Investigation of synthesized ethyl-(2-(5-arylidine-2,4-dioxothiazolidin-3-yl) acetyl) butanoate as an effective corrosion inhibitor for mild steel in 1 M HCl: A gravimetric, electrochemical, and spectroscopic study

Abstract

The corrosion inhibition effects of five concentrations (5E-5 M to 9E-5 M) of ethyl-(2-(5-arylidine-2,4-dioxothiazolidin-3-yl) acetyl) butanoate, a novel thiazolidinedione derivative code named B1, were investigated on mild steel in 1 M HCl using gravimetric analysis, electrochemical analysis and Fourier transform infrared spectroscopy. After synthesis and purification, B1 was characterized using nuclear magnetic resonance spectroscopy. All gravimetric analysis experiments were carried out at four different temperatures: 303.15 K, 313.15 K, 323.15 K and 333.15 K, achieving a maximum percentage inhibition efficiency of 92% at 303.15 K. The maximum percentage inhibition efficiency obtained from electrochemical analysis, conducted at 303.15 K, was 83%. Thermodynamic parameters such as ΔG°_ads showed that B1 adsorbs onto the MS surface via a mixed type of action at lower temperatures, transitioning to exclusively chemisorption at higher temperatures.

1. Introduction

Mild steel (MS), also known as plain-carbon steel, is an important metal with a variety of applications in industry [1]. It is used in the fabrication of reaction vessels, storage tanks, construction of petroleum refineries, as a building material, in machinery and construction of pipes [[2], [3], [4], [5]]. Mild steel frequently encounters corrosive conditions in industry, primarily brought about by acids such as hydrochloric acid (HCl) [6,7]. Hydrochloric acid is used extensively in industry for processes such as acid pickling and descaling, well acidizing and electrochemical etching [[8], [9], [10]]. The acid induced corrosion of MS results in several negative effects such as reduction in mechanical strength, crumbling of MS structures and fluid intrusion in containers and pipelines [[11], [12], [13]]. The need to reduce HCl induced corrosion of MS has led to the use of corrosion inhibitors (CIs), typically added to the acid in small concentrations. This results in a significant reduction in corrosion, prolonging the useful life of the metal [14].

To achieve the desired protective effect, CIs must meet certain structural requirements: they must have an abundance of heteroatoms (P, N, S and O) and must also possess multiple bonds [15]. Such compounds have been used extensively as CIs for metals in aqueous environments [16,17]. Heteroatoms and multiple bonds are electron rich centres that can potentially donate electron density to the metal surface, increasing the likelihood that dative covalent bonds can be formed between the CI and the metal. This results in the formation of a strong and durable adsorption film on the metal surface, protecting it from corrosion. Organic compounds are thus desirable as CIs as they possess all the necessary structural qualities [18]. The novel thiazolidinedione derivative (TZD) highlighted in this article has all the structural qualities mentioned above: the presence of heteroatoms (N, O, and S) and double bonds. Extensive corrosion research has been carried out on other TZDs [[19], [20], [21]]. Gravimetric analysis experiments by Chaouiki et al. [19] indicated that the TZDs MeOTZD and MeTZD showed good inhibition for carbon steel in 1 M HCl, with maximum percentage inhibition efficiencies (%IE) of 94% and 83% respectively. Potentiodynamic polarization (PDP) and electrochemical impedance spectroscopy (EIS) experiments by Lgaz et al. [20] showed that the TZDs MTZD and ATZD attained maximum %IE of 90% and 96% respectively for PDP and 94% and 98% respectively for EIS. These results are in keeping with data presented in this paper and show that TZDs are potent CIs for metals in aggressive environments.

However, when selecting a potential CI, its molecular structure should not be the only consideration. Many CIs in use today are toxic and thus harmful to human health and the environment. Some of these toxic CIs include polyphosphates, chromates and benzothiazoles [[22], [23], [24]]. Birkholz et al. [25] stated that benzothiazoles are used extensively as anticorrosive agents in engine coolants, aircraft de-icers, antifreeze liquids and dishwashing liquids, where they are used to protect silver from corrosion. Benzothiazoles have been associated with dermal sensitization, respiratory tract irritation, endocrine disruption, genotoxicity, and carcinogenicity [26]. Liao et al. [26] noted that rubber factory workers exposed to 2-mercapto-benzothiazole had a high prevalence of cancers such as bladder cancer, lung cancer and leukaemia. Toxic effects of polyphosphates and chromates include nose bleeds, sores, major organ toxicity and increased risk of strokes and heart attacks [27,28]. The TZD discussed in this work is neither a benzothiazole, polyphosphate nor a chromate, making it far more desirable as a CI.

