The Inhibition of Mild Steel Corrosion in Sulfuric Acid by New Dapsone Derivatives

Dapsone derivatives Viz., 4, 4’-sulfonylbis(N-((1H-pyrrol-2-yl)methylene)aniline) (SBPMA) and 2-dimethylamino-5-[(4-{4[(4-dimethylamino-benzylidene)-amino]-benzenesulfonyl}-phenylimino)-methyl]-benzene (DBBPB), a new class of corrosion inhibitors have been synthesized and their corrosion inhibition efficiency on mild steel in 0.5 M H2SO4 was investigated by mass loss, Tafel polarization and AC impedance measurements. Potentiodynamic polarization studies showed that, these compounds behave as mixed type of corrosion inhibitors. The adsorption process was found to obey Langmuir isotherm model. Electrochemical impedance spectroscopy (EIS) studies revealed that polarization resistance (RP) increases and double layer capacitance (Cdl) decreases as the concentration of the inhibitors increases. Various thermodynamic parameters for the adsorption of inhibitors on mild steel were computed and discussed. FTIR, EDX and SEM analyses were performed to study the film persistency of the inhibitors.


INTRODUCTION
Mild steel (MS) is an alloy of iron, carbon and several other elements, which hardens above its critical temperature, and is a class of strong, tough and low cost steel with an excellent mechanical property. It has good physical and chemical properties such as toughness, welding, stamping and cutting performance. It is the most extensively used alloy in a wide spread spectrum of many industrial units for structural, instrumental and automobile applications. But it is highly susceptible for corrosion in humid air especially in acidic media [1]. In order to prevent corrosion, the primary strategy adopted is to isolate the metal from corrosive media. Amongst the various methods available, the use of inhibitors is one of the most practical and economical methods for protection against corrosion. The use of inhibitors for the corrosion inhibition of MS is the subject of tremendous technological importance due to the increased industrial applications of MS and other related materials especially in acid media [2]. Therefore, a wide variety of corrosion inhibitors ranging from rare earth elements [3] to organic compounds [4][5][6][7] have been used. There is a continuing effort to find corrosion inhibitors that exhibit greater effect with smaller quantity in the corrosive media. This is a challenging problem in the steel industry because corrosion over MS surfaces affects long term industrial projects. The performance of corrosion inhibitors based on organic compounds containing nitrogen, sulphur and oxygen atoms shows promising results. The corrosion inhibition property of inhibitors depends on their molecular structures, planarity and the lone pairs of electrons present on the hetero atoms, which determine the adsorption of these molecules on the metallic surfaces. Corrosion inhibitors block the active sites and enhance the adsorption process, thus decreasing the rate of dissolution and extending the life span of the equipment [8,9].
The use of organic compounds as corrosion inhibitors is the main choice in various industrial processes such as acid pickling of steels, scale removal in metallurgy, acid cleaning of boilers and oil-well acidizing. Generally, a variety of organic compounds containing heteroatoms such as O, N, S and multiple bonds in their structure are of particular interest as they give better inhibition efficiency than those containing N or S alone [10,11]. Compounds rich in heteroatoms can be regarded as environmental friendly inhibitors because of their characteristic strong chemical activity and low toxicity [12]. The adsorption characteristics of organic molecules are also affected by size, electron density at the donor atoms and orbital character of the donating electrons [13][14][15][16]. Organic compounds containing hetero atoms in the functional groups, pi-electron in triple or conjugated double bonds and presence of aromatic rings in their structure are the major adsorption centers and are usually good inhibitors [17]. Several nitrogen, oxygen and sulphur containing heterocyclic compounds such as indole derivatives [18], acetyl pyridine derivatives [19], thiosemicarbazone derivatives [20], sulphonamide compounds [21], thiadiazoles derivatives [22], imidazole derivatives [23], triazoles [24], pyrazole derivatives [25], pyridazine derivatives [26] and triazines [27] have been reported as anticorrosion substances in acid media.

