Corrosion and Natural Corrosion Inhibitors: A Case Study for C. microphyllus

Worldwide, corrosion causes the value of the gross domestic product to decrease in industrialized countries by 4.26% and causes significant losses to industries including infrastructure. As a result, corrosion prevention and research related to it are extremely important. Some researchers are working to develop plant-based natural corrosion inhibitors, and experimental and computational studies are being conducted widely to prevent corrosion through cheap and environmental friendly coatings. A case study of Convolvulus microphyllus (C. microphyllus) extract was examined as eco-friendly for bio-corrosion inhibitor of mild steel in 0.5 M H2SO4 by using conventional weight loss, electrochemical polarization measurements, and electrochemical impedance spectroscopy (EIS) techniques. The compounds responsible for decreasing the rate of corrosion are kaempferol and phydroxycinnamic acid present in the extract. This inhibitor slows down the corrosion rate. Out of many observations, the best result 89.87% corrosion resistance efficiency was obtained at 600 mg/L of C. microphyllus as extract for mild steel in 0.5 M H2SO4 by applying electrochemical and weight loss measurements. The presence of a heteroatom in the main component of C. microphyllus as extract is believed to be an excellent inhibitor. Theoretical research revealed an entirely important report about comparative inhibition effect of different phytochemicals.


Introduction
The process of deposition of oxide layer on the surface of metals is called corrosion. It usually occurs when metal is exposed to moisture or water and gases such as dioxygen, dihydrogen, dichloride, hydrogen sulfide. It is a spontaneous chemical reaction with a slow rate. It usually occurs over days or weeks, and as a result of corrosion, the refined metal is turned into a more stable form such as oxide, hydroxide, or sulfide. The net worth of the corrosion prevention industry was estimated to be around $2.5 trillion (USD) in 2018 and is expected to cross $3.0 trillion (USD) by 2022, as per the National Association of Corrosion Engineers [1,2]. The electrochemical phenomenon of degradation of material over course of time due to exposure toward the environment is called corrosion. Common types of corrosion for rusting of steel and internal polymeric pipeline are wet corrosion and dry corrosion [3]. In today's world, corrosion is no longer merely a chemical degradation

Corrosion
Corrosion is a surface process where the oxide layer is formed on the surface of the metal. There is various types of corrosion such as oxidation corrosion, corrosion by other gases such as Cl 2 , SO 2 , H 2 S, NO x , liquid metal corrosion, differential metal corrosion, differential aeration corrosion, crevice corrosion, and pitting corrosion. If we discuss pitting corrosion as an example, it is due to the formation of small holes or pits on the metal surface. Pitting corrosion is highly destructive as pits are very small to be observed and usually covered with corrosion products. The pitting corrosion is the result of depassivation of a small area on the metal surface, which acts as an anode, while the rest of the undefined and large surface acts as a cathode. It is preceded by a spontaneous galvanic reaction with very limited diffusion of ions (Figure 1) [6]. DOI: http://dx.doi.org/10.5772/intechopen.100505

Electrochemical aspects of corrosion
Most of the metals that we humans commonly use are unstable in the atmosphere as they were obtained from respective ores by artificial reduction through a chemical process. Thus, such metals undergo a reaction very easily to get converted into a stable form. The chemical reactions that do not require any external medium or reagent like energy or catalyst to proceed but occur on their own are called as spontaneous reactions. For spontaneous reactions, the formed products are more stable than reactants. The value for change in Gibb's free energy is always negative, and change in enthalpy is also negative, while the change in entropy is positive for such reactions.
The reactions in which oxidation and reduction both occur simultaneously are called as redox reactions. Corrosion also involves a redox reaction in which one specie or metal or part of the metal is oxidized and it acts as an anode, while the other specie or metal or part of the metal is reduced and acts as a cathode. At anode loss of electrons takes place and loss of mass occurs, while at cathode gain of electron takes place and deposition of corrosion products occurs.
The most primitive corrosion is where anodic oxidation reactions involve a pure iron when it is exposed to a strong acid such as hydrochloric acid. The reaction occurs with the formation of bubble violently. It is given as follows: Another example is the exposure of iron toward moisture or water (Figure 2).
Some other reactions involve in the formation of anode and cathode that leads to corrosion are given below.
Examples of anode reactions: Corrosion reactions are often electrochemical in nature. The reaction is timeconsuming and may take few weeks to months to occur, it is a time and temperature-dependent reaction.
If corrosion reactions occur in aqueous media, then they are similar to that of Leclanche cell. As shown in Figure 3, a zinc container act as an anode, it gets oxidized. Graphite rod coated with carbon and MnO 2 paste acts as a cathode, it gets reduced, whereas both anode and cathode are joined by means of NH 4 Cl and ZnCl 2 paste that serves the role of electrolyte. The greater the flow of electrons, the greater is the corrosion of the zinc electrode [7,8].

