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Thesis: Coordination chemistry and schiff bases - Essay UK Free Essay Database

Coordination chemistry is the rapid growing branch of chemistry and deals with the interaction between the metal and ligand. It was Werner, a Swiss chemist who first recognized such a class of compounds and awarded Nobel Prize in chemistry in 1913 for his invaluable contribution to coordination chemistry. Metal complexes possess wide variety of applications in pharmaceutical, agricultural and various industrial fields. Cis’platin is a platinum based chelate and acts as a well known anti cancer drug due to the high DNA binding capacity of the metal chelate. Oxygen has inevitable role in the life of every living organism and it is carried by haemoglobin present in blood, which is an iron complex. Right from textile to petroleum industry numerous metal chelates have got wide range of applications. Schiff Bases Among the metal chelates studied, Schiff base complexes have got great attention. A Schiff base is a compound with a functional group that consists of an azomethine linkage or an imino group. It is formed by the condensation reaction between an aldehyde or a ketone with a primary amino group as shown below. R-NH2 + >C=O ‘ > C=N-R + H2O Primary amine Aldehyde / Ketone Schiff Base where R may be an alkyl or aryl group. Schiff bases possessing aryl groups are easier to synthesize due to their extra stability through conjugation effect. Comparatively less stable alkyl Schiff bases may polymerize or decompose to their parent compounds in the presence of moisture. The formation of Schiff base from an aldehyde or ketone is a reversible process and takes place generally in the presence of acidic or basic or neutral media. In the first part of the mechanism, the amine reacts with the aldehyde or ketone to give an unstable addition compound called carbinolamine. The carbinolamine loses water by either acid or base catalyzed pathways. In the second step carbinolamine undergoes acid catalyzed dehydration. The second step is the rate determining step of the process which is catalyzed by acid. But too much acid concentration will adversely affect the nucleophilicity of amine and hence the synthesis is to be best carried out in mild acidic condition. Many Schiff bases may be hydrolyzed back to their parent compounds in the presence of acid or base. In some cases it is better to remove the water formed during the reaction by distillation using azeotrope forming solvent [1].

In most cases, especially the condensation between the aromatic aldehyde or ketones with various amines are not reversible and the resultant Schiff bases can be easily separated from the reaction mixture. The mechanism for the formation of Schiff base is depicted in Figure 1.1.

Figure 1.1 Mechanism of Schiff base formation The name Schiff base is given to these classes of compounds after the German chemist Hugo Schiff (1864). Schiff bases are very popular in coordination chemistry due to their potential chelating ability to the metal ions through azomethine moiety. A variety of methods including direct synthesis, template methods, microwave assisted synthesis etc were developed by scientists for the synthesis of Schiff bases [2]. The amazing capacity of several Schiff base molecules in participating chelation process is due to the easiness of providing the lone pair of electron on nitrogen atom which in turn arises due to the low electronegative nature of nitrogen atom. Metal chelates having five or six membered ring system acquires high stability. The stability of metal chelates depends on the strength of the azomethine linkage, basicity of the imino group and sterric factors due to the other groups present in the ligand. Because of the relative easiness of preparation, synthetic flexibility and the peculiar binding nature of the azomethine linkage make the Schiff bases excellent chelating molecules [3]. Schiff bases and their metal chelates can be synthesized by different ways. Important methods are discussed below. Direct synthesis This method involves the condensation reaction between carbonyl compound and amino compound in alcohol or a mixture of alcohol and water. Azeotropic distillation followed by the treatment with molecular sieves ensure the complete removal of water molecules from the reaction mixture [4,5]. Dehydrating solvents such as tetramethyl orthosilicate or trimethyl orthoformate can also be used for the removal of water [6,7]. The synthesized Schiff base can be separated and purified by suitable techniques. The purified Schiff base is then allow to react with the metal salts in aqueous or alcoholic medium to obtain chelates. The main advantage of this method is that the minimization of impurities during the metal chelate synthesis. It is clear from the mechanism of formation of Schiff bases that the efficiency of direct condensation involves the presence of highly electrophilic carbonyls and strongly nucleophilic amino compounds, which can be accelerated by the use of compounds that act as Br??nsted-Lowry or Lewis acids, to activate the carbonyl group, accelerating the nucleophilic attack by amines and dehydrate the system by removing water. Br??nsted-Lowry or Lewis acids used for the synthesis of Schiff bases include ZnCl2, TiCl4, Ti(OR)4, alumina, H2SO4, NaHCO3, MgSO4, Mg(ClO4)2, CH3COOH, Er(OTf)3, P2O5/Al2O3 and HCl [8-16]. 2) In situ method This method is employed only if the recovery of the Schiff base from the reaction mixture is tedious. At first the parent aldehydes or ketone is allow to react. After the completion of the reaction, metal salts in aqueous or alcoholic solution are added and reflux the mixture for a particular period.

Template synthesis:

