Electrochemical, surface morphological and computational evaluation on carbohydrazide Schiff bases as corrosion inhibitor for mild steel in acidic medium

Anticorrosion and adsorption behaviour of synthesized carbohydrazide Schiff bases, namely (Z)-N′-(4-hydroxy-3-methoxybenzylidene)-6-methyl-2-oxo-4-phenyl-1,2,3,4-tetrahydropyrimidine-5-carbohydrazide(MBTC) and (Z)-N′-(3,4-dichlorobenzylidene)-6-methyl-2-oxo-4-phenyl-1,2,3,4-tetrahydropyrimidine-5-carbohydrazide (CBTC) was examined for mild steel (MS) in 15% HCl medium. The corrosion inhibition study was performed by using gravimetric, thermodynamic, electrochemical and theoretical studies including density functional theory (DFT), molecular dynamic simulation (MDS) and Monte Carlo simulations (MCS). The outcomes in terms of corrosion inhibition efficiency using electrochemical impedance spectroscopy (EIS) method at 303 K and 150 ppm concentration were 96.75% for MBTC and 95.14% for CBTC. Both inhibitors adsorbed on the MS surface through physical as well as chemical adsorption and followed the Langmuir isotherm. The mixed-type nature of both inhibitors was identified by polarization results. Surface analysis was done using FESEM, EDX, AFM and XPS studies and results showed that a protective layer of inhibitor molecules was developed over the surface of MS. The results of DFT, MCS and MDS are in accordance with experimental results obtained by weight loss and electrochemical methods.

Material.All investigations were conducted using MS with elemental percentage composition, as; Sn: 0.03, Ni: 0.02, Mn: 0.13, Cu: 0.01, P: 0.02, C: 0.13 along with Fe (iron: rest elemental percentage).MS was cut into the dimension of 3.5 cm × 3.0 cm × 0.1 cm for gravimetric method and for electrochemical analysis 1.0 cm × 1.0 cm × 0.1 cm individually.Several grades (i.e., 80 to 2000) of emery paper were used to clean the MS surface before each experiment successively so that homogeneous surface was obtained.These samples were then thoroughly cleaned in an ultrasonic cleaner for 15 min, cleaned with acetone, and dried.The solution of inhibitor concentrations ranging from 0 to 150 ppm was made in 15% HCl by dilution of analytical grade 37% HCl with distilled water.www.nature.com/scientificreports/Gravimetric Analysis.Gravimetric analysis was used to investigate how temperature and concentration affected the examined Schiff base ability to prevent corrosion.MS specimens were submerged in 500 mL 15% HCl solution for 6 h at different temperatures ranging from 303 to 333 K, without and with several concentration of synthesized carbohydrazide Schiff base inhibitors.The specimens were weighed before and after submersion and weight loss was calculated.Each experimental value was calculated from the average data of triplicate value of weight loss 32 .The corrosion rate (CR), efficiency ( η%) and surface coverage ( θ) were calculated as follows: where average weight loss is represented by W, sample area by A, immersion time by T (in hours), and sample density by D (in gm cm −3 ).The efficiency (η %) along with surface coverage (θ) of the investigated inhibitor are calculated by utilizing the Eqs.( 2) and (3), as below: where CR 0 and CR I , stands for corrosion rates without and with synthesized carbohydrazide Schiff base inhibitors.
Electrochemical analysis.Electrochemical analysis was executed in conventional 3-electrode system cell by the electrochemical corrosion analyzer (CS-350) monitoring through CS Studio.Saturated calomel electrode act as the reference electrode, platinum electrode serves as the counter electrode and MS with surface area of 1cm 2 serve as working electrode.Before starting the experiment, the cells' working electrode is maintained in contact with 200 mL of acidic solution for 1800s for attaining steady state condition.Later potentials were examined in the range of ± 2.5V with a scan rate of at 0.5 mV s −1 from OCP which provides the polarization curves.Subsequently, at an open circuit voltage and frequency range of 100 kHz to 1Hz, electrochemical impedance spectroscopy (EIS) was evaluated 33 .

