Introduction

Mild steel is employed extensively in many chemical industries due to its low cost, good tensile strength and availability for the fabrication of various reaction vessels such as cooling tower tanks, pipelines, etc. [1]. Indeed, acid solutions are also widely used in industry, for applications including acid pickling of steel and iron, ore production, chemical cleaning and processing and oil well acidification [2–4]. The corrosion of steel particularly in acidic environments and its inhibition constitute a complex progressive problem. During the past decade, corrosion inhibition of iron or steel in acid solutions by various types of organic and inorganic inhibitors has attracted much attention [5–10]. However, most of the available inhibitors are toxic and quite expensive compounds that should be replaced by new inhibitors for environmental safety. Hence, looking for safe and effective corrosion inhibitors becomes necessary in the inhibition of steel or iron corrosion in acidic medium. Generally, organic compounds containing electronegative functional groups (C=O, N–H, O–H and C=S) and carbon–carbon multiple bonds (C=C and C≡C) are good inhibitors for corrosion of steel in acid media. Heteroatoms such as sulphur, phosphorus, nitrogen and oxygen (S, P, N and O) and aromatic rings serve as major adsorption centers, which effectively retards corrosion [11].

Moreover, the orientation, shape, size, and electric charge on the molecules determine the degree of adsorption on the steel surface and the effectiveness of the inhibitor. It is well known that, plant extracts are composed of numerous naturally occurring organic compounds which are mostly nontoxic, biodegradable and plentiful. Therefore, instead of toxic inhibitors, researchers are currently focusing their interest on environmentally friendly inhibitors. In addition, various plant parts such as seeds, fruits, leaves, flowers etc. have been reported to be effective corrosion inhibitors [12], and a number of scientific studies have been carried out to determine the inhibitory action of various plant extracts for the corrosion of mild steel in acidic media. Extracts from the various plant sources have been studied and shown to serve as effective corrosion inhibitors; Justicia gendarussa [13], Lawsonia inermis [14], Ocimum viridis, Telfairia occidentalis, Azadirachta indica, Hibiscus sabdariffa and Garcinia kola [15], Phyllanthus amarus [16], Olea europaea, Ferula hermonis, Chamaemelum mixtum, Cymbopogon proximus, Nigella sativa, Phaseolus vulgaris [17], Zanthoxylum alatum [18], Piper nigrum, Mentha pulegium [19] and Lupinus albus [20].

Polycarpaea corymbosa (PC) and Desmodium triflorum (DT), belonging to the family of Caryophyllaceae and Fabaceae, respectively, are renewable resources and are easily biodegradable. The important constituents of the PC extract are the metabolites alpha-1-barrigenol, camelliagenin A, stigmasterol [21], flavonoids, steroids, terpenoids and cardiac glycosides which have been isolated from this plant [22–24] and used in the treatment for strangury, urinary calculi and ulcers. The important constituents of the DT extract are alkaloids and flavonoids compounds [25, 26] which are known to have many medicinal properties and are used for the treatment of muscle spasms, diarrhea, dysentery, wounds, and abscesses [27, 28], and as an analgesic and anti-inflammatory [29, 30].

In the present study, the corrosion inhibition properties of ethanolic extracts of Polycarpaea corymbosa (PC) and Desmodium triflorum (DT) on MS in 1.0 M HCl was investigated by weight loss, electrochemical impedance spectroscopy and potentiodynamic polarization measurements. The inhibition mechanism was supported by UV–Visible, FT–IR, WAXD, adsorption isotherms and SEM–EDS techniques. The choice of these green inhibitors was based on molecular structure considerations (i.e., these are green inhibitors having numerous naturally occurring organic compounds with delocalized ~π electrons (aromaticity), heteroatom’s (nitrogen and oxygen) with lone pairs of electrons which can serve as centers for adsorption). Additionally, in order to understand the inhibition mechanism of inhibitor molecules on MS surface in 1.0 M HCl solution, MS surface scrapes were subjected to spectroscopic investigation.

