Apparatus and Procedure

The cell and electrical circuit used in the present work showed in previous work [6]. The cell consists of a rectangular plastic container having the dimensions 5.1×5×10 cm with electrodes fill-in the whole cross section. The electrodes were rectangular copper sheets of 10 cm height and 5 cm width. Electrode separation was 5 cm. A porous PVC diaphragm was used to prevent the effect due to H2 bubbles. The electrical circuit consists of 6V D.C. power supply, while a voltmeter is connected in parallel to the cell to measure the voltage and multirange ammeter connected in series with cell to measure the current.

Measurements of the Limiting Current

Polarization curves were obtained byincreasing the cell current step wise and the steady state anode potential against a reference electrode consisted of a wire immersed in a cup ofLuggin tubefilled with phosphoric acid concentration similar to that in the cell, the tip of the Luggin tube was placed 0.5-from the anode. The potential difference between the anode and the reference was measured by high impedance potentiometer. Five phosphoric acid concentrations (6, 8, 10, 12 and 14 M) were prepared from Analar grade phosphoric acid distilled water. The anode height varied from 1-5 cm. Before each run, the back part of the anode was insulated with polystyrene lacquers and the active surface of the anode was polished with fine emery paper, decreased with trichloroethylene, washed with alcohol and finally rinsed in distilled water. Electrode treatment was similar to that used by Wilke [18]. Organic acid concentration were 10^-5 - 10^-2 mol l^-1. The rate of electropolishing of copper was determined at 25 °C.

Results and Discussion

Effect of alcohol concentration on the limiting current density

The observed limiting current density, which represents the rate of copper metal corrosion in phosphoric acid at 25 °C , decrease with increasing the concentration of the organic alcohols under study.

Effect of alcohol concentration on the limiting current density

It found that the limiting current decreases with increasing the concentration of alcohols. From the practical point of view and on the basis of results obtained the uses of alcohols studied in the concentration range between (2, 5, 10, 15, 20, 25 and 30 %) inhibit the corrosion of copper metal in 8 M H3PO4acid [19, 20]. The limiting current in the absence of organic alcohols (I), and in the presence of organic alcohols (IL), the percentage inhibition can be calculated from the following equation:

Adsorption Isotherm

The adsorption of the inhibitor molecules from aqueous solutions can be regarded as a substitution adsorption process between the organic compounds in the aqueous phase and water molecules adsorbed on the electrode surface (H2O(s)).

where x (the size ratio) is the number of water molecules displaced byone molecule of inhibitor. The above process reaches equilibrium when the chemical potential on the left hand equal to that of the right hand side.

Adsorption isotherms are very important in determining the mechanism of electrochemical reactions. The most frequently used isotherms are those of Langmuir . Frumkin, Parsons, Temkin, Flory-Huggins, and Bockris-Swinkels [21].

These entire isotherms are of the general form:

where f(θ, x) is the configuration factor which depends essentially on the physical model and assumption underlying the derivation of the isotherm. The mechanism of inhibition is generally believed to be due to the formation and maintenance of a protective film on the metal surface [22,23].

Application of Isotherm

The degree of surface coverage (θ) at constant temperature was determined from the following equation[24,25].

Inhibitor adsorption characteristics can be estimated by using the Langmuir isotherm given by the following equation:

The langmuir adsorption isotherm for copper electrode in H3PO4plotted as θ /1-θ against C at 25 °C which give as a straight line that does not pass from the origin, i.e. Langmuir isotherm does not verify.

A straight line is obtained with a slope X and intercept log × K. The experimental data fit the Flory-Huggins adsorption isotherm, which represented by:

where x is the number of water molecules replaced by one molecule of the inhibitor. It is clear that, the surface coverage data are useful for discussing adsorption characteristics. The adsorption of inhibitors at the metal-solution interface may be due to the formation of electrostatic or covalent bonding between the adsorbs and the metal surface [26]. The kinetic adsorption isotherm maybe written in the form [27]:

From equation (6) a plot of log θ /1-θ against log C should yield a straight line with intercept of log K' and slope = y, where y is the number of inhibitory molecules occupy one active site. The binding constant of adsorption K= k' l/y, where 1/y is the number of the active surface sites occupied by one molecule of the inhibitor, and k is the binding constant.

And from this table, it is obvious that the value of 1/y for all compounds are higher than one indicating that, the given inhibitors molecules are attached to more than one active site. The free energy of adsorption (ΔG_ads). At a different concentration of the alcohols as calculated from the following equation:

In all cases, the (ΔG_ads) values are negative and lie in the range of 10.2 - 16.1 KJ/mol. The most efficient inhibitor shows the most negative (ΔG_ads) value. This suggests that, they are strongly adsorbed on metal surfaces. The negative values of (ΔG_ads) indicate that the spontaneous ion of the inhibitors. It's found that (ΔG_ads) values are more positive than (-40) indicating that the inhibitors are physically adsorbed on the metal surface. Similar results have also been reported by Talati [27].

