INTRODUCTION:

Soft steel is an alloy of iron which is extensively utilised as important structural and industrial material in various chemical factories such as pharmaceutical, fertilizer, petrochemical, and other chemical industries because of its natural availability toughness and possessed better-quality mechanical properties and of less priced value¹. These materials exposed under various aggressive conditions such as alkaline, acid and salt solutions in various industrial applications. The treatment of various acidic conditions during drilling operations, gas exploration, pickling, descaling, oil-well acidizing, industrial cleaning results development of the unnecessary deterioration of metal, which leading to the failure of materials as it undergoes metal dissolution in various industrial applications^2-6.

The chemical compounds are widely used to reduce the dissolution of metals in acid media. Organic molecules possessed electron donating sites like hetero atoms were proved effectively retard the corrosion of materials in various corrosive media^7,8. In acidic solutions, heterocyclic compounds possessed polar groups and/or p electrons exhibited efficient corrosion inhibitors^9,10. Several plant extracts developed as effective corrosion inhibitors for many metals and alloys^11-16. Schiff bases exhibited superior corrosion protection effect than its corresponding carbonyl and amines. Organic compounds formed by the condensation reaction of amines and carbonyl groups strongly interact on the metal surface and effectively minimize the deterioration of soft steel. Several authors investigated many Schiff base compounds are effectively interacting on metal surface in acid media and potentially control the corrosion of soft steel, aluminum and copper metals^17-20.

In the current investigations, it is focused on the effect of (E)-2-Furan-2-ylmethyleneamino acetic acid (FMAA) in hydrochloric acid solution on corrosion protection of soft steel applying mass change and electrochemical techniques. In this investigations pertain into assess the soft steel corrosion rate with and without the addition of FMAA by different temperature, also study the modification of soft steel face during corrosion and protection through SEM analysis.

MATERIALS AND METHODS:

Preparation of Inhibitor:

The Schiff base (E)-2-Furan-2-ylmethyleneamino acetic acid (FMAA) was synthesized in the laboratory and used as a corrosion inhibitor. The FMAA is synthesized according to the described procedure: Glycine (0.75g, 10m. mol) and Furfuraldehyde (3ml, 10 m.mol) were dissolved in (20mL) of ethanol in the presence of KOH (0.56g, 10m. mol) as a catalyst. The mass was vigorously agitated at laboratory temperature, refluxed for 4 hour and progress of reaction monitored by TLC, on cooling, a brownish precipitate was formed, filtered off, washed with cold ethanol and dried. Further recrystallized the crude product in ethanol and pure Schiff base, (E)-2-Furan-2-ylmethyleneamino) acetic acid was obtained. Reaction for the synthesis of FMAA is:

Samples preparation:

Commercial grade soft steel strips of 0.2cm thickness was selected for corrosion studies with the chemical compositions: P = 0.06, Mn = 0.15, Ti = 0.05, Cr = 0.04, C = 0.20 and Fe = 99.50. The specimens were subject to polishing and cleaning process followed by the removal of grease, oil, oxide film and undesirable scale present on its surface. The cleaned, rinsed and dried samples were stored in the desiccator. For mass change measurements rectangular strips of 5cm² specimen surface area was exposed. For electrochemical studies, soft steel specimen exposed area 1cm² used as a working electrode.

Solution preparation:

AR grade hydrochloric acid used to prepare different strength of HCl solutions and used as corrosion test solutions for soft steel. Various strength of inhibitor (FMAA) were prepared in 1M and 2M HCl solutions and subjected for corrosion protection studies. Double distilled water used to prepare the test solutions.

Mass change measurement:

Pretreated soft-steel coupons were weighed accurately using electronic balance and suspended in container filled with 100mL test solution. After the specified testing time, the coupon removed from the beaker and washed thoroughly using distilled water, dried and weighed accurately. All the experiments done in duplicate and the mean values of specimen mass change recorded. Soft steel corrosion rate (W_Corr) determened by the equation (1) and percentage inhibition efficiency (% IE) of inhibitor determined by the equations (2) ²¹.

