Research On The Corrosion Inhibition Of N,S Heterocyclic Organic Compounds For Mild Steel In Acid Solution

Mild steel is widely used in the field of manufacturing industry and construction business since it's affordable and can be machined easily.Before processing,mild steel is usually cleaned by acid.How to reduce wastage of mild steel during Industrial pretreatment has always been a hot issue.Lots of researchers focus their attention on adding corrosion inhibitors into acid solution since some of them perform quite well.Heterocyclic compounds such as indole,imidazole,triazole and thiazole are studied.Because of having the?electrons of heterocycles and a lonely pair of electron of heteroatoms,heterocyclic compounds can adsorb on the surface of mild steel to form a protective film against corrosion.But till now,no one studies corrosion inhibition efficiency and mechanism of indazole systemically.Meanwhile,most of theoretical studies were based on the assumption that inhibitor molecules would retain original form in the test media,ignoring the effect caused by the real existing form.In this study,the inhibition efficiency and mechanism of 2-?2-Hydroxyphenyl?benzothiazole as a new inhibitor was researched by weight loss and electrochemical measurements.pKa was introduced to calculate the real existing form of the inhibitor in the test media.The conclusion was the inhibitor was protonated.And it was accordance with the data attained by theoretical studies.On prime of this conclusion,the researcher studied adsorption behaviors of indazoles affected by different substituents on metal,and also compared the synergistic mechanisms of protonated and original inhibitor compounds working respectively with different halogen ions.The main work and conclusions are as follows:?1?The inhibition efficiency of 2-?2-Hydroxyphenyl?benzothiazole?HBT?as a corrosion inhibitor of mild steel in 1M HCl solution was studied.It increases with the increasing concentration of HBT,and reach the value⁹5%at 0.07 mM.In the study,it illustrates that HBT is a mixed-type inhibitor with high inhibition efficiency,and the adsorption behavior of it coincides with the Langmuir adsorption isotherm.Furthermore,computational calculations reveal that the inhibitor is protonated and protonated HBT?HBT-H⁺?molecules are the main substance constituting adsorption film on metal surface.?2?The inhibition efficiency of Indazole?IA?as a corrosion inhibitor of mild steel in 0.5M H₂SO₄ solution was studied.Meanwhile 5-bromoindazole?BIA?and5-iodoindazole?IIA?were explored as corrosion inhibitors of mild steel in the same test solution respectively,to study the effect of two halogen substituents?Br,I?on the corrosion inhibition efficiency of Indazole.It shows that IA is a mixed-type inhibitor,and the adsorption behavior of it coincides with the Langmuir adsorption isotherm.The value of IA's inhibition efficiency reaches 89.5%at 4 mM?concentration of IA?.are mixed-type inhibitor,whose inhibition efficiency?reach 91.9%and 98.1%respectively,at 0.8 mM?is better than IA's and the adsorption behavior of them coincides with the Langmuir adsorption isotherm,too.The order of inhibition efficiency is IIA>BIA>IA.Theory calculations indicate that the three inhibitors are protonated in test solution,working in the form of IA-H⁺,BIA-H⁺and IIA-H⁺,respectively.The relatively excellent inhibition efficiency of IIA and BIA owe to the strong electron-accepting effect of Br and I ions which cause changes of frontier orbital distribution and energy parameters.Moreover,the relationship between quantum-chemical parameters and inhibition efficiency is established on prime of computational chemistry.?3?5-aminoindazole?AIA?and 5-nitroindazole?NIA?were explored as corrosion inhibitors of mild steel in 0.5M H₂SO₄ and 1M HCl solution respectively,in order to study the effect of two Nitrogen-containing substituents?-NO₂,-NH₂?on the corrosion inhibition efficiency of Indazole.It shows that both AIA and NIA are mixed-type inhibitor in test solution,and the inhibition efficiency of they are better than IA?76.8%and 80.5%,at 2 mM?.It indicates by theory calculations that AIA and NIA in acid solution are protonated,AIA-2H⁺and NIA-H⁺.The relatively low inhibition efficiency of NIA-H⁺attributes to the–NO₂ substituent which has stronger electron-accepting capacity.Its substitution at position 5 decreases the electron cloud density around benzene rings,resulting the changes of distribution in the frontier track.It also indicates by theory calculations that AIA-2H⁺and NIA-H⁺adsorb parallel on the surface of the mild steel.?4?Indazole-3-carboxaldehyde?3-CIA?and indazole-3-carboxylic acid?3-CAIA?were explored as corrosion inhibitors of mild steel in 0.5M H₂SO₄ solution,in order to study the effect of two Carbonyl substituents?-CO-H,-CO-OH?on the corrosion inhibition efficiency of Indazole.The experimental results show that both 3-CIA and3-CAIA are mixed-type inhibitor with low inhibition efficiency?<80%at 4 mM?,while inhibition efficiency of 3-CAIA is relatively higher than that of 3-CIA.Theory calculations indicate that aldehyde and carboxyl groups are electron-absorbing groups.Their substitution at the third position makes the molecular frontier orbital deviate to the five-membered ring,which results in the change of energy parameters of the frontier orbital.It is the reason for the low corrosion inhibition efficiency of the two inhibitors.While the reaction activity parameters are similar,the adsorption configurations of3-CIA and 3-CAIA are different,which results different corrosion inhibition efficiency.?5?Indazole?IA?coordinating with halogen ions?Cl^-,Br^-,I^-?were explored as corrosion inhibitors of mild steel in 0.5M H₂SO₄ solution,in order to study the effect of halogen ions on the corrosion inhibition efficiency of Indazole.The difference of inhibition performance between indazole?IA?and indole?IO?under the same conditions was compared,too.Experimental results show that the three halogen ions tested can play a synergistic corrosion inhibition role with IA and IO and the synergistic inhibition efficiency follow the order of I^->Br^->Cl^-.Coordinating with Cl^-and Br^-,IA performs better than IO.The result will reverse when the two work with I^-.Theory calculations indicate IO retain its original form while IA protonate into IA-H⁺in tested media.They have different adsorption configurations when they adsorb on mental surface with halogen ions.It's the major cause for different inhibition efficiency.In summary,the existence form of inhibitor molecule in test solution plays an important role on its corrosion inhibition performance.Theoretical parameters getting from real existing form?infered by pKa?agree well with inhibition efficiency.Studies on indazole derivatives with different substituents and substituted positions show that the inhibition efficiency of the inhibitor is closely related to the frontier orbital of the molecule,and the quantum parameters.When corrosion inhibitor particles adsorb on metal surface,the effect of adsorption configuration on corrosion inhibition efficiency is higher than that of quantum parameters and adsorption energy parameters.