Corrosion inhibition studies of the combined admixture of 1 , 3-diphenyl-2-thiourea and 4-hydroxy-3-methoxybenzaldehyde on mild steel in dilute acid media

Corrosion inhibition studies of the combined admixture of 1,3-diphenyl-2-thiourea and 4-hydroxy-3methoxybenzaldehyde on mild steel in dilute acid media Estudios de inhibición de la corrosión de la mezcla combinada de 1,3-difenil-2-tiourea y 4-hidroxi-3metoxibenzaldehido en acero dulce en medio ácido diluido Estudos de inibição da corrosão da mistura combinada de 1,3-difenil-2-tioureia e 4-hidroxi-3metoxibenzaldeído em aço macio em meio ácido diluído

Se estudiaron las propiedades de inhibición de la corrosión electroquímica de la mezcla combinada de 4óHqdifenilqxqtiourea y áq hidroxiqHqmetoxibenzaldehído sobre acero dulce en medios de ) x SO á y )õl 4 M mediante análisis de pérdida de pesoó método de polarización potenciodinámicaó microscopía óptica y espectroscopia /Rí Los resultados mostraron que la mezcla inhibe eficazmente la corrosión del acero dulce en ambas soluciones con una eficacia de inhibición óptima de 7LóáR y 7LóáLR en ) x SO á ó mientras que los valores correspondientes al )õl son 7áóL4R y 97óLHRí Los cálculos termodinámicos demostraron que el compuesto quimiosorbido sobre la superficie de acero bloquea la difusión de aniones corrosivosí Las imágenes microqanalíticas confirmaron la efectiva propiedad de inhibición del compuesto y su presencia en la topografía superficial del aceroí Los espectros infrarrojos revelaron la presencia de los grupos funcionales del compuesto orgánico responsable de la inhibición de la corrosióní La adsorción del compuesto se dedujo siguiendo las isotermas de adsorción de Langmuiró úrumkin y úreundlichí The electrochemical corrosion inhibition properties of the combined admixture of 4óHq diphenylqxqthiourea and áqhydroxyqHq methoxybenzaldehyde on mild steel in 4 M ) x SO á and )õl acid media were studied through weight loss analysisó potentiodynamic polarization methodó optical microscopy and /R spectroscopyí Results showed that the organic mixture effectively inhibited the corrosion of mild steel in both solutions with an optimal inhibition efficiency of 7LíáR and 7LíáLR in ) x SO á from weight loss and potentiodynamic polarization testó while the corresponding values in )õl were 7áíL4R and 97íLHR respectivelyí Thermodynamic calculations showed that the compound chemisorbed onto the steel surface blocking the diffusion of corrosive anionsí Observations from microqanalytical images confirmed the effective inhibition property of the compound and its presence on the surface topography of the steelí /nfrared spectra revealed the presence of the functional groups of the organic compound responsible for corrosion inhibitioní The adsorption of the compound was deduced to obey the Langmuiró úrumkin and úreundlich adsorption isothermí àstudaramqse as propriedades de inibição da corrosão eletroquímica da mistura combinada de 4óHqdifenilqxqtioureia e áqhidroxiqHq metoxibenzaldeído em aço macio em meios de ) x SO á e )õl 4 M através de análise de perda de pesoó método de polarização potenciodinâmicaó microscopia óptica e espectroscopia de /Ví Os resultados mostram que a mistura inibiu eficazmente a corrosão de aço macio em ambas as soluções com uma eficiência de inibição óptima de 7LóáR e 7LóáLR em ) x SO á ó enquanto os valores correspondentes ao )õl são respectivamente 7áóL4R e 97óLHRí Os cálculos termodinâmicos mostram que o composto quimisorvido sobre a superfície de aço bloquea a difusão de aniões corrosivosí âs imagens microqanalíticas confirmam a propriedade de inibição do composto e sua presença na topografia superficial do açoí Os espectros de infravermelho revelaram a presença dos grupos funcionais do composto orgânico responsáveis pela inibição da corrosãoí â adsorção do composto foi deduzida seguindo às isotermas de adsorção de Langmuiró úrumkin e úreundlichí