Dissimilar electrode cells
When two dissimilar metals come in direct contact with each other and an electrolytic substance is present between them, then dissimilar electrode cells are formed. It is basically a Galvanic cell, for example, we have a Daniel cell in which zinc and copper are in contact. In daily life, a copper pipe connected to a steel pipe provides an example of this type of corrosion cell. This cell is also referred to as galvanic coupling cell, the less noble metal becomes the anode whereas others act as a cathode (Figure 4).

Concentration cells
When two similar electrodes dipped in solutions of different compositions come in contact, then concentration cells are formed. The electrode dipped in dilute solution acts as an anode, while the electrode that is in contact with concentrated solution acts as a cathode ( Figure 5).

Differential aeration cells
When two similar electrodes exposed to different aeration conditions come in contact with one another, then differential aeration cells are formed. The electrode exposed to lesser aeration conditions will act as an anode, while the electrode exposed to higher aeration conditions will act as a cathode. This type of corrosion is very common and is responsible for the significant economic loss (Figure 6) [9, 10].

Natural corrosion inhibitors
Natural corrosion inhibitors reduced or controlled the rate of corrosion by the addition of a natural product in cleaning or pickling solution. It reduces the rate of corrosion either by inhibiting oxidation at the anode or inhibiting reduction at the   cathode, or both. It forms a protective layer at the metal surface, either by physical (means electrostatic attraction between natural product and metal surface) adsorption or chemisorption (means coordination bonds between natural product and metal surface).
Studies of natural corrosion inhibitors are one of the methods for protecting metal from corrosion. So here I am discussing a specific plant C. microphyllus as a natural corrosion inhibitor.

Preparation of materials
The C. microphyllus is a hairy herb from the Convolvuaceae family, which is also known as Shankhpushpi and wind weed. The leaves are linear to oblong, small, and subsessile. About 1-3 flowers are produced, which are stalked. It is easily available all over the world. C. microphyllus contains alkaloids kaempferol, and p-hydroxycinnamic acid as its main phytochemical constituent [11,12]. Figure 7 shows the image of C. microphyllus aerial parts and its main phytochemicals.
The soxhlet apparatus is used for plant extraction. Fresh C. microphyllus aerial parts were collected from our surrounding and then put them in a shaded area for few days, and after drying them properly, they were grinded to convert into powder material. Now, the powder was packed in a soxhlet apparatus and refluxed for 72 hours; then, the diluted extract was collected in the round bottom flask and transformed into a gel like extract by distillation. In the next step, the gel extract is kept in the desiccators packed with silica for few days so that the extract converts into solid extract without moisture. After that, it is preserved in other desiccators for experimental studies. In this work, for weight loss measurement and electrochemical impedance studies a piece of 1 cm 2 dimension is taken as a sample. These samples were mechanically polished with emery paper of different grades and subsequently cleaned with acetone, once again clean distilled water before inserting them into the pickling or cleaning solution. Here for study of C. microphyllus diluted solution of conc. H 2 SO 4 having a molarity of 0.5 is used as a corrosive medium (Figures 8 and 9).

Weight loss studies
The best method for this measurement is the ASTM standard G 31-72 [13]. In this method, all measurements are carried out at room temperature in the thermostat.   Here minimum three times repeating the same measurement for the average value. The weight loss values, surface coverage (θ), inhibition efficiency (η%), and corrosion rate (CR) are shown in Table 1 at different concentrations of C. microphyllus extract for mild steel. At different concentrations of C. microphyllus extract, the value of weight loss, surface coverage (θ), inhibition efficiency (η%), and corrosion rate (CR) for mild steel were calculated by equations that are as follows: where, • C R = corrosion rate (mmy −1 ) • W = weight loss of iron alloy (g) • K = 8.76 × 10 4 (constant) • ρ = 7.86 g cm −3 (density of Fe) • t = immersion time (h) • A = area of the iron alloy coupon • θ shows surface coverage values.
• i R C shows the corrosion rate with inhibitor.
• 0 R C shows the corrosion rate of the iron alloy without the inhibitor.
From the above data, it is clear that the concentration of C. microphyllus extract is inversely proportional to the rate of corrosion, which is used as an inhibitor. Adsorption of active phytochemicals of C. microphyllus extract occurs on the surface of mild steel, which reduces the surface area available for corrosion that is why the rate of corrosion decreases. The highest inhibition efficiency 88.08% is obtained at 600 mg/L.