Table 1 shows the total cost of corrosion (CoC) incurred by different countries and regions around the world. Table 1 also shows the percentage of gross domestic product (GDP) lost to corrosion. The data was obtained from sectors such as infrastructure, utilities, transportation, production, and manufacturing [29]. All these sectors make extensive use of MS. Table 1 emphasizes the importance of discovering new, effective CIs to protect MS. Developing effective CIs will result in significant savings by different economic sectors, leading to a 3%–4% improvement in GDP for countries [30]. There are also environmental benefits to using more effective CIs, primarily the reduction of CO₂ emissions associated with the production of more steel to replace corroded steel [31]. Due to the high %IE attained by B1 in protecting MS in 1 M HCl, it can be said that such a CI has been developed.

Table 1.

Global cost of corrosion [29].

Economic regions	Total CoC (USD Billion)	CoC (% GDP)
United States	451.3	2.7
India	70.3	4.2
Europe	701.5	3.8
Arab world	140.1	5.0
China	394.9	4.2
Russia	84.5	4.0
Japan	51.6	1.0

This paper represents the first time B1 has been used in corrosion studies. Originally synthesized as a potential antidiabetic agent by Tshiluka et al. [32], the molecular structure of B1 made it an ideal candidate for use as a CI for MS in 1 M HCl. In addition, B1 is not commercially available. This necessitated the addition of an extra dimension to the study, that of organic synthesis. An exhaustive search of literature showed that B1 has never been synthesized let alone used in anyway outside of the University of Venda (UNIVEN) Chemistry Department, of which Tshiluka et al. [32] and the authors of this paper are members. This adds yet another novelty aspect to this paper.

2. Experimental

2.1. Materials

Most of the mass of the MS coupons was made up of Fe, with the balance shared amongst a variety of different elements (Table 2). Each of the MS coupons had a 6 mm hole in the centre through which a glass hook was inserted for gravimetric analysis (GA) and for preparing MS coupons for Fourier transform infrared spectroscopy (FTIR). The length and width of the MS pieces were 3 cm and 2 cm, respectively.

Table 2.

Elemental composition of mild steel coupons.

Element	Fe	C	Ni	Mn	S	P	Mo
Mass %	99.32	0.21	0.04	0.37	0.03	0.02	0.01

2.2. Preparation of solutions

Each 1 M HCl solution was prepared using ultrapure de-ionized water and fuming, 32% HCl. All in all, five different solutions of B1 were prepared: 5 E-5M, 6 E-5M, 7 E-5M, 8 E-5M, and 9 E-5M. B1 is insoluble in water. Therefore, dissolving B1 in an organic solvent was necessary before commencement of corrosion studies. Even so, gentle heating with magnetic stirring was needed to ensure B1 remained dissolved. The organic solvent used to dissolve B1 was anhydrous dimethyl sulfoxide (DMSO) (Sigma Aldrich, St. Louis). The total amount of DMSO required to dissolve B1 ranged from 1 to 2 mL. Table 3 shows the percentage purity of all reagents used throughout the study.

Table 3.

Percentage purity of reagents.

Reagents	% Purity
Bromoacetyl chloride	≥98
Dichloromethane	≥99.8
Dimethyl sulfoxide (anhydrous)	≥99.5
Dimethyl sulfoxide (NMR solvent)	99.9
DL-2-aminobutyric acid	99
Ethanol	95
Hydrochloric acid	32
Para-anisaldehyde	98
Piperidine	≥99
Potassium chloride	≥99
Potassium hydroxide pellets	≥85
Tetrahydrofuran	99.9
2,4-thiazolidinedione (Technical grade)	90
Thionyl chloride	≥99

2.3. Synthesis and characterization of B1

The synthetic steps followed to synthesize B1 (Fig. 1) are highlighted in the work done by Tshiluka et al. [32]. All reagents used were sourced from Sigma-Aldrich (St. Louis, United States). After purification, 50 mg of B1 was dissolved in DMSO‑d₆ and characterized using a Bruker Ultra shield plus 400 MHz nuclear magnetic resonance (NMR) spectrometer (Billerica, United States). Specifically, B1 was subjected to three NMR characterization techniques: proton NMR (¹H NMR) at 400 MHz, carbon-13 NMR (¹³C NMR) at 100 MHz and DEPT-135, also at 100 MHz.

Fig. 1.

B1 reaction scheme.

2.4. Gravimetric analysis

Gravimetric analysis experiments were conducted at four different temperatures in Memmert water baths (Buchenbach, Germany): 303.15 K, 313.15 K, 323.15 K and 333.15 K. All MS coupons were gently polished using Struers 200 mm silicon-carbide (SiC) emery paper attached to a Struers LaboSystem instrument (Cleveland, United States). After polishing, all MS coupons were rinsed with acetone, dried then weighed before immersion in both uninhibited and inhibited 1 M HCl solutions. After an immersion period of 5 h, the MS coupons were gently brushed, re-rinsed with acetone, re-dried and re-weighed. The difference in masses was recorded and used in subsequent calculations.