Mild steel specimen preparation
MS specimens used in the present study having the following chemical compositions (in wt %) : C -0.051; Mn -0.179; Si -0.006; P -0.005; S -0.023; Cr -0.051; Ni -0.05; Mo -0.013; Ti -0.004; Al -0.103; Cu -0.050; Sn -0.004; B -0.00105; Co -0.017; Nb -0.012; Pb -0.001 and the remainder iron. For all experiments, MS specimens of dimension 1 cm × 1 cm × 0.1 cm were used. The specimens were mechanically well abraded with different grades SiC (1200 -1600) emery papers, degreased with benzene, washed with triply distilled water and finally dried. All the solvents and chemicals used were of AR grade and used as such. Doubly distilled water was used in the preparation of the various concentrations of test solutions.   (6.28). Melting range was determined by Veego Melting Point VMP III apparatus. FTIR spectra were recorded using a Jasco FTIR 4100 double beam spectrophotometer. 1 H-NMR spectra were recorded on Bruker DRX-500 spectrometer at 400 MHz using DMSO-d6 as solvent and TMS as an internal standard. Mass spectra were recorded by LC/MSD Trap XCT. Elemental analyses were recorded on VarioMICRO superuser V1.3.2 Elementar.

Mass loss measurements
Mass loss measurements were performed by weighing cleaned and dried MS specimens before and after immersion in 0.5 M H2SO4 solutions from one to five hours in the absence and presence of various concentrations of SBPMA and DBBPB at different temperatures (30 -60 °C). Triplicate experiments were performed in each case and the mean value of the mass loss was noted. Corrosion rate (CR) in mg cm -2 h -1 and inhibition efficiency IEML (%) were calculated using the following equations: (1) where ΔM is the mass loss, S is the surface area of the specimen and t is immersion time. (2) where (CR)a and (CR)p are the corrosion rates in the absence and the presence of inhibitor, respectively. J u l y 15, 2 0 1 4

Potentiodynamic polarization measurements
The potentiodynamic polarization studies were carried out with MS specimen as working electrode in 0.5 M H2SO4 solutions with different inhibitor's concentrations (200 -500 ppm) with an exposed area of 1cm 2 and this working area was remained precisely fixed throughout the experiment. A conventional three electrode cell consisting of MS as working electrode, platinum foil as counter electrode and saturated calomel electrode as reference electrode was used. Potentiodynamic polarization studies were carried out using CH-instrument (model CHI660D). Before each Tafel experiment, the MS electrode was allowed to corrode freely and its open circuit potential (OCP) was recorded as a function of time up to 30 min. After this time, a steady state OCP corresponding to the corrosion potential (Ecorr) of the working electrode was obtained. The IEP (%) was calculated from corrosion currents determined from the Tafel extrapolation method using the experimental relation (3): where (Icorr)a and (Icorr)p are the corrosion current density (µA cm -2 ) in the absence and presence of the inhibitor, respectively.

Electrochemical impedance spectroscopy (EIS)
Electrochemical impedance measurements were carried out using the same CH-instrument. The EIS data were taken in the frequency range 10 kHz to 100 mHz. The double layer capacitance (Cdl) and the polarization resistance (Rp) were determined from Nyquist plots [28]. The percentage inhibition efficiency, IEEIS (%) was calculated from Rp values using the following expression: (4) where (Rp)a and (Rp)p are polarization resistances in the absence and presence of inhibitor, respectively.

FTIR, EDX and SEM studies
The MS specimens were immersed in 0.5 M H2SO4 in the presence of inhibitors (500 ppm) for a period of 5 hr. Then the specimens were taken out and dried. The surface adheared film was scratched carefully and its FTIR spectra were recorded using a Jasco FTIR 4100 double beam spectrometer. The surface feature of the MS specimens in the absence and presence of inhibitors was studied by energy dispersive X-ray spectroscopy (EDX) and scanning electron microscopy (model JSM-5800).

Mass loss studies
The CR and IEML (%) in the absence and presence of various concentrations of SBPMA and DBBPB in 0.5 M H2SO4 solution and at different temperatures (30 -60 °C) are presented in Table 1. The mass loss was found to be decreased and IEML (%) observed to be increased with increasing concentration of dapsone derivatives. The studied inhibitors were found to attain the maximum inhibition efficiency at 500 ppm. There is no appreciable increase in the inhibition efficiency after 5 hr of immersion time, this is due to desorption of the inhibitor molecules from the metal surface with increasing immersion time and instability of inhibitor film on the metal surface [29,30]. The IEML (%) of DBBPB found to be greater than that of SBPMA.