Where ‘Δm’ is the mass change in mg, ‘S’ is the exposed surface area in cm² and‘t’ is the immersion time in hour.

Where, W_corr is the corrosion rate of soft steel in free acid and W¹_corr is corrosion rate in presence of inhibitor.

Electrochemical measurements:

CHI (608D) electrochemical workstation employed for electrochemical measurements. Three-electrode electrochemical cell was used to measure the anodic and cathodic polarised potentials of working soft steel electrode against reference saturated calomel electrode (SCE). Platinum electrode used as a counter electrode and hydrochloric acid solution used as electrolyte as well as corrosive medium at room temperature (303 ± 1K). Prior to polarization, the experiment conducted after the specimen immersed for 30 minutes in HCl solutions without and with addition of inhibitor. Tafel curves consructed by varying electrode potential from -0.2 V to + 0.2 V versus open circuit potential (OCP) at a scan rate of 1.0 mVs^-1. Various parameters like corrosion potential (E_corr.), cathodic (β_c) and anodic (β_a) Tafel slopes and corrosion current density (I_corr.) obtained by extrapolating the Tafel lines. The % IE obtained by the equation (3):

Where, I_corr and I¹_corr represents corrosion current density in free HCl solution and HCl solution containing different concentration of inhibitor respectively.

In electrochemical impedance spectroscopy (EIS), without altering external source the electrode allowed to attain steady state potential (OCP) as a function of 180 seconds corresponding to reference electrode. Electrochemical impedance measurements made from 100 kHz to 10 MHz frequency range with 10 mV amplitude AC signal. Double layer capacitance (Cdl), resistance polarization (R_p) and solution resistance (R_s) determined from Nyquist plots and percentage inhibition efficiency calculated from (R_p) using the equation (4):

where R⁰_pand R_p are the resistance polarization in presence and absence of the inhibitor respectively^22-24.

Activation Parameters:

Specimen dissolution at varied temperature from 303±1 K to 343±1 K in presence and absence of inhibitor studied by mass change method. Different activation parameters like energy of activation (E_a), activation enthalpy (∆H^*) and activation entropy (∆S^*) were derived from the dissolution of soft- steel in blank 2M HCl and with inhibitor in 2 M HCl at immersion time of 2h.

Adsorption study:

Adsorption studies were very much useful to confirm the mode of corrosion protection by the used inhibitors. Experimental results used to fit with Temkin, Freundlich and Langmuir’s adsorption isotherms. From the obtained data, thermodynamical adsorption parameters like K_ads, and free energy of adsorption (∆G⁰_ads), were evaluate and interpreted²⁵.

SEM and FT-IR Studies:

Surface morphology change of soft steel during corrosion and corrosion protection with FMAA was monitored by scanning electron microscopy (SEM: Model-JEOL make, JSM-IT 500LA). The corrosion protection of soft steel in attendance of FMAA established via adsorption process. The interaction of the FMAA functional groups on mild steel surface confirmed through FT-IR analysis using a Nicolet 6700 FT-IR spectroscopic analyzer with a working range of 400 - 4000 cm^-1 wavenumber.

RESULT AND DISCUSSION:

Mass change experiment :

Mass change measurement procedure conducted to obtain the corrosion protection of soft steel in 2M HCl solution using different concentration of FMAA inhibitor with immersion duration of 2 h at 303±1 K. The results obtained are taken to compute the soft steel corrosion rate and percentage inhibition efficiency (%IE) of inhibitor (FMAA) and these values are reported in (Table 1).

Table 1. Corrosion rate of soft steel and % IE of inhibitor.