Experimental

Specimen and Inhibitor Preparation

MS specimens containing C = 0.13%, Mn = 0.39%, Cr = 0.25% and Fe = 99.23% were used in the present study. MS specimens of size 2.5 × 2.0 × 0.06 cm were used in the weight loss study. Prior to all measurements, the MS specimens were abraded with a series of emery papers (1/0, 2/0, 3/0 and 4/0 grade) and then the MS specimens were washed thoroughly with double distilled water, degreased with acetone and dried in a vacuum. MS specimens with an exposed surface area of 1 cm2 were used for both electrochemical studies and SEM–EDS analysis. Hydrochloric acid was purchased from Wako Pure Chemicals, Japan, and all other chemicals were purchased from Aldrich and used as received.

Fresh leaves (1 kg) of Polycarpaea corymbosa (PC) and Desmodium triflorum (DT) were collected at Maruthamalai from the Coimbatore district, Tamil Nadu, India, and extracted with ethanol by the cold percolation method according to a procedure in the literature [31]. In a typical procedure, the fresh leaves were dried in the shade for 10 days, ground to half dust (0.25 kg) and soaked in 500 mL of ethanol for 48 h at room temperature with occasional shaking. The extracts were filtered and the residues were soaked with the same volume of fresh solvent. A rotary evaporator at 40 °C was used to evaporate the ethanol. Water was removed using a high vacuum lyophilized. The crude brownish color solid masses (2.5 g) were collected and tested for their anticorrosion properties on MS. Various concentrations of plant extracts (inhibitors) were prepared by dissolving a known quantity of powdered plant extract in the acid medium. The important constituents of the extracts were Camelliagenin A and stigmasterol [21], the chemical structures of which are shown in Fig. 1.

Fig. 1
figure 1

Chemical structures of a stigmasterol and b Camelliagenin A

Full size image

Weight Loss Method

MS specimens were abraded by a series of emery papers, washed with distilled water and dried using acetone then used for weight loss measurement. After accurately weighing, three parallel MS specimens were completely immersed in an open beaker containing 100 mL of 1.0 M HCl with and without different concentrations of inhibitors at different temperatures (300, 310 and 320 K) for 2 h. Then, the MS specimens were removed, washed with distilled water, dried and the mass of the MS specimen was determined using an analytical balance. The experiments were done in triplicate, and the weight losses were averaged. Inhibitor concentrations starting from 100 to 400 ppm were used, and inhibition efficiency (I.E) was monitored based on different concentrations at different temperatures. The degree of surface coverage (θ) and percentage inhibition efficiency (I.E, %) were calculated by the following equations:

$$ {\text{I}}.{\text{E}}\,(\% ) = \frac{{W_{0} - W}}{{W_{0} }} \times 100 $$
(1)
$$ \theta = \frac{{{\text{I}}.{\text{E}}\,(\% )}}{100} $$
(2)

where W 0 and W are the weight loss of the MS in the absence and presence of inhibitors, respectively.

Electrochemical Studies

Electrochemical measurements were carried out in a conventional three-electrode cylindrical glass cell, using a CH electrochemical analyzer Model 604B electrochemical workstation. MS specimens with an exposed area of 1 cm2 were used as the working electrode, a saturated calomel electrode (SCE) was used as the reference electrode, and a platinum electrode was used as an auxiliary electrode. The MS surfaces were exposed to various concentrations (100–400 ppm) of PC and DT in 100 mL of 1.0 M HCl at 300 K. The potentiodynamic polarization curves were recorded by changing the electrode potential automatically from −350 to +350 mV versus the open–circuit potential (OCP) with a scan rate of 20 mV/min. The linear tafel segments of the anodic (b a ) and cathodic (b c ) curves were extrapolated to the corrosion potential to obtain the corrosion current densities (I corr). Inhibition values were calculated from the I corr values. The inhibition efficiency (I.E, %) was calculated using the equation;

$$ {\text{I}}.{\text{E}},\% = \frac{{(I_{\text{corr}}^{\prime } - I_{\text{corr}} )}}{{(I_{\text{corr}}^{\prime } )}} \times 100 $$
(3)

