Enhancement of corrosion protection of metal carbon steel C45 and stainless steel 316 by using inhibitor (Schiff base) in sea water

This research has presented a solution to the problem faced by alloys: the corrosion problem, by reducing corrosion and enhancing protection by using an inhibitor (Schiff base). The inhibitor (Schiff base) was synthesized by reacting of the substrates materials (4-dimethylaminobenzaldehyde and 4-aminoantipyrine). It was diagnosed by infrared technology IR, where the IR spectrum and through the visible beams proved that the Schiff base was well formed and with high purity. The corrosion behavior of carbon steel and stainless steel in a saline medium (artificial seawater 3.5%NaCl) before and after using the inhibitor at four temperatures: 20, 30, 40, and 50 C° was studied by using three electrodes potentiostat. The corrosion behavior was studied by cathode and anode polarization through which all corrosion parameters were investigated which include: corrosion current icorr (1341× 10-7-5393 × 10-9A/cm2), corrosion potential Ecorr (-1.031--0.227 mV vs SCE) , corrosion rates CR (0.658-0.007 mm.y-1), inhibition efficiency %IE (92-98%), and energy activation barriers Ea (4.709-26.733 kJ/mole). The thermodynamic and kinetic properties of the corrosion behavior of these two metals under study, which include: enthalpy ∆H*(2.153-24.176 kJ/mole), entropy ∆S*(-197 -156 J/mole), and free Gibbs energy ∆G*(59.87-74.56 kJ/mole) before and after using the inhibitor, were also studied.

Uniform (general) corrosion, galvanic (Metal-Metal) corrosion, pitting corrosion, selective corrosion, crevice corrosion, intergranular corrosion, stress corrosion cracking, erosion corrosion 2 . Corrosion processes can be explained briefly, as they are considered part of the electrochemical processes, which are illustrated in the Fig.  1 below:

Figure 1. Corrosion cell 2 .
The Fig. 1, above shows us the oxidation and reduction reactions that occur at the anode and cathode electrodes, respectively.
Resorting to solving the corrosion problem has attracted great interest from many researchers, as there are several methods used to reduce corrosion that affect alloys, including the use of corrosionresistant materials, and there are two ways to use those materials that are resistant to corrosion to reduce corrosion and enhance protection; the first is the use of the coating method, which is coating these materials on the surface of the metal and making a thin layer on its surface that acts as an insulating layer that isolates the metal surface from the corrosive surroundings as much as possible and through physical adsorption.
Recently, nanomaterials have been used as a coating material on metal surfaces because of the unique properties that distinguish them from other materials. The most important of these materials are TiO2, ZnO, Al2O3, ZrO2…etc 3-5 , nanopolymers [6][7][8] , carbon nanomaterials (as reduced graphene oxide, graphene, and carbon nanotubes) [9][10][11][12] , drugs [13][14][15][16] , and environmentally friendly materials 17 all of which are reported. The second method that is used to reduce corrosion is the use of the so-called corrosion inhibitors 18 , which are chemical compounds, especially organic compounds (drugs or ligands; Especially ligands that contain nitrogen atoms [19][20][21] , such as Schiff bases [22][23][24][25] or other ligands [26][27][28][29] or nanoparticles 30 , that are added in small concentrations to the corrosion medium, they reduce corrosion by decreasing cathodic reduction or anodic oxidation, or both, by making a film on the metal surface. It is assumed that the adsorption of inhibitors on the surface of the metal is either through physical adsorption or chemical adsorption 31 . There is a lot of research in this field has been recorded. In this work, Schiff base was used as an inhibitor for several reasons, including: The adsorption of inhibitors occurs through the presence of heteroatoms such as nitrogen, oxygen, phosphorus and sulfur. The inhibition efficiency of these heteroatom's have been reported to follow the sequence O<N<S<P 32,33 . Now days, the condensation products of aldehydes or ketones with amines i.e., Schiff base have been reported with good inhibitive properties. These are the important class of ligands in co-ordination chemistry. This ability is due to the presence of C=N groups. Schiff bases become effective by adsorption on the metal/solution surface and protect it from the hazardous effect of saline media.
The Schiff base was prepared from the reaction of 4-dimethylaminobenzaldehyde with 4aminoantipyrine as follows: First, the reactants were dissolved in methanol solvent (1 mmol \ 0.149 g) of (4dimethylaminobenzaldehyde was dissolved in 15mL methanol and (1 mmol/0.203 g) of (4aminoantipyrine) was dissolved in 10mL methanol. After that, in a round flask, the reactants were added with the addition of two drops of glacial acetic acid, and the mixture was left with continuous stirring for two hours, after the completion of the reaction period, yellow crystals are formed. The Product was collected by filtration, washed several times with ethanol and recrystallized from hot ethanol and air dried. The Fig.2, below shows the chemical reaction preparation of the Schiff base.