Conc.of FMAA (M)	Corrosion rate (mg/cm²/h)	(%IE)
Blank	3.4	-
0.001	0.55	83.33
0.002	0.45	86.76
0.003	0.4	88.05
0.004	0.15	95.58

The data indicated in 2M hydrochloric acid solution, W_corr of soft steel is higher and W_corr decreased with the addition of FMMA in 2M HCl. It is noticed in the Table 1 the % IE of the inhibitor is increased with increased in inhibitor concentrations from 0.001M to 0.004M. The highest 95.58% inhibition efficiency obtained at maximum 0.004 M inhibitor concentration. Further increasing concentration of inhibitor, the %IE is negligibly increased and 0.004 M is considered as optimum concentration of inhibitor. The added FMMA molecules are reducing the hydrogen evolution reaction by forming protonated molecules in acid solution; it leads decreased in the rate of cathodic reaction²⁶. Further soft steel surface adsorbed by the FMMA molecules and formed thin film between soft steel surface and corrosive medium. This thin layer act as a barrier and restrict the electron transfer speed by soft steel, which leading to slowdown the corrosion of soft steel in addition of FMAA molecules.

Electrochemical Experiments:

Polarisation studies:

Figure 1(A and B) presented the typical polarization graph for soft steel in 1M and 2M HCl solution and addition of various strength of FMAA inhibitor in hydrochloric acid solution. The data obtained from the polarization measurements such as corrosion potential (E_corr), corrosion current density (I_corr), tafel slopes (β_a and β_c) and percentage inhibition efficiency calculated and reported in (Table 2). The soft steel corrosion current density (I_corr.) in blank acid solution noticed high values, whereas I_corr values were minimized in adding FMAA in HCl solution. Continuously enhanced the FMAA concentrations, I_corr decreased to reach minimum value at the concentration of 0.004 M in 1M HCl. On the other hand FMAA molecules blocked both anodic and cathodic reaction sites without much alter the cathodic and anodic Tafel slopes and FMAA molecules worked as mixed type of inhibitor²⁷.

The results from the table 2 noticed that, E_corr values are not many changes in among different inhibitor concentration. Suppose E_corr displacement value of inhibitor had more than 85 mV then inhibitor act as a cathodic or anodic type. If the E_corr displacement value below 85 mV, then consider the inhibitor coluld as mixed type²⁸. The mode of corrosion protection was interpreted through adsorption of FMAA molecules on specimen surface. The added FMAA molecules in acid solution are readily adsorbed on metal specimen surface due to the presence of lone pair of electrons (hetero atoms) and covered larger surface area and retard the anodic metal dissolution reaction. Meanwhile, FMAA molecules are got protanated in hydrochloric acid solution and slow down the hydrogen reduction reaction at the cathodic site ^{5, 9}. Similar trend was observed in 2M HCl solution and the maximum %IE is found to 90.12 and 92.22 at 0.004 M inhibitor concentration in 1M and 2M HCl solution respectively.

Figure 1. Polarization graphs of soft steel: A) in 1M HCl solution B) in 2M HCl solution

Impedance measurements (EIS):

Electrochemical impedance spectroscopy is an electrochemical techniques used to measure the impedance of the system in dependence of the AC potentials frequency. EIS allows seperating the influence of different components that means the contribution of the electron transfer resistance, double layer capacity, etc. EIS response (Nyquist plots) of soft steel in 1M and 2M HCl solution at 303 ±1 K represented in (figure 2A andB). Using the obtained results, resistance polarisation (R_p) and solution resistance (R_s) evaluated by constructing an equivalent circuit of the electrical double layer, which consists of constant phase element (CPE) in parallel with a resistor R_p and R_s (figure 3) ^{29, 30}.

Figure 2 (A and B). Nyquist graphs for soft steel.

Figure 3. Equivalent circuit for the impedance measurement

The soft steel impedance response after the addition of FMAA in HCl solution was markedly changed. The addition of concentration of FMAA raised, the impedance response such as semicircle radii also increased, as a result capacitance values decreased and the resistance polarisation values increased. When the addition of concentration of FMAA increased, progressively enhanced the thickness of the electrical double layer formed due to the adsorption of the inhibitor molecules at the elctrode – electrolyte interface. The capacitance value in the circuit decreased due to the decline of local dielectric constant, which is suggested that, the adsorbed FMAA molecules formed a thin film on soft steel surface ².Various parameters obtained from the EIS measurements such as R_p, R_s, CPE, %IE and n (CPE exponent) were entered in (Table 3). Variation of n values with the addition of FMAA to the corrosive medium is an indication of change in the surface homogeneity due to adsorption of inhibitor on the most active sites of the soft steel.

