Synthesis and applications of novel Schiff base derivatives as corrosion inhibitors and additives for improvement of reinforced concrete

The studied Schiff-base compounds in this work are multitasked investigated as corrosion inhibitors and also, to improve the physical and mechanical properties of reinforced concrete. The efficiency inhibition of the two novel Schiff-base compounds named (DHSiMF) and (DHSiB) for corrosion of carbon-steel in aqueous media of 1 M HCl was assessed via electrochemical methods and loss in weight. FT-IR, 1H-NMR spectra and elemental analysis were used to confirm the structure of such compounds. It was found to have successful inhibition even at low concentrations in tested media, as an increase in inhibitor concentration led to an improvement in the inhibition efficiency. The weight loss results clearly demonstrate that DHSiMF of C-steel in 1 M HCl has a higher inhibition efficiency than DHSiB, with a maximum inhibition efficiency (85%) attained at 1 × 10–2 M from DHSiMF. Electrochemical experiments likewise revealed the same order, but with a maximal inhibitory efficiency of 98.1%. The addition of inhibitors to the corrosive media dramatically changed the anodic Tafel constants (βa) and cathodic Tafel constants (βc), indicating a mixed type nature. Electrochemical polarization curves illustrated the functions of mixed-type inhibition and the action of adsorption matching with the Langmuir adsorption isotherm. The ∆Gads values for DHSiMF and DHSiB at temperatures (ranging from 303 to 333 K) are − 34.42 kilojoule/mole to − 37.51 kilojoule/mole. These values indicate that the compounds’ adsorption types are chemo-physical adsorption. X-ray diffraction (XRD) and Scanning electron microscopy (SEM) experiments were used to check the existence of the protection layer on the surface of carbon steel by analyzing the morphologies of the corrosion effects and the formed chemical compositions of the corrosion outcomes. For the concrete, the findings suggest that the chemical reaction that takes place between the DHSiMF and DHSiB and the concrete mix will result in an increase in the flexural strength, the compressive strength, and the indirect tensile strength of the concrete that is made of the gravel and dolomite aggregate.

www.nature.com/scientificreports/Solutions were subsequently put into 150 mL sealed glass bottles and the coupons were suspended without stirring at room temperature in these solutions.After 24 h immersion time, the samples were collected, cleaned, dried with warm air of 40 °C and exactly weighed.The Eqs. (1), (2) estimated the surface coverage degree (Ө) and the efficiency of inhibition (η w %) respectively 27 ; where the values of weight loss given the symbol ( W o and W inh. ) without and with definite inhibitor concentra- tions individually.Equation (3) estimated the rate of corrosion C R (mg cm -2 h -1 ) 28 .
where S and t are express the surface area of each tested specimen and the time of exposure respectively.

Electrochemical measurements
For electrochemical tests, a cylinder of rebar with a surface area of 1 cm 2 was coated with epoxy resin and then used as a working electrode.The electrode layer was abraded using a sequence of emery paper (320:1200) and cleaned with bi-distilled H 2 O, and acetone.
For polarization and impedance testing, a Volta Lab Master Radiometer (model PGZ 301) equipped with the Zsimpwin software application was used.Three electrode systems were deployed against a carbon steel working electrode, including a saturated calomel electrode reference system and a platinum wire counter electrode.To measure impedance, a 10 mV AC sine wave with an open circuit potential and a frequency range of 100 kHz to 50 mHz was used.For polarization measurements, a potential of (− 800:− 300 mV) was required at a scanning rate of 0.2 mVs -1 .The electrochemical experiments were carried out at 30 °C (ASTM G3-74 and G-87).By extending the linear Tafel part of the anodic and cathodic curves to the corrosion potential axis, you can figure out how much corrosion is going on.

Monte-Carlo (MC) and molecular dynamics (MD) simulations
The Fe (110) model was used to mimic the interaction of the two Schiff base molecules with the steel surface in the corrosive environments (1.0 M HCl) as it is more stable than the other (1 0 0) and (1 1 1) Fe planes 29,30 .Fe crystal split along (110) plane.The Fe (110) plane was expanded to 10 × 10 supercell contact using a vacuum slap of 30 Å at the top of the plane to achieve dimension of 22.341 × 22.341 × 48.422 Å 31 .The aqueous solution was introduced as 1 inhibitor molecule, 200 H 2 O, 20 H 3 O + and 20 Cl -.MC and MD simulations were performed using the adsorption locator and Forcite Dynamics tools respectively implemented in BIOVIA Material Studio 2017 software 32 .Utilizing a Nose thermostat with a 298 K temperature setting, MD was performed.It is decided to use an NVT ensemble with a simulation duration of 50 ps and a time step of 1 fs.The force field used for the energy was COMPASS II.

X-ray diffraction (XRD) measurements
It was utilized to describe the corrosive conditions of the metal and to identify the corrosion products that were spread throughout its surface.Used metal coupons were soaked in an offensive solution for 48 h, both with and without a properly determined concentration of DHSiMF and DHSiB.The coupons' rust was eliminated, finely ground, and then homogenized.The phases were identified using X-ray powder diffractometry (X'PERT-PRO-MPD-PANalytical, Netherlands).At 40 kV/40 mA and 1.5406A o , the resultant patterns for Cu K radiation were obtained.The samples were examined incrementally in the range of 4°-80° with 0.02-degree step sizes and a phase time 0.40-s phase period.

