1 Introduction

Steel has a favorable role owing to its superior technical characteristics. Nevertheless, it has the major drawback of possessing a fairly strong reactivity with the surrounding environment, which contributes more or less to their demise [1,2,3].Metal corrosion is a phenomenon in which metals and alloys tend to return to their original state of oxide, sulfide, carbonate, or any other salt that is more stable under the influence of chemical reagents or environmental agents [4, 5]. Petroleum corrosion is certainly much more common than elsewhere, considering the complexity of the conditions faced and the manufacturing situations in which prevention is practiced [6, 7]. Petroleum sites are the key victims of corrosion phenomena, particularly when utilizing acids that are frequently used in this industry, particular H2SO4 and HCl, in steel pickling, chemical washing, and boiler decaling [8,9,10,11]. Organic inhibitors, whose mode of action usually comes from their adsorption on the metal surface, are more commonly used to solve this unfavorable tendency owing to their excellent biodegradability, easy availability, and non-toxic nature [12, 13]. Such compounds must fulfill other requirements: prevent the metal dissolution, slow the acid action, and remain stable at low concentrations [14,15,16,17].Successful organic inhibitors usually have in their structure either N, S, or O atoms and π-electrons in triple or conjugate double bonds [18,19,20].

Chalcone oxime has been used as selective retinoid receptor agonists RARγ, and tyrosinase inhibitors, and also applied to synthesize isoxazolines and isoxazoles, both of which are heterocycles very useful in organic and medicinal chemistry [21, 22]. Until now there is no research on the usage of chalcone oxime as a corrosion inhibitor, as well the cheap and straightforward synthesis of the product. Thus, in this situation, the investigation seems to be a significant study.

In this paper, chalcone was synthesized by the Claisen–Schmidt reaction and chalcone oxime derivatives via the condensation of chalcone and hydroxylamine hydrochloride (Scheme 1). The compounds inhibitory effect was inspected against the corrosion of carbon steel in 0.5 M H2SO4 solution at different temperatures (293–323 K). Various chemical and electrochemical techniques were used in this perspective. The surface morphology was examined using scanning electron microscopy to reveal the nature of the protective layer developed on carbon steel substrate. UV–visible technique was used to understand the adsorption action of the synthesized inhibitors over the carbon steel surface. To gain basic findings about interactions and surface adsorption of CO–H and CO–OMe, molecular dynamics (MD) simulation simulations were applied.

Scheme 1
scheme 1

Synthesis of the chalcone 3 and chalcone oxime 4

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2 Materials and measurements

2.1 Preparation of chalcone 3 and chalcone oxime 4

  • Synthesis of chalcone

In a round bottom flask (50 mL), placed in an ice bath, 20 mL of EtOH, 10 mmol of acetophenone 1 (11 mmol) of benzaldehyde derivatives 2 and 15 mL of 10% NaOH solution were added. Then kept on magnetic stirring at 40 °C for 4 h. The mixture was cooled slowly and left in freezer for 48 h and filtered under vacuum. The crude product was recrystallized from ethanol [23].

  • Synthesis of chalcone oxime

A mixture of chalcone derivatives 3 (1 mmol), hydroxylamine hydrochloride (3 mmol), potassium carbonate (1 mmol), and anhydrous sodium sulfate (1.5 mmol) was refluxed in ethyl acetate (15 mL). After the reaction completion, the mixture was filtered, and the solvent was evaporated under reduced pressure. Then distilled water (15 mL) and dichloromethane (15 mL × 3) were added to extract organic compounds. The organic extracts were combined, dried over anhydrous MgSO4, filtered, and concentrated under reduced pressure. The crude mixture was obtained and purified by silica gel column chromatography eluted with a mixture of n-hexane–EtOAC (80/20, v/v) to afford pure product 4 [24].

