Evaluation of the anticorrosion performance of Tamsulosin as corrosion inhibitor for pipeline steel in acidic environment: experimental and theoretical study

Abstract

The inhibition performance of expired Tamsulosin (TAM) was studied on St52 pipeline steel in 1 M HCl by gravimetric, electrochemical impedance spectroscopy (EIS), potentiodynamic polarization (PDP), scanning electron microscopy (SEM) and energy-dispersive X-ray (EDX) spectroscopic analysis. It was seen that the presence of the studied expired drug decreased the corrosion rate of the pipeline steel. TAM’s efficiency as an inhibitor increased as its concentration increased but decrease with temperature rise. The expired drug showed maximal inhibition efficiency of 98.1% from weight loss measurement at 2.0 × 10⁻³ M TAM concentration and 303 K. PDP results showed that TAM acted as a mixed-type inhibitor. The adsorption study revealed that Langmuir isotherms gave the best fit. The surface morphological study revealed that TAM formed an adsorbed protective layer on the St52 steel surface in the acid medium. The theoretical study performed using the density functional theory (DFT) allowed correlation with an experimental study.

KEYWORDS:

• Tamsulosin
• St52 steel
• SEM
• corrosion inhibition
• adsorption
• DFT

1. Introduction

Carbon steel with different grades is exploited as pipework steel in the petroleum industry due to its good mechanical properties. The exposure of pipeline steel to aggressive environments as a result of different industrial applications endangers the steel. One of such industrial applications is the use of acids usually hydrochloric acids in oil-well acidizing. This process, as good as it is, leads to acid corrosion of the steel. Therefore, efforts are being made to prevent these unwanted reactions. In the petroleum industry, the corrosion inhibitor is usually introduced to the solution containing the acid in the acidizing process to prevent the dissolution of the steel [1]. The interaction of inhibitor with corroding steel surface significantly modifies the rate of the corrosion process. Immense efforts have been put in by researchers at developing appropriate corrosion inhibitors in various corrosive media [2,3].

Different types of organic compounds comprising π-electrons, aromatic rings and heteroatoms like S, O and N have been applied as inhibitors of metal corrosion in acidic environments [4–7]. The presence of these heteroatoms and the π electrons provides adsorption sites for the inhibitor on steel surfaces, thereby creating an impediment to acid corrosion attack [8]. The adsorption of organic inhibitors onto metal surfaces depends mostly on the various factors which includes the inhibitor molecules' electronic structure, functional groups, aromaticity, steric factors, the donating electrons p-orbital character and electron density at the donor atoms [9]. These inhibitors form complexes when they react with the Fe atom at the steel surface, blocking the steel surface from further deterioration [10].

Several research works have been carried out to ascertain the inhibitive performance of various drugs on various metals [11–20]. Recently, some fresh and expired pharmaceutical compounds have been investigated as potential inhibitors both in our laboratory and by other researchers, including carvedilol [21], glimepiride [22], domperidone [23] and ethambutol [24]. In continuance of our previous research, the evaluation of the performance of expired tamsulosin (TAM) as corrosion inhibitor for St52 pipeline steel in 1M HCl medium was examined using weight loss, EIS, PDP and scanning electron microscopy (SEM) techniques. Tamsulosin is a medication used in the treatment of chronic prostatitis and also to help with kidney stones passage, among others [25]. Considering the high cost of most pharmaceutical drugs, we opted to study the inhibitive performance of the expired drug which is useless as it were. Reports show that above 90% of pharmaceutical drugs are still stable after expiration [26]. TAM has the formula of C₂₀H₂₈N₂O₅S and a molar mass of 408.51 g/mol. The molecular and 3D ball and stick structures of TAM are shown in Figure 1. The comparison of the inhibitive performance of TAM with other drugs and with already reported standard corrosion inhibitors are presented in Table 1.

Figure 1. (a) Molecular and (b) 3D ball and stick structures of TAM.

Table 1. Comparison of the inhibitive performance of TAM with other reported drug corrosion inhibitors.

