Corrosion inhibition of ferrite bainite AISI1040 steel in H2SO4 using biopolymer

Abstract The purpose of this study is to investigate the corrosion behavior of the ferrite-bainite phase of AISI1040 steel and its corrosion inhibition using pectin in 0.5 M sulphuric acid medium. The corrosion study was conducted through weight loss, potentiodynamic polarization, and electrochemical impedance spectroscopy (EIS) measurements. The study was conducted within the temperature range of 313–343 K and immersion time of 1 h— 7 h. The inhibition performance of pectin has boosted with an increase in the concentration of pectin and decreased with the temperature and time of exposure. From the weight loss study, the highest inhibition efficiency of 85% was achieved at 5.0 g/L at 1 h of exposure at 40 °C. The activation, thermodynamic, and adsorption isotherm were adapted for the experimental results. The energy and enthalpy of activation emphasized on energy barrier for the corrosion process. The entropy of activation values is negative, indicating the association of molecules. The physical adsorption of pectin on the metal surface was confirmed by the free energy of adsorption (close to-20kj mol−1). The enthalpy of adsorption indicated the exothermic process and, arrangement of the molecules on the surface of the metal expressed by the entropy of adsorption. The optimum efficiency of 74.9% is achieved for the addition of 5.0 g/L of pectin at 313 K by the EIS technique. Surface analyses involving a scanning electron microscope and atomic force microscopy were carried out to understand the nature of the surface in the presence and absence of pectin.


PUBLIC INTEREST STATEMENT
Corrosion is a phenomenon that produces a less desirable material due to the deterioration of the component/system in a corrosive medium. The purpose of the investigation was to control the corrosion of heat-treated applicable material in a corrosive medium (0.5 M sulphuric acid) using a green inhibitor (pectin). To understand the nature of inhibitor and optimization, the study was carried out at various conditions of inhibitor concentration, range of temperature, and time of exposure. The measurements were done using weight loss, potentiodynamic polarization, and electrochemical impedance spectroscopy. The surface analysis was carried out by scanning electron microscope and atomic force microscopy. From the output achieved from the experiment, the pectin inhibitor was effective at the lowest time of exposure and temperature considered in the experiment at the optimum concentration of inhibitor. Thus, pectin is an efficient inhibitor for the corrosion inhibition of the ferrite-bainite phase of AISI1040 steel.