Mix design and preparation of test specimens
On the other side, the mix design and preparation depended on the ACI Committee was used to compute the quantities of materials required for the test batch.Many trials mixes were made to adjust the proportions of the used materials to give the 250 kg/cm 2 compressive strengths and suitable workability.The concrete mixes proportions required for one cubic meter of concrete are Cement 250 (kg), fine aggregate 670 (kg), coarse aggregate 890 (kg), water 150 (liter), and siloxane concentration 100, 200, 300, 400, and 500 ppm.A homogeneous concrete mixture was required, thus mixing procedures were carried out at a temperature of 20 to 25 °C.The specimens were de-molded and submerged in water for 7 and 28 days after being exposed for 24 h.At each of the test ages, each water-cured specimen was removed from the water, then it was rubbed with a clean and dry cloth to give it a saturated and dry surface.

The compression tests
The compression test was carried-out on 72 cubes specimens in which three specimens for each group of two ages at age of 7 and 28 days with using two kinds of coarse aggregate were tested and their average strength was taken into consideration.
On the other hand, the indirect tensile strength is the capacity of a material or structure to withstand indirect loads and the ultimate tensile strength of a material is calculated from the following Eq.( 4):

The indirect tension tests
The indirect tension test was carried-out on 72-cylinder specimens in which three specimens for each group of two ages at age of 7 and 28 days with using two kinds of coarse aggregate were tested and their average strength was taken into consideration.In the other side, the procedure for conducting a flexural strength test in compliance with the requirements was outlined.

The flexural tests
The authors chose the four-point flexural test over the three-point test because it allows the specimens to experience a pure bending moment in the middle third of their length, as well as pure compressive and tensile stress in their cross-section due to the absence of shear force in that area.During a four-point flexural test, the crosssections of the specimens are both compressed and stretched at the same time.The Flexural strength test was carried-out on 36 beam specimens 100*100*500 mm in which three specimens for each group at age of 28 days with using two kinds of coarse aggregate were tested and their average strength was taken into consideration due to following Eq.(5).

Confirmation of chemical structure of the prepared Schiff bases
The accompanying OH stretching vibrations of the associated alcohols, phenolic benzene ring, and C-H furan ring, respectively, are responsible for the broad band seen in the FTIR spectra of (DHSiMF and DHSiB) in the 3348-3100 cm -1 range.The aliphatic C-H stretching vibration is what causes the two bands in the region of 2941 and 2887 cm -1 .In addition to the C-H stretching vibration of the ethylene groups, the bands in the www.nature.com/scientificreports/region of 1639 cm -1 that are indicative of C=N and may overlap and absorb in this range of 1700-1630 cm -1 .The stretching vibration of the C furan =C furan and the C Aro =C Aro in the polar aliphatic methyl alcoholic type causes the 1620-1600 cm -1 and a shoulder at 1517 cm -1 .The peak at 1372 cm -1 corresponds to CH 2 and C-H groups bending, and the band at 1440 cm -1 is caused by in-plane deformation of -CH 2 (CH 2 -C-N).The big peak at 1032 cm -1 caused by (C=S) and the absorption band at 1151 cm −1 are both related to ether vibrations (C-O-C).Finally, Si-OCH 2 could be responsible for the band at 1071 cm -1 .The majority of two novel compounds' 1 HNMR spectra are found at the triplet at δ1H  Table 1 and Fig. 2 show that the physical properties for the aggregate and cement respectively.Bulk density or unit weight of an aggregate is the amount or weight required to fill a container to a specific unit volume, Mass/ Volume equals bulk density 33 .Common building materials used to make normal-weight concrete have a bulk density of between 1200 and 1750 kg/m 3 .
From Table 1, the values of bulk density are 1634, 1357, and 1542 for sand, gravel, and dolomite respectively, these values for bulk density are agree with the value for the bulk density for common materials used in concrete.On the other side, the proportion of an aggregate's mass to the mass of an equivalent volume of water is the relative density (specific gravity) of the substance 34 .The majority of aggregates have a relative density of 2.4 to 2.9 and a particle density of 2400 to 2900 kg/m 3 .The values for specific gravity computed in Table 1 are 2.33, 2.7, and 2.5 for sand, gravel, and dolomite respectively, these values are agreed with the standard values.On the other hand, the cement properties (Consistency of standard cement paste, Table 1 calculates setting time and compressive strength, these values for cement properties are agreement with the ESS limits.Figure 2 shows the plot between the particle size for sand, gravel, and dolomite with the sieve opening (mm).The particle size for dolomite is 0, 4.55, 58.7 and 100 with the sieve opening is 4.75, 9.5, 19 and 38 respectively.The particle size for sand is 0, 0, 21, 79, 91 and 96 with the sieve opening is 0.13, 0.25, 0.5, 1, 2 and 4 respectively.

Open circuit potential measurements
Figure 3a and b shows the relationship between the potential and the time at a constant current in 1 M HCl in the absence and presence of the corrosion inhibitors DHSiMF and DHSiB, respectively.From Fig. 3a, we find that the behavior of DHSiMF concentrations differs at the time from 0 to 400 s.The potential of high concentrations start from less negative values then decreases until it reaches a steady state, while the low concentrations start from the more negative potential, then the potential increases until it reaches a steady state.It is also clear from Fig. 3a shows that the behavior of the inhibitor DHSiMF tends towards the cathodic direction concerning the blank, that is, the potential difference of the anticorrosion is greater than the voltage of the blank, and this indicates that the anticorrosion tends to prevent the cathodic reaction.On the other hand, it appears from Fig. 3b that the anticorrosion behavior of DHSiB at different concentrations is less in the anodic direction than the blank potential until it reaches a steady state, and this indicates that the anticorrosion prevents the anodic reaction.This behavior of corrosion inhibitors may be explained by the presence of furan and benzene rings with their contact with the methanol and hydroxyl groups, which work to increase and decrease the voltage in order.

