The protection effect of malonic acid on carbon steel corrosion was studied in aerated stagnant 1M HCl solutions at 250C. Measurements were conducted under different experimental conditions using weight loss, Tafel polarization, electrochemical impedance spectroscopy (EIS) and electrochemical frequency modulation (EFM) techniques. malonic acid was found to be good inhibitor of carbon steel corrosion in1 M HCl. The adsorption of this inhibitor is found to obey the Langmuir adsorption isotherm. The calculated activation energies proposed that the inhibitor molecules being physically adsorbed onto the metal surface. Polarization data revealed that this compound behave as mixed type inhibitor.
Malonic acid is a substance that has uses both in medicine and wider industries. The study of carbon steel corrosion phenomena has become important especially in acidic media because of the increased industrial applications of acid solutions. The corrosion of carbon steel is an essential academic and industrial interest that has received a large amount of interest . The use of inhibitors is one of the most practical methods for protecting materials against corrosion, particularly in acidic media . Literatures show that many organic compounds mainly contain phosphorous, sulphur, nitrogen and oxygen as a part of their structures are effective in the corrosion control of various metals and alloys exposed to acid solutions. Schmitt employed ethoxylated amines as corrosion inhibitors for iron and steel in acid media . Many studies have been published on the use of natural products as corrosion inhibitors in various media [4–12]. Most of the natural products are nontoxic, capable of being decomposed and readily available in overflow. The efficiency of an inhibitor is very much dependent on its adsorption on the metal surface, which consists of replacement of water molecules by the organic inhibitor at the interface [13-15]. The adsorption of these molecules relies on mainly on specific physicochemical properties of the inhibitor molecule like functional groups, steric factors, and aromaticity, electron intensity at the donor atoms and p-orbital character of donating electrons and the electronic structure of the molecules .
In this present work, malonic acid was tested as green corrosion inhibitor using chemical and electrochemical techniques. Surface examination by scanning electron microscopy (SEM) and energy dispersive X-ray (EDX) was also analyzed.
The specimens of carbon steel with the following composition (weight %): 0.200 C, 0.350 Mn, 0.024 P, 0.003 Si and the remainder Fe were covered by epoxy resin leaving a working area of 1 cm 2. The working surface was thereafter abraded with 400 to 1200 grit grinding papers, rinsed by bidistilled water and acetone.
Malonic acid was purchased from Aldrich-Sigma Company. Its molecular structure, molecular weight and molecular formula are shown below.
1 M HCl was used as a corrosive solution, and was prepared by diluting concentrated HCl (37%) to a required concentration using bidistilled water and its concentration was checked using standard Na2CO3 solution.
2.4. Weight loss measurements
Samples of the carbon steel were abraded with Sic emery papers (400-1200grit), wash with bidistilled water to remove, detergent, dirt, or purities, dried in a desiccator, weighed by an analytic balance (0.0001 g), and dipped in the experiment solutions for 30 min at 250C. Then the specimens were removed, dipped in water and acetone, dried in a desiccator, and weighed. The variation of the mass of the specimens before and after the dipping in the corrosive solution was determined. The inhibition efficiency IE and corrosion rate from loss of mass are calculated by:
The corrosion rate (Ύ) was calculated from the following equation :
Where W is the average weight loss of three parallel carbon steel sheets, S the total area of one carbon steel specimen and t is immersion time. From the calculated corrosion rate, the inhibition efficiency (IE) and the degree of surface coverage are determined as follows:
% IE = surface coverage x 100 = [1- (W1/W2)] x 100 (2)
Where W1 and W2 are the weight losses of the carbon steel in the presence and absence of inhibitor, respectively.
2.5. Electrochemical measurements
Electrochemical measurements were carried out in a traditional electrochemical cell containing three compartments for working, a platinum foil (1 cm2) as counter as well reference electrodes. A Luggin–Haber capillary was also included in the design. The tip of the Lugging capillary is made very close to the surface of the working electrode to minimize IR drop. The reference electrode was a saturated calomel electrode (SCE) used directly in contact with the working solution. Tafel polarization curves were determined by polarizing to ±250 mV with respect to the free corrosion potential (E vs SCE) at a scan rate of 0.5 mV/s. A 10 mV peak to-peak sine wave over ac frequency range extending from of 100 kHz to 100 mHz was used for the impedance measurements. Carried out using Gamry Potentiostat/Galvanostat/ZRA (modelPCI4/ 300) with a Gamry framework system based on ESA400. Gamry applications include software DC105 for polarization, EIS300 for EIS and EFM140 for EFM measurements; computer was used for collecting data. Echem Analyst 5.5 software was used for plotting, graphing and fitting data. Each experiment was repeated at least three times to confirm the reproducibility. All tests have been performed in non-deaerated solutions under unstirred conditions at 250C.
