Inhibition of Acid Corrosion of Mild Steel Using Delonix regia Leaves Extract

. The inhibition efficacy of aqueous extracts of the leaves of Delonixregia (DR) in 1 M HCl and 0.5 M H 2 SO 4 have been investigated using weight loss, electrochemical and surface probe techniques. DR extract inhibited mild steel corrosion in both acidic environments via adsorptionof the extract organic matter on the metal/solution interface.Potentiodynamic polarization results indicate that DR is a mixed type inhibitor in both acidic environments, whereas the impedance results revealed adsorption of the DR species on a corroding steel surface. Increase in inhibition efficiency was concentration dependent. The adsorption followed Langmuir adsorption isotherm. Scanning electron microscopy (SEM) results revealed the formation of a protective layer adsorbed on a mild steel surface in the acid solutions.


Introduction
Introduction of corrosion inhibitors into acid solutions commonly employed in cleaning iron and steel surfaces is a useful method to protect such surfaces from corrosion damage [1,2]. As regards to the adsorption strength of these inhibitors, certain factors are of significant consideration, including: the nature and surface charge of the metal, composition of the electrolyte and the structure of the inhibitor [3,4]. Most effective inhibitors are organic compounds that contain heteroatoms like nitrogen, sulfur and oxygen in a conjugated system [5][6][7][8][9][10]. The inhibitors function at the interface between the metal and aqueous corrosive solution, their interaction with the metal surface through adsorption, results in a modification in the mechanism of the electrochemical process. Polar functional groups are regarded as the reaction center that stabilizes the adsorption process [11], this helps to reduce the corrosion susceptibility of the metal surface [9,12]. As a result, the service life of the metal is prolonged. The requisite electronic structural characteristics of corrosion inhibitors such as substituted heterocycles like imidazoles, pyridines, furans, thiophenes, isoxazoles, and others are readily found in plant extracts [13].
Considering the increasing ecological awareness and strict environmental regulations, attention is now focusing on the development of substitute environmentally friendly alternatives to hazardous chemical processes. Such development will definitely depend on the only practical sustainable source of materials -plant(biomass) extracts -and encourage the use of these extracts as sources of alternative benign chemical substances. Considering this fact, we have over time studied the possibility of applying biomass extracts directly in solving materials corrosion problems and as potential replacements for the costly and toxic chemicals in use presently. The justification here is that some phytochemical constituents of plant extracts including tannins, proteins, polysaccharides, polycarboxylic acids, alkaloids, etc., possess electronic structures similar to those of conventional organic corrosion inhibitors and some have been reported to function as inhibitors of metal corrosion [12][13][14][15][16][17][18][19][20]. Most organic inhibitors are known to function via adsorption, involving both specific and non specific interactions with the metal surface. Therefore, all the components are involved in the inhibition process. The leaf extract of Delonixregia (DR) has been studied in this work for its inhibiting efficacy on mild steel corrosion in aqueous acidic environments. Delonixregia possesses a lot of medicinal qualities and the decoction of the leaves is traditionally used in treating gastric problems, body pain, and rheumatic pains of joints. Ethanolic extracts of the flower and bark were investigated to anti-inflammatory activity. The leaves are reported to possess antibacterial and antimalarial properties. Delonix regia contains carbohydrates, alkaloids, flavanoids, cardiac glycosides, anthraquinone glycosides, saponins, steroids and tannins [21 -25].

Materials preparation
Mild steel specimens with chemical composition (weight %) C -0.30, Si -0.30, Mn -0.30, P -0.045, S -0.050, Cr -0.064, Cu -0.040, Ti -0.04 and the balance Fe were used for the experiments. The metal specimens were wet-polished with silicon carbide abrasive paper (from grade #150 -#1000), degreased in acetone and dried in warm air. All chemicals and reagents used in preparation of the test solutions were of analytical grade (Sinopharm Chemical Reagent Co., Ltd). The aggressive solutions were prepared using HCl and H 2 SO 4 (Sinopharm Chemical Reagent Co., Ltd) and double distilled water. Stock solutions of the crude extract were prepared by refluxing weighed (40g) amount of the dried and ground leaves of DR in absolute ethanol (500 ml) for three hours. This was used without further purification. The amount of material extracted into solution was quantified by comparing the weight of the dried residue with the initial weight of the dried plant material before extraction. Inhibitor test solutions were prepared in the concentration range 50-1200 mg/L from the stock solution.

