Introduction

Corrosion research has been among most trending study topics in the past few years, due to its diverse usage in constructional and industrial applications ^1,2. Recently, a study revealed that corrosion causes economic losses of about 2.5 trillion US dollars a year, constituting almost 3.4% of the worldwide GDP ³. As corrosion is associated with economic and safety issues, it should be highly addressed by the researchers throughout the world.

Metals and alloys are exposed to hostile environments during their industrial usage, including manufacturing, processing and transportation, which accelerate their degradation. In particular, steel and its alloys are the most utilized metals in renowned industries, for different applications, due to their low cost and good mechanical strength, with high thermal and mechanical conductivity ^4-6. However, their practical utility in industries is threatened during cleaning, pickling and oil well oxidization in acidic solutions that corrode them ^7,8. This natural phenomenon can be minimized to a great extent through the use of corrosion inhibitors. The development of new, in particular, eco-friendly and green corrosion inhibitors is therefore urgently required, for ensuring humankind and environmental safety.

Natural products, such as plant extracts, are highly demanded, due to ecological consciousness and eco-friendly regulations in different fields of science and technology, since they are ecologically acceptable and renewable sources ^9-11. Plant extracts corrosion inhibition activity is correlated to the presence of abundant chemical constituents, such as alkaloids, flavonoids and tannins, which have large potential to prevent corrosion ^12,13. Alireza et al. have made a comparative study of henna extract and its constituents, namely, gallic and tannic acids, a-D-Glucose and Lawsone, for MS corrosion inhibition in HCl. Henna inhibitor molecules were chemisorbed on the MS surface, but the corrosion inhibition was slightly enhanced by oxygen scavenging ¹⁴. Mohsen et al. have analysed the effect of two oleo-gum resin exudates on MS corrosion inhibition in aggressive media. They acted as mixed-type inhibitors, predominantly controlling corrosion. EIS showed that corrosion was controlled by a charge transfer process ¹⁵.

Although many plant extracts have shown potential to mitigate corrosion, no study has yet investigated the effect of harsh conditions, such as high T, on their IE(%).

SVLE, which has been widely used in numerous applications ¹⁶, due to its significant nutritional value, lesser water requirements and high adaptability to a broad range of growing conditions, is greatly available. It is expected to show high corrosion IE(%), since it contains phenolic compounds, such as flavonoids ¹⁷.

Prompted by SVLE abovementioned properties, the present study aimed to measure the feasibility of using this natural, inexpensive and green corrosion inhibitor. Electrochemical measurement results showed good agreement with the MS surface examination by AFM and SEM. Furthermore, thermodynamic parameters that govern MS corrosion have also been evaluated, and showed SVLE great potential as corrosion inhibitor.

Results and discussion

PDP study

The MS surface exhibited better corrosion inhibition with SVLE, as demonstrated by electrochemical PDP results from Tafel curves in H₂SO₄ (Fig.1 (a-d)).

Corrosion electrochemical parameters values, such as E_corr and I_corr, were calculated from Tafel curves (Table 1), following our previous article ¹⁸.

IE% values were obtained from PDP, using the following relation ¹⁹:

where I_corr and I_corr(Inh) (in uninhibited and inhibited solutions, respectively) were evaluated by the intersection of extrapolated Tafel lines. β_a and β_c varied with the addition of different SVLE C, implying controlled reactions. Additionally, the extract not only hindered cathodic HER, but also retarded MS anodic dissolution. As the extract C increased, inhibition of cathodic and anodic reactions became more visible.

Recent studies have shown that, if the inhibited solution E_corr values are ± 85 mV with respect to the corrosive medium, the inhibitor is categorized as of the anodic or cathodic types ^20-22. The observed data (Table 1) depict that E_corr values varied from 10 to 75 mV, towards less negative values. E_corr value did not change significantly by adding SVLE, which indicates that the inhibition behavior was of the mixed type, but with a slight predominance towards the cathodic direction, at lower C. A prominent decreasing trend in I_corr values was observed with SVLE in H₂SO₄. I_corr values decreased with higher SVLE C, and increased with a rise in T, which indicates that the extract is an effective inhibitor in higher C for MS corrosion in H₂SO₄, at lower T. It is noticeable that, at SVLE lowest and highest C (10 and 40%), IE(%) was 70.1 and 93.6%, respectively. This could be due to the better ( accomplished by SVLE molecules onto the MS surface ²³. Further, SVLE IE(%) showed a decreasing trend with an increase in T, but it still was 86%, at 328 K (Fig. 1). A similar decreasing trend in IE(%) was seen for the other three C.

