Electrochemical And Spectroelectrochemical Study Of Corrosion Inhibition Of Copper By Phytate And AMT

Phytic acid, known as inositol hexakisphosphate (IP6), is an organicphosphoric acid compound that occurs naturally in plant seeds. It is widely used inthe fields of food, medicine and daily chemicals because of its inoxidizability andexcellent chelate ability with various metal ions. Also it has been proposed as anenvironment-friendly corrosion inhibitor for many metals.2-Amino-5-mercapto-1,3,4-thiadiazole (AMT) is a heterocyclic compound with afive-membered ring, which has been used in protection of bronze cultural relics as anew corrosion inhibitor due to its special structure. In this thesis, the corrosioninhibition of copper by phytate and AMT were studied using cyclic voltammetry(CV), Tafel plot, electrochemical impedance spectroscopy (EIS) and in situ UV-visspectroelectrochemical techniques, especially cyclic voltabsorptometry. Theconditions for corrosion inhibition were respectively optimized for the twoinhibitors and the inhibition mechanisms were discussed.Initially, the corrosion inhibiting effects of the two inhibitors wereinvestigated in0.4mol dm–3NaSO4medium. The CV curves of copper in0.4moldm–3NaSO4showed a large oxidation peak and a small reduction peak. A passivepotential region occurred after the oxidation peak. The inhibition efficiencies wereevaluated from the decrease in peak current of the oxidation peak due to theaddition of the inhibitors. The CV results indicate that the appropriate pH range isnot lower than7, and the optimal concentration is10mmol dm–3for IP6and4mmol dm–3for AMT, respectively. This result is supported also by the Tafel plots.The in situ UV-vis spectra under constant potential electrolysis of copper in NaSO4solution showed that the soluble cupric species were generated and transformedinto its stable form by chemical reaction. The small reduction peak on the CVcurvers corresponds to the reduction of solid cuprous oxide on copper surface.Further, the corrosion inhibiting effect of IP6was investigated in NaOHsolutions using CVA. The ability to in situ monitor the formation and transition ofsoluble corrosion products generated during metal electrochemical processes isimportant for elucidating the corrosion mechanism. The combination of CV andCVA allows simultaneous measurements of the potential-dependent current and absorbance, the latter of which corresponds to the characteristic wavelength of eachlight-absorbing species. For this purpose, a long-optical-path thin-layerelectrochemical cell (LTE-cell) was fabricated to study the corrosion inhibition ofcopper by phytate in NaOH solutions. A conventional electrochemical cell was alsoused for common CV and EIS. The morphology of the corroded copper surfaceswas characterized using field-emission scanning electron microscopy. A highcorrosion inhibition efficiency (ca.90%) of phytate on copper was achieved in0.1mol dm–3NaOH solution. Phytate was shown to promote the formation of thesoluble intermediate cuprous ions and hinder the generation of the cupric ions frompitting corrosion of the initially deposited cuprous oxide layer. The integrity of thecuprous passive layer was therefore maintained, which strongly suppressed furtherformation of various cupric corrosion products. The results of this study indicatethat cyclic voltabsorptometry based on a LTE-cell is an effective method forstudying metal corrosion processes that involve light-absorbing ions.