Green synthesis of cadmium oxide nanoparticles (CdO-NPS) using syzygium cumini: exploring industrial applications of CdO NPs as a corrosion inhibitor of mild steel in the acidic environment

Abstract

Potentiodynamic polarization (PDP), electrochemical impedance spectroscopy (EIS) and weight loss measurements were used to assess the effectiveness of CdO-NPs as a corrosion inhibitor for carbon steel in 0.5 M H₂SO₄. It was amply shown that as the concentration of CdO-NPs increased, the cathodic currents decreased and the active corroding sites were blocked completely. Moreover, a decrease in the mass of mild steel in an aggressive environment was reduced gradually with an increase in the concentration (ppm) of CdO-NPs inhibitor, resulting in an increase in the inhibition efficiency. The novel synthesized CdO-NPs were characterized by FT-IR, XRD and SEM-EDX spectroscopy.

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1. Introduction

In recent times, acids have found different roles in various industries such as refining crude oil, pickling, cleaning, and acid scaling.^1,2 Amongst various commercial acids used, hydrochloric acid is the most frequently used acid in the oil and gas industry due to its high solubility in water. It removes scale, rust, and carbonite deposits.^3–5 The major disadvantage of hydrochloric acid is that it causes corrosion of metals and their alloys.^6,7 Corrosion has become a worldwide problem causing a huge destruction of life and property. Hence, in the last few decades, untiring efforts and attempts have been made to discover novel, economical, efficient, eco-friendly methods to eradicate and reduce corrosion. The use of corrosion inhibitors is one of the convenient methods to protect the metal. A corrosion inhibitor is a substance when added in low quantities to a corrosive environment, effectively reduces the corrosion of metal in an acidic medium.^8–10 It reduces the rate of metal dissolution by adsorbing ions or molecules to form a protective layer on the metal surface.¹¹ Different compounds such as chromates, silicates, phosphates, and arsenate-based inorganic inhibitors were tried as high-performance inhibitors due to their toxic nature, and their application has been restricted by various environmental laws.¹² Synthetic organic inhibitors were also developed and tried but their high toxicity and manufacturing cost have restricted their usage.¹³ Hence, researchers started focusing on eco-friendly, inexpensive, and biodegradable corrosion inhibitors.^14,15 This led to the concept of green inhibitors, which offer numerous benefits such as high accessibility, easy production, renewability, and more efficiency.^16,17 Corrosion inhibitive property of plant extracts is attributed to the electro-active organic functional groups such as amine, hydroxyl, and carboxyl, or compounds containing functional groups such as phenols in the phytochemical extract. These compounds contain electro-active atoms such as nitrogen, oxygen, and sulfur as well as C–C bonds with π electrons interacting with vacant d-orbitals of metals.^17,18 In this research work, the leaf extract of Syzygium cumini (Java plum) was used, which is a potential natural corrosion inhibitor. In the experimental work, Syzygium cumini is preferred because of its biodegradability and high solubility in water. It is an evergreen tropical tree in the family of Myrtaceae. It is mostly present in the region of South America and Southeast Asia.¹⁹ Most of the plant part is associated with multiple applications such as food, medicine, and furnishings.²⁰ However, without any effective use, the leaves are usually discarded as waste. Java plum leaves have a high amount of ferulic acid, rutin, catechin, and limonene.²¹ Nanoparticles are extensively used in various fields due to their exceptional characteristic features.^22,23 However, the cost factor and eco-friendly aspects are the biggest concerns in green synthesis. In recent years, seeds, leaves, barks, plant biomass and micro-organisms have become the economical and eco-friendly solutions to synthesize nanoparticles.²⁴ Plant-based nanoparticle synthesis has a number of advantages over other methods, including ease of preparation, processing, cost-effectiveness, rapid development, reproducible, stable components, environmental friendliness, and the avoidance of harsh and toxic chemicals. Furthermore, the particle size is influenced by compounds such as phenols and polyphenols in the leaf extract, which play a crucial role in the reduction of ions to form stable NPs.²⁵ There is a significant variation in the chemical compositions of plant extract of the same species when it is collected from different parts of the world and which may lead to different results. So, these results indicate that there is a need for an in-depth study of the compounds present in the plant materials that play a crucial role in the synthesis of NPs.²³

In the present work, we have synthesized cadmium oxide nanoparticles (CdO NPs) using the aqueous extract of Syzygium cumini leaves as a reducing agent at room temperature. Then, the characterization of CdO NPs was performed using several techniques such as XRD, FTIR, and SEM-EDX spectroscopy. Most industrial machine parts are made from mild steel, and many industries maintain machinery parts for cleaning processes such as descaling and acid pickling. To overcome the drawbacks, our research team decided to prefer CdO NPs to form a complex as it is less toxic compared with other metal oxides and also eco-friendly to our environment Also, an attempt has been made to find the corrosion resistance ability of CdO-NPs layer formed on mild steel substrate in the acidic medium. Corrosion techniques such as potentiodynamic polarization (PDP) and electrochemical impedance spectroscopy (EIS) were used to study the corrosion resistance ability.