2.2 Physical and spectroscopic data for chalcone oxime

1,3-diphenylprop-2-en-1-one oxime (CO–H); white solid, yield: 93%; mp 90–91 °C; 1H NMR (300 Hz, CDCl3): δ (ppm) 11.29 (s, 1H, OH), 7.78–7.56 (m, 4H), 7.46–7.31 (m, 6H), 7.29–6.38 (m, 2H). 13C NMR (75 Hz, CDCl3): δ (ppm) 157.74 (C=N); 139.76 (CH–CAr), 137.04 (CAr); 136.19 (Cq–CH=CH); 134.79 (Cq–C=N); 129.25–125.74 (CAr); 117.15 (CH=CH–Cq). IR (KBr) cm−1: 3590 (OH); 2985 (CH); 1620 (C=N); 1514 (C=C); 945 (N–O).

4-methoxyphenyl-1-phenylprop-2-en-1-one oxime (CO–OMe); white solid, yield: 88%; mp117–118 °C 1H NMR (300 MHz, CDCl3):δppm: 10.98 (s, 1H), 7.61–7.40 (m, 8H), 7.30–7.11 (m, 2H), 6.95–6.85 (m, 2H), 3.80 (s, 3H).13C NMR (75 MHz, CDCl3):δppm: 161.33 (Cq–O) 154.07 (C=N);136.29 (CH–CAr); 131.57 (Cq–CN), 130.85 (Cq–CH=CH); 128.62 (CAr); 128.60 (CAr); 127.24 (CAr); 123.18 (CH–CN); 114.28 (CAr); 55.31 (C–O).IR (KBr) cm−1: 3600 (OH); 2995 (CH); 1640 (C=N); 1520 (C=C); 1310 (Ar–OMe).

2.3 Materials

The chemical composition of carbon steel operated as working electrode in this study is enclosed in Table 1. The corrosive solution (0.5 M H2SO4) was prepared by diluting an analytical reagent grade 98% H2SO4 with distilled water. In the meanwhile, various concentration gradients of chalcone oxime derivatives (1.12 × 10–4, 2.24 × 10–4, 3.36 × 10–4, 4.48 × 10–4 M) at temperatures of 293 K were prepared to assess inhibition efficiency. Before each test, the electrode was polished by empery paper (200, 600, 800, and 1200), then washed with distilled water, acetone and dried.

Table 1 Chemical composition of carbon steel C38
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2.4 Weight loss measurements

Weight loss was accomplished by exposing carbon steel specimens to 100 mL of 0.5 M H2SO4 solution for 6 h with differing chalcone oxime derivatives concentrations at 293 K. The surface of specimens (1 cm × 1 cm × 0.3 cm) underwent polishing pre-treatment prior to each trial. Afterward, carbon steel was washed by hot ethanol, distilled water, and acetone to remove all organic and inorganic matter, and then the samples were weighed. The corrosion rates (ν) were computed according to the following equation [25]:

$$\nu = \frac{\Delta W}{{S \times t}},$$
(1)
$$\eta_{W} \left( \% \right) = \frac{{\nu^{0} - \nu }}{{\nu^{0} }} \times 100,$$
(2)

where ΔW denotes the average weight loss, S represents the sample surface area (cm2), t stands for the immersion time (h), νo and νdesignate the values of corrosion rate (mg cm–2 h–1) in uninhibited and inhibited media, respectively.

2.5 Electrochemical measurements

In this study, the electrochemical measurements in this study were performed using PGZ301 electrochemical workstation with a classical three-electrode system: Pt as the counter electrode, a saturated calomel reference electrode (SCE), and carbon steel as a working electrode (0.282 cm2). The carbon steel was maintained immersed in the corrosive medium until the open-circuit potential (OCP) was stabilized (40 min) before the electrochemical measurements. The potentiodynamic polarization (PDP) curves were plotted by a scan rate of 0.5 mV s–1 and the range of OCP ± 200 mV. The electrochemical impedance spectroscopy (EIS) was recorded at various frequencies between 100 kHz and 10 mHz in the OCP with an amplitude of 10 mV. The inhibition efficiency (η) was estimated using both techniques PDP and EIS as following [26, 27]:

$$\eta_{P} \left( \% \right) = \frac{{i_{corr}^{0} - i_{corr} }}{{i_{corr}^{0} }} \times 100,$$
(3)