Surface analysis.
For surface analysis the MS specimen was prepared by pervious procedure as described from weight loss analysis.Before and after adding 150 ppm of inhibitor at 303 K, the MS surface was submerged for 6 h in 15% HCl medium.The specimen was removed after 6 h and washed with distilled water and then dried and further sampled for FESEM, EDX, AFM and XPS analysis 34 .
Computational studies.Quantum chemical calculations.All quantum chemistry calculations have been implemented with full geometric optimizations utilizing the Dmol3 module integrated into the Biovia Materials studio software.The geometric structures of the studied molecules were attained through global optimizations characterized by a calculation of the vibration frequencies using the DFT method by utilizing Dmol3, including the B3LYP/DND/COSMO (water) model [34][35][36][37][38][39][40] .
MC and MD simulations.Materials Studio 8.0, programmed created by Accelrys Inc, was used to run MC and MD [41][42][43][44] .A three-dimensional geometry simulation box whose dimensions are (24.823752× 24.823752 × 18.241 658) was used for simulations of the interaction between the MBTC along with CBTC molecules and the surface of Fe (110).The MDS were worked at a temperature of 298 K and the motion equations were integrated under the NVT canonical ensemble utilizing the COMPASS force field, with a time step of 1 fs and a simulation time of 800 ps.This model included Fe layer, 15 hydronium, 15 chloride, 700 H 2 O molecules and 1 inhibitor molecule and a 35 vacuum layer were involved in the simulation box.

Results and discussion
Gravimetric analysis.The gravimetric analysis is a first step in the investigation of corrosion inhibition.
The effect of concentration on the inhibition efficiency (η %) and corrosion rate (CR) for the MS in 15% HCl at various temperatures (303 K-333 K) was done and obtained corrosion parameters are specified in Table 2.
According to Table 2 and Fig. 2, it is found that, as the surface coverage of MS grew, the CR gradually dropped and corrosion inhibition (η %) gradually increased with rising inhibitor concentration.This indicated that sample surface is protected through the molecule of inhibitors during the increase in concentration as they are absorbed on the MS surface.Thus, the highest inhibitory efficiency obtained was at 150 ppm of MBTC as 96.78%, and it was 95.09% for CBTC.The higher efficiency of MBTC is because of the presence of electron donor group -OCH 3 , imine, carbonyl groups and heterocyclic rings (Fig. 1) which improved the efficiency level in the transfers of unpaired electrons from inhibitor molecules to the vacant d-orbital of Fe, which develops a coordinate-bond and thus slows the rate of corrosion (CR) 45 .The decrease in the inhibition potency on increase in temperature was observed due to removal of adsorbed inhibitory layer from MS surface.As per literature review, corrosion inhibition studies of MBTC and CBTC is not reported and both inhibitors offered better efficiency than most of the similar type of inhibitors reported in literature [16][17][18][19][20][21][22][23][24][25][26][27][28][29][30]   where activation enthalpy and entropy are represented by ∆H * and ∆S * while A is the pre-exponential component called Arrhenius constant respectively.T refers to the absolute temp., and H, N, R, refers to Planck constant, Avogadro number and gas constant respectively.By fitting the Arrhenius and transition plots shown in Figs. 3  and 4 respectively, the values for the activation parameters ( E a ) and ∆H * and ∆S * were determined and provided in Table 3.
The E a value of inhibitors MBTC and CBTC for all concentrations are greater than uninhibited value.These demonstrate that a shielding layer was created on the MS surface and makes more resistant to corrosive process.From Table 3, the negative values of ∆S* reflect that the rate-determining step is the association rather than dissociation of activated complex, and the positive data of ∆S* denotes an increment in disorder because desorption of more number of water molecules from the surface of MS by adsorption of one molecule of inhibitor while the positive values of ∆H indicate the difficulty in MS dissolution 46 .

