3.2.1. Corrosion Potential (Ecorr)
Table 4 shows the corrosion potential values obtained after conducting the OCP and polarization resistance tests followed by potentiodynamic scans, for 316L stainless steel tested in different solutions at room temperature. Corrosion potential (E
corr) is essential for understanding the mechanisms of corrosion inhibition with the addition of species to electrolyte solutions. E
corr values are obtained from the polarization curve with the extrapolation of the Tafel lines. Comparing E
corr values of solutions with and without CNTs can help identify the dominant effect of corrosion inhibition due to the addition of the CNTs. Having no change or small change (less than 85 mV) in E
corr with the addition of a component is an indication that this component affected both reactions. A change or a shift in the E
corr value signifies that the added species affected one of the reactions more than the other, depending on the direction of shifting [
63,
64,
65].
In this neutral solution, the anodic reaction is the dissolution of 316L stainless steel ions into the solution from the metal surface. Cathodic reactions are overall associated with the reduction of metal cations and water to deposit metal and form hydrogen gas, respectively. A compound can be classified as anodic type or cathodic type if the change in E
corr values is higher than 85 mV [
65,
66]. Since the displacement for E
corr between GA-water solutions and the corresponding E
corr values for CNT nanofluids was less than 85 mV, the CNT in the solution was affecting both anodic and cathodic reactions. The presence of CNTs and GA in deionized water solution reduces the dissolution of 316L SS and retards the hydrogen evolution and hydroxyl formation reactions.
The addition of CNT affected both reactions, but there was a dominant effect towards one of the cathodic or anodic reactions. A positive displacement of E
corr values was observed for CNT-water nanofluids compared with their corresponding GA-deionized water solutions. The highest positive displacement (59 mV) was observed in 0.3 wt% CNT-water nanofluid, compared with that for the corresponding 0.9 wt% GA-deionized water solution. This signifies that CNTs were more affective towards the anodic reaction; that is, CNTs occupied anodic sites and decreased metal dissolution at a higher rate than they decreased cathodic hydrogen evolution reaction. With the positive shift of E
corr values, decreasing anodic reaction might have occurred as a result of the adsorption of GA and CNTs on the surface of the metal, which led to an enhancement of the stability of the metal oxide layer and the CNTs layer [
67]. A small displacement of E
corr was also noted with the addition of CNTs and SDS surfactant to deionized water, but it was negative displacement [
68].
GA is a natural inhibitor that is soluble in water and contains a mixture of polysaccharides, sucrose, oligosaccharides, arabinogalactan and glucoproteins [
69,
70]. GA plays a great role in providing the needed corrosion inhibition in the surface of metallic substrates. It is an anionic surfactant with a hydrophobic tail and hydrophilic head, and it is connected to metal surfaces though its hydrophilic head, which is attracted to the polar negatively charged surface of the oxides [
61]. GA contains constituents in the form of hetero atoms such as oxygen, which will provide active adsorption sites [
71]. With the structure of GA shown in
Figure 4 [
71], the interaction happens between the ferrous ions and the oxygen atom in the backbone of the polymer, which provides the right coordination for bonding [
72]. On the other hand, CNTs are hydrophobic in nature, hence their surfaces need to be modified by attachment with some groups in order to overcome their weak interfacial interactions with aqueous solutions and allow their dispersion in such solutions [
73]. GA is added to CNT-water solution as a surfactant to allow homogenous dispersion of the hydrophobic CNT in water.
Adsorption on the surface of the metal is recognized as the mechanism to explain the retardation action of organic inhibitors such as GA [
64,
75]. Adsorption of inhibitors usually decreases the corrosion rate in two ways [
63]: (1) by decreasing the available surface area for reactions, which is called the geometry blocking effect; and (2) by changing the activation energy needed for cathodic/anodic reactions. Determining the dominant effect is difficult, as more than one parameter can influence the result. Theoretically, no change in the E
corr for the solution after the addition of the inhibitor indicates that the geometry blocking effect is stronger than the activation energy effect [
63,
64]. E
corr values for all GA-water solutions had a small shift towards more negative values compared with those for deionized water, highlighting the blocking’s role in the inhibition of GA-only solutions in addition to the activation energy effect. Furthermore, the small positive displacement observed for E
corr values of CNT nanofluids also indicated the small role of the surface energy effect in CNT adsorption, and hence in corrosion inhibition. It can be said that the blocking effect of CNTs had a major contribution to corrosion inhibition, without ignoring the surface energy effect.
