Synergistic effect of KI on the corrosion inhibition efficiency of 3-acetylpyridine phenylhydrazone (3APPH) on carbon steel (CS) in 0.5 M sulphuric acid solution has been investigated using gravimetric studies, electrochemical impedance spectroscopy (EIS), and potentiodynamic polarization studies. Gravimetric corrosion studies revealed that 3APPH showed moderate corrosion inhibition efficiency up to 8 h and beyond this period it showed corrosion accelerating behavior. This antagonistic effect of 3APPH is due to the hydrolysis of the molecule in acidic medium. A very high percentage of inhibition efficiency at 24 h was obtained on the addition of KI due to the synergistic effect of iodide ions. The adsorption of 3APPH and 3APPH + KI on the surfaces of the corroding metal obey Langmuir isotherm as obtained by impedance measurements. Polarization studies revealed that 3APPH act as a mixed type inhibitor. Thermodynamic parameters (
Industrial processes such as pickling and descaling will escalate the corrosion rate of metals enormously and the addition of certain organic compounds which contain
The heterocyclic phenylhydrazone (3APPH) was obtained by the condensation of equimolar mixture of 3-acetylpyridine and phenylhydrazine hydrochloride in ethanol. The reaction mixture was refluxed for 2 hours, cooled to obtain yellow precipitate, filtered, washed with water, and dried. Figure
Molecular structure of 3APPH.
The aggressive solution of 0.5 M H2SO4 was prepared by the dilution of A.R grade 98% of H2SO4 (Merck) with deionized water. Solutions of 3APPH were prepared in the range of 0.2 mM–1.0 mM concentrations in 0.5 M H2SO4.
Carbon steel specimens of dimension
The impedance measurements were performed in a three electrode assembly. Saturated calomel electrode (SCE) was used as the reference electrode. Platinum electrode having 1 cm2 area was taken as the counter electrode. Metal specimens with an exposed area of 1 cm2 were used as the working electrode. The EIS experiments were carried out on an Ivium compactstat-e electrochemical system. 0.5 M H2SO4 acid was taken as the electrolyte and the working area of the metal specimens were exposed to the electrolyte for 30 minutes prior to the measurement. Impedance measurements were performed at constant potential (OCP) in the frequency range from 1 KHz to 100 mHz with an amplitude of 10 mV as excitation signal. The percentage of inhibition from impedance measurements were calculated using charge transfer resistance by the following expression [
Electrochemical polarization studies of CS specimens in 0.5 M H2SO4 with and without 3APPH and 3APPH + KI were performed by recording anodic and cathodic potentiodynamic polarization curves. Polarization plots were obtained in the electrode potential range from −100 to +100 mV versus corrosion potential (
Surface analyses of CS specimens were performed using scanning electron microscope (model Hitachi SU6600). SEM images of CS surface were taken by treating 0.5 M H2SO4 solutions in the absence and presence of the 3APPH and 3APPH (0.8 mM) + KI (0.2 mM) for 24 h at 30°C.
Weight loss of CS specimens in 0.5 M H2SO4 at 30°C was determined at 24 h in the presence of various concentrations of 3APPH and 3APPH + KI (0.2 mM). The corrosion rates and inhibition efficiencies are listed in Table
Inhibition efficiency (
System/conc. | Corrosion rate ( |
% of inhibition efficiency ( |
---|---|---|
Blank | 23.53 | — |
Blank + 0.2 mM KI | 19.40 | 24.0 |
0.2 mM 3APPH | 27.83 | −18.28 |
0.4 mM 3APPH | 32.67 | −38.83 |
0.6 mM 3APPH | 44.77 | −90.26 |
0.8 mM 3APPH | 45.31 | −92.54 |
1.0 mM 3APPH | 45.48 | −93.27 |
0.2 mM 3APPH + 0.2 mM KI | 2.84 | 87.90 |
0.4 mM 3APPH + 0.2 mM KI | 2.50 | 89.38 |
0.6 mM 3APPH + 0.2 mM KI | 1.06 | 95.47 |
0.8 mM 3APPH + 0.2 mM KI | 0.99 | 95.80 |
1.0 mM 3APPH + 0.2 mM KI | 0.82 | 96.50 |
Comparison between the corrosion inhibition efficiencies of various systems.
To emphasize the role of iodide ions on the corrosion inhibition of CS by 3APPH, impedance study was performed by exposing the corroding metal surface towards the 3APPH and 3APPH + KI solutions for 30 minutes. Figures
Nyquist plots for CS specimens in 0.5 M H2SO4 and 3APPH.
Nyquist plots for CS specimens in 0.5 M H2SO4 and 3APPH + KI.