2. Materials and methods

2.1 Chemicals and instrumentation

Sulphuric acid (32%), sodium hydroxide, acetone and ethanol were purchased from Merck Chemicals. Cd(NO₃)₂·6H₂O was commercially obtained from Sigma-Aldrich Chemicals, India, and diluted with distilled water to prepare 0.5 M H₂SO₄ solution, used as a corrosion inhibitor. The study investigated the structural morphology of pellets made with a CdO-Syzygium cumini complex and KBr using FTIR and XRD techniques. The XRD patterns were recorded using a Rigaku Maniflex diffractometer and JSM-6390 Scanning Electron Microscope. The EIS and potentiodynamic polarization were analyzed with an EC-LAB 10.37 model instrument with a cell of three-electrode system. The mild steel was used as the working electrode, the platinum electrode was used as the counter electrode, and SCE as the reference electrode. The open circuit potential OCP was stabilized up to 1800 s by keeping the scan rate at 0.5 mV s⁻¹, and potentiodynamic polarization plots were observed for the applied potential value between −200 mV and +200 mV. Electrochemical impedance spectroscopy (EIS) was performed in the frequency range of 100 kHz to 10 mHz AC sine wave possessing 10 mV amplitude. The weight loss method was used for studying corrosion inhibition efficiency.

2.2 Preparation of aqueous leaf extract of syzygium cumini (java plum)

Syzygiumn cumini leaves were collected from Vel Tech University, Chennai, India. The leaves were washed several times with double distilled water to remove the impurities. Note that 10 g of leaves was soaked with 100 mL of boiled DD water for 30 min. The boiled leaves solution changes from colourless to brown colour. Then, the leaf extract was filtered using Whatman grade No. 1 filter paper. The filtrate was centrifuged at 7500 rpm for 5 min. The aqueous leaf extract was used for further analysis.²⁶

2.3 Green synthesis of cadmium oxide nanoparticles (CdO-NPs)

The study involved adding 20 mL of Syzygium cumini leaf extract to 200 mL of 0.1 M cadmium nitrate solution maintained at room temperature. The reaction solution was further heated on a hot plate at 60 °C for 2 h with a continuous magnetic stirrer. Afterward, 1.0 M of NaOH was added to the solution to maintain the pH at 10.0 until a grayish-green powder was obtained. The solution was allowed to cool and centrifuged at 8000 rpm for 10 min. The precipitate formed was maintained at 200 °C in a hot air oven for 2 h. Then, the resulting powder samples were used for further characterization, revealing the formation of cadmium oxide nanoparticles (CdO-NPs).²⁷ The formation of green synthesized cadmium oxide nanoparticles is shown in Scheme 1.

Scheme 1 The mechanism for the formation of green synthesized cadimum oxide nanoparticles.

2.4 Surface pretreatment of a mild steel electrode

The Mild Steel (MS) electrode (percentage by weight) consisted of C: 0.21%, Si: 0.035%, Mn: 0.25%, P: 0.082% and Fe: 99.28% used for corrosion studies. The mild steel electrode, having 4 cm × 2 cm × 0.2 cm in sizes, abrades with disparate grade (600, 800 up to 1200) and the exposed surface area was 1 cm². Subsequently, the surface of the electrode was washed under running water and degreased in ethanol, acetone, and desiccated. With recently polished coupons, gravimetric measurements to gauge reluctance were carried out.^28–33

2.5 Preparation of hostile solutions

The aggressive solution (0.5 M) for metal dwindling was prepared by diluting AR-grade sulphuric acid, while inhibition measurements were performed by adding CdO-NPs (100 ppm to 500 ppm) to the hostile solutions.

3. Results and discussion

3.1 FTIR analysis of CdO-NPs

The synthesis of CdO-NPs was confirmed by Fourier transform infrared spectroscopy (FT-IR spectrum). Fig. 1 reveals the spectrum recorded in the range of 400–4000 cm⁻¹. The FT-IR spectrum of CdO-NPs showed a broad absorption band at 3435 cm⁻¹ for hydroxyl ions and 2040 cm⁻¹ for aliphatic –CH₂– whereas carbonyl stretching was observed at 1700–1642 cm⁻¹. The bands at 1444 cm⁻¹ and 1072 cm⁻¹ indicate the presence of N=C and C–O bands, respectively. The FT-IR band at 670 cm⁻¹ is attributed to the stretching band of cadmium oxide.