\(i_{corr}^{0}\) and \(i_{corr}\) denote the corrosion current densities values in absence and presence of the inhibitory molecule, respectively.

$$\eta_{EIS} \left( \% \right) = \frac{{R_{p}^{i} - R_{p}^{b} }}{{R_{p}^{i} }} \times 100,$$
(4)

where \(R_{p}^{b}\) and \(R_{p}^{i}\) are polarization resistances obtained in the blank and the inhibited solutions, respectively.

The surface coverage, θ is expressed as:

$$\theta = \frac{{\eta_{P} \left( \% \right)}}{100},$$
(5)

\(\eta_{P}\) is the inhibitory efficiency determined by PDP.

2.6 Scanning electron microscopic measurement (SEM) and electrolyte analysis

The surface of carbon steel was polished with abrasive papers of different grades (200–1200) and sprayed with water and acetone, then the carbon steel was immersed for 6 h in 0.5 M H2SO4 with and without adding 4.48 × 10–4 M of synthesized inhibitory molecules. The specimens were extracted from the study solution and rinsed with distilled water and dried. The SEM images were obtained on an SEM FEI FEG 450. Weight reduction testing electrolytes are subject to an evaluation by means of UV mini-1240/UV–VIS Spectrophotometer with a view to confirming the iron-inhibitor complex formation that could happen upon dipping steel in a sulfuric acid electrolyte comprising inhibitor molecules.

2.7 Molecular dynamics (MD) simulation details

The adsorbed inhibitors CO and CO-NO2neutral and unnatural forms on the metal surface was established using the MD simulation using Forcite module implemented in Materials Studio 8.0 software [28, 29]. The study of these interactions of the molecules with the Fe (1 1 0) surfaces was carried out from a simulation box (22.34 × 22.341 × 35.13 Å3) with periodic boundary conditions. The Fe (110) surface was presented with a 6-couche slab model in each layer representing a (9 × 9) unit cell. The constructed simulation box is emptied by 20.13 Å3. This vacuum is occupied by 500H2O, 10H3O+, 5 \(SO_{4}^{2 - }\) and the inhibitory molecule. The temperature of the simulated system of 293 K was controlled by the Andersen thermostat, NVT ensemble, with a simulation time of 400 ps, and a time step of 1.0 fs, all under the COMPASS force field [30].

3 Results and discussion

3.1 Weight loss measurements

The protection performance of CO–H and CO–OMe against carbon steel corrosion in 0.5 M H2SO4 at 293 K was calculated gravimetrically. Table 2 provides the values of corrosion rate and the inhibition activity. It can be noticed that the corrosion rate decreases with increasing CO–H and CO–OMe concentration. As a result, the inhibiting efficiency increases and reaches an inhibiting power of 93% and 95% in the presence of 4.48 × 10–4 M of CO–H and CO–OMe, respectively. The maximum protection efficiency for CO–OMe was due to the presence of electron donating methoxy group[31].This indicates that CO–H and CO–OMe covered the carbon steel surface through the adsorption and from it prevent the attack by the corrosive solution on the surface[32].The inhibition activity can be explained by the following equations[33]:

Table 2 Weight loss, corrosion rate, and inhibitory efficacy of CO–H and CO–OMe at different concentrations in 0.5 M H2SO4 medium
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The iron dissolution reaction.

$$Fe + SO_{4}^{2 - } \rightleftharpoons \left( {FeSO_{4}^{2 - } } \right)_{ads},$$
(6)
$$\left( {FeSO_{4}^{2 - } } \right)_{ads} \rightleftharpoons \left( {FeSO_{4} } \right) + 2e^{ - },$$
(7)
$$\left( {FeSO_{4} } \right) + 2e^{ - } \rightleftharpoons Fe^{2 + } + SO_{4}^{2 - },$$
(8)

