1 Introduction

Corrosion is the degradation of metals and alloys by interaction with their surroundings. Corrosion failures need for expensive replacements may occur, even though there was small destruction by corrosion. In industries, acid pickling, acid cleaning, acid de-scaling and oil well acidizing are very common and hydrochloric acid is used to employ it [1,2,3]. In this processes, steel undergoes corrosion by interacting with an acidic solution [4, 5]. To control the corrosion, among the various methods, the use of inhibitor is efficient and flexible. A corrosion inhibitor is an organic or inorganic compound, and it controls the corrosion by adsorbing on the metal surface [6,7,8,9,10,11,12].

Nowadays the use of polymers as corrosion inhibitors concerned considerable attention due to low cost, stable and non-toxic nature. Polymers find applications as efficient corrosion inhibitors for steel [13, 14]. The high surface energy of polymers has a more tendency to make complex with metals [15]. Only few polymers exhibit interaction with metal; this is attributed to high charge density on their polymer ion leading to complex with metal ion [16]. Polyaniline has been used as an effective corrosion inhibitor for mild steel in HCl [17].

The influence of poly (styrenesulfonic acid)-doped polyaniline polymer on the inhibition of corrosion of mild steel in HCl was investigated [18]. Badran et al. [19] reveals the effect of different polymers on the efficiency of waterborne methylamine adduct as corrosion inhibitor for surface coatings. Varsha et al. [20] developed the poly (aniline-co-o-toluidine) as inhibitor. Several reports have been shown that the polymer is a possible protective material, especially for mild steel [21,22,23].

Polymers are used as corrosion inhibitors because of their functional groups. They form complexes with metal ions, and these complexes occupy a large surface area, thereby blanketing the surface and protecting the metal from corrosive agents present in the solution. The inhibitive power of these polymers is related structurally to the cyclic rings, heteroatoms (oxygen and nitrogen), that are the major active centers of adsorption.

The present work is aimed at investigating the corrosion inhibition characteristics of sulphonated polysulfone polymer (SPS) on corrosion of mild steel in 1 M HCl solution. SPS is a big molecule and nowhere has tried as corrosion inhibitor for any metals. This polymer contains sulfur and oxygen atoms, and these are mainly donating the electron to make a chemical bond with metals. These are mainly responsible to choose this polymer as corrosion inhibitor for these studies. Weight loss, potentiodynamic polarization and EIS techniques were performed to evaluate the results. Adsorption isotherm and thermodynamic parameters were studied to establish the mechanism of corrosion inhibition. Quantum study was also conducted to verify the experimental results with theoretical results.

2 Experiments

2.1 Materials

Mild steel specimens of dimension 4 cm × 2.5 cm × 0.05 cm were used for weight loss method. Specimens with an exposed area of 1 cm2 (rest was covered with araldite resin) with 3.0 cm long stem were used for electrochemical methods. The specimens were abraded by using different grade emery papers, washed thoroughly with distilled water, degreased with acetone and dried at room temperature. The corrosive medium was HCl and was prepared by double distilled water.

2.2 Test Solution

SPS was synthesised and characterized by adopting the standard and reported procedure [24]. The structure of the polymer is shown in Fig. 1.

Fig. 1
figure 1

The molecular structure of sulfonated polysulfone polymer (SPS)

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The stock solution was prepared in DMSO (dimethyl sulfoxide) and then diluted to required concentration in the HCl medium (0–5000 ppm). However, the same concentration of DMSO was added to the blank solution to cancel its effect.

2.3 Methods

2.3.1 Weight Loss Measurements

Weight loss measurements were carried out by immersing the mild steel specimen in a 100 cm3 of corrosive media (1 M HCl) in the presence of different concentrations of the inhibitor. Specimens were weighed before immersing into the corrosive medium, and after 4 h of immersion, the specimen was washed with water, dried and weighed accurately (ASTM standards). The difference between the weight before and after the experiment was recorded. All these experiments were conducted twice, and average values were reported.

2.3.2 Electrochemical Measurements

The electrochemical measurements were performed twice by CHI 608D electrochemical workstation (CH Instruments, Austin, USA) at different temperatures. Mild steel, platinum and silver–silver chloride electrodes were used as working electrode, a counter electrode and reference electrode, respectively. One hour time was given to the electrodes to achieve the equilibrium potential. Polarization experiments were conducted from −0.2 V anodically to +0.2 V versus the OCP at a scan rate of 0.001 V s−1. Electrochemical experiments were conducted from 30 to 60 °C. These experiments were conducted at different temperatures, to know the performance of the inhibitor at different temperatures. Above 60 °C, mild steel dissolution is very high so it is conducted up to 60 °C.