Drug name	Metal and electrolyte	Inhibitor concentration	Inhibition efficiency, %	Reference
Tramadol	Aluminium in 1 M HCl	500 ppm	96	[11]
Chloraphenicol	Mild steel in 0.1 M HCl	10% v/v	85	[12]
Cephapirin	Carbon steel in 2M HCl	600 ppm	83	[13]
Chemically modified Dapsone	Mild steel in 0.5 M H2SO4	0.219 nM	95	[14]
Irbesartan	Mild steel in 0.5 M H2SO4	300 mg/L	83	[15]
Dexamethasone	Mild steel in 2 M HCl	0.4 g/L	83	[16]
Ranitidine	Aluminium in 1 M HCl	400 ppm	90	[17]
Cefixime	Mild steel in 1 M HCl	8.8 × 10^−4 M	62	[18]
Cefazolin	Mild steel in 1 M HCl	11 × 10^−4 M	68	[19]
Methocarbamol	Mild steel in 2.5 M H2SO4	2 × 10^−3 M	67	[20]
TAM	St52 steel in 1 M HCl	2.0 × 10^−3 M	94	This work

2. Experimental

2.1. Materials preparation:

St52 steel sheets specimens with composition (weight %), C 0.18, Si 0.50, P 0.05, Mn 1.50, S 0.05 and the balance Fe were used for the experiments (surface morphology, electrochemical and weight loss measurements). The sheets were cut into dimensions of 4 cm × 2 cm × 0.02 cm, and for the electrochemical study, the steel samples were enclosed with epoxy resin exposing 1 cm² area. The samples were abraded successively with SiC₂ paper (800, 1000 and 1200 grades), to secure a smooth surface. The St52 steels were thereafter washed with water, degreased in acetone, and then, finally dried in an oven before weighing and use. The acid test solution (1M HCl) was made by using 37% HCl analytical grade from Merck and distilled water.

2.2. Corrosion inhibitor

The expired Tamsulosin (TAM) drug (Exp. Date: July 2019) was supplied by Naiviv Pharmacy Limited. The drug was crushed into powder and used without further purification. Different concentrations of TAM ranging from 4.9 × 10⁻⁴ to 2.0 × 10⁻³ M were prepared by the dissolution of the weighed amount of the expired drug in 1M HCl.

2.3. Weight loss studies

Pre-treated and accurately weighed St52 steel samples were submerged in 1M HCl aggressive solutions comprising different TAM concentrations and bare acid at 30°C. The samples being removed after 4 h were washed thoroughly, dried and accurately reweighed. The difference in the steel weights before immersion and after was taken as weight loss. The weight loss tests were repeated thrice, and the mean value was recorded to ensure reproducibility of the results. The tests were repeated at 40, 50 and 60°C. From weight loss data, the inhibition efficiency ($\eta \mathrm{\%}$) and surface coverage ($\theta$) of each concentration of TAM were calculated utilizing the equation: (1) $\begin{array}{rl}{\eta }_{WL}\mathrm{\%}& =\frac{w_o-w_i}{w_o}\times 100,\end{array}$(1) (2) $\begin{array}{rl}\theta & =\frac{w_o-w_i}{w_o},\end{array}$(2) where $w_o$ and $w_i$ represent the weight loss without and in presence of TAM, respectively. The corrosion rate ($C_R$) with and in the absence of different concentrations of TAM were computed by using the equation: (3) $C_R(mgc{m}^{-2}{h}^{-1})=\frac{\mathrm{\Delta }W}{At},$(3) where $\Delta W$ represents weight loss, A represents an area of St52 steel (cm²) exposed and $t$ stands for time ($h$) of immersion.

2.4. Electrochemical measurements

Electrochemical studies were carried out in a three-electrode electrochemical cell of 500 mL capacity using VERSASTAT 400 full set DC voltammetry and a potentiodynamics/Galvonstat system with E-chem software. The cell consists of St52 steel with 1 cm² area exposed as working electrode, platinum counter electrode, and the reference electrode is SCE – saturated calomel electrode. Before the experiment, the St52 steel was immersed in 1M HCl test solution with and in the absence of TAM for sufficient time (3600s) until steadying open circuit potential (OCP) is achieved. For each inhibitor concentration, the experiments were performed three times for reproducibility. EIS tests were performed in a range of 0.1–100 kHz frequency at AC amplitude of 10 mV. The charge-transfer resistance ($R_{ct}$) values were deduced from Nyquist plots. Inhibition efficiency (${\eta }_{EIS}\mathrm{\%}$) of TAM was computed from the $R_{ct}$ values utilizing the expression [27]: (4) ${\eta }_{EIS}\text{\%}=\frac{R_{ct}^i-R_{ct}^0}{R_{ct}^i}\times 100$(4) where $R_{ct}^0$ and $R_{ct}^i$ are the charge-transfer resistances with and without TAM, respectively.