Electrochemical impedance measurements
Parameters of electrochemical kinetic resulted by EIS-technique such as solution resistance (R S ), charge transfer resistance (R ct ), double layer capacitance (C dl ), and inhibition efficiency (IE) evaluate the electrochemical properties of samples 35,36 .The impedance data of carbon-steel was clarified as Nyquist plots in Fig. 4 and reported in Table 2 after 30 min.as an inundation time in 1 M HCl with and without DHSiMF and DHSiB 10 -6 -10 -2 mol L -1 concentrations at ambient temperature.At the higher and lower frequencies, the semicircles were produced, which cut the real axis.The solution resistance (Rs) is equal to the intercept produced from cutting the semicircles with the real axis at higher frequencies.While the summation of the solution resistance and charge transfer is equal to the intercept produced from cutting the semicircles with the real axis at lower frequencies.The difference between the two values at low and high frequencies equals charge transfer 37 .At the interface between the solution and metal, on the solution side, the ions govern the charge distribution, while on the metal side, the electrons govern the charge distribution.In other words, the charge of the metal will occupy a large volume on the solution side of the double layer because the ions are much larger than the electrons.So, CPE is used to talk about the non-ideal capacitive behavior of the double layer instead of C dl , which is the capacitance of the double layer 38,39 .The capacitance of double layer (C dl ) can be determined based on the following equation 40 : where (ω max.= 2πf max.and f max.are the frequencies at which the imaginary component of the impedance is maximal, respectively, and Q is a proportional factor (frequency independent constant).Figure 4 shows that the semi-circle diameter increases with the increase of the electrolyte concentration of DHSiMF and DHSiB detecting an enhancement in the resistance of corrosion.
Table 2 shows also the C dl values at various DHSiMF and DHSiB concentrations.The electric capacitance of the electrode-electrolyte interface, or the rate of the charge/discharge process at the interface, is found to fluctuate in the presence of prepared inhibitors in the corrosive medium.As the concentration of the studied compounds rose, the C dl values dropped.Furthermore, the capacitance outcomes may indicate an increase in the layer deposited by the DHSiMF and DHSiB concentration.This is described by the creation of multiple layers of investigated inhibitors on the c-steel.The IE% values were estimated from Eq. ( 7) 41,42 : where the charge transfer resistance in the free and inhibitor-containing states is symbolized as R free ct and R inh.ct respectively.It was found that the increase in DHSiMF and DHSiB rise the R ct values determining a charge transfer resistance in the C-steel dissolution reaction 43 .The progressive removal of H 2 O reduced C dl and the corrosion ( 6)  www.nature.com/scientificreports/activity.The increase of IE values indicates the inhibiting influence of DHSiMF and DHSiB on the C-steel/solution interface is may due to the adsorption on surface with thin organo-silicon layers formation.The inhibition efficiency of DHSiMF of C-steel in 1 M HCl is more than DHSiB.Table 2 displays the n values derived from EIS measurements.The n value is around 0.86 when DHSiMF and DHSiB are absent.The formation of impurities, roughness, crystalline network flaws, active site size and irregular distribution, and other surface defects have all been connected to surface heterogeneity, which has been shown to be clearly visible at n values smaller than one 44 .The n values show that the presence of these chemicals impaired surface homogeneity because they were present in higher concentrations than those in the corrosive media without inhibitors.A more erratic metalsolution interface was produced by the adsorption of the DHSiMF and DHSiB, which have high molecular sizes and are asymmetric in relation to the corrosion products.
It is useful to get a hypothetical electrical equivalent circuit (EC) as mentioned above to investigate the frequency rate changes with electrochemical impedance 45 .As shown in Fig. 4, the EIS obtained from DHSiMF and DHSiB at different concentrations has been fitted to electrical EC.The literature has defined the EC of CPE parallel to (R ct ), which was used to represent the iron/acid contact, and the analogous circuit for the suppression of steel acid corrosion 46 .Double layer capacitors are equiltative with constant phase elements (CPE) and some pore quantities are similar n quantities to 0 ˃ n < 1. Due to the charged surfaces being covered and the capacitive effects being diminished, the addition of (DHSiMF and DHSiB) causes the CPEs to drop.
For the Bodes plots (Fig. 5), the values of impedance at 0.01 Hz are important measurable parameter that may be used to evaluate the resistance to corrosion in the presence of inhibitors 47 .The strong corrosion resistance propensity of the PMS treated alloys is shown by high values of |Z| 0.01 Hz.It is clear from Fig. 5 that the results obtained from |Z| 0.01 Hz agree well with the Nyquist plots.Additionally, the presence of DHSiMF and DHSiB in the corrosive solution exhibit excellent corrosion resistance as evidenced by the high impedance level |Z| in the low frequency region 48 .For a pure capacitor, the slopes of lines at middle frequencies are − 1.In our situation, the improved capacitive behavior is indicated by the slopes shifting towards -1 following the addition of DHSiMF and DHSiB molecules according to S values in Table 2.In contrast to the blank sample, the DHSiMF and DHSiB containing solutions have the greatest phase angle (α°) values, which achieved − 66.78 in case of highest concentration of DHSiMF.Since the phase angle value (α°) for perfect capacitive behavior is − 90°, this demonstrates the stability and insulation of the passivation and therefore the growth of capacitance behavior by addition of the inhibitor's molecules 49 .

