الفهرس 
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Abstract
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Summary
The thesis consists of three chapters. Chapter I is the introduction, Chapter II represents the experimental techniques, and Chapter III contains the results and their discussion.
Chapter I: Introduction, in this chapter the following fields of interest were given: electrochemical nature of corrosion, potential of zero charge, structure and properties of the electrical double layer (E.D.L), forms of corrosion of aluminium, passivity and its theories, pickling of aluminium, corrosion inhibitors and its classification, factors affecting the adsorption process, effect of anion on the corrosion of aluminium in acidic solutions, adsorption isotherms, literature survey on inhibition of the acidic corrosion of aluminium metal. The aim of the work was also given in this part.
Chapter II: Experimental, in this chapter the experimental techniques were obtained: Electrochemical impedance spectroscopy (EIS), potentiodynamic polarization techniques and optical microscopic method. The detailed description of electrodes, solutions and their preparation, electrochemical cell, the main chemical constituents of plants extracts (damsissa, Lupine and Halfabar), the method of extraction and experimental procedure were included. .
Chapter III: Results and Discussion, in this chapter the experimental results were discussed in five parts:
Part III.1: Represents the corrosion behavior of aluminium in 0.1 M HCl, 0.1 M HClO4 and 0.05 M H2SO4 solutions using potentiodynamic polarization and electrochemical impedance spectroscopy techniques. The results revealed the following :
The polarization curves indicated that the acids have a predominate effect on the anodic polarization curves, the anodic Tafel lines showed activation behavior where active dissolution of aluminium takes place followed by breakdown potential in case of HCl. The rate of corrosion of aluminium decreased in the order: HCl > HClO4 > H2SO4.
The Nyquist impedance plots showed that the impedance response consisted of depressed capacitive semicircle. The experimental data were analyzed by fitting the data to the equivalent circuit model. The results showed that Rct increased in the order: HCl < HClO4 < H2SO4.
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Part III.2: Represents the effect of Damsissa, Halfabar and Lupine as corrosion inhibitors for aluminium in 0.1M solutions of HCl, HClO4 and 0.05 M H2SO4.
III.2.a. Effect of Damsissa, Halfabar and Lupine as inhibitors for the corrosion of aluminium in 0.1 M HCl.
The results showed that:
The anodic polarization curve with no additives showed that on going from cathodic towards more anodic potential, an active region of dissolution of aluminium was recorded followed by break down potential Eb at -775 mV indicating pitting corrosion as a result of the aggressive attack of Cl- anions. The behavior of different plant extracts was generally in similar manner, the break down potential Eb is shifted to more noble value by increasing extracts concentration. These extracts act predominately as anodic inhibitors.
The Nyquist impedance plots showed only one depressed capacitive semicircle. The diameter of the semicircle increased with increasing the extracts concentrations.
Kinetic-Thermodynamic model is applicable to fit the data of all plant extracts. According to the numerical values of K obtained from this model, the inhibition efficiency of the extracts for the corrosion of aluminium in 0.1 M HCl decreased in the order:
Damsissa > Lupine ≈ Halfabar
III.2.b. Effect of Damsissa, Halfabar and Lupine as inhibitors for the corrosion of aluminium in 0.1 M HClO4
The results showed that:
The polarization curves showed that these extracts affect both the anodic and the cathodic polarization curves indicating that these extracts act as mixed type inhibitors.
The Nyquist impedance plots showed only one depressed capacitive semicircle. The size of the semicircle increases with increasing concentration of Damsissa, Halfabar and Lupine extracts.
The Kinetic-Thermodynamic model was found to fit the data of all plant extracts. According to the numerical values of K obtained from this model, the inhibition efficiency of different plant extracts decreased in the order:
Lupine > Damsissa > Halfabar
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III.2.c. Effect of Damsissa, Halfabar and lupine as corrosion inhibitors for
aluminium in 0.05 M H2SO4.
The results showed that:
The polarization curves showed that the Damsissa do not show any significant effect on the shape of the polarization curves indicating that this extract has no significant effect on corrosion behaviour of aluminium. On contrast, Halfabar enhances the corrosion attack on aluminium by affect the hydrogen evolution reaction and consequantly increase the corrosion rate. On the other hand, Lupine affect both the cathodic and anodic Tafel lines indicating that lupine may act as mixed type inhibitor.