The inhibition mechanism

$$Fe + \left( {CO - H} \right)H^{ + } \rightleftharpoons \left[ {Fe\left( {} \right)\left( {CO - H} \right)H^{ + } } \right]_{ads},$$
(9)
$$Fe + \left( {CO - OMe} \right)H^{ + } \rightleftharpoons \left[ {Fe\left( {} \right)\left( {CO - OMe} \right)H^{ + } } \right]_{ads},$$
(10)

First of all, the protonation of CO

$$CO - H + H^{ + } \rightleftharpoons \left( {CO - H} \right)H^{ + },$$
(11)
$$CO - OMe + H^{ + } \rightleftharpoons \left( {CO - OMe} \right)H^{ + },$$
(12)

3.2 Potentiodynamic polarization (PDP)

PDP curves were plotted for carbon steel in H2SO4 environment in with and without different concentrations of CO–H and CO–OMe at 293 K. As seen in Fig. 1, the cathodic curves are in the form of a Tafel line, indicating that the H+ reduction reaction on the steel surface takes place according to a pure activation mechanism [34]. As seen in Fig. 1, the addition of the CO–H and CO–OMe to the acid solution caused a significant reduction in the anodic and cathodic current densities. Concerning anodic polarization plots and especially in the higher concentrations for CO–H and CO–OMe inhibitors, it should be noted that one areas (1, and 2) are observed, which show the inhibited area, and uninhibited area, respectively. With augmenting anodic potentials, the anodic currents augmented with anodic Tafel slope βa1 in area 1. Next, for the rest of potential values, the desorption potential (Edsp), the anodic currents rise sharply and dissolve at βa2 slope in area 2 and stabilized until the end forming a considerable flat. This flat can be ascribed to the shift of adsorption–desorption equilibrium toward the desorption of CO–H and CO–OMe molecules on the electrode surface [35].

Fig. 1
figure 1

PDP plots of carbon steel in 0.5 M H2SO4 solution containing CO–H and CO–OMe at 293 K

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Examination of Table 3 leads to the conclusion that the corrosion current densities in presence of the inhibitor are lower than those found in the blank test solution. It can be suggested that the inhibitors adsorb first on the carbon steel surface before acting by simple blocking through its active sites. Furthermore, on may notice that the corrosion potential gap is less than 85 mV for all concentrations and that both the anodic and cathodic partial currents are also reduced. These outcomes demonstrate the mixed nature with an anodic predominance of the inhibitors under study [36, 37], and show that the inhibitors CO–H and CO–OMe reduces both rates of the anodic dissolution of carbon steel and the reduction of H+.

Table 3 Electrochemical descriptors estimated from PDP for carbon steel in 0.5 M H2SO4 with various concentrations of CO–H and CO–OMe at 293 K
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The fact that the Tafel slopes (βa and βc) do not present a defined behavior pattern with respect to the blank again support the claim that the adsorption process of CO–H and CO–OMe on carbon steel surface in 0.5 M H2SO4 solution is due to the blockage of active sites by the CO–H and CO–OMe compounds.

3.3 Electrochemical impedance spectroscopy

The study of electrochemical impedance diagrams at the corrosion potential for different concentrations was conducted to complete the understanding of the corrosion mechanisms of carbon steel and their prohibition in 0.5 M H2SO4 medium. From Fig. 2, one may see that the recorded impedance spectra consist of a unique capacitive loop that is not a perfect semicircle, which is attributed to the dispersion of the interfacial impedance frequency owed to the heterogeneity of the electrode surface [38, 39]. This heterogeneity may be the result of roughness, dislocations, impurities, adsorption, and can also be caused by the electrode's surface [40]. In general, this type of spectrum is assigned to a charge transfer mechanism taking place on a heterogeneous and irregular substrate [41].