Effect of temperature and thermodynamic treatment of the reaction

The effect of temperature on the Cu electro polishing rate in absence and presence of organic additives was determined in the temperature ranges between (25 - 40 °C). It was observe that the electro polishing rate increases with temperature for different concentrations of organic additives. Values of Ea that have been derived from the slopes of Arrhenius [28].

It is obviously seen that the Ea values in the absence and presence of organic additives are less than 40 kJ mol^-1, also indicating that the diffusion processes are controlling the electropolishing reaction [29]. In this research, the thermodynamic parameters such as change in free energy ΔG^*, enthalpy ΔH^* and entropy ΔS^* were calculated in the same way as the related researches did in literature [30-33]. From transition state equation [37] a straight line was obtained, from which can ΔH^* and ΔS^* be calculated from the slope and intercept, respectively. The free energy change, ΔG^*, can be represented as follows:

The result indicated by that the tested compounds acted as inhibitors through adsorption on copper surface, which resulted in the formation of a barrier to mass and charge transfer. The values of ΔH^* reflect the strong adsorption of these compounds on copper surface. The negative values of ΔS^* pointed to a greater order produced during the process of activation.

This can be achieved by the formation of an activated complex representing the association or fixation with consequent loss in the degree of freedom of the system during the process. ΔG^* values showed a limited increase with a rise in the concentration of organic additives revealing that weak dependence of ΔG^* on the composition of the organic additives can be attributed largely to the general linear composition between ΔH^* and ΔS^* for the given temperature [28, 34].

The operator, δ, concerns the difference between any two reactions in the series. Substituting from the equation (11) into the familiar relationship:

we obtain

These values are much lower than that of the experimental temperature 298 K indicating that the rate of the reaction is Entropy controlled.

Effect of dielectric constant

The absence of linearity suggests a large differential effect of solvent structure acting on the initial and transition states. This was shown from the extension of the equation of Laidler and Landskroaner [37], which allows changes in solvent structure with varying solvent composition [38].

Solvent effect on limiting current

Protic solvent: The variation of the physicochemical properties of the medium such as viscosity, density and dielectric constant, as a result of adding organic solvents, were expected to affect the rate of mass transfer process by affecting the diffusivity and the activity of the metal ions. The rate of corrosion of copper was studied in methanol, ethanol, propanol and isopropanol-H2O mixtures of different compositions at 298-313 K. The addition of organic solvent maychange the reactive species existing in solution by changing the composition of the salvation shell. Copper ions are solvated with water molecules to form [Cu (H2O)4]2+ in aqueous solution, alcohol molecules mayreplace some of the ligated water molecules and this would affect the mobility of copper ion [39], but in many cases this cannot be separated from the effect of the viscosity of the solvent. Another explanation for the decrease in the rate of corrosion would be that the addition of alcohol has a marked effect on increasing the basicity of the medium. The relative affinities of H2O or the non aqueous component toward Cu2+ may be expected to arise from the relative salvation capacities of solvent dipoles exerted largely through hydrogen bond formation. The strength of hydrogen bonding should be largely guided by the relative charge densities on the oxygen and hydrogen charge centers of the isolated dipoles. To a first approximation, the isolated dipoles of H2O and ROH, indicated by Feakins [40] can be represented as follows:

Owing to the inductive effect of the R-group in alcohols, the negative charge density on the oxygen Atom (RΔ-) is presumably somewhat greater than the corresponding quantity in water (wΔ-). As a result, the protonic character of the H atom of -OH group in alcohols (Rδ+) is weaker than that in water (wδ+), i.e., RΔ- > wΔ- and Rδ+ > wδ+. Furthermore, alcohol molecule is also expected to induce, in water molecule hydrogen bonded to it, a positive charge on the hydrogen atom. These effects are likely to be relayed over several molecular diameters, rendering all molecules in methanol, ethanol, propanol or isopropanol -H2O mixtures more basic and less acidic than in pure water. The viscosity is high because the relatively large number of hydrogen bonds would require more energy to be ruptured in the formation of the activated state. The inhibitory action of alcohols depends on stress effect taking part in the absorption of the inhibitor molecule and their electron charge density, molecular size of such alcohols. The order of inhibition efficiencyfor the alcohols is

isopropanol > n-propanol > ethanol > methanol.

Conclusions

Electropolishing of copper in phosphoric acid is considered as corrosion rate of copper. It is measured by measuring the limiting current . The rate of corrosion decreases by adding of organic alcohols ( methanol, ethanol, propanol, and isopropanol). The rate of inhibition ranged from 8.7 -53.8%, depending on the type of the alcohols and its concentration. The corrosion rate increases by increases the temperature and the activation energy of the reaction is less than 40 kJ; i.e., the reaction is diffusion controlled. All alcohols verifyLangmuir, Flory-Huggins and Kinetic Adsorption Isotherm.

References