2.5. Electrochemical analysis

A BioLogic SP-150 instrument (Seyssinet-Pariset, France) was used for all electrochemical experiments. All MS coupons were cut into squares with a surface area of 1 cm². All cut MS coupons were attached to electrical conducting wires, embedded in a synthetic resin, and polished before being used as working electrodes (WE). As with GA, the polishing was done using a Struers LaboSystem instrument (Cleveland, United States) with 200 mm SiC emery paper. The other two electrodes used were a silver/silver chloride (Ag/AgCl) reference electrode (with a saturated KCl filling solution) and a graphite counter electrode. Thirty minutes was allotted for the WE to reach open circuit potential (OCP), also known as corrosion potential (E_corr), before commencement of experiments.

2.6. Fourier transform infrared spectroscopy

An ALPHA FT-IR Brucker platinum 22 vector instrument (Billerica, United States) was used for all FTIR experiments. After pre-treatment of two MS coupons (see Section 2.4), one coupon was immersed in an inhibited 1 M HCl solution and the other in an uninhibited 1 M HCl solution. After an immersion period of 5 h, both metal coupons were removed from the aggressive solutions and allowed to corrode over a sufficient period. The corrosion products formed on the metal surfaces were then scrapped off and subjected to FTIR analysis.

3. Results and discussion

3.1. B¹ NMR spectra

The NMR spectra below show that the synthesis of B1 was successful. Fig. 2 shows the full ¹H NMR spectrum of B1, encompassing the aliphatic, heteroatomic and aromatic regions. The expanded aliphatic to heteroatomic region of the ¹H NMR spectrum (Fig. 3) shows a total of eight peaks, with the two protons on C-5 (H-5a and H-5b) being non-equivalent due to the stereogenic centre at C-4. The aliphatic region of the ¹H NMR spectrum shows three methyl group peaks at 0.89 ppm, 1.19 ppm and 3.84 ppm ascribed to H-6 (doublet), H-1 (doublet) and H-18 (singlet) respectively. The aromatic region of the ¹H NMR spectrum shows four peaks: H-7 (doublet) at 8.70 ppm, H-15 (doublet) at 7.62 ppm, H-16 (doublet) at 7.13 ppm and H-13 (singlet) at 7.92 ppm (Fig. 4). Proton H-7 has the highest chemical shift due to the electronegative nature of the N atom to which it is bonded [33]. The DEPT-135 spectrum, which shows methylene (negative peak), methine (positive peak) and methyl (positive peak) carbons in the molecular structure of a compound [34], confirmed that the structure contains three methylene groups: C-2 at 60.98 ppm, C-9 at 43.56 ppm and C-5 at 24.88 ppm (Fig. 5). Also, a combination of DEPT-135 and ¹³C NMR spectra confirmed that the structure contains seven quaternary carbons: C-11 at 172.03 ppm, C-3 at 167.54 ppm, C-8 and C-10 at 165.80 ppm, C-17 at 161.73 ppm, C-14 at 125.81 ppm and C-12 at 118.24 ppm. The ¹³C NMR spectrum (Fig. 6) which, unlike the DEPT-135 spectrum, includes quaternary carbons [35], also showed the presence of one methine carbon at 54.03 ppm (C-4) and three methyl carbons at 56 ppm (C-18), 14.50 ppm (C-1) and 10.57 ppm (C-6).

Fig. 2.

B1 full ¹H NMR spectrum.

Fig. 3.

B1 ¹H NMR spectrum (aliphatic and heteroatomic region).

Fig. 4.

B1 ¹H NMR spectrum (aromatic region).

Fig. 5.

B1 DEPT-135 spectrum.

Fig. 6.

B1 ¹³C NMR spectrum.

3.2. Gravimetric analysis

Three sets of important data were obtained from GA: %IE, metal corrosion rate (C_R) and thermodynamic data. Fig. 7, Fig. 8 show the relationship between %IE Equation (1), C_R Equation (2) and inhibitor concentration (C_inh) [36]. As the name suggests, %IE is a measure of how effective an inhibitor is at protecting a substrate, in this case MS, from corrosion. The higher the %IE, the greater the degree of protection. Under isothermal conditions, an increase in C_inh results in an increase in %IE. This is due to an increase in fractional coverage (θ) Equation (3) [36]. An increase in %IE also corresponds with a decrease in C_R. The above-mentioned trends in %IE and C_R have been reported elsewhere [[37], [38], [39]]. Olomola et al. [39] noted that all three CIs studied exhibited higher %IE with an increase in their concentrations.