The polarization study was performed by adjusting potential from −0.25 to +0.25 V versus SCE at 1 mV/s scan rate with respect to OCP. The current density ($i_{corr}$) was deduced by the Tafel plots cathodic and anodic linear segments extrapolation. The inhibition efficiency (${\eta }_{PDP}\mathrm{\%}$) was computed from the $i_{corr}$ values using the relation [27]: (5) ${\eta }_{PDP}\text{\%}=\left(1-\frac{i_{corr}^{\text{i}}}{i_{corr}^0}\right)100$(5) where $i_{corr}^0$ and $i_{corr}^i$ represent corrosion current densities without and with TAM, respectively.

2.5. SEM/EDX studies

Ziess 50 XVP scanning electron microscope was utilized to perform the SEM/EDX study. The pre-treated St52 steel samples were immersed in 1M HCl medium without and with 2.0 × 10⁻³ M of TAM at 30°C. The samples were removed after 6 h, rinsed in water, dried prior to SEM/EDX analysis.

2.6. Theoretical calculation

Quantum chemical calculations were performed to correlate experimental results with theoretically calculated properties. Quantum chemical parameters were computed utilizing DFT. The geometrical optimizations of the studied TAM molecules (neutral and protonated) were performed with B3LYP along with basis set 6$-$31$\text{G}$ (d, p), implemented in the Spartan 14.0 package. The following parameters were obtained: $E_{HOMO}$, highest occupied molecular orbital energy; $E_{LUMO}$, lowest unoccupied molecular orbital energy; and $\mu$, dipole moment. The obtained $E_{LUMO}$ and $E_{HOMO}$ were utilized in calculating other quantum chemical parameters which include energy band gap ($\mathrm{\Delta }E$), electronegativity ($\chi$), global softness ($\sigma$) and hardness ($\eta$) as expressed in the following equations [28,29]: (6) $\begin{array}{rl}\mathrm{\Delta }E& =E_{LUMO}-E_{HOMO},\end{array}$(6) (7) $\begin{array}{rl}\chi & =-\frac{1}{2}(E_{LUMO}+E_{HOMO}),\end{array}$(7) (8) $\begin{array}{rl}\eta & =\frac{1}{2}(E_{LUMO}-E_{HOMO}),\end{array}$(8) (9) $\begin{array}{rl}\sigma & =\frac{1}{\eta }.\end{array}$(9)

The fraction of transferred electron from TAM molecule to the Fe surface ($\mathrm{\Delta }N$) was computed using the expression [30]: (10) $\mathrm{\Delta }N=\frac{{\chi }_{Fe}-{\chi }_{inh}}{2({\eta }_{Fe}+{\eta }_{inh})},$(10) where ${\chi }_{Fe}$ and ${\chi }_{inh}$ are respectively the Fe and inhibitor electronegativity values, while ${\eta }_{Fe}$ and ${\eta }_{inh}$ represent the global hardness for Fe and the inhibitor (TAM). Pearson's scale of electronegativity designate $7\text{ ev}$ for ${\chi }_{Fe}$ and $0\text{ ev}$ for ${\eta }_{Fe}$ bulk Fe [31].

3. Results and discussion

3.1. Gravimetric measurements

3.1.1. Effect of TAM concentration

The gravimetric method of monitoring metal corrosion is used due to its reliability and simple application. The variation of $C_R$ and ${\eta }_{WL}\mathrm{\%}$ of TAM with inhibitor concentrations at varying temperatures is shown in Figure 2(a,b). Figure 2(a) result clearly shows that the C_R declines while increasing TAM’s concentration, and consequently, ${\eta }_{WL}\mathrm{\%}$ increases with inhibitor concentrations (Figure 2(b)). This observation indicates the effectiveness of the expired drug in the protection of St52 steel in 1 M HCl because of enhanced surface coverage and TAM’s adsorption on the St52 steel surface. This adsorption of TAM molecules on the St52 steel surface reduces the available surface area for corrosion.

Figure 2. Variation of (a) $C_R$ and (b) ${\eta }_{WL}\mathrm{\%}$ of TAM with inhibitor concentration in 1 M HCl at various temperatures.

