The corrosion inhibition of mild steel in 1.0 M H2SO4 solution by ethyl hydroxyethyl cellulose has been studied in relation to the concentration of the additive using weight loss measurement, EIS, polarization, and quantum chemical calculation techniques. The results indicate that EHEC inhibited corrosion reaction in the acid medium and inhibition efficiency increased with EHEC concentration. Further increase in inhibition efficiency is observed in the presence of iodide ions, due to synergistic effect. Impedance results reveal that EHEC is adsorbed on the corroding metal surface. Adsorption followed a modified Langmuir isotherm, with very high negative values of the free energy of adsorption
Organic compounds containing polar functional groups such as nitrogen, sulphur, and/or oxygen in a conjugated system have been reported to be effective as corrosion inhibitors for steel [
Some authors have reported on the effectiveness of polymeric corrosion inhibitors [
In order to support experimental studies, theoretical calculations are conducted in order to provide molecular-level understanding of the observed experimental behaviour. The major driving force of quantum chemical research is to understand and explain the functions of ethyl hydroxyethyl cellulose in molecular forms. Among quantum chemical methods for evaluation of corrosion inhibitors, density functional theory (DFT) has shown significant promise [
The present study presents the appraisal of inhibitive capability of ethyl hydroxyethyl cellulose (EHEC) on mild steel corrosion in 1.0 M H2SO4 solution using weight loss measurements and quantum chemical calculations techniques.
Tests were performed on mild steel specimens of the following percentage chemical composition: Si: 0.02; C: 0.05; Mn: 0.18; Cu: 0.02; Cr: 0.02 and the remainder Fe. This was machined into test coupons of dimensions 3 × 2 × 0.05 cm and a small hole drilled at one end of the coupon to enable suspension into the test solution in the beaker. The metal specimens were polished with fine emery paper, degreased, and cleaned as described elsewhere [
Weight loss experiments were conducted on test coupons. Tests were conducted under total immersion conditions in 200 mL of test solutions at ambient temperature,
Electrochemical experiments were performed using a
All theoretical quantum chemical calculations were performed using the density functional theory (DFT) electronic structure programs, Forcite and DMol3 as contained in the Materials Studio 4.0 software.
The corrosion rates of metals and alloys in aggressive solutions can be determined using different electrochemical and nonelectrochemical techniques. The mechanism of anodic dissolution of iron in acidic solutions corresponds to [
As a consequence of these reactions, including the high solubility of the corrosion products, the metal loses weight in the solution. The results of the gravimetric determination of mild steel corrosion rate as a function of time and concentration of the additive are given in Table
Calculated values of corrosion rate of mild steel in 1.0 M H2SO4 in the absence and presence of EHEC and KI.
System | Corrosion rate (mm/y) | ||||
---|---|---|---|---|---|
Day | |||||
1 | 2 | 3 | 4 | 5 | |
Blank | 25.27 | 22.36 | 20.16 | 19.03 | 17.99 |
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0.5 g/L EHEC | 14.50 | 12.07 | 10.93 | 10.39 | 10.10 |
0.5 g/L EHEC + KI | 12.20 | 10.59 | 9.50 | 9.27 | 9.06 |
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1.0 g/L EHEC | 12.92 | 10.78 | 9.74 | 9.23 | 8.97 |
1.0 g/L EHEC + KI | 9.39 | 7.97 | 7.25 | 7.17 | 7.19 |
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1.5 g/L EHEC | 13.22 | 11.43 | 10.31 | 9.79 | 9.51 |
1.5 g/L EHEC + KI | 9.85 | 8.22 | 7.37 | 7.16 | 7.05 |
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2.0 g/L EHEC | 11.84 | 10.08 | 9.20 | 8.79 | 8.59 |
2.0 g/L EHEC + KI | 10.47 | 8.37 | 7.31 | 6.81 | 6.60 |
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2.5 g/L EHEC | 11.40 | 9.92 | 9.35 | 8.80 | 8.59 |
2.5 g/L EHEC + KI | 10.06 | 8.21 | 7.70 | 7.56 | 7.48 |
These results show that the corrosion rate of mild steel in 1.0 M H2SO4 decreases with time in systems with additive and the blank acid solution. The effects of addition of different concentrations of EHEC on corrosion rates in the acid solution after 5 days of exposure are shown in Table
A quantitative evaluation of the effect of EHEC on mild steel corrosion in 1.0 M H2SO4 solution was achieved from appraisal of the inhibition efficiency (
Calculated values of inhibition efficiency of mild steel in 1.0 M H2SO4 in the presence of EHEC and KI.