Potentiodynamic polarization measurements
Figure 6 shows the Potentiodynamic polarization curves for solutions of 1 M HCl with and without DHSiMF and DHSiB.The cathodic curves do not clearly demonstrate of Tafel regions therefore, the linear sections of the anodic regions have been used to determine the Potentiodynamic characteristics 50 .Tafel plots were used to evaluate the corrosion properties provided in Table 3: anodic and cathodic Tafel slopes (β a and β c ), inhibition efficiency (IE %), surface coverage degree (θ), corrosion current density (i corr ), and corrosion potential (E corr ).The polarization diagram shows the actions of the Tafel form.
Table 3 shows the (β a and β c ) values of that are not inhibitors concentration features and do not give a welldefined character.It also identifies the existence of DHSiMF and DHSiB in HCl medium not only decreases the anodized oxidation of C-steel but also slows down the cathodic hydrogen evolution.Both the cathodic and anodic   www.nature.com/scientificreports/overvoltage changed due to the presence of DHSiMF and DHSiB, this largely created a parallel displacement to the more negative and positive values connected with the blank curve 51,52 .
A further indication that DHSiMF and DHSiB molecules adsorb to the surface and lower the reaction after blocking the carbon steel surface reaction sites without altering the anodic and cathodic reaction mechanism is the small shift that the DHSiMF and DHSiB generate in (β a and β c ). From Eqs. ( 8), ( 9), the inhibition efficiency was computed 53,54 : where the corrosion current densities expressed by (i corr , and i corr ) while the corrosion rate symbolized in (CR corr, and CR corr ) after and before inhibitor addition respectively.It is clear from Table 3 values that i corr drops significantly for the studied media when DHSiMF and DHSiB are present, indicating that compounds adsorb on the surface thus suppressing both reduction of oxygen and dissolution of metals.It's also reported that the E corr values of the non-inhibitor systems have changed with the introduction of DHSiMF and DHSiB to the corrosive medium.Corrosion potential relocations (ΔE corr ) caused by the existence of the tested inhibitors were determined according to the following equation: where the corrosion potentials of the sample before and after the inhibitor, respectively, are E inh.
corr and E o. corr .Several papers have demonstrated that the E corr value can also be used to determine the corrosion inhibition effect.If the E corr values are less than 85 mV, the DHSiMF and DHSiB are considered inhibitors of mixed type.The DHSiMF and DHSiB can be either anodic or cathodic, depending on whether the E corr is greater than + 85 mV or less than -85 mV 55 .According to E corr values, DHSiMF for the current work may be classified as a mixed-type inhibitor and DHSiB as an anodic inhibitor.While the amount of additives was increased, the corrosion rate (i corr ) went down and the efficiency of stopping corrosion (% IE) went up.This shows that DHSiMF and DHSiB work as corrosion inhibitors on carbon steel in a 1 M HCl solution.(

Adsorption considerations
Adsorption isotherms can be used if the inhibitors' efficacy is attributable to adsorption on the metallic substrate, as proven by electrochemical and weight loss data 56 .Adsorption isotherms are commonly acknowledged to explain the nature of the interaction between of active metal surface sites with inhibitor molecules 57 .To determine the covered surface (θ) as a function of inhibitor concentration (C inh ), weight loss tests were utilized, which had already been graphically assessed by the several isotherms that are appropriate to pick the optimal convenient presented in this report.Based on R 2 values closest to unity, the Langmuir isotherm has offered the finest overview of carbon steel adsorption and may be described as: where K ads. is the equilibrium adsorptive constant.The DHSiMF and DHSiB adsorption on the metal surface in HCl conforms to the Langmuir isotherm depending on the relationship of C ihb /θ vs C ihb in Fig. 7.
(11)  www.nature.com/scientificreports/ The Langmuir isotherm argues that a fixed fraction of active locations on a solid surface contain a single adsorbed species.The equation depicts the relationship between K ads.. and the standard free energy of adsorption ΔG ads is 58 : where R is the universal gas constant and T is the absolute temperature.(55.5) is the concentration of water in solution mol L -1 .
Table 5 illustrated the high K ads.and negative G o ads.values which show that DHSiMF and DHSiB molecules are spontaneously and intensively adsorbed to the metal surface.The adsorption process will be compatible with the electrostatic interaction between charged molecules and the charged metal surface (physic-sorption) if thermodynamic calculations yield G o ads.values of -20 kJ mol -1 .Charge transfer from DHSiMF and DHSiB molecules to the metal surface occurs during chemisorption 59 at G o ads.−40 kJ mol -1 .For this study, the values of G o ads.are between-37.51kJ mol -1 and − 34.42 kJ mol -1 for DHSiMF and DHSiB at temperatures ranging from 303 to 333 K.The physical and chemical adsorption are thus thought to be the best explanation for the adsorption process for the complex interactions of DHSiMF and DHSiB on the carbon steel surface 60 .Table 5 further implies that chemical adsorption is more likely for DHSiMF and DHSiB in examined solutions, which may be confirmed by comparing the (%IE) for each inhibitor at the above-mentioned temperatures.The Van't Hoff Eq. ( 13) enabled us to calculate the adsorption heat ( H o ads. ) 61,62 : When lnK ads.and 1/T are plotted, H o ads can be produced (Fig. 8).(− H o ads /R) denotes the slope of the straight lines created.
Adsorption heat under test circumstances can be generally characterized as the standard heat of adsorption H o ads .Finally, ( S o ads ) the standard adsorption entropy may be calculated 62 using the Eq. ( 12): The value of H o ads.reveals more about the mechanism of corrosion inhibition.When ( H o ads.< 0), the adsorption process is followed by the release of heat (an exothermic process), which can be chemi-sorption, physicsorption, or both, whereas the endothermic process is associated with chemi-sorption 63 .The values of H o ads.may also be used to forecast whether the exothermic process is chemi-sorption or physic-sorption.Adsorption can be divided into two types: physic-sorption ( H o ads.− 40 kJ mol -1 ) and chemi-sorption ( H o ads.− 100 kJ mol -1 ) 64 .Table 5 displays the thermodynamic values obtained.The values of H o ads.indicate that inhibitor adsorption is an exothermic process, with IE% decreasing as temperature rises.

