3.1 Potentiodynamic polarization techniques
Tafel plots for aluminium in 1 M HCl in the absence and presence of different concentrations of the inhibitors are presented in Fig. 2; the plots show that the trend adopted by the uninhibited processes is similar to that adopted by the inhibited processes. This behaviour can be interpreted to mean that the inhibition mechanism that is followed by all utilized inhibitors is similar to that of the corrosion process [31−34]. In addition, the anodic and cathodic processes are altered. Since corrosion is an electrochemical process, altering either anodic or cathodic half-reactions brings about a delay in the rate of corrosion [35, 36], which implies that the inhibitors have a tendency to slow down the corrosion process on the Al surface. Another interesting observation from Fig. 2 is the elongated section on the upper solid curve which is also referred to as the anodic branch of the experimental polarization. This elongation effect on the anodic section may be attributed to the formation of the passivation film on the aluminium surface during the reaction, which aides in the protection of the Al surface. Extrapolation on the Tafel slopes. gives rise to certain crucial PDP parameters such as Tafel anodic slope (ba), Tafel cathodic slope (bc), corrosion current density (icorr) and corrosion potential (Ecorr), which are all collectively reported in Table 1.
Table 1: Potentiodynamic polarization (PDP) parameters such as corrosion potential (Ecorr), corrosion current density (icorr) and anodic and cathodic Tafel slopes (ba and bc) using different inhibitors.
Inhibitor
|
Inhibitor
Conc. (M)
|
-Ecorr(mV)
vs Ag/AgCl
|
icorr
(mA.cm-2)
|
Rp
(10-1) (Ohm)
|
ba
(V.dec-1)
|
-bc
(V.dec-1)
|
%IEPDP
|
Blank
|
|
712
|
8.63
|
4.99
|
9.82
|
8.95
|
-
|
SNA
|
1.0×10-5
|
650
|
2.53
|
3.18
|
1.52
|
0.69
|
70.68
|
2.0×10-5
|
648
|
1.85
|
2.53
|
0.21
|
0.05
|
78.56
|
3.0×10-5
|
637
|
1.47
|
3.29
|
0.11
|
0.14
|
82.96
|
4.0×10-5
|
641
|
1.43
|
3.14
|
0.10
|
0.10
|
83.42
|
5.0×10-5
|
644
|
1.27
|
3.22
|
0.09
|
0.09
|
85.28
|
SBZ
|
1.0×10-5
|
621
|
1.99
|
3.65
|
1.13
|
0.66
|
76.94
|
2.0×10-5
|
599
|
1.61
|
3.22
|
1.52
|
0.56
|
81.34
|
3.0×10-5
|
666
|
1.13
|
3.12
|
1.51
|
0.88
|
86.90
|
4.0×10-5
|
614
|
1.10
|
2.56
|
0.99
|
0.95
|
87.25
|
5.0×10-5
|
621
|
1.01
|
3.68
|
0.41
|
0.65
|
88.29
|
SCP
|
1.0×10-5
|
631
|
2.64
|
1.98
|
2.14
|
1.45
|
69.40
|
2.0×10-5
|
645
|
2.81
|
2.01
|
1.99
|
0.98
|
67.43
|
3.0×10-5
|
615
|
2.60
|
2.14
|
1.84
|
1.02
|
69.87
|
4.0×10-5
|
608
|
2.51
|
2.56
|
1.95
|
0.78
|
70.91
|
5.0×10-5
|
645
|
2.43
|
2.41
|
2.54
|
1.44
|
71.84
|
SDM
|
1.0×10-5
|
566
|
2.99
|
2.98
|
1.98
|
1.54
|
65.35
|
2.0×10-5
|
614
|
2.85
|
2.36
|
1.97
|
1.56
|
66.97
|
3.0×10-5
|
598
|
2.61
|
3.12
|
1.69
|
2.08
|
69.75
|
4.0×10-5
|
601
|
2.60
|
3.01
|
1.65
|
0.96
|
69.87
|
5.0×10-5
|
613
|
2.51
|
3.11
|
2.05
|
1.55
|
70.91
|
SSZ
|
1.0×10-5
|
589
|
0.91
|
3.61
|
2.01
|
1.66
|
89.45
|
2.0×10-5
|
601
|
0.87
|
2.98
|
1.99
|
1.54
|
89.91
|
3.0×10-5
|
578
|
0.78
|
2.52
|
1.85
|
1.21
|
90.96
|
4.0×10-5
|
612
|
0.74
|
3.12
|
1.84
|
1.81
|
91.42
|
5.0×10-5
|
613
|
0.69
|
3.14
|
1.47
|
1.68
|
92.00
|
SMZ
|
1.0×10-5
|
618
|
1.69
|
2.96
|
1.99
|
1.87
|
80.41
|
2.0×10-5
|
598
|
1.45
|
2.87
|
1.67
|
1.85
|
83.19
|
3.0×10-5
|
612
|
1.11
|
2.55
|
1.55
|
1.99
|
87.13
|
4.0×10-5
|
615
|
0.98
|
3.10
|
1.41
|
1.84
|
88.64
|
5.0×10-5
|
599
|
0.81
|
3.12
|
1.58
|
1.78
|
90.61
|
The obtained corrosion current densities were further utilized to calculate the %IEPDP for all inhibitors at various temperatures according to Eq. 1. As it is observed in Table 1, the values of ba and bc for the uninhibited processes are less in comparison with those of the inhibited processes by a margin. This information suggests that the cathodic and anodic half-reactions were altered through the introduction of the individual inhibitor molecules. A closer inspection into the results involving the corrosion current density shows that the addition of all inhibitors utilized shifted the polarization curves towards lower regions of the current. This is a characteristic trend for favourable adsorption by inhibitors on the metal surfaces [37, 38]. The magnitude of the displacement in the Ecorr values between the inhibited and uninhibited systems can be used to classify the mode of inhibition as either anodic, cathodic or mixed-type. An inhibitor can be classified as anodic or cathodic type if the change in the Ecorr values is greater than ± 85mV [38, 39]. A mixed-type adsorption mechanism is further proposed from the values of corrosion potential, which are almost constant with negligible variation [38, 39].