System | Inhibition efficiency ( |
||||
---|---|---|---|---|---|
Day | |||||
1 | 2 | 3 | 4 | 5 | |
0.5 g/L EHEC | 42.62 | 46.02 | 45.78 | 45.40 | 43.86 |
0.5 g/L EHEC + KI | 51.72 | 52.64 | 52.88 | 51.29 | 49.64 |
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1.0 g/L EHEC | 48.87 | 51.79 | 51.69 | 51.50 | 50.14 |
1.0 g/L EHEC + KI | 62.84 | 64.36 | 64.04 | 62.32 | 60.03 |
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1.5 g/L EHEC | 47.69 | 48.88 | 48.86 | 48.55 | 47.14 |
1.5 g/L EHEC + KI | 61.02 | 63.24 | 63.44 | 62.38 | 60.81 |
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2.0 g/L EHEC | 53.15 | 54.92 | 54.37 | 53.81 | 52.25 |
2.0 g/L EHEC + KI | 58.57 | 62.57 | 63.74 | 64.21 | 63.31 |
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2.5 g/L EHEC | 54.89 | 55.64 | 53.62 | 53.76 | 52.25 |
2.5 g/L EHEC + KI | 60.19 | 63.28 | 61.81 | 60.27 | 58.42 |
The plots show that
Variation of inhibition efficiency with concentration of EHEC.
To further clarify the modes of inhibitor adsorption, experiments were conducted in the presence of iodide ions, which are strongly adsorbed on the surface of mild steel in acidic solution and facilitate adsorption of organic cation-type inhibitors by acting as intermediate bridges between the positive end of the organic cation and the positively charged metal surface. Specific adsorption of iodide ions on the metal surface leads to recharging the electrical double layer [
Thus, an improvement of
Synergistic effect between KI and EHEC on the variation of inhibition efficiency with time.
Basic parameters which are descriptors of the nature and modes of adsorption of organic inhibitor on the corroding metal surface can be provided by adsorption isotherms which depend on the degree of surface coverage,
Langmuir isotherm for EHEC adsorption on mild steel surface in 1.0 M H2SO4.
In general,
Adsorption parameters from modified Langmuir isotherm.
Day |
|
|
|
|
---|---|---|---|---|
1 | 0.990 | 1.702 | 4.515 | −45.381 |
2 | 0.988 | 1.692 | 5.795 | −58.246 |
3 | 0.993 | 1.772 | 7.982 | −80.228 |
4 | 0.993 | 1.764 | 7.412 | −74.498 |
5 | 0.992 | 1.834 | 7.627 | −76.659 |
The free energy of adsorption
In addition, it is important to note that adsorption free energy values of −20 kJ mol−1 or less negative are associated with an electrostatic interaction between charged molecules and charged metal surface (physical adsorption). On the other hand, adsorption free energy values of −40 Kj mol−1 or more negative values involve charge sharing or transfer from the inhibitor molecules to the metal surface to form a co-ordinate covalent bond (chemical adsorption) [
Electrochemical impedance spectroscopy analyses provide insight into the kinetics of electrode processes as well as the surface characteristics of the electrochemical system of interest. Figure
Nyquist impedance spectra of mild steel corrosion in 1.0 M H2SO4 in the absence and presence of EHEC and EHEC + KI.
The presence of a single time constant may be attributed to the short exposure time in the corrosive medium which is not adequate to reveal degradation of the substrate [
The values of the impedance parameters derived from the Nyquist plots using the selected equivalent circuit model
Impedance and polarization parameters for mild steel in 0.5 M H2SO4 in the presence and absence of EHEC and EHEC + KI.