3.1.2. Temperature effect and thermodynamics of corrosion process

The ${\eta }_{WL}\mathrm{\%}$ and $C_R$ values deduced from gravimetric measurement without and with optimum concentration (2.0 × 10⁻³ M) of TAM at various temperatures are given in Table 1. It shows clearly from the table that $C_R$ of St52 steel in both absence and presence of TAM increased with temperature rise while ${\eta }_{WL}\mathrm{\%}$ decreased with temperature rise. This could be because the inhibitor molecules get desorbed at a higher temperature from the metal surface [32]. The dependency of C_R on temperature is given by Arrhenius and Eyring’s equations (Equations 11 and 12), from which the activation energy and some thermodynamics parameters were deduced [27]. (11) $\begin{array}{rl}{C}_R& =Aexp\left(\frac{-E_a}{RT}\right),\end{array}$(11) (12) $\begin{array}{rl}{C}_R& =\frac{RT}{Nh}\exp \left(\frac{\mathrm{\Delta }{S}^{\ast }}{R}\right)\exp \left(\frac{-\mathrm{\Delta }{H}^{\ast }}{RT}\right),\end{array}$(12) where $A$ stands for pre-exponential factor; $E_a$, activation energy; $R$, gas constant; $T$, absolute temperature; $N$ stands for Avogadro’s number; $h$ represents Plank’s constant; $\mathrm{\Delta }{S}^{\ast }$ represents activation entropy; and $\mathrm{\Delta }{H}^{\ast }$ the activation enthalpy. Arrhenius plot ($\log C_R$ versus $1/T$) and Eyring’s plot ($\log C_R/T$ versus $1/T$) are shown in Figures 3 and 4. Linear curves were gotten with slope $-\Delta E_a/2.303R$ for Arrhenius plot and $-\mathrm{\Delta }{H}^{\ast }/2.303R$ for transition state plot, from where $E_a$ and $\mathrm{\Delta }{H}^{\ast }$ were respectively obtained and given in Table 2. The values of $\mathrm{\Delta }{S}^{\ast }$ given in Table 2 were calculated from the intercept of $[\text{log }(R/Nh)+(\mathrm{\Delta }{S}^{\ast }/2.303R)]$ from Figure 4. The $E_a$ in the absence of TAM is 31.32 kJ/mol whereas in the presence of different concentrations of TAM, the values ranged from 36.71 to 45.92 kJ/mol. The higher $E_a$ values in inhibitor presence when compared with its absence show the difficulty in the dissolution of St52 steel in the presence of TAM due to TAM molecules’ adsorption on the St52 steel surface [32,33]. The calculated $\mathrm{\Delta }{H}^{\ast }$ values are positive and follow the pattern of $E_a$, which indicates the endothermic character of the St52 steel corrosion and the slow corrosion of the steel in TAM’s presence. The negative values of activation entropy ($\mathrm{\Delta }{S}^{\ast }$) both with and without TAM indicate that activated complexes in the rate deciding step address dissociation, suggesting a diminishing in disorderliness on proceeding from reactants to activated complex [34].

Figure 3. Plot of $\log C_R$ versus $1/T$ for St52 steel corrosion in 1M HCl and inhibited solutions at various concentrations of TAM.

Figure 4. Plot of $\log C_R/T$ versus $1/T$ for corrosion of St52 steel in 1 M HCl and solution and inhibited solutions at different concentrations of TAM.

Table 2. Computed ${\text{E}}_{\text{a}}$ values and thermodynamics parameters for St52 steel corrosion in 1 M HCl solution without and with various TAM concentrations.

Conc. (M)	${\text{E}}_{\text{a}}(\text{kJ}/\text{mol})$	$\mathrm{\Delta }{\text{H}}^{\ast }(\text{kJ}/\text{mol})$	$\mathrm{\Delta }{\text{S}}^{\ast }(\text{J}/\text{mol}/\text{K})$
Blank	31.32	26.69	−146.19
4.9 × 10^−4	36.71	29.05	−141.83
9.8 × 10^−4	40.90	34.73	−136.05
1.5 × 10^−3	43.74	36.64	−131.62
2.0 × 10^−3	45.92	40.88	−127.81