3. Results and discussion
3.1. Mass loss measurements
Figure 2 shows the mass loss (mg cm−2,min-1) of carbon steel after 3 h dipping in 1 M HCl in the presence of different concentrations of malonic acid as corrosion inhibitor. A decrease in the mass loss of carbon steel in the presence of malonic acid with respect to the blank was observed. By increasing the concentration of malonic acid, a furthermore decrease in mass loss is observed. Also, it is obvious that weight loss of C-steel in the presence of malonic acid varies linearly with time. This indicated the absence of the insoluble surface film during corrosion.
Table 1 presents the results of mass loss measurements for the corrosion of C-steel, in 1M HCl solutions devoid of and containing various concentrations of the malonic acid. The data in the Table 1 indicate that the increase in the concentration of malonic acid decreases markedly the rate of corrosion of carbon steel. This result refers to the inhibitive influence of the added malonic acid on C-steel corrosion in the acidic solution. The inhibition efficiency increases by the increasing the concentration of added malonic acid. The inhibitory action of malonic acid against C-steel corrosion can be attributed to the adsorption of malonic acid molecules on C-steel surface, which limits the dissolution of the latter by blocking its corrosion sites and hence decreasing the weight loss. Similar reports have been decomended elsewhere .
3.2. Polarization measurements
Figure 3 shows the polarization curves after the increment of malonic acid. From the curves, it is observed that the current density of the anodic and cathodic sections is displaced to lower values, which refers to corrosion decrease. This displacement is more obvious with the increase in concentration of malonic acid when compared relative to the blank. The inhibition efficiency (IE) and the degree of surface coverage (θ) can be calculated using the following equation [35 -38]:
% IE = θ x100 = [Icorr-(Icorr (inh) / Icorr)] (3)
where Icorr and Icorr are the corrosion current densities in the absence and presence of malonic acid, respectively.
The electrochemical parameters obtained for malonic acid such as current density (icorr ), anodic (βa) and cathodic (βc) slopes were obtained by Tafel extrapolation at the corrosion potential (Ec) and are reported in Table 2. The data of Table 2 show that when the concentration of malonic acid increased, the values of corrosion current density decrease and hence the inhibition efficiency will increased. This may be due to the adsorptive ﬁlm of malonic acid on surface of carbon steel. This film becomes more perfect as well stable with the increase in the concentration of malonic acid. These results indicate that this inhibitor acts by simple blocking the available sites of corrosion on the surface area . In the other words, this inhibitor decreases the surface area for corrosion without influencing the mechanism of corrosion. The Tafel slopes change slightly and Ecorr changes within 45 mV with the increasing malonic acid concentration. These indicate that this inhibitor acts as mixed type inhibitors. This advantage indicates that corrosion inhibitor has minor influence on both evolution of hydrogen as well carbon steel dissolution [20–22].
3.3. Electrochemical impedance spectroscopy (EIS) measurements
The representative Nyquist plots of carbon steel in 1 M HCl solution in absence and presence of different concentrations of malonic acid after 30 min exposure are given in Figure 4. As it is observed from Figure 4, the Nyquist plots of carbon steel in 1M HCl solution do not yield perfect semicircle as expected from the theory of EIS. The deviation from ideal semicircle is generally attributed to the frequency dispersion as well as to the in homogeneities of surface and mass transport resistant [23-26]. The Nyquist plots are analyzed in terms of the equivalent circuit composed with classic parallel capacitor and resistor (shown in Figure 5)  the impedance of a CPE is described by the equation 4:
ZCPE = Y0-1 (jω)-n (4)
Where Y0 is the magnitude of the CPE, j is an imaginary number, ω is the angular frequency at which the imaginary component of the impedance reaches its maximum values and n is the deviation parameter of the CPE: -1 ≤ n ≤ 1. The values of the interfacial capacitance Cdl can be calculated from CPE parameter values Y0 and n using equation 5:
Cdl = Y (ωmax) n-1 (5)
The inhibition efficiency (IE) and the degree of surface coverage (θ) can be calculated using the following equation:
% IE = θ x100 = [1-(Rºct / Rct)] (6)
where Rºct and Rct are the charge transfer resistance in the absence and presence of malonic acid, respectively.