3.2 Electrochemical impedance spectroscopy (EIS)
EIS plays an integral role in studying the properties of metal/solution and inhibitor boundary [40, 41], in that it provides information in connection to the effect of the increase on the concentration of each of the inhibitor molecule on the extent of adsorption of the inhibitor on the metal surface. Moreover, this technique also permits for the investigation of the corrosion dynamic behaviour of aluminium surface as time changes [42]. The effects of introduction of various concentrations of the selected sulphonamide inhibitors on decreasing the corrosion of the aluminium surface are better discussed through the use of the Nyquist plots, which are shown in Fig. 3, and the corresponding Bode plots, which are shown in Fig. 4. Both plots (uninhibited and inhibited ones) are characterized by imperfect semicircles coupled with curved-like end, which represent the formation of the passivation protective film on the aluminium surface. This characteristic nature of the Nyquist plots is commonly due to the impurities on the metallic surface, inhomogeneity of the aluminium surface, adsorption of sulphonamides on aluminium surface and the roughness of the electrode surfaces [42–45]. Additionally, Nyquist plots are composed of inductive loops at low frequency and capacitive loops at high frequency. The inductive loop at low frequency is due to the bulk relaxation process which is a result of the formation of the adsorption film on the charged metal surface induced by the existence of charged species such as hydrogen ions, hydroxide ions, protonated inhibitor or chloride ions [43, 44]. The capacitive loop at high frequency is due to the transfer of charge and double layer capacitance (Cdl) [43, 44]. The experimental data obtained herein was treated and fitted utilizing an electrical equivalent model circuit shown in Fig. 5. The diameters of the semicircles of the inhibited processes are larger than those of the uninhibited process and increases as the inhibitor concentration increases. This observation signifies that sulphonamide inhibitors slow down aluminium corrosion in 1.0 M HCl and the corrosion rate decreases with increase in inhibitor concentration. Table 2 lists the results in relation to significant impedance parameters such as the solution resistance (Rs), the resistance of the charge transfer (Rct) and the exponent of the constant phase element (n). These results were obtained as a consequence of fitting the EIS data onto an electrical circuit as shown in Fig. 5. The solution resistance is related to the conductivity of the solution or system; the results shows that Rs values corresponding to the blank experiments are much less than those corresponding to the inhibited experiments, which implies that the conductivity of the solution decreases as the inhibitor molecules are introduced into the system. The fact that the geometry of the area in which the movement of the current is located within the systems varies causes an increase in the inhibitor concentration to bring about a no particular trend in the values corresponding to Rs. The effect of the inhibitor concentration on the Rct values is such that as the inhibitor concentration increases the corresponding Rct values also increase. This phenomenon may be attributed to the fact that the surface coverage occupied by the sulphonamides on aluminium increases as the number of inhibitor molecules increases in the system [44, 45]. In other words, increasing the concentration of the inhibitor molecule has a direct relationship with the increase in the coverage of the aluminium surface. The values of n corresponding to all utilized inhibitors do not present a specific trend but each has a value that is close to unity, which implies that the interface is attributed to capacitive trend. The %IEEIS increases with increase in inhibitor concentration due to the fact that as the number of inhibitor molecules are increased within the system, the amount of aluminium surface covered is maximized thereby increasing the inhibitor-aluminium surface adsorption.