In situ one-pot template condensation reactions lie at the heart of macrocyclic chemistry. Therefore, template reactions have been widely used for the synthesis of macrocyclic complexes, in which transition metal ions are generally used as the template agent [17-19]. For instance, D. Singh et al have synthesized a novel series of complexes of the type M(C28H24N4)X2], where M = Co(II), Ni(II), Cu(II), Zn(II) and Cd(II), X = Cl’, NO3′, CH3COO’ and C28H24N4 corresponds to the tetradentate macrocyclic ligand, were synthesized by template condensation of 1,8-diaminonaphthalene and diacetyl in the presence of divalent metal salts in methanolic medium [20]. The reaction pathway is depicted in Figure 1.2. Fig. 1.2 Example for template synthesis Applications of Schiff Bases and Their Metal Chelates Many Schiff base and its metal chelates possess wide variety of application in the pharmaceutical field, catalysis reactions, analytical field and anti corrosion compounds. The subsequent paragraphs describe few examples. Schiff base ligands containing various donor atoms (like N, O, S, etc.) show broad biological activities and are of special interest due to variety of ways in which they can bond to metal ions. It is known that the existence of metal ions bonded to biologically active compounds may enhance their activities [21]. Schiff bases derived from sulfane thiadiazole and salicyladehyde and thiophene-2-aldehyde and their metal chelates exhibit toxicities against insects [22-23]. Zhu et al have reported that fluorination of the aldehyde part of the Schiff base enhance activity against insects [24]. Many thiazole, benzothiazole, pyran and quinazole derived Schiff bases posses effective antifungal activity and the efficiency is enhanced when groups such as methoxy, halogen and naphthyl are attached [25-27]. Several Schiff bases derived from furan and their transition metal chelates found to be very efficient against fungi such as A. niger, A. solani etc [28,29]. Large number of heterocyclic and non heterocyclic Schiff bases and their metal complexes display antibacterial activity. Schiff bases derived from furfural, pyridine aldehydes, salicylaldehyde, thiazole, amino acids, taurine, glucosamine, aminopyridine, aminothiazole, pyrazolone, indole and benzaldehyde and their metal chelates are found to be efficient inhibitors towards the growth of different bacteria [30-41]. A lot of Schiff bases acquire anti-inflammatory, allergic inhibitors reducing activity, radical scavenging and anti-oxidative action. Thiazole and furan based Schiff base and their metal chelates display analgesic activity [42-45]. Several Schiff bases exhibit anti-cancer activity and sometimes the activity enhanced upon complexation with transition metal ions. It is reported that Schiff bases derived from quinoline, pyridine, nitrophenols, vanillin and benzene sulfonanilide, anthracene carboxaldehyde and their metal complexes show significant anti-cancer activity [46-50]. Many Schiff bases containing aromatic ring system and their metal chelates are found to catalyze various reactions such as oxygenation, hydrolysis, reduction and decomposition reactions [51,52]. It was reported by S. Forster et al [53] that some cobalt Schiff base complexes can catalyze the oxidation of anilines with tert-butyl hydroperoxide to give nitrobenzenes. A selective chromogenic chemosensor was designed by M. X. Liu et al using a novel 5-mercapto triazole derived Schiff base. The sensing of Cu2+ by this sensor was found to be reversible, with the Cu2+-induced color being lost upon addition of EDTA [54]. Copper(II) and iron(III) chelates were synthesized from 4-formyl-3-hydroxy benzamidine or 3-formyl-4-hydroxy benzamidine and various L- or D-amino acids and their inhibitory activities for bovine alpha-thrombin were explored by E. Toyota et al [55]. Amine terminated liquid natural rubber(ATNR) on reaction with glyoxal yield poly Schiff base [56], which improves aging resistance of rubber. Organocobalt complexes with tridentate Schiff base act as initiator of emulsion polymerization and co-polymerization of dienyl and vinyl monomers [57]. It has been reported that Zinc(II) complexes with Schiff bases type chelating ligands can be used as an effective emitting layer [58]. Amino acid Schiff base complexes derived from 2-hydroxy-1-naphthaldehydes are important due to their use as radiotracers in nuclear medicine [59]. Many organic molecules containing hetero atoms and compounds containing azomethine linkage (C=N) were reported to act as good corrosion inhibitors for carbon steel, aluminum, copper and zinc in acidic media. A detailed survey regarding the corrosion inhibition capacity of the Schiff bases is given in Part II. Schiff Bases Derived From Pyridine and Their Metal Chelates- A Review Novel potential Schiff bases derived from acetylpyridine and their transition metal chelates have been synthesized by various researchers and characterized. Some of them have got properties including pharmaceutical agents. A series of Schiff bases derived from 2-acetylpyridne and 4-(2-aminoethyl)morpholine, and 4-(2-aminoethyl)piperazine and their transition metal complexes were synthesized and characterized by N. S. Gwaram et al using elemental analysis, NMR, FT-IR and UV-Vis spectral studies. Zn (II) complex displayed square pyramidal geometry while Cd(II) complexes exhibited polymeric structure. Ni(II) complexes possessed an octahedral geometry [60]. R.H. Prince and D. A. Stotter [61] reported of a series of metal(II) complexes of a quinquidentate ligand produced in situ or by complexation with the Schiff-base, condensation-product of two moles of 2-acetylpyridine with 3,3′-iminobispropylamine. From the in situ synthesis of the Ni(II) compound only a quadridentate, mono-Schiff-base complex with coordinated acetyl-pyridine is isolable. Analytical data, infra-red studies, magnetic moments and solution-spectra of the complexes were described, and the interconversion of the two types of Ni(II) complex investigated. Binuclear Schiff base complexes derived from glycine (Gly) and 3-acetylpyridine (3-APy) in the presence of M(OAc)2 [M = Co(II), Ni(II), Cu(II), Zn(II) and Cd(II)] have been synthesized by N. A. Nawar [62]. The role of pH in promoting the condensation of glycine and 3-acetylpyridine, as well as the substitution of acetates by hydroxide ion, has been discussed. The reaction of glycine with 3-acetylpyridine in the presence of MCl2 [M = Co(II) and Ni(II)] and MCl3 [M = Fe(III) and Cr(III)] yields mono- and/or binuclear complexes containing both of glycine and 3-acetylpyridine without condensation. Both types of complex were isolated and characterized by chemical analysis, conductance, spectral, magnetic and thermal measurements. Recently, N. M. Hosny et al [63] synthesized metal chelates of Cu(II), Co(II), Ni(II), Cr(III) and Fe(III) chlorides with a Schiff base ligand derived from 2-acetylpyridine and leucine. The IR spectra show that the Schiff base can act as a neutral tridentate ligand to Cu(II), Co(II) and Ni(II) through the pyridyl nitrogen, azomethine nitrogen and carbonyl oxygen. Another mode of chelation has been established that the Schiff base can act as mononegative tridentate ligand to Fe(III) and Cr(III) through pyridyl nitrogen, azomethine nitrogen and the carbonyl oxygen after the displacement of hydrogen from hydroxyl group. The synthesized metal chelates were subjected to elemental, spectral, thermal, magnetic and molar conductance studies. The results suggest that Co(II) and Ni(II) metal chelates posses tetrahedral geometry while Fe(III) and Cr(III) acquire octahedral geometry. A square planar geometry was assigned to Cu(II) complex. Semi empirical calculation of the complexes was also performed. In 2007, N. M. Hosny [64] synthesized Schiff-base complexes [ML(H2O)2(Ac)]nH2O (M=Co(II), Ni(II) and Zn(II); L= novel heterocyclic Schiff-base ligand derived from 2-acetylpyridine and alanine and n= 1’3/2) were synthesized and characterized by elemental analysis, spectral (FTIR, UV/Vis, MS, 1Hnmr), thermal (TGA), conductance and magnetic moment measurements. The results suggest octahedral geometry for all the isolated complexes. IR spectra show that the ligand coordinates to the metal ions as mononegative tridentate through pyridyl nitrogen, azomethine nitrogen and carboxylate oxygen after deprotonation of the hydroxyl group. Semi-empirical calculations PM3 and AM1 have been used to study the molecular geometry and the harmonic vibrational spectra to assist the experimental assignments of the complexes. A new Cd(II) complex with a tridentate Schiff base derivative of gallic hydrazid with 2-acetylpyridine has been prepared by A. A. Alhadi [65]. The structure of the ligand 3,4,5-trihydroxybenzoic acid[1-(pyridyl)-ethylidene]hydrazone (GAPy) was confirmed using the X-ray structure analysis. The elemental analysis, FTIR, UV-Vis, 1Hnmr spectral studies and thermal analysis indicate that the Schiff base ligand GAPy is a tridentate ligand which is coordinated with the Cd(II) complex through N, N and O atoms. They confirm that acetate ion is a bidentate ligand which is coordinated with the metal ion through two O atoms. A series of new Zn(II) complexes of 2-acetylpyridine thiosemicarbazone/semicarbazone Schiff base complexes have been synthesized and characterized by elemental analysis, IR, electronic and 1H NMR spectral studies by R. Manikandan et al [66]. The thiosemicarbazone/semicarbazone ligand coordinates to zinc as tridentate N, N and S/O donors. Based on the analytical and spectral results, tetrahedral geometry has been tentatively proposed by them for all the complexes. V. R. Souza [67] reported the synthesis and spectroscopic/electrochemical properties of iron(II) complexes of polydentate Schiff bases generated from 2-acetylpyridine and 1,3-diaminopropane, 2-acetylpyrazine and 1,3-diaminopropane, and from 2-acetylpyridine and L-histidine. The complexes exhibit bis(diimine) iron(II) chromophores in association with pyrazine, pyridine or imidazole groups displaying contrasting pi-acceptor properties. In spite of their open geometry, their properties are much closer to those of macrocyclic tetraimineiron(II) complexes. An electrochemical/spectroscopic correlation between E degrees (FeIII/II) and the energies of the lowest MLCT band has been observed, reflecting the stabilization of the HOMO levels as a consequence of the increasing backbonding effects in the series of compounds. They also reported the M??ssbauer data which confirm the similarities in their electronic structure, as deduced from the spectroscopic and theoretical data. [Cu(2AcPh)Cl]2H2O, [Cu(2AcpClPh)Cl]2H2O, [Cu(2AcpNO2Ph)Cl], [Cu(2BzPh)Cl], [Cu(2BzpClPh)Cl] and [Cu(2BzpNO2Ph)Cl] complexes were synthesized and characterized by A. Angel et al [68], with 2-acetylpyridine-phenylhydrazone (H2AcPh), 2-acetylpyridine-para-chloro-phenylhydrazone (H2AcpClPh), 2-acetylpyridine -para-nitro-phenylhydrazone (H2AcpNO2Ph), 2-benzoylpyridine-phenylhydrazone(H2BzPh), 2-benzoylpyridineparachloro-phenyl hydrazone (H2BzpClPh) and 2-benzoylpyridine-para-nitro-phenylhydrazone (H2BzpNO2Ph) . Schiff Bases Derived From Furan-2-Aldehyde and Thiophen-2-Aldehyde and Their Metal Chelates – A Review Many researchers have synthesized and characterized Schiff bases derived from furan-2-aldehyde and thiophene-2-aldehyde. The chelating ability of the newly synthesized Schiff bases was exploited and different transition chelates were prepared and characterized. Synthesis and characterization of some thiocarbohydrazone Schiff bases derived from pyrole, thiophene and furan carbaldehyde and their complexes with Cu(II), Ni(II), Zn(II), Co(II) and Fe(II) were done by F. Esmadi et al [69]. The prepared Schiff bases are bis(pyrrole-2-carboxaldehyde)thiocarbohydrazone (Pytch), bis(thiophene-2-carboxaldehyde)thiocarbohydrazone (Thtch) and bis(furfuralthiocarbohydrazone (Futch). They found that Futch ligand produced tetracoordinate complexes of the general formual [MLCl2] where they act as neutral bidentates bonding through the two imine nitrogens. Thtch ligand acted as neutral bidentate producing a tetracoordinate complex of the formula [Fe(Thtch)Cl2] or