Adsorption isotherm.
Adsorption isotherm studies provide information on the interactions between the inhibitor and the MS surface.The following equation is representation of the Langmuir isotherm,  www.nature.com/scientificreports/Different adsorption models were tried with the experimental data, Langmuir isotherm gave a linear plot between C inh θ vs Concentrations which are shown in Fig. 5, through which K ads value was calculated.Using K ads value, G 0 ads value was derived from the following Eq. 7.
R, T and 55.5 denote the gas constant, absolute temp., and the conc.(mol l −1 ) for water.Since the R 2 as well as slope values for the Langmuir isotherm are very close to one, it has been verified to be a superior fit for the examined value displayed in Table 4.
Experimental data from Table 4, has larger value of K ads indicating superior inhibition efficiency and good interaction between carbohyrazide Schiff base and MS surface whereas lower value of K ads indicates weaker interaction so that adsorbed molecules can be easily removed from MS surface.
The negative G 0 ads values suggests that the adsorption of carbohydrazide Schiff base MBTC and CBTC on the surface of MS was proceeding spontaneously and the value of G 0 ads obtained for both inhibitors verified physisorption as well as chemisorption adsorption at MS surface.According to the review of literature, G 0 ads value less negative than − 20 kJ mol −1 results physisorption whereas more negative than − 40 kJ mol −1 results chemisorptions.Table 4 shows that the values of G 0 ads for synthesized carbohydrazide Schiff base in 15% HCl at various temperatures attain the range showing the spontaneous chemisorption and physisorption, which is a mixed nature of adsorption 47 .( 6)  6, displays that the addition of MBTC and CBTC inhibitors shifted the potential to the negative side when compared with pure 15% hydrochloric acid corrosive medium, indicating that both inhibitors are predominantly cathodic.
Whereas the obtained data of OCP for the working electrode with various dosages of both inhibitors alter OCP to an extent of less than ± 85 mV vs. OCP data as compared to without inhibitor, indicating that the studied inhibitors are mixed type and dominant towards cathodic nature 48 .