Electrochemical measurements
An electrochemical workstation is used for the electrochemical measurement. The workstation is made up of three electrodes: (a) the working electrode of metal which works like anode (b) saturated calomal electrode as the reference electrode work like cathode, and (c) platinum electrode as the counter electrode for collect current. In this measurement, metal or alloy was immersed in cleaning or pickling solutions and different concentrations of C. microphyllus aerial part extract were added immersed for a specific time. At room temperature, the values of the open circuit potential (OCP) were noted with respect to the reference electrode. The scanning frequency from 100 kHz to 0.01 Hz is used for recording the Nyquist plot. A 5 mV signal amplitude perturbation at OCP was considered in EIS measurement. The authentic values were taken after three-time repeat measurement.
In Figure 10 shown Nyquist plot explained that 600 mg/L C. microphyllus extract provided larger radius semicircles. The diameter of the capacitive loop was enlarged by increasing C. microphyllus extract concentration, highest at 600 mg/L explaining also the highest inhibition effect. The radiuses of the semicircles in the case of using the C. microphyllus extract were larger than the blank. Because of same shapes of semicircles, it means the mechanism is not change. Calculated EIS results from the following equation provided the percentage inhibition efficiency of bioinhibitor in the electrolytes [14]:  where, R ct and 0 ct R represent the charge transfer resistance in inhibitor and blank, respectively [15]. From Table 2, it is clear that maximum inhibition efficiency of 89.87% was obtained at 600 mg/L of C. microphyllus extract, where the highest R ct value was 155.13 [15]. In Table 3, value of the double-layer capacitor (C dl ) gradually decreasing means protective layer gradually form at the metal surface [16].

DFT optimization
For the evaluation of electronic features of C. microphyllus extract molecules (including hydroxylcinnamic acid and kaempferol shown in Figure 11), the structures were initially optimized by density functional theory (DFT) using Gaussian 09 program. Optimization was done via widely used functional of B3LYP, which was combined with 6-311G** basis function. By a combination of SCRF theory and PCM formula, these calculations were continued in the liquid phase. After optimization, the following properties were analyzed: graphics, energies, and gap of molecular orbitals of highest occupied and lowest unoccupied (HOMO, LUMO, E HOMO , E LUMO , and E LUMO -E HOMO = ΔE L-H ) and partial charges [17].
Electronic level calculations (i.e., DFT) were performed so as to obtain insights into the reactive sites of hydroxylcinnamic acid and kaempferol molecules. The optimized hydroxylcinnamic acid and kaempferol geometries resulted from chemical DFT calculations are plotted in Figure 11. For these energy-minimized geometries, the analysis of frontier orbitals was done as these orbitals play a crucial role in  Table 3. donor-acceptor interactions of inhibiting extract molecules. The HOMO and LUMO pictures of hydroxylcinnamic acid and kaempferol are provided in Figure 11. As displayed, the phenyl ring, carbonyl/hydroxyl O atoms, and ethylene bond of hydroxylcinnamic acid appeared in the role of HOMO implying their strong electron supplying affinity toward a potential electron acceptor, for instance metal atoms containing unoccupied orbitals. According to the LUMO plot, all carbon and oxygen atoms of hydroxylcinnamic acid contributed to LUMO distribution, which is a signification of their tendency for getting electrons provided by filled orbitals of metal atoms. In the case of the kaempferol compound, it is seen that the HOMO placed over the entire aromatic heterocyclic skeleton and on the other hand the carbon as well as oxygen atoms behaved as reactive LUMO locations. As a consequence, the kaempferol component is able to involve in electronic interactions (i.e., donor-acceptor) with the metal surface through the donation of electrons (lone pair in hydroxyl/carbonyl O and π electrons of heterocycles) and receiving of electrons from filled iron orbitals by its C and O atoms. The eigenvalues of HOMO/LUMO and the energetic gap of frontier orbitals were examined, and the quantitative results are tabulated in Table 3 for hydroxylcinnamic acid and kaempferol. From this table, it is evident that the kaempferol species have higher E HOMO and lower E LUMO in comparison with the other compound (i.e., hydroxylcinnamic acid). These results suggest the stronger electron donation and receiving capacity of kaempferol. In addition, the optimized kaempferol molecule owned a lower energetic gap (ΔE L-H ), an observation reflecting better electron sharing and subsequent stronger corrosion inhibition manner of this compound of C. microphyllus extract. This trend of DFT calculated electronic parameters and simulated adsorption energies discloses the stronger kaempferol interactions and adsorption on the steel surface leading to its more effective corrosion prevention influence as compared with the inhibiting molecule of hydroxylcinnamic acid [18,19]. The electrophilic/nucleophilic interactions of studied molecules are summarized in Figure 12. From the given pictures, it is obvious that the electrophilic nature of hydroxylcinnamic acid is graphically distributed on benzene ring and double bond of carbon atoms (with a rich source of π electrons) and heteroatom of O (owing lone pair electrons). These electrons could be donated to an appropriate acceptor of electrons like metal atoms that are composed of empty orbitals. Moreover, similar to LUMO distribution, almost all carbon atoms along with three O heteroatoms are the most reactive sites for nucleophilic behavior clarifying electron getting affinity of these atomic sites. Also, the electrophilic interactions of equilibrated kaempferol compound could likely happen via its electron-rich places including heterocycles and the oxygen atoms of surrounding oxygenated fragments. On the other side, the O and C atoms of the whole backbone could accept electrons when attacked by a nucleophile [20,21].