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

Electrochemical impedance measurements (EIS) were carried out in the range from 0.1 to 10,000 Hz at an amplitude of 10 mV. The impedance diagrams are given by Nyquist representation. Charge transfer resistance (R ct) values were obtained by deducting the high-frequency impedance from the low-frequency impedance. Inhibition efficiency (I.E, %) was calculated from charge transfer resistance (R ct) values according to the following equation;

$$ {\text{I}}.{\text{E}}(\% ) = \frac{{(R_{\text{ct}} - R_{\text{ct}}^{\prime } )}}{{(R_{\text{ct}} )}} \times 100 $$
(4)

The double layer capacitance (C dl) was calculated by the following equation:

$$ C_{\text{dl}} = \frac{1}{{(2\pi f_{\hbox{Max} } R_{\text{ct}} )}} $$
(5)

where f max is the frequency at apex on the Nyquist plot.

In the given electrical equivalent circuit (Fig. 2), R ct is the charge transfer resistance, R s is the solution resistance and C dl is the double layer capacitance. Inhibition efficiency was calculated from the charge transfer resistance (R ct) values.

Fig. 2
figure 2

The equivalent circuit model of electrochemical ac impedance measurements

Full size image

FT–IR and UV–Visible Spectral Studies

The MS specimens were immersed in a saturated solution of the inhibitors with 1 M HCl for 2 h at room temperature [32]. Then the MS specimens were air dried and the surface film was scratched carefully with a knife [33] and the resultant powder (scraped sample) was used for recording FT–IR spectra using a PerkinElmer FT–IR spectrophotometer and UV–Vis spectra using a PG instrument model T90+ spectrophotometer in DMF solvent.

Wide Angle X-ray Diffraction

In order to confirm the productive film (adsorbed layer) formation by inhibitor molecules (PC and DT) on the MS surface, the diffraction pattern of the MS surface and the MS after a 2-h immersion in 1.0 M HCl with 400 ppm concentration of inhibitors were examined using a Rotaflex RTP300 X-ray diffractometer (Rigaku Co., Japan). The X-ray diffractometer was operated at 50 kV and 200 mA. Nickel filtered Cu Kα radiation was used for the measurements with an angular range of 5° < 2 θ < 70° at room temperature.

SEM–EDS Analysis

The MS specimens were immersed in 100 mL of 1.0 M HCl with the inhibitors (PC and DT) at a concentration of 400 ppm for 2 h. After termination of the experiment, the specimens were dried and their surface morphology was examined using a Hitachi–3000H Scanning Electron Microscope (SEM–EDS).

Result and Discussion

Weight Loss Measurements

Different Inhibitor Concentrations

The values of percentage inhibition efficiency (I.E, %) and surface coverage (θ) were obtained from weight loss measurements for MS at different concentrations of inhibitors (PC and DT) in 1.0 M HCl at 300 K and are shown in Table 1. These data revealed that PC and DT inhibited the corrosion of MS in 1.0 M HCl solution, for the various concentrations used in this study (100–400 ppm). It is clear that the addition of inhibitors increased the inhibition efficiency in the 1.0 M HCl through adsorption of the basic constituents of inhibitors (PC and DT) on the MS surface and provided protection against corrosion [34]. This may be due to the presence of heteroatoms like oxygen, and nitrogen and to aromatic rings. The extent of inhibition seems to depend on the nature and mode of adsorption of the inhibitor molecules on the MS surface. In addition, protonated inhibitor species may also assist in the physical interaction of the inhibitors with Cl −ads ions which are adsorbed on the MS surface. Hence, the plant extracts showed maximum corrosion inhibitions of 84.6 % for PC and 91.9 % for DT.

Table 1 Weight loss and inhibition efficiency for various concentrations of inhibitors for the corrosion of MS in 1.0 M HCl from weight loss measurements at 300 K
Full size table

Different Acid Concentrations

The values of percentage inhibition efficiency (I.E, %) and surface coverage (θ) were obtained from weight loss measurements for MS at different concentrations of HCl in 400 ppm inhibitor (PC and DT) concentrations at 300 K. The data are summarized in Table 2. It can be seen that inhibition efficiency and surface coverage decreased with an increase in HCl concentrations (1, 1.5, 2.0 and 2.5 M). This is may be due to the increased aggressiveness of the HCl solution [35].

