الفهرس 
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Abstract
The aim of the present work is to study the electrochemical behavior of C-steel in phosphoric acid solution in the absence and presence of some selected inhibitors.
This thesis contains three main chapters;
The first chapter: Deals with the following fields of interest
1) General aspects of corrosion.
2) Corrosion inhibitors.
3) Adsorption isotherm.
4) Literature survey on corrosion of C-steel.
5) Aim of the present work.
The second chapter: Deals with the experiment part
It includes the chemical composition of the investigated material, preparation of phosphoric acid, the used inhibitors solutions and the experimental techniques contain the instruments and the procedures used for the corrosion measurements.
The third chapter: Deals with the results obtained and their discussion under three separated sections, (A), (B) and (C).
Section A:
Contains the results of potentiodynamic polarization measurements for C-steel in 2 N H3PO4 solutions containing different concentrations of first and second group inhibitors. The results revealed that the inhibition
efficiency increases with increasing the concentration of these compounds. The order of increased inhibition efficiency for the additives is:
First group:
(B) > (A) Second group:
(2) > (1)
Potentiodynamic polarization studies were shown that the investigated compounds are mixed type inhibitors. The results showed that the adsorption of first group compounds and compound (2) obeyed to Langmuir adsorption isotherm while compound (1) obeyed to Temkin adsorption isotherm. The effect of temperature on the corrosion rate of C- steel in 2 N H3PO4 solutions over the temperature range 25-55oC in the absence and presence of selected inhibitors have been studied.
Arrhenius plots of logarithm corrosion current (log icorr) against reciprocal of absolute temperature (1/T) were found to be linear and
obeyed the following equation:
log k = log A – E *
/ 2.303 RT
where k is corrosion current. Thermodynamic activation parameters (ΔH* and ΔS*) are calculated and discussed. The inhibition efficiency was found to decrease with rise in temperature.
Section B:
Contains the results of electrochemical impedance spectroscopy
measurements for C-steel in 2 N H3PO4 solutions in the absence and presence of different concentrations of first and second group inhibitors. The results revealed that the inhibition efficiency increaseswith increasing the concentration of these compounds. The presence of these compounds in the solution decrease the double layer capacitance and increase the charge transfer resistance. The order of increased inhibition efficiency for the additives is:
First group: (B) > (A)
Second group: (2) > (1)
Section C:
In order to support experimental data, theoretical calculations were conducted in order to provide molecular- level understanding of the observed experimental behavior.
Quantum chemical calculations were used to predict the efficiency of the investigated compounds as corrosion inhibitors. The inhibition efficiency of first group inhibitors increases with decreasing negative values of the highest occupied molecular orbital (EHOMO) and energy gap (∆E) but for second group inhibitors the inhibition efficiency increases with decreasing ∆E and increasing dipole moments. The order of increased inhibition efficiency for the additives is:
First group: (B) > (A)
Second group: (2) > (1)
This is in a good agreement with the experimental data obtained by potentiodynamic polarization and EIS techniques.
In conclusion, potentiodynamic polarization and electrochemical impedance spectroscopy measurements support the assumption that corrosion inhibition primarily takes place through adsorption of the selected inhibitors on C-steel surface.
Agreement among these different techniques indicates the validity of the obtained results.