3.2.2. Polarization Parameters
Table 5 shows some corrosion parameters for 316L SS tested in different solutions. Polarization parameters of anodic (β
a) and cathodic (β
c) Tafel constants, corrosion current density (i
corr), polarization resistance (R
p), pitting potential (E
pit) and corrosion rate were gathered after conducting three tests in series: OCP, polarization resistance test and the potentiodynamic scan.
Potentiodynamic scans of CNT-water nanofluids and of the GA-water solutions are shown in
Figure 1 and
Figure 2, respectively. A passive–active–transpassive–active behavior was observed, with or without CNTs. Typically, adding an inhibitor affects one of the redox reactions, or both reactions. An inhibitor is supposed to decrease the anodic Tafel slope and slightly increase the OCP after the immersion. No change or negligible change in the cathodic Tafel slope indicates that the hydrogen evolution is diminished by the surface blocking effect of the inhibitor [
63].
Tafel constant values β
a and β
c were found to change with the addition of GA to deionized water, indicating that GA affected both reactions and confirming that GA is a mixed-type inhibitor [
43,
75,
76]. Addition of CNTs to the GA-water solution decreased both β
a and β
c values; however, the change in β
a was higher. This confirms the same finding as the slight shift of E
corr of a higher influence on the anodic reaction. At the same time, the decrease in β
c was not significant with the addition of CNTs, indicating the blocking effect of CNT in diminishing the hydrogen evaluation reaction. Hence, it can be acknowledged that presence of CNT inhibited the corrosion of the stainless steel by blocking parts of the exposed surface, with some influence on the surface energy of the steel.
Pitting potential (E
pit) is also a corrosion parameter that can give insight into understanding the susceptibility of the tested sample to corrosion in a specific condition. E
pit is usually obtained from the potentiodynamic scan curve where an abrupt increase in current density is detected. In the current solutions, an abrupt increase in current density was not noticeable, so E
pit was considered as the potential at which the current density increased at a higher rate and kept on increasing without any passivation state (re-passivation is where there is retarding in current density value). However, it is important to mention that an increase in the anodic current does not entirely represent pitting; it can also result from oxygen evolution (water oxidation) reaction, although for this instance we can assume that pitting was more dominant. Therefore, the average pitting potential values (E
pit) can be estimated from the curves, and are listed in
Table 5. It is noticed from curve (1) in
Figure 5 that the pitting potential in the NaCl solution was the lowest compared to all other solutions due to the presence of aggressive chloride ions. When tested in NaCl solution, pitting initiated at an average potential of 0.187 V
SCE. This value is lower than the 0.286 V
SCE obtained by Saadi et al. when they tested 316L in 3.5 wt% NaCl solution [
56]. On the other hand,
Table 5 shows that lower concentrations of CNT nanofluids (0.05 and 0.1 wt%) had the highest E
pit values, which were higher than those for deionized water and tap water. This was due to the inhibition effect and adsorption of the GA and CNT on the surface of the metal, which enabled the surface of the metal to sustain higher potentials. Higher CNT concentrations are associated with lower pitting potential values and with higher corrosion rates. Such increases in metal dissolution with higher CNT concentrations might be due to the formation of active anodic sites in the stagnant regions between the accumulated CNTs and the surface of the metal. It might be also because of the desorption of the CNTs from the surface with increased CNT concentration [
64].
Another parameter to investigate is polarization resistance (R
p).