Fig. 1 FT-IR spectra of CdO-NPs.

3.2 XRD analysis of CdO-NPs

The formation of the CdO-NP composite was characterized by XRD analysis. Fig. 2 demonstrates the X-ray diffraction pattern of CdO-NPs.^33–35 The characteristic peak of CdO-NPs observed at plane values of (100), (110) and (214), which correspond to the cubic phase with a = 0.465 nm (JCPDS Card No: 75-0592).

Fig. 2 XRD patterns of CdO-NPs.

The crystallite size (D) was calculated using Debye–Scherrer's formula shown in eqn (1).

D = k/βcosθ (1)

where λ is the wavelength of X-ray (1.5406 Å), k is a constant (0.94), β is full width at half maximum (FWHM) and θ is the glancing angle. The average crystallite size (D) is 24 nm.

3.3 SEM with EDX analysis of CdO-NPs

Fig. 3A shows SEM analysis of CdO-NPs that revealed a granular structure with a diameter of 49.84 nm to 72.06 nm, with slight modifications in the synthesis and EDX analysis indicating a protective layer, in the aggressive environment.^23,27,28,33

Fig. 3 SEM Image and EDX of CdO-NPs.

3.4 Exploration of the inhibition property

3.4.1 Corrosion measurements. The study evaluates the impact of aggressive environments on mild steel surfaces by assessing weight loss in blank and test solutions. Corroded steel is cleaned acetone, dried and reweighed to determine the inhibition efficacy (IE%) and surface coverage (θ). The results are presented in Table 1.³⁶where W_i and W_o are weight loss values (g) on mild steel in 0.5 M H₂SO₄ with and without CdO-NPs, respectively (Fig. 4).³⁶

Table 1 The values of IE and θ were determined from weight loss measurements in 0.5 M blank and test solutions

Conc. of inhibitor solution (ppm)	2 hours			4 hours			6 hours			8 hours
	CR mm py	I.E (%)	(θ)	CR mm py)	I.E (%)	(θ)	CR mm py)	I.E (%)	(θ)	CR mm py	I.E (%)	(θ)
Blank	99.67	—	—	82.821	—	—	69.945	—	—	65.81	—	—
100	13.861	86	0.8609	12.956	86	0.8608	12.121	83	0.8267	12.085	81	0.8163
200	12.189	88	0.8777	11.427	86	0.8620	11.308	84	0.8383	10.971	83	0.8332
300	11.354	89	0.8870	10.866	87	0.8687	10.774	85	0.8459	10.744	84	0.8367
400	9.891	90	0.9013	9.562	88	0.8847	8.3355	88	0.8802	10.256	84	0.8441
500	8.707	91	0.9124	8.185	90	0.9011	7.2910	89	0.8957	8.6370	86	0.8687

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Fig. 4 Inhibition efficiency portrayal by bar chart for 0.5 M test solution.

The corrosion rate (CR) of mild steel was calculated using the relationship in eqn (3).

where W is the weight loss of mild steel (mg), A is the area of the mild steel (cm²), t is the exposure time (h) and D is the density of mild steel (g cm⁻³).

Inhibition efficiency (%) = CR_B − CR_I/CR_B × 100 (4)

where CR_B and CR_I are the corrosion rate of mild steel in the presence and absence of inhibitors in eqn (4).

According to the information in the above table, CdO-NPs are capable of protecting against 0.5 M acidic solution with a 91% efficacy with 500 ppm of the inhibitor solution at 2 h and 87% efficiency at 8 h. The above results suggested that cleaning agents with an acid concentration of less than or equal to 0.5 M could be added to the CdO-NPs combination.^28,37

3.4.2 Potentiodynamic polarization studies. Fig. 5 shows the Tafel plot for CdO-NPs adsorbed on mild steel in 0.5 M sulphuric acid test solution for different concentrations (100 ppm to 500 ppm). The cathodic and anodic polarization curves were deduced to determine the corrosion current density (I_corr) using corrosion potential (E_corr). The values obtained from the plot of corrosion potential and corrosion current density are provided in Table 2. From the eqn (5), the percentage efficiency (η%) was calculated. The η% was increased by increasing the concentration of the inhibitor from 100 to 500 ppm. Therefore, the results reveal 79% to 91% efficiency, which indicates that the CdO-NPs have a good absorption behavior on mild steel in a corrosive environment. The values of polarization resistance increase and corrosion current (I_corr) decrease substantially (0.1036 μA cm⁻² to 0.0097 μA cm⁻²). Therefore, it can be concluded that CdO-NPs adsorbed on the surface of the mild steel exhibit an excellent corrosion resistance performance in an extremely aggressive acidic medium. From Table 2, the documented values of the electrochemical parameters such as R_p, β_a, β_c, E_corr, I_corr, C. Rate mm year⁻¹, (η%) percentage efficiency, and surface coverage area (θ) were obtained from the corrosion process; moreover, the calculation of inhibition efficacy, η, was performed using eqn (5):

Fig. 5 Potentiodynamic polarization curve of mild steel in 0.5 M blank and test solutions.