3.2. EIS measurements

EIS measurements were undertaken to better understand the electrochemical processes that occur at the surface of the corroding steel without and with the addition of TAM. The impedance spectra of St52 steel presented as Nyquist diagrams for TAM in 1M HCl solution without and with low and highest concentrations of the inhibitor are displayed in Figure 5(a). The spectra possess a single depressed semicircle, indicating the existence of a charge-transfer corrosion process that is not affected by the introduction of TAM. However, the semicircle’s diameter increased with TAM’s concentration, suggesting that the presence of TAM increased resistivity towards charge-transfer at the steel/1M HCl interface and resulted in a protective film of TAM on the St52 steel surface [22,35,36]. Furthermore, the EIS spectra are imperfect semicircles, which show the frequency scattering consequent to the inhomogeneity or roughness of the steel surface [37,38]. More so, phase angle and $\log Z$ are displayed for higher values by the higher concentration than the lower concentration (Figure 5(b,c)). This observation is reflective of better inhibition performance of higher concentration than lower concentration. The inhibition phenomenon could be explained further on the premise that more TAM molecules were adsorbed and a larger surface area covered upon the addition of a higher concentration of TAM to the corrosive media. The EIS data were fitted with an equivalent circuit depicted in Figure 5(d). In the model, $R_s$ is the solution resistance and $R_p$ represents charge-transfer resistance while CPE stands for constant phase element. The use of CPE in place of the pure capacitor is due to the non-ideal activity of steel surfaces and the CPE will give a better fit of the impedance data. The CPE impedance $Z_{CPE}$ is computed using the following equation [27,39]: (13) $Z_{CPE}=\frac{1}{Q{(j\omega )}^n},$(13)

Figure 5. (a) Nyquist, (b) phase angle and (c) Bode modulus plots of St52 steel in 1M HCl without and in different concentrations of TAM.

where $Q$ is the CPE magnitude, $\omega$ represents angular frequency, $j^2$ = −1 is an imaginary number and $n$, related to phase shift, is the CPE exponent which gives information on the unevenness of the steel surface. The double-layer capacitance ($C_{dl}$) values were deduced from the CPE parameter $Q$ and $n$ using the expression [40]: (14) $C_{dl}=Q({\omega }_{max}{)}^{n-1},$(14) where ${\omega }_{max}$ is the angular frequency (rad s⁻¹) at maximum imaginary impedance. The impedance parameters including $Q$, $n$, $R_{ct}$, $R_s$, $C_{dl}$ and ${\eta }_{EIS}$ are presented in Table 3. The $C_{dl}$ values were seen to decrease while the $R_{ct}$ values and ${\eta }_{EIS}$ increase with the increase in TAM concentration. $C_{dl}$ decrease could be attributed to decreased dielectric constant coupled with increased electric double-layer thickness while $R_{ct}$ increase could be due to surface coverage increase which propels an inhibition efficiency increase [41,42].

Table 3. Impedance data for St52 steel in 1M HCl and with various concentrations TAM at 303 K.

Conc. (M)	$R_s$ (Ω cm^2)	$R_{ct}$ (Ω cm^2)	$Q$ (μΩ^−1 s^n cm^−2)	$C_{dl}$ (μF cm^−2)	$n$	${\eta }_{EIS}$ (%)
Blank	1.17	60	189.3	130.4	0.88	—
4.9 × 10^−4	0.94	372.5	160.1	86.6	0.89	83
2.0 × 10^−3	1.25	519.6	90.2	10.3	0.89	88

3.3. Open circuit potential/ polarization measurement

Figure 6(a) shows the variation of Eocp with respect to time for St52 steel in 1 M HCl in the absence and presence of TAM. It was observed that the blank solution progressed in a positive direction until a steady-state process was attained. The observed phenomenon is attributed to the corrosion process of pre-exposure, air-formed oxide layer and surface dissolution of the steel. Compared to the Eocp vs time plot of the blank, the Eocp in the inhibited systems is shifted in the anodic direction, which indicates adsorption of TAM molecules on the anodic sites of the steel.

Figure 6. (a) Open circuit potential and (b) polarization curves of St52 steel in1 M HCl and in the presence of different TAM concentrations.

Furthermore, polarization curves of St52 steel in 1 M HCl without and with various concentrations of TAM at 303 K are shown in Figure 6(b). The polarization parameters like $i_{corr}$, corrosion potential ($E_{corr}$), anodic and cathodic Tafel slopes (${\beta }_a\text{ and }{\beta }_c$) and inhibition efficiency (${\eta }_{PDP}$) deduced by extrapolation of the cathodic and anodic current potential curves are given in Table 4. Figure 7 and Table 4 clearly show that ${\beta }_a\text{ and }{\beta }_c$ decreased with increase TAM concentration, implying the suppression of both anodic St52 steel dissolution and cathodic H₂ evolution. Table 4 clearly shows that $i_{corr}$ values decreased from 182.4 µA cm⁻² for 1 M HCl without TAM to 26.2 µA cm⁻² in the presence of 4.9 × 10⁻⁴ M of TAM and continually reduces to 10.8 µA cm⁻² as the inhibitor concentration is increased gradually to 2.0 × 10⁻³ M. This also leads to a significant increase in ${\eta }_{PDP}$ as the concentration of TAM increases and reaches a maximum of 94.1% at maximum inhibitor concentration. Again, the $E_{corr}$ is seen to shift towards a positive direction, suggesting that TAM effectively retarded the dissolution of St52 steel [43]. Based on the maximum shift in $E_{corr}$ values which is less than 85 mV, the inhibitor TAM drug in the present study behaves as a mixed-type inhibitor [44]. The polarization and EIS techniques complement each other with their results being in good agreement.