Table 2: Electrochemical impedance (EIS) parameters such as the resistance of charge transfer (Rct) and the CPE exponent (n) using different inhibitors.
Inhibitor
|
Inhibitor
Conc.
(M)
|
Rs
(W cm2)
|
Rct1
(W cm2)
|
Rct2
(Ω cm2)
|
Y01×10-8
(μF cm-2)
|
Y02×10-5
(μF cm-2)
|
n1
|
n2
|
%IEEIS
|
Blank
|
|
-63.70
|
06.64
|
08.54
|
1.91
|
2.50
|
1.100
|
0.955
|
-
|
SNA
|
1.0×10-5
|
-8.199
|
29.29
|
26.38
|
6.50
|
3.33
|
0.944
|
1.044
|
77.33
|
2.0×10-5
|
-9.661
|
31.56
|
26.59
|
3.72
|
7.07
|
0.948
|
1.045
|
78.96
|
3.0×10-5
|
-1.392
|
33.54
|
48.90
|
6.52
|
3.31
|
0.993
|
0.912
|
80.20
|
4.0×10-5
|
-66.34
|
121.4
|
20.08
|
9.52
|
3.56
|
1.000
|
1.052
|
94.53
|
5.0×10-5
|
-90.12
|
256.9
|
57.83
|
1.45
|
3.12
|
0.561
|
1.082
|
97.41
|
SBZ
|
1.0×10-5
|
-8.199
|
29.29
|
26.38
|
6.51
|
3.31
|
0.945
|
1.041
|
77.33
|
2.0×10-5
|
-7.577
|
39.26
|
59.60
|
1.11
|
3.56
|
0.880
|
1.023
|
80.06
|
3.0×10-5
|
-7.895
|
40.80
|
56.70
|
1.12
|
3.62
|
0.981
|
1.036
|
83.72
|
4.0×10-5
|
-84.41
|
122.2
|
17.65
|
2.28
|
3.75
|
1.096
|
0.978
|
94.56
|
5.0×10-5
|
-64.57
|
137.2
|
48.30
|
9.11
|
3.34
|
0.999
|
1.044
|
95.16
|
SCP
|
1.0×10-5
|
-65.62
|
123.6
|
32.81
|
1.96
|
3.54
|
0.995
|
1.063
|
94.62
|
2.0×10-5
|
-64.80
|
123.8
|
41.49
|
1.98
|
2.54
|
0.956
|
1.123
|
94.63
|
3.0×10-5
|
-64.09
|
124.7
|
47.14
|
2.91
|
2.88
|
0.925
|
1.095
|
94.88
|
4.0×10-5
|
-56.99
|
129.9
|
105.2
|
1.63
|
4.62
|
0.963
|
0.987
|
95.16
|
5.0×10-5
|
-64.51
|
137.2
|
48.30
|
9.12
|
3.26
|
0.999
|
1.056
|
94.53
|
SDM
|
1.0×10-5
|
-66.34
|
121.4
|
20.08
|
9.51
|
3.56
|
1.000
|
1.056
|
94.64
|
2.0×10-5
|
-87.07
|
124.1
|
18.94
|
9.52
|
3.56
|
1.088
|
1.056
|
94.88
|
3.0×10-5
|
-56.99
|
129.9
|
105.2
|
1.63
|
4.61
|
0.963
|
0.985
|
95.73
|
4.0×10-5
|
-73.65
|
155.6
|
37.80
|
1.32
|
4.36
|
0.785
|
1.000
|
95.83
|
5.0×10-5
|
-53.46
|
159.3
|
31.83
|
1.56
|
3.45
|
0.985
|
1.042
|
94.51
|
SSZ
|
1.0×10-5
|
-61.64
|
121.0
|
69.79
|
2.40
|
5.91
|
0.945
|
0.995
|
94.56
|
2.0×10-5
|
-65.64
|
122.2
|
32.81
|
1.65
|
3.75
|
0.963
|
1.056
|
95.33
|
3.0×10-5
|
-64.76
|
142.2
|
41.49
|
1.95
|
2.48
|
0.951
|
1.120
|
95.16
|
4.0×10-5
|
-64.51
|
137.2
|
48.30
|
9.12
|
3.26
|
0.999
|
1.012
|
95.83
|
5.0×10-5
|
-53.44
|
159.4
|
31.02
|
1.46
|
3.56
|
0.956
|
1.046
|
94.63
|
SMZ
|
1.0×10-5
|
-64.09
|
123.8
|
47.14
|
2.96
|
2.88
|
0.925
|
1.096
|
94.88
|
2.0×10-5
|
-56.99
|
129.9
|
105.2
|
1.62
|
4.62
|
0.965
|
0.958
|
94.93
|
3.0×10-5
|
-57.76
|
131.0
|
17.43
|
1.27
|
5.61
|
0.978
|
1.000
|
95.72
|
4.0×10-5
|
-73.44
|
155.5
|
37.80
|
1.30
|
4.32
|
0.789
|
1.026
|
95.83
|
5.0×10-5
|
-57.76
|
159.3
|
31.83
|
1.56
|
3.28
|
0.958
|
1.098
|
83.94
|
3.3 Surface morphology from scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS)
Scanning electron microscopy images corresponding to the aluminium samples are shown in Fig. 6 while their respective energy dispersive spectroscopy spectra data are shown in Fig. 7. The main objective of the SEM technique is to provide some in-depth knowledge regarding all kinds of surface interactions at the aluminium-sulphonamide/hydrochloric acid interfaces. EDS spectra provide some valuable information regarding the molecular functionality variations [46−50]. The results show that aluminium surfaces prior to contact with the corrosive environments are of a smooth nature coupled with some minor roughness that may be attributed to sample preparations that involve thorough abrasions. The smoothness is compromised as soon as the samples are introduced to the corrosive environments, as shown in Fig. 6. The main cause associated with this observation is the