Introduction
Corrosion inhibitors remain an important way of protecting metals and their alloys from corrosion processes in a corrosive environment (Nathan, 1973;T. Olugbade & Lu, 2019b;T. O. Olugbade, 2021;Shinato et al., 2020;Yu. M. Panchenkoa, A. I. Marshakova, L. A. Nikolaevaa, 2020). Carbon steel finds numerous applications in many industries due to its excellent mechanical properties, low cost, and easy workability (T. O. Olugbade, 2021). However, they exhibit poor corrosion resistance, which results in a metal loss. The use of corrosion inhibitors is a very effective and economic way of protecting metals in industrial processes, such as pickling, industrial acid cleaning, and oil and gas well acidizing (T. Olugbade et al., 2019;T. Olugbade & Lu, 2019a). The heat treatment methods are applied to increase the mechanical properties thereby increasing the applicability of the metal. Different materials like steel, aluminum, titanium, etc. undergo corrosion when it comes in contact with corrosive environments (Ali, 2019;Deepa & Padmalatha, 2017). AISI1040 steel (also known as EN 8 steel) is a plain medium carbon steel with moderate strength. The material properties can be reinforced by the thermal treatment process, where the metal is heated and then cooled at the required temperature, to obtain the desired microstructure. This steel can be utilized in the manufacturing of automobile parts. Dual-phase (DP) steels have received massive consideration in the past few decades because of increased application in the automotive industry (Abedini et al., 2019;Salamci & S. Candan, 2017). The dual-phase structure of steels has a place with a gathering of present-day multi-phase steels described by ferrite matrix containing different phases (martensite, bainite, and leftover austenite) (Abedini et al., 2019;Gerengi et al., 2020;Keleştemur et al., 2009;Song et al., 2019). The concurrence of delicate/hard phases and their connection in working conditions permit both high strength and malleability to be accomplished. DP steels comprising of delicate ferrite and hard martensite show better mechanical properties compared to regular HSLA (High Strength Low Alloy) steels (Abedini et al., 2019;song et al., 2020). That has driven the experts to investigate their appropriateness for various structural and constructional purposes (Katiyar et al., 2019). Although a lot of research articles have come out concerning the excellent blend of mechanical properties in DP steels (A. Karimi, S. Kheirandish, 2017;Basantia et al., 2021;Bilal et al., 2019;Fereiduni & Ghasemi Banadkouki, 2014;Kumar et al., 2019;Shahzad et al., 2018;Xiong et al., 2019;Zhao et al., 2019;Zhu, 2019), further examinations are important to discover the influence of phase constituents on the corrosion resistance properties (Bignozzi et al., 2020;Gürkan Aydın, 2019;Ma et al., 2020;P. R. Prabhu, Prabhu, Sharma et al., 2020;Zhang et al., 2020). Furthermore, the temperature is generally reported to be one of the important factors that can influence metal corrosion and metal-inhibitor interaction (Bentiss et al., 2005;Mert et al., 2011). Numerous studies have examined the effect of steel corrosion inhibitors in aggressive environments where green inhibitors are present (Loto & Loto, 2020;Lowmunkhong et al., 2010;Obi-Egbedi et al., 2012;D;D Prabhu & Rao, 2013;Qiang et al., 2018;Solomon et al., 2010). The corrosion study of heat-treated applicable material using green inhibitors are limited. Therefore, the present study was done on the heat-treated AISI1040 steel to arrive at ferrite bainite phase followed by its corrosion behavior in 0.5 M sulphuric acid using weight loss, potentiodynamic polarization, and electrochemical impedance spectroscopy measurements. The corrosion inhibition studies were performed using pectin biopolymer varying time, temperatures, and inhibitor concentrations. The parameters of activation and thermodynamics of the material in the absence and presence of pectin have been calculated and discussed. The confirmation of the adsorption of pectin on the metal surface was carried out using a scanning electron microscope and atomic force microscopy.
The heat-treated test samples were prepared in a cylindrical rod shape of 19 mm diameter and 8 mm height. The specimen was abraded using different grades of emery paper (80-1200) and finally on the polishing disk with different grades (3 microns to 0.25 micron) of diamond paste. The polished sample was then washed with double distilled water, cleaned with acetone, and eventually dried before being immersed in the acidic environment.

Preparation of medium
The medium used in the investigation is standard sulphuric acid. A large quantity, solution of sulphuric acid was made ready by diluting an analytical reagent grade sulphuric acid to an appropriate volume. The solution was standardized by the volumetric method. The solution of concentration (0.5 M) was made and prepared from the stock solution. The experiments were conducted at temperatures ranging from 313 K, 323 K, 333 K to 343 K.

Inhibitor used
The inhibitor used in our experiments is commercially available pectin. It is a structural acid heteropolysaccharide that is found in the medium and primary lamellar cell walls of terrestrial plants. The main constituent is galacturonic acid, which is a sweet acid (a derivative of galactose).

Heat treatment
The samples were heated in a muffle furnace to the predetermined austenitization temperature (900 °C). The process PQ (Figure 1) represents the austenitization process. The samples were held at the austenitization temperature for a fixed duration of 2 h. After isothermal holding for 2 h, specimens are quickly taken out from the furnace for air cooling to get the room temperature structure.
The normalized specimens are used for the ferrite-bainite heat treatment. Here, the austenitizing is carried out at 750°C for 2 h followed by quenching in a salt bath containing an equal proportion of molten sodium nitrate and sodium nitrite mixture held at a specific temperature of 350°C isothermally. After isothermal holding for about 30 min, specimens are cooled to room temperature in still air. Figure 2 shows the dual-phase heat treatment procedure to obtain an F-B structure.
Figure 3(a) shows the microstructure of AISI1040 steel in as-received condition. The steel shows ferrite and lamellar pearlite grains. The steel rods might have undergone air cooling, resulting in the formation of fine grains. Figure 3(b) shows the microstructure of steel in the normalized condition. The microstructure displays fine and lamellar pearlite and proeutectoid ferrite. The interlamellar distance in normalized pearlite is smaller when compared to the as-bought steel as a result of normalizing heat treatment. Fine pearlite enhances the mechanical properties of the steel. Figure 3 (c) shows the duplex structure in the steel containing ferrite and feathery bainite. The steel is heated in between lower and upper critical temperatures of the iron carbide phase diagram where homogeneous austenite is not formed. During this process, the pearlite of asreceived steel converts into austenite first and proeutectoid ferrite remains unchanged. When the steel is quenched from this temperature, the austenite converts to lower temperature structures like bainite. When the steel is quenched in subcritical temperatures (350°C), the cooling curve enters the bainite zone. When austenite is cooled in this temperature range, carbon atoms redistribute in austenite. Low carbon regions transform to ferrite by diffusionless processes and result in fine needles of ferrite. As time passes, carbon diffuses out and precipitates in the form of fine carbides, the arrangement of carbides here is not in the form of lamellar structure. In this study, the steel displays upper or feathery bainite along with proeutectoid ferrite.