Figure 7. SEM/EDX images of St52 steel surface in (a) 1 M HCl without and (b) with 2.0 × 10⁻³ M of TAM.

Table 4. Polarization data for St52 steel corrosion in 1.5 M HCl in the absence and presence of TAM.

Concentration (M)	$-E_{corr}$ (mV/SCE)	$i_{corr}$ (µA cm^−2)	${\beta }_a$ (mV dec^−1)	−${\beta }_c$ (mV dec^−1)	${\eta }_{PDP}$ (%)
Blank	−503.4	182.4	120.2	130.5	—
4.9 × 10^−4	−500.2	26.2	110.4	127.3	85.6
2.0 × 10^−3	−491.6	10.8.	82.8	128.6	94.1

3.4. SEM/EDX studies

The morphological analysis using SEM/EDX gives useful qualitative evidence about the extent of corrosion attack and adsorption of inhibitor molecules on the steel surface. Figure 7(a) shows the SEM image of corroded St52 steel surface in 1 M HCl without TAM while Figure 7(b) represents the SEM image in the presence of 2.0 × 10⁻³ M of TAM with the corresponding EDX spectrum. The image without TAM reveals a steel surface that is greatly damaged with visible corrosion deposits and cracks. These observations were caused by the aggressive attack of the studied electrolyte (1 M HCl) on the St52 steel surface. On the contrary, the SEM image in the presence of TAM presents a smoother surface that appears less corroded, suggesting adsorption and protective film formation by the molecules of TAM on the steel/acid solution interface [45]. The EDX also shows the adsorption of expected compounds on the steel surface which proves good inhibition action of TAM molecules.

3.5. Adsorption isotherm

To ascertain the best-fitted adsorption isotherm for TAM’s adsorption on the corroding St52 steel surface, various isotherms such as Temkin, Langmuir, Frumkin and Freundlich were tested. The Langmuir isotherm was best fitted for all the tested isotherms and is expressed as follows [27]: (15) $\frac{C_{inh}}{\theta }=C_{inh}+\frac{1}{K_{ads}},$(15)

where $C_{inh}$ is TAM concentration, $\theta$ represents surface coverage, and $K_{ads}$ represents adsorption equilibrium constant. The Langmuir isotherm plot for TAM is shown in Figure 8. The regression coefficients ($R^2$) and the slope of the isotherm are listed in Table 5. The results clearly show that the $R^2$ and slope values are very near to unity, implying that TAM’s adsorption obeys Langmuir isotherm. The little drifting of the slopes from one is due to mutual attraction or repulsion between the inhibitor molecules resulting from intermolecular interaction between the adsorbed TAM molecules. From the intercept of the linear graph, $K_{ads}$ values were deduced. The $K_{ads}$ is known to represent the adsorption or desorption strength between the metal surface and the adsorbate. Large $K_{ads}$ values are consistent with effective inhibitor adsorption [46]. The $K_{ads}$ is related adsorption free energy ($\mathrm{\Delta }{G}_{ads}^0$) as: (16) $\mathrm{\Delta }{G}_{ads}^0=-RT\ln (55.5K_{ads}),$(16)

Figure 8. Langmuir isotherm plots for TAM’s adsorption on St52 steel surface at various temperatures.

Table 5. Langmuir adsorption isotherm data for TAM at various temperatures.