Table 3. Impedance parameters for corrosion protection of soft steel at 303 ±1 K

Concentration (M)	R_S Ώ (cm^-2)	Rp Ώ(cm^-2)	CPE (µF)	%IE	N
1M HCl	1.995	56.9	392.3	-	0.7667
0.001	4.317	276.4	258.3	79.41	0.6716
0.002	4.728	279.6	24.6	79.64	0.7723
0.003	1.368	280.2	227.8	79.69	0.8042
0.004	4.247	281.1	216.3	79.75	0.7588
2M HCl	1.387	38.01	605.1	-	0.8805
0.001	1.438	194.9	277.8	80.49	0.8087
0.002	1.371	205.1	270.8	81.46	0.7945
0.003	1.002	224.4	219.3	83.06	0.7782
0.004	1.269	280.1	217.8	86.42	0.8042

Effect of temperature:

A temperature effect study plays a vital role to assess the stability of the adsorbed layer and performance of the corrosion protection at higher temperature. Temperature effect on corrosion rate of soft steel in 2M HCl solution with 2 h exposure time carried out using mass change method and the results listed in (Table 4).

Table 4. Wcorr, % IE and surface coverage (Ɵ) of soft steel at different temperatures.

Temp. (K)	Concentration (M)	W_corr (mg/cm²/h)	%IE	Ɵ
303	Blank	3.38	-	-
	0.002	0.45	86.76	0.8676
	0.003	0.4	88.05	0.8805
	0.004	0.15	95.56	0.9556
313	Blank	5.5	-	-
	0.002	1.25	71.59	0.7159
	0.003	0.75	82.95	0.8295
	0.004	0.35	92.06	0.9206
323	Blank	10.6	-	-
	0.002	2.5	76.41	0.7641
	0.003	2.1	80.18	0.8018
	0.004	1.0	90.56	0.9056
333	Blank	18.28	-	-
	0.002	4.85	73.46	0.7346
	0.003	4.4	75.92	0.7592
	0.004	3.15	82.76	0.8276
343	Blank	32.36	-	-
	0.002	10.1	68.78	0.6878
	0.003	8.05	75.12	0.7512
	0.004	6.0	81.45	0.8145

Data from the table 4 revealed that, the corrosion rate is increased and percentage inhibition efficiency decreased as the temperature increased from 303±1 to 343±1 K²⁷. It revealed that, as the temperature increased the migration of H⁺ ions enhanced towards soft steel surface and induced to increase reduction reaction rate. At higher temperature the stability of the adsorbed layer weakened due to increase in the local thermal energy of the adsorbed molecules and start molecules detaching from the specimen surface, as a result corrosion rate increased and %IE was decreased³¹.

Arrhenius theory is employed to calculate the apparent activation energy and other activation parameters ³² such as ∆H* and ∆S* for corrosion process of soft steel in 2M acid solution using Eq. (5) and (6).

Where, E_a is the activation energy, A is the pre-exponential factor, h is the Planks constant, N is the Avogadro number and R is the universal gas constant. Using equation (5) Arrhenius graph was constructed to determine E_a values by ploting a graph of W_corr versus 1/T (figure 4A). Similarly ∆H* and ∆S* were obtained using transition state equation (6) by ploting graph log (W_corr /T) versus 1/T (figure 4B). The values of various activation parameters recorded in (table 5).