$$\%\mathrm{I}\mathrm{E}=\mathrm{\theta }\times 100$$ (1)

$${\mathrm{C}}_{\mathrm{R}}=\frac{{\mathrm{W}}_{\mathrm{b}}-{\mathrm{W}}_{\mathrm{a}}}{\mathrm{A}\bullet \mathrm{t}}$$ (2)

$$\mathrm{\theta }=\frac{{\mathrm{w}}_{\mathrm{o}}-{\mathrm{w}}_{\mathrm{i}}}{{\mathrm{w}}_{\mathrm{o}}}$$ (3)

were W_b, W_a, A, t, w_o and w_i are the metal weight before immersion, metal weight after immersion, total metal surface area, immersion time and the weight loss values in the absence and presence of B1 respectively [[40], [41], [42], [43]].

Fig. 7.

Relationship between inhibitor concentration and percentage inhibition efficiency.

Fig. 8.

Relationship between inhibitor concentration and metal corrosion rate.

At much higher temperatures, the %IE is significantly lower (Fig. 7). Some of the B1 molecules desorb from the adsorption sites on the metal surface, leading to a decrease in both θ and %IE. This has also been reported elsewhere [38,39,44,45]. Olomola et al. [39] stated that one of the CIs studied, an inhibitor code named 4a, showed higher %IE at lower temperatures. This drop in θ exposes parts of the metal surface to more frequent and forceful collisions from corrosive species in the aggressive environment, especially highly corrosive Cl⁻ ions, increasing the metal C_R [[46], [47], [48]]. Noor [48], observed that HCl is more corrosive than H₂SO₄ at all studied temperatures (30°C–70 °C) because of the more corrosive nature of Cl⁻ ions on MS.

3.3. Adsorption and activation parameters

3.3.1. Adsorption parameters

Adsorption isotherms are a key tool in the toolbox of corrosion scientists as they provide a simplified model of the pathway through which corrosion inhibitors adsorb onto a metal surface [49]. The Langmuir adsorption isotherm (LAI), which is the simplest adsorption isotherm developed, was the best fit for B1. Using Equation (4), the LAI (Fig. 9) is obtained from plotting C_inh/θ (y-axis) and C_inh (x-axis), with a y-intercept of 1/K_ads [50,51]:

$$\frac{{\mathrm{C}}_{\mathrm{i}\mathrm{n}\mathrm{h}}}{\mathrm{\theta }}=\frac{1}{{\mathrm{K}}_{\mathrm{a}\mathrm{d}\mathrm{s}}}+{\mathrm{C}}_{\mathrm{i}\mathrm{n}\mathrm{h}}$$ (4)

were K_ads is the adsorption equilibrium constant.

Fig. 9.

B1 Langmuir adsorption isotherms.

Once K_ads is obtained, the standard Gibbs free energy change of adsorption (ΔG°_ads) is calculated using Eq. (5) [52]:

$${\mathrm{K}}_{\mathrm{a}\mathrm{d}\mathrm{s}}=\frac{1}{55.5}\bullet {\mathrm{e}}^{-\frac{{\Delta \mathrm{G}}_{\mathrm{a}\mathrm{d}\mathrm{s}}^{°}}{\mathrm{R}\mathrm{T}}}$$ (5)

were R, T and 55.5 are the universal gas constant (8.314 J mol⁻¹ K⁻¹), the temperature (in Kelvin) and the molar concentration of pure water respectively [53]. Using the van't Hoff equation (Eq. (6)), a plot of lnK_ads vs 1/T (Fig. 10) is used to obtain the standard enthalpy change of adsorption (ΔH°_ads) [54,55]:

$$\ln {\mathrm{K}}_{\mathrm{a}\mathrm{d}\mathrm{s}}=\frac{{-\Delta \mathrm{H}}_{\mathrm{a}\mathrm{d}\mathrm{s}}^{°}}{\mathrm{R}\mathrm{T}}+\frac{{\Delta \mathrm{S}}_{\mathrm{a}\mathrm{d}\mathrm{s}}^{°}}{\mathrm{R}}$$ (6)

were ΔS^°_ads is the standard entropy change of adsorption.

Fig. 10.

B1 van't Hoff plot.