Effect of temperature
The effect of temperature on CR and IEML (%) was studied in 0.5 M H2SO4 in the temperature range of 30 -60 °C in the absence and presence of different concentrations of inhibitors ( Table 1). The inhibition efficiencies were found to decrease with increasing temperature from 30 -60 °C. It may be explained with desorption of adsorbed inhibitor on the MS surface. This proves that the inhibition occurs through the adsorption of the inhibitors on the metal surface, and desorption is aided by increase in temperature. The activation parameters for the corrosion process were calculated from the Arrhenius plot according to the following equation:  where Ea is the activation energy, k is the frequency factor, T is the absolute temperature and R is the universal gas constant. The values of Ea for MS in 0.5 M H2SO4 without and with various concentrations of inhibitors are obtained from the slope of the plot of log CR versus 1/T (Fig. 2) and are shown in Table 2. Inspection of Table 2 revealed that, the activation energy increasees in the presence of inhibitors, which indicated physical adsorption process (electrostatic) in the first stage [30]. The activation energy rises with increasing inhibitors concentration, suggesting the strong adsorption of inhibitor molecules at the metal surface [31].
An alternative Arrhenius plots of log CR/T versus 1/T (Fig. 3) for MS dissolution in 0.5 M H2SO4 medium in the absence and presence of different concentrations of SBPMA and DBBPB were used to calculate the values of activation thermodynamic parameters such as enthalpy of activation (∆Ha) and entropy of activation (∆Sa) using the relation (6)   (6) where R is the Universal gas constant, T is the absolute temperature, N is the Avogadro's number, h is Planks constant. The values of ∆Ha and ∆Sa were obtained from the slope and intercept of the above plot, and presented in Table 2. The obtained ∆Ha values are in good agreement with those calculated from the equation (7), The positive values of enthalpy of activation in the absence and presence of inhibitors indicate an endothermic nature of MS dissolution process [32]. The entropy of activation values are less negative in the presence of the inhibitors compare to free acid solutions, this suggests that an increase in randomness occurred while moving from reactants to the activated complex [33]. J u l y 15, 2 0 1 4

Adsorption isotherm
The adsorption isotherm can give important information about the interaction of the inhibitor molecules and the metal surface. The adsorption of inhibitor molecules from aqueous solution is a quasi-substitution process, and was found to be highly pH dependent [34]. The surface protection of MS depends upon how the inhibitor molecule will adsorbed on the metal surface, and also ionization and polarization of molecules [35]. The degree of surface coverage (θ) as function of concentration (C) of the inhibitor was studied graphically by fitting it to various adsorption isotherms to find the best adsorption isotherm. The Langmuir adsorption isotherm model was proposed since equilibrium adsorption of inhibitors was found to obey this adsorption isotherm model on MS in 0.5 M H2SO4 medium. According to this adsorption isotherm, θ is related to the inhibitor concentration, C and adsorption equilibrium constant Kads through the following expression: (8) The surface coverage was tested graphically by fitting a suitable adsorption isotherm. In these cases the plots of C/θ versus C (Fig. 4) yield straight lines with the linear correlation coefficient (R 2 ) values close to unity, which suggests that the adsorption of SBPMA and DBBPB in 0.5 M H2SO4 medium on MS surface obeys the Langmuir isotherm model. The free energy of adsorption was calculated using the following relation: (9) where R is the universal gas constant, T is the absolute temperature, Kads is the equilibrium constant for adsorptiondesorption process and 55.5 is the molar concentration of water in solution (molL -1 ). The adsorption thermodynamic parameters such as enthalpy of adsorption (∆H 0 ads) and entropy of adsorption (∆S°ads) were obtained from the slope and intercept of the plot of log Kads versus 1/T (Fig. 5) using the equation (10). (10) The calculated values of Kads, ∆H°ads, ∆G°ads and ∆S°ads over the temperature range of 30 -60 °C are recorded in Table 3. The negative values of ∆G°ads indicate the spontaneous adsorption of inhibitors on the MS surface [36]. In the present study, ∆G°ads values for SBPMA and DBBPB are found to be in the range -36.62 to -38.96 kJ mol -1 and -37.02 to -39.30 kJ mol -1 , respectively indicating that the adsorption is more physisorption than chemisorption [37][38][39][40]. J u l y 15, 2 0 1 4

FTIR spectral studies
FTIR spectra were recorded to understand the interaction of inhibitor molecules with the metal surface. Figs. 6a and 7a show the FTIR spectra of pure SBPMA and DBBPB, and Figs. 6b and 7b represent the FTIR spectra of the scratched samples obtained from the metal surfaces after corrosion experiments. It was found that peaks in the spectrum of pure compounds were changed in the spectrum of scratched samples. The azomethine group stretching frequencies for pure SBPMA and DBBPB were found to be at 1615 and 1626 cm -1 , respectively. In the FTIR spectra of scratched samples, the stretching frequencies of the azomethine group were found to be disappeared in both SBPMA and DBBPB confirming that the azomethine groups are involved in the complex formation with the metal.