In presence of FMAA in acid solution, the apparent activation energy (E_a) values found to be higher comparatively acid solution alone. These enhanced E_a values guided that, adsorbed FMAA molecules develop a physical barrier and restrict the charge and mass transfer processess. As a result, corrosion rate of soft steel decreased³. Positive ∆H^*values inferred that, metal dissolution process takes place through endothermic nature, which indicated that metal dissolution process further decreased in presence of inhibitor. The values of ∆S^* is negative indicated that the metal dissolution process proceeds through decrease in disordering takes place on going from reactant to the activated complex. In the presence of FMAA inhibitor ∆S^* values increased (lower negative values) compare to blank HCl solution due to transition state is high orderly arranged with respect to the initial state during recombination rate determining step.

Table 5. Values of soft steel activation parameters.

Concentration (M)	E_a (kJ /mol.)	∆H^* (kJ /mol.)	∆S^* (kJ /mol.)
2M HCl	49.60	48.85	-86.82
0.002	66.08	63.32	-54.04
0.003	67.62	64.94	-45.59
0.004	83.27	80.55	-78.1

In acid solution, hydrogen ion discharge is the rate-determining step and the specimen surface covered with the adsorbed hydrogen atoms. In presence of inhibitor, specimen surface covered with FMAA molecules, this effect slow down the H⁺ ions discharge at the specimen surface, this leading to the system pass from random arrangment and hence ∆S^* values increased³³.

Figure 4 A and B. Arrhenius graphs for soft steel in presence and absence of inhibitor in 2M HCl.

Adsorption study:

Corrosion protection of soft steel in 2M HCl solution containing different concentration of FMAA interpreted through adsorption isotherm. The interaction of metal – inhibitor was fitted Langmuir adsorption isotherm given by the equation (7). This was confirm by the graph plotted Log [Ɵ /1-Ɵ] against Log C_inh (figure 5) give a curve fitted in the graph is a straight line with regression coefficient is around unity (R = 0.997). Langmuir adsorption isotherm studies support that corrosion control tendancy by the FMAA molecules takes place via adsorption process^{34, 35}.

Log [Ɵ/1-Ɵ] = Log K_ads + Log C (7)

Where C is the molar inhibitor concentration, K_ads is the adsorption equilibrium constant and ‘Ɵ’ is the degree of surface coverage and determined from the equation (8):

Where W₀ represents the mass loss of soft steel in uninhibited solution and W is the mass loss of soft steel in inhibited solution ³⁶.

Free energy of adsorption (∆G⁰_ads) of inhibitor molecules on soft steel calculated by the equation (9):

-∆G⁰_ads = RT ln (55:5 K_ads) (9)

Where R is the gas constant (8.314 J K^-1mol^-1), and K_ads is adsorption equilibrium constant and obtained by equation (10)³⁷.

K_ads = Ɵ / C_inh(1- Ɵ) (10)

Negative ∆G⁰_ads value of FMAA was around 30-32 kJ/ mol supported to the spontaneity of the adsorption process under investigated experimental conditions and occurs by chemisorption. The chemisorption process involving through coordinate type bond formation via charge transfer/sharing by FMAA molecules to the soft steel surface^38,39. The values of free energy of adsorption, adsorption equilibrium constant and regression coefficient listed in (table 6)

Table 6. Free energy of adsorption, K_ads and regression coefficient

Temp. (K)	K_ads (10⁴/mol)	∆G⁰_ads (kJ /mol)	R²
303	5380.63	-31.76	0.94179
313	2898.61	-31.20
323	2398.30	-31.69
333	1200.11	-30.75
343	1097.70	-31.42

Figure 5. Langmuir adsorption isotherm for the adsorption of FMAA molecules on soft- steel surface

Surface Characterization

SEM analysis:

The change in surface morphology of corroded and corrosion protected soft-steel surface is characterized by scanning electron microscopy (SEM). Figure 6A SEM image of polished soft- steel surface appeared smooth thin bars originated during polishing from the emery paper. Figiure 6B SEM image of soft- steel surface immersed in blank 2M HCl solution for 2 h. This surface contained large number of cavities distributed over the entire surface and absence of corrosion product deposited over it. This amount to be metal surface is severely affected by the acid solution⁴⁰. Figure 6C is SEM image of soft steel surface dipped 2 h in 2M HCl solution containing 0.004mol /dm³ of FMAA. The specimen is not severely affected by the acid solution and possessed smooth surface compared with that of surface treated in blank 2M HCl solution. On observation of the figure 6C the specimen is slightly corroded and partial deposition of FMAA molecules on soft steel surface^41,42.

