Effect of Iodide on the Corrosion Inhibitive Behaviour on Carbon Steel by an Azomethine Compound Derived from Anthracene-9 ( 10 H )-one

The inhibition effect of (s)-2-(anthracene-9(10H)-ylideneamino)-5-guanidinopentanoic acid (A9Y5GPA) on carbon steel (CS) in 0.5 M sulphuric acid solution has been investigated using weight loss measurements, electrochemical impedance spectroscopy (EIS) and potentiodynamic polarization studies. The addition of KI to A9Y5GPA enhanced the inhibition efficiency due to synergistic effect. The adsorption of A9Y5GPA and A9Y5GPA+ KI on the carbon steel surface obeys Freundlich and Langmuir adsorption isotherm respectively. The result showed that compound studied was acted as a mixed type inhibitor causing blocking of active sites on the metal. Surface morphology of the carbon steel specimens was evaluated by SEM analysis.


INTRODUCTION
An effective method employed by corrosion engineers to control the corrosion of metal is the use of certain organic inhibitors 1 .Organic molecules possessing azomethine linkage (C=N) act as effective potential corrosion inhibitors [2][3][4][5][6] .The presence of halide ions in solutions has been found to stabilize the adsorption of some organic cations, leading to improved inhibition efficiency 7 .In continuation of our work 8 , we report in this paper the corrosion inhibition behaviour and synergism The aggressive solution of 0.5M H 2 SO 4 was prepared by the dilution of A.R grade 98% of H 2 SO 4 (Merck) with de-ionized water.A9Y5GPA solutions were prepared in the range, 0.2mM-1mM concentrations.0.2 mM KI solution was also prepared.

Weight Loss Measurements
Weight loss experiments were performed with CS specimens of dimension 1.5x 2x 0.1 cm (Composition: C,0.5%; Mn,0.07%;P,0.02%; S,0.015%; Si,0.02% and rest Fe).The specimens were first polished with different grades of emery papers, rinsed with ethanol and water, respectively.Then dried, weighed and immersed in 50 ml corrosive medium with and without different concentrations of A9Y5GPA and A9Y5GPA + 1ml KI solutions.Weight loss of metal specimens was noted after 24 h.The corrosion rate (½) and the percentage of inhibition efficiency (w%) were calculated by the following equations 9, 10 .... (1)   ... (2)   where W is the weight loss (g) of coupon, S is the total area (cm 2 ) of specimens, t is the time of treatment (24 hrs), ½ 0 and ½ are the corrosion rates of uninhibited and inhibited specimens respectively.

Electrochemical Investigations
The impedance measurements and polarization curves were performed in a three electrode assembly.Saturated calomel electrode (SCE) was used as the reference electrode.Platinum electrode having 1cm 2 area was taken as counter electrode.Metal specimens with an exposed area of 1cm 2 were used as the working electrode.The EIS experiments were carried out on an Ivium compactstat-e electrochemical system.100 ml of 0.5M H 2 SO 4 with A9Y5GPA and A9Y5GPA + 2ml KI (no stirring) were taken as the electrolyte and the working area of the metal specimens were exposed to the electrolyte for 1 h prior to the measurement.EIS measurements were performed at constant potential (OCP) in the frequency range from 1 KHz to 100 mHz with amplitude of 10 mV as excitation signal.The percentage of inhibitions from impedance measurements were calculated using charge transfer resistance values by the following expression 11 : ... (3)   where R ct and R' ct are the charge transfer resistances of working electrode with and without inhibitor respectively.
Polarization plots were obtained in the electrode potential range from -100 to +100 mV Vs equilibrium potential (E corr ) at a scan rate of 1mV/ sec.Tafel polarization analysis were done by extrapolating anodic and cathodic curves to obtain corrosion current densities ( I corr ).The percentage of inhibition efficiency (• pol% ) was evaluated from the measured I corr values using the relation : ... (4)   where I corr and I' corr are the corrosion current densities of the exposed area of the working electrode in the absence and presence of inhibitor.Surface analyses were performed using scanning electron microscope (model Hitachi SU6600).

Weight Loss Measurements
The calculated values of inhibition efficiency (• W %) and surface coverage (¸) at 30 0 C from the losses of weight of CS after immersing in solutions of 0.5 M H 2 SO 4 containing different concentrations of the A9Y5GPA are given in table 1.
From table 1 it is clear that the inhibition efficiency increases with increasing the concentration of the compound.Various concentrations of A9Y5GPA alone show very low inhibition values.This behaviour indicates that the A9Y5GPA can act as a moderate inhibitor for CS corrosion in 0.5M H 2 SO 4 solution.However the A95GPA + KI system shows high inhibition values.The inhibitive action of the used compound is attributed to the adsorption of molecules on CS surface, forming a barrier between the metal surface and the corrosive environment.