Thermodynamic activation parameters
The findings in Table 6 show that (CR) and (IE %) are directly and inversely proportional to temperature respectively, with absence and presence of DHSiMF and DHSiB.This is possibly because of the reduced rate of adsorption of DHSiMF and DHSiB compounds on the metal surface at elevated temperatures.The Arrhenius Eq. ( 15) represents the relation between the corrosion rate of the metal (CR) and temperature (T) 65     where Ea, A, and R are the apparent activation energy, frequency factor, and gas constant (8.314)J K -1 mol -1 .The plot between an ln CR vs 1/T for carbon steel in the tested HCl medium is shown in Fig. 9, both after and before the addition of varied inhibitor doses.Table 6 shows the data for calculating E a using the derived regression equation between ln CR and 1/T.Each straight-line slope denotes a different value of (E a ).Table 6 shows that the Ea values achieved in the absence and presence of an inhibitor (DHSiMF and DHSiB) were 16.09, 50.89, and 54.70 kJ mol -1 , respectively.The greater energy barrier for the process of dissolving steel and the adsorbed DHSiMF and DHSiB layer on the surface of the steel, which slows down the process of charge/mass transfer, are the two causes of the rise in Ea values in the presence of DHSiMF and DHSiB 66 .Due to the formation of a steel-Schiff bases complex or a slowdown in the rate of steel dissolution, Ea values grew as organic inhibitors concentration rose 67 .The transition state equation was used along with an extra version of the Arrhenius equation 68 : where h is the Plank constant (6.626176 × 10 -34 Js) and N denotes Avogadro's number (6.02252 × 10 23 mol -1 ).The plot draw of ln CR/T vs 1/T for metal corrosion before and after the addition of DHSiMF and DHSiB is shown in Fig. 9. Table 6 lists the enthalpy of activation ( H * a ) and entropy of activation ( S * a ) which were calculated from the slope and intercept of straight lines, respectively.It was found that ( H * a ) values for DHSiMF are positive, which refers to the endothermic nature of the metal oxidation process, making it difficult to dissolve depending in the analyzing the results are given in Table 6.These results demonstrate that both H * a and S * a values in the presence of DHSiMF and DHSiB rise above those in the blank, indicating an increase in the energy barrier for corrosion reaction in the presence of DHSiMF and DHSiB 69 .The difficult and slow endothermic nature of the dissolving process was demonstrated by the positive values of H * a 70 .The molecules are orientated on the surface and ordered by adsorption through the active center, which results in a rise in the value of S * a (less negative), which indicates a decrease in disorder 71 .

Molecular dynamics and Monte Carlo simulations
In order to comprehend the inhibitory phenomena on the steel surface and identify the low E ads locations on the surface, MC and MD simulations were carried out.This allowed identification of the preferred adsorption sites and geometry of DHSiMF and DHSiB compounds either in the neutral or protonated form [72][73][74] .By optimizing the entire system, different forms of energies for DHSiMF and DHSiB inhibitors on the Fe (1 1 0) surface in the simulated corrosive fluid were determined.The findings are shown in Fig. 10.Also, through the analysis of T (K) variations in MD simulation, the most stable adsorption sites of DHSiMF and DHSiB were found shown in Fig. 11. Figure 11 illustrates how little T(K) fluctuation there is, demonstrating the accuracy of our system's MD estimates 75 .
The abundance of the E ads for the examined compounds DHSiMF and DHSiB on the surface of Fe (110) substate is shown in Fig. 12.The values of the several types of adsorption energies (total, rigid, deformation) are    collected in Table 7.The adsorption process is spontaneous, as indicated by the negative E ads values.Compared to other molecules precent in then corrosive media, the DHSiMF and DHSiB molecules have substantially larger E ads distributions.The E ads distribution in Fig. 10 shows that the DHSiMF and DHSiB molecules may progressively substitute the adsorbed corrosive ions (H 3 O + , Cl -) and water molecules from the steel surface 76 .
According to data in Table 7, DHSiMF and DHSiB molecules either in neutral or protonated forms can be ranked in order of their potency as inhibitors for the gas or aqueous phase adsorption: DHSiMF > DHSiB.The exceptionally high levels of binding energies show the DHSiMF and DHSiB molecules' extraordinary ability to resist corrosion for steel 77,78 .The equilibrium configurations of both inhibitors' adsorption on the surface of Fe (1 1 0) are depicted in Fig. 13.The fact that each inhibitor molecule is adsorbed on the surface of Fe (1 1 0) in www.nature.com/scientificreports/almost parallel mode (position) of the inhibitor's rigid structure with regard to the surface of the metal is clearly shown in Fig. 13, which supports the strong connection and extra surface coverage between the inhibitor and the Fe atoms 79,80 .By analysis of the molecular structures of DHSiMF and DHSiB, we can note that sharing the electrons of the N, O, S, and aromatic rings present in the two inhibitors can facilitate the adsorption on the surface of steel.This is done by forming a strong Fe-N, Fe-S, and Fe-O chemical bond between the molecules and the Fe (1 1 0) surface 81,82 Also, the Van der Waals dispersion forces and electrostatic attraction can govern the physical interactions between the investigated inhibitors and the mild steel surface 83 .The bond length between the Fe (110) and the atoms of the DHSiMF and DHSiB molecules was calculated using the radial distribution function (RDF) analysis of the MD data (Fig. 13).By determining the values of the bond lengths, the various types of bonds formed can be determined.The sort of adsorption activity taking place on the metal is described by peaks of the RDF graph that appear at specific distances from the metal surface 84,85 .The chemisorption mechanism is represented as a process when the peak is present between 1 and 3.5 Å, however for physisorption, the RDF peaks are expected to be present at distances larger than 3.5 Å 86,87 .DHSiMF and DHSiB molecules have relatively strong contact with the steel surface because of the close position of their heteroatoms to the steel surface, as seen in Fig. 14, which supports their reflected inhibitory performance 88 .

