الفهرس 
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Abstract
In the present study the effectiveness of four inhibitors namely 2-mercaptobenzimidazole, 2-mercaptobenzoxazole, isatin and thiourea to inhibit the general corrosion of low alloy steel ASTM A213 grade T22 in 0.5 M HCl solution at different temperatures were investigated.
The present thesis comprises five chapters:
The first chapter includes a literature survey on the types and uses of low alloy steels in the boilers and thermal liquid heater generate heat for use on other operations. Also this chapter contains a short review about the meaning and types of corrosion especially the pitting corrosion, the types of corrosion inhibitors and theories of corrosion protection.
The second chapter is concerned with the experimental work, the electrodes (working electrode, counter electrode and reference electrode), electrolytic cell, solutions, procedures and instruments used in different parts of this thesis. The electrochemical techniques for investigation and characterization of low alloy steel were described.
The third chapter contains the results and their discussion. It divided into four parts:
Part (I) is concerned with the studying of pitting corrosion of T22 low alloy steel in different concentrations of HCl solution and at four different temperatures values (298-333 K). The effect of different concentrations of HCl solution on the corrosion rate of T22 low alloy steel has been studied using open circuit potential and potentiodynamic techniques. The results indicate that an increase in HCl concentration increases the corrosion rate. The adsorbed Cl- ions participate directly in the ionization of metal atoms. These results were in good agreement with scanning electron microscopy (SEM).
Part (II) is concerned with the studying of pitting corrosion inhibition of T22 low alloy steel in HCl solution by using different concentrations of two organic inhibitors, 2-mercaptobenzimidazole and 2-mercapto- benzoxazole at four different temperature values (298-333 K). The effect of these compounds on the corrosion of T22 low alloy steel in HCl solution has been studied using open circuit potential, potentiodynamic and SEM techniques. The addition of increasing concentrations of these two compounds decrease the corrosion rate at one temperature value, indicating corrosion inhibition effect, while the corrosion rate increases as the temperature increases. The inhibition efficiency became in the following order MBI > MBO. The adsorption characteristics of these inhibitors on the surface of T22 low alloy steel play a significant role in the inhibition process so thermodynamic activation parameters (Ea , ΔS°, ΔH°) of the corrosion process and thermodynamic adsorption parameters (ΔG°ads, ΔS°ads, ΔH°ads) of the adsorption process have been calculated. The results obtained from potentiodynamic and SEM was in a good agreement with each other.
Part (III) is concerned with the studying of pitting corrosion inhibition of T22 low alloy steel in HCl solution by using different concentrations of isatin at four different temperature values (298-333 K). The effect of isatin on the corrosion of T22 low alloy steel in HCl solution has been studied using open circuit potential, potentiodynamic and SEM techniques. The addition of increasing concentrations of isatin decrease the corrosion rate at one temperature value, indicating corrosion inhibition effect, while the corrosion rate increases as the temperature increases. The adsorption characteristics of this inhibitor on the surface of T22 low alloy steel play a significant role in the inhibition process so thermodynamic activation parameters (Ea , ΔS°, ΔH°) of the corrosion process and thermodynamic adsorption parameters (ΔG°ads, ΔS°ads, ΔH°ads) of the adsorption process have been calculated. The results obtained from potentiodynamic and SEM was in a good agreement with each other.
Part (IV) is concerned with the studying of pitting corrosion inhibition of T22 low alloy steel in HCl solution by using different concentrations of thiourea at four different temperature values (298-333 K) . The effect of thiourea on the corrosion of T22 low alloy steel in HCl solution has been studied using open circuit potential, potentiodynamic and SEM techniques. The addition of increasing concentrations of thiourea decrease the corrosion rate at one temperature value, indicating corrosion inhibition effect, while the corrosion rate increases as the temperature increases. The adsorption characteristics of this inhibitor on the surface of T22 low alloy steel play a significant role in the inhibition process so thermodynamic activation parameters (Ea , ΔS°, ΔH°) of the corrosion process and thermodynamic adsorption parameters (ΔG°ads, ΔS°ads, ΔH°ads) of the adsorption process have been calculated. The results obtained from potentiodynamic and SEM was in a good agreement with each other.
The fourth chapter contains the summery and conclusions.
The fifth chapter contains a list of references.
The principle conclusions are:
1. The corrosion rate of T22 low alloy steel increases with increasing the HCl concentration and temperature.
2. The used four inhibitors have been shown good inhibiting properties for T22 low alloy steel in 0.5 M HCl solution.
3. The inhibition efficiency for all inhibitors increases with increase of inhibitor concentration but decreases with the increase of solution temperature.
4. Polarization curves in 0.5 M HCl solution with absence and presence of the four inhibitors indicated that these compounds act as mixed type inhibitors; they increase both the anodic and cathodic overpotentials and shift the position of the Tafel lines to both directions. This means that these compounds suppress both the cathodic evolution of oxygen and anodic dissolution of the alloy. Therefore, there is no effect on the corrosion mechanism and so the adsorbed species exert their action by simple blocking of the anodic and cathodic active sites of the steel surface and hence act as blocking type inhibitors.
5. Thermodynamic activation parameters for corrosion process of HCl in absence and presence of the four tested inhibitors were calculated from the data obtained from potentiodynamic polarization curves at four temperatures and different concentrations of the inhibitors. The activation parameters showed higher activation energies and enthalpies indicative of the high protection efficiency as the concentration of inhibitor increased.
6. The adsorption of the three inhibitors (2-mercaptobenzimidazole, 2-mercaptobenzoxazole and isatin) on the steel surface in 0.5 M HCl solution gave a good fit on Langmuir isotherm, but the adsorption of thiourea on steel surface in 0.5 M HCl gave a good fit on Temkin isotherm. The adsorption parameters for corrosion process of HCl in absence and presence of the four tested inhibitors were calculated.
(a) The values of the equilibrium constant of adsorption process (Kads) for the four inhibitors are relatively small, decrease with the temperature indicative of weaker binding to the metal surface and hence a lower inhibition efficiency at higher temperatures.
(b) The values of the standard free energy of adsorption (ΔG°ads) acquire a negative sign ensure the spontaneity of the adsorption process and stability of the adsorbed layer on the steel surface. Also the values of (ΔG°ads) are less than -40 KJmol-1 indicating that the adsorption mechanism of the four inhibitors on the steel surface in 0.5 M HCl solution take place via electrostatic interaction between the adsorbent and adsorbate (typical physisorption).
(c) The negative values of enthalpy of adsorption (ΔH°ads) show that the adsorption of the four inhibitors is an exothermic process which indicates that inhibition efficiencies decrease with increasing temperature.
(d) The negative values of entropy of adsorption(ΔS°ads) for isatin and thiourea show that before the adsorption of inhibitors on the steel surface, the inhibitor molecules might freely move in the bulk solution, but with the progress of the adsorption, inhibitor molecules were orderly adsorbed onto the steel surface, as a result, a decrease in entropy. The positive value of (ΔS°ads ) for 2-mercaptobenzimidazole and 2-mercaptobenzoxazole can be interpreted on basis that the disordering of desorbed water molecules (which follow the adsorption of inhibitor molecules) overweights the ordering resulting from adsorption of inhibitors on the metal surface.
7. Surface morphological of the steel surface confirms the existence of a protective adsorbed film at the low alloy steel surface. These data support the results obtained from electrochemical method that the four compounds are good inhibitors for T22 low alloy steel in HCl solution. The difference in inhibitors efficiencies is probably associated with the difference in chemical structure.