The subject of this thesis consisted of two main parts: _1_- Which dealt with the interaction of gaseous chloride with evaporated metal films of Ca, Cd, Sn, Pb, Sb and Bi. The experiments have been out in a high vacuum glass apparatus capable of obtaining and maintaining pressures as low as 10-6 Nm-². Each film was sintered at as high temperature as possible (343 K) for 30 min. and its surface area was determined from Physical adsorption of krypton at 78 K. Adsorption and incorporation of HCl gas in the pressure range 1-10 Nm-² have been carried out on the films in successive stages in the temperature range 193K 473K. Analysis of the gas phase was carried out using three different techniques involving : (i) Mass- Spectromatric analysis of the gas phase mixture using a Quadrupole Mass Spectrometer - partial pressure gauge , (ii) a pre -calibrated piranl gauge, and (iii) by condensing HCl gas at 78K and measuring the pressure of the remaining gaseous hydrogen. An attempt was made to estimate on theoretical treatments the hermodynamic functions of HCl adsorption on the metals. The kinetics of HCl absorption on the metals have also been studied by measuring the rates of adsorption at several temperatures in the experimental temperature range.2.Part 2- Which dealt with the potentiostatic investigation of the corrosion of the Metals (excluding Ca) in oxygen-free O.I M HC1 solution over the temperature range 298-313K. The potential scan started one hour after the specimen Immersion in the acid solution begining at -l.O V and proceeded through to 1.0 V versus a saturated calomel electrode. A potentiodynamic potential sweep rate of 15mV min-¹ was used recording the current density continuously with the change of potential.The corrosion current densities (ia) and corrosion potentials (Ea ), the cathodic (ba ) and anodic (ba) Tafel slopes have been derived from the polarization curves of the various metals in the HC1 solutions. The cathodic (aa)and anodic (aa ) transfer coefficients have been calculated from the corresponding cathodic and anodic Tafel slopes. Other informations regarding the critical, passivity and pitting behaviours of Sn, Pb, Sb,and Bi metals have been erived from the appropriate polarization curves of those metals. Data have been derived
concerning the polarization resistance (Rp) and the equilibrium current density (i о) of each metal from low- field polarization plots of the metals. The measured corrosion potentials and current densities enabled a thorough investigation of the thermodynamic and kinetic aspects of the metal corrosion in the HC1 solution. An attempt was made thereafter to compare, on thermo-dynamic and kinetic grounds, the behaviours of the metals towards the gaseous HC1 with those for the metals towards aqueous HC1. It was thus possible to derive information concerning the influence of water in changing the tendency and the rate of HC1 reactions with the metal surfaces. Adsorption and extensive incorporation of hydrogen chloride occurred on the metal
films over the temperatures 193 to 473K. The interaction of HC1 in most cases was accompanied by the evolution of hydrogen but a part of the hydrogen of the adsorbed HC1 was retained by the metals. The rate-determining step was considered to be the adsorption and the subsequent dissociation of the chemi-sorbed HC1 molecules, subsequently some surface atoms recombined to form gaseous hydrogen and the surface chlorine with some hydrogen atoms were also Incorporated. Kinetic data for the adsorption of HC1 on the metal films indicated the direct dependence of the adsorption rate on HC1 pressure. A compensation effect was found to operate throughout the interaction of gaseous HC1 with the metal films; the variation of the rate was attributed to simultaneous variation of the activation energy (Ea ) and the pre-exponential factor (A) in the Arrhenius equation. The compensation effect was likely to arise from the presence of energetically heterogeneous reaction sites on the metal surfaces. A higher rate of HC1 interaction was found to occur on Ti, Zn, Cd and Pb metals as compared with the rate of interaction with Ca, Sn, Sb, and Bi. The thermodynamic data indicated the feasibility of HC1 interaction with the metals and that adsorption was substantially exothermic. The results of the corrosion experiments showed a shift of the corrosion potentials towards more negative values with the rise of temperatures except in the case of Ti metal where the shift was in less negative direction. The corrosion current densities generally increased with the rise of temperature reflecting the increasing rate of metal corrosion with increasing temperature. The linear sections of the cathodic Tafel lines shifted almost parallel to each other, and n similar behaviour was found for the linear sections of the anodic Tafel lines. The Tafel lines maintained their linearity and slope at all the experimental temperatures suggesting no alterations in the mechanisms of the cathodic and anodic reactions with the variation of temperature. The values of the Tafel slopes or of the corresponding transfer coefficients suggested that proton discharge at the cathode and the metal dissolution at the anode were the prevailing rate-determining steps of the reactions which occurred at the cathode/solution and anode/ solution interfaces respectively. Four of the metals (Sn, Pb, Sb, and Bi) showed passivity at some or all the experimental temperatures. The values of the resistance polarization and exchange current density varied with temperature. The behaviours of the metals accordingly were interpretated on the basis of the extent of the polarization of the metal/solution races which were located adjacent to the metal electrodes. Thermodynamics of metal corrosion have been from their corrosion potentials and the resulting data showed generally the feasibility of the corrosion reactions and that the formation of the corrosion products was companied by the decrease of both the enthalpy and entropy of corrosion. The kinetic data showed the operation of a compensation effect in the corrosion reaction of Sn, Pb, Sb and Bi metals, and the operation of anticompensation mechanism in corrosion reaction of Ti, Zn and Cd metals. The reactivity of the metals for corrosion in HC1 -Ion, as deduced from the kinetic results, followed the sequence Zn > Cd > Sb > Sn > Bi > Pb > Ti Zn was the most and Ti the least reactive metals in the corrosion reactions. A comparision of the reactivity of the metals towards gaseous HC1 was made with that in the aqueous HC1 on both thermodynamic and kinetic results, and the derived formation revealed the following: 1. The thermodynamic data showed the increasing tendency and the greater feasibility of the corrosion reaction in aqueous HC1 as compared with the metals interaction with gaseous HC1 Ti metal was the only metal to show a reverse behaviour. 2. A comparison on kinetic grounds indicated a higher reaction rates of the metals in the aqueous HCl than with gaseous HCl. Pb and Ti metals showed slower rates of corrosion in aqueous HCl than with gaseous HCl. The not results of the thermodynamlc and the kinetic comparison results showed that water molecules enhanced the reactivity of Zn, Cd> Sn> Sb and Bi metals for interaction with HC1. The wet corrosion of these metals proceeded considerably faster than their dry corrosion. The reverse of this conclusion was true in the case of Ti and Pb metals.