Gravimetric experiments
Gravimetric experiments were conducted on test coupons of dimension 3 x 3 x 0.14 cm, these were abraded using Silicon carbide paper (up to 1000 grit), rinsed with distilled water, dried in acetone and warm air, weighed and stored in moisture free desiccators prior to use, these were suspended using glass rods and hooks under total immersion conditions in 300ml beaker containing the test solutions at room temperature. All tests were made in aerated and unstirred test solutions. In order to ascertain the weight loss with respect to time, test coupons were retrieved at 24 h intervals progressively for 168 hrs; immersed in 20% NaOH solution containing 200g/L of zinc dust, scrubbed with bristle brush, washed, dried and reweighed. The weight loss was taken as the difference between the initial and final weights of the coupons. Measurements were undertaken using a FAJA weighing balance of range 0.0001 to 200g.

Electrochemical measurements
Electrochemical tests were conducted using a PAR-2273 Advanced Electrochemical System workstation, with a conventional three-electrode corrosion cell. A platinum sheet and a saturated calomel electrode (SCE) were used as a counter and reference electrodes, respectively. A metal specimen fixed in epoxy resin with a surface area of 1 cm 2 exposed to the test solution, served as the working electrode. Electrochemical measurements were carried out in aerated and unstirred solutions at the end of 1800 s of immersion, which allowed the OCP values to attain steady state. Temperature was fixed at 30±1 O C. Impedance measurements were performed at corrosion potentials (E corr ) over a frequency range of 100 kHz -0.1 Hz, with a signal amplitude perturbation of 5 mV. Potentiodynamic polarization studies were conducted in the potential range ±250 mV versus corrosion potential at a scan rate of 0.333 mV/s [28].

Scanning Electron Microscopy
The XL-30FEG scanning electron microscope was used to study the surface morphology of the metal specimens after immersion in 1 M HCl and 0.5 M H 2 SO 4 in the absence and presence of DR. Mild steel specimens of dimensions 3 cm x 3 cm x 0.25 cm were prepared as mentioned earlier (Section 2.1) and immersed for 24 h in both acidic solutions, in the absence and presence of 1200 mg/L DR. The specimens were then removed and cleaned with double distilled water, dried in warm air and submitted for SEM analysis.     where W 1 and W 2 are the weight losses in inhibited and uninhibited corrodents, respectively. The high IE% values obtained indicate a strong adsorption of the constituents of DR species on a corroding metal surface.

Electrochemical Measurements
It has been established the corrosion reaction is an electrochemical process, for this reason electrochemical measurements are the most appropriate for understanding in-depth mechanistic insights into corrosion systems.