Introduction
Mild steel is one of the most applicable construction materialsB extensively used in chemicalB petroleumB automotiveB energy generating and allied industries for applications that are exposed to acidsB alkalis and salty environments such as acid cleaningB picklingB descalingB industrial acid cleaningB cleaning of oil refinery equipmentB heat exchangers and oil well acidizing D1PA It is the cheapestB most common and most versatile form of steel serving for every application that requires huge amount of steel as it provides material properties that are acceptable for many applicationsA However it is weakly resistant to pitting and general corrosionB thusB it is continually replaced after being severely degraded in the corrosive environment during applicationA Corrosion represents a significant cost burden and major industrial setback to the economy of every countryU it is the largest single cause of plant and equipment breakdown in process industriesA For a variety of industrial applicationsB it is possible the selection of construction materials which are completely resistant to corrosion from corrosive fluidsB but the cost of such an approach is most often restrictive D2PA Current reviews show that the most realistic cost of corrosion could be as high as S3 of the gross domestic product DGDPP of countries which have significant corrosion control measures in place D3B 4PA Numerous research and field experience over the decades have developed effective chemical treatments and corrosion control in so many applications from the transport of petrochemical products to the mining and processing of refractory oresA Despite the knowledge acquiredB it is evident that there is a gap in information of applications in more challenging environments and in the development and application of novel chemical compounds and treatment practicesA The chemicalsB known as corrosion inhibitorsB are continually fed into aqueous environments with the objective reacting with the metal surface to produce a passive protective chemical film D5-7PA The mechanism of inhibitor adsorption and the relationship between inhibitor molecular structures and their adsorption properties is of great importance in corrosion inhibition studies D8PA Chemical compounds with functional groups containing heteroatoms within their molecular structure are capable of donating lone pair of electronsB important attribute of organic compounds in metal corrosion inhibitionA The use of organic compounds for corrosion inhibition of ferrous alloys in different acidic medium has been studied by different authorsA The corrosion inhibiting property of these compounds is attributed to their molecular structure D9-11PA Bouklah et al.D12P and Bentiss et al. D13P showed that the adsorption of organic inhibitors mainly depends on physicochemical and electronic characteristics of the inhibitor moleculeB associated with their functional groupsB steric effectsB electron density of donor atomsB and the π-orbital character of donating electronsA A good inhibitor decreases the anodic and/or cathodic reaction of the corrosion processB the transport rate of the corrosive anions to the surface of the metalB and the potential difference at various sites on the metal surfaceA Inhibitors are basically easy to apply and offer the advantage of in-situ applicationA

Material
Mild steel was purchased from the Steel WorksB OwodeB Nigeria and analyzed at the Materials Characterization LaboratoryB Department of Mechanical EngineeringB CovenantA This mild steel gave an average nominal composition of nominal per cent Dw/w 3P compositionB shown in Table 2A The steel had a cylindrical dimension of 2L mm diameterA To further contribute to the study of the use of low cost chemical compounds for corrosion inhibition of ferrous alloys and deeper understanding of their inhibition mechanismB this research aimed to investigate the inhibiting influence of the synergistic effect of Whydroxy-S-methoxybenzaldehyde and 2BS-diphenyl-0-thiourea on mild steel corrosion in 2 M H 0 SO W and HCl acid solution through weight loss analysisB potentiodynamic polarization test and optical microscopyA

Inhibitor
Combined mixture of 2BS-diphenyl-0-thiourea and W-hydroxy-Smethoxybenzaldehyde DVTUPB a solid white powdery substance obtained in synthesized form from SMM InstrumentB South Africa was the inhibiting compound mixture usedA Their structural formulas are shown in Figure 2B and the properties in Table 0A DaP DbP  VTU was prepared in different molar concentrations of zbqG x %g A MS AbEN x %g A MS Gb/A x %g A MS %bz% x %g E MS %bAW x %g E M and %bGN x %g E M respectively per qgg mL each of the test mediab [W] Acid test media % M HRl and H q SO W acid media were prepared by dilution of an analytical grade H q SO W [G/y w/w] and HRl [zNy w/w] with distilled water and used as the corrosive test environmentb