as monobasic tridentate producing tetracoordinate [Zn(Thtch)Cl] complex or as monobasic bidentate forming tetracoordinate [M(Thtch)2] complexes where M = Co, Ni and Cu. Complexes of Co(II), Ni(II), Cu(II), Zn(II) and Mn(II) of a Schiff base derived from o-phenyldiamine and furfural were synthesized and characterized by F. Dianzhong et al [70] using various physical and chemical methods. Electronic spectra, magnetic moment studies, EPR and XPS studies revealed that these metal chelates have octahedral geometry. The non-electrolytic nature of the complexes was verified by the molar conductance measurements. In 2009, P. Mittal et al [71] have synthesized and characterized novel Schiff bases derived from furan-2-aldehyde and thiophene-2-aldehyde with vinyl aniline. The chelating ability of these Schiff bases were screened by preparing the transition metal chelates of metal ions such as Co(II), Ni(II), Cu(II) and Mn(II). They verified the octahedral geometry of these complexes with various physico-chemical methods. The complexes were also screened for their antimicrobial activity. New two nickel(II) and copper(II) complexes of two Schiff base ligands formed by condensation of furfural and benzil with S-benzyldithiocarbazate have been synthesized and characterized by elemental analysis, magnetic and spectroscopic measurements by M. A. Ali et al [72]. The geometries of nickel(II) complexes, were square planar and octahedral, respectively. Cu(II) complexes acquired dimeric/polymeric structure due to low magnetic moment values. A new series of transition metal complexes of Mn(II), Co(II), Ni(II), Cu(II) and Zn(II) were synthesized from the Schiff base ligand derived from 4-aminoantipyrine, furfural and o-phenylenediamine by M. S. Suresh et al [73]. The structural features were derived from their elemental analyses, infrared, UV-visible spectroscopy, NMR spectroscopy, thermogravimetric analyses, ESR spectral analyses and conductivity measurements. The data suggested square planar geometry for the complexes having metal ions with primary valency two. In 2013, Y. Harinath et al [74] have synthesized a new Schiff base bidentate ligand (L), 5-methyl thiophene-2-carboxaldehyde carbohydrazone and its metal [Cu(II), Cd(II), Ni(II) and Zn(II)] complexes with general stoichiometry [M(L)2X2] (where X=Cl). The ligand and its metal complexes were characterized by elemental analyses, IR, 1Hnmr, ESR spectral analyses, and molar conductance studies. The molar conductance data revealed that all the metal chelates are non-electrolytes. IR spectra showed that ligand (L) is coordinated to the metal ions in a bidentate manner with N and O donor sites of the azomethine-N, and carbonyl-O. ESR and UV-Vis spectral data showed that the geometrical structure of the complexes are orthorhombic. Scope and Objectives of the Present Investigation The quest for novel Schiff bases and their metal chelates is the perpetual phenomenon for the chemists and scientists. By the exploration of the chelating ability of the Schiff bases, a novel class of metal complexes may be opened, which may possess potential applications in industrial, pharmaceutical and catalytic fields. By elucidating the structures of the metal chelates and ligands using advanced tools and techniques, a proper correlation can be made with the structure and activity. Even though large number of Schiff bases was prepared and chelating efficacy of the compounds was exploited, still remain wide scope for the synthesis of novel Schiff bases which possesses hetero atoms and their metal chelates. Literature survey showed that Schiff bases which are primarily derived from heterocyclic compounds such as acetylpyridines, furan-aldehydes and thiophene-aldehydes and their metal chelates were not much explored and reported. In the present course of investigation it is proposed to synthesize and characterize novel heterocyclic Schiff bases derived from 3-acetylpyridine, furan-2-aldehyde and thiophene-2-aldehyde using various spectroscopic techniques such as IR, UV-vis, NMR and mass. It is also proposed to synthesize new transition metal complexes of these Schiff bases by exploiting their chelating ability. The geometry and structure of the metal chelates are to be recognized by analytical tools like as spectral, magnetic, electrical and elemental analyses. ””””””””””””””””’ The slow destruction of metal under environmental conditions such as acidic gases, humidity etc is termed as metallic corrosion. Metallic corrosion takes place naturally and usually the metals will be converted into their most stable oxides. Some metals such as silver, copper etc slowly changes into their sulphides and basic carbonates respectively. The slow rusting of the iron that takes place naturally is a well known example for the corrosion in the world. In the chemical point of view, corrosion is an electrochemical phenomenon. The rate of corrosion enhances rapidly in the presence of acidic environment and electrolytes. Metallic corrosion has got a great attention among scientists and technologists, primarily due to its economic impact and secondly due to its safety consequences. Corrosion will lead to the lowering of the efficiency of plants, increasing the maintenance cost, contamination or loss of the products and may lead to the catastrophic damages. India has been losing around 1.52 lakh crore annually due to corrosion in various sectors such as infrastructure, production and manufacturing, defense, petrochemicals, railway, metal industries and nuclear power plants. In developed countries such as U.S. and Japan, the estimated loss due to corrosion is approximately 3% of their respective gross domestic product (GDP) which is about half of the loss estimated in India.[1] It is estimated that about 10-15% of the globally extracted iron from its ores will turn back to the nature per annum in the form of rust as a result of natural as well as accelerated corrosion. Apart from the natural and unavoidable corrosions, manmade activities such as acid pickling, de-scaling, oil-well acidizing will escalate the rate of corrosion considerably and these activities are considered as the major reasons for the corrosion problems in the metal industries and oil industries. De-scaling and Acid Pickling De-scaling and pickling are the metal surface cleaning techniques which consumes enormous quantities of hydrochloric acid and sulphuric acid. The thin oxide film (e.g., hydrated ferric oxide or rust), organic and inorganic stains and other impurities on the metal surfaces can be eliminated by treating the metal specimens for a stipulated time in acidic solutions and thus the original metallic appearance can be reinstated. Similarly the basic carbonate formed on the surface of copper and brass can be easily removed by treating the surface with mild acids. Treating aluminium with mild acids regains its original metallic luster. To minimize the corrosion during surface cleaning process, it is customary to add certain inhibitors into the aggressive solutions [2-5]. Petrochemical Industry and Corrosion The economic losses in oil industry due to corrosion mainly occur by the direct contact between the metallic oil pipelines and equipment with the aggressive media. The prolonged interaction between the aggressive media and pipelines is unavoidable during the production of oil, refining and transportation. The addition of corrosion inhibitors during these processes will help to decrease the corrosion rate considerably [6]. The acidic environment in the oil industry arises mainly due to two major reasons. a) Dissolution of the corrosive gases such as H2S and CO2 and b) hydrolysis of the acidic salts present in the aqueous phase to produce hydrochloric acid. Enormous amount of concentrated acids have been used for stimulating the oil wells and to obtain the unrecovered hydrocarbons. The underground rocks which are basic in nature (e.g., limestone) can be destroyed by the treatment with concentrated hydrochloric acid or acetic acid on injection. The hydrocarbons trapped between the rocks will be easily ejected by this treatment. Hydrofluoric acid is commonly employed for silica or sand stone based rocks. These acidizing process will cause to shoot up the corrosion rate of the metallic pipes inside the oil wells. In the presence of hydrogen sulphide, the dissolved metal in acidic medium will cause to precipitate the iron oxide and iron sulphide. These precipitates will negatively affect the quality of crude oil and oil production equipments. The addition of corrosion inhibitors is very essential to reduce the rate of the corrosion considerably. Prevention of Corrosion It is a fact that corrosion can’t be prevented completely, but the most economical solution is to adopt more practical techniques for controlling the rate of corrosion. There are number of corrosion controlling techniques available depending upon the type and nature of corrosion. Surface coating is the most widely accepted method for controlling natural corrosion. Galvanizing, anodizing etc are used for decreasing the rate of galvanic corrosion. Accelerated corrosions such as acid pickling, de-scaling, oil well corrosion etc are chiefly controlled by the addition of certain corrosion inhibitors into the acidic solutions. Corrosion Inhibitors The use of corrosion inhibitors is the most practical way for decreasing the rate of corrosion especially in acidic media. Since corrosion is an electrochemical phenomenon, oxidation and reduction are the two major processes taking place during the corrosion. Metal atoms which undergo oxidation will act as the anodic regions and the electrons released by the same atoms will be accepted by the protons, which are at the immediate vicinity of the metal surface and will get reduced to hydrogen atoms. This region of the metal is behaving as cathode. Corrosion inhibitors are classified into three according to their inhibitive mechanism. They are a) anodic inhibitors b) cathodic inhibitors and c) mixed type inhibitors. The role of a corrosion inhibitor is to protect the metallic surface by interacting with metal atoms directly or reacting the environment by which the surface is exposed. The inhibitive action of a corrosion inhibitor on the metal surface in a homogeneous liquid corrosive medium may be due to Increasing the anodic or cathodic polarization Reducing the diffusion of H+ ions from the bulk to the metal surface or Increasing the electrical resistance and thus by reducing the corrosion current density on the metallic surface. Anodic corrosion inhibitors The action of these inhibitors is to control the rate of anodic oxidation and thus prevent corrosion [7]. They can make large anodic shift of the corrosion potential. These types of inhibitors can passivate the steel by making passive oxide layers on the metal surface. They may be oxidizing (e.g., chromates, nitrites and nitrates) or non oxidizing type (e.g., ortho phosphate, tungstate and molybdates). The first type make protective layer in the absence of oxygen, while the second type require oxygen for making the passive layer [8,9]. These inhibitors are sometimes referred as ‘dangerous inhibitors’ since, small pores and defects on the oxide layer of these inhibitors, may lead to the accelerated corrosion of the metals [10]. Cathodic corrosion inhibitors These inhibitors will decrease the rate of reduction process taking place on the cathodic sites, by shifting the potential more towards negative direction (cathodic side). The localized precipitation of species on cathodic site will enhance the corrosion resistance and thus reduce the migration of ions towards cathodic region considerably. The reduction of oxygen will be difficult in this scenario. Some cathodic inhibitors can act as oxygen scavengers and thus help to control the corrosion by preventing the cathodic depolarization caused by oxygen. Examples for cathodic inhibitors are metal ions (calcium, zinc etc), bicarbonates, polyphosphates, sulphites etc. Organic compounds such as imidazole and benzamide are usually used as cathodic inhibitors in boilers, which will help to prevent the deposition of calcium and magnesium [11].