Molecular simulations
The hydroxylcinnamic acid and kaempferol adsorption and their interactions with the substrate (i.e., steel) were examined applying molecular simulations. These atomic simulations include Monte Carlo (MC) and molecular dynamics (MD). For the representation of steel substrate in these simulations, the frequently applied iron surface Fe (110) was used. The thickness, vacuum region, and periodic replication of this surface were set as 1.5 nm, 4 nm, and (14 × 14), respectively. To examine the adsorption preference of extract components, which are output from DFT calculations, on the iron substrate the MC-based simulation was carried out by adsorption Locator module (Materials Studio software). The MC simulations convergency was controlled with a level of fine. The last cell with the lowest potential energy generated by MC was the starting simulation box of the next modeling, that is, MD. For performing this simulation in solution phase-like experiments, water molecules were also added to the MC-generated cell. Thereafter, an optimization of 20,000 steps and NVT MD simulation of 1 ns were successively performed via COMPASS force field and Forcite module. The time step and temperature of NVT were set as 1 fs and 298 K. The electrostatic interactions of hydroxyl cinnamic acid and kaempferol were modeled by the Ewald scheme along with their charges calculated by DFT. The interactions of van der Waals form were evaluated by the atom-based cutoff. All metal atoms of the substrate were kept fixed [22,23].

Molecular simulation
Simulations at molecular scale namely MC and MD were conducted for the analysis of the ability of C. microphyllus extract molecules adsorption above the metal surface (Fe (110)). The equilibrated cells of hydroxyl cinnamic acid and kaempferol generated by MC-based molecular simulations are displayed in Figure 13. The depicted MC snapshots clearly demonstrate that the C. microphyllus extract species have the ability of approaching and adsorption to iron surfaces. It is observed from the presented top views that hydroxylcinnamic acid and kaempferol adsorption on the surface happened through preferred flat orientation. The aromatic backbones of these adsorbed C. microphyllus extract molecules aligned parallel relative to the outmost iron atoms. The adsorption of hydroxylcinnamic acid and kaempferol took place with energies of −96.65 and − 165.51 kcal/mol, respectively. The adsorption energies of hydroxyl cinnamic acid and kaempferol with negative values further declare their adsorption over the steel substrate. The molecular adsorption was more evaluated through conducting liquid-phase dynamics (MD) simulations. The resulting snapshots of such simulations for hydroxyl cinnamic acid and kaempferol compounds are depicted in Figure 13. The obtained side views of ultimate simulation snapshots indicate that the chosen C. microphyllus extract compounds equilibrated in the vicinity of the crystalline surface (i.e., Fe (110)) under the wet conditions of the interface. This observation points to the fact that hydroxyl cinnamic acid and kaempferol as organic constituents of the green extract could bind to the surface of iron even in the presence of water molecules. Such surface adsorption affinity