Using the Gibbs-Helmholtz equation (Eq. (7)), together with the ΔH°_ads obtained from Equation (6), ΔS^°_ads data is calculated at all four different temperatures:

$${\Delta \mathrm{G}}_{\mathrm{a}\mathrm{d}\mathrm{s}}^{°}={\Delta \mathrm{H}}_{\mathrm{a}\mathrm{d}\mathrm{s}}^{°}-\mathrm{T}{\Delta \mathrm{S}}_{\mathrm{a}\mathrm{d}\mathrm{s}}^{°}$$ (7)

Table 4 shows a summary of the LAI parameters and adsorption data obtained from Equations (4), (5), (6), (7). The coefficient of determination (r²) values are close to unity and thus show that the LAI is a good fit for the experimental data obtained [56]. Bashir et al. [56] indicated that if ΔH°_ads is negative, the adsorption of the CI onto the metal surface is exothermic. Therefore, a positive ΔH°_ads (38.67 kJ mol⁻¹) is indicative of endothermic adsorption. This shows that B1 molecules, once adsorbed onto the metal surface, repel one another (due to their bulky nature) [57]. This necessitates the supply of additional heat energy (q) to override the increased energy generated by the repulsive forces, hence the endothermic nature of the adsorption.

Table 4.

B1 adsorption parameters.

Temperature (K)	r2	Kads	ΔG°ads kJ·mol−1	ΔH°ads kJ·mol−1	ΔS°ads
		L·mol−1			J·mol−1∙K−1
303.15	0.9516	33717.5	−36.40	38.67	178.83
313.15	0.9735	66811.96	−39.38	38.67	181.57
323.15	0.9069	93773.44	−41.55	38.67	181.63
333.15	0.9692	131485.52	−43.77	38.67	181.85

The additional q not only increases the spontaneity of adsorption of B1 onto the metal surface but, also facilitates the formation of additional dative covalent bonds between B1 and the metal surface. This is shown by the increasingly negative ΔG°_ads values, as B1 transitions from being an inhibitor that adsorbs in a mixed manner (−40 kJ mol⁻¹ < ΔG°_ads ≤ −20 kJ mol⁻¹) [[58], [59], [60]], to one that adsorbs primarily via chemisorption (ΔG°_ads ≤ −40 kJ mol⁻¹) [61]. This increase in the spontaneity of adsorption as the temperature is increased also explains the increase in the K_ads data from 33,718 L mol⁻¹ to 131,486 L mol⁻¹. The more the adsorption of the adsorbate onto the adsorbent is favoured, the higher K_ads becomes [56].

An increase in temperature results in a steady increase in ΔS°_ads. Nwosu and Muzakir [62], reported negative ΔS°_ads values, stating they represent the adsorption of the CI onto the MS surface. Therefore, positive ΔS°_ads data is indicative of desorption from the metal surface. This is supported by Atkins and De Paula [57], when they stated that positive ΔS°_ads data indicates an increase in the translational energy of inhibitor molecules at the metal surface. This increase in translational energy results in a weakening of the bonds formed between B1 molecules and the MS surface. However, given the slight increase in ΔS°_ads, most of the bonds formed between B1 and the MS surface remain largely intact and unaffected. This emphasizes why chemisorption is the most ideal mode of adsorption as it forms resilient and tough protective films on the metal surface [63].

3.3.2. Activation parameters

Activation parameters are a key component in describing the metal corrosion process. These parameters consist of activation energy (E_a), standard enthalpy change of activation (ΔH°_act) and standard entropy change of activation (ΔS°_act). All activation parameters are shown in Table 5. Activation energy is calculated using the Arrhenius equation (Eq. (8)) whereas ΔH°_act and ΔS°_act are obtained using the transition state equation (Eq. (9)), an alternative form of (Eq. (8)) [54].

$$\mathrm{Log}{\mathrm{C}}_{\mathrm{R}}=\mathrm{L}\mathrm{o}\mathrm{g}\mathrm{A}-\frac{{\mathrm{E}}_{\mathrm{a}}}{2.303\bullet \mathrm{R}\bullet \mathrm{T}}$$ (8)

were A is the Arrhenius constant, susceptible to the type of metal being used and the aggressive environment into which it has been immersed [64].

$$\log \left(\frac{{\mathrm{C}}_{\mathrm{R}}}{\mathrm{T}}\right)=\left[\log \left(\frac{\mathrm{R}}{\mathrm{N}\mathrm{h}}\right)+\left(\frac{{\Delta \mathrm{S}}_{\mathrm{a}\mathrm{c}\mathrm{t}}^{°}}{2.303\mathrm{R}}\right)\right]+\left(\frac{{-\Delta \mathrm{H}}_{\mathrm{a}\mathrm{c}\mathrm{t}}^{°}}{2.303\mathrm{R}}\right)\left(\frac{1}{\mathrm{T}}\right)$$ (9)

were N and h are the Avogadro and Planck constants respectively. A plot of log (C_R/T) against 1/T yields a straight line with slope (-ΔH°_act/2.303 R) and a y-intercept of [log (R/Nh)+(ΔS°_act/2.303 R)] [65].

Table 5.

B1 activation parameters.