Electrochemical impedance spectroscopy (EIS) method
For the studies of EIS and Potentiodynamic Polarization (PDP), the working electrode was made of DP AISI1040 steel (the molded specimen has been exposed to corrosive media with 0.7 cm 2 surface area), a platinum electrode as a counter electrode, and a calomel electrode as a reference electrode. CH600E potentiostat was the workstation used for obtaining the EIS and PDP plots. The electrochemical experiments were carried out at 313 ± 1 K. All electrochemical measurements were performed in an aerated solution in unstirred conditions. The test samples were able to attain stable and consistent open-circuit potential (OCP) in 30 min. EIS measurements were done over a frequency range of 100 kHz to 10 mHz; small amplitude (10 mV) sinusoidal ac voltage was used. Experiments are repeated three times and the average of the best three agreeing values was reported.
The polarization resistance (R p ) values were derived from the EIS by adding the charge transfer resistance (R ct ) and resistance of the solution (R s ). The inhibition efficiency (IE EIS %) of the inhibitor was derived from R p values using the following equation 1: where R pðinhÞ and R p are the polarization resistance in the presence and absence of pectin, respectively.

Potentiodynamic polarization technique
The samples were polarized relative to the OCP with a potential of −250 mV cathodic and +250 mV anodically at a scan level of 1.0 mVs −1 . The anode and cathodic curves were extrapolated to obtain the corrosion potential (E corr ) as well as corrosion current densities (i corr ). The inhibition efficiency (IE PDP %) was assessed using i corr values using equation 2:  where i corr and i corrðinhÞ are the corrosion current densities in the absence and the presence of pectin, respectively.

Weight-loss method
Weight-loss corrosion measurements are very simple because they do not require specialized instruments apart from an accurate balance. Experiments are generally performed as per the standard method (A. G1-90, 1999). Weight-loss methods are predominantly slower than other techniques, but a good number of samples can be run at the same time. This method obtains the average corrosion rate over extended periods, according to the following equation 3: where K = a constant (3.45 x 10 6 ), W = difference in weight of the sample in gms, D = density of the sample in g/cm 3 , A = sample surface area exposed to a corrosive medium in cm 2 , t = exposed time in hours.
From the weight-loss method, the inhibition efficiency (IE w ) is calculated by using the following equation 4: where W 1 = the difference between the initial and final sample weights in a corrosive environment without an inhibitor (gms), W 2 = the difference between the initial and final weight of the sample in the presence of inhibitor (gms)

Effect of temperature and kinetic factors
The CRs measured at various temperatures (313-343 K) by the weight loss method were applied for the calculation of activation energy E a , activation enthalpy ΔH #, and activation entropy ΔS # . The determination of E a is done using equation 5 (Deepa Deepa Prabhu & Rao, 2019).
where R = 8.314 J mol −1 K −1 , T = absolute temperature (in Kelvin), and B = a constant which differs on metal type The ΔH # and ΔS # were calculated using equation 6:

Adsorption and thermodynamic factors
The isotherm is presented according to the best fit and correlation factor (R 2 ) to obtain adsorption equilibrium constant (K) from equation 7.
where C inh = inhibitor concentrations (in g/L) and θ = surface coverage With the value of K, the standard free energy of adsorption (∆G o ads ) was calculated using equation 8 (D D Prabhu & Rao, 2013).
where R = 8.314 J mol −1 K −1 , T = absolute temperature The plot of ∆G°a ds vs T was then used to obtain the standard enthalpy of adsorption (ΔH°a ds ) standard entropy of adsorption (ΔS°a ds ) using equation 9.