SEM results
Figure 15 displays a variety of SEM pattern reported for C-steel specimen processed for 24 h in various media studied.The characterization of the C-steel surface immersed in tested HCl solution was seen in Fig. 15a.The images show that the metal is corroded with localized zones in the absence of investigated Schiff base.With addition of DHSiMF and DHSiB, (Fig. 15b,c), the rate of corrosion is suppressed causing a reduction in the region affected by corrosion and the protective layer covers much of the C-steel surface.The images show that when DHSiMF and DHSiB are present, a protective coating formed on the surface of carbon steel that makes it look like it almost doesn't rust.It is also evident from the figure with the sample treated with x, the formation of more than one layer of inhibitor.This is logically consistent with the results of the chemical and electrochemical tests.
On the other hand, this study performed SEM analysis on samples after 28 days of curing to better understand the changes in internal micros morphology of concrete following the addition of (DHSiMF and DHSiB chemicals).Before the test, the sample is gold-plated to enhance its electrical conductivity, providing for a clearer image of the micromorphology.Several SEM images were produced in this investigation, and the best of them were chosen, as shown in Fig. 15d-f for concrete, without and with 400 ppm of DHSiMF, and DHSiB, respectively.Figure 15d shows that the concrete mixture free of amide compounds, consisting of fine and coarse aggregates, sand, cement, and water, was bonded by the formation of hydration bonds.These bonds covered the concrete aggregates in the form of ginned cotton.One can notice found the deep gaps between the coarse, fine aggregates, and sand.On the other hand, Figs.15e,f for concrete blended with amide compounds is based on silaspiro group (DHSiMF and DHSiB) respectively.It has been found that from these figures, the hydration formation is increased due to the abundance of bond formation between cement and amide compounds.Furthermore, increasing the bond covered for the concrete aggregates in the form of ginned cotton and decrease the gaps between the aggregates.

XRD results
The technique of XRD was employed to offer methodological validation of the inhibitory characteristics of synthesized compounds DHSiMF and DHSiB for carbon steel in 1 M HCl.
The diagrams of XRD are shown in Fig. 16 for carbon steel immersion in 1 M HCl solution for 24 h without and with inhibitor compounds (DHSiMF and DHSiB).The production of magnetite peaks attributable to iron oxides (Fe3O4 and FeOOH) appearing at 2θ = 14.73°, 26.97°, 37.12°, 53.54°, and 61.34° due to the carbon steel surface obviously subjected to corrosion in the absence of an inhibitor.On the other hand, the iron maximum peak occurs at 2θ = 45.6°.XRD analysis revealed no magnetite on any of the specimens treated with DHSiMF and DHSiB.An oxide film on carbon steel will eventually form magnetite, which will convert from (γ -FeO(OH)) to goethite (α -FeO(OH)).

Tensile strength
The results of the tensile strength test for concrete of the 7 and 28 days are shown in Table 8 and Fig.   at 7 and 28 days at the addition of different concentrations of (DHSiMF and DHSiB) is shown in Fig. 10.It has been added at these concentrations of 0, 100, 200, 300, 400, and 500 ppm of (DHSiMF) to the concrete.The tensile strength of the concrete samples made of gravel and dolomite that were immersed for 28 days and 7 days increased by 1.6-2.2,1.9-2.2,1.3-1.7,1.5-1.73,and 1.9-2.2,respectively.In other words, the indirect tensile strength values for concrete containing gravel and dolomite aggregates when adding the same concentration of (DHSiB) to the concrete at 28-days and 7-days were reinforced by 1.7-1.96,1.95-2.1,1.3-1.5, and 1.53-1.7 (MPa), respectively.Overall, the results indicate that the chemical interaction between (DHSiMF and DHSiB) and the concrete will increase in the indirect tensile strength of the concrete made from gravel and dolomite aggregate.The effect of the addition (DHSiMF and DHSiB) to the concrete on the indirect tensile strength, such as compressive strength, when the concrete is immersed at 28-days-old was quite greater than those immersed at 7-days-old.For both 7-day and 28-day immersed concretes, 400 ppm is the best amount of (DHSiMF and DHSiB) to get the best tensile strength.Just like with compressive strength, the tensile strength of concrete keeps going down at any concentration of DHSiMF and DHSiB more than 400 ppm is added 91 .