aggressive ions such as Cl- that are available within HCl. Further analysis suggests that all surfaces of aluminium samples in the presence of utilized inhibitors are of a more smooth nature than those associated with uninhibited processes. This means that all utilized inhibitors have the ability to minimize aluminium corrosion through adsorption on the surfaces [51 − 53]. From the EDS spectra shown in Fig. 7, the presence and absence of Cl− is somewhat observable for uninhibited and inhibited processes, respectively. These results in conjunction with the FTIR data further support the idea that sulphonamide inhibitors possess the tendency to reduce the rate of corrosion on metals through the formation of the adsorption type bond [22].
3.4 Surface morphology from fourier transform infrared spectrometry (FTIR)
The variation in molecular functional groups resulting from the adsorption of the inhibitor molecule on the aluminium surface was studied using FT-IR technique. This technique is often utilised to analyse the adsorption film formed on the aluminium surface by making use of molecular functional group studies [54 − 57]. The presence of the certain molecular functional groups of the inhibitor on the surface of the metal surface confirms that fact that inhibitor molecule adsorbs on the surface. The FT-IR spectra of both the corrosion inhibitor and film formed on the surface are shown in Fig. 8. The absorption bands at the fingerprint region could signify the molecular functional groups that are involved in the adsorption of the inhibitor on the aluminium surface. The absorption band at around 1550 cm−1 and 1000 cm−1 correspond to aromatic conjugated C = C and C−N (stretch) respectively. The loss of both C = C and C−N functional groups on the corrosion inhibitor spectrum as compared to one of adsorption film is an indication of the interaction between the corrosion inhibitor and aluminium surface [58, 59]. The band at around 3300 cm− 1 represent O−H functional group. The O−H group at this region is due to the presence of water molecules during the analysis of adsorption film which may be due to the fact that the current corrosion studies are conducted in aqueous HCl solutions [60, 61].
3.5 Adsorption isotherms and thermodynamic parameters
The adsorption process is the primary step for the interaction of organic inhibitor with the metal surface and it depends on the type of metal, chemical structure of the inhibitor as well the type of electrolyte [62]. Analysis of the various plots of the various adsorption isotherms provide useful insight into the type of adsorption followed by the selected inhibitor molecules on the aluminium surface. In the current study, various adsorption isotherms were tested including Langmuir, Temkin, Frumkin, and Freundlich. The degree of surface coverage obtained from the weight loss at different concentration of the inhibitor and different temperature was used to determine the type of adsorption isotherm which correlates with the experimental results. The results of the investigation suggest that the selected sulphonamide molecule best adsorption on the aluminium surface following the Langmuir adsorption isotherm pattern. This is due to the fact that out of all fitted adsorption isotherms, Langmuir produced the best linear relationships with regression coefficient close to unity. Langmuir adsorption equation relating the concentration of the inhibitors to their surface coverage at constant temperature is shown as Eq. 4 and was used to plot adsorption isotherms shown in Fig. 9.