According to Le-Chatelier’s principle, the adsorption exothermic reaction started to move in the opposite direction, leading to the extract molecules desorption from the MS surface, as the system T increased ²⁴. Therefore, ( by the extract molecules decreased with increasing T, thereby diminishing IE(%). These results suggested that SVLE adhered onto the MS surface, which hindered anodic reaction kinetics and retarded MS dissolution, even under high T conditions.

Adsorption isotherm

( is an important parameter for studying the adsorption process involved during corrosion inhibition. ( values for various SVLE C in 0.5 M H₂SO₄ were evaluated from PDP studies, in the T range from 298 to 328 K (Table 1). The best correlation between the electrochemical investigational outcomes and adsorption functions was evaluated by the Langmuir’s adsorption isotherm, according to the relation ²⁵:

The plot of C/(, as a function of SVLE C in H₂SO₄, with a raise in T from 298 to 328 K, exhibits a straight line (Fig. 2).

The linear regression parameters obtained at different T are given in Table 2.

It is apparent, from the results, that all R² values were approximately equal to 1, which signifies that SVLE molecules that were adsorbed onto the MS surface followed the Langmuir’s adsorption isotherm. Calculations for the adsorption process from the straight line obtained by plotting C/θ versus C (Fig. 2) show the correlation between K_ads and ΔGº_ads²⁶.

The determined thermodynamic parameters are shown in Table 3.

It is clearly indicated that K_ads decreased slightly as T increased, which confirms that SVLE molecules were strongly adsorbed onto the MS surface. Adsorbed SVLE molecules tended to desorb from the MS surface with elevated T, and IE(%) also decreased ^27,28.

Further, ΔH°_ads was evaluated from the plot of K_ads log variation against 1/T (Fig. 3).

ΔH°_ads negative value (-10.01 kJ/mol) indicates that SVLE molecules adsorption onto the MS surface was a spontaneous and exothermic process ^29,30, which concurs with the decrease in IE(%) at elevated T. Usually, ΔG°_ads values up to -20 kJ/mol signify that the interaction between the inhibitor and the charged metal was electrostatic ^31,32. Calculated ΔG°_ads values ranged from -17.01 to -17.27 kJ/mol, which means that SVLE physisorption occurred onto the MS surface immersed in H₂SO₄^33,34.

Additionally, ΔS°_ads positive values indicate an increase in the adsorption process entropy, which can be explained by considering a substitution mechanism. When SVLE molecules in the aqueous phase are getting adsorbed onto the MS surface, they and H₂O molecules at the electrode surface swap sites with each other. This way, the inhibitor molecules adsorption takes place when H₂O molecules simultaneously leave their position and enter in to the solution. ΔS°_ads and desorption process algebraic sum create a net positive ΔS change. ΔS value reveals that SVLE molecules adsorption was not reversible, which substantiates the inhibitor good thermal stability ³⁵.

Arrhenius equation was used to calculate MS E_a in H₂SO_4, which was given by equation ³⁶:

where A is the Arrhenius factor and R is the universal gas constant. Fig. 4 presents the Arrhenius plots of I_corr natural logarithm versus T reciprocal.

CR decreased in SVLE presence, with a marked increase in E_a values (Table 4).

E_a was found to be 22.68 and 17.61 kJ/mol, at 40 and 10% SVLE, respectively. As already reported, based on E_a value, the adsorption phenomena can be classified into physisorption (E_a < 40 kJ/mol) and chemisorption (E_a > 80 kJ/mol) ³⁷. This confirms that SVLE was physically adsorbed onto the MS surface, acting as a protective coverage against H₂SO₄.

EIS

EIS measurements were conducted for examining the corrosion process quantitative kinetics. The impedance measurements were carried out in a 0.5 M H₂SO₄ solution without and with SVLE, at 40, 30, 20 and 10%, in a three electrode cell assembly, as shown in Fig. 5(a).

EIS results are presented in the form of Nyquist and Bode plots (Fig. 5(b) and (c)). Nyquist plot possesses a depressed capacitive loop, which is attributed to a single time constant for H2SO4 and SVLE solutions, at 298 K. The diameter of these semicircle increased with higher SVLE C, indicating the effective extract inhibition of MS surface corrosion. This single capacitive loop suggests that MS corrosion was primarily controlled by a charge transfer process. However, the deviation from a perfect semicircle is attributed to porosity in mass transport effects, and frequency dispersion and relaxation ³⁸.