Table 2 Weight loss and inhibition efficiency for various concentrations of HCl for the corrosion of MS in the presence of 400 ppm of inhibitors from weight loss measurements
Full size table

Different Temperatures

The values of percentage inhibition efficiency (I.E, %) and surface coverage (θ) were obtained from weight loss measurements for MS at different concentration of inhibitors (PC and DT) in 1.0 M HCl at different temperatures (300, 310 and 320 K) and the data are shown in Table 3. Results indicated that when the temperature increased the inhibition efficiency decreased. This is may be due to the equilibrium that exists between adsorption and desorption of inhibitor molecules that continuously occurs at the metal surface at a particular temperature. With the increase in temperature, the equilibrium between adsorption and desorption is shifted towards desorption until equilibrium is reestablished. Therefore, the lower inhibition efficiencies were obtained at higher temperatures (310 and 320 K), which indicates that the mechanism of adsorption of the inhibitor is physisorption [36]. In fact, the physisorption may be due to electrostatic interactions which disappear at elevated temperatures. Furthermore, slight decreases in the inhibition efficiency at higher temperatures (310 to 330 ± 1 K) may also be attributed to the disappearance of physical forces between protonated inhibitor species and adsorbed Cl −ads species on the MS surface.

Table 3 Weight loss and inhibition efficiency for various concentrations of inhibitors for the corrosion of MS in 1.0 M HCl at different temperatures
Full size table

Electrochemical Impedance Spectroscopy

The corrosion behavior of MS in 1.0 M HCl in the absence and presence of inhibitors (PC and DT) was also investigated by the EIS method at 300 K. The charge transfer resistance (R ct) was calculated from the difference in impedance at lower and higher frequencies from Nyquist plots. The representative impedance data obtained for inhibitor PC is shown in Fig. 3 and for inhibitor DT is shown in Fig. S1 of the supporting information. Using R ct values, C dl values were calculated. The double layer capacitance (C dl) and inhibition efficiency (I.E, %) were calculated by using Eqs. (4) and (5) [37].

Fig. 3
figure 3

Nyquist plots for MS in 1.0 M HCl without and with various concentrations of PC at 300 K

Full size image

The curves in Fig. 3 showed a similar type of Nyquist plot for MS in the presence of various concentrations of inhibitors. Moreover, the Nyquist plots contain a depressed semi-circle, with the center below the real X-axis, when increasing the inhibitor concentration, which indicated that corrosion was mainly a charge transfer process [38, 39]. It is clear that the impedance response of MS was significantly changed after the addition of inhibitors. The depressed semi-circle was the characteristic of solid electrodes and often refers to frequency dispersion which arises due to the roughness and homogeneities of the surface [40]. The impedance parameters derived from these plots are given in Table 4. The R ct values of inhibited substrates are increased as the concentration of inhibitors increases. On the other hand, the values of C dl are decreased as the concentration of inhibitors increases which probably happened due to the decrease in local dielectric constant and/or increase in thickness of the electrical double layer. It is suggested that the PC and DT act via adsorption at the metal/solution interface [41–43]. It may be assumed that the decrease of C dl values is due to the gradual replacement of water molecules by adsorption of organic molecules on the MS surface, which decreases the extent of the MS dissolution [44].

Table 4 Electrochemical impedance parameter values for the corrosion of MS in 1.0 M HCl in presence and absence of inhibitor
Full size table