The impedance measurements were conducted by using AC signal with 5 mV amplitude at OCP in the frequency range of 100 kHz–10 MHz. Polarization results and impedance results were consistent during the measurements.

2.3.3 Surface Morphology Studies

The surface morphology of the mild steel specimens in the presence (3000 ppm) and absence of SPS at 30 °C was investigated using scanning electron microscopy (JEOL JSM-6380LA). 3000 ppm shows maximum inhibition efficiency, and it covers almost all parts of the metal so it is choosing for studies.

2.3.4 Quantum Chemical Studies

Quantum chemical calculations for SPS inhibitor were carried out in the gas phase by using parametric method 3 (pm3). These calculations were performed by using Hyperchem 7.5 package program (Hypercube Inc., Florida, 2003).

3 Results and Discussion

3.1 Weight Loss Studies

The corrosion inhibition efficiency (% η w) values are reported in Table 1.

Table 1 Weight loss measurements results
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The  % η w was calculated from the following relationship

$$\% \eta_{\text{w}} = \frac{{W^{o} - W}}{{W^{o} }} \times 100$$
(1)

where W° and W are a weight loss of mild steel in the absence and presence of inhibitor.

The result indicates that  % η w was increased with inhibitor concentration up to 3000 ppm, and above this concentration, no changes in efficiency were observed.

3.2 Potentiodynamic Polarization Studies

The polarization behavior of mild steel in 1 M HCl at different temperatures in the presence of different concentrations of SPS is shown in Fig. 2.

Fig. 2
figure 2

Tafel plots for mild steel at a 30 °C, b 40 °C, c 50 °C, d 60 °C

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Table 2 gives the electrochemical parameters obtained by Tafel method.

Table 2 Polarization results
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The  % η p was calculated from the following relation

$${\text{\% }}\eta_{\text{P}} = \frac{{i_{\text{corr}}^{ \circ } - i_{\text{corr}}}}{{i_{\text{corr}}^{ \circ } }} \times 1 0 0$$
(2)

where i corr and i corr° are corrosion current densities in the presence and absence of inhibitor, respectively.

Corrosion current values (i corr) were decreased with inhibitor concentration, and it decreases up to 3000 ppm. Corrosion current values were decreased from 302 to 138.9 µA cm−2 at 30 °C. In the same way, corrosion current decreased from 1007 to 159 µA cm−2 at 60 °C. In this study, 3000 ppm of the SPS was considered as an optimum concentration for achieving maximum inhibition. The variation of E corr values shows a steady trend with different temperatures and concentrations of the inhibitor. It is known that if the displacement of E corr is smaller than 85 mV, then it is considered mixed-type inhibitor [25]. In our studies, changes in E corr are around 15–35 mV, and in average displacement it is around 20 mV.

3.3 Electrochemical Impedance Spectroscopy (EIS) Studies

Figure 3 gives the EIS data of mild steel in 1 M HCl with different concentrations of SPS at a different temperature.

Fig. 3
figure 3

Nyquist plots for mild steel at a 30 °C, b 40 °C, c 50 °C, d 60 °C

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Table 3 gives data obtained by the impedance method. R p values were increased with inhibitor concentration, and it is also supported by the diameter of the semicircle in Fig. 3 [26, 27]. Equivalent circuit model was used to analyze the EIS data (Fig. 4). This circuit was established by using Zsimp software. R p is polarization resistance, and R s is solution resistance. C dl is double-layer capacitance. Inhibition efficiency (%η z) was calculated using following equation.

Table 3 EIS results
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Fig. 4
figure 4

Equivalent circuit for EIS

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$$\% \eta_{\text{Z}} = \frac{{R_{\text{p}} - R_{\text{p}}^{ \circ } }}{{R_{\text{p}} }} \times 100$$
(3)

where R p and \(R_{\text{p}}^{\text{o}}\) are polarization resistance values in the presence and absence of inhibitor.

R p values increased and capacitance values decreased with inhibitor concentration. Decreased capacitance values confirm the adsorption of the inhibitor on metal surface. Which is due to reduce in the local dielectric constant with increase in the thickness of the electrical double layer. The effect of concentration of inhibitor and temperature on the inhibition efficiency was followed the same trend like polarization method. The obtained η z are in good agreement with η p and η w.

