الفهرس 
Corrosion inhibitionOnly 14 pages are availabe for public view
 |  | from | 280 |  |  |
| | | from | 280 | | |
Abstract
The thesis comprises three main chapters: The first chapter contains an introduction about the electrochemical nature and kinetics of corrosion. Corrosion inhibitors were defiled and - systematically classified. Experimental observation of corrosion Inhibition and the effect of inhibitors on polarization behaviour were defined. Adsorption isotherms were discussed together with the effect of temperature on the action of inhibitors. The synergistic effect of anions on the acidic corrosion inhibition of iron and a literature survey on inhibition of iron corrosion in acidic and alkaline media were reviewed. Chapter I1 deals with the experimental chemical ( weight-loss and thermometric ) and electrocl~emical ( potentiodynarnic ) techniques used for measuring the corrosion rate and various electrochemical corrosion parameters of iron metal immersed in nitric, sulphuric acids ( 2M ) and in 2M sodium hydroxide solutions in the absence and in the presence of the tested hydantoin compounds. The investigated compounds employed in this study as corrosion inhibitors were: ( i ) Phenylhydantoin compounds: 3 -etllylcarboxy-1 -phenylhydantoin ( A ) 3 -11ydroxymethyl- 1 -phenylhydantoin ( B ) 5-benzylidenehydantoin ( C ) 1 - phenylhydantoin ( D ) ( ii ) Thiohydantoin compounds: 5-methyl-2-th~ohydantoin ( E ) 1,3-dimethyl-5-phenylazo-2- thiohydantoin ( F ) ( iii ) Dithiohydantoin compounds: 5,5-dimethyl-2,4- dithiohydantoin ( G ) In the experimental part a complete description of apparatus, working procedure and preparation of solutions was given. Electrode preparation, electrolytic cell and polarization measurements were described. Methods of calculating corrosion currents (icorr), corrosion potentials ( Eco,, ), corrosion rates (Rco,) and degree of surface coverage ( 0 ) were also indicated. Chapter III presents the results and discussion of the data obtained. These data are reported in ( 49 tables ) and illustrated in ( 127 figures ). This chapter is devided into four parts as follows: I ) Inhibition of the iron corrosion by hydantoin substituted compounds in nitric acid solution, I1 ) Inhibition of the iron corrosion by hydantoin substituted compounds in sulphuric acid solution, 111 ) Mubition of the iron corrosion by hydantoin substituted compounds in sodium hydroxide solution, and IV ) The synergistic effect between some anions and the hydantoin derivatives for Inhibition of iron corrosion. Weight-loss, thennometric and polarization measurements revealed that the presence of the tested hydantoin compounds in acidic and alkaline solutions retard the corrosion of iron and the Inhibition efficiency depends on the type and concentration of the idubitors. The action of the hydantoin inhibitors in the aggressive acidic and alkaline media was assumed to be due to their adsorption at the metal / solution interface. The order of increasing inhibition efficiency for the investigated hydantoin compounds either in acidic or in alkaline solutions was as follows: A > B > C > D > G > E > F The hgher action of compound ( A ) than the other derivatives was attributed to the presence of an extra ethylcarboxy group which increases the electron density on the molecule and provides additional active center of adsorption through the oxygen atom. Moreover, increased molecular size besides to the presence of two nitrogen adsorption sites enhances better coverage at the iron1 solution interface. Also, compound ( A ) could form insoluble complexes with iron especially at higher concentrations, this increases covering of the iron su-fkce with an orderly arranged adsorbed layer. Ths could be the main reason for the higher dubition effect of compound ( A ) than the other derivatives. Although compound ( F ) has three active centers ( three nitrogen atoms ). It shows, the lowest mhibition efficiency percentage compared with all the tested hydantoin derivatives. This can be explained on the basis that, compound ( F ) could form soluble complex with iron which hampers covering of the iron surface. This interpretation is supported experimentally by spectrphotometric measurements. The three thecniques applied (weight-loss, thermometric and polarization measurements ) gave consistent results. The higher inhibition efficiency of the investigated compounds in acidic than in alkaline media may be due to the less negative potential of iron in acidic solutions, favouring adsorption of additive. The adsorption of these inhibitors were found to obey Ternkin adsorption isotherm. A kinetic relationship of the corrosion reaction in the presence of the hydantoin inhibitors ( A - G ) was deduced by examining the reaction data. A computer programme was used to fit the obtained experimental data with the proper adsorption isotherm equations. The programme , facilitated the computations of the number of active sites, the equilibrium constant of the adsorption process ( B ), binding constant ( K), the change in the fiee energy of adsorption (-AG’,~, ) and the lateral interaction (-f ) between molecules. Comparison between the results obtained •’tom the model and those obtained using the well known adsorption isotherm has been examined in details. For the tested hydantoin inhibitors it was found that the proposed kinetic model and Temkin adsorption isotherm fit well the experimental data. The mechanism of corrosion of iron in presence of the hydantoin inhibitors was investigated by potentiodynamic polarization measurements. The results indicated that all the compounds act as mixed - type inhibitors in acid media, whereas, act as cathodic type inhibitors in alkaline solutions and that the corrosion current ( i,,,, ) decreases as their concentration increases. The results obtained from weight-loss and thermometric measurements were in excellent agreement. A kinetic relation between the cathodic current density and concentration of the inhibitors similar to that proposed for weight-loss and thermometric was found valid. The experimental results indicate that the presence of adsorbed inhibitors on iron surface does not affect the mechanism of hydrogen evolution. Skeletal representation of the proposed mode of adsorption of the investigated hydantoin compounds was shown and the order of inhibition efficiency was clarified in its terms. The adsorption of organic molecules over the metal surface takes place through already adsorbed solution anions. The present study revealed that nitrogen quaternary hydantoin salt compounds behave as more effective corrosion inhibitors in acid media.