According to Heckerman et al. [44], the adsorption of organic molecules on metal surfaces can occur either directly on the basis of donor–acceptor interactions between the π-electrons of the heterocyclic compound and the vacant d-orbitals of metal surface atoms or interaction of organic molecules with already adsorbed chloride ions (FeCl −ads ) on the steel surface. Similarly, the inhibitors (PC and DT in 1.0 M HCl) may exist as neutral molecule and/or protonated species in the HCl medium. It can be seen from Table 4 that the value of charge transfer resistance (R ct) increases with increasing inhibitor concentration, indicating that hetero atoms of neutral inhibitors species (N and O) may be adsorbed on the MS surface by charge transfer processes and protonated inhibitor species may also be adsorbed due to the electrostatic interaction with the negatively charged MS surface (FeCl −ads ) [45]. Consequently, both physical (aromatic rings and protonated inhibitor species) and chemical (C=O and NH) inhibitor interactions may be responsible for the higher inhibition efficiency, thus effectively inhibiting MS from corrosion in the acidic medium at 300 K. In this method the plant extracts showed maximum inhibition of 90.79% for PC and 92.99% for DT in a 1.0 M HCl solution at 300 K.

Bode plots are presented in Fig. 4. A continuous increase in the phase angle shift and a new phase angle shift at higher frequency range with increasing concentration of inhibitors was observed. According to Tan et al. [46], this phase angle shift is due to the formation of the productive layer on the MS surface by inhibitor molecules which change the electrode interfacial structure. The continuous increase in the phase angle shift is obviously correlated to the progress of surface coverage by the inhibitor molecules on the MS surface.

Fig. 4
figure 4

Bode plots for MS in 1.0 M HCl in the presence and absence of different concentrations of a PC and b DT

Full size image

Potentiodynamic Polarization Technique

Figures 5 and S2 of the supplementary material show the cathodic and anodic polarization plots of MS immersed in 1.0 M HCl at 300 K in the absence and presence of different concentrations of PC and DT, respectively. Electrochemical parameters such as corrosion potential (E corr), cathodic and anodic Tafel slopes (b c and b a ) and corrosion current density (I corr) were extracted by Tafel extrapolating of the anodic and cathodic lines and are listed in Table 5. The percentage of inhibition efficiency (I.E, %) and surface coverage (θ) were calculated using Eqs. (2) and (3) [20].

Fig. 5
figure 5

Tafel plots of MS in 1.0 M HCl without and with various concentrations of PC at 300 K

Full size image
Table 5 Tafel polarization parameter values for the corrosion of MS in 1.0 M HCl in presence and absence of inhibitor
Full size table

It can be seen from Table 5 that the values of corrosion current density (I corr) were decreased by increasing the concentrations of inhibitors, which indicates the inhibiting nature of the inhibitors. As seen from Fig. 5, the anodic (b a ) and cathodic (b c ) Tafel slopes were almost unchanged, which indicated that the inhibitory role of these compounds is not through the interference on the reactions of metal dissolution and reduction of protons. These results indicated that the green inhibitors (PC and DT) act as adsorptive inhibitors (i.e., they reduced cathodic dissolution and also retarded the hydrogen evolution reaction by blocking the active reaction sites on the MS surface). Furthermore, the values of b a and b c (Table 5) showed no significant changes, which suggests that the inhibitors are mixed type inhibitors and inhibited corrosion by blocking the active sites of the MS [43].

Singh et al. [47] proposed the inhibition mechanism of interactions of protonated and neutral organic compounds with iron surface. Accordingly, the anodic dissolution mechanism of iron:

$$ {\text{Fe}} + {\text{Cl}}^{ - } \leftrightarrow \left( {{\text{FeCl}}^{ - } } \right)_{\text{ads}} $$
(6)
$$ \left( {{\text{FeCl}}^{ - } } \right)_{\text{ads}} \leftrightarrow \left( {\text{FeCl}} \right)_{\text{ads}} + {\text{e}}^{ - } $$
(7)
$$ \left( {\text{FeCl}} \right)_{\text{ads}} \to \left( {{\text{FeCl}}^{ + } } \right) + {\text{e}}^{ - } $$
(8)
$$ {\text{FeCl}}^{ + } \leftrightarrow {\text{Fe}}^{2 + } + {\text{Cl}}^{ - } $$
(9)