Scanning electron microscopy (SEM) and atomic force microscopy (AFM) analysis
The SEM of DP AISI1040 F-B steel immersed in 0.5 M H 2 SO 4 solution with and without 5.0 g/L pectin inhibitor was investigated by capturing the images with the magnification of 1 K X using EVO MA18. The AFM spectral images were taken using IB342 Innova model for the surface of corroded and inhibited surfaces of DP AISI1040 F-B steel in the corrosive medium of 0.5 M H 2 SO 4 with and without 5.0 g/L pectin.
The summary of methodology adopted during the experimental section has been presented in the form of flow chart as shown in Figure 4.

EIS method
The EIS measurements for the corrosion of DP AISI1040 F-B steel were performed in 0.5 M H 2 SO 4 and with different amounts of pectin at 313 K is given in Figure 5. The Nyquist plots exhibited a depressive capacitive loop in the high frequency (HF) range indicating the Faradic process at the electrode locations (Deepa & Padmalatha, 2017). Due to the non-uniform nature of the surface and surface inhomogeneity, the constant phase element (CPE) was used as a replacement for ideal capacitance/double-layer capacitance (C dl ) (Majd et al., 2019). The HF capacitive loop can also be attributed to the charge transfer reaction and the time constant of the dual electrical layer. The equivalent circuit is made up of the constant phase element (CPE) in parallel with the charge transfer resistance (R ct ). Figure 6 is obtained by adjusting the experimental data to the equivalent circuit model through ZSimpWin version 3.1 software. Comparing the experimental and calculated spectra confirms the relevance of the equivalent circuit adopted. The impedance parameters obtained are calculated and listed in Table 1. in 0.5 M H 2 SO 4 at 313 K A double-layer capacitance (C dl ) is replaced by a CPE in the circuit used is to take into account the depressed capacitive nature of Nyquist plotted and the C dl can be computed using the equation. 10 (Machnikova et al., 2008;Toppo et al., 2013).
where Q = CPE, ω max = maximum impedance rate and n = CPE exponent. If the value of n is 1, then CPE acts as a capacitor. The average value of n is 0.82. The slight change in capacity relative to its actual value was calculated using Equation 11 .
f max = is the frequency at the top of the semicircle (where-Z" is maximum). The polarization resistance (R P ) was calculated using equation 12.
By adding pectin compound to the sulphuric acid solution, there is a decrease in the corrosion rate indicated by the increase in the R p values of DP AISI1040 F-B steel through (i) pectin molecules adsorption onto the surface of the metal, (ii) decreasing in the rate of corrosion reaction, (iii) reduction in the rate of corrosive species diffusion into the metal surface and (iv) decreasing the electrical resistance of the metal surface (Izadi et al., 2017).
The value of R p increased and C dl declined with increased pectin concentration. Inhibitor molecule's adsorption on the surface forming a physical barrier between the metal and corrosive. The increased thickness of the double layer at the metal-electrolyte interface caused a decrease in C dl values. The C dl reduction is attributed to the local dielectric constant reduction. Water molecules can be replaced by pectin molecules on the metal surface, leading to the local dielectric constant reduction (Izadi et al., 2017).

