Corrosion Inhibition of Iron in 0 . 5 mol L-1 H 2 SO 4 by Halide Ions

O efeito de inibição de íons haletos, tais como iodeto, brometo e cloreto, na corrosão de ferro em solução 0,5 mol L de H 2 SO 4 , e o comportamento da adsorção desses íons na superfície do eletrodo, foram estudados pelos métodos de polarização e de impedância. Foi observada uma inibição de aproximadamente 90% para íons iodeto a 2,5 × 10 mol L e para íons brometo a 10 × 10 mol L, e de 80% para íons cloreto a 2,5 × 10 mol L. O efeito da inibição aumenta com o aumento da concentração dos íons haletos I e Br, mas decresce no caso do Cl, para concentrações maiores que 5 ×10 mol L. Os valores de capacitância de dupla camada diminuíram consideravelmente na presença dos íons haletos, o que indicou que esses ânions são adsorvidos no ferro no potencial de corrosão.


Introduction
2][3][4][5][6] It is well known that the dissolution of iron in H 2 SO 4 solutions occurs in four different states viz.active, passive, transpassive and brightening states as determined by the nature and kinetics of reaction involved, which depend on the potential and electrolyte composition.Combined adsorption of anions and cations together on the surface have also been studied.Electrostatic interaction is the main reason for the specific adsorption of anions on metal surface. 7,8Possibility of chemisorption of the anions on metal by the formation of a covalent type bond is also suggested by Grahame. 9Quantum chemical calculations have been used to describe the chemisorption of the halide ions on the metal electrode surface by the formation of partial charge transfer bonds. 10Pearson suggested that the stability of the anion adsorption bond over metal surface should resemble to hard and soft acids and bases principle if the adsorption occurs by forming a donoracceptor type bond. 11,12The specific adsorption behaviour of some of the anions on metal electrode surface and their effects on corrosion have been qualititatively related to this HSAB principle. 13,146][17][18][19][20] Due to the complex physico -chemical reaction involved, the mechanism and kinetics of passivity and pitting intiation are not fully understood.2][23][24][25][26][27][28][29][30] It is reported that, the aggressive anions like Cl -, Br -and I -are found to catalyze the iron dissolution reaction in higher concentrations. 31But some studies [32][33][34][35][36] have shown that halide ions in lower concentrations inhibit the corrosion of iron in sulphuric acid.However the adsorption characteristics of halide ions on iron surface has not been well established.Hence a study has been made to find the adsorption characteristics of halide ions on the iron surface in 0.5 mol L -1 H 2 SO 4 and the effect of adsorbed halide ions on corrosion.

Experimental
Experiments were made using a conventional three electrode cell assembly at 28 ± 1 o C. The working electrode was a pure iron sample (99.99% purity, Johnsons Mattey Ltd., UK) of one cm 2 area with the rest being covered with araldite epoxy and a large rectangular platinum foil was used as counter electrode and saturated calomel electrode as reference electrode.The reference electrode was connected to the main cell through a luggin capillary in order to avoid the contamination of 0.5 mol L -1 H 2 SO 4 with chloride ions.The working electrode was polished with 1/0 to 4/0 grades of emery papers, washed with water and degreased with trichloroethylene.All solutions were prepared using AR grade chemicals using triple distilled water and deaerated by purging purified nitrogen for half an hour before the start of the experiment under stationary condition.Solartron Electrochemical analyzer (Model 1280 B) interface with an IBM computer and Corrware and Z plot corrosion software were used for data acquisition and analysis.The polarization and impedance studies were made after 30 minutes of immersion since the specimen attained a steady state potential (± 0.005 V).The polarization was carried out using a Corrware software from a cathodic potential of -0.2V to an anodic potential of +0.2 V with respect to the corrosion potential at a sweep rate of 0.5 mV s -1 .E vs log I curves were plotted.The linear tafel segments of the anodic and cathodic curves were extrapolated to corrosion potential to obtain the corrosion current densities.For linear polarization resistance measurements, polarization was done from -0.020 V to + 0.020 V with respect to corrosion potential at a sweep rate of 0.5 mV s -1 and the slope of the linear segment at corrosion potential was obtained as polarization resistance R p .AC signals of 10 mV amplitude and a frequency spectrum from 10 KHz to 0.01 Hz was impressed and the Nyquist representations of the impedance data were analysed with Zview software using the following equivalent circuit due to the presence of single semi circle in the impedance diagram.
where R s is the solution resistance, R ct is the charge transfer resistance and C dl is the double layer capacitance.
The diameter of the semicircle was measured as the charge transfer resistance R ct .For Tafel polarization method, the corrosion inhibition efficiency was evaluated from the measured i corr values using the relationship where i corr and i corr ' are the corrosion current densities without and with the addition of halide ions.The inhibition efficiencies were evaluated from the measured R p values in linear polarization resistance method as Where R p and R p ' are the polarization resistance values without and with the addition of halide ions.In the impedance method, the inhibition efficiency was evaluated from the measured charge transfer resistance R ct values as where R ct and R ct ' are the charge transfer resistance values in the absence and presence of halide ions. [39]