Cinh (M)	Eact (kJ·mol−1)	ΔH°act (kJ·mol−1)	ΔS°act (J·mol−1∙K−1)
Blank	54.69	54.68	−118.29
5E-5	68.87	68.86	−82.66
6E-5	68.89	68.87	−81.79
7E-5	69.54	69.53	−81.20
8E-5	74.30	74.29	−65.18
9E-5	101.14	101.13	17.05

All E_a values for the inhibited 1 M HCl solutions are larger than the E_a of the uninhibited 1 M HCl solution [66,67]. This is an indication that in the presence of B1 molecules the corrosion process is suppressed significantly [68]. As C_inh is increased, E_a also increases [55,69]. As a result, 9E-5 M has the highest E_a of all the studied concentrations, as can be deduced from its much steeper Arrhenius plot (Fig. 11). Plotting LogC_R vs 1/T yields straight lines with a slope of -E_a/R [64].

Fig. 11.

B1 Arrhenius plots.

All ΔH°_act values are approximately the same as their corresponding E_a values [67]. Diki et al. [67] attributed the closeness of E_a and ΔH°_act data to the equation ΔH°_act = E_a-RT. However, Table 5 shows that E_a and ΔH°_act are virtually identical. Since all experiments were carried out under constant pressure conditions and, q = E_a, it follows that ΔH°_act = q. Therefore, E_a = ΔH°_act. This explains the similarity in E_a and ΔH°_act data. In addition, all ΔH°_act values are positive. Thoume et al. [70] attribute this to chemisorption.

As C_inh is increased, ΔS°_act becomes more positive, indicating an increase in the extent of disorder at the metal surface [56,67]. Diki et al. [67] attribute this trend to the formation of an activated complex between the metal surface and the inhibitor molecules, resulting in the displacement of H₂O molecules from the MS surface (Eq. (10)). Therefore, the adsorption of B1 molecules onto the MS surface can be regarded as a quasi-water displacement process governed by the equilibrium equation below [71]:

$${\mathrm{I}\mathrm{n}\mathrm{h}}_{\mathrm{s}\mathrm{o}\mathrm{l}}+{\mathrm{n}\mathrm{H}}_2{\mathrm{O}}_{\mathrm{a}\mathrm{d}\mathrm{s}}\to {\mathrm{I}\mathrm{n}\mathrm{h}}_{\mathrm{a}\mathrm{d}\mathrm{s}}+{\mathrm{n}\mathrm{H}}_2{\mathrm{O}}_{\mathrm{s}\mathrm{o}\mathrm{l}}$$ (10)

were Inh_sol, H₂O_ads, Inh_ads and H₂O_sol are the inhibitor molecules in solution, adsorbed water molecules, adsorbed inhibitor molecules and displaced water molecules respectively.

3.4. Potentiodynamic polarization

The corrosion inhibition effects of five concentrations of B1 were studied at 303.15 K. The potential of the WE was varied both in the cathodic and anodic directions over a pre-set potential range of -1 V to +1 V. Tafel plots (Fig. 12, Fig. 13) were then plotted, enabling the calculation of all the parameters shown in Table 6. The logarithm of corrosion current (Log_icorr) values for the inhibited 1 M HCl solutions are lower than that of the uninhibited solution (Fig. 12, Fig. 13), as reported elsewhere [[72], [73], [74]]. This is an indication that, in the presence of B1, MS corrosion is suppressed [75]. Fig. 13 shows a shift to more positive and negative potentials by anodic and cathodic curves respectively. This shows that B1 is a mixed action inhibitor, preventing corrosion at both anodic and cathodic sites on the MS surface [76].

Fig. 12.

B1 Tafel plot (Full scale plot).

Fig. 13.

B1 Tafel plot (Expanded version).

Table 6.

B1 PDP parameters.

Concentration (M)	CR (mpy)	Ecorr (mV)	icorr (μA)	βa (mV·dec−1)	βc (mV·dec−1)	Rp (Ω)	%IE
0	380	−426	866	70	127	180	–
5E-5	155	−455	339	53	94	379	61
6E-5	135	−456	295	60	126	415	66
7E-5	123	−439	270	59	104	437	69
8E-5	88	−435	191	58	120	449	78
9E-5	67	−438	146	63	123	690	83

All C_R values for inhibited solutions are significantly lower than the C_R for the uninhibited solution (Table 6) [66], [77]. This shows that corrosion of MS is suppressed when B1 is added to the 1 M HCl solution. A decrease in C_R corresponds to a decrease in i_corr (Table 6). In turn, a decrease in i_corr results in an increase in %IE (Table 6). Corrosion current can therefore be used to calculate %IE according to Eq. (11) [78]:

$$\%\mathrm{I}\mathrm{E}=\left(1-\frac{{\mathrm{i}}_{\mathrm{c}\mathrm{o}\mathrm{r}\mathrm{r}}^{\mathrm{i}}}{{\mathrm{i}}_{\mathrm{c}\mathrm{o}\mathrm{r}\mathrm{r}}^{\mathrm{o}}}\right)\times 100$$ (11)

were i^o_corr and iⁱ_corr are the corrosion currents in the uninhibited and inhibited solution respectively.