The electrochemical impedance parameters of the corrosion process in the presence of the malonic acid in 1 M HCl solution include the charge transfer resistance as well double layer capacitance (Rct, Cdl, respectively) which were calculated and listed in Table 3. The charge-transfer resistance (Rct) values were calculated from the difference in impedance at low and high frequencies [28, 29]. It is obviously from Table 3 that Rct values increase and Cdl values decrease with increasing the concentration of malonic acid. Lowering in Cdl, is due to a decrease in dielectric constant and/or an increase in the thickness of the electrical double layer, suggests that the inhibitor molecules act by adsorption at the metal/solution when metal and solution interact . As consequence of increasing of Rct values with the concentration of malonic acid, the inhibition efficiency increases signiﬁcantly, revealing that malonic acid shows inhibiting effect for the corrosion of carbon steel. Both simulated and measured data are ﬁtted well. The high frequency part in Nyquist describes the behavior of an inhomogeneous surface layer of the electrode surface, while the low frequency contribution shows the kinetic response for the charge transfer reaction .
3.4. Electrochemical frequency modulation (EFM) measurements
Figure 6a/b shows plots of current as a function of frequency in absence and presence of 300 ppm malonic acid. Similar curves were obtained for other concentrations but not shown. EFM is a nondestructive corrosion measurement technique that can immediately give values of the corrosion current in the absence of knowledge of Tafel constants. As can be observed from Table 4, the corrosion current densities decrease with increase in malonic acid concentration. The causality factors in Table 4 refers to the measured data are of good perfect quality. The criterion values for CF-2 and CF-3 are 2.0 and 3.0, respectively. The causality factor is calculated from the frequency spectrum of the current response. If the causality factors differ signiﬁcant from the theoretical values of 2.0 and 3.0, then it can be inferred that the measurements are inﬂuence by noise. If the causality factors are approximately equal to the theoretical values of 2.0 as well 3.0, there is a causal relationship between the perturbation signal and the response signal. Then the data are presumed to be credible . The causality factors in Table 4 are very relative to theoretical values which according to the EFM theory should warranty the veracity of Tafel slopes and corrosion current densities .The great strength of the EFM is the causality factors which serve as an interior check on the veracity of the EFM measurement .
3.5. Adsorption isotherms
Adsorption isotherms are very important in understanding the mechanism of organic electrochemical reactions . In order to obtain the isotherm, degree of surface coverage (θ), as a function of inhibitor concentration, must be obtained. θ can be calculated from the corrosion current density values using equation (3). Assuming the increase in inhibition is caused by adsorption of inhibitor on the steel surface and obeys the Langmuir adsorption isotherm equation [39-41]:
C/θ =1/Kads + C (7)
where Kads the binding constant of the adsorption reaction and C is the inhibitor concentration in the bulk of the solution.
Plots of C/θ vs. C straight line was obtained, the correlation coefficient (R) was used to choose the isotherm that best ﬁt experimental data. This proposes that the adsorption of malonic acid on metal surface obeys the Langmuir adsorption isotherm (Figure 7).
From the intercepts of the straight lines C/θ -axis, K values were calculated and are given in Table (5). The most important thermodynamic adsorption parameters are the free energy of adsorption. Standard free energy (ΔG0ads) was obtained according to the following equation :
ΔG°ads = – RTln (55.5Kads) (8)
where 55.5 is the molar concentration of water in solution expressed in g-1L. Using this equation, the standard Gibbs free energy of adsorption of malonic acid on the carbon steel surface at 298K was found to be 18.7 kJ mol-1. Literature illustrate that values of standard Gibbs free energy of adsorption in aqueous solution about 20 kJ mol-1 or lower (more positive) refers that adsorption with electrostatic interaction between the adsorbent and adsorb ate (physisorption), while those about or higher (more negative) than 40 kJ mol-1 involve charge sharing between the molecules and the metal (chemisorptions) [43-46]. The negative value of standard free energy of adsorption (18.7 kJ mol-1) refers that spontaneous adsorption of malonic acid on carbon steel surface and the larger values of Kads (18.8 g L-1) also indicate the strong interaction as well stability of the adsorbed layer with the steel surface [47, 48].
3.6. Effect of temperature
Comparing the corrosion rates of the carbon steel in absence and in the presence of the tested inhibitor showed the inhibitive tendency of this inhibitor towards the corrosion reaction of carbon steel in the acidic medium. The apparent activation energy (E*a) for dissolution of carbon steel in 1M HCl was calculated from the slope of plots by using Arrhenius equation :
(Rate) k = A exp (−E∗a /RT) (9)
where k is rate of corrosion, E*a is the apparent activation energy and A is the Arrhenius pre-exponential factor. By plotting log k against 1/T the values of activation energy (E*a) has been calculated (E*a = (slope) 2.303·R) (Figure 8).