The Nyquist impedance plots showed that only one depressed capacitive semicircle. Damsissa has no significant effect on the depressed capacitive semicircle by increasing concentration. In contrast, increasing the concentration of Halfabar extract, decrease the diameter of the capacitive semicircle. On the other hand, the diameter of the semicircle increase with increasing the lupine extract concentration.
The Kinetic-Thermodynamic model is found to be applicable for lupine extract. According to the numerical values of K obtained from the model, the adsorption of lupine extract on aluminium surface is very weak compared to other acids.
Part III.3: Represents the role of acid anion on the inhibition of the acidic corrosion of aluminum by lupine extract.
Potentiodynamic polarization curves for aluminum in 0.1 M HCl, 0.1 M HClO4 and 0.05 M H2SO4 acids in the absence and presence of different lupine extract concentrations show that the addition of lupine affect both the cathodic and anodic Tafel lines indicating that lupine extract act as mixed type inhibitor.
The Nyquist impedance plots for aluminium in 0.1 M HCl, 0.1 M HClO4 and 0.05 M H2SO4 acids in the absence and presence of different lupine extract concentrations showed that the dissolution process of aluminium is under activation control. The impedance response consisted of semicircles of capacitive type whose size increased with increasing lupine concentration.
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The percentage inhibition values obtained from both the polarization and impedance measurements confirming that the inhibition action of lupine extract for aluminium corrosion in acid media is decreasing in the order:
HClO4 > HCl > H2SO4
The Kinetic-Thermodynamic model is found to be applicable for lupine extract indifferent acids.
Part III.4. Represents the effect of etching on the inhibition efficiency of different plant extracts on the corrosion of aluminium in 0.1 M HCl and 0.1 M HClO4.
III.4.a. Inhibition of the Corrosion of etched Aluminium in 0.1 M HCl by lupine, damsissa and halfabar.
The results showed that:
Potentiodynamic anodic polarization curves showed activation behaviour for etched aluminium followed by break down potential Eb in absence and presence of Damsissa, Halfabar and Lupine extracts indicating pitting corrosion as a result of the aggressive attack of Cl- anions. All examined extracts shift the corrosion potential towards less negative values indicating that these extracts act predominately as anodic inhibitors.
The Nyquist impedance plots for etched aluminium in the absence and presence of different concentrations of Damsissa, Halfabar and Lupine extracts showed only one depressed capacitive semicircle. The inhibition efficiency increased with increasing extract concentration which indicates that the Damsissa, Halfabar and Lupine extracts act as corrosion inhibitors for aluminium.
The Kinetic-Thermodynamic model was found to fit the data of all plant extracts for both etched and non-etched aluminum. According to the numerical values of K obtained from the model, stronger adsorption of the inhibitors on to the non-etched aluminium surface compared to that etched one.
III.4.b. Inhibition of the Corrosion of etched Aluminium in 0.1 M HClO4 by lupine, damsissa and halfabar.
The results showed that:
Potentiodynamic anodic polarization curves showed activation behaviour for etched aluminium in absence and presence of Damsissa, Halfabar and Lupine extracts. All examined extracts shift the corrosion potential towards less negative values indicating that these extracts act predominately as anodic inhibitors.
The Nyquist impedance plots for etched aluminium in the absence and presence of different concentrations of Damsissa, Halfabar and Lupine extracts showed only one depressed capacitive semicircle. The inhibition efficiency increased with increasing extract concentration which indicates that the Damsissa, Halfabar and Lupine extracts act as corrosion inhibitors for aluminium.
The Kinetic-Thermodynamic model was found to fit the data of all plant extracts for both etched and non-etched aluminium. According to the numerical values of K obtained from the model, stronger adsorption of the inhibitors on to the non-etched aluminium surface compared to etched one.
Part III.5: Represents the Optical microscopy studies of the etched and non-etched aluminium surface in absence and presence of damsissa extract in 0.1 M HCl.
The results showed that the surface photographs for non-etched aluminium have clearly visible polishing marks on the surface either before or after dipping in acid free or acid containing the extract. The photographs obtained after surface pre-treatment by alkaline etching indicated that the etching process reveals the surface to be a porous-like structure and both the acid and extract has limited effect on the size of pores. These results indicate that the number of the active sites of the etched aluminium surface is less than those for non-etched one. This argument explains the data obtained from the polarization and impedance measurements which indicate that the inhibition efficiency and the value of the binding constant K of extract is higher in case of non-etched aluminium.