Novel high-performance and cost-effective green inhibitor based on fundamental theoretical evaluations combined with electrochemical/surface examinations of Arachis hypogaea shell extract for pipeline steel corrosion in 1 M H₂SO₄ solution

For corrosion studies, the steel specimen utilized was API X 65 pipeline steel with the following compositions (%): Si 0.186, C 0.09, Ni 0.016, Nb 0.023, Cu 0.01, Al 0.016, Mn 0.867, P 0.006, S 0.003 and Fe Balance. A 1 mol L⁻¹ H₂SO₄ solutions was prepared by diluting analytical grade H₂SO₄ and used as the electrolytic solution. With different grades of emery papers, the scale and oxide was cautiously rubbed from the steel surface, dipped in acetone to remove the organic contamination, and dried, then stored in moisture-free desiccators. The Arachis hypogaea shell was collected from Gauteng, South Africa. The extract was dried and weighed, and the shell was powdered for the extraction process. Afterward, 1 L distilled water was refluxed to 40 g of Arachis hypogaea shell dry powder, the resulting solution was continuously stirred at 70 ^◦C for 3 h. The solution was then passed from the filter to separate the undesirable parts. The clear solution was centrifuged at 4500 rpm power for 20 min. Then, the solution was heated at 70 ^◦C for 24 h, and the un-dissolved specie was separated by a filter. Numerous solvents can be used in extracting green inhibitors. Nevertheless, water solvent, which is eco-friendly and cheaper than other solvents ( i.e., cyclohexane, methanol, and ethanol), was used in this study. Also, only the component totally soluble in 1 mol L⁻¹ H₂SO₄ solution is essential. A specific quantity of Arachis hypogaea shell extracts was dissolved in test solutions for gravimetric, morphological, and electrochemical studies. The chemical structures of Arachis hypogaea is displayed in figure 1.

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2.2.1. Potentiodynamic polarization measurements

2.2.2. Gravimetric measurements

Weight loss was measured for 456 h exposure time in 1 mol L⁻¹ H₂SO₄ solutions protected by AHS (0, 1, 2, 3, 4, and 5 g L⁻¹). At 24 h intervals, the specimen was removed, washed with distilled water to remove corrosion product, dried, and weighed carefully. Equation (1) was used to compute the average weight loss of the un-inhibited and inhibited samples [30]:

$$\begin{eqnarray}&&{\rm{\Delta }}W={W}_{0}-W\end{eqnarray} \tag{ 1 }$$

Where W₀ and W are the steel weight before and after immersion

The corrosion rate (CR) was estimated using equation (2):

$$\begin{eqnarray}&&CR=\displaystyle \frac{87.6W}{DAT}\end{eqnarray} \tag{ 2 }$$

Also, the inhibition efficiency was estimated using equation (3):

$$\begin{eqnarray}&&IE( \% )=\displaystyle \frac{C{R}_{0}-CR}{C{R}_{0}}\times 100\end{eqnarray} \tag{ 3 }$$

Where, CR₀ and CR is the corrosion rate of uninhibited and inhibited sample.

2.2.3. Surface characterizations

The effects of AHS on the cathodic and anodic reactions was established by potentiodynamic polarization study in 1 mol L⁻¹ H₂SO₄ solution and the obtained results are shown in figure 2. By extrapolating the cathodic and anodic part of polarization curve, the corrosion current densities, icorr, anodic Tafel slopes, βa, corrosion potentiasl, Ecorr, and cathodic Tafel slopes, βc, were achieved and listed in table 1. It is evident from the data given in figure 2 that by using AHS, the corrosion current density and the corrosion rate were reduced significantly compared to the uninhibited sample. From the polarization curves, the major shift of cathodic and anodic branches to lower current densities values is obvious signifying that the AHS reduced the cathodic and anodic corrosion reactions via mixed inhibition mechanisms. The displacement in the Ecorr towards more negative value is lower than ±85 mV as shown in table 1, signifying that the AHS behaves as mixed inhibitor [31]. The cathodic branch of the inhibited and uninhibited sample is approximately parallel, denoting the adsorption of the inhibitor on the surface of the metal on the hydrogen evolution reactions on cathodic site [32]. Conversely, the iron dissolution mechanisms was influenced with the inhibitor via observation of noticeable change in anodic branch slope and shape [33]. The impacts of AHS on the corrosion of X 65 steel can be observed from the obvious decline in icorr with 4 g/l AHS. The polarization plots results show the AHS's high efficiency block the X 65 steel reaction site. As shown in table 1, corrosion rate and icorr declines denote the effect of the AHS layer against steel corrosion with increasing AHS concentrations [34]. Conversely, the change in βa value is principally because of the SO₄ ^2—ions adsorbed from corrosive solution to the metal substrate.