An increase in C_R also corresponds to a decrease in polarization resistance (R_p), as reported by Thoume et al. [70] and Jalab et al. [54]. A decrease in R_p is an indication of decreasing %IE [79]. Therefore, R_p can also be used to calculate %IE according to Eq. (12) [78]:

$$\%\mathrm{I}\mathrm{E}=\left(1-\frac{{\mathrm{R}}_{\mathrm{p}}^{\mathrm{o}}}{{\mathrm{R}}_{\mathrm{p}}^{\mathrm{i}}}\right)\times 100$$ (12)

were R^o_p and Rⁱ_p are the polarization resistances for the uninhibited and inhibited solutions respectively. All R_p values for inhibited solutions are significantly larger than the R_p for the uninhibited solution, another positive sign that corrosion inhibition of MS is taking place in the presence of B1 [80].

The deviation from E_corr is < 85 mV, indicating that B1 is a mixed action inhibitor, inhibiting the corrosion process at both the anodic and cathodic sites on the WE surface [[81], [82], [83]]. However, the cathodic Tafel constants (β_c) change more significantly than the anodic Tafel constants (β_a) on addition of B1 to the 1 M HCl solution, an indication that the formation of H₂(g) at the cathodic sites is impaired more than the dissolution of MS at the anodic sites [84].

3.5. Electrochemical impedance spectroscopy

The EIS parameters that define the adsorption of B1 onto the MS surface are shown in Table 7. These parameters were obtained by fitting a modified Randels circuit (Fig. 14) to real (Z′) and imaginary (-Z″) impedance data points. The fit was done using ZSimDemo 3.30 d software. A plot of the (Z’; -Z″) coordinates, using OriginPro graphing and analysis software, produced Nyquist plots (Fig. 15).Were R1, R2 and Q2 are the solution resistance (R_s), charge transfer resistance (R_ct) and the constant phase element respectively [85].

Table 7.

B1 EIS parameters.

Concentration (M)	Rct (Ω)	Rs (Ω)	Χ2	n	Qdl (μF)	%IE
0	15.27	0.73	0.0008478	0.7951	225.14	–
5E-5	42.82	0.856	0.007498	0.9035	129.44	64.34
6E-5	43.41	0.865	0.01525	0.8817	104.13	64.82
7E-5	53.22	0.897	0.0009147	0.8649	101.81	71.31
8E-5	61.03	0.932	0.0006241	0.8861	96.00	74.98
9E-5	73.69	1.156	0.001487	0.8594	84.33	79.29

Fig. 14.

Modified Randels circuit.

Fig. 15.

B1 Nyquist plots.

Table 7 shows that all R_ct values for the inhibited 1 M HCl solutions are much larger than that of the uninhibited solution [86]. Table 7 also shows that an increase in C_inh results in an increase in R_ct [79,87]. This is because, as C_inh is increased, more and more molecules of B1 adsorb onto the surface of the WE, protecting it from corrosion [86]. For this reason, any increase in R_ct corresponds to an increase in %IE (Table 5). Therefore, R_ct can be used to calculate %IE according to Eq. (13) [88]:

$$\%\mathrm{I}\mathrm{E}=\left(1-\frac{{\mathrm{R}}_{\mathrm{c}\mathrm{t}}^{\mathrm{o}}}{{\mathrm{R}}_{\mathrm{c}\mathrm{t}}^{\mathrm{i}}}\right)\times 100$$ (13)

were R^o_ct and Rⁱ_ct are the charge transfer resistances of the uninhibited and inhibited solutions respectively.

The Solution resistance also increases as C_inh is increased, a trend also reported by Bashir et al. [56] and Mashuga et al. [89]. Although in Mashuga et al. [89], only one of the four compounds studied (P4) follows this trend perfectly. Mashuga et al. [89] also notes that R_s for the blank is lower than that of the inhibited solutions. Mashuga et al. [89] and Daoud et al. [90] put this down to a decrease in the solution conductivity and the adsorption of inhibitor molecules onto the metal surface.

Since a modified Randles circuit (Fig. 14) was the best fit for the impedance data obtained, the constant phase element (CPE), denoted as Q2 in Fig. 14, was used in place of an ideal capacitor. The CPE compensates for the complexity of the system brought about by the roughness of the WE surface [91]. Double layer capacitance (C_dl) was calculated as shown (Eq. (14)) [54]:

$${\mathrm{C}}_{\mathrm{d}\mathrm{l}}=\frac{{\left({\mathrm{Y}}_0{\mathrm{R}}_{\mathrm{c}\mathrm{t}}\right)}^{\raisebox{1ex}{$1$}\!\left/ \!\raisebox{-1ex}{$\mathrm{n}$}\right.}}{{\mathrm{R}}_{\mathrm{c}\mathrm{t}}}$$ (14)

were n and Y₀ are the phase shift and CPE constant respectively.