Potentiodynamic polarization (PDP).
PDP is a significant electrochemical method to know about inhibition efficiency of inhibitor.PDP gives value of corrosion current and corrosion potential of the electrode immersed in corrosive solution.Here.PDP curves were plotted to know the impact of various dosages of carbohydrazide Schiff base inhibitor at MS/15% HCl interface as shown in Fig. 7. PDP variables such as, corrosion potential ( E corr ) , anodic and cathodic slopes (β a & β c ), and corrosion current density were observed from PDP curves and arranged in Table 5.The efficiency of corrosion prevention was obtained from Eq. ( 8) below.
It is noticed that by increase of the conc. of MBTC and CBTC, there is a gradual decrease of i corr value.This observation shows the improvement of a protection film on the MS surface by the adsorption of inhibitor molecules that decreases the dissolution of MS surface.This process is more effective with the increment in the conc. of MBTC and CBTC and reaches maximum at 150 ppm 97.46% and 95.65%, individually.Furthermore, PDP shows very less shift in E corr data ( lessthan 85 mV) towards more negative side compared to the uninhibited MS, indicating that both inhibitor molecules control anodic as well as cathodic reactions and act as mixed inhibitor.In addition the shift in E corr values and the variation of β a and β c values with inhibitor concentration w.r.t.without inhibitor also suggested that studied inhibitors are mixed type inhibitors.From Table 5, the data of β a and β c do'nt show regular trend which indicate that both physisorption and chemisorption plays an important role in the corrosion inhibition 49  EIS analysis.EIS is used to correctly measure the inhibition efficiency of an inhibitor.The Nyquist and Bode plots were obtained without and with MBTC and CBTC inhibitors as depicted in Figs. 8 and 9, respectively.Nyquist semicircles were fitted against electrical circuit, shown in Fig. 10.This circuit consist of solution resistance (R S ) , a double layer capacitance ( C dl ) and a charge transfer resistance (R ct ) .The obtained data are shown in Table 6.The obtained Nyquist plots have depressed semi-circles with a single loop, suggesting single time constant participation in the EIS spectra 50 .The depression in semi-circle is because of inhomogeneity of electrode surface.The rise in diameter of capacitive loops with an increment in carbohydrazide Schiff base inhibitor concentration suggests an increase in the charge transfer resistance as a result of inhibitor adsorption on MS surface.The impedance efficiency was obtained by utilizing the accompanying equation: where,R As the inhibitor concentration increases, C dl reduces and R ct increases as represented in Table 6, the results confirmed the high degree of adsorption of carbohydrazide Schiff base at MS/HCl interface by sharing of nonbonding heteroatoms electrons of inhibitor molecules to the vacant d-orbital of Fe.MBTC shows maximum 96.75% inhibition efficiency at 150 ppm against 320.67 Ωcm 2 R ct value, Whereas CBTC shows maximum 95.14%  where (Y 0 ) for proportional factor and (n) for phase shift and it is measured for the surface inhomogeneity.
In Bode plots continuing increment in the phase angle with concentration up to 150 ppm is related to the development of surface coverage by the inhibitors MBTC and CBTC molecules.The maximum height in the phase angle towards − 90°, the existence of single maxima demonstrates presence of one time constant related to electrical double layer at the surface solution interface 52 .
The inhibition efficiency calculated by gravimetric, PDP, & EIS methods for MS in presence and absence of inhibitors in 15% HCl solution are in good agreement to each other.The high corrosion inhibition proficiency value of both studied MBTC and CBTC even at lower concentration suggested that MBTC and CBTC are good corrosion inhibitors for MS in acidic solution.
Surface morphology study.FESEM analysis.The surface morphology of polished MS, after 6 h of immersion in 15%HCl solution in the absence and presence of 150 ppm for MBTC and CBTC inhibitors are depicted in Fig. 11.From Fig. 11B, it is observed that the metallic surface is extremely corroded in HCl solution without inhibitor whereas polished sample (Fig. 11A) is smooth due to minor corrosion.However, Fig. 11C and D containing inhibitors show very smooth MS surface and it contain less cracks and pits due to adsorption of MBTC and CBTC inhibitors, respectively, at MS surface.Consequently, these inhibitors demonstrated that MS is protected against corrosion by forming protective layer of inhibitor at the surface of mild steel 53 .( 10)    AFM analysis.AFM analysis is an important technique for 3-D representation of surface with appropriate height profile of inhibitors.It consists of the important parameter of average roughness (R max ), average of maximum height (R a ), and the greatest separation among the maximum point and minimum points on the profile roughness (R q ).The AFM of samples are shown in Fig. 13.The result for the height profile for polished surface (Fig. 13A) are R max = 57.3nm, R q = 6.07 nm, R a = 4.92 nm.For blank sample (Fig. 13B) R max = 567 nm, R q = 77.9nm, R a = 62.6 nm.From Fig. 13C the surface with MBTC containing R max = 107 nm, R q = 10.and is associated with the inhibitory capacity 65 .A high value of the μ results in significant inhibitory efficacy and increases the interaction (adsorption) of inhibitor compounds with a metal surface 42 .Table 7 shows that the μ of the MBTC (9.562 Debye) is greater than μ of the CBTC (4.126 Debye).As well as, both inhibitors are greater than that of the H 2 O molecule (1.88 Debye), which explains the significant inhibitory efficacy of the MBTC inhibitor.
According to Table 7, the data of ∆N for both inhibitor is positive and greater than 3.6 (N > 3.6), which shows that electrons are transferred from both inhibitors studied to iron, promoting the development of coordination bond and thereby helping the formation of an adsorbed layer against corrosion.
To determine the active sites accountable for nucleophilic and electrophilic attacks, decided to use ESP surfaces as a useful descriptor.Figure 16 shows the ESP of the MBTC and CBTC molecules inhibitors study, the blue color shows the positive zones with nucleophilic reactivity, whereas yellow and red colors represent the negative zones with electrophilic reactivity.
As displayed in Fig. 16, the two inhibitors studied have 4 oxygen atoms possible electrophilic attack sites for the MBTC molecule and 2 possible electrophilic attack sites for the CBTC molecule.On the other hand, the two aromatic rings for the two compounds studied have a negative region.There are also on the two molecular structures of our inhibitors the 4 nitrogen atoms which are 4 other possible sites for the electrophilic attack.From the calculated zones of ESP, we notice that the potential negative zones possible are located on the electronegative atoms (N and O) and the unsaturated double bonds along with potential positive zones possible were located around the hydrogen atoms.
MC and MD simulations.The study of the adsorption behavior of the MBTC and CBTC molecules were examined on the surface of MS using MC and MD simulations [66][67][68] .
Figure 17shows the adsorption equilibrium configurations for the two MBTC and CBTC molecules on the Fe (110) surface, thus two inhibitory molecules are adsorbed on the Fe (110) surface in almost parallel mode position (almost horizontal), indicating that the adsorbed inhibitor molecules cover the Fe surface well by developing a horizontal protective layer on the steel surface, which confirms the strong interactions among the heteroatoms (O, N) and the conjugated system of these inhibitors and atoms of iron.
Figure 18 displays the adsorption energy (E ads ) that was calculated by MC simulations of the MBTC and CBTC molecules with the surface of Fe (110).The compounds investigated are listed in the following order according to their effectiveness at inhibiting growth: MBTC > CBTC.
The most energetic stable positions of inhibitors are found by studying the temperature variations in MD simulation analyses.The little temperature drift in Fig. 19 indicates that the MD of our system was successful 69,70 .
The bond length among the iron and the atoms of the MBTC and CBTC was determined by utilizing the radial distribution function (RDF) analysis of the MD trajectory in Fig. 20.Through estimating bond length data, different types of bonds formed was determined 39,71 .The type of adsorption activity occurring on the metal is shown as peaks in the RDF plot at specific locations relative to the metal surface 41,72,73 .The mechanism of chemisorption is reflected, when the peak is present between 1 and 3.5, but the RDF peaks are expected for physisorption at distances larger than 3.5 [74][75][76] .
The inhibition mechanism.The hypothesized corrosion mechanism of MBTC for MS in 15% HCl solutionis presented in Fig. 21, as determined by the experimental and theoretical investigations.Inhibitor molecules, by virtue of their adsorptive affinity, reject the pre-adsorbed H 2 O molecules and corrosive species, finally adsorb on the surface of MS, therefore sheltering cathodic along with the anodic site 77,78 .This protects the surface against corrosion.In particular, because the compound possesses both a heterocyclic ring and other heteroatoms, as shown by the Mulliken charges, there is an increased predisposition toward surface adsorption 79 .These heteroatoms (O, N) and the conjugated system are able to donate lone pair electrons to the empty d-orbit of the iron atom 80 .
Adsorption has an important role in corrosion prevention because the organic molecules are effectively and efficiently adsorbed on the surface, thus stop further deterioration on the surface of MS.All of the information www.nature.com/scientificreports/
ct are the polarization resistance data in the inclusion along with exclusion of an inhibitor individually.
3 nm, R a = 8.14 nm and Fig. 13D, in presence of CBTC, R max = 156 nm, R q = 20.0 nm, R a = 15.4 nm.The lower value of roughness in presence of inhibitors is because of the adsorption of inhibitor molecules on the MS surface 55 .XPS analysis.XPS is used to know the composition of adsorbed inhibitor layer on MS surface.Result of XPS analysis of MBTC and CBTC inhibitor are shown in Figs. 14 and 15 respectively.

