm 2 . When the inhibitor was added to the blank system, R ct value increased while the C dl value decreased. The thickness of a safe layer formation by adsorption of the inhibitor molecule is enhanced along CS/solution connection (82,83) . The investigative inhibitor system acts as a good prevention activity on the CS surface in well water. The EIS spectra are given in Figure 13.
https://www.indjst.org/  Figure 14a displays the FTIR spectra for pure AO extract. A peak at 3441cm -1 is associated with -OH bond vibration. The stretching frequency at 1609cm -1 is assigned for C=C bond vibration (84) . The peak around 1122cm -1 indicates C-O stretching vibration. The protecting layer produced on the CS surface in well water for AO-Zn 2+ system is depicted in the figure. A band around 3469cm -1 and 1617cm -1 suggests the existence of -OH and also C=C groups sequentially. A band vibration of C-O shows around 1121cm -1 . These shifts confirm that the electron cloud density of C-O, -OH and C=C functional groups in AO. A peak around 620cm -1 is assigned for the Zn-O groups (Figure 14b).

Scanning Electron Microscopy study
The SEM is an effective tool for examining the surface morphology of CS (83) . The corrosion of uninhibited and inhibited of the CS is investigated by using SEM. As illustrated in Figure 15a, the CS (polished) specimen is smooth and has no corrosion product on the CS site (85) . As depicted in Figure 15b, the CS immersed in well water is found to be rough with corrosion product on the CS surface, as well as cracks and damages (86) . Because the CS immersed in blank system contains AO (500ppm) and Zn 2+ (30ppm), the surface of the CS becomes smoother and less damaged, as shown in Figure 15c. A highly compact protective film completely covered on the CS surface through adsorption of insoluble the ethanolic extract of AO leaves-Zn 2+ composition existing in anodic position and zinc hydroxidedeposits in cathodic position (85) .

AFM analysis
2D AFM images were analyzed the roughness parameters of pure CS (uncorroded), CS submerged in well water (corroded) and CS submerged in a blank system (well water) with inhibitor systems (protected) are coordinated to visualize the smoother CS surface through the adsorption of inhibitor molecules (55,87,88) . Data of AFM parameters are shown in Table 14 . Figure 16a shows the CS surface has a smooth texture and lower R a , R rms and P-V values. Figure 16b shows a severely damaged surface following the corrosive attack without AO leaves extract, R a (average roughness), R rms (root mean square roughness) and P-V (maximum peak-to-valley height) values are increased. Figure 16c, depicts a smoother CS surface that is obviously a smaller amount attacked by the corrosive medium and has R a , R rms roughness part that is greatly closer to the polished CS surface. AFM parameters reveal that a safe film forms onto CS surface from corrosion process (59,(89)(90)(91) , when exposed to the blank solution occurs in the existence of ethanolic extract of AO leaves. https://www.indjst.org/

Conclusion
Corrosion kinetic and adsorption thermodynamic effect for AO leaves extract onto CS surface in well-water were investigated using WL measurement. The IE and CR were computed for varied immersion period and temperatures using various concentrations of AO with Zn 2+ ions. The effect of adding Zn 2+ as an additive to an ethanolic extract of AO leaves has been explored. The inhibition efficiency (IE) has been found to increase appreciably when Zn 2+ ions are added. The AO-Zn 2+ (500ppm-30ppm) system has a highest IE of 93%. A synergistic effect between the AO and Zn 2+ is proven by the obtained SI and F-values.Polarization curves reveal that AO-Zn 2+ system behave as a mixed-type inhibitor.Electrochemical impedance spectroscopy directs the CS corrosion is primarily controlled through R ct . The Langmuir model governs the adsorption of ethanolic extract of AO leaves. Activation energy values for AO-Zn 2+ systems are greater than that of the blank system, implying physisorption. The enthalpy values (positive) imply the reaction on the CS surface is endothermic. The ∆G ads and q ads values (negative) reflect that the adsorption of AO leaves extract on the CS surface is a spontaneous activity. The data of ∆G ads ranges from -16.4KJ/mol to -18.4KJ/mol, the ∆G ads values are less than -20KJ/mol. Hence, the adsorption of ethanolic extract of AO leaves is physisorption. The formation of Fe 2+ -AO complex with a CS surface is probed using FT-IR data. SEM studies indicate that the barrier layer formed over the CS surface.AFM findings suggest that the smooth surface on the CS is due to the creation of Fe 2+ -AO complex and also zinc hydroxide coatings on the CS surface.