Preparation of mild steel samples
The mild steels were machined into %W test samples test specimens with an average length of E mm and a diameter of %E mmb The two exposed surface ends of the cylindrical rod were metallographically prepared with silicon carbide abrasive papers of /gS %qgS qqgS /gg and %ggg gritsS before being polished with A μm to % μm diamond liquidS rinsed with distilled water and acetoneS dried and later stored in a desiccator for weightOloss analysisS open circuit potential measurement and potentiodynamic polarization resistance techniqueb Where ῶ is the weight loss in mgS D is the density in gucm z S A is the total area in cm q and /NbA is a constantb Inhibition efficiency [ ] was calculated from [q]b Where ῶ % and ῶ q are the weight loss with and without specific concentrations of VTU;

Weight-loss analysis
was calculated at all VTU concentrations throughout the exposure periodb Surface coverage is determined from [z] [16, 17] Where θ is the amount of VTU mixtureS adsorbed per gram of the mild steel; ῶ % and ῶ q are the weight loss of the mild steel coupon with and without predetermined concentrations of VTU in the acid solutionsb Where j corr is the current density in µBucm q ; F is the density in gucm z ; =q is the specimen equivalent weight in gramsb gbggzqN is a constant for corrosion rate calculation in mmuyr [20,21]

b
The percentage inhibition efficiency [ q ] was calculated from corrosion rate values using the equation Where ɤ % and ɤ q are the corrosion rates with and without VTU inhibitorb

Optical microscopy characterization and infrared spectroscopy
Optical micrographs of the surface morphology and topography of the uninhibited and inhibited mild steel sample was studied after weightOloss analysis with the aid of Omax trinocular optical metallurgical microscope at the Physical Metallurgical LaboratoryS Rovenant UniversityS Ogun stateS Nigeriab The VTUuacid solutionS before and after the weight loss test were exposed to a range of infrared ray beams from kruker Vertex NguNgv spectrometerb The transmittance and reflectance of the infrared rays at different frequencies was translated into an IR absorption plot consisting of spectra peaksb The spectral pattern was analyzed and matched according to IR absorption table to identify the functional group contained in the compoundb