Equivalent circuit fitting for EIS.
The impedance of CPE can be expressed as
Electrochemical impedance parameters of CS specimens in 0.5 M H2SO4 at 30°C in the absence and presence of 3APPH (a) and 3APPH + 0.2 mM KI (b).
|
|
|
|
---|---|---|---|
0 | 9.72 | 133 | — |
0.2 | 19.54 | 134 | 50.25 |
0.4 | 22.77 | 121 | 57.31 |
0.6 | 24.09 | 119 | 59.65 |
0.8 | 41.39 | 118 | 76.51 |
1.0 | 48.22 | 116 | 79.84 |
|
|
|
|
---|---|---|---|
Blank + KI | 12.21 | 128 | 20.39 |
0.2 + KI | 518.4 | 28.9 | 98.13 |
0.4 + KI | 1071 | 26.2 | 99.09 |
0.6 + KI | 1301 | 20.2 | 99.25 |
0.8 + KI | 1499 | 21.5 | 99.35 |
1.0 + KI | 1607 | 14.3 | 99.39 |
In order to reveal the mechanism of inhibition, various adsorption isotherms such as Langmuir, Freundlich, Temkin, and Frumkin were plotted and the most suitable one was selected with the aid of correlation coefficient. Among the isotherms considered above, the most adequate one for both experiments, that is, impedance measurements in the presence and absence of KI, was Langmuir adsorption isotherm. Figures
Thermodynamic parameters from adsorption isotherms.
System | Adsorption isotherm |
|
|
---|---|---|---|
3APPH | Langmuir |
|
−30.9 |
3APPH + KI | Langmuir |
|
−43.17 |
Langmuir adsorption isotherm for CS specimens in 0.5 M H2SO4 and 3APPH.
Langmuir adsorption isotherm for CS specimens in 0.5 M H2SO4 and 3APPH + KI.
It is evident from Table
Potentiodynamic polarization curves for 3APPH in 0.5 M H2SO4 at 30°C for CS specimens in the presence of various concentrations of 3APPH and 3APPH + KI are shown in Figures
Potentiodynamic polarization parameters of CS specimens in 0.5 M H2SO4 at 30°C in the absence and presence of 3APPH.
|
|
|
|
|
|
---|---|---|---|---|---|
Blank | −468 | 1041 | 84 | 122 | — |
0.2 | −461 | 591 | 67 | 131 | 43.1 |
0.4 | −464 | 432 | 63 | 123 | 58.5 |
0.6 | −455 | 395 | 68 | 124 | 62.0 |
0.8 | −465 | 255 | 54 | 114 | 75.5 |
1.0 | −462 | 175 | 46 | 106 | 83.2 |
Potentiodynamic polarization parameters of CS specimens in 0.5 M H2SO4 at 30°C in the presence of KI and 3APPH + KI.
|
|
|
|
|
|
---|---|---|---|---|---|
Blank + KI | −475 | 799 | 80 | 123 | 23.3 |
0.2 + KI | −507 | 18.8 | 85 | 102 | 98.2 |
0.4 + KI | −540 | 12.6 | 118 | 121 | 98.8 |
0.6 + KI | −526 | 11.8 | 117 | 105 | 98.9 |
0.8 + KI | −515 | 8.9 | 102 | 119 | 99.1 |
1.0 + KI | −534 | 8.5 | 127 | 106 | 99.2 |
Tafel plots of CS specimens in 0.5 M H2SO4 at 30°C, with and without 3APPH.
Tafel plots of CS specimens in 0.5 M H2SO4 at 30°C, with KI and 3APPH + KI.
To examine the effect of 3APPH molecules on CS surface, SEM analyses were performed [
(a) SEM image of bare CS surface (b) SEM image of CS surface in 0.5 M H2SO4, (c) SEM image of CS surface in 0.5 M H2SO4 and 3APPH (0.8 mM), and (d) SEM image of CS surface in 0.5 M H2SO4 and 3APPH (0.8 mM) + 0.2 mM KI.
3APPH undergo slow hydrolysis in sulphuric acid medium and showed moderate/poor corrosion inhibition efficiency up to 8 h. Beyond this period, 3APPH shows corrosion antagonistic character. In the presence of KI, the 3APPH molecules are firmly attached on the CS surface and do not undergo hydrolysis and act as excellent corrosion inhibitor. Adsorption studies revealed that chemical interaction occurs between the 3APPH molecules and CS surface trough I− ions. This causes the enhancing of the inhibition efficiency greatly. Parent ketone and parent amine exhibit poorer corrosion inhibition efficiency than the 3APPH molecules. Polarization studies reveal that 3APPH act as a mixed type inhibitor.