System | Impedance data | Polarization data | |||||
---|---|---|---|---|---|---|---|
|
|
I.E. % |
|
|
|
I.E. % | |
Blank | 11.931 | 0.889 | — | 14.65 | −468.35 | 16.27 | — |
EHEC | 37.502 | 0.876 | 68.19 | 5.13 | −477.87 | 58.04 | 71.97 |
EHEC + KI | 133.37 | 0.854 | 91.05 | 2.41 | −489.10 | 177.51 | 90.83 |
The double layer capacitance values of the systems were also examined and calculated using the expression:
Lower double layer capacitance suggests reduced electric charge stored, which is a consequence of increased adsorption layer that acted as a dielectric constant. The increase in
Since adsorption of an organic inhibitor on a metal surface involves the replacement of adsorbed water molecules on the surface, the smaller dielectric constant of the organic molecule compared to water as well as the increased thickness of interfacial layer due to inhibitor adsorption acted simultaneously to reduce the double layer capacitance. This provides experimental evidence of adsorption of EHEC on mild steel surface. The significantly lower
Figure
Polarization curves of mild steel corrosion in 1.0 M H2SO4 in the absence and presence of EHEC and EHEC + KI.
Inhibition efficiency was calculated from the polarization data as follows:
The cooperative effect between EHEC and KI in hindering the corrosion of mild steel in 1.0 M H2SO4 solution is also evident in both the Nyquist and Tafel polarization plots. Addition of KI resulted in a significant increase in the diameter of the Nyquist semicircle and hence an increase in
The inhibition effectiveness of inhibitors has been reported to correlate with the quantum chemical parameters such as HOMO (the highest occupied molecular orbital), LUMO (the lowest unoccupied molecular orbital), and the energy gap between the LUMO and HOMO
According to the frontier molecular orbital theory of chemical reactivity, transition of electrons is due to an interaction between the frontier orbitals, HOMO and LUMO, of reacting species. The energy of HOMO is directly related to the ionization potential and characterizes the susceptibility of the molecule toward attack by electrophiles. The energy of LUMO is directly related to the electron affinity and characterizes the susceptibility of the molecule toward attack by nucleophile. The lower the values of
The electronic structure of EHEC, the distribution of frontier molecular orbital, and Fukui indices have been modeled in order to establish the active sites as well as local reactivity of the inhibiting molecules. This was achieved using the DFT electronic structure programs, Forcite and DMol3, and using a Mulliken population analysis. Electronic parameters for the simulation include restricted spin polarization using the DND basis set as the Perdew Wang (PW) local correlation density functional. The geometry optimized structures of EHEC, HOMO and LUMO orbitals, Fukui functions, and the total electron density are presented in Figure
Electronic properties of ethyl hydroxyethyl cellulose (EHEC) [C, grey; H, white; O, red].
Optimized structure
Total electron density
HOMO orbital
LUMO orbital
Fukui function for nucleophilic attack
Fukui function for electrophilic attack
Local reactivity was analyzed by means of the Fukui indices to assess the active regions in terms of nucleophilic and electrophilic behaviour. Thus, the site for nucleophilic attack will be the place where the value of
Calculated values of quantum chemical properties for the most stable conformations of EHEC.
Property | EHEC |
---|---|
|
−6.154 |
|
−2.323 |
|
3.831 |
Maximum |
0.015 O(12) |
Maximum |
0.165 O(12) |
Ethyl hydroxyethyl cellulose was found to be an effective inhibitor of mild steel in 1.0 M H2SO4 solution and its inhibition efficiency increased with increasing concentration. The corrosion process is inhibited by adsorption of EHEC on the mild steel surface following the modified Langmuir isotherm. The inhibiting action is attributed to general adsorption of both protonated and molecular species of the additive on the cathodic and anodic sites on the corroding mild steel surface. In addition, corrosion inhibition is due to the formation of a chemisorbed film on the mild steel surface. The EIS measurement confirmed the adsorption of EHEC and EHEC + KI on the mild steel surface. Polarization studies showed that EHEC and EHEC + KI were mixed-type inhibitor systems with predominant cathodic effect. The theoretical study demonstrated that the inhibition efficiency is related to molecular structure of inhibitor whereby increase in
The authors hereby declare that there is no conflict of interests regarding the publication of this paper.
The authors are grateful to Udemmadu Tochukwu and Ochiogu Valentine for assistance in carrying out some measurements.