Generally cathodic corrosion inhibitors are termed as ‘safe inhibitors’ since they can reduce the rate of cathodic process even at low concentrations.

Mixed inhibitors These inhibitors influence both anodic and cathodic processes of corrosion. Many organic molecules come under this category. Various amines, triazoles, thiourea, quinolines can act as mixed corrosion inhibitors especially for steel and copper in acidic media [12,13]. Organic inhibitors There are several natural as well as synthetic organic molecules, which act as corrosion inhibitors for different metals such as iron, copper, zinc etc in acidic, basic and neutral media. Majority of the organic inhibitors are acting as mixed type corrosion inhibitor. At sufficient concentrations they can make good protective film via adsorption on the surface of the corroding metal. Adsorption may be physical or chemical depending upon the molecular structure of the compound. In general, inhibition efficiency of the organic inhibitors is found to increase with the concentration. Even though most of the organic molecules are acting as the mixed type inhibitors, some of them may affect more at anodic or cathodic site. It was well established that the organic molecules containing hetero atoms such as O, S, N etc and compounds possess azomethine linkage (>C=N-) i.e., Schiff bases act as good corrosion inhibitors for various grades of steels, zinc and copper in acidic as well as NaCl solution. The efficiency of these compounds depend on the number of active probes on the molecule, charge density, molecular size, concentration, nature of adsorption and ability to form metallic complexes. The unshared pair and ?? electrons on the molecule can interact well with the empty orbitals of the metal atoms and cause to the firm adsorption of aromatic Schiff bases on the metal surface. In addition to this processes, the back donation of the electrons from the filled metal orbitals to the unoccupied ??*-orbitals of the Schiff base also come into play during the interaction, which will help the molecule to make good protective layer on the metal surface. Schiff Bases as Corrosion Inhibitors- A Review Large numbers of