Fig. 2
figure 2

Nyquist diagrams of carbon steel in 0.5 M H2SO4 solution containing CO–H and CO–OMe at 293 K and the employed circuit for fitting EIS spectra

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Table 4 exhibits the data achieved following experimental results simulation with an equivalent circuit described in Fig. 2. In the 0.5 M H2SO4 alone and with 1.12 × 10–4, 2.24 × 10–4, 3.36 × 10–4, and 4.48 × 10–4 M of CO–H and CO–OMe, the circuit consisted of an electrolyte resistance (Re) in series with a polarization resistance (Rp) that is in parallel with a constant phase element (CPE), as illustrated in Fig. 2. The CPE is inserted to substitute a double layer capacitance (Cdl) to provide a more precise fit. The impedance of the CPE is established as [42]:

$$Z_{CPE} = A^{ - 1} {\text{(i}}\omega {)}^{ - n},$$
(13)

where A is the CPE modulus and n is a measure that reflects a deviation from the ideal behavior ranged from − 1 to 1. Besides, the CPE is linked with the double layer capacitance (Cdl) by the next relationship [42]:

$$C_{dl} = \left( {A_{{}}^{1/n} R_{P}^{{{{1 - n} \mathord{\left/ {\vphantom {{1 - n} n}} \right. \kern-\nulldelimiterspace} n}}} } \right),$$
(14)
Table 4 Electrochemical impedance descriptors of carbon steel in 0.5 M H2SO4 solution comprising varied concentrations of CO–H and CO–OMe at 293 K
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The chi-squared was utilized to appraise the precision of the fitting outcomes, the small chi-squared values (Table 4) acquired for all the outcomes show that the fitted results have a great concurrence with the experimental findings. The estimation of A for the reference electrolyte is larger than that of inhibited electrolyte; this proposes that the CO–H and CO–OMe molecules interacted with the electrode surface, thus reducing the destruction of exposed sites of the electrode. The highest Rp (137.3 Ω cm2 for CO–H and 176.2 Ω cm2 for CO–OMe) have been found at 0.448 mM. Indeed, increases in Rp values and the simultaneous reduction in Cdl as CO–H and CO–OMe concentration increased demonstrates the substitution of corrosive ions and water molecules from substrate surface with the inhibitor molecules, which rise the thickness of the electrical double layer, and decreases the local dielectric constant, and it is a sign that CO–H and CO–OMe acted at the steel/acid interface [43]. However, the augments of the n value after the addition of CO–H and CO–OMe in 0.5 M H2SO4 electrolyte (0.856–0.884) relative to that achieved in reference electrolyte (0.853) can be interpreted as a certain diminution of the surface heterogeneity [44]. The corresponding values of ηEIS(%) for carbon steel in presence of CO–H are above that of CO–OMe and attaints 94% and 95% at 0.448 mM, respectively. These outcomes affirm once again that the synthesized compounds display potent inhibitive activity for carbon steel in 1/2 M H2SO4. It is worth noting that EIS trials confirm the superiority attached to the protection capacity of CO–H in relation to CO–OMe, which is consistent with the data of weight reduction and PDP measurement.

3.4 Effect of temperature

Temperature can be considered as one of the factors that likely alter both the behavior of the steel in an aggressive environment and the nature of the interaction between the metal and the inhibitory molecules. In order to analyze the temperature effect on the inhibitory efficacy of the molecules under study, the polarization curves were plotted for carbon steel in 0.5 M H2SO4 with and without the addition of the CO–H and CO–OMe, in the temperature range going from 293 to 323 K (Fig. 3). The electrochemical descriptors derived from the polarization curves and the inhibiting efficiency are given in the Table 5. It can be noted that the increase in temperature causes an increment of icorr value without and with the introduction of the inhibitors, while the values of the inhibiting efficiency are almost constant in the temperature range studied (Table 5). This reflects strong adsorption of CO–H and CO–OMe (chemisorption) [45]. To understand the adsorption ability of the examined inhibitors, a detailed investigation was done using different activation and adsorption parameters.