Figure 6 A. Polished untreated mild steel surface

Figure 6 B. Corroded mild steel surface in 2M HCl at 2 hrs immersion time

Figure 6 C. Corroded mild steel surface in 2 M HCl with 0.001M FMAA at 2 hrs immersion time

FT-IR Studies:

FT-IR spectral technique used to know the possible interactions between the organic inhibitor adsorbed on soft steel surface. Figure 7A present FTIR spectrum of pure FMAA compound and Figure 7B is that of spectrum of scratched product from corroded soft-steel surface in 2M HCl containing optimized concentration of the FMAA inhibitor. In figure 7A the characteristic stretching peak at 1611.40 cm^-1 that is represented (CH=N) cm^-1, broad peak at 2500 cm^-1 to 3300 cm^-1 and 1463 cm^-1 were indicated O-H stretching vibrations and C-O groups respectively. In Figure 8b the characteristic stretching bands are slightly varied and appeared at 1629.87cm^-1, 3418.95 cm^-1 and 1452.22 cm^-1 corresponding to -C=N, O-H and C-O groups respectively.

These alteration or modifications of the corresponding peaks from the pure compounds to the scratched compounds indicated that N and O atoms could act as active sites for adsorption of the inhibitor molecules to the specimen surface in HCl medium. Such alteration of the peaks revealed that, FMAA molecules chemically interacted on soft- steel surface and control corrosion in acid medium^43-45.

CONCLUSION:

The synthesised (E)-2-Furan-2-ylmethyleneamino acetic acid (FMAA) developed as an effective corrosion inhibitor for soft steel in hydrochloric acid medium. Mass change and electrochemical techniques were opted to assess the corrosion rate and corrosion inhibition performance of the inhibitor. Electrochemical polarisation experiment proved that FMAA acts as mixed type inhibitor. FMAA inhibitor control corrosion of soft steel maximum of 95.58% in 2M hydrochloric acid solution at 303±1 K. Corrosion protections achieved through an adsorption of FMAA molecules on soft steel surface that obeys Langmuir adsorption isotherm. The percentage inhibition efficiency of the inhibitor depends on amount of inhibitor, temperature of the medium and molecular structure of the compound. The inhibition action of FMAA against corrosion is due to the formation of thin film on soft-steel surface, this confirmed by SEM and FT-IR reports.

CONFLICT OF INTEREST:

The authors have no conflicts of interest regarding this investigation.

ACKNOWLEDGMENTS:

Authors are thankful to University Science Instruments Centre, Karnatak University, Dharwad, India for providing SEM facility and College of Engineering and Technology, Srinivas University, Mangalore -574 146, for providing CHI 608D CH work station to carry out the research work.

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Received on 30.05.2021 Modified on 25.10.2021

Asian J. Research Chem. 2022; 15(1):10-18.

DOI: 10.52711/0974-4150.2022.00002

Concentration (M)	I_corr (µA cm^-2)	E_corr (mV vs. SCE)	β_c (mV dec^-1)	β_a (mV dec^-1)	% IE.
1M HCl	162.03	-351	15.07	-11.62	-
0.001	29.50	-454	19.47	-18.46	81.79
0.002	28.58	-459	26.93	-22.27	82.36
0.003	18.12	-462	24.89	-21.83	88.81
0.004	16.0	-468	26.24	-26.07	90.12
2M HCl	207.00	-431	20.38	-15.59	-
0.001	37.14	-424	17.25	-16.01	82.05
0.002	21.17	-461	20.91	-20.07	89.77
0.003	18.56	-455	26.62	-31.05	91.03
0.004	16.10	-457	14.74	-20.87	92.22