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Schiff bases were screened for their corrosion inhibition capacity in acidic media. Many of them were effective against the corrosion of mild or carbon steel in acidic media. Few of them were acted as efficient inhibitor against the corrosion of copper and zinc in aggressive medium. The subsequent paragraphs explore the ability of certain newly synthesized Schiff bases to act as good inhibitors against the metallic corrosion in acidic media, which was reported by the previous researchers. Two newly synthesised Schiff bases N,N’-ortho-phenylene(salicylaldimine-acetylacetone imine) and N,N’-ortho-phenylene(salicylaldimine-2-hydroxy-1-naphthaldimine) were studied as inhibitors for the corrosion of mild steel in 0.5 M sulphuric acid by M. Hosseini et al [14]. They confirmed by weight loss studies, electrochemical impedance and Tafel polarization measurements that both compounds act as good inhibitors, with efficiencies of around 95% at a concentration of 400 ppm. The nature of inhibition in both cases was mixed type (anodic and cathodic). Temkin isotherm is found to provide an accurate description of the adsorption behaviour of the investigated Schiff bases. N. Saxena et al investigated the corrosion performance of mild steel in nitric acid solution containing various concentrations of Schiff bases derived from anisaldehyde such as N-(4-nitro phenyl) p-anisalidine, as N-(4-chloro phenyl) p-anisalidine, as N-(4-phenyl) p-anisalidine, as N-(4-methoxy phenyl) p-anisalidine, as N-(4-hydroxy phenyl) p-anisalidine using mass loss, thermometric and potentiostatic polarization studies [15]. All compounds exhibited appreciable corrosion inhibition efficiencies. The inhibition efficiency was found larger than their parent amines and a maximum of 98.32% of efficiency was obtained. 2-alkyl-N-benzylidenehydrazinecarbothioamide of fatty acid hydrazides from nontraditional oils (neem, rice bran and karanja) have been synthesized and evaluated as corrosion inhibitors for mild steel in hydrochloric acid solution using weight loss method by Toliwal et al. Adsorption of all Schiff bases on mild steel surface in acid solution obeyed Temkin adsorption isotherm. Inhibition efficiency of these compounds was increased with the concentration of the compound, and varies with solution temperature, immersion time and concentration of acid solution. Various thermodynamic parameters were also calculated to investigate the mechanism of corrosion inhibition [16]. The inhibiting effect of (NE)-4-phenoxy-N-(3-phenylallylidene) aniline (PAC) on the corrosion of mild steel in 1.0 M HCl has been studied by H. Keles et al, very recently, by electrochemical impedance spectroscopy and Tafel polarization measurements. They determined the corrosion rate theoretically in terms of mm per year, using current density values of mild steel in 1.0 M HCl medium. It was found that PAC has remarkable inhibition efficiency on the corrosion of mild steel especially at high temperatures. By thermometric studies they proved that transformation of physical adsorption into chemical adsorption took place as the temperature of the system increased. The thermodynamic functions of adsorption processes were also evaluated. Scanning electron microscope observations of the electrode surface confirmed the existence of a protective adsorbed film of the inhibitor on the electrode surface [17]. D. Gopi et al has reported the corrosion inhibition efficiency of of 3,5-diamino-1,2,4-triazole Schiff base derivatives, based on the effect of changing functional groups. An attempt has been done to establish a relationship between inhibitor efficiency and molecular structure using weight loss method, electrochemical and Fourier transform infrared spectral techniques. They found that the molecules containing more electron donating groups have higher inhibition efficiency than the corresponding compounds with low electron donating groups. The results indicated that the order of inhibition efficiency of the triazole and its Schiff bases in solution and the extent of their tendency to adsorb on mild steel surfaces were as follows: vanilidine 3,5-diamino-1,2,4-triazole > furfuraldine 3,5-diamino-1,2,4-triazole > anisalidine 3,5-diamino-1,2,4-triazole > 3,5-diamino-1,2,4-triazole [18]. A novel Schiff base 2,2′-[bis-N(4-choloro benzaldimin)]-1,1′-dithio has been synthesized by S. M. A. Hosseini et al and its inhibiting action on the corrosion of mild steel in 0.5 M sulfuric acid was investigated by various corrosion monitoring techniques, such as weight loss and potentiodynamic polarization techniques. They showed that this compound acted as a good corrosion inhibitor for mild steel and the inhibition efficiency increased with the inhibitor concentration. This organic compound behaved as mixed type inhibitor in the acid solution, and its adsorption on the mild steel surface was found to obey the Langmuir adsorption isotherm [19]. Recently A. J. A. Nasser et al have investigated the influence of N-[morpholin-4-yl(phenyl)methyl]benzamide (MPB) on corrosion inhibition of mild steel in 1.0 M HCl by weight loss, effect of temperature, potentiodynamic polarization and electrochemical impedance spectroscopic studies. They found that the adsorption of MPB on the mild steel surface obeyed the Temkin adsorption isotherm. Potentiodynamic polarization curves showed that MPB act as a cathodic inhibition predominantly in hydrochloric acid [20]. Schiff base N-[(2-chloroquinolin-3-yl) methylidene ]-2-methylaniline (CQM) was synthesized by S. Jauhari et al and its inhibitive effect on mild steel in 1.0 M HCl solution was investigated by weight loss measurement and electrochemical tests. From the studies, they observed that the inhibition efficiency increased with the Schiff base concentration and reached a maximum at the optimum concentration. This was further confirmed by the decrease in corrosion rate of mild steel with the inhibitor concentration. They also proved that the system follows Langmuir adsorption isotherm [21]. A. S. Fouda et al [22] have investigated the corrosion behavior of carbon steel in 0.5 M HCl solution in the absence and presence of new five Schiff bases of indole derivatives by electrochemical impedance spectroscopy (EIS), electrochemical frequency modulation (EFM) and potentiodynamic polarization techniques. All the experimental results showed that these Schiff bases have excellent corrosion inhibition performance. The polarization curves showed that these compounds act as mixed type inhibitors. The adsorption of these Schiff bases on carbon steel surface is consistent with Langmuir adsorption isotherm. The effect of temperature on the rate of corrosion in the absence and presence of these compounds were also studied. The inhibiting action of 4-amino-antipyrine (AAP) and its Schiff bases 4-[(benzylidene)-amino]-antipyrine (BAAP), 4-[(4-hydroxy benzylidene)-amino]-antipyrine (SAAP) and 4-[(4-methoxy benzylidene)-amino]-antipyrine (AAAP) which are derived from 4-amino-antipyrine with benzaldehyde, salicylaldehyde and anisaldehyde, towards the corrosion behavior of mild steel in 1.0 M HCl solution was investigated by K. M. Govindaraju et al [23] using weight loss, potentiodynamic polarization, electrochemical impedance and FT-IR spectroscopic techniques. They found that all the synthesized Schiff base compounds were behaved well to retard the corrosion rate very effectively. The inhibitor efficiencies calculated from all the applied methods were in good agreement and were found to be in the order: AAAP > SAAP > BAAP > AAP. R. K. Upadhyay et al have reported the corrosion inhibition capacity of Schiff bases N-(furfurlidine)-4-methoxy aniline, N-(furfurlidine)-4-methylaniline, N-(salicylidine)-4-methoxy aniline, N-(cinnamalidine)-4-methoxy aniline, and N-(cinnamalidine)-2-methylaniline. They adopted mass loss and thermometric studies to evaluate the inhibition of corrosion of mild steel in hydrochloric acid. Results of inhibition efficiency yielded by the two methods were in good agreement and depend on the inhibitor and acid concentration. Maximum inhibition efficiency of 98% was reported by them [24]. Two series of long chained Schiff base amphiphiles were prepared by condensation of benzaldehyde or anisaldehyde with three different alkyl chain length fatty amines namely: dodecyl, hexadecyl and octadecyl amine by I. A. Aiad et al. The synthesized Schiff bases were evaluated as corrosion inhibitors for low carbon steel in various acidic media (HCl and H2SO4) using weight loss technique. The corrosion inhibition measurements of these inhibitors showed high protection against corrosion process in the tested acidic media at different doses. Attempts to correlate the inhibition efficiency of these compounds with their chemical structures have also been done [25]. Recently S. Issaadi et al have reported the corrosion inhibition studies of novel thiophene based Schiff bases. The Schiff bases, 4,4′-bis(3-carboxaldehyde thiophene) diphenyl diimino ether and 4,4′-bis(3-carboxaldehyde thiophene) diphenyl diimino ethane, were obtained by the condensation of 3-carboxaldehydethiophene and its corresponding amine. Polarization curves revealed that both compounds were mixed type (cathodic/anodic) inhibitors and inhibition efficiency (%IE) increases with increasing concentration of compounds. They suggested that corrosion inhibitive response of the compounds depend on their concentrations and the molecular structures. Adsorption of compounds on mild steel surface was spontaneous and obeyed Langmuir isotherm [26]. The behavior of the Schiff base N,N’-bis(salicylidene)-1,2-ethylenediamine (Salen), and a mixture of its parent molecules, ethylenediamine and salicylaldehyde, as carbon steel corrosion inhibitors in 1.0 M HCl solution was studied by A. B. da Silva et al [27] using corrosion potential measurements, potentiodynamic polarization curves, electrochemical impedance spectroscopy and spectrophotometry measurements. They reported that results obtained in the presence of Salen were similar to those obtained in the presence of the salicylaldehyde and ethylenediamine mixture, showing that in acid medium the Salen molecule undergoes hydrolysis, regenerating its precursor molecules. Corrosion inhibition investigations of pyridine based Schiff bases were reported by A. Yurt et al [28] on carbon steel in HCl medium using potentiodynamic and ac impedance studies. The Schiff bases under examination were synthesized by the condensation between pyridine-2-carboxaldehyde and respective amines. All compounds were found to act as good corrosion inhibitors. Scope and Objectives of the Present Investigation Vigorous research on corrosion and corrosion prevention techniques are undergoing globally by various scientists and surface engineers to minimize the rate of corrosion, since it is a potential threat which may affect directly or indirectly the economy and safety measures. The need for novel corrosion prevention techniques and corrosion inhibitors are increasing day by day. To reduce rate of corrosion of a metal in an aggressive medium with the aid of corrosion inhibitors is a challenging and interesting area of research. Synthesizing novel molecules and monitoring their corrosion inhibition capacities on various metals, especially mild and carbon steels and to implement these molecules as useful corrosion inhibitors are of keen interest for researchers and corrosion/ surface engineers related to metal and petroleum based industries. Even though a large number of organic molecules especially Schiff bases were screened for their corrosion inhibition capacity on metals in acidic media, still remains unanswered questions about the corrosion behavior of various heterocyclic Schiff bases. A very few of the articles have been reported by the previous researchers on the corrosion inhibition behavior of pyridine, thiophene and furfural based Schiff bases which was confirmed by thorough literature survey. In the present course of investigation it is proposed to determine the corrosion inhibition properties of eight different heterocyclic Schiff bases derived from 3-acetyl pyridine, furan-2-aldehyde and thiophene-2-aldehyde on carbon steel in hydrochloric acid and sulphuric acid by the conventional mass loss studies, electrochemical studies such as Tafel polarization and ac impedance measurements. It is also proposed to investigate the mechanism of corrosion inhibition by plotting various adsorption isotherms. Thermodynamic parameters such as adsorption equilibrium constant and free energy of adsorptions are also proposed to evaluate from adsorption isotherms.

