الفهرس 
METALLIC CORROSIONOnly 14 pages are availabe for public view
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Abstract
The present thesis includes studies on the corrosion and corrosion inhibition of zinc and nickel in some aqueous solutions under the influence of various experimental conditions using different electrochemical methods. The thesis contains the following basic chapters.
Chapter I ”Introduction”
The introduction contains brief account of corrosion of metals, classification of corrosion, pitting corrosion, corrosion inhibitors and types of inhibitors. Importance of zinc and nickel. Literature survey on the corrosion and corrosion inhibition of zinc and nickel in different media.
Chapter II ”Experimental”
This chapter includes the nature of electrodes, solutions, instruments and procedures used in potentiodynamic polarization, potentiostatic current-time transient, EIS and SEM measurements.
Chapter III” Results and Discussion”
This chapter is divided into three parts.
Part III.1:
1- General corrosion of zinc in different concentrations of NaOH solutions ,free from or containing different concentrations of NaClO4 was investigated by means of potentiodynamic polarization and potentiostatic current- time transient. The effect of addition of various concentrations of Na2MoO4 and Na2Si2O5 as inorganic inhibitors on corrosion behaviour of zinc also was examined.
2- Presence of ClO-4 in NaOH solutions enhances the corrosion current density(jcorr) and shifts the corrosion potential (Ecorr) towards more negative direction. The value of (jcorr) and hence the rate of corrosion of zinc in NaOH solutions increases with increasing ClO-4 concentration but decreases with increasing NaOH concentration.
3- Addition of the cited inorganic inhibitors decreases the values of jcorr and hence the rate of corrosion of zinc in NaOH solutions contaning ClO-4. The inhibition action of these inorganic inhibitors enhances with increasing their concentrations.
4-The anodic responses of zinc reveal that the presence of ClO-4 in NaOH solutions can induces pitting corrosion of zinc at certain critical pitting potentials(Epit). The susceptibility of zinc towards pitting corrosion increases with increasing ClO-4 concentration and applied anodic potential but decreases with increasing NaOH concentration.
5- Addition of the cited inorganic inhibitors to NaOH solution containing ClO-4 enhances the pitting corrosion resistance of zinc to an extent depends upon the nature and concentration of each inhibitor added.
Part III.2:
1- The general and pitting corrosion of zinc in sodium gluconate solutions was studied by various electrochemical methods under different experimental conditions.
2- The values of( jcorr) and hence the rate of corrosion of zinc increases with increasing the concentration of gluconate ions and temperature.
3- The anodic excursion curves of zinc in gluconate solutions exhibit active/passive transition. The active dissolution region involves one anodic peak.
4- The passivity is attributed to the formation of ZnO film on the anode surface. The passive region extends up to a certain critical pitting potential (Epit) at which initiation and propagation of pitting corrosion occurs.
5- The values of Epit shifts to more negative (active) direction with increasing gluconate ion concentration and temperature but shifts to less negative direction with increasing scan rate.
6- Potentiostatic current- time transients reveal that an incubation time (ti) is necessary before pit initiation. The rate of pit initiation(nucleation) (ti-1) increases with increasing gluconate ion concentration, temperature and applied anodic potential.
7- The inhibiting functions of sodium tungstate, sodium molybdate and sodium silicate on anodic behaviour and pitting corrosion of zinc in 0.1M sodium gluconate and at 20oC was investigated. The inhibition function of these anions decreases in the order :
Si2O2-5 > MoO2-4 > WO2-4
Part III.3:
1- The general corrosion behavior of nickel was studied in 1.0M H2SO4 solutions in absence and presence of different concentrations of citric acid and at different temperatures, by means of potentiodynamic polarization and impedance techniques.
2- Addition of citric acid to H2SO4 solutions inhibits the corrosion of nickel. The corrosion inhibition occurs through physical adsorption of citrate anions on positively charged nickel surface without modifying the mechanism of corrosion process.
3- The data reveal that citrate anions act as mainly as anodic type inhibitor and its inhibition efficiency increases with increasing its concentration and immersion time, but decreases with rising temperature.
4- Thermodynamic activation parameters (Ea, ∆Ha and ∆Sa) for dissolution of zinc in 1.0 M H2SO4 in the absence and presence of different concentration of citrate anion and temperatures were calculated. The values of Ea and ∆Ha in the inhibited solution are higher than those in the uninhibited solution indicating that more energy barrier for corrosion process in the presence of the inhibitor is attained. The positive sign for ∆Ha reflects the endothermic nature of corrosion process.
5- The adsorption of citrate anions on nickel surface in sulphuric acid solution follows Temkin adsorption isotherm.
6- The values of adsorption equilibrium constant (Kads) and free energy of adsorption (∆Gads) confirm the suggestion that adsorption of citrate anions on nickel surface is mainly physiorption and spontaneous.
7- The results obtained from potentiodynamic polarization and (EIS) techniques are compatible and run in parallel.