Potentiodynamic polarization (PDP) technique
PDP measurements for the corrosion of DP AISI1040 F-B steel were made in 0.5 M H 2 SO 4 and with different quantities of pectin. PDP plots of corrosion are shown in Figure 7; it represents the corrosion of DP AISI1040 F-B steel in 0.5 M H 2 SO 4 having various concentrations of pectin at 313 K. For the anodic Tafel slopes obtained (β a ) and the cathodic slope of Tafel (β c ), no substantial variation in values occurred, suggesting that there has not been a change in the mechanism of the anodic and cathodic process with pectin (Li & Deng, 2012). The corrosion potential value (E corr ) was on average below ± 85 mV (Li et al., 2011;Nathan, 1973;Deepa Deepa Prabhu & Rao, 2019) Table 2. The i corr values decreased with increased pectin concentration indicating increased effectiveness of pectin inhibition. The inhibition efficiency achieved from the PDP measurements was consistent with that achieved from the EIS studies. increasing time of exposure as well with increased temperature suggesting the enhanced corrosiveness to the metal on exposure. Figure 9 reflects the inhibition efficiency for pectin with the selected metal and medium. There was an increase in efficiency with the increase in pectin concentrations. Increased pectin concentration increases inhibition effectiveness to approximately 85% at 5.0 g/L for 1 h exposure time. The inhibitory action of the natural compound relative to the sample may be attributed to the adsorption of the pectin component onto the metallic surface. This adsorption confines the disintegration of the material by blocking the corrosion process. The inhibition efficiency decreased with time of exposure could be due to the degradation of pectin when exposed for a longer time. Thus from Figure 9, it is clear that the plots have shown lesser inhibition efficiency (%) from 1 h to 7 h exposure time as well as with temperature from 313 K to 348 K.

Activation parameters and effect of temperature and immersion time
Arrhenius plot and ln (CR/T) vs 1/T in 0.5 M H 2 SO 4 comprising pectin on DP AISI1040 F-B steel is shown in Figure 10. Activation parameters for degradation of DP AISI1040 F-B steel in 0.5 M H 2 SO 4 comprising pectin are depicted in Table 3. The value of E a increased with increasing concentrations of pectin at each immersion time, showing that the energy barrier for the corrosion process (Amin et al., 2011). The values of E a were larger for lesser exposure time, which could be due to lesser corrosion of metal at lesser immersion time. The ∆H # values varied correspondingly to that of E a , matching the suggested mechanism (Deepa Deepa Prabhu et al., 2021). The ∆S # values were negative, characterizing the molecular association and decrease in disorder during the formation of the activated complex. (Oguzie et al., 2008). Figure 11 shows the three-level effect, different immersion times, weight loss, and inhibition efficiency for different concentrations of pectin. Increased weight loss and reduced inhibition efficiency were observed with higher exposure time and temperature due to an increased corrosion process at a higher exposure time and physical adsorption of the inhibitor. There is an electrostatic interaction between pectin molecules and the metal surface. These electrostatic interactions between the inhibitor and the metal surface reduce their attractive force with raise in temperatures.

Adsorption and thermodynamic factors
Langmuir adsorption isotherm (Akinbulumo et al., 2020) observed with pectin for all the studied temperatures and the time of exposure is illustrated in Figure 12 (a). A plot of ∆G°a ds versus T for pectin on DP AISI1040 F-B steel in 0.5 M H 2 SO 4 for the above-mentioned conditions is given in Figure 12(b). Thermodynamic values obtained for the pectin adsorption are given in Table 4.
The obtained values of ΔG°a ds at every immersion time were less than to À 20kJmol À 1 (Galo et al., 2020) and the value went up with the increase in temperature, which suggested the physical adsorption of pectin on the metal surface (Fouda et al., 2009). The ΔH°a ds values were negative, which is the indication of the exothermic process, which in addition supports the physisorption. ∆S°a ds values were significant and negative, indicating the orderly placement of the inhibiting molecules on the metal surface. (Tang et al., 2003).