Temperature effect on corrosion was investigated in order to determine thermodynamic parameters such as activation energy, enthalpy and entropy. Present investigation also aims to improve the corrosion inhibition capacities of certain organic molecules by utilizing the synergistic properties of iodide ions. In the present study, an attempt was also made to correlate the corrosion inhibition capacity of these molecules with their structural interactions on carbon steel surface.

””””””””””’.. It is not a tedious job to create a natural corrosive environment in the experimental settings of a laboratory. At the same time, since natural corrosion is a slow phenomenon and the monitoring of the rate of decay of a metal is very time consuming process, it is customary to adopt accelerated corrosion techniques which will mimic the corrosive environment. Accelerated corrosion tests of various metals are mainly performed in acidic (aggressive) solutions. To investigate the rate of corrosion and the behavior of corrosion inhibitors, conventionally accepted acceleration tests are mass loss or gravimetric studies and electrochemical studies. Electrochemical studies are mainly subdivided into Tafel polarization studies and electrochemical impedance spectroscopy (EIS). This chapter describes the preparation of metal specimens used for corrosion studies, its composition, aggressive solutions, details of corrosion monitoring techniques employed for the investigation and the electrochemical instrumental set up used for corrosion measurement. Metal Specimens Carbon steel (composition: 0.58 %; Mn, 0.07 %; P, 0.02 %; S, 0.015 %; Si, 0.02 % and the rest Fe, determined by EDAX method) were cut in the dimension 1.5x 1.5x 0.114 cm and abraded with various grades of silicon carbide papers (120, 400, 600, 800, 1000 and 1200) to obtain well polished surfaces as per ASTM standards. The total surface area of the metal specimens was accurately determined using vernier calipers and screw gage. Metal specimens were degreased with acetone, washed with detergent and distilled water, dried and finally weighed. Specimens were immersed in aggressive solutions with and without the inhibitor in different concentrations using hooks and fishing lines. Aggressive Solutions HCl and H2SO4 (Merck samples) were diluted to 1.0 M and 0.5 M concentrations respectively using distilled water. A stock solution of the inhibitor was first prepared and diluted with respective acidic solutions to obtain inhibitor solutions having concentrations 0.2 mM ‘1.0 mM for performing the corrosion studies of Schiff bases derived from 3-acetylpyridine and solutions in the concentration 0.1 mM- 0.5 mM for Schiff bases derived from furan-2-aldehyde and thiophene-2-aldehyde. The total volume of the medium was 50ml for gravimetric studies but 100ml was maintained for all electrochemical investigations. Gravimetric Corrosion Studies Gravimetric corrosion inhibition studies were performed by immersing the well polished carbon steel (CS) specimens in aggressive solutions having different concentrations of the inhibitor for 24 hours. A blank experiment was also conducted without adding the inhibitor. The weight loss occurred for metal specimens were measured after 24 h. For good reproducibility, all experiments were carried out in duplicate and the average values were reported. The corrosion rates and percentage of inhibition efficiencies were calculated by the following equations. The corrosion rates were expressed in mm/y and the inhibition efficiencies were obtained from corrosion rates. Rate of corrosion W= (K??wt.loss in grams)/(Area in sq.cm ??time in Hrs ??Density) (1) where ‘K’ =87600 (This is a factor used for the conversion of cm/hour into mm/year) Density of CS specimen= 7.88g/cc Percentage of inhibition or the inhibition efficiency (??) was calculated by ??=(W-W’)/W??100 (2) where W & W’ are the corrosion rate of the CS specimen in the absence and presence of the inhibitor respectively. Corrosion Inhibition Studies of Parent Compounds