The Bode plot represents an increase in the absolute impedance value |Z|, at low frequencies, after MS preliminary treatment with SVLE. An increase in the extract C, from 10 to 40%, led to its effective adsorption onto the MS surface, which protected it more effectively from corrosion. Fig. 5(d) is in agreement with Bode plot, showing a continuous well-pronounced phase angle shift at higher frequencies, which indicates that the protection barrier was less stable. It also suggests that the pseudo-capacitive film has been formed onto the MS surface, inhibiting its corrosion in H₂SO₄³⁹.

Impedance parameters, such as C_dl, R_ct, frequency at maximum Z’’_max (f_max) and IE(%), are listed in Table 5. Further, IE(%) was calculated by using the following formula ⁴⁰:

where R_{ct (Extract)} and R_{ct (Acid)} are charge transfer resistances with and without SVLE, respectively.

From Table 5, it was noted that, with higher SVLE C, R_ct increased, and its highest value, obtained with 40% SVLE, was 396.65 Ω/cm², and IE(%) was 93.4%. 30% SVLE gave a R_ct value of 191.79Ω cm², with an IE(%) of 86.4%. A lower SVLE C produced decreased R_ct values of 127.74 and 88.83 Ω/cm², with an IE(%) of 79.5 and 70.6%, respectively.

On the contrary, SVLE absence gave a very lower R_ct value of 26.06 Ω/cm². It is noticeable from the results that C_dl decreased with an increase in R_ct. Due to C_dl reduction, it can be ascertained that SVLE molecules were adsorbed onto the MS surface, forming a protective layer on the metal-electrolyte surface ^41,42.

Morphological characterization

Complimentarily, AFM micrographs investigated MS topography and surface properties. In the present study, MSR was quantitatively evaluated (Table 6), in order to predict its morphological changes after SVLE addition.

The plain MS surface morphology shown in Fig. 6(a) clearly displays almost no roughness. The micrographs further revealed that the MS surface immersed in H₂SO₄ with 10 and 40% SVLE was smoother and less damaged than the one dipped into the blank aggressive medium (Fig. 6(b-d)). Fig. 6(b) and Table 6 show the corroded MS bumpy structure, with an average roughness of 964.47 nm, without SVLE. However, a smoother MS surface morphology was obtained with higher (40%) than with lower (10%) SVLE C, as can be noticed by their MSR values of 204.0 and 328.97 nm, respectively.

In addition to this, SEM micrographs (Fig. 7) investigated 10 and 40% SVLE molecules interaction with the MS surface in H₂SO₄ (Fig. 7(c-d)).

As shown in the micrographs, SVLE mitigated MS corrosion process in H₂SO₄. The SEM image of the extract lower C (10% (Fig. 7(c)) displayed MS surface slight roughness, with some cracks and pits, while, at a higher C (40 % (Fig. 7 (d))), it is as smooth as the polished one shown in Fig. 7(a).

Conclusions

SVLE is the most abundant, economical, easy processable, effective and efficient eco-friendly inhibitor for MS corrosion in aggressive media. Electrochemical measurements have showed that SVLE retarded MS corrosion in 0.5 M H₂SO₄, with high IE(%) of 93.6%, which was 86%, even at elevated T. SVLE is a mixed type inhibitor, with a slight predominance towards the cathodic direction, at lower C. SVLE molecules absorbed onto the MS surface followed Langmuir’s isotherm. ΔG°_ads values signify that SVLE mechanism of adsorption onto the MS surface in a H₂SO₄ solution involved physisorption. IE(%) data obtained from EIS and PDP measurements showed analogous results, in agreement with the diminished MSR shown by AFM and SEM micrographs.

Acknowledgements

Authors thank USIC, DU for characterization facility.

Competing financial interests

The authors declare no competing financial interests.

Authors’ contributions

S. Sharma: was in the main charge of this research; conceived the idea and supervised the whole project; performed the experiments; analyzed the experimental data; prepared the paper. M. Sharma: prepared the paper. N. Dheer: discussed the results; helped in the analysis; edited the paper. S. K. Ujjain: discussed the results; helped in the analysis; edited the paper. P. Ahuja: discussed the results; helped in the analysis; edited the paper. G. Singh: edited the paper. R. Kanojia: conceived the idea; supervised the whole project; prepared and edited the paper.

Abbreviations

Symbols definitions