3.4 Effect of Inhibitor Concentration

Inhibition efficiency at various temperatures is given in Tables 2 and 3. In the studied temperature, range inhibition efficiency of the inhibitor molecules increases up to 3000 ppm, and above this concentration, the inhibition efficiency is almost same or less. It suggests that 3000 ppm of the inhibitor concentration is the optimum level for achieving good inhibition. SPS covers entire metal surface at 3000 ppm and gives maximum efficiency. Above 3000 ppm concentration, inhibitor may come out from the metal surface and exposes of the metal surface to the corrosive environment. So, corrosion rate increases above 3000 ppm inhibitor concentration.

3.5 Effect of Temperature

Tables 2 and 3 give the variation of inhibition efficiency with temperature in the range 30–60 °C. Inhibition efficiencies of SPS were increased with the temperature up to 60 °C. Physisorption and chemisorption are the possible methods of adsorption of the inhibitor on the metal surface. In this case, the inhibitor is adsorbed on the metal surface by chemisorption alone or a combination of both physisorption and chemisorption method. SPS may form a strong bond with the metal surface. This phenomenon holds good up to 60 °C, and above this temperature, adsorbed molecules may be desorbed and the metal surface is exposed to the acid environment, so corrosion is increased. SPS gains threshold energy efficiently for making a strong chemical bond to a metal surface with a rise in temperature up to 60 °C. Above 60 °C, the time gap between adsorption and desorption of inhibitor molecules on the metal surface is too small. Hence, the metal surface remains exposed to the acid environment for a longer period. Therefore, inhibition efficiency falls at elevated temperature.

Polymers are bigger molecules than organic molecules, and they cover most of the metal compared to other organic inhibitors. In this case, SPS is adhered to the metal firmly by both chemisorption and physisorption method. At 50 and 60 °C, chemisorption is dominated and shows higher inhibition efficiency. SPS blocks active sites of the metal by forming a passive layer, and it acts as a barrier between metal and acid. It indicates that SPS is an excellent corrosion inhibitor at a higher temperature.

3.6 Quantum Chemical Studies

Quantum chemical method is a theoretical method to elucidate the structure and mechanism of corrosion inhibition of the inhibitor. This method is extra evidence to the experimental results. The optimized molecular structure of monomer unit of sulfonated polysulfone polymer (SPS) is given in Fig. 5. The evaluated quantum chemical data are provided in Table 4. Adsorption of the inhibitor on metal surface takes place by donating the electrons by the inhibitor molecule to the metal surface. Higher the E HOMO, and lower the E LUMO values related to the higher tendency of donation and acceptance of electrons from the inhibitor to the metal surface. The compound with lower ΔE values gives higher inhibition efficiencies because the excitation energy gap is more polarizable and is associated with chemical reactivity. SPS ΔE value is 5.02 eV, and it justifies the results. Corrosion inhibition efficiency increases with decrease in ΔE values [28, 29]. In this present study, the calculated ΔE value is 5.02 eV and this value is small and indicates as a good corrosion inhibitor. Dipole moment (μ) plays a significant role in deciding the inhibition efficiency of the adsorbed inhibitor molecule on mild steel surfaces. The evaluated dipole moment (μ) of SPS is 4.606 debye. It shows that SPS is an excellent corrosion inhibitor for mild steel in 1 M HCl medium. Quantum studies give the further evidence and confirm the experimental results.

Fig. 5
figure 5

Optimized monomer molecular structure of SPS

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Table 4 Quantum chemical parameters
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3.7 Surface Morphology Study

SEM images were taken for the mild steel specimen in 1 M HCl with and without SPS inhibitor.

Figure 6 shows some flake, and almost all part of the metal is corroded. These heterogeneous surfaces undergo corrosion easily. In the presence of the inhibitor, metal is completely covered by the inhibitor (SPS), and it strongly adheres to the metal surface. It gives uniform surface and covered the metal surface by avoiding the interaction between metal and corrosive environment so it gives more inhibition efficiency. Polymers are larger in size, so they cover the metal surface thoroughly and uniformly [30].

Fig. 6
figure 6

SEM images of the surfaces of mild steel after immersion in 1 M HCl solution at 30 °C a blank and b with SPS

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FTIR spectra of the pure SPS and scraped compound from the metal surface were recorded and are given in Fig. 7.

Fig. 7
figure 7

FTIR spectra of pure SPS and scrapped SPS from the metal

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In the scraped compound, stretched frequency of the compound is completely merged. It indicates the strong interaction between metal and SPS so it gives more inhibition efficiency in the presence of SPS.