To compare the corrosion inhibition efficiency of Schiff base and its parent aldehyde/ketone and amine, gravimetric corrosion studies of the parent compounds were performed in aggressive solutions for 24 h. This study has considerable significance in two aspects. At first, one can validate the higher inhibition efficiency of the Schiff base when compared to the corrosion inhibition efficiency of parent compounds. Sometimes Schiff base molecules undergo hydrolysis in the acidic media into their parent compounds and an appreciable change in the inhibition efficiency occur with time. In such cases the corrosion inhibition efficiency of the mixture of parent compounds were performed and compared with the inhibition efficiency of Schiff bases. The information regarding the hydrolysis and inhibition efficiency of the hydrolyzed product is the second aspect of this study.

Synergistic Effect Studies Synergistic effect study was conducted with aggressive solutions (sulphuric acid) together with 0.2 mM KI solutions. Gravimetric studies and electrochemical studies were performed separately to check the synergistic effect of iodide ions with the Schiff base molecules on carbon steel surface. If synergism plays, the addition of KI (1ml, 0.2 mM KI for gravimetric and 2ml for electrochemical studies) into the aggressive solution will raise the corrosion inhibition efficiency drastically. Adsorption Isotherms The mechanism of inhibition of various organic molecules on the surface of a corroding metal can be well explained by adsorption. To verify the nature of interaction between the metal surface and inhibitor molecules, adsorption isotherms were plotted by calculating the surface coverage from the inhibition efficiency. The different models of adsorption isotherms proposed was Langmiur, Freundlich, Temkin and Frumkin and the recently formulated thermodynamic/kinetic model, El-Awady isotherm. Among the isotherms mentioned above, the most suitable one was chosen with the help of correlation coefficient. The important thermodynamic parameters such as adsorption equilibrium constant (Kads) and free energy of adsorption (‘Goads) were calculated from the adsorption isotherms. These parameters are of key important in predicting the spontaneity of the process and the nature of adsorption i.e., physisorption or chemisorption or a combination of both. The important models of adsorption isotherms considered and the equation for the free energy of adsorption isotherm are given as follows [29-32]. Langmiur adsorption isotherm C/?? = 1/K_ads + C (3) Freundlich adsorption isotherm ??=K_ads C (4) Temkin adsorption isotherm ef?? = Kads C (5) Frumkin adsorption isotherm ??/(1-??) exp'(f??)=K_(ads )C (6) El-Awady adsorption isotherm log ??/(1-??)=’logK’_ + y logC (7) In the above equations C represents the concentration of the inhibitor, ?? is the surface coverage and Kads is the adsorption equilibrium constant. In El-Awady isotherm, Kads = K1/y, where y= number of active sites. If 1/y is less than 1, it implies multilayer adsorption and if 1/y is greater than 1, suggests that a given inhibitor molecule occupies more than one active site. Free energy of adsorption is related to adsorption equilibrium constant by the following equation. ‘G0ads = -RTln(55.5 Kads) (8) Surface Analysis Using SEM Surface morphological studies give insight to the mechanism of inhibition by which an organic molecules decrease the rate of corrosion. This was done by taking the scanning electron micrographs (SEM) of the metal surfaces at different conditions. SEM images of well polished bare metal specimen, metal specimen in acid solution (blank, treated for 48 h) and specimens in the inhibitor solution (treated for 48 h) were taken in the resolution 2.00x and compared. Hitachi SU6600 model scanning electron microscope was used for performing the surface morphological studies. Temperature Studies Gravimetric corrosion inhibition studies were performed in the temperature range 30-600C for investigating various thermodynamic parameters of corrosion such as enthalpy of corrosion (‘H*), entropy of corrosion (‘S*), activation energy (Ea) and Arrhenius parameter (A). The rate of corrosion is related to the energy of activation by the well known Arrhenius equation K=A exp'(-E_a/RT ) (9) where K is the rate constant, A is pre exponential or Arrhenius factor, Ea is the activation energy, R is the universal gas constant and T is the temperature in Kelvin scale. From the above equation it is evident that a plot of logK Vs 1000/T will be a straight line having slope -Ea/2.303R and intercept log A. The enthalpy and entropy of activation (‘H*, ‘S*) were calculated from the transition state theory [33] K= (RT/Nh ) exp ((‘S*)/R) exp ((-‘H*)/RT) (10) Here, N is the Avogadro number and h is the Planks constant. The equation can be rewritten in the form y= mx +c to obtain log K/T=log R/Nh+’S/(2.303 R)-‘H/(2.303 R T) (11) The slope of the above equation is -‘H/2.303R, from which enthalpy of activation can be calculated. Entropy of activation can be calculated from the intercept of the above equation i.e., log R/Nh+’S/(2.303 R) (12) Electrochemical Investigations It is well known that corrosion is an electrochemical phenomenon. The measurable electrochemical parameters such as corrosion current density, corrosion potential, charge transfer resistance, cathodic and anodic slope values (from current-potential response) etc will quantify the corrosion and help one to predict the rate of corrosion and to determine the mechanism of the corrosion. In the presence of corrosion inhibitors, the electrochemical parameters change considerably and hence affect the rate of corrosion. Exploiting these responses of corroding metals in the presence and absence of the inhibitor with the help of sophisticated electrochemical systems, is the major strategy adopted by the corrosion researchers to predict the inhibitive capacity of the various organic molecules in acidic media. Widely practiced electrochemical corrosion measurement techniques are Electrochemical Impedance Spectroscopy (EIS) and polarization studies. Polarization techniques are further classified into Tafel polarization analysis and linear polarization resistance analysis. Applications of the electrochemical methods are widely accepted for the investigation of corrosion [34-39]. The main advantage of the electrochemical investigation than the conventional gravimetric studies is that the former one require short span of time. All electrochemical measurements are computer assisted, and most modern softwares and electrochemical systems are using for the corrosion analysis. More refined and accurate values for the electrochemical analyses is thus possible than the conventional time consuming gravimetric investigations. In the present corrosion investigation, Ivium compactstat-e (made in Netherlands) electrochemical system was used. Latest version of the software ‘IviumSoft’ was powered the electrochemical analysis. Various analytical procedures like selection of proper equivalent circuit, simulation of curves obtained by the analysis, calculation of resistance and current densities etc can be easily performed with the software. For all electrochemical measurements a cell with three electrode assembly was used. Platinum electrode having area 1cm2 was used as the counter or auxiliary electrode and saturated calomel electrode (SCE) was the reference electrode. Well polished metal surface having an exposed area of 1cm2 towards the corroding medium acted as the working electrode.

