الفهرس 
IntroductionOnly 14 pages are availabe for public view
 |  | from | 201 |  |  |
| | | from | 201 | | |
Abstract
The present work was aimed to study the corrosion protection of AA6061 in naturally aerated aqueous solutions namely Na2HPO4 and NaClby using nonionic surfactant (polysorbate 80) and cerium conversion,two cerium salts (cerium chlorideand cerium sulfate).
The present thesis contains three chapters:
Chapter 1
This chapter contains the introduction and the aim of the work. The introduction deals with the following topics: corrosion of metals and alloys, types of corrosion, corrosion protection, corrosion inhibition, rare earth metals, Egyptian sources of monazite and recent literature survey concerning this subject.
Chapter 2
This chapter deals with electrodes, solutions, equipments used. The corrosion evaluation was performed by electrochemical methods (potentiodynamic polarization, cyclic voltammetry, chronoamperometry, Electrochemical impedance spectroscopy (EIS) and complemented with EDX and SEM examination). Methods of of preparation and characterization of CeCl3 and Ce2(SO4)3 from Egyptian monazite.
Chapter 3
This chapter deals with the results and discussion, and can be divided into five parts as follows:
Part 1
The corrosion behavior of AA6061 in 1M Na2HPO4 solution in the absence and presence of polysorbate 80 was investigated under different
182
experimental conditions.It is found that the addition of polysorbate 80 to the phosphate solution inhibits the alloy corrosion.
Polysorbate 80 acts as a cathodic inhibitor.
The corrosion inhibition is due to physical adsorption of the inhibitor molecules on the alloy surface.
The inhibition efficiency enhances with increasing the inhibitor concentration but decreases with rising temperature.
Langmuir and Flory–Huggins adsorption isotherms fit well the experimental data.
Some thermodynamic functions of alloy dissolution and inhibitor adsorption were calculated and the calculated data reveal that the corrosion process is endothermic process and the adsorption is spontaneous and exothermic process.
Part 2
The corrosion behavior of AA6061 in 3.5% NaCl solution in the absence and presence of polysorbate 80 was investigatedby different electrochemical techniques under different experimental conditions.
The presence of this inhibitor in the chloride solution mitigates the uniform corrosion of the alloy.
Polysorbate 80 acts as a cathodic inhibitor.
The inhibition action of this inhibitor is ascribed to physical adsorption of the inhibitor molecules on the alloy surface forming a barrier film separating the alloy surface from the corrosive medium.
The pitting corrosion of the alloy is due to local dissolution and removal of the passive Al2O3 film especially at defects and thin areas of the passive film by the aggressive action of adsorbed Cl¯ ions.
183
The inhibition function of the surfactant for both uniform and pitting corrosion increases with increasing its concentration but decreases with rising temperature.
Some thermodynamic parameters of dissolution and adsorption processes were calculated and discussed.
Part 3
Addition of CeCl3 or Ce2(SO4)3to 1M Na2HPO4 solution inhibits the alloy corrosion.The inhibition is due to the formation of a mixture Ce2O3/Ce(OH)3 conversion film on the electrode surface as a result of the partial cathodic reaction.
In this medium, reduction of dissolved oxygen in the solution is the mean partial cathodic reaction at active cathodic sites. This reduction reaction yields OH¯ ions which react with Ce (III) in the solution to give cerium conversion film on the electrode surface which blocks the active cathodic sites. These events retard the rate ofcathodic reaction and consequently the rate of corrosion reaction.
These two cerium salts act as cathodic inhibitors. Their inhibition efficiency increases with increasing their concentration and with ageing in the phosphate solution but decreases with rising temperature.
In all cases, Ce2(SO4)3provides better corrosion inhibition than CeCl3.
Part 4
Addition of CeCl3 or Ce2(SO4)3to NaCl solution inhibits the AA6061 corrosion. The inhibition in both cases is attributed tothe formation of a conversion formed by Ce2O3/Ce(OH)3coat covers the electrode surface. This film blocks the active cathodic sites and reduces the rate of cathodic reaction and consequently the uniform corrosion rate of the alloy.
184
The inhibition function of thesecerium salts increases with increasing their concentration, concentration of NaCl and the immersion time but decreases with rising temperature.
The two cerium salts also inhibit the initiation and the growth of pitting corrosion. The polarization data showed larger widening of passivity field (Ecorr – Epit) in the presence of the inhibitor indicating an increase in pitting corrosion resistance.
Part 5
In this part, extraction of two Cerium salts (CeCl3 and Ce2(SO4)3) from Egyptian monazite was carried out using Solvent extraction technique under different experimental conditions to achieve the optimum extraction conditions. The preparedtwo salts were characterized by different techniques. As an example, the prepared Ce2(SO4)3 were tested as a corrosion inhibitor for the used aluminum alloy in the two corrosive media. The obtained results reveal that the prepared cerium salts extracted from Egyptian monazite can be used as green non toxic corrosion inhibitors for AA6061 alloy in aqueous solutions.