The cathodic hydrogen evolution mechanism is:

$$ {\text{Fe}} + {\text{H}}^{ + } \leftrightarrow \left( {{\text{FeH}}^{ + } } \right)_{\text{ads}} $$
(10)
$$ \left( {{\text{FeH}}^{ + } } \right)_{\text{ads}} + {\text{e}}^{ - } \to \left( {\text{FeH}} \right)_{\text{ads}} $$
(11)
$$ \left( {\text{FeH}} \right)_{\text{ads}} + {\text{H}}^{ + } + {\text{e}}^{ - } \to {\text{Fe}} + {\text{H}}_{2} $$
(12)

In HCl solution, physical interactions may take place between protonated inhibitor species and adsorption species such as H +ads and Cl −ads due to electrostatic interactions. This suggests that the inhibitors may inhibit both anodic dissolution and the cathodic hydrogen evolution by blocking active sites on the MS surface (mixed mode of inhibition) [43]. Chemical interactions (coordinated covalent bond formation) may take place between unprotonated O and N atoms of inhibitors and the MS surface. Additionally, protonated inhibitor species may also physically interact with the adsorbed Cl −ads species on to the MS surface. Moreover, the electrochemical measurement results were well correlated with the weight loss method.

Adsorption Isotherms

In order to study the interaction between the inhibitors and the MS surface, various adsorption isotherms were plotted using weight loss and electrochemical measurement data. In general, the most frequently used isotherms are Langmuir, Frumkin, Hill de Boer, Parsons, Temkin, etc. To test graphically by fitting to various isotherms, the degrees of surface coverage (θ) for different concentrations of inhibitors as well as at different temperatures were calculated by using weight loss and electrochemical data. Generally, adsorption of organic molecules can occur through the interactions by the replacement of the adsorbed water from the iron surface:

$$ {\text{Inhibitor}}_{{({\text{sol}})}} + {\text{nH}}_{2} {\text{O}}_{{({\text{mild}}\;{\text{steel}})}} \to {\text{nH}}_{2} {\text{O}}_{{({\text{sol}})}} + {\text{Inhibitor}}_{{({\text{mild}}\;{\text{steel}})}} $$
(13)

where ‘n’ is the number of water molecules removed from the metal surface for each molecule of inhibitor adsorbed.

Basic information on the interaction between inhibitor molecules and the surface of MS can be provided by adsorption isotherms. A correlation between surface coverage (θ) and the concentration of inhibitor (C inh) in an electrolyte was represented by the Langmuir adsorption isotherm;

$$ \frac{{C_{\text{inh}} }}{\theta } = \frac{1}{K} + C_{\text{inh}} $$
(14)

where, K is the adsorption constant.

From the intercepts of the straight lines C inh/θ-axis, K values were calculated and are given in Table 6. Figure 6 shows the Langmuir plots for both inhibitors on MS in 1.0 M HCl at different temperatures. It showed that the expected linear relationship was well approximated in the Langmuir isotherm (correlation coefficient r 2 is 0.999 for PC and 0.998 for DT), and the lines have slopes of 1.147 and 1.113 for PC and DT at 300 K, respectively. The deviation of the slope from unity was interpreted as a sign that the adsorbing species occupy more or less than a typical adsorption site at the metal/solution interface. The standard free energy of adsorption (ΔG ads) and the adsorption constant (K ads) are related by the equation:

$$ \Updelta G_{\text{ads}} = - RTIn(55.5K_{\text{ads}} ) $$
(15)

where T is temperature and the constant value of 55.5 represents the concentration of water in solution in mol/dm3.

Table 6 Thermodynamic parameters for MS in 1.0 M HCl in the absence and presence of the inhibitor at different concentration
Full size table
Fig. 6
figure 6