The enthalpy of activation (ΔH*) and entropy of activation (ΔS*) for the corrosion of carbon steel in HCl were obtained by applying the transition-state equation :
Rate (k) = RT/Nh exp (ΔS*/R) exp (− ΔH*/RT) (10)
where h is Planck’s constant, N is Avogadro’s number, ΔH*is the activation enthalpy and ΔS* is the activation entropy. A plot of log (Log k/ T) versus 1/ T for carbon steel in 1 M HCl in the absence and presence of different concentrations of malonic acid gives straight lines as shown in Figure 9.
Temperature plays a important role on the progress of the corrosion reactions of carbon steel in HCl solutions. Increasing the temperature increases the energy of the reactants to form the activated complex which dissociates to yield the corrosion products . The corrosion rates of carbon steel were calculated at 25-450C in the absence and the presence of the malonic acid in 1M HCl solution. Data in Table 5 revealed that increasing the temperature increases the mass lose as well corrosion rates of the carbon steel. Activation energy for the reaction of carbon steel in 1 M HCl increases in the presence of inhibitor (Table 5). The increase in activation energy refers to the retardation in corrosion rate which could have taken place because of adsorption of the inhibitor at the surface of the metal . The negative value of ∆S* (Table 6) for the inhibitor indicates that activated complex in the rate determining step represents an association rather than a dissociation step, meaning that a decrease degree of freedom during the course of transition from reactant to the activated complex .The positive sign of ∆H*reﬂects the endothermic nature of the carbon steel dissolution process.
3.7. Scanning electron microscopy (SEM)
Figure 10 shows the SEM image of carbon steel surface in 1 M HCl. It can be seen from Figure 10a that the carbon steel; specimen before dipping appears smooth as well shows some abrading scratches on the surface. However, it also appears small black holes, which may be attributed to the defect of steel. Inspection of Figure 10b shows that the carbon steel surface after dipping in uninhibited 1M HCl shows an aggressive attack of the corroding medium on the steel surface. Furthermore, the corrosion products appear very uneven. Figure 10c shows a smooth surface with deposited inhibitor on the surface of carbon steel after the addition of 300 ppm of malonic acid to the 1M HCl. It is clearly observed from SEM images that the irregularities on the surface due to corrosion are decreased to a greater extent in the inhibited surface and the surface is now almost free from corrosion. It reveals that a good protective adsorbed ﬁlm is formed on the sample.
3.7.2 Energy dispersive X-ray (EDX)
Figure 11a/b/c EDX spectrum of the carbon steel surface after dipping in 1 M HCl, for a period of 24 hrs, in absence and presence of 300 ppm of malonic acid. Figure 11b the EDX spectrum shows the characteristics peaks of some of the elements constituting the steel sample after 24 hrs immersion in 1 M HCl without malonic acid. Figure 11c in presence of 300 ppm of malonic acid the EDX spectrum shows extra lines of carbon, nitrogen and oxygen, due to the adsorbed layer of inhibitor that covered the electrode surface. In addition, the Fe peaks are considerably suppressed relative to uninhibited steel surface specimen. This suppression of the Fe lines occurs because of the overlying inhibitor ﬁlm.
3.8. Mechanism of inhibition
As shown from weight loss, potentiodynamic polarization, EIS and EFM measurements, corrosion of carbon steel in 1 M HCl is retarded in the presence of the malonic acid. The results clearly showed that the inhibition mechanism involves blocking of carbon steel surface by inhibitor molecules via adsorption. In general, the phenomenon of adsorption is inﬂuenced by the nature of metal and chemical structure of inhibitor. The values of thermodynamic parameters for the adsorption of inhibitor can provide valuable information about the mechanism of corrosion inhibition . So malonic acid can absorb onto C-steel surface and this adsorption through the carboxylate group [55, 56]. This is an acid–base reaction, and the driving force is the formation of a surface salt between the hydroxyl anion and a surface metal cation . Or due to the complex formation  of Fe2+- and malonic acid and this formed complex will adsorb on the metal surface thereby inhibiting the corrosion of carbon steel which confirms the formation of the film on the metal surface, which is protective in nature.
The following could be concluded:
EIS, EFM, Tafel polarization and weight-loss measurements demonstrated that malonic acid is good and effective corrosion inhibitor. An excellent agreement among the four experimental techniques used in research was obtained. The malonic acid corrosion inhibition efficiency decrease with an increase temperature. The malonic acid adsorption process was found to be highly spontaneous, reversible and of a physisorptive type. It was successfully described by the Langmuir isotherm. SEM reveals the formation of a smooth surface on carbon steel in presence of malonic acid probably due to the formation of an adsorptive ﬁlm of electrostatic character.
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