Iodide ions
The potentiodynamic polarisation behaviour of iron in 0.5 mol L -1 H 2 SO 4 without and with the addition of iodide ions is shown in Figure 1.The corrosion kinetic parameters derived from these curves are presented in Table 1.From the table it is clear that the addition of iodide ions in the concentration range 0.5 × 10 -3 to 5 × 10 -3 mol L -1 markably reduces the dissolution rate of iron in 0.5 mol L -1 H 2 SO 4 .The corrosion current density, i corr , decreases from 410 μA cm -2 for the inhibitor free solution to 46 μA cm -2 .Beyond 5 × 10 -3 mol L -1 concentration, the increase of iodide ion concentration leads to a slight increase in corrosion current density.The steady state corrosion potential E corr shifts to the more anodic value.This shows that iodide ions act as anodic inhibitor.
The Nyquist representation of the impedance values of the iron in 0.5 mol L -1 H 2 SO 4 with and without the addition of iodide ions is shown in Figure 2. The existence of a single semi circle depicts the presence of single charge transfer process during dissolution which is unaffected by the presence of halide ions.The slightly depressed nature of the semi circle which has the center below the real axis indicates the generation of micro roughness surface heterogeneities at the surface during the corrosion process. 40,41The charge transfer resistance R ct and the interfacial double layer capacitance C dl derived from these curves are given in Table 2.
It is observed that the R ct values increase from 33 Ω cm 2 to 288 Ω cm 2 at 2.5 ×10 -3 mol L -1 KI where the highest inhibition efficiency of 89% is observed.The C dl values are also decreased from 2244 μF cm -2 in the presence of iodide ions.The higher surface capacitance values for iron is due to the micro roughness of the surface during corrosion process.Similar higher values of 1775 μF cm -2 , 42 750 μF cm -2 43 and 1504 μF cm -2 44 have been reported for iron in 0.5 mol L -1 H 2 SO 4 solutions.Typical linear polarization curves for iron in 0.5 mol L -1 H 2 SO 4 with and without the addition of various concentrations of iodide ions is shown in Figure 3.The slope of these curves, the polarization resistance R p showed an increase values from 34 Ω cm 2 to 327 Ω cm 2 and then decreases with the further addition of iodide ions (Table 2).As observed in the case of potentiodynamic method, linear polarization method and impedance method have showed that further increase of iodide ion concentrations leads to gradual decrease in inhibition efficieny.Surface coverage "θ" values suggest that uniform adsorption of iodide ions on iron surface at lower concentrations and a small amount of desorption of the same at higher concentrations.

Bromide ions
Figure 4 shows the potentiodynamic polarization behaviour of iron in 0.5 mol L -1 H 2 SO 4 with the addition of bromide ions.The corrosion kinetic parameters derived from these plots are presented in the Table 3.It is seen from the Table 3 that the bromide ions are not as effective as iodide ions even though the inhibition efficiency is increased with the increase in the concentration of the bromide ions.The corrosion current values decrease from 410 μA cm -2 to 52 μA cm -2 with the maximum concentration of bromide ions (i.e.10.0 ×10 -3 mol L -1 ) where the inhibition efficiency is 87%.The corrosion potential E corr remains unaffected by the added bromide ions which indicates the mixed mode of action.
The charge transfer resistance (R ct ) values derived from the Nyquist plots (Figure 5) are given in Table 4 along with the polarization resistance values obtained from the linear polarization resistance method.The R ct values are increased from 33 Ω cm 2 to 201 Ω cm 2 with a corresponding increase of inhibition efficiency to 84%.The interfacial double layer capacitance C dl values are decreased from 2244 μF cm -2 to 131 μF cm -2 while the surface coverage θ values are increased from 0.47 to 0.94 indicating the uniform adsorption of Br -ions on the iron electrode.