3.8 Adsorption Isotherm and Thermodynamic Parameters

Adsorption isotherm provides the basic information on the interaction between the inhibitor and metal surface. Polarization measurements values were used to evaluate the degrees of surface coverage (θ) in the presence of different concentrations of inhibitor. The data were applied to different isotherms including Langmuir, Temkin, Frumkin and Flory–Huggins isotherms. The Langmuir adsorption isotherm gave the best fit. The equation could represent the Langmuir adsorption isotherm:

$$\frac{C}{\theta } = C \, + \, \frac{1}{K}$$
(4)

where K is the adsorption/desorption equilibrium constant, C is the corrosion inhibitor concentration in the solution and θ is the surface coverage, which is calculated using equation

$$\theta = \frac{{\% \eta_{p} }}{100}$$
(5)

where  % η p is the inhibition efficiency as calculated using Eq. 2.

The plot of C/θ versus C gives a straight line with intercept 1/K of Langmuir adsorption isotherm is shown in Fig. 8.

Fig. 8
figure 8

Langmuir adsorption isotherm

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The thermodynamic parameter, standard free energy of adsorption (ΔG °ads ), is calculated from the thermodynamic equation

$$K = \frac{1}{55.5}\exp \left( {\frac{{ - \Delta G_{\text{ads}}^{ \circ } }}{RT}} \right)$$
(6)

where K is the equilibrium constant for the adsorption/desorption process, 55.5 mol dm−3 is the molar concentration of water in the solution, T is temperature and R is the gas constant.

Standard enthalpy of adsorption (ΔH °ads ) and standard entropies of adsorption (ΔS °ads ) were obtained from the plot of (ΔG °ads ) versus T according to the basic thermodynamic equation

$$\Delta G_{\text{ads}}^{ \circ } = \, \Delta H_{\text{ads}}^{ \circ } - \, T \, \Delta S_{\text{ads}}^{ \circ }$$
(7)

The thermodynamic data obtained from adsorption isotherm are tabulated in Table 5

Table 5 Thermodynamic parameters
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The correlation coefficient (R 2) was used to choose the isotherm. The linear regression coefficients are also close to unity, and the slopes of straight lines are nearly unity, suggesting that the adsorption of SPS obeys Langmuir’s adsorption isotherm. SPS has strong adsorption on the metal surface and is proved by high K values. Negative ΔG °ads indicates that adsorption of the inhibitor on the metal surface is spontaneous and stable. The ΔG °ads values for SPS on the mild steel surface are −30.02 to −37.89 kJ mol−1, indicating the chemisorption with slight effect of physisorption. SPS polymer gained threshold energy at a higher temperature and adsorbed strongly on the metal surface by making coordinate covalent bond. Therefore, inhibition efficiency increases up to 60 °C, but, above this desorption, will play a dominant role, and adsorbed molecules may be desorbed from the metal surface. Therefore, inhibition efficiency is decreased.

A plot of ΔG ads° V/s T was used to calculate the heat of adsorption ΔH ads° and the standard adsorption entropy ΔS ads°. According to Eq. 7 shows the good dependence of ΔG ads° on temperature. The thermodynamic data are given in Table 5. Thermodynamic parameters can obtain mechanism of corrosion inhibition. The enthalpy of adsorption could be approximately regarded as the standard enthalpy of adsorption (ΔH ads°) under experimental conditions. The positive sign of ΔH ads° indicates that the adsorption of inhibitor molecule is an endothermic process. In general, an exothermic adsorption process signifies either physisorption or chemisorption, while the endothermic process is attributable unequivocally to chemisorption [31]. The ΔH ads° values obtained for the studied inhibitor on the mild steel surface in 1 M hydrochloric acid ranges is around 56.57 kJ mol−1, indicating chemical adsorption. The negative value of entropies (ΔS ads°) is −0.28 Jmol−1K−1 which implies that an ordering takes place when the inhibitor gets adsorbed on the metal surface.

4 Conclusions

SPS (sulfonated polysulfone) polymer acts as a good corrosion inhibitor for mild steel in acid medium. Corrosion inhibition efficiency of SPS increases with temperature and concentration. The adsorption mechanism of SPS on the mild steel surface obeys Langmuir’s adsorption isotherm. The strong interaction between SPS and the mild steel surface is proved by negative values of Gibbs free energy of adsorption (ΔG ads). Electrochemical results show that SPS is a mixed-type inhibitor. A quantum study proves the experimental results.