Temp. (K)	Intercept	Slope	$R^2$	$K_{ads}$ (M^−1)	$\mathrm{\Delta }{G}_{ads}$ (kJ/mol)
303	3.36 × 10^−4	0.8472	0.9967	2976	−30.27
313	4.04 × 10^−4	0.8741	0.9969	2475	−30.79
323	4.51 × 10^−4	0.9368	0.9992	2217	−31.47
333	5.23 × 10^−4	0.9073	0.9982	1912	−32.04

where $T$ stands for temperature, $R$ represents gas constant and 55.5 is ${\text{H}}_2\text{O}$ concentration (M) in solution. The computed $K_{ads}$ and $\mathrm{\Delta }{G}_{ads}^0$ values are displayed in Table 5. Negative $\mathrm{\Delta }{G}_{ads}^0$ suggest that the adsorption of TAM on the St52 steel surface is self-generated and with stable adsorbed film. Literature of previous studies indicates that values of $\mathrm{\Delta }{G}_{ads}^0$ within −40 kJ mol⁻¹ or negatively higher are consistent with chemisorption [47]. On the other hand, values of $\mathrm{\Delta }{G}_{ads}^0$ around −20 kJ mol⁻¹ are consisted with physisorption. The values of $\mathrm{\Delta }{G}_{ads}^0$ as shown in Table 4 from −30.27 to −32.04 kJ mol⁻¹ indicate that the adsorption of TAM on the St52 steel surface is competitive of chemisorption and physisorption [47].

3.6. Proposed inhibition mechanism

The corrosion inhibition mechanism of organic compounds based on literature reports and our present study is generally due to adsorption. TAM displayed a $\mathrm{\Delta }{G}_{ads}^0$ value comprising chemisorption and physisorption mechanism. Chemisorption involves the donor–acceptor interaction between vacant d-orbitals of iron and lone pairs of electrons of heteroatoms together with $\pi$ electrons of multiple bonds [48]. Physisorption involves the existence of both electrically charged steel surface and charged molecules of the inhibitor. The inhibition mechanism of TAM will depend greatly on its molecular structure and the functional groups present. The molecules of TAM in the acidic solution can be protonated, and the resulting cations exist in equilibrium with its molecular form: ${\text{C}}_{\text{20}}{\text{H}}_{\text{28}}{\text{N}}_{\text{2}}{\text{O}}_{\text{5}}\text{S + x}{\text{H}}^{\text{ + }}\rightleftharpoons [{\text{C}}_{\text{20}}{\text{H}}_{\text{28 + x}}{\text{N}}_{\text{2}}{\text{O}}_{\text{5}}\text{S}{]}^{\text{x + }}$

The protonation of amine could improve the solubility of TAM molecules in the aggressive acid solution and the chloride ion, Cl⁻ (counter anion) will enhance the inhibitor adsorption [49]. The protonated amine group will interact with the adsorbed Cl⁻ on the St52 steel surface through electrostatic interaction. The various spatial adsorption interactions of TAM on the St52 steel surface are illustrated in Figure 9. The molecules of TAM can also adsorb through the interaction of Fe atom vacant d-orbital with $\pi$-electrons of TAM molecules and non-bonded electrons of heteroatoms [50]. There is also a possibility of TAM forming a smooth homogenous monolayer of protection on the St52 steel surface as shown in Figure 10. This leads to high surface coverage and effective adsorption [51].

Figure 9. Spatial adsorption interactions of TAM on St52 steel surface.

Figure 10. Systematic and staggered arrangement of TAM on St52 steel surface.

3.7. Theoretical calculations

To have a better understanding of the donor–acceptor interactions between surface iron atoms and the frontiers molecular orbital (FMO) of TAM, quantum chemical calculations were performed. According to the FMO theory of chemical reactivity, the interaction between the LUMO and HOMO of the reacting species results in transition formation [52]. The HOMO energy ($E_{HOMO}$) is related to the capacity of the molecule to give electrons to the available metal surface vacant d-orbital. Therefore, higher $E_{HOMO}$ leads to better adsorption. Then, again, the energy of LUMO ($E_{LUMO}$) provides information on electron acceptance by the molecule. Decreased $E_{LUMO}$ shows the capability of TAM molecules to receive an electron from the steel surface and increase inhibition efficiency. Reports have shown that effective inhibitors are those that will not only donate an electron to the corroding metal but can also accept an electron from the metal. The energy gap $\mathrm{\Delta }E$ is a measure of the reactivity or efficacy of the inhibitor, and small $\mathrm{\Delta }E$ values suggest good inhibition efficiency of the molecule [53].