Fig. 3
figure 3

PDP plots for carbon steel in the absence and presence of various concentrations of CO–H and CO–OMe at different temperature

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Table 5 Electrochemical parameters derived from PDP plots for carbon steel in the absence and presence of various concentrations of CO–H and CO–OMe at different temperature
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Activation thermodynamic parameters were assessed using Arrhenius and transition state equations [46, 47]:

$$i_{corr} = A \times \exp \left( {\frac{{ - E_{a} }}{R \times T}} \right),$$
(15)
$$i_{corr} = \frac{R \times T}{{N \times h}} \times \exp \left( {\frac{{ - \Delta S_{a} }}{R}} \right)\exp \left( {\frac{{ - \Delta H_{a} }}{R \times T}} \right),$$
(16)

where A represents a pre-exponential factor, ∆Ha designates the enthalpy, ∆Sa denotes the entropy of activation, T is the absolute temperature in Kelvin, h represents the Plank constant, N refers to the Avogadro number, and R is the molar gas constant. Figure 4 shows the Arrhenius plots of Ln icorr versus 1/T.

Fig. 4
figure 4

Arrhenius plots for carbon steel in 0.5 M H2SO4 in the absence and the presence of CO–H and CO–OMe

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It can be seen that the plot yields straight lines with almost unitary regression coefficients, which reveals that the corrosion mechanism obeys to the Arrhenius equation with a slope equal to ( − Ea/R). The apparent activation energies (Ea) were computed and gathered in Table 6. The Ea values of CO–H and CO–OMe were found lower than those of the blank test solution which reveals that the adsorption occurs upon the active centers having the greatest energies, whereas the corrosion mechanism takes place on the active centers possessing the lowest energies, and form it CO–H and CO–OMe are typically chemisorbed on the carbon steel surface [48]. From Fig. 5, △Ha and △Sa values were calculated and are listed in Table 6.

Table 6 Activation parameters values of carbon steel in blank and with 4.48 × 10−4 M of CO–H and CO–OMe
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Fig. 5
figure 5

Alternative Arrhenius plots for carbon steel in 0.5 M H2SO4 in the absence and the presence of CO–H and CO–OMe

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The positive values of △Ha confirm that the formation of activated complex is an endothermic process, while the negative values of △Sa suggest that the order takes place, through transformation from reactants to activate complex [49, 50].

3.5 Adsorption isotherm

The action of organic inhibitors occurs generally through adsorption onto the metallic surface. This adsorption can be considered as a quasi-replacement process betwixt the organic product in the aqueous phase Org(aq) and H2O molecules following the reaction below [51]:

$$Org_{\left( S \right)} + xH_{2} O_{{\left( {ads} \right)}} \to Org_{{\left( {ads} \right)}} + xH_{2} O_{\left( S \right)},$$
(17)

where x refers to the number of H2O molecules displaced by the inhibitory molecules. The investigation of the adsorption type and the determination of the thermodynamic parameters characterizing this adsorption often enable to elucidate the action mode of these inhibitory compounds. To identify the adsorption type corresponding to the inspected inhibitors, diverse forms of isotherms were tested: Langmuir, Frumkin, and Temkin. It can be noticed that the Langmuir isotherm has the correlation nearly equal to the unity (Fig. 6), which is expressed by the subsequent equation [52]:

$$\frac{C}{\theta } = \frac{1}{{K_{ads} }} + C,$$
(18)

where θ represents the surface coverage, C refers to the inhibitor’s concentration, and Kads denotes the adsorption equilibrium constant. The high values of Kads (Kads(CO–H) = 1.51 × 104 M−1and Kads(CO–OMe) = 2.12 × 104 M−1) are an indication of a stronger adsorption of CO–H and CO–OMe on the carbon steel surface [53, 54].In a way to obtain a deeper insight of the CO adsorption mechanism, the standard adsorption free energy (\(\Delta G_{ads}^{^\circ }\)) associated with Kads were estimated by this equation [55]:

$$K_{ads} = \frac{1}{55.5} \times \exp \left( {\frac{{ - \Delta G_{ads}^{^\circ } }}{R \times T}} \right),$$
(19)

where 55.5 represents the water concentration in mol/L, R denotes the universal gas constant, and T refers to the absolute temperature. The negative value of \(\Delta G_{ads}^{^\circ }\) indicates the stability of the double layer adsorbed on the metal surface [56]. According to literature, the values of \(\Delta G_{ads}^{^\circ }\) near to − 20 kJ/mol or more positive, corresponds to charged molecules/charged metal interaction (physical adsorption), whereas values close to − 40 kJ/mol or more negative implies a charge transfer between the organic molecules and the metallic substrate (chemisorption) [57, 58]. The values of \(\Delta G_{ads}^{^\circ }\) show that the studied inhibitors (\(\Delta G_{ads}^{^\circ }\)(CO–H) =  − 33.2 kJ/mol and \(\Delta G_{ads}^{^\circ }\)(CO–OMe) =  − 34.3 kJ/mol), is chemisorbed on the carbon steel surface and also uses the electrostatic interactions betwixt the charged molecules/charged metal interface, which points out to a mixed adsorption process [59].

Fig. 6
figure 6

Langmuir adsorption isotherm of CO–H and CO–OMe on carbon steel in 0.5 M H2SO4

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3.6 Surface analysis

Scanning electron microscopy (SEM) was used to assess the morphology of the steel surface and confirm whether inhibition is owed to the development of an organic film on the surface. On one hand, the image of the electrode surface after an immersion time of 6 h at 293 K in 0.5 M H2SO4 (Fig. 7a), show that the dissolution of the metal makes the substrate very rough. While in the presence 4.48 × 10−4 M of CO–H or CO–OMe (Fig. 7a, c) the surface is covered with a platelet-shaped product reflecting the presence of an organic compound. From these observations, it seems that that the inhibition is owed to the development of an adherent and stable deposit that hinders the electrolyte's arrival to the steel surface [60, 61].

Fig. 7
figure 7

Scanning electron micrographs of carbon steel samples at 293 K: a after 6 h immersion in 0.5 M H2SO4, b after 6 h immersion with 4.48 × 10−4 M of CO–H in 0.5 M H2SO4, c after 6 h immersion with 4.48 × 10−4 M of CO–OMe in 0.5 M H2SO4

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3.7 UV–visible spectroscopic studies

UV–visible spectroscopic measurements were conducted for 0.5 M H2SO4 solution in absence and presence of 4.48 × 10−4 M of either CO–H or CO–OMe before and after an immersion time of 48 h immersion of carbon steel at 293 K. The electronic absorption spectra (Fig. 8) of CO–H and CO–OMe before the steel immersion display a main visible band at 200, 238, 290, 315 nm, and 311,214 nm, respectively. The observed band can be attributed to π–π× transition related to aromatic ring and n–π× electronic transition from oxime groups. After 48 h of steel immersion, it can be seen that the UV–visible spectra of CO–H seem to change. In fact, there is a slight shift from 315 to 318 nm and disappear two bands at 238,290 nm with an appearance at 257 nm. For CO–OMe UV–visible spectra show a shift from 311 to 303 nm and an appearance band at 257 nm. These experimental findings may be assigned to the creation of a complex among Fe2+ and CO–H and/or CO–OMe in 0.5 M H2SO4 [62, 63].