Corrosion measurement
The corrosion tests (as shown in Fig.3,and Fig.4) were conducted with the use of an advanced potentiostat with all accessories of the cell body made of Pyrex consisting of internal and external bowls, a host computer, thermostat, magnetic stirrer, electrodes (reference electrode, auxiliary electrode working electrode) and working electrode holder. The corrosive media was artificial seawater 3.5% NaCl prepared by dissolving 35 gm of sodium chloride in 1000 mL deionized water, the inhibitor solution was prepared by dissolving 10 mg of the inhibitor in100 mL (50%dimethylformamide (DMF)+ 50% deionized water) 100 ppm (The concentration of the inhibitor is directly proportional to the inhibition efficiency, that is, the higher the concentration of the inhibitor, the greater the inhibition efficiency). The two metals under study were made in a circular shape with a diameter of 2.5 cm and a thickness of 1 mm and were polished by silicon carbide (SiC) grit abrasive paper from 400, 600, 800, 1200 and 2000, then they were washed with acetone and distilled water DW and with ethanol, then, they were introduced into desiccators to prevent their oxidation (the Table 1 below shows the chemical composition of the two metals, carbon steel, and stainless steel). The polarization curves (Tafel plots) were scanned between -200 to + 200 mv from the open circuit potential and the corrosion currents (icoor) plus the corrosion potentials (Ecoor) of the metals under study (before and after using inhibiter) 50 ml of inhibitor was added, which were estimated by extrapolating the cathodic and anodic Tafel lines at four different temperatures of 20, 30, 40, and 50ᴼC.

Schiff base characterization
The prepared Schiff base was diagnosed by Fourier Transform Infrared Spectrophotometer, as well as the substrates materials from which the Schiff base was prepared for comparison (as shown in Figs. 5, 6, and 7. In general, the band at 1608.63 cm -1 belongs to the azomethine group ν(HC=N-) , which is present in the prepared Schiff base and its absence in the substrates materials from which the Schiff base was prepared, and this indicates the formation of the Schiff base and the two bands at 3433 cm -1 and 3325cm -1 belong to the amine group ν(NH2), which appeared in 4-aminoantipyrine (the basic material from which the Schiff base was prepared) and disappeared in the Schiff base this indicates the formation of the azomethine ν(HC=N-) group and thus indicates the formation of a Schiff base. The bands at (1647, 1662, and 1647.21) cm -1 belong to the carbonyl group υ(C=O) of the compounds 4-aminoantipyrine, 4dimethylaminobenzaldehyde, and prepared Schiff base, respectively.

Corrosion Parameters Measurement
From the Tafel polarization curves (Tafel plots shown in Figs. 8, 9, 10, and 11) the corrosion parameters were calculated which include: The corrosion current density (icorr), corrosion potential (Ecorr), anodic (βa), cathodic (βc), polarization resistance (Rp), the inhibition efficiency (%IE) and the rates of corrosion (CR) in absence and presence the inhibitors molecules in NaCl 3.5% solution (artificial seawater). Tafel plot reveals that Ecorr for carbon steel or stainless steel in the presence of the inhibitors (50 ml of inhibitor was added) shifts to a higher (noble) position compared with blank solution, which means the protection works as anodic protection. The inhibition efficiency (%IE) was calculated by the following Eq.1 5 .
Where (icorr)ois the corrosion current density in the absence of inhibitors (blank), (icorr) is the corrosion current density in the presence of inhibitors (100 ppm) (50 ml of inhibitor was added).
Rp in Ω⋅cm 2 was calculated from the Stern-Geary Eq. 2 16 : Where; Rp is the polarization resistance in Ω.cm 2 , icorr is the corrosion current density, βa, and βc are the anodic and cathodic Tafel slopes in V.dec.
The rates of corrosion, it is calculated by Faraday's law 9 at four temperatures for carbon steel and stainless steel alloys without and with the inhibitor.
Where (e) is the chemical equivalent of the metal, (d) its density, and icorr is the corrosion current density at four temperatures. All the abovementioned parameters that were calculated are shown in Table 2.     The data mentioned in the above Table 2 shows the following: We note that the corrosion current icoor values increase with increasing temperature for the two alloys under study without and with the inhibitor, the reason is because corrosion is an electrochemical reaction and the rate of the reactions increases with increasing temperature. Also, a significant decrease in the values of the corrosion currents icoor of the alloys is observed with the use of the inhibitor compared to the values without the use of the inhibitor, and this leads to a high inhibition efficiency. The polarization resistance values Rp of the two alloys without the use of the inhibitor decrease with increasing temperature, it is caused by the formation of a corrosion product (iron oxide), while the values of the polarization resistance of the alloys with the use of a high inhibitor (Schiff base) are greater than the values without the inhibitor and also decrease with increasing temperature, This explains why corrosion products are not formed on the surface of the alloys (carbon steel and stainless steel) 1 . The values of corrosion rates for alloys without the inhibitor are high compared to the values when adding the inhibitor, and these values also increase with the increase in temperature, and this is due to the role of the inhibitor in reducing the corrosion process significantly (the inhibitor is adsorbed on the surface of the metal and forms an insulating layer that isolates the surface of the metal from the surroundings and stops the oxidation of the metal). Also, a significant decrease in the corrosion potential values of the alloys was observed when the inhibitor was added 1 .

