الفهرس 
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Abstract
SUMMARY : Metal corrosion is a great problem, which faced the world from the last years until now. We cannot hide this problem from our live but we can reduce “inhibit” it by several methods as the environment need. -This work discusses the corrosion of C-steel in 0.5M H2SO4. -This work contains three basic chapters: Chapter one: “INTRODUCTION” This chapter discusses :Definition of corrosion ,the corrosion theory ,classification of corrosion ,forms of corrosion ,cost of corrosion, factors influencing corrosion ,methods of protection, type of inhibitors ,electrochemical methods, corrosion inhibitors and literature survey on corrosion inhibition of C-steel. Chapter two: “EXPERIMENTAL AND TECHNIQUES ” It includes the chemical composition of the C-steel ,the preparation of the test specimens for the different techniques of measurements and methods of preparation of materials of various solutions used ,preparation of the used sulphuric acid solution ,preparation of the used plant extracts, solutions and procedures used for the corrosion measurements such as a weight loss and electrochemical techniques (potentiodynamic, EIS, EFM) and surface analysis (FT-IR, AFM). Chapter three :”RESULTS AND DISCUSSION” This chapter is divided into three parts: Part(1) The first one is the chemical techniques which make the evaluation of the inhibitor efficiency by the weight loss method for investigated extracts (50-300ppm) in 0.5 M H2SO4 at (25-45OC) revealed that the inhibitor efficiency increases with the increasing of the concentration. This study also confirmed that the corrosion inhibition occurred through chemical adsorption of the additives on C-steel surface. Including the calculation and discussion of thermodynamic kinetic parameters (ΔH*, Ea*, ΔS*) of the investigated inhibitors on C-steel surface by plotting various Transition –state and Arrhenius curves. Also these plant extracts obey Temkin adsorption isotherm. The degree of surface coverage (θ) for the inhibitors on the metal surface increases with increasing the concentration in the investigated inhibitors. The action of the inhibitors in the aggressive acid was assumed to due to their adsorption at the metal / solution interface. The results were treated to obtain the thermodynamic parameters for the adsorption of inhibitors from Temkin isotherm model and kinetic-thermodynamic model (ΔG°ads , ΔH°ads , ΔSo ads ) and the adsorption constant kads .This results indicating that the inhibition occurs through chemical adsorption of the additives on C-steel surface.The negative values of ΔG°ads indicate the corrosion of C-steel in 0.5 M H2SO4 is spontaneous. These studies approved that: Citrus Sinensis and Ecballium extracts are good green inhibitors. Part(2) 1. Potentiodynamic polarization measurements were carried out for C-steel in 0.5M H2SO4 in the absence and presence of different concentrations of Citrus Sinensis and Ecballium extracts. The polarization curves indicated that these plant extracts are a mixed type inhibitors which inhibit both the cathodic and anodic processes. Corrosion rate decreased with increasing the concentration of the plant extracts and together with increase in both cathodic and anodic polarization, but the corrosion inhibition has a great effect on the cathodic polarization .Variation of inhibitor efficiency with the structure of Citrus Sinensis and Ecballium extracts was interpreted in terms of the number of adsorption sites in the molecule and their electron charge density ,molecular size, mode of adsorption and the polar effect of the substituent groups. 2. AC impedance spectroscopy measurements were carried out for C-steel in 0.5MH2SO4 in the absence and presence of different concentrations of the investigated inhibitors under charge transfer control. The %IE obtained from EIS measurements are close to those deduced from polarization. 3. The electrochemical frequency modulation (EFM) technique was used as a rapid and non-destructive technique for corrosion rate measurements. It was carried out for C-steel in 0.5 MH2SO4 in the absence and presence of different concentrations of the investigated plant extracts. Corrosion current densities (icorr) obtained with this technique was in good agreement with those obtained from weight-loss , polarization and EIS methods. In addition of the causality factors were good internal check for verifying of data obtained by this technique. The corrosion rate decrease with increasing the concentration of the inhibitors and the inhibition efficiency (%IE) is found to increase with increasing the inhibitors concentrations. Part (3): Surface examination of C-steel includes the following methods: 1. AFM spectroscopy used for investigation the roughness of C-steel surface in the non-existence and existence of the plant extracts (300 ppm) .The surface roughness decreased in the presence of the inhibitors. 2. The metal specimen was analyzed by ATR-FTIR technique. Chemical bonds and Functional groups in a molecule had been detected by generate an infrared spectrum. This also confirmed the presence of chemical adsorption of the inhibitors on the C-steel surface. In conclusion : The weight loss, polarization, electrochemical impedance spectroscopy (EIS) and EFM measurements support the assumption that corrosion inhibition primarily takes place through adsorption of the inhibitors on the C-steel surface. Agreement amoung these different independent techniques indicates the validity of the obtained results. This thesis also contains references, Arabic and English summaries.