Table 5 shows that lower R
p is associated with higher corrosion rates. Testing in NaCl solution resulted in a lowest resistance of 839 kohm, while deionized water had the highest R
p value of 7994 kohm. In addition, testing in GA-water had R
p values in the range of 2634–3667 kohm, while nanofluids and tap water had R
p values in the range of 3777–5950 kohm. Presence of CNTs in nanofluids resulted in higher resistance in the electrolyte solution. Among all concentrations of CNT nanofluids, 0.1 wt% resulted in the highest polarization resistance, while further increases in CNT concentration led to a decrease in that resistance. In addition to solution resistance and charge transfer resistance, CNTs formed a thin film carbon coating on the surface of the metal that created extra resistance. However, at higher concentrations of CNTs, anodic reaction accelerated at a higher rate and increased the corrosion rate. This is in agreement with the increase in β
a values obtained for the mentioned two highest CNT concentrations.
Table 5 shows the corrosion rate values of all tested solutions. As can be seen from the presented average values, the highest corrosion rate was obtained with the NaCl solution, with a value of 44.34 mmpy (milli mil per year, equivalent to 0.0011 mm/yr) and a current density of 0.1488 µA/cm
2. Such a high corrosion rate was associated with the highest i
corr and lowest R
p values among all other tested solutions. Testing samples using deionized water resulted in lower corrosion rates than in tap water, which was as expected since the latter solution contained more ionic species than deionized water. For the same reason, the corrosion rate for GA in deionized water was higher than the corrosion rates of deionized and tap water, despite the fact that GA is an inhibitor. Furthermore, adding CNTs to the GA solution decreased the corrosion rate at all concentrations of CNT nanofluids. A minimum corrosion rate of 6.43 mmpy was obtained with 0.1 wt% CNTs nanofluid. Increasing CNTs loading higher than 0.1 wt% caused the corrosion rate to increase with increasing CNTs concentration. This indicates that introducing CNTs to the GA-water solution increased the resistance of the bulk solution and lowered the corrosion rate due to the adsorption of CNTs and the GA on the metal surface. The presence of such species covered part of the metal’s exposed surface and decreased the anodic reaction. However, if the coverage was not uniform it could accelerate the corrosion by providing areas of different potential values within the metal surface.
Previous work related to the present study was limited in scope and volume. Rashmi et al. tested stainless steel in 0.1 wt% CNT-water nanofluid and achieved a corrosion rate of 2.48*10
−5 mm/yr [
40], which is relatively close to our value of 1.63*10
−4 mm/yr. In addition, an immersion test was used to measure the corrosion rate of 316L stainless steel in de-mineralized water and in distilled water. It was found that the corrosion rate in both de-mineralized water and distilled water were around 0.0608 and 0.0476 mm/yr, respectively [
77]. Both values are higher than the corrosion rate obtained in the current research (equivalent to 1.15 × 10
−4 mm/yr), which might have been due to the method followed in measuring the corrosion rate. An immersion test method is usually not recommended when the material is expected to have low corrosion rates in the tested media [
78]. Furthermore, Rashidi et al. measured the corrosion rate of carbon steel in 0.1 wt% MWCNT dispersed in distilled water and found corrosion rate values of 3.3402 and 1.643 mpy for SDBS and SDS surfactants used to stabilize the CNTs, respectively [
68].
Figure 6 shows corrosion rate values for samples tested in CNT nanofluids and for GA-water solutions. A linear relation was proposed between the corrosion rate and the CNT loading that appeared to be a positive relation. On the other hand, GA was revealed to have a negative linear relation as the corrosion rate decreased with increasing GA concentrations. Similar results of decreasing corrosion rates were found with increasing GA concentrations in potable water [
76] and in an acidic medium [
42,
75]. Furthermore, other surfactants such as SDS and SDBS used to disperse CNTs in distilled water did not show an inhibition effect. Both surfactants were found to have higher corrosion rates compared to those for distilled water [
68].