Kinetic and Thermodynamic parameters
The kinetic and thermodynamic values include: the activation energy ( a), the entropy (Δ * ), the enthalpy (Δ *), and the Gibbs free energy(∆G*). From Arrhenius plot Eq. 5 10 estimated the activation energy ( a) while the entropy (Δ * ) and the enthalpy (Δ *) were estimated from derivative formulation Eq.6, called the transition state 10 .

∆G*= ∆H* -T∆S* -----------7
Where (icorr) is the corrosion current density, h is the Planck's constant (6.626 × 10 -34 J.s ), N is Avogadro number (6.022 × 10 23 Mol-1 ), R is the universal gas constant (8.315 JK -1 mol -1 ), T: Absolute temperature (K), ΔH* is the enthalpy of activation, ΔS* is the entropy of activation and ∆G* is the Gibbs free energy. A plot of log (icorr) Vs 1/T of Eq. 5, provides straight lines with a slope of (-Ea / 2.303R) and an intercept of (log A) for blank and with 100ppm inhibitor for two alloys carbon steel and stainless steel as shown in Fig. 12, 13, 14 and 15 respectively. A plot of log ( icorr / T) Vs 1/T of Eq. 6, provides straight lines with a slope of (-ΔH*/2.303R) and an intercept of (log R/Nh) + ΔS*/ 2.303R) for blank and with 100ppm inhibitor for two alloys carbon steel and stainless steel as shown in Fig. 16, 17, 18 and 19 respectively. All the values of thermodynamics for the corrosion process of blank and inhibitor solutions for carbon steel and stainless steel at temperature range 293-323 K shown in Table 3.         The results are shown in the Table 3 above show facts that support the success of the inhibition process: The values of the activation energies of the alloys with the use of the inhibitor are much higher than their values without the inhibitor and this is due to the increase of the inhibitor molecules covering the surface of the metal 1 . Inhibitors in aqueous solutions alter the corrosion reaction kinetics by suggesting a different reaction pathway with lower activation energies, and this behavior is beneficial in inhibiting the corrosion rate of metal 1 . It was also noticed that the values of the activation enthalpy of the stainless steel alloy increased with the use of the inhibitor more than of the finding in the values of the activation enthalpy of carbon steel, and this supports that the inhibition efficiency of the stainless steel alloy is higher than the inhibition efficiency of the carbon steel alloy. The enthalpy of activation for the transition process takes positive values, and this indicates that the corrosion reaction is endothermic. Chemical adsorption requires activation energy values of more than 80 KJ/mol and since the activation energy values for the metals under study showed less than 80 KJ/mol (from 10KJ/mol to 24 KJ/mol), this indicates that it is a physical adsorption 10 . The action of the inhibitor in reducing metal corrosion is its physical adsorption on the surface of the metal and it forms an insulating layer on the surface of the metal isolating it from the environment causing the corrosion and prevents the oxidation of the metal.
There is further evidence that it is physical adsorption is the enthalpy (Δ * ) values of less than 40 KJ/mol for the studied metals, because when the enthalpy values are 100 KJ/mol, the adsorption is chemical 10 . The values of Δ * for in the presence and absence of the inhibitor are negative, this means that the activated complex, which is the ratelimiting step of the reaction, represents an association rather than a dissociation, this indicates a decrease in disorder taking place by going from the reactants to the activated complex 10 . The values of activation Gibbs free (∆G*) energies take positive values and most of them change slightly with increasing temperature, and this indicates that the activated complex is unstable and the probability of its formation decreases with increasing temperature, it also describes that the process is non-spontaneous one for transition reaction 5 .

Conclusion
Through this work, the significant effect of the inhibitor was observed in reducing corrosion of the two metals, stainless steel and carbon steel, with high reinforcement of corrosion protection when using a concentration of 100ppm and at temperatures from 20 -50C° this is supported by the thermodynamic kinetic properties of the inhibition process.

Acknowledgment
The author thanks Dr. Haider, director of the continuing education center at the University Al-Turath College, for stealth check for search, and lecturer Ziad Mohsen Bandar, lecturer in University Al-Turath College, for his high efforts in proofreading the research.

Author's Contribution Statement
This work was carried out in collaboration between all authors. M H the owner of the research idea and made measurements of corrosion with wrote and edited the manuscript. N A, made measurements of corrosion with revisions idea. T H, S R, and Sh M, prepared of inhibitor. H H, Check the linguistics.