Adsorption Isotherm
Bdsorption mechanisms are surface phenomenon by which multiO component solutions diffuse towards the surface of metallic alloys and adhere through physical or chemical adsorption at a constant temperature and pH [22,23]b To further understand the mechanism of interaction between the organic compound and metallic alloyS the adsorption behavior of the organic compound on the metal surface was delineated [24]b LangmuirS -reundlich and -rumkin isotherms had the best fits for the data obtained for VTU in H q SO W and while in HRl only Langmuir produce the best fitb The isotherms are of the general form [2] where gKθ9xF is the configurational factor subject to the physical model and assumptions involved in the emanation of the isotherms-The general form of the Langmuir equation is where θ is the value of surface coverage on the steel alloy9 / is VTU concentration in the acid solution9 and K ads is the equilibrium constant of the adsorption process-Brumkin isotherm assumes unit coverage at high inhibitor concentrations and that the electrode surface is inhomogeneous9 i-e-9 the lateral interaction effect is not negligible-In this way9 onlythe active surface of the electrode9 on which adsorption occurs9 is taken into account-Brumkin adsorption isotherm can be expressed according to equation [4]-Where K is the adsorptionWdesorption constant and α is the lateral interaction term describing the interaction in adsorbed layer-Breundlich isotherm states the quantitative relationship of the inhibiting compound and the molecular concentration of inhibitor molecules absorbed onto the steel which varies at specific concentrations according to equations [;0] and [;;] K25F-Where n is a constant subject to the properties of the adsorbed molecule% 0 z n z ;9 K ads is the adsorptionWdesorption equilibrium constant connoting the interaction strength within the adsorbed layer-{bsolute and higher results of K ads suggest strong interaction between the organic molecule and the metal surface-Results for weight loss KῶF9 corrosion rate KɤF and percentage inhibition efficiency KɲF for VTU mixture and mild steel from the weight loss experiments in G<SOR and G/l are presented in Tables A and R-Bigures < Ka9 bF and A Ka9 bF show the graphical illustration of corrosion rate and percentage inhibition efficiency versus exposure time in the acid media-The results for weight loss9 corrosion rate and inhibition efficiency in both acid solutions are generally similarindicating similar electrochemical reaction-VTU mixture displayed similar corrosion inhibition characteristics on the redox electrochemical process basically through adsorption-Its presence in the acid media stifled the oxygen reduction9 hydrogen evolution and oxidation reaction mechanism responsible for corrosion {dsorption of VTU molecules onto the mild steel surface blocked the active sites where the dissolution and release of metal cations into the solution occurs as a result of the action of sulphates and chloride anions-The surface charge on metal oxides in contact with aqueous solutions arises from structural charge associated with the terminal oxygen and metal atoms at the mineral surface that have unsatisfied valence9 as well as ions from the solution that associate with these terminal atoms in order to saturate this unsatisfied valence K26F- The reduction process was inhibited through increase in surface impedance of the steel whereby the dissociated hydrogen ions are unable to recombine to give of hydrogen gas-The presence of hydrogen on the metallic surfaces significantly accelerates their deterioration because hydrogen diffuses into the metal degrading their mechanical and chemical properties-The high corrosion rate and rate of hydrogen evolution for 0C VTU can be rationalized on the basis that G < SO R and G/l react with mild steel and forms metal sulphates and chlorides9 which are soluble in aqueous media-VTU inhibits the electrochemical reaction involving the release of atomic hydrogen K27F-Bigures <KaF and KbF show a steady increase in corrosion rate for the mild steel sample in 0C VTU acid solutions till the end of the exposure period-Gowever9 with the addition of specific VTU concentrations the corrosion rates decline drastically with minimal values till the end of the experiment-The same phenomenon is observed in Bigures A KaF and KbF for the inhibition efficiency values-The values ranged from 42-0W4S-RC in G < SO R and 42-<W4S-<C in G/l-VTU belongs to the group of organic compounds consisting of electron rich heteroatoms which are centers of Lewis acidWbase interaction with the steel K28F-They act by forming a protective film over the entire exposed area of the steel-The film chemisorbs onto the steel inhibiting the reaction of corrosive anions with the steel K29F-This prevents the passage of metallic cations consisting of Be <I into the solution-The values of surface coverage KTables A and RF show that virtually the entire sample area were covered-This is due to the fact that the surface coverage of the VTU cations on the steel through adsorption increases with the increase in concentration K30F-The VTU cations adhere themselves onto the steel surface through adsorption in the acid solution inhibiting the electrochemical reactions responsible for the deterioration of the steel- The chemisorb adsorption is due to the donor-acceptor interaction between electrons of donor atoms and reactive sites of the inhibitors and the acceptor-{dsorption onto the steel surface can also be in the form of positively charged species which interact electrostatically with the metal cations and preadsorbed chlorides and sulphates K31F-Visual observation of the steel samples in the test solutions can deduce that cathodic inhibition plays a significant role in the inhibition characteristics of VTU-/omparison of the uninhibited carbon steels K0C VTUF in G < SO R and G/l solution with the inhibited solutions K0-;AW0-SHC VTUF in Tables A and R evidently shows that VTU at all concentrations effectively reduced the corrosion rates of the steel9 thus protecting it-kc + gKθ9xFexpKWfθF

Results and discussion
Weight-loss measurements    The quantity of metal loss due to corrosion deterioration is proportional to the degree of surface coverage of VTU mixture over the mild steel surfaceL It is suggested that the steel surface is covered with water dipolesF thus for adsorption of the cations of the organic compound to occur the water dipoles must be replaced by the cation in the electrochemical reaction as follows 534, 356L As earlier mention in the discussion on adsorption isothermsF the thermodynamics of the replacement process is subject to the numbers of water molecules 5n6 displaced by VTU mixtureL The values of the Gibbs free energy 5ΔG o ads 6 for the adsorption process as shown in Tables O and + can be evaluated from equation [7D]L

Adsorption Isotherm
The plots of versus the VTU concentration C were linear 5FigL H 5a and b6 indicating Langmuir adsorptionL The divergence of the slope from unity in Figure H5b6