Fig. 8
figure 8

UV–visible spectra of the carbon steel in the presence of CO before and after 48 h immersion in 0.5 M H2SO4

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3.8 MD simulation

In this last section, we have exploited molecular dynamics simulation as a very useful and efficient approach to clarify the reactivity or the action mode of the studied molecules in relation to the steel surface used in a sulfuric acid (H2SO4) environment [64,65,66]. Figures 9 and 10 give the preferred adsorption configuration of adsorbents in both neutral and protonated forms and their radial distribution functions (RDF) on the first well-ordered layer of iron. The configurations presented inform us about the adoption performance of each species on the simulated iron substrate. As visualized by each configuration (Figs. 9 and 10) that the compounds CO–H, CO–HH, CO–OMe, and CO–OMeH adsorb through all their skeletons onto the substrate, showing that these species favor the reduction of steel metal degradation of iron in H2SO4.

Fig. 9
figure 9

MD snapshots and RDF of the preferred adsorption configuration of adsorbents in both CO–H and COHH on the first well-ordered layer of iron

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Fig. 10
figure 10

MD snapshots and RDF of the preferred adsorption configuration of adsorbents in both CO–OMe and CO–OMeH on the first well-ordered layer of iron

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With respect to the overviews obtained from the RDF in Figs. 9 and 10, we can see that the values of the first peaks are lower than 3.5. These properties make it possible to reinforce the adsorption of the investigated inhibitor molecules on the surface of the metal; consequently, there is a reduction in the active sites on this area [67]. Additionally, protonation of CO–H, CO–H, and CO–OMe leads to the elongation of the studied bond lengths, which negatively influences on the rigidity of the adsorbed layer on the surface of the iron.

The interfacial interactions of species Co and CO–OMe no-charged and loaded forms with the Fe-surface were assessed using the interaction energy (E interaction) [68]:

$$E_{{\text{interaction}}}^{{}} = E_{total} - (E_{surface + solution} + E_{inhibitor} ),$$
(18)

Based on the simulation data to calculate the interaction energy (E interaction), the values of this descriptor are collected in Table 7. The negative value of E interaction confirms a large spontaneous adsorption of the selected molecules on the Fe (110) substrate [69,70,71,72]. Thus, neutral and protonated inhibitors tend to adsorb in the acid solution thermodynamically on the Fe of the steel in question. The minimum value of CO–OMe/Fe (110) shows that this molecule reacts strongly with the iron surface. This data validates the results obtained by the experiments.

Table 7 Einteraction of systems inhibitors/Fe (110)
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Based on the MD simulation data, the neutral and protonated forms of the studied molecules adsorb in parallel on the steel surface, which shows a strong adsorption of these inhibitors. The nature of this adsorption can be chemical by the coordination bonds made between the electronic doublet of oxygen (O) of the tested inhibitors and the vacant orbitals found in the iron atoms (donation effect), the reverse effect (donation) allows the chemical adsorption to be reinforced. Physical adsorption occurs between the protonated nitrogen atom of the cationic forms (COHH and CO–OMeH) and the chlorine anions (Cl−) adsorbed on the iron surface. Therefore, if we assume that the inhibitor CO–OMe does not protonate in the acidic medium, in this case chemical adsorption will be in the majority which leads to a better inhibition against corrosion.

4 Conclusion

CO–H and CO–OMe were synthesized and characterized. Various techniques such as weight loss, PDP, and EIS were operated to evaluate the inhibitory efficacy on the carbon steel in 0.5 M H2SO4. Several conclusions are found and are summarized as follows:

  • CO–OMe has slight advantage as a good inhibitory efficacy of carbon steel in 0.5 M sulfuric acid solution than CO-H and both acts as a mixed type.

  • The temperature effect indicates the CO–H and CO–OMe is chemisorbed on the carbon steel surface.

  • The adsorption of CO–H and CO–OMe on the carbon steel surface obeys Langmuir isotherms.

  • The scanning electron microscopy (SEM) show the formation of a protective film on the surface of carbon steel.

  • UV–visible spectra shown the formation of a complex between Fe2+ and CO–H and/or CO–OMe in 0.5 M H2SO4

  • Molecular dynamics simulations reveal the strong interaction between the CO–H and CO–OMe and the iron surface.