Potentiodynamic polarization measurements
Polarization experiments were carried out to investigate the influence of DR on the anodic and cathodic half reactions of the corrosion process. Parts a and b of Figure 7show typical polarization curves for mild steel specimens in 0.5 M H 2 SO 4 and 1 M HCl, in the absence and presence of DR. Accordingly, the mild steel specimen in both acidic environments shows active dissolution with no distinctive transition to passivation within the studied potential range.
The obtained electrochemical parameters, namely, corrosion potential (E corr ), and corrosion current density (I corr )were obtained and their values are presented in Table1. The data presented therein reveals that in both acidic environments the I corr decreases in the presence of the inhibitor compared to the uninhibited solution and the trend continues with an increase in the concentration of the inhibitor. In 0.5 M H 2 SO 4 and 1 M HCl the polarization curves in presence of DR (Figure 7 a and b) show evidence of inhibition. It is obvious that DR has no significant effect on E corr , however, a shift of both the cathodic and anodic curves to lower corrosion current densities was observed and this effect becomes more significant with increasing DR concentration in the studied environments.
The results obtained show that the rate of mild steel dissolution in 0.5 M H 2 SO 4 is higher than that in 1 M HCl, which is in agreement with previous reports [26,27]. The values obtained are presented in Table 1. The observed reduction in anodic and cathodic corrosion current densities shows that the inhibitor reduced the mechanism of H 2 gas evolution reaction [28] and also the anodic dissolution of mild steel. If the displacement in E corr is greater than 85 mV with reference to E corr (in the absence of the inhibitor) the inhibitor may act as a cathodic or anodic type and if the displacement is less than 85 mV the inhibitor may be regarded as a mixed-type. In our result, displacement of E corr is less than 85 mV; therefore, DR is classed as a mixed-type inhibitor.    [29]. In 1 M HCl environment, the corresponding Nyquist plots show single semicircles for all systems over the frequency range studied, relating to one time constant in the Bode plots. The high frequency intercept with the real axis in the Nyquist plots is assigned to the solution resistance (R s ) and the low frequency intercept with the real axis is ascribed to the charge transfer resistance (R ct ). It is evident in Figure 7 (a) and (b) that introduction of DR to the acidic environments result to an increase in the charge transfer resistance which points towards inhibition of the corrosion process.
In order to determine the numerical values of the various impedance parameters presented in Table 2, the impedance spectra were analyzed by fitting to the equivalent circuit modelR s (Q dl R ct ) which was used before to model the mild steel/acid interface [30 -33].An example of the equivalent circuit model is presented in Figure 9. From the data presented in Table 2, it is clear that introduction of DR lead to an increase in the R ct values at all concentrations in 0.5 M H 2 SO 4 and 1 M HCl. The values of double-layer capacitance (C dl ) were determined using the relation; Due to a modification on introduction of DR, the values of C dl decrease more than observed in the absence of the inhibitor. Charge transfer resistance and double layer capacitance also show the opposite trend, according to Helmholtz model: = (Eq. 4) ε represents the dielectric constant of the medium, ε o is the vacuum permittivity, A is the electrode area, and is the thickness of the interfacial layer. The decrease in C dl value which occurs due to a decrease in the dielectric constant and/or an increase in the double layer thickness can be related to the adsorption of DR species on a corroding metal surface. An increase in resistance with inhibitor concentration, suggests enhanced adsorption of DR species on a steel surface and efficient blocking of the steel surface [34]. Also, the inhibition efficiency (IE %) from EIS data was determined by comparing the values of the charge transfer resistance in the absence and presence of DR using relation; ) 100 (Eq. 5) The obtained inhibition efficiency values are shown in Table 2. Though the result follows the same trend, however, a slight variation in the computed corrosion rate and inhibition efficiency values was observed from polarization and impedance measurements. This could be ascribed to the processes associated with the different techniques. For example, the mild steel surfaces were held close to the equilibrium corrosion potential for impedance measurements, but taken far away from the equilibrium potential during polarization measurements, which should influence the measured values from both techniques (35).

Scanning electron microscopy
The surface micrographs of mild steel dipped in the absence and presence of optimum concentration of DR (1200 mg/L) in both acidic solutions are shown in Figure 10. Careful scrutiny of the picture, reveal that the surface of the mild steel appears rough due to active dissolution in the absence of the inhibitor. However, the roughness reduced significantly after the introduction of DR to the acid environments. The findings show that the presence of DR molecules reduced the active dissolution of mild steel by formation of a protective film on its surface. Examination of the images presented in Figure 10 shows  Adsorption of DR species on a corroding metal surface can be explained by using a suitable isotherm, which explains the variation of experimentally obtained values of the amount of adsorbed substance by unit area of the metal surface with its concentration in bulk solution at a constant temperature. For us to determine the adsorption mode of DR molecules on the metal surface in the different acid environments experimental data obtained from gravimetric result were tested by fitting to several adsorption isotherms (not shown) and the best fit was obtained from the Langmuir isotherm. If we assume a direct relationship between inhibition efficiency and the degree of surface coverage values (θ) defined as θ = IE/100 for different inhibitor concentrations, data derived from gravimetric measurements were adapted to determine the adsorption characteristics of DR on mild steel in different acid solutions according to the Langmuir equation.
C/Ө = 1/b + C (Eq. 6) Figure 11 shows the plot of C/θ against C to be linear in both acid environments, with slopes of 0.9942 (0.5 M H 2 SO 4 ), and 1.318 (1 M HCl) respectively, showing that the adsorption of DR species onto a mild steel surface obeys Langmuir adsorption isotherm. The observed deviations of the slopes from the predicted values of 1.00 can be attributed to the interactions between adsorbate species on the metal surface as well as changes in the adsorption heat with increasing surface coverage, factors which were not taken into consideration in deriving the isotherm.

Conclusions
The Delonixregia extract was found to be a good inhibitor for corrosion of mild steel in the acidic environments (0.5 M H 2 SO 4 and 1 M HCl). The inhibition efficiency was found to increase with increased inhibitor concentration. Potentiodynamic polarization results indicate that DR is a mixed type inhibitor, affecting both the anodic metal dissolution reaction and the cathodic hydrogen evolution reaction, whereas the impedance results revealed adsorption of the DR species on a corroding steel surface. The adsorption of the DR on mild steel surface followed Langmuir adsorption isotherm. Thus, indicating that there was a homogeneous layer of adsorption on the mild steel surface.