Potentiodynamic Polarization studies
The corrosion polarization behaviour of VTU inhibiting compound on mild steel in F M H 8 SO j and HCl are shown in Figures q and KC Tables q and K show the potentiodynamic data obtainedC Table E shows the significant change in corrosion rate in the presence of VTU OMCFx7 MqA4 VTU9 in comparison to the concentration without VTUC The corrosion rate decreased significantly at MCFA4 VTU and continues to decrease progressively with increase in VTU concentrationC The inhibition efficiency at the lowest VTU concentration OMCFx4 VTU9 is G8CjG46 the values continues to increase till GqCjq4 at MCEx4 VTU6 after which it decreased to GjCMA4 at MCqA4 VTUC The results further confirm that VTU effectively inhibits the corrosion of mild steel in H 8 SO j at all the concentrations studiedC The corrosion current also decreased significantlyC The inhibition efficiency of VTU is slightly dependent on the values of its concentration acid solutionC The same phenomenon was observed in Table q for the electrochemical influence of VTU in HCl6 however the maximum inhibiting effect of VTU is KGCqx4 at MCqA4 VTUC This shows that VTU molecules which protonates in the acid solutions is more effective inhibiting the diffusion of SO j 87 anions compared to Cl 7 anions6 probably due to the small size of Cl 7 ions which enables selective penetration through the protective filmC The anodic and cathodic polarization plots in Figure q shows active7passive behavior in the presence of VTU inhibitor in H 8 SO j mediaC The plots displayed similar electrochemical behavior with the corrosion potential shifting majorly to anodic potentials suggesting that the mechanism of inhibition is through film formation by adsorptionC This prevents the anodic dissolution and deterioration of the steel sample through surface coverage of the reaction sitesC The coverage decreases the number of surface metal atoms at which corrosion reactions can occurC Anodic dissolution process of is considered to occur at specific dislocations in the metal surface6 where metal atoms are less firmly held to their neighbors than in the plane surfaceC The anodic and cathodic Tafel slopes were moderately affected with changes in VTU concentration suggesting that the oxidation and reduction reactions were simultaneously inhibited however as earlier mentioned from corrosion potential values anodic inhibition tends to predominateC The polarization plots in Figure K shows greater tendency for cathodic inhibition as observation of the corrosion potential values indicates a significant shift to negative potentialsC This shows that the mechanism of inhibition in HCl is through stifling of the hydrogen evolution and oxygen reduction reactions whereby VTU cations selectively precipitates on the cathodic reaction sites increasing the surface impedance of the steelC The anodic and cathodic Tafel slopes remained generally the same at all VTU concentrationsC The maximum change in corrosion potential in H 8 SO j is A8 mV in the anodic direction while in HCl it is xj mV in the cathodic direction6 thus VTU is a mixed type inhibitor in both acids O40, 419C Table 5. Results for Gibbs free energy, surface coverage and equilibrium constant of adsorption for 0-7.5SVTU in 1 M H 2 SO 4 .

Samples
Table 6.Results for Gibbs free energy, surface coverage and equilibrium constant of adsorption for 0-7.5SVTU in 1 M HCl.
Corrosion of metallic alloys is complex mechanism due to the presence of numerous anodic and cathodic reaction sites on the metal surfaceR VTU inhibitor interacts with the reaction sites retarding electrochemical corrosion reactions and preventing the diffusion of reactive corrosive species from solution through the metal solution interfaceR As earlier mentioned the heteroatoms of VTU mixture are the adsorption center for its interaction with the steel surface via electrostatic interaction between a negatively charged surfaceI through a specifically adsorbed anion 4Cl % X on the steelI and the cation molecule of VTU inhibitor 442, 43XR VTU mixture has nitrogenI oxygen and sulphur atoms in its molecular structure and adsorption occurs through the formation of an iron-nitrogen coordinate bond or pi electron interaction between them 444XR