The decrease in corrosion rate due to the presence of CNT can be evaluated by calculating the inhibition efficiency (
IE). Inhibition efficiency is the percentage of corrosion rate decrease with the presence of CNT with respect to the corrosion rate value in CNT-free solution. Inhibition efficiency (
IE%) can be calculated as in Equation (6):
where
CR (CNT free solution) is the corrosion rate of 316L stainless steel samples tested in GA-water solution, and
CR (CNT nanofluid) is the corrosion rate of the same steel tested in CNT-water nanofluid. The inhibitor’s surface coverage (
θ) can also be calculated as the fraction of the inhibition efficiency (IE%/100), as listed in
Table 6. A highest inhibition efficiency of 61% was obtained with 0.1 wt% CNT nanofluid, while a lowest inhibition of 24% with 0.5 wt% CNT nanofluid. Comparing with the inhibition effect of GA in the literature, it was found that a maximum inhibition of 96.8% was achieved with the adsorption of acacia gum (600 ppm) on mild steel where the testing media were potable water and at 80 °C [
76].
3.2.3. Adsorption Isotherm
In order to evaluate the adsorption process of CNTs and the surfactant GA on the surface of the metal surface, it is important to determine the adsorption isotherm that best fits our data. Adsorption isotherms provide insights into the interactions between adsorption molecules and the surface by plotting the surface coverage of the adsorbed inhibitor (
θ) versus the equilibrium concentration of the inhibitor (
C). Different adsorption isotherms were checked for their applicability on the results using Langmuir, modified Langmuir, Langmuir-Freundlich, Freundlich, Temkin, Flory–Huggins and Frumkin equations. It was found that only the modified Langmuir type I, modified Langmuir type II and Flory–Huggins equations were applicable to our experimental data. Adsorption isotherms were obtained according to Equations (7)–(9) [
79]:
Modified Langmuir isotherm type I:
Modified Langmuir isotherm type II:
Flory–Huggins isotherm:
where
C is the equilibrium inhibitor concentration in g/L,
θ is the surface coverage of the adsorbed inhibitor,
Kads is the adsorption equilibrium constant and
n and
x are parameters associated with the number of displaced water molecules per active site on the surface of the metal and consequently the number of adsorbed inhibitor molecules.
To determine which model would best fit the surface coverage of the adsorbed CNTs and GA species, the linearized forms of these equations are plotted in
Figure 7. The modified Langmuir type I isotherm showed the best representation of the data, as it had a regression constant value (R
2) of 0.9865. The Flory–Huggins isotherm matched the data better than modified Langmuir type II, with both having R
2 values of 0.9134 and 0.8543, respectively. The parameter
n was obtained from the slope of the modified Langmuir type I correlation, and had a value of 1.38, indicating that there was more than one CNT and/or GA adsorbed at a particular active site on the surface. In addition, having an
n value in that model that was not equal to unity signifies that there were interactions between adsorbed CNTs and GA species [
79]. The adsorption constant
Kads was also obtained from the intercept of the same correlation, and had a value of 0.61 L/g. The value of
Kads represents how strongly and efficiently the adsorbed species are attached to the surface [
79,
80]. A low value of
Kads is expected with physically adsorbed molecules.
Finding the value of
Kads can also provide the amount of adsorption free energy (ΔG°
ads) according to Equation (10):
where
R is the gas constant,
T is absolute temperature and
Csolvent is the concentration of solvent (equal to 999 g/L for water). A negative value of Δ
G°ads indicates that the adsorption of the surfactant is spontaneous and that its interaction with the surface is strong. Having Δ
G°ads that is more positive than −20 kJ/mol indicates that the adsorption is a physisorption process. On the other hand, if the process is more negative than −40 kJ/mol, then the adsorption will be classified as chemisorption, as it involves charge sharing or transfer to form bonds between surfactant and solid surface [
61,
64,
65]. According to our data, Δ
G°ads had a value of −15.73 kJ/mol, indicating it was a spontaneous adsorption. Since the value of Δ
G°ads was more positive than −20 kJ/mol, then CNTs and GA were adsorbed physically to the surface of the metal. Electrostatic interactions occurred between the negatively charged surface of the metal and the adsorbed molecules, rather than chemical bonding. As the mechanism of adsorption was a physical one, the process could be reversed, and a desorption of the molecules might happen, especially at higher temperatures or concentrations. A physical adsorption of GA on the surface of steel was obtained in 0.1 M H
2SO
4 solution [
71], in 0.1 M HCl solution [
81] and in potable water [
76].