(4)
The adsorption or desorption equilibrium constant was obtained by using equation 5.
(5)
where Cinh is the concentration of the inhibitor, θ is the degree of the surface coverage, Kads is the adsorption equilibrium constant, ΔGoads is the standard free energy of adsorption and the value 55.5 represents the molar concentration of water in solution.
The values of change in Gibbs free energy of adsorption (∆Gads) are recorded in Table 3. ∆Gads values obtained are of negative nature, which is indicative of a spontaneous process. Desorption process of inhibitor from the aluminium surface is also indicated by the decrease of ∆Gads as reaction temperature increase from 30°C to 50°C.
The values of ∆Gads may also provide an indication on the type of adsorption followed by the inhibitor molecules when binding on the metal surface; it is understood that ∆Gads values starting from − 40 kJ/mol and above symbolize chemisorption process. Such energetic values represent possibility of the chemical interaction between the inhibitor molecule and the Al molecule. However, values of ∆Gads ranging from – 20 kJ/mol and lower symbolize physisorption process; a physisorption process is described by the existence of weak van der Waals interaction rather than chemical bonding between the inhibitor and the metal surface [63−67]. Table 3 shows that all ∆Gads values increase with an increase in temperature, and they range from – 16.82 kJ/mol to – 7.15 kJ/mol, which signifies mixed type of adsorption with physical adsorption predominant.
Table 3: Thermodynamic and adsorption parameters (Langmuir adsorption isotherms) for aluminium in 1.0 M HCl at various temperatures for the utilized corrosion inhibitors.
Inhibitor
|
Temperature (°C)
|
r2
|
Slope
|
Kads (M-1)
|
ΔG0ads (kJ.mol-1)
|
SNA
|
30
|
0.999
|
0.995
|
13.15
|
-16.60
|
40
|
0.999
|
1.107
|
4.890
|
-14.54
|
50
|
0.998
|
1.249
|
5.285
|
-15.25
|
SBZ
|
30
|
0.999
|
0.997
|
14.35
|
-16.82
|
40
|
0.999
|
1.112
|
5.154
|
-15.18
|
50
|
0.998
|
1.249
|
6.553
|
-15.83
|
SCP
|
30
|
0.999
|
1.369
|
2.861
|
-12.76
|
40
|
0.998
|
1.833
|
1.239
|
-11.01
|
50
|
0.998
|
3.167
|
0.254
|
-07.15
|
SDM
|
30
|
0.999
|
1.155
|
1.672
|
-11.79
|
40
|
0.999
|
1.981
|
3.279
|
-13.97
|
50
|
0.999
|
2.050
|
0.719
|
-09.01
|
SSZ
|
30
|
0.999
|
1.052
|
14.24
|
-16.80
|
40
|
0.999
|
1.194
|
3.921
|
-14.01
|
50
|
0.998
|
1.318
|
1.237
|
-11.35
|
SMZ
|
30
|
0.999
|
1.023
|
6.808
|
-14.94
|
40
|
0.996
|
1.174
|
2.807
|
-13.55
|
50
|
0.999
|
1.344
|
1.433
|
-11.75
|
3.6. Structures of the Al⋅⋅⋅sulphonamide and their corresponding binding energy
All interaction sites between each of the given sulphonamides on the Al (111) were tested. The preferred site for interaction between each of the inhibitor with the Al (111) surface is shown in Fig. 10. The results shows that the highest binding energy corresponds to the interaction between Al (111) and SSZ inhibitor, which can be considered to lie in the range of mixed inhibitor type of interaction. The second inhibitor in terms of high binding energy is SMX. All other inhibitors have binding energy less than 30 kcal/mol, which would suggest that their mode of interaction with the Al(111) surface is strongly physisorption type in nature.
In structures that show mixed type of interactions (SSZ and SMX) with the Al (111) surface, the inhibitor molecule interacts with the surface through the N atom while in all the other physisorption-favored interactions, the molecule interacts with the Al (111) surface largely through the O atom. This result suggest that the N atom is more preferred center for donating electrons towards the Al (111) surface. The bond length value between the inhibit molecule and the Al (111) is consistent with the intermolecular type of interaction rather than covalent interaction. The results reported herein are therefore consistent with the experimental findings, which also suggest that the interaction energies between the selected sulphonamide inhibitors and the Al surface are largely through the physisorption type of interaction, with slight inclination towards chemisorption for the SSZ and SMX inhibitors.