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Abstract
The aim of the present work is to study the reactivity of some compounds as corrosion inhibitors for aluminium in phosphoric acid solution. The thesis comprises three main chapters.
The first chapter:
Deals with the Introduction, which includes corrosion theories, types of corrosion, corrosion protection, literature survey on corrosion behavior of aluminium in acidic solutions and aim of the present work.
The second chapter:
Deals with the experimental techniques, which includes the chemical composition of the investigated material and preparation of the used solutions.
Also the experimental techniques contain the instruments and the procedures used for the corrosion measurements such as weight loss and galvanostatic polarization techniques.
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 weight loss measurements for aluminium in 1M H3PO4 containing different concentrations of investigated compounds. These results revealed that these compounds behave similarly and the weight loss is generally decreases with increasing the concentration of these compounds and also depends upon the nature and type of the investigated compounds. The order of decreasing inhibition efficiency of these compounds is:
i- For the first group compounds
III > II > I
ii- For the second group compounds
VI >V > IV
The synergistic effect of potassium iodide, potassium bromide and potassium chloride was examined by addition of 1x10-2M of KI, KBr and KCl, individually with different concentrations of the selected compounds to enhance the inhibitive effect of these compounds. The values of Synergism parameters, S, was also calculated and discussed.
A number of mathematical relationships for the adsorption isotherms have been suggested to fit the experiment data of the present work. The simplest equation that fit our results is:
• For the first and second groups compounds:
The simplest equation that fits our results is that due to Freundlish and is given by the general equation:
θ = K Cn
Thermodynamic parameters for the adsorption of the first and second group compounds in 1M H3PO4 on aluminium surface were also calculated.
The effect of temperature on the corrosion rate of aluminium in 1M H3PO4 over the temperature range (30-50oC) in absence and presence of different concentrations of the investigated compounds has been studied. The % inhibition efficiency is found to decrease with increasing the temperature; this indicated that, these compounds are physically adsorbed on the aluminium surfaces. Arrhenius plots of logarithm corrosion rate (log k) against reciprocal of absolute temperature (1/T) were found to be linear and obeyed the following equation:
Log k = log A - (Ea* / 2.303 RT )
The values of the activation energy are calculated in absence and presence of different concentrations of the investigated compounds and are found to decrease with increasing the concentration of these compounds.
Plots of logarithm corrosion rate divided by absolute temperature (log k/T) against reciprocal of absolute temperature (1/T) were found to be linear and obeyed the following transition state equation:
Rate = RT/ Nh exp (S*/ R) exp (-H*/ RT)
Thermodynamic activation parameters (ΔH* and ΔS*) are also computed and discussed. The values of the activation energy, Ea*, and the activation enthalpy, ΔH*, are increased with increasing inhibitor concentration while the value of the activation entropy, ΔS*, is decreased at the same time.
Section B:
Contains the results of galvanostatic polarization measurements for aluminium in 1M H3PO4 in absence and presence of different concentrations of the investigated compounds. The polarization curves indicated that these compounds influence both cathodic and anodic processes. The order of decreasing inhibition efficiency for the additives is:
i- For the first group compounds:
III > II > I
ii- For the second group compounds:
VI >V > IV
This is also in agreement with the observed order of corrosion inhibition determined by the weight loss technique.
The results obtained from this technique give further support to that obtained from the previously mentioned weight loss measurements.
Section C:
The influence of the chemical structure of the used compounds on their % inhibition efficiencies was discussed. The order of these % inhibition efficiencies depends mainly upon the type and kind of the substituents groups, skeletal representation of the molecules supported this examination.
In conclusion the polarization and weight loss measurements support the assumption that corrosion inhibition primarily takes place through adsorption of the inhibitors on aluminium surface. Agreement among these different independent techniques indicates the validity of the obtained results.
This thesis contains also references, Arabic and English summaries.