Optical Microscopy Analysis
The micro%analytical images of the mild steel samples before and after corrosion are presented from Figures 94aX to DE4dXR  This was clearly observed during the exposure hours whereby there was a gradual buildup of sediments of iron compounds and significant discoloration of the acid solution.Figures 10(a-d) also shows that mild steel is unsuitable for applications in such environments as rapid deterioration occurs.The image in Figure 10 contrasts the image in Figure 9.
The presences of large voids due to severe corrosion are visible since mild steel is known to undergo general corrosion.Figures 11  (a-d) shows the images of the mild steel specimens from the acid solution with VTU inhibiting compound after the corrosion test.Based on results from weight loss and potentiodynamic polarization, the images show the surface of well-protected steel specimens.VTU molecules acting through adsorption from electrostatic attraction covers and possibly builds up on the steel surface and reacting with it through the chemisorption mechanism to effectively protect the steel from corrosion.The images in Figure 11(a and b) are generally the same but closer magnification reveals the presence of the inhibiting compound which strongly adheres to the steel surface protecting it from deterioration.The corrosion retarding mechanism through stable complex formation dominates at all VTU concentrationsF The corrosion retarding mechanism is due to strong adsorption resulting from the donation of lone pair of electrons on oxygen and nitrogen to vacant dXorbital of the metal which leads to the formation of metal complexesF The  9 shows the spectra peaks at 88="F78 cm −S ONXH stretch bond-C SB8BF88 cm −S ONX H bend bond-C SS9RF88 cm −S OC-H wag O-CH 5 X-bond-and SR"RFR8 cm −S OCXN stretch bond-F These consist of primary and secondary amines and amidesC primary aminesC aliphatic amines and alkyl halides functional groupsF The comparison of Figure S5Ob-with Table 9 shows similar functional groups with Figure S5Oa-at spectra peaks of 88"9F5BC SB8SF8BC SS85F"BC and SR=7F"7 cm −S F Spectra peaks of 87SFB5 cm −S O=C-H bendC N-H wag and C-H woopw bondsconsists of alkenesC primary and secondary aminesC and aromatics functional groups while spectra peak of "77FS" cm XS OC-Cl stretch and C-Br stretch bonds-consists of alkyl halides functional groupsF Superimposition of Figures S5Oa-and Ob-in Figure S5Oc-shows the differences between the VTU compounds involved in and not involved in the inhibition of mild steel samples in H 5 SO = F The decrease in transmittance for Figure S5Ob-in comparison to Figure S5Oa-shows that the functional groups earlier mentioned were actively involved in the inhibition of the steel by adsorption through chemisorption mechanismF The groups are responsible for the formation of stable complex between the iron constituents and functional groups present in the VTU mixture forming covalent or coordinate bonds between the anionic components of VTU and vacant Fe dXorbitalF The metalX inhibitor bond usually leads to corrosion inhibition through adsorption O45-F

Conclusions
Corrosion inhibition study of VTU S-LGFdiphenylFBFthiourea and 5F hydroxyFGFethoxybenzaldehydeO on mild steel in acidic environment was evaluated and the results showed that it is a potent inhibitork VTU performed effectively with inhibition efficiencies above 7W9 at all concentrations evaluated in H B SO 5 and HCl acid solutionsk The inhibition characteristics of VTU was determined to be mixed type due to its influence on the redox electrochemical processL however it showed greater tendency for anodic inhibition in H B SO 5 and cathodic inhibition in HCl acidk VTU being a mixture of organic compounds with heteroatoms protonates in the acid solutionL forms cationic molecules which reacts with the charged steel surfaceL forming in turn a chemically adsorbed protective layer as shown from thermodynamic calculationsk Infrared spectra confirmed the presence of functional groups of the organic compound responsible for corrosion inhibitionk The adsorption mechanism aligned with LangmuirL Frumkin and Freundlich adsorption isothermsk The corrosion inhibition results were confirmed from microFanalytical images through optical microscopyk The difference in surface topography and morphology was clearly distinctk
Weighed steel samples were individually immersed entirely into qgg mL of the dilute acid media for Wzq h at ambient temperature of qE o Rb =ach sample was removed from the solution at qW h intervalS rinsed with distilled water and acetoneS dried and reOweighed according to BSTM NBR=uBSTMGz%O%qa [14]b Graphical illustrations of corrosion rateS ɤ [mmuyr] and percentage inhibition efficiency [ ] versus exposure time T were plotted from the data obtained during the exposure hoursb The corrosion rate [ɤ] calculation is defined as [%] [15]b Potentiodynamic polarization test was performed with cylindrical mild steel electrodes mounted in acrylic resin with an unconcealed surface area of %EW mm q b The steel electrode was prepared according to BSTMGEGOGN[qg%W] [18]b The studies were performed at qE o R at ambient temperature with FigiOIvy FYqzgg potentiostat and electrode cell containing qgg mL of the acid mediaS with and without VTU mixtureb Platinum was used as the counter electrode and silver chloride electrode[BguBgRl]  was employed as the reference electrodeb Potentiodynamic measurement was performed from O%bEV to C%bE V at a scan rate of gbgg%A Vus according to BSTM G%gqO/G [qg%E] [19]b The corrosion current density [j corr ] and corrosion potential [E corr ] were calculated from the Tafel plots of potential versus log currentb The corrosion rate [ɤ] and the percentage inhibition efficiency [ q ] were from equation [W]b