Figure 17 .
Figure 17.Results of MC (a) and MD (b) simulations: The equilibrium configurations of MBTC and CBTC molecules adhering to the surface of iron (110).

Figure 18 .
Figure 18.Distribution of the E ads of MBTC and CBTC:MC simulations of the molecules on Fe (110) plane.

Figure 19 .
Figure 19.Temperature fluctuation at 298 K during the MD run of MBTC and CBTC molecules on the Fe(110) plane.

Figure 20 .
Figure 20.RDF of O, N and Cl atoms of MBTC and CBTC in the simulated corrosion media on the Fe plane via MD.

Table 1 .
Summary of already reported Schiff bases as corrosion inhibitors for MS in HCl solution.

Table 2 .
as shown in Table1.Therefore the work reported in the present investigation is novel and has potential industrial application.Gravimetric parameters of carbohydrazide Schiff base inhibitor.

Table 5 .
Polarization data of carbohydrazideschiff base inhibitors.
Conc. (ppm)E corr (mV/SCE) i corr (µA cm −2 ) β a (mV dec −1 ) −β c (mV dec −1 ) IE T % against 214.48Ω cm 2 R ct value with R ct value.It confirms that MBTC has better inhibition performance against the 15% HCl corrosive medium.The value of C dl was obtained by equation

Table 6 .
EIS data for carbohydrazideschiff base inhibitors.

Table 7 .
Calculated theoretical parameters for MBTC and CBTC molecules.