Chloride ions
Table 4 shows the corrosion kinetic parameters derived from the polarization curves (Figure 6) after the addition of various concentrations of chloride ions.It is observed that the i corr values are decreased to 90 μA cm -2 for 5.0 ×10 -3 mol L -1 of KCl corresponding to an inhibition efficiency of 78% and after that a sharp rise in corrosion current density is observed with further increase in concentration of chloride ions.As in the case of bromide ions, here also, the E corr values are not changed significantly with the addition of chloride ions.The charge transfer resistance R ct values derived from the electrochemical impedance spectroscopy (Figure 7) and the polarization resistance R p values obtained from LPR studies are given in Table 6.It is found that increase in inhibition efficiency up to a specific concentration of chloride ions which is very much agreeing with that of polarization measurements.Chloride ions inhibit the iron dissolution at lower concentrations more effectively than at higher concentrations.The role of halide ions in the iron dissolution is still a matter of dispute.According to the present study, all halide ions influence the kinetics of metal dissolution to some extent depending on their nature and concentration.At lower concentrations and near the corrosion potential, the halide ions are chemisorbed more strongly on the iron electrode surface thereby reducing the free surface area of the metal for metal dissolution reaction to a larger extent than for hydrogen evolution. 45The specific adsorption of halide ions on the iron surface gives rise to the inhibition of iron dissolution [32][33][34][35][36] and the inhibitory action of these halide ions on the active dissolution of iron in H 2 SO 4 has been reported by Walpert. 46] The inhibition behaviour of I -ions at lower concentrations is mainly due to the strong adsorption of these ions on the electrode surface at E corr . 49The adsorption ability of halide ions on the iron surface has been estimated in the order 2, 50-52 I -> Br -> Cl - Generally the adsorbability of anions is related to the degree of hydration; the less hydrated ion is preferentially adsorbed on the electrode surface. 53,54The ease of adsorption in the case of iodide ions may be due to its less degree of hydration.The inhibitive effect of halide ions is found to be in the same order as that of adsorption ability.The anodic tafel slope values in the presence of halide ions are 70 ± 10 mV and cathodic tafel slope values are 100 ± 10 mV.The anodic tafel slopes have been reported as 30 mV 55 and 40 mV 56 for iron in acid solutions The higher anodic tafel slopes are attributed to the measurements made in shorter immersion time. 3e corrosion potential E corr values are found to be shifted in noble direction in the case of iodide ions where as, the values remain unaffected in the case of bromide and chloride ions.This shows that iodide ions affect the anodic reaction significantly whereas bromide and chloride ions affect both the reactions.
There is a marked decrease in C dl values in the presence of halide ions.This decrease in the C dl , which can result from a decrease in local dielectric constant and / or an increase in the thickness of the electrical double layer, signifying    that the halide ions act by adsorption at the solution/ interface. 57Further, it is reported that these adsorbed halide ions do not participate in iron dissolution reaction since negative reaction orders have been observed in sulphate solutions. 32,58,59Cathodic polarization studies on the effect of addition of chloride and iodide ions on iron dissolution in H 2 SO 4 solutions have revealed that the adsorbed halide ions inhibits the hydrogen evolution reaction predominantly. 49,60Hence the mechanism of inhibition of iron dissolution in sulphuric acid solution by halide ions is mainly due to blocking of surface by adsorption.

Conclusions
The halide ions are found to inhibit the corrosion of iron in 0.5 mol L -1 H 2 SO 4 to the extent of 80 to 90% at concentrations less than 5 x 10 -3 mol L -1 .The order of inhibition is I -> Br -> Cl -.The inhibition of halide ions is mainly due to adsorption on iron surface at the corrosion potential.

Figure 2 .
Figure 2. Nyquist plots of iron in 0.5 mol L -1 H 2 SO 4 with and without the addition of iodide ions.

Figure 3 .
Figure 3. Linear polarization curves for iron in 0.5 mol L -1 H 2 SO 4 with and without the addition of iodide ions.

Figure 1 .
Figure 1.Potentiodynamic polarisation behaviour of iron in 0.5 mol L -1 H 2 SO 4 without and with the addition of iodide ions.

Figure 4 .
Figure 4. Potentiodynamic polarisation behaviour of iron in 0.5 mol L -1 H 2 SO 4 without and with the addition of bromide ions.

Figure 5 .
Figure 5. Nyquist plots of iron in 0.5 mol L -1 H 2 SO 4 with and without the addition of bromide ions.

Figure 6 .
Figure 6.Potentiodynamic polarisation behaviour of iron in 0.5 mol L -1 H 2 SO 4 without and with the addition of chloride ions.

Figure 7 .
Figure 7. Nyquist plots of iron in 0.5 mol L -1 H 2 SO 4 with and without the addition of chloride ions.

Table 1 .
Corrosion kinetic parameters of pure iron in 0.5 mol L -1 H 2 SO 4 with I -ions

Table 3 .
Corrosion kinetic parameters of pure iron in 0.5 mol L -1 H 2 SO 4 with Br -ions

Table 2 .
Electrochemical impedance and linear polarization parameters for pure iron in 0.5 mol L -1 H 2 SO 4 with I -ions

Table 4 .
Electrochemical impedance and linear polarization parameters for pure iron in 0.5 mol L -1 H 2 SO 4 with Br -ions

Table 5 .
Corrosion kinetic parameters of pure iron in 0.5 mol L -1 H 2 SO 4 with Cl -ions

Table 6 .
Electrochemical impedance and linear polarization parameters for pure iron in 0.5 mol L -1 H 2 SO 4 with Cl -ions