Langmuir plot of a PC and b DT on MS in 1.0 M HCl at different temperature

Full size image

In Table 6 the negative values of ΔG ads indicated the stability of the adsorbed layer on the MS surface and spontaneity of the adsorption process [43]. The dependence of ΔG ads on temperature could be explained by two cases as follows: (a) ΔG ads may increase (becomes less negative) with the increase of temperature which indicates the occurrence of the exothermic process, (b) ΔG ads may decrease (becomes more negative) with increasing temperature indicating the occurrence of the endothermic process. In the present study, when increasing the temperature, ΔG ads values (Table 6) increased since inhibitors reflect the exothermic nature of the adsorption process on the MS surface [43]. In addition, if the magnitude of ΔG ads is around −20 kJ/mol it was assumed that electrostatic interactions exist between inhibitor molecules and the charged metal surface (i.e., physisorption). Those ΔG ads values around −40 kJ/mol are more indicative of charge sharing or transferring from inhibitor molecules to the metal surface to form a coordinate type of metal bond (i.e., chemisorptions). In our study, ΔG ads values (Table 6) were around −30 kJ mol−1 for the inhibitors (PC and DT) which revealed that the inhibitors adsorb on the MS surface by both electrostatic (л electrons of aromatic rings) and charge transfer (C=O and –NH) processes [41]. Moreover, adsorption of inhibitors on the MS surface in 1.0 M HCl solution was supported by FT–IR, UV–vis, WAXD and SEM–EDS studies.

The other important thermodynamic functions can also be calculated from the following equation:

$$ \Updelta G_{\text{ads}} = \Updelta H_{\text{ads}} - T\Updelta S_{\text{ads}} $$
(16)

where ∆H ads and ∆S ads are the enthalpy and entropy of adsorption respectively.

The calculated values of free energy of adsorption (∆G ads) were plotted against temperature; straight lines were obtained as shown in Fig. S3 of the supporting information. ∆H ads was determined from the graph and the calculated values given in Table 6. The negative sign of ∆H ads (Table 6) reveals that the adsorption of inhibitors on the MS surface is exothermic while its absolute value (around −50 kJ mol−1) suggested that the adsorption of green inhibitors was not merely physical or chemical but a combination of physisorption and chemisorption [48, 49].

FT–IR Spectra

FT–IR spectral studies were carried out for inhibitors (PC and DT) and scraped samples (Fe–PC and Fe–DT) (Fig. 7). In Fig. 7, the inhibitors showed a C=O stretching peak at 1,665 and 1,672 cm−1 for PC and DT, respectively, and an O–H stretching peak at 3,401 and 3,998 cm−1 for PC and DT respectively. The IR spectra of scraped samples (Fe–PC and Fe–DT) indicated the presence of functional groups whose absorption frequencies correspond to C=O (shifted from 1,665 to 1,651 cm−1 for PC and from 1,672 to 1,652 cm−1 for DT, respectively) and O–H (shifted from 3,368 to 3,401 cm−1 for PC and from 3,323 to 3,398 cm−1 for DT, respectively). The shift in the absorption frequencies of the inhibitors on the MS surface strongly suggested that the inhibitors (PC and DT) make a coordination bond with the MS surface through the C=O and O–H. In addition, the curve broadening of the O–H bond at around 3,400 cm−1 and the C=O band at around 1,670 cm−1 for scraped samples suggested that the inhibitors make (PC and DT) a coordination bond with the MS surface through the heterocyclic atoms [50].

Fig. 7
figure 7

FT–IR spectra of the inhibitors and inhibitors adsorbed on MS surface a PC and b DT

Full size image

UV–Visible Spectra

UV–Visible spectral studies were carried out for inhibitors (PC and DT) and scraped samples (Fe–PC and Fe–DT) (Fig. 8). The two spectral bands at around 280 nm and 320 nm in the spectra of inhibitors (PC and DT) were assigned to π → π* and n → π* transitions due to C=O and N–H groups, respectively. According to Abboud et al. [51], the change in position of the absorbance maximum and change in the value of absorbance indicate the formation of a complex between two species in solution. In the UV–Vis spectra of scraped sample (Fe–PC and Fe–DT), it could be clearly seen that the band in the region of 320 nm has completely disappeared and the absorbance maximum also changed when compared to that of the inhibitors (PC and DT), which ascribed to the charge transfer process probably from inhibitors (C=O and N–H) to the MS surface. These experimental results give strong support to the possibility of chemisorbed film formation on the MS surface in 1.0 M HCl solution [52].