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The plot of the weight loss, corrosion rate, degree of surface coverage and inhibition efficiency were summarized in figure 3. Evidence from figure 3 proved that after adding the AHS to the aggressive solutions, the steel corrosion significantly reduced. By increasing the AHS concentrations, the corrosion rate was suppressed, and the inhibition performance was enhanced. This increase could be ascribed to the AHS molecules adsorbed on the surface active site [35]. The corrosion rates of the uninhibited samples are much higher than the inhibited sample, as shown in figure 3.

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Also, by increasing the exposure time, the value increased remarkably. These increases ensure that the uninhibited samples were corroded severely. Though, the corrosion rate diminished remarkably by increasing the AHS concentration in the aggressive solution. This reduction is directly ascribed to the AHS molecule's adsorbed on the steel active site. Also, increasing the AHS concentrations can enhance the surface coverage and improve the performance of higher inhibition [36]. According to figure 3, after adding 4 g/L AHS in the acidic solutions, the rate of corrosion value reduced from 69.88 to 7.59 mm/year. The inhibition performance increased to 98.42% for the specimen in H₂SO₄ solutions with 4 g/L AHS.

The corrosion rate value was also determined at various AHS concentrations (1, 2, 3, 4, and 5 g L⁻¹). Figure 3 reflects that the corrosion rate was remarkably enhanced by increasing the AHS concentrations. The computed corrosion rate value for the uninhibited sample is higher than the inhibited samples (particularly at 4 g L⁻¹). As discussed previously, the uninhibited sample surface was covered by corrosion product (iron oxides), which can rarely protect the steel from corroding. Nevertheless, with immersion time, iron oxides can easily detach from metal substances.

Therefore, the steel substrates can be highly corroded by aggressive ion attack, resulting in notably increase in the corrosion rate value. Conversely, the corrosion rate did not change considerably at a concentration above 4 g L⁻¹. This observation denotes that the AHS molecule can protects the steel surface from corrosion even at low concentrations [37]. This finding supports that the AHS act via geometric blockage of the metal surface through adsorption mechanisms.

Inhibitor molecule adsorption can be influenced by the inhibitor molecule size, active site number, charge densities, metal surface interaction, and the film stability formed on the metal surface. The molecule adsorbed can bond with the surface of the metal via different mechanism. Accordingly, the organic molecules of Arachis hypogaea can contribute to interaction with metal surfaces via the electron sharing between the electron pair heteroatoms, the vacant iron atoms orbital, and π-electrons. Additionally, inhibitors molecule can interacts with the surface of the metal through van der Waals force and hydrogen bonding mechanism. The isotherms in the Arachis hypogaea shell extracts adsorption on the steel surface was also investigated. Various models, including Frumkin Langmuir, Temkin, and Freundlich was utilized in describing the interactions between AHS molecule and steel. The equation related to this isotherm is as follows:

$$\begin{eqnarray}&&\displaystyle \frac{C}{\theta }=\displaystyle \frac{1}{{K}_{ads}}+C\,{\rm{Langmuir}}\end{eqnarray} \tag{ 4 }$$

$$\begin{eqnarray}&&\displaystyle \frac{\theta }{1-\theta }\exp (-2\alpha \theta )=bXC\,{\rm{Frumkin}}\end{eqnarray} \tag{ 5 }$$

$$\begin{eqnarray}&&\exp (-2\alpha \theta )=bXC\,{\rm{Temkin}}\end{eqnarray} \tag{ 6 }$$

$$\begin{eqnarray}&&\mathrm{Log}(\theta )=\mathrm{Log}({K}_{ads})+n\mathrm{Log}(C)\,{\rm{Freundlich}}\end{eqnarray} \tag{ 7 }$$

In the presented isotherms, θ, C, and Kads denotes the surface coverage, the Arachis hypogaea shell extracts concentrations in H₂SO₄ media, and kads is the adsorption equilibrium constant, respectively. Figure 4 illustrates that the best-fitted isotherm is a linear connection between C/θ versus C (g/L) with a 0.99996 slope value. Hence, the AHS adsorption on the steel surface fits the Langmuir isotherms, and a protected monolayer was generated on the steel surface, leading to a wholly smooth surface [38]. The increased Kads with inhibitor concentrations suggest the presence of inhibitor adsorption on the steel surface [39].