Table 2 Corrosion resistive parameters in 0.5 M H₂SO₄ and test solution

Conc. Of inhibitor (ppm)	−Ecorr (mV vs. SCE)	RP (Ω)	βa (mV dec^−1)	βc (mV dec^−1)	Icorr (μA cm^−2)	C-rate (mm year^−1)	Inhibition efficiency (η%)	Surface coverage (θ)
Blank	0.443	3.018	0.116	0.250	0.1036	229.8	00	00
100	0.014	3.303	0.275	0.296	0.0217	160.3	79	0.7905
200	0.027	3.327	0.164	0.286	0.0131	157.2	87	0.8735
300	0.009	4.585	0.239	0.300	0.0121	141.1	88	0.8832
400	0.090	4.750	0.219	0.296	0.0114	133.5	89	0.8899
500	0.006	5.837	0.240	0.290	0.0097	114.1	91	0.9063

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In the above equation, I_corr⁰ and I_corr are corrosion current densities in the absence and presence of inhibitors.^38–41

3.4.3 Electrochemical impedance spectroscopy (EIS) measurements. According to the impedance spectra data, the impedance curve and Bode plots shown in Fig. 6A and B, respectively, were obtained when mild steel was dissolved in 0.5 M H₂SO₄ without the presence of an inhibitor and at different concentrations of the CdO-NPs. It is evident that the addition of an inhibitor to an aggressive solution causes a significant alteration in the inhibition property of mild steel. Close analysis reveals that, in the inhibitor solution, the semicircles' diameter is larger than what it is in the inhibitor-free solution. The CdO-NPs inhibitor gets adsorbed over the metal surface and inhibits the flow of corrosion current, acting as a particularly effective inhibitor in very low pH environments. This is due to the increase in the capacitive loop size with an increase in inhibitor concentration. As stated in the impedance results shown in Table 3, the value of the double-layer capacitance (C_dl) decreases, and the charge transfer resistance (R_ct) value rises as the CdO-NPs inhibitor concentration increases. The results of EIS parameters such as charge transfer resistance (R_ct) and double layer capacitance (C_dl) were determined using the following electrical equivalent circuit diagram as depicted in Fig. 7.^22,41 The percentage efficiency (η) was calculated using eqn (6).

Fig. 6 Impedance plot for mild steel in 0.5 M blank and test solution (A) and (B) Bode plots.

Table 3 Impedance parameters derived for 0.5 M blank and test solution

Conc. of inhibitor (ppm)	Rs (Ω)	Cdl (μF cm^−2)	Rct (Ω cm^2)	Inhibition efficiency (%)	Surface coverage (θ)
Blank	1.78	5.36	0.5071	00	00
100	1.41	4.12	0.99	49	0.4877
200	1.57	3.47	1.21	58	0.5809
300	1.51	3.12	2.05	75	0.7526
400	2.62	1.98	3.25	84	0.8439
500	2.41	1.87	4.95	90	0.8976

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Fig. 7 Electrical equivalent circuit diagram.

In the above equation, R_ct⁰ and R_ct are the charge transfer resistance of the blank and the inhibitor solution, respectively.

4. Conclusions

The anti-corrosive electrochemical performance of green synthesized CdO-NPs as corrosion inhibitors on mild steel in a low pH environment was evaluated. The percentage inhibition efficiency of 87% was observed even with an immersion time of up to 8 h and 91% for 2 h using the weight loss method and it increased with surface coverage area. The potentiodynamic polarization study presented a 91% efficacy 500 ppm test solution. The CdO-NPs system worked mostly as a mixed type of inhibitor. The electrochemical impedance measurements indicated that owing to the engorged thickness of the surface layer on the mild steel surface, an increase in R_ct value and a decrease in C_dl and I_corr values due to charge transfer were observed. Therefore, the result revealed that the inhibitor material forms a defensive barrier film on the mild steel surface. Moreover, there was an increasing trend of the safeguard efficiency with respect to the increase in the concentration of the inhibitor (100 ppm to 500 ppm) observed in weight loss, and PDP and EIS studies implies that the water-soluble inhibitor material can act as a good protecting agent during the industrial cleaning, pickling and descaling process.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

The authors are thankful to the Dean R and D and University Management Committee Members of Vel Tech Rangarajan Dr. Sagunthala R and D Institute of Science and Technology, Avadi, Chennai. for providing necessary instrumental facilities to carry out research work.

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