Fig. 8
figure 8

UV–Visible spectra of the inhibitors and inhibitors adsorbed on MS surface; a PC and b DT

Full size image

Wide Angle X-ray Diffraction Study

In order to confirm the productive layer formation of the inhibitors (PC and DT) on the MS surface, XRD patterns were recorded for the MS surface and MS after a 2-h immersion in 1.0 M HCl with 400 ppm concentration of inhibitors and are shown in Fig. 9. XRD pattern for polished MS specimen (pattern a), revealed the characteristic values for Fe (44.8° and 64.9°). The dramatic decrease in the intensity at 44.8° and the absence of the peak at 64.9° for MS in the presence of inhibitors (PC and DT) clearly showed the formation of a protecting layer on the MS [53]. In addition, peaks around 28.4 and 40.8 for MS in the presence of inhibitors strongly support the possibility of chemisorbed layer formation by organic molecules (C=O and N–H) on the MS surface in 1.0 M HCl solution [9].

Fig. 9
figure 9

WAXD pattern of MS surface and MS exposed to 1.0 M HCl in the presence of a PC and b DT

Full size image

SEM–EDS Analysis

The SEM images were recorded (Fig. 10) to establish the interaction of inhibitors (PC and DT) with the MS surface [54]. Figure 10a indicated the finely polished characteristic surface of MS and shows some scratches which had arisen during polishing. Figure 10b reveals that the MS specimens were highly damaged and appeared full of pits and cavities in the presence of 1.0 M HCl, which may be due to the direct attack of aggressive acids. Figure 10c and d shows the formation of a protective film by inhibitors (PC and DT) on the MS surface which significantly inhibits corrosion in 1.0 M HCl [55]. SEM images (Fig. 10c, d) also showed the scratches formed during metal polishing covered well by the inhibitors. Hence the plant inhibitors protect MS effectively in 1.0 M HCl solution at 300 K.

Fig. 10
figure 10

SEM images and their corresponding EDS spectra (inserts) of a MS, b MS in 1.0 M HCl, c PC +1.0 M HCl, and d DT +1.0 M HCl for 2 h immersion

Full size image

The EDS spectra were recorded (Fig. 10, inserts) in order to determine the percentage of chloride present on the MS surface in the presence and absence of inhibitors (PC and DT). The data in Fig. 10a and Table 7 revealed that the MS surface was chlorine free. However in the case of MS with 1.0 M HCl solution (Table 7) a very high chloride content (7.2 %) was present, which may be due to the aggressive attack of the 1.0 M HCl solution. Figure 10c and d and Table 7 referred to the EDS spectrum of the MS immersed for the same period of time in 1.0 M HCl solution containing 400 ppm of inhibitors; this spectrum revealed that inhibitors protected the MS surface from 1.0 M HCl, which indicates very low chloride content compared to the MS in 1.0 M HCl [55].

Table 7 EDS analysis result of MS, MS in 1.0 M HCl in the absence and presence of inhibitors
Full size table

Conclusions

In conclusion, the plant extracts (PC and DT) acted as effective corrosion inhibitors for MS in 1.0 M HCl. The inhibition efficiency of the inhibitors studied was increased when the concentration of inhibitors was increased. There was good agreement between the results obtained from weight loss and electrochemical measurements. At the concentration of 400 ppm, the plant extracts showed a maximum inhibition of 90.79 % for PC and 92.99 % for DT in 1.0 M HCl at 300 K. Adsorption isotherms and Langmuir provided the evidence for both physisorption and chemisorption mechanism. Thermodynamic adsorption parameters (ΔH ads and ΔG ads) revealed that the adsorption process of inhibitors was spontaneous and exothermic. FT–IR and UV–Visible spectra provided evidence for the chemisorption mechanism and different temperatures were evidence for a physisorption mechanism. WAXD was evidence for adsorbed layer formation on the MS surface by the inhibitors (PC and DT). SEM–EDS images provided information of protective film formation on the MS surface. As a final point, the plant extracts of Polycarpaea corymbosa (PC) and Desmodium triflorum (DT) are cheap, easily available and environmentally friendly green inhibitors for 1.0 M HCl corrosion of MS.