Since the CPE was used in place of an ideal capacitor, C_dl has been replaced by Q_dl (Table 7). Table 7 shows a decrease in Q_dl as C_inh is increased. This is because B1 molecules displace H₂O molecules from the WE surface as they adsorb onto it, lowering Q_dl [92]. The low chi-squared (Χ²) values show that the modified Randles circuit (Fig. 14) fits the EIS data very well [70].

3.6. Adsorption film analysis

Fourier transform infrared spectroscopy is a simple, quick, non-destructive, and affordable technique for characterizing the functional groups of molecules present in a sample [93]. It can be used for the analysis of solids, liquids, and gases [94]. Only molecules with a net dipole moment can absorb infrared radiation and therefore only such molecules can be characterized using FTIR [95].

The FTIR spectrum for B1 (Fig. 16) shows two sets of peaks between 2500 cm⁻¹ and 3500 cm⁻¹. The first intense peak just before 3500 cm⁻¹ (green square) represents the N–H amine stretch whereas the second, less intense cluster of peaks at approximately 3000 cm⁻¹ (yellow square) corresponds to aromatic and alkyl C–H stretches [96,97]. Both peaks are absent in the spectrum of the adsorption layer, an indication that both the N atom and the aromatic ring take part in the formation of an adsorption film on the MS surface. The carbonyl (C O) peak at 1700 cm⁻¹ (purple square) experiences a significant reduction in its intensity as B1 adsorbs onto the metal surface. This shows that the C O groups also participate in the formation of an adsorption film on the metal surface. A reduction in the intensity of the C–O ester stretch (1100 cm⁻¹ to 1200 cm⁻¹), as shown by the B1 adsorption layer spectrum, suggests the O heteroatoms found in the ester functional group of B1 take part in the formation of an adsorption film on the MS surface.

Fig. 16.

Ftir spectra for B1.

4. Conclusions

• •Gravimetric analysis results show that B1 is a very good corrosion inhibitor for MS in 1 M HCl, achieving a maximum %IE of 92%.
• •The ΔS°_act data shows the displacement of water molecules from the metal surface as C_inh increases and the formation of a protective adsorption film.
• •The ΔG°_ads data shows that the adsorption of B1 onto the metal surface is not only spontaneous but shifts from a mixed type of adsorption (mostly chemisorption in nature) to chemisorption as the temperature is increased.
• •A positive ΔH°_ads value (38.67 kJ mol⁻¹) is indicative of the endothermic nature of the adsorption of B1 onto MS, showing that chemisorption is the favoured mode of adsorption.
• •The E_corr data obtained from PDP analysis shows that the WE is polarized by < 85 mV, showing that B1 is a mixed action inhibitor, suppressing MS corrosion at both anodic and cathodic sites on the WE surface. However, both Tafel plots and the β_c constants show that B1 suppresses MS corrosion by acting mainly at the cathodic sites on the WE surface, hindering the evolution of H₂ gas.
• •As C_inh is increased, i_corr, R_p, C_R and %IE decrease, increase, decrease and increase respectively. This shows the success of B1 in suppressing the corrosion of MS in 1 M HCl.
• •EIS analysis shows that a modified Randels circuit is the best fit for the impedance data obtained.
• •EIS data also shows that as C_inh increases, parameters such as R_ct, %IE, and Q_dl increase, increase, and decrease respectively. This is further evidence of the ability of B1 to suppress MS corrosion in 1 M HCl.
• •The (n) data, which is close to unity, is indicative of pseudo-capacitive behaviour of the constant phase element.

Author contribution statement

Nhlalo M. Dube-Johnstone: Conceived and designed the experiments; Performed the experiments; Analyzed and interpreted the data; Wrote the paper.

Unarine Tshishonga: Performed the experiments; Analyzed and interpreted the data; Wrote the paper.

Simon S. Mnyakeni-Moleele: Conceived and designed the experiments; Analyzed and interpreted the data.

Lutendo C. Murulana: Conceived and designed the experiments; Analyzed and interpreted the data; Contributed reagents, materials, analysis tools or data; Wrote the paper.

Funding statement

This research was supported by the National Research Foundation (NRF) of the Republic of South Africa Thuthuka (Grant 107,431) and the South African Synthetic Oil Limited (SASOL) University Collaboration Programme Grant.

Data availability statement

Data will be made available on request.

Declaration of interest's statement

The authors declare no competing interests.

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.