SEM and AFM analysis
The metal AISI1040 dual-phase F-B steel undergoes severe corrosion in the 0.5 M H 2 SO 4 medium, which is observed in the two-dimensional SEM image. SEM is operated at a temperature below 298 K and humidity below 60 gm −3 , the current is varied during scan and voltage of 10-15 kV under vacuum environment. In Figure 13 (a) the metal surface seems to be even and smooth for the polished sample. Figure 13 (b) shows the DP AISI1040 F-B steel in 0.5 M H 2 SO 4 for 3 h, depict the uneven surface as a result of the corrosion of the metal and the deposition of the products of corrosion. Figure 13 (c) is the image of the surface of the metal in presence of 5.0 g/L of pectin. The addition of pectin produced a uniform layer covering the surface and forming a barrier between the metal and the medium preventing deterioration.
The AFM was operated at similar conditions of temperature and humidity as that of SEM, the current is varied at the probe and voltage varies between 1.6 and 1.8 V in a normal air environment. The AFM represents the three-dimensional images of DP AISI1040 F-B steel for the freshly polished, sample in Figure 14 (a) represents the smooth uniform surface before the corrosion. Figure 14 (b) represents the metal immersed in 0.5 M H 2 SO 4 . The rough surface is observed in the 3D representation of corroded metal. Figure 14 (c) is for inhibited metal surface in the presence of 5.0 g/L pectin. The values of maximum roughness R max were 197 nm for polished, 5025 nm for corroded, and 1990 nm for the inhibited surface. This is a clear indication of the adsorbed pectin on the surface of the AISI1040 steel (Pais & Rao, 2020), due to a decrease in the R max value for the inhibited sample surface when compared with the corroded sample roughness value.

Conclusions
In this investigation, corrosion behavior and adsorption ability of pectin on heat-treated dualphase AISI1040 steel with ferrite-bainite structure in 0.5 M sulphuric acid solution were done using weight loss and electrochemical techniques. Based on the data obtained, it is possible, to sum up, the following important points: • The pectin acts as a worthy eco-friendly inhibitor for the control of corrosion of DP AISI1040 F-B steel in 0.5 M H 2 SO 4 solution at 313 K.
• The optimum efficiency of 74.9 % is achieved for the addition of 5.0 g/L of pectin at 313 K by the EIS technique • PDP studies showed that pectin reduces both anodic and cathodic reactions thus acting as a mixed inhibitor on the surface of DP AISI1040 F-B steel.
• In the weight-loss study, the highest inhibition efficiency of 85 % was achieved at 5.0 g/L at 1 h of exposure at 40 °C.
• The IE of pectin increases with the increase in concentration and decreases with temperature, due to physical adsorption to the surface of the DP AISI1040 F-B steel and obeys Langmuir adsorption isotherm.
• The energy and enthalpy of activation confirmed the formation of a protective barrier for the corrosion process.
• The entropy of activation values was negative, indicating the association of molecules.
• The free energy of adsorption values was close to À 20kJmol À 1 , which suggested spontaneous physical adsorption of pectin on the metal surface.
• The enthalpy of adsorption indicated the exothermic process, and entropy of adsorption, indicating the arrangement of the molecules on the surface of the metal.
• SEM and AFM studies have shown that the surface inhibitor film reduces the corrosion rate of DP AISI1040 F-B steel in 0.5 M sulphuric acid solution.
Nomenclature EIS -Electrochemical Impedance Spectroscopy PDP -Potentiodynamic Polarization DP -Dual Phase HSLA -High Strength Low Alloy F-B -Ferrite Bainite H 2 SO 4 -Sulphuric Acid OCP -open circuit potential R p -Polarisation resistance R ct -Charge transfer resistance R s -Resistance of the solution R pðinhÞ -Polarization resistance in the presence of pectin R p -Polarization resistance in the absence of pectin IE -Inhibition Efficiency i corr -Corrosion current density in the absence of pectin i corrðinhÞ -Corrosion current density in the presence of pectin K -Constant W -Difference in weight of the sample D -Density of the sample A -Sample surface area exposed to a corrosive medium T -Exposed time W 1 -Difference between the initial and final sample weights in a corrosive environment without an inhibitor W 2 -Difference between the initial and final weight of the sample in an inhibitory corrosive environment E a -Activation energy ΔH # -Activation enthalpy ΔS # -Activation entropy R 2 -Correlation factor K -Adsorption equilibrium constant C inh -Inhibitor concentration θ -Surface coverage CR -Corrosion Rate T -Absolute temperature ∆G o ads -Standard free energy of adsorption ΔH°a ds -Standard enthalpy of adsorption ΔS°a ds -Standard entropy of adsorption SEM -Scanning Electron Microscopy AFM -Atomic Force Microscopy HF -High frequency CPE -Constant phase element C dl -Double layer capacitance ω max -Maximum impedance rate β a -Anodic Tafel slope β c -Cathodic slope of Tafel E corr -Corrosion potential R max -Maximum roughness