The use of inorganic inhibitors as an alternative to organic compounds is based on the possibility of degradation of organic compounds with time and temperature. The inhibition effect of potassium iodide on the corrosion of pure iron in 0.5 M H2SO4 has been studied by weight loss. It has been observed from the results that the inhibition efficiency (IE%) of KI increases from 82.17% to 97.51% with the increase in inhibitor concentration from 1·10−4 to 2·10−3 M. The apparent activation energy (
Corrosion is the deterioration of materials by chemical interaction with their environment [
The weight loss experiments were conducted in a 150 mL beaker; the electrolyte volume is 100 mL. Julabo thermostat brand keeps the electrolyte at the desired temperature (±0.1°C).
The test pieces were mechanically polished with emery paper (a coarse paper was used initially and then progressively finer grades were employed, 400 to 1200 grade). The specimens were weighed by electronic digital analytical balance with five decimal accuracies before and after exposure.
The corrosive solution, 0.5 M H2SO4, was obtained by dilution of analytical grade 98% sulphuric acid with bidistilled water. The concentration range of inhibitor employed was 1·10−4 to 2·10−3 M in the sulphuric acid.
The test pieces were washed with bidistilled water, degreased with acetone, washed again with bidistilled water, dried between two filter papers, and weighed. After specified periods of time, 3 test pieces were taken out of the test solution, rinsed with bidistilled water, dried as before, and weighed again. The average weight loss at a certain time for each set of three samples was taken. The weight loss experiments were performed after an exposure of 2 h. The inhibition efficiency of potassium iodide was expressed in terms of percentage inhibition, calculated using
The corrosion rate (
The degree of surface coverage (Θ) was calculated using the following equation:
The values of percentage inhibition efficiency (IE%) and corrosion rate (
Corrosion parameters for pure iron in aqueous solution of 0.5 M H2SO4 in presence and absence of different concentrations of KI at 303 K for 2 h.
|
|
IE (%) | |
---|---|---|---|
H2SO4 | 0.5 | 1.60383 | — |
|
|||
KI |
|
0.28593 | 82.17 |
|
0.10956 | 93.16 | |
|
0.07787 | 95.15 | |
|
0.05445 | 96.60 | |
|
0.04169 | 97.11 | |
|
0.03986 | 97.51 |
Variation of inhibition efficiency with KI concentration.
Figure
Plot of corrosion rate versus inhibitor concentration for pure iron test specimens after 2 hrs of exposure.
The weight loss measurements were performed in 0.5 M H2SO4 in absence and presence of KI at 2·10−3 M concentration for 30 min to 6 h immersion time at temperature of 303 K. Inhibition efficiencies were plotted against immersion time as seen in Figure
Variation of inhibition efficiency of KI with immersion time.
In order to study the effect of temperature on the inhibition efficiencies of potassium iodide, weight loss measurements were carried out in the temperature range 293–323 K in absence and presence of inhibitor at optimum concentration during 2 hours of immersion. Table
Effect of temperature on pure iron in the presence and absence of KI, at 2 h.
|
|
|
IE (%) |
---|---|---|---|
293 | 0.15107 | 0.00622 | 95.88 |
303 | 0.34152 | 0.01329 | 96.10 |
313 | 1.32256 | 0.05035 | 96.19 |
323 | 3.43109 | 0.11280 | 96.71 |
The apparent activation energy
The plots of
The values of activation parameters for pure iron in 0.5 M H2SO4 in the absence and the presence of inhibitor of 2·10−3 M concentration at 2 h.
Concentration of inhibitor (M) |
|
|
|
---|---|---|---|
0.5 H2SO4 | 84.32 | 81.65 | 2.67 |
|
78.89 | 76.22 | 2.67 |
Adsorption isotherm plots for ln
A plot of
Adsorption isotherm plots for
The
Adsorption isotherms provide information about the interaction of the adsorbed molecules with the electrode surface [
Relationship between (
The
The intercept permits the calculation of the equilibrium constant
Generally, the standard free energy values of −20 kJ/mol or less negative are associated with an electrostatic interaction between charged molecules and charged metal surface (physical adsorption); those of −40 kJ/mol or more negative involves charge sharing or transfer from the inhibitor molecules to the metal surface to form a co-ordinate covalent bond (chemical adsorption).
Note that our measures were carried out without inert atmosphere. In 1 M H+, the iodide ion becomes hydrogen iodide which reacts with oxygen to form molecule iodide I2. The formed molecule could be adsorbed onto iron and occupying the surface, it strikes the adsorption of iodide. At the end, a lower efficiency of adsorption of iodide was obtained but the inhibition efficiency of the studied inhibitor KI increases due to adsorption of I2.
Otherwise the value of
On the basis of the experimental results obtained in the present study, the following conclusions can be drawn. Potassium iodide is a good inhibitor for pure iron corrosion in 0.5 M H2SO4 solution. The inhibition efficiency increases with increased KI concentration to attain a maximum value of 97.51% at 2·10−3 M. The adsorption of KI on pure iron obeyed Langmuir adsorption isotherm. At higher experimental temperature, inhibitor molecules are adsorbed into the metal surface. The negative value of
The authors declare that there is no conflict of interests regarding the publication of this paper.