Flexural strength
The results for the flexural strength test of concrete, which includes different concentrations of DHSiMF and DHSiB, which were immersed for 7 and 28 days, are shown in Table 8 and Fig. 17 This concentration works well with the concrete specimen that is immersed in a 28-day-old.Furthermore, the final result, the 500 ppm concentration of (DHSiMF and DHSiB), was not the optimal content for improving the compressive strength, indirect tensile strength, and flexural strength of concrete made of gravel and dolomite aggregate at 28 days, despite the fact that the improvement in the compressive strength, indirect tensile strength, and flexural strength was still greater than that of concrete made of gravel and dolomite aggregate without (DHSiMF and DHSiB).This happens because DHSiMF and DHSiB have such a large specific surface area that they give off a lot of surface energy 92 .
Improving the mechanical properties of reinforcement Concrete requires a good curing process, and the curing process is carried out in several ways: water curing, membrane curing, and steam curing.Water curing reduces water loss from the concrete surface by maintaining continuous wetting of the concrete's exposed surface.This is accomplished by spraying or sprinkling water or curing ingredients over the concrete surface in order to maintain a continuously moist surface.Moisture in the body of the concrete is prevented from evaporating, thereby strengthening the concrete.Ponding, sprinkling, fogging, mist curing, and wet covers are water curing techniques 93,94 .The process of curing by water spray leads to the exposure of the concrete reinforcement rebar to an aqueous medium, which makes it susceptible to corrosion.
Two inhibitor materials for corrosion (DHSiMF and DHSiB) were added to the concrete mix in which the rebar is protected from corrosion.Figure 18 shows the effect of concrete without and with corrosion inhibitors on protecting rebar from corrosion.Figure 18a shows the effect of chlorine and oxygen ions on the formation of the rust layer on the surface of the rebar without concrete mixed and inhibitor materials.Figure 18b shows the layer formation as calcium silicate hydrate-based cementitious compounds from concrete mixed on top of the rebar.These layers behave like cotton layers that surround the reinforcing rebar, which increases its protection from the influence of the aqueous medium.On the other hand, Fig. 18c shows that adding the inhibitor compounds to concrete during the concrete mixing and casting led to film formation adsorbed under the cotton layer, which is attached to the rebar surface.The rebar has been protected from corrosion by these layers.

Mechanism of corrosion inhibition
The schematic design in Fig. 19 may be used to understand how DHSiMF and DHSiB prevent steel corrosion.Aggressive species (Cl -and H 3 O + ) may easily access the steel surface in the blank media, causing significant corrosion of the metal.The cathodic process involves the reduction of H + ions in the HCl-containing aqueous environment, whereas the anodic process involves the dissolving of steel ions.Anodic reaction mechanism without inhibitor: Cathodic reaction mechanism without inhibitor: After the addition of Schiff base derivatives into the HCl solution, DHSiMF and DHSiB could adsorb on steel surface by mixed chemisorption and physisorption as indicated from experimental ΔG values.The adsorbed H 2 O, H3O + and Cl -molecules on the steel surface may be replaced by molecules of DHSiMF and DHSiB.
Anodic reaction mechanism after adding DHSiMF and DHSiB inhibitor: According to Molecular simulations, DHSiMF and DHSiB could progressively pass through the aqueous layer and adsorb on the surface of steel in a pattern that was virtually parallel.This was made possible through several active adsorption sites like hetero atoms, unsaturated bonds and aromatic moieties adhering to the steel surface.The protonated parts in DHSiMF and DHSiB molecules can be adsorbed on the steel surface via electrostatic interaction with a little amount of Cl -in acidic solution, which is attributed to physical adsorption.Also, the unprotonated reactive adsorption sites like the heteroatoms and π electrons will then interact with the vacant d-orbitals of Fe to form coordination bonds which is attributed to chemical adsorption.Additionally, a certain quantity of electrons accumulates on the surface of carbon steel due to the sharing of electrons from the DHSiMF and DHSiB with the vacant d-orbitals of Fe.This additional negative charge on the surface of carbon steel has to be discharged by retro-donation from the unoccupied d-orbitals of Fe to a vacant * (antibonding) orbital in the  www.nature.com/scientificreports/DHSiMF and DHSiB molecules.This improves DHSiMF and DHSiB absorption and gives the carbon surface better corrosion protective capabilities.Also, Table 9 compares the examined Schiff bases with other inhibitors of steel in acidic media that have been reported.The comparative table makes it evident that the compounds we evaluated had higher efficiency and stronger protective capabilities.

Conclusions
Two novel silicon-based Schiff-base compounds, (DHSiMF) and (DHSiB), were evaluated as corrosion inhibitors for carbon-steel corrosion by using electrochemical and weight loss methods.The obtained efficiency results for both inhibitors are excellent in an aqueous medium of 1 M HCl.The inhibition efficiency of DHSiMF of carbon steel is greater than DHSiB.According to ΔE corr values from the extrapolation of the Tafel curve, the DHSiMF and DHSiB may be categorized as mixed-type and anodic inhibitors, respectively.The isotherm of Langmuir has provided the best overview of the adsorption on carbon steel.The values of ∆Goads for DHSiMF and DHSiB at temperatures (ranging from 303 to 333 K) are between-34.42kJ mol -1 and − 37.51 kJ mol -1 , so Physical and chemical adsorption is therefore considered to be the best explanation for the adsorption process.It was also noted that ( H * a ) values for DHSiMF are positive and refer to the endothermic nature of the system, while the negative values of ( H * a ) for DHSiB refer to the exothermic nature of the metal oxidation process.On the other hand, the best (DHSiMF and DHSiB) content for the 28 and 7-day specimens was 400 ppm, but at 28 days, the mechanical properties for the concrete mix were better.One can observe that the reasons for these improvements in the compressive strength of concrete containing gravel and dolomite aggregate in mixed concrete are that (DHSiMF and DHSiB) act as a quite increased bond of contact between gravel, dolomite aggregate, and cement molecules, preventing the occurrence of cracks in the concrete.On the other hands, the presence of (DHSiMF and DHSiB) in concrete encourages the formation of microcrystals in the micro-voids of calcium silicate hydrate gel.The compressive strength has increased as a result.