The three electrode system eliminates the limitations of the conventional two electrode system. The conventional two electrode set up consists of a working electrode and a reference electrode only. A desired potential is applied in a controlled way on the working electrode to facilitate the charger transfer process during the electrochemical experiments. A second electrode having a fixed potential must be used in conjunction with the working electrode to gauge the exact potential of working electrode by balancing the charge added or removed by the working electrode. This setup has serious shortcomings since practically it is very difficult to maintain the constant potential of the reference electrode during the passage of current to the working electrode. These limitations can be overcome with the help of three electrode assembly (Figure 2.1)

Fig. 2.1 Three electrode circuitry In the above figure, CE, RE and WE represent counter, reference and working electrodes respectively. Additional electrodes namely counter or auxiliary electrode is inserted into the cell assembly. Now the role of the reference electrode is to control and measure the electrode potential of the working electrode only and practically no current is passed through the it. The auxiliary electrode allows the passage of whole current required for balancing the current of the working electrode. Polarization studies Polarization studies are performed by changing the applied potential of the working electrode in a controlled manner and scanned at constant rate (potentiodynamic). Mainly, Polarization studies can be divided into two a) Tafel extrapolation technique (Stern method) and b) Polarization resistance studies (Stern and Geary method) [40-43]. Tafel extrapolation technique The basis of the polarization techniques is mainly derived from mixed potential theory proposed by Wagner and Traud [44,45]. According to this theory, an overall electrochemical reaction can be algebraically divided into half cell reactions i.e., oxidation and reduction half cell reactions. In the cases of iron/copper based metal specimens, the reaction usually takes place at anodic areas is M ‘M2+ + 2e-, where ‘M’ represents iron/copper. The main cathodic reaction takes place during corrosion is the reduction of H+ ions to H2. Since the cathodic reaction is slower than the anodic process, the rate is usually controlled by the cathodic reaction. If the rate of anodic and cathodic processes is equal, there will be no charge accumulation. The mixed potential at this moment is called the open circuit potential (OCP) and commonly designated as corrosion potential or Ecorr. This corrosion potential is distinctly different from the reversible potential of the corroding metal or the species in the solution that is reduced at the cathode. The current at this mixed potential is designated as corrosion current density and denoted by icorr. An electrode can be polarized by the application of external voltage. The magnitude of polarization can be measured in terms of overvoltage i.e., the difference between the equilibrium potential and the external potential. Polarization can be either anodic direction (noble) or cathodic direction (active). To get iapp (measured or applied current density) as a function of E (applied potential), applied potential between the reference electrode and the working electrode is controlled and scanned at constant rate. The important types of polarization that occur during electrochemical measurements are activation polarization and concentration. Since the main steps in the electrochemical corrosion are controlled by activation or charge transfer, the effect due to concentration polarization can be neglected. For the reversible electrodes which are controlled by activation process, the polarization can be best described by the equation similar to Butler-Volmer equation [46] i_app= i_corr {exp[ (??_(a ) )/RT zF(E-E_corr ) ]-exp[-(??_(c ) )/RT zF(E-E_corr ) ] } (13) iapp is applied or measured current density; icorr is corrosion current density; ??a and ??c are the charge transfer coefficients for anodic and cathodic reactions, respectively. E-Ecorr is the polarization or the over potential obtained by the difference between applied and corrosion potential; z is metal valence; F is Faraday constant; R, the gas constant and T is the absolute temperature. When the polarization is in the anodic direction (positive), E>> Ecorr and the second term in the above equation can be neglected. Now equation 13 can more simply expressed as i_app= i_corr {exp[ (??_(a ) )/RT zF(E-E_corr ) ] } (14) The Tafel slope for the anodic process from the above equation is b_a=(2.303 RT)/(??_a zF) Similarly, for cathodic process, Ecorr>> E, then the first term in equation 13 can be neglected. The simplified equation for the cathodic reaction can be expressed as i_app= i_corr {exp[- (??_(c ) )/RT zF(E-E_corr ) ] } (15) Tafel slope for cathodic reaction can be expressed as b_c=(2.303 RT)/(??_c zF) The mixed potential diagram and Tafel extrapolation are described in the Figure 2.2. From the obtained current densities, the percentage of inhibition can be calculated by the following equation ‘ ‘??_(pol )%=(I_corr-‘I^”_corr)/I_corr X100 (16) where icorr and i’corr are uninhibited and inhibited corrosion current densities respectively. . Fig. 2.2 Tafel extrapolation method Fig. 2.3 Linear polarization method The slopes of Tafel lines have a significant role in interpreting the mechanism of inhibitor. By comparing the Tafel slopes of the uninhibited and the inhibited solutions, one will get the idea about the nature of inhibition. If the anodic slope (ba) of the inhibited solution only deviates considerably from the anodic slope of the uninhibited solution, it can be assumed that inhibitor molecule affect on anodic process of corrosion. Similarly, the change of cathodic slope (bc) alone is an indication of the adsorption of the inhibited molecules on the cathodic sites. An appreciable change in both ba and bc assures that the inhibitor molecule affect both anodic and cathodic process of corrosion. Tafel extrapolation as well as other polarization techniques badly affect by several factors such as ohmic resistance (arises from the resistivity of the solution) cell geometry, location of the reference electrode, magnitude of the applied current etc, which are few among this. Ohmic resistance may also contribute to overvoltage error sometimes [35,36,47,48]. Linear polarization resistance method For small overpotential reactions with respect to Ecorr, Stern and Geary modified the kinetic equation [43]. For the activation controlled process the equation can be linearized as i_corr=((‘i_app)/(2.303 ‘E))((b_a b_c)/(b_a+b_c )) (17) Rearranging the above equation i_corr=(1/(2.303 R_p ))((b_a b_c)/(b_a+b_c ))=B/R_p (18) where Rp is ‘E/’i, called the polarization resistance and ‘B’ is a constant obtained by all the constant terms in the above equation. Most often iapp shows approximate linearity with potential. When one determine the slope of this plot(Figure 2.3) at Ecorr, it may call as polarization resistance and it will be inversely proportional to the corrosion rate [49-50]. Linear polarization technique can be applied for the corrosion monitoring studies in the sense that the polarization resistance (Rp) increases with the inhibitor concentration. The rate of charge transfer process will be decreased considerably as the inhibitor molecules ‘work’ on the surface of the corroding metal by adsorption. In this scenario the rate of corrosion decreases appreciably. From the slope analysis of the linear polarization curves at the corrosion potential, the polarization resistances were obtained. The corrosion inhibition efficiency can be calculated using the equation ??_(R_p )%=(‘R^”_p-R_p)/’R^”_p X100 (19) where R’p and Rp are the polarization resistance in the presence and absence of inhibitor respectively [51] Electrochemical impedance spectroscopy (EIS) Impedance measurements are of key importance in predicting the corrosion rate of the metal in the aggressive solutions. Studies like corrosion behavior of metals which are coated with protective layers and of the inhibitive role of corrosion inhibitors on the metal surface etc can be easily performed with EIS measurements. In EIS technique, the working electrode is subjected to a small amplitude sinusoidal potential at a number of discrete frequencies ??. The resulting currents at each frequency will display sinusoidal response that is out of phase with applied potential signal by an amount ??. The amplitude of the current is


Category: Dissertation

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