Potentiodynamic polarization measurements
Polarization measurements were carried out to know the kinetics of anodic and cathodic reactions. Fig. 8 reveals the polarization curves for MS in 0.5 M H2SO4 in the absence and presence of different concentrations of SBPMA and DBBPB. Inspection of Fig. 8 revealed that the addition of inhibitors hindered the acid attack on the MS electrode. In all the cases, the addition of inhibitors reduces both anodic and cathodic current densities indicating that these inhibitors exhibit cathodic and anodic inhibition effects, hence they are relatively mixed type of corrosion inhibitors [41,42]. Table 4 clearly highlights that there is a gradual decrease in the corrosion potential and corrosion current values as the inhibitor's concentration was raised from 200 -500 ppm. The values associated with electrochemical polarization measurements such as corrosion current density (icorr), corrosion potential (Ecorr) and inhibition efficiency (IEP %) were determined from the polarization plots are given in Table 4. The values of corrosion current densities were obtained by extrapolating the current-potential lines to the corresponding corrosion potentials. It is evident that IEP (%) increases with inhibitors concentration, and protection action of SBPMA and DBBPB can be attributed to the electron density of the azomethine (-C=N-) group and this electron density varies with the substituents in the inhibitor molecules. The imine J u l y 15, 2 0 1 4 nitrogen can donate the lone pair of electrons to the metal surface more easily and hence reduce the corrosion rate. The higher IEP (%) of DBBPB compare to that of SBPMA, can probably be explained on the basis of the additional functional groups and also the nature of the hetero atoms the inhibitor molecule contain in its structure.

Electrochemical impedance spectroscopy
The Nyquist plots for MS in 0.5 M H2SO4 solution without and with different concentrations of SBPMA and DBBPB are presented in Fig. 9. The electrochemical impedance parameters derived from the Nyquist plots and IEEIS (%) are listed in Table 5. From the plots it is clear that the impedance response of MS in uninhibited acid solution has significantly changed after the addition of inhibitors to the corrosive solution. This indicates that the impedance of the inhibited metal has increased with increasing concentration of inhibitors. The measured impedance data were based upon the equivalent circuit given in the Fig. 10, consists of double layer capacitance (Cdl), polarization resistance (Rp) and solution resistance (Rs).
It was observable that, the increase in the Rp values in the presence of different concentrations of SBPMA and DBBPB indicate the reduction in the MS corrosion rate with the formation of adsorbed protective film on the metal-solution interface [43,44]. When the concentration is raised from 200 -500 ppm, there was a gradual increase in the diameter of semi-circle of the Nyquist plot reflecting increasing Rp values from 27.81 to 234.5 and 363.7 Ω cm 2 for SBPMA and DBBPB, respectively, which indicates the adsorption of inhibitors on the metal surface. The double layer capacitance (Cdl) values were decreased due to decrease in local dielectric constant and / or increase in the thickness of the electrical double layer, indicating that the inhibitor molecules adsorbed at the metal-solution interface [45,46]. Decrease in the surface area [47] and imperfections of the metal surface may also be the reason for decrease of Cdl values. Addition of inhibitors provided lower Cdl values because of the replacement of water molecules by inhibitor molecules at the electrode surface [48].

EDX analysis
EDX spectra were used to determine the elements present on MS surface before and after exposure to the inhibitor solution. Fig. 11a is the EDX spectrum of abraded MS sample and it is notable that the peak of oxygen is absent which confirm the absence of air formed oxide film. However, for inhibited solutions (Figs 11b and 11c) additional lines characteristic for the existence of N, O and S (due to the N, O and S atoms of the SBPMA and DBBPB) in the EDX spectra are obtained. These data showed that the N, O and S atom of inhibitors are involved in bonding with the MS electrode. J u l y 15, 2 0 1 4

SEM analysis
The surface morphology of MS surface due to corrosion process was confirmed by the SEM images of the abraded and corroded MS surface in the absence and presence of inhibitors (Figs. 12a-12d). Fig. 12a represents the SEM image for abraded MS surface. Fig. 12b is the SEM image of MS surface in 0.5 M H2SO4 without inhibitor. It was found that the corroded MS surface contains large number of pits. However, SEM images of MS surface in the presence of inhibitors (Figs. 12c and 12d) were observed to be smoother than that of MS surface in 0.5 M H2SO4 alone. These observations reveal that, the inhibitors form protective layer on the MS surface which prevents the attack of acid as well as the dissolution of MS by forming surface adsorbed layer and thereby reducing the corrosion rate.