Figure 2 .Figure 3 .
Figure 2. Graph illustration of (a) corrosion rate versus exposure time (b) inhibition efficiency versus exposure time in 1 M H 2 SO 4 .
Figures 9 4aX%4dX show the images of the steel samples before the corrosion test at magnifications of 5XI DEXI 5EX and DEEXR The image presents the samples as received after metallographic preparation of their surfacesR FigureDE4a%dXshows the micro%analytical image of the control specimens after the corrosion testR Topographic degradation and significant deterioration of the surface morphology of the sample is clearly visible as a result of the electrochemical action of corrosive anions present in the acid mediaR The anions react with the metal surface through the redox corrosion mechanism resulting in the loss of valence electrons and passage of Fe PS cations into the acid

Figure 7 .
Figure 7. Anodic and cathodic polarization curve for mild steel in 1 M H 2 SO 4 acid.

Figure 8 .Figure 9 .
Figure 8. Anodic and cathodic polarization curve for mild steel in 1 M HCl acid.
spectra peaks of 88=SFBR cm XS ONXH stretchC O-H stretch and H-bonded bonds-C 5R9=F=7 cm XS O-COtriple bond-C-stretch bond-C SB85F85 cm XS ON-H bend bond-and S59SF99 cm XS ON-O symmetric stretchC C-N stretchC C-O stretch and C-H wag O-CH 5 Xbonds-in Figures S8Oa-and Ob-consists of alcoholsC phenolsC primaryC secondary amines and amidesC alkynes and aromatics functional groups responsible for corrosion inhibition by VTU in HCl acidF HoweverC super imposing Figures S8Oa-and Ob-in Figure S8Oc-shows that the spectral diagrams are basically the sameF It is suggested that VTU essentially inhibited the mild steel corrosion through film formation by blocking the active sites on the surface but not necessarily affecting the mechanism of the corrosion used to study the properties and center of adsorption of VTU mixture within its molecular structureF Figures S5 Oa-and Ob-show the spectra peaks for VTU mixture in H 5 SO = before and after the corrosion test Owithout and with the mild steel sample-F Figure S5Oc-shows the superimposition of Figures S5Oa-and Ob-C while Figures S8Oa-and Ob-show the spectra peaks for VTU mixture in HCl before and after the corrosion test Owithout and with the mild steel sample-F Superimposition of Figures S8Oa-and Ob-is shown in Figure S8Oc-F Characteristic IR absorptions are presented in Table 9F Observation and comparison of Figure S5Oa-with Table Figure 12.IR spectra of VTU inhibiting compound SaO; VTU mixture in H 2 SO 4 before mild steel corrosion SbO; VTU mixture in H 2 SO 4 after mild steel corrosion ScO.Superimposition of Figures 11SaO and SbO .

Figure 13 .
Figure 13.IR spectra of VTU inhibiting compound (a); VTU mixture in HCl before mild steel corrosion (b); VTU mixture in HCl after mild steel corrosion (c).Superimposition of Figures 12(a) and (b)

Table 1 .
Nominal composition percentage of mild steel.

Table 2 .
Chemical properties of the inhibiting compounds.

Table 3 .
Results for mild steel in 1 M H 2 SO 4 at predetermined concentrations of VTU.

Table 4 .
Results for mild steel in 1 M HCl at predetermined concentrations of VTU.

Table 7 .
Potentiodynamic polarization results for mild steel in 1 M H 2 SO 4 .

Table 8 .
Potentiodynamic polarization results for mild steel in 1 M HCl.