Surface Enhanced Raman Spectroscopy (SERS) and Electrochemical Investigation of the Adsorption and Corrosion Inhibition Performance of 4-methylimidazole on Copper Corrosion in Chloride Media

摘要

Copper is a noble metal, but it is susceptible to Cl~- ions attack regardless of the solution pH andcopper alloy elements. Heat exchangers, pipes and other Copper alloys structures are oftensubmitted to acidic pickling in order to remove corrosion products. Thus, it becomes essential toprotect the metal during such procedures and the use of corrosion inhibitors is usually the chosenstrategy. Benzotriazole is the best-known inhibitor for copper due to its very high inhibitionefficiency (IE) both in acidic and neutral chloride media. Unfortunately, it is also toxic to plants andaquatic microorganisms, thus demanding that less aggressive alternatives to protect the metal. Inthis context, Imidazole (Im) and derivatives, such as 4-methylimidazole (4-MeIm), are proposed ascorrosion inhibitors for copper and its alloys [1].Adsorption of 4-MeIm on copper electrodes was investigated by potential(E)-dependent SurfaceEnhanced Raman Spectroscopy (SERS), electrochemical and gravimetric measurements. SERSdata from a 4-MeIm 0.001 M + KCl 0.1 M (pH = 8.8) solution revealed that 4-MeIm chemisorbs oncopper in a perpendicular orientation at cathodic overpotentials. As E is moved to less negativevalues, spectral changes show that it bends towards the surface. Tautomeric equilibria of the twoneutral forms of 4-MeIm is also affected by potential changes. Additionally, the spectra showedthat 4-methylimidazolium (4-MeImH+), a product of 4-MeIm hydrolysis in water, does not interactdirectly with the metal, in contrast to imidazolium (ImH~+), that coadsorbed with Im even in alkalinemedia[2]. Formation of Cu_2O and CuCl/CuCl_2- were evidenced by both SERS and cyclicvoltammetry (CV) results, suggesting low IE in this media. SERS spectra acquired at the opencircuit potential (OCP) and at anodic overpotentials showed that 4-MeIm physisorb on the electrodesurface, lying almost flat on it at the OCP. As the potential becomes more positive, 4-MeIm reactswith Cu_2O generating a soluble complex that leaves the metal completely exposed. Weight lossexperiments performed in different immersion times revealed that corrosion rates obtained in 4-MeIm solution are always higher than in the blank KCl solution, result confirmed by CV. Thus, 4-MeIm may be considered as a good cathodic inhibitor but has very low overall IE, in fair agreementwith literature results [3]. To the best of our knowledge, this is the first report on formation of asoluble complex of 4-MeIm with Cu_2O and such an explanation could only be attained by acombination of spectroscopic and electrochemical experiments, thus emphasizing the importanceof our methodology in studying the IE of a molecule.