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Figure 5 shows the SEM and EDX result for the API X 65 steel surface after 456 h exposure time in 1 mol L⁻¹ H₂SO₄ solutions inhibited by AHS with 4 g L⁻¹ concentration. The SEM image shown in figure 5 reflects that the AHS makes the steel surface smooth with few corroded areas and damages. Observation from figure 5 shows that without AHS, severe damage, with pit and cavity, showed on the steel surface. This illustration discloses that the passive films covered the steel surface effectively and blocked the active site [40]. It is obvious from this result that the AHS molecules adsorbed on the steel substrates results to a reduced roughness on the metal surface [41, 42]. The reduced surface roughness is obvious from the SEM image in figure 5 with inhibitor. The lower surface roughness indicates the AHS molecules' efficiency in reducing iron dissolution rates. This observation is in close accordance with the gravimetric and electrochemical test result. This result proves the role of Arachis hypogaea shell molecule adsorption on the API X 65 steel surface and its efficiency in iron dissolution rate reduction.

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As mentioned in the literature calcium, thiamine, and niacin are three significant components present in the Arachis hypogaea. The main absorption band in this compound is −OH, C=O, and −CH₃. The organic molecule adsorption in the Arachis hypogaea extract on the API X 65 steel surface depends on the inhibiter molecules chemical structure. To figure out the presence of this band in the Arachis hypogaea shell extracts, the FTIR analysis was performed. From figure 6, the absorption band located at 2355 cm⁻¹ is linked to the –OH stretching vibration. The C=C stretching and –CH₃ bending vibration is observed at wavenumber 1606 cm⁻¹ and 1012 cm⁻¹, respectively. The absorption peak at 3390 cm⁻¹ is associated with alkane stretching, respectively. This finding reflects that all significant bonds and functional groups in Arachis hypogaea shell extract are observable from the FTIR spectrum.

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Recently, corrosion inhibitors have become extremely common in industrial application. On the other hand, in some cases, this inhibitor can negatively impact the environment and human health. Also, conventional and synthetic inhibitors are generally not cost-effective and eco-friendly. Hence, scientists now focus on green corrosion inhibitors. Owing to its adverse impacts and utterly toxic nature on human health and the environment, the application of inhibitor such as chromate has been limited, and environmentally friendly inhibitor such as seeds of fruits, roots, leaves of plants and shells has received much attention as possible replacements. Organic compound with active functional groups and heteroatoms provide high inhibitive effectiveness for green inhibitors. In this study, from the gravimetric test with 4 g/L Arachis hypogaea shell extract, 98.42% efficiency was derived. This efficiency is greater than most of the reported green inhibitors [24–27]. The process of extracting most inhibitors with efficiency lower than Arachis hypogaea shell extracts are expensive. 1 kg of Arachis hypogaea shell extracts needs only R0.00, while most green inhibitors' extraction process is higher than $50 per kg.

Conversely, high-performance inhibitor such as Ceratonia siliqua L seed oil [43–45], and Polysaccharide [46–48] are costly and, hence, are not recommended for large scale because of high preparation prices. It is worth noticing that most inhibitors reached highest performance at high concentration, increasing the related cost. Also, extraction of Arachis hypogaea shell is green extraction, non-aqueous solvent (for instance, methanol cyclohexane, and ethanol) are essential to extract most green inhibitors, which leaves negative impacts on the environmental and increase the cost also. Most of the reported environmentally friendly inhibitors are scarce, which restricts the extensive utilization of the inhibitors. Hence, the Arachis hypogaea shell extracts can be used for large-scale application because of its high availability, low cost, high performance, and eco-friendly nature.