Synergistic Effect
From table 1 it is also evident that • W % for KI in combination with A9Y5GPA is higher than the sum of • W % for single KI and single A9Y5GPA in all investigations, which is synergism in nature.Aramaki and Hackerman 12 calculated the synergism parameter S ¸ using the following equation: ... (5)   where 1+2= (1+2) -12;  1 = surface coverage by anion;  2 = surface coverage by cation; ' 1+2 = measured surface coverage by both anion and cation.S ¸ approaches unity when there are no interactions between the inhibitor compounds, while S ¸ > 1 points to a synergistic effect; in the case of S <1, the antagonistic interaction prevails.The values of the synergism parameter for the various concentrations of A9Y5GPA studied from the gravimetric analysis are presented in table 2.
All values shown in this table are greater than unity.This is an indication for the enhanced inhibition efficiency obtained by the addition of iodide ions to A9Y5GPA is synergistic in nature 13 .

Adsorption Studies
The best description of the adsorption behavior of A9Y5GPA and A9Y5GPA + KI on CS specimens in 0.5M H 2 SO 4 was Freundlich and Langmiur adsorption isotherms respectively.These models are expressed as 14 Freundlich adsorption isotherm =Kads ...( 6) Langmiur adsorption isotherm = ... (7)   where C is the concentration of the inhibitor, ¸ is the fractional surface coverage and K ads is the adsorption equilibrium constant.Figs. 2 and 3 represent the adsorption plots of A9Y5GPA and A9Y5GPA+ KI obtained by the weight loss measurements of CS steel specimens in 0.5M H 2 SO 4 at 30 0 C for 24 h respectively.
The adsorption equilibrium constant K ads is related to the standard free energy of adsorption G 0 ads , by G 0 ads = -RTln(55.5K ads ) ... (8)   Where 55.5 is the molar concentration of water, R is the universal gas constant and T is the temperature in Kelvin 15 .In the present study, A9Y5GPA and A9Y5GPA + KI molecules showed G 0 ads -24.2 KJ and -27.5 KJ respectively, suggesting that the adsorption of inhibitor involves both electrostatic and chemical interactions.

Electrochemical Investigations
Figs.4 and 5 represent the Nyquist plots of CS specimens in 0.5M H 2 SO 4 in the presence of various concentrations of A9Y5GPA and A9Y5GPA + KI respectively.It is evident from the plots that the impedance response of metal specimens has marked difference in the presence and absence of the KI with A9Y5GPA.
Impedance behaviour can be well explained by pure electric models that could verify and enable to calculate numerical values corresponding to the physical and chemical properties of electrochemical system under examination 16 .The simple equivalent circuit that fit to many electrochemical systems composed of a double layer capacitance (C dl ), solution resistance (R s ) and charge transfer resistance (R ct ) 17,18 .The impedance parameters are tabulated in the table 3 (3a and 3b).
The capacitance values C dl decreases with A9Y5GPA concentration and this decrease in C dl is enhanced upon addition of I " ions to the corrosive environment.These results suggest that the A9Y5GPA molecules function by adsorption at the metal/solution interface 19 and this adsorption is reinforced by I " ions.The • EIS% data reveal that the corrosion inhibition capacity of A9Y5GPA is markedly enhanced by the addition of KI.   4 .The data show that, addition of the A9Y5GPA to acid media affected both the cathodic and anodic parts of the curves.Addition of I -ions to A9Y5GPA -H 2 SO 4 systems results in marked decrease in the corrosion current density (I corr ).The maximum shift of E corr is 74mV, suggesting that A9Y5GPA acts as a mixed type inhibitor for CS specimens in 0.5M H 2 SO 4 .From the values it is clear that the inhibition efficiency of A9Y5GPA alone is increased in presence of KI.These results also confirm the existence of strong synergism between A9Y5GPA and KI in the corrosion inhibition of CS in these solutions.

SEM Studies
The surface morphology of carbon steel surface was evaluated by scanning electron microscopy (SEM).Figures 8 (A, B, C and D) show the scanning electron micrographs of the bare carbon steel surface, CS specimens in H 2 SO 4 , with A9Y5GPA and with A9Y5GPA+ KI in H 2 SO 4 medium for 24 h respectively.The morphology revealed that in the absence of A9Y5GPA, the surface is highly corroded.However in the presence of A9Y5GPA the rate of corrosion is suppressed and the SEM image D shows that inhibition action enhanced by forming a protective layer on the surface that prevents the attack of acid on the corroding metal CS.

Mechanism and Explanation for synergism
The synergistic inhibition brought about by the combination of A9Y5GPA and iodide ions for the corrosion of CS in 0.5 M H 2 SO 4 can be explained on the basis that halide ions have a greater tendency to be adsorbed on the surface in attraction with organic cations.
The protonated Schiff base (A9Y5GPA + ) is adsorbed by coulombic attraction at the steel surface, where iodide ions are already adsorbed by chemisorptions.Greater surface coverage from the stabilization of adsorbed iodide ions by means of electrostatic interaction with A9Y5GPA + facilitates corrosion inhibition synergism.

1.
Inhibition efficiency increases with increase in concentration of inhibitor.2.
The addition of iodide ions to A9Y5GPA enhanced the inhibition efficiency due to synergistic effect.

3.
The adsorption of A9Y5GPA alone and in combination with iodide ions obeys Freundlich and Langmiur adsorption isotherms respectively.4.
The thermodynamic parameters calculated from the adsorption isotherms showed that both physisorption and chemisorptions are involved in the inhibition process.