The LUMO and HOMO surfaces of the neutral (TAM) and protonated (TAM-H⁺) species of the inhibitor are displayed in Figure 11. It is clear that the LUMO and HOMO orbitals are separately distributed on amino group, phenyl ring and oxygen atoms both in protonated and in species non-protonated. However, the LUMO orbital of TAM-H⁺ involved sulphur atoms as well. This indicates that the oxygen atoms, nitrogen, sulphur atoms and phenyl rings are among the active adsorption centres. The quantum chemical parameters computed from the DFT are shown in Table 6. From the table, the values of $E_{HOMO}$ suggest TAM to be superior over TAM-H⁺ while values of $E_{LUMO}$ suggest TAM-H⁺ is superior. However, the $\mathrm{\Delta }E$ which probably is the most important DFT reactivity parameter supports the superior effect of TAM-H⁺ over TAM. The smaller value of the electronegativity ($\chi$) of TAM indicates its higher reactivity and higher ability over TAM-H⁺ in donating electrons to the metal surface. The total energy of TAM-H⁺ was more negative than that of TAM, suggesting molecular stability of TAM-H⁺ when interacting with the St52 steel. Another computed parameter, the dipole moment ($\mu$) which shows molecular polarity, was found to be higher in TAM-H⁺ than in TAM. There are conflicting reports in literature on the influence of $\mu$ on corrosion inhibition. While some authors suggest that a decrease in $\mu$ leads to an increase in the efficacy of the inhibitor, others rather think the reverse is the case [54]. Low hardness ($\eta$) value for an organic molecule is indicative of an efficient corrosion inhibitor while a higher value of $\eta$ indicates a less potent inhibitor [55]. The inverse of harness ($\eta$) is known as softness ($\sigma$), and it has a reverse effect on corrosion inhibition with the $\eta$ values. The values of $\eta$ and $\sigma$ from the present study also support the superiority of TAM-H⁺ over TAM. The fraction of electron transfer ($\mathrm{\Delta }N$) is a measure of the extent of metal–inhibitor interactions, and studies have shown that values of $\mathrm{\Delta }N$ greater than 0 and less than 3.6; that is, $\Delta N<3.6$ indicates increasing inhibition efficiency with an increase in the electron-donating capability of the inhibitor at the steel surface [55]. More so, the computed value of $\mathrm{\Delta }N$ in the present study is $<$3.6 for both pronated and neutral species of TAM which confirms the capability of the inhibitor to donate an electron to the steel surface and inhibitive effect of the studied inhibitor.

Figure 11. The FMO density distribution of HOMO and LUMO of neutral and protonated TAM.

Table 6. DFT parameters for neutral (TAM) and protonated (TAM-H⁺) form of the inhibitor.

Inhibitor	$E_{HOMO}$ (eV)	$E_{LUMO}$ (eV)	$\mathrm{\Delta }{E}_{gap}$ (eV)	$\chi$ (eV)	$\eta$ (eV)	$\sigma$ (eV^−1)	$\mathrm{\Delta }N$	$\mu$ (Debye)	Total energy (au)
TAM	−5.37	−0.87	4.50	3.12	2.25	0.44	0.86	7.27	−1622
TAM-H^+	−11.05	−8.24	2.81	3.65	1.41	0.71	1.19	10.22	−1624

4. Conclusions

The present study considers the evaluation of the performance of expired tamsulosin (TAM) as a corrosion inhibitor for St52 steel in a 1.5 M HCl medium. The studied drug was found to inhibit the corrosion of the steel in the aggressive acid solution with an increase in its efficiency as TAM concentration increases. Gravimetric studies reveal that TAM’s efficiency decreases with temperature rise. EIS measurement indicates that the presence of TAM in the aggressive acid medium significantly increased $R_{ct}$ due to TAM’s adsorption at the metal–electrolyte interface. PDP studies disclose that TAM is a mixed-type corrosion inhibition. SEM image in the presence of TAM reveals a smoother surface that appears less corroded, suggesting adsorption and protective film formation by the molecules of TAM on the steel/acid solution interface. The inhibitor followed Langmuir isotherm and $\mathrm{\Delta }{G}_{ads}^0$ values suggest competitive chemical and physical adsorption mechanism. Theoretical calculations support the corrosion inhibitive effect of the inhibitor.

Acknowledgments

Council of Scientific and Industrial Research (CSIR), India, and The World Academy of Sciences (TWAS), Italy, are gratefully acknowledged by Valentine Chikaodili Anadebe for the CSIR-TWAS Postgraduate Fellowship (Award No.22/FF/CSIR-TWAS/2019) to pursue his Ph.D. research programme in CSIR-CECRI, India. In addition, V.C.A is grateful to Alex Ekwueme Federal University Ndufu-Alike Ebonyi State, Nigeria, for the research leave to visit CECRI, India.

Disclosure statement

No potential conflict of interest was reported by the author(s).