Figure 1 .
Figure 1.The chemical formula of the prepared Schiff base compounds.

Figure 2 .
Figure 2. The diagram for the aggregate and sand grading used in the concrete mix.

Figure 3 .
Figure 3. OCP-time curves for steel in 1.0 M HCl solution without and with different concentrations of DHSiMF (a) and DHSiB (b) at 30 °C.

Figure 4 .
Figure 4. Nyquist plots for carbon steel in 1.0 M HCl in absence and presence of different concentrations of the (DHSiMF and DHSiB) compounds.

Figure 5 .
Figure 5. Bode and phase angle plots for carbon steel in 1.0 M HCl in absence and presence of different concentrations of the (DHSiMF and DHSiB) compounds.

Figure 6 .
Figure 6.Potentiodynamic polarization curves for the corrosion of Steel in 1.0 M HCl in absence and presence of different concentrations of (DHSiMF and DHSiB).

Figure 8 .
Figure 8. Van't Hoff equation represents the relation between ln K ads and 1/T for (DHSiMF (a) and DHSiB (b)) in 1.0 M HCl solution.

Figure 9 .
Figure 9. Arrhenius plots and Transition state plots for carbon-steel dissolution with and without different concentrations of (DHSiMF and DHSiB) in 1.0 M HCl solution.

Figure 10 .
Figure 10.Distribution of the different energy terms during the process of optimization of the adsorption of for (DHSiMF and DHSiB (protonated)/200 H 2 O/19 H3O + / 20 Cl -) systems onto the Fe (110) surface obtained by via MC.

Figure 14 .
Figure 14.RDF of the O, N and S heteroatoms for DHSiMF and DHSiB inhibitors/Fe (110), obtained via MD.

Figure 15 .
Figure 15.SEM images of carbon-steel and concrete specimen with and without (DHSiMF and DHSiB).

17
. The results show that (DHSiMF and DHSiB) increase the indirect tensile strength of concrete by a large amount.The concrete tensile strength test, the concrete is made of gravel and dolomite aggregate, and has a strength of up to 1.8, 2.3, 2.4, 2.6, 1.6, 1.8, 2.1, and 2.32 MPa, which includes 400 ppm from (DHSiMF and DHSiB) and is higher than that of conventional control concrete at 7 and 28 days.The concrete indirect tensile strength (MPa) curing

Figure 17 .
Figure 17.The diagram for the relationship between the concentration of (DHSiMF and DHSiB) and the values of the mechanical properties for concrete.
. The flexural strength of the concrete improved when (DHSiMF and DHSiB) were added to the concrete samples.Flexural strength is one of the mechanical properties of concrete.When adding 400 ppm of (DHSiMF and DHSiB) to the control concrete sample, the flexural strength reached to the maximum increased.The values of flexural strengths of up to 34, 47.2, 35, and 28% for concrete made of gravel and dolomite, respectively.The flexural strengths of concrete made of gravel and dolomite are higher than those of ordinary concrete.Figure17shows how the improvement of the flexural strength of concrete with the concentration's changes (DHSiMF and DHSiB).Different concentrations of (DHSiMF and DHSiB) added to gravel and dolomite aggregate concrete improved flexural strength by 20-42%, 11-29%, 15-35%, and 7-24%, respectively, at 28 days.The 400 ppm concentration of (DHSiMF and DHSiB) is a good concentration which leads to the improvement in the flexural strength of the concrete, shown in Fig.10.

Figure 18 .
Figure 18.The corrosion reaction mechanism (a) without inhibitor (b) concrete mixed without inhibitor (c) concrete mixed with inhibitor.

Figure 19 .
Figure 19.The inhibitive adsorption mechanism of DHSiMF for steel corrosion in HCl-containing environment.

Table 2 .
EIS parameters for the carbon steel in 1.0 M HCl in the absence and presence of different concentrations of the inhibitor compounds.

Table 3 .
Polarization parameters for carbon steel in 1 M HCl in the absence and presence of different concentrations of the inhibitor compounds.

Table 4 .
8) IE icorr % = 1 − i inh, corr./i Corrosion Table 4 lists the values of (CR , mg cm -2 h -1 ), (Ө) and (% IE) for various concentrations of DHSiMF and DHSiB at 303, 313, 323, and 333 K.The data clearly show that as DHSiMF and DHSiB levels are increased and CR values are decreased, weight loss reduces, and at higher temperatures, the (%IE) increases.Once more, it appears that electrochemical measures may be relied upon to accurately predict the order of inhibitors' efficacy in weight loss tests.The presence of benzene rings, Si, S, N, and O atoms in DHSiMF and DHSiB contributes to the %IE of carbon steel.Table 4 further shows that rising temperatures have an impact on CR and %IE values.

Table 5 .
Thermodynamic parameters for the adsorption of inhibitor compounds in 1 M HCl on carbon steel surface.

Table 6 .
Activation parameters of dissolution reaction of carbon steel in 1 M HCl with at different concentrations of inhibitor compounds.

Table 7 .
The outputs and descriptors calculated by the Monte Carlo simulations for adsorption of DHSiMF and DHSiB on Fe (110) (in kcal/ mol).

Table 8 .
Compressive strength, indirect tensile strength, and flexure strength for gravel and demode concrete at different concentrations of inhibitor compounds.