In Situ and Ex Situ Studies of Imidazole and Its Derivatives as Copper Corrosion Inhibitors. II. AC Impedance, XPS, and SIMS Studies

Abstract

Copper is a material with excellent electrical and thermal conductivity and is often used in heating and cooling systems. Scale and corrosion products cause a decrease in the heating efficacy of the equipment, which is why periodic descaling and cleaning in acid-pickling solutions are necessary. In order to reduce the corrosion of copper during such surface treatments, a corrosion inhibitor is typically added. In general, most acid corrosion inhibitors are hetero-cyclic compounds containing nitrogen, sulfur, or oxygen. Imidazole derivatives are of interest as potential corrosion inhibitors for copper and its alloys, 1-7 as well as for silver, 8 zinc, 9 iron, 10 and gold. 11 Imi-dazole is a planar, aromatic heterocyclic organic compound with imino and tertiary nitrogen atoms forming part of a five-membered ring. 12 In aqueous solutions imidazole (Im) behaves like a weak organic base with pK a 6.99. 13 In a previous paper we investigated the efficiency of imidazole and its derivatives 4-methylimidazole, 4-methyl-5-hydroxyimida-zole, 1-phenyl-4-methylimidazole, 1-(p-tolyl)-4-methylimidazole as copper corrosion inhibitors in 0.5 M HCl. 5 We investigated the activation energies for the corrosion procssses, and we explored the ther-modynamics of the adsorption of these inhibitors to the Cu surface. We deduced from the adsorption energies that in 0.5 M HCl all of the inhibitors in this series are physisorbed to the copper surface. 5 We have used X-ray photoelectron spectroscopy (XPS) and secondary ion mass spectrometry (SIMS) here to explore this issue of physisorb-tion vs. chemisorption of these inhibitors. These studies show that 1-(p-tolyl)-4-methylimidazole is the best inhibitor in this series. Because 1-(p-tolyl)-4-methylimidazole was found to be the best inhibitor, 5 we have conducted further ac impedance studies on this in-hibitor. AC impedance data were obtained at three different concentrations of the inhibitor and at three different potentials. These studies have shown that 1-(p-tolyl)-4-methylimidazole acts as mixed inhibit-or. Furthermore, the ac impedance data clearly show that inhibition efficiency improves with the concentration of this inhibitor. Experimental Materials.-Concentrated HCl (Merck), concentrated HNO 3 (Merck), and imidazole (Aldrich) were used as received. The inhibit-ors 4-methylimidazole, 1-phenyl-4-methylimidazole, and 1-(p-tolyl)-4-methylimidazole were obtained from the pharmaceutical company Pliva (Zagreb, Croatia). Ethanol (Merck) and purified water (Milli-pore) were used to rinse the samples; Millipore water was used to prepare all solutions. The copper working electrode was of 99.98% purity. XPS and SIMS studies were done on Cu/epoxy composites consisting of two 18 m thick Cu films sandwiching an 2 mm thick film of epoxy resin. These Cu samples were obtained from Ericsson Nikola Tesla (Zagreb, Croatia). Methods.-AC impedance measurements were conducted at 20C using an EG&G PARC potentiostat/galvanostat 283A, EG&G PARC frequency response detector model 1025 and EIS M398 software. An ac sinusoid of 5 mV was applied at the corrosion potential (E E corr) and at E E corr 30 mV. The frequency range 100 kHz to 0.02 Hz was employed. Instabilities in the impedance data were observed at lower frequencies. A conventional three-electrode electrochemical cell of volume 100 mL was used. The working electrode was prepared from a cylindrical copper rod insulated with polytetrafluoroethylene tape (PTFE) such that the area exposed to solution was 0.785 cm 2. The hydrophobicity of the PTFE tape insured that solution did not have access to the sides of the copper rod. A saturated calomel electrode (SCE) was used as the reference; a Pt plate electrode was used as the counter. All potentials are reported vs. SCE. The working electrode was etched for 30 s in 7 M HNO 3 , rinsed in redistilled water and ethanol, and then used immediately. 14 The supporting electrolyte used for all studies was aerated 0.5 M HCl. The inhibitor 1-(p-tolyl)-4-methylimidazole was added to concentrations ranging from 1 10 3 M to 0.1 M. XPS and SIMS were done on the copper samples exposed to 0.5 M HCl with and without addition of the inhibitor and to Milli-pore water with the desired inhibitor. XPS measurements were performed with a Surface Science Instruments (SSI) S-Probe ESCA instrument using of a Mg K (1253.6 eV) or Al K (1486.6 eV) monochromatized X-ray source to stimulate photoemission. Survey and high resolution spectra were obtained with a hemispherical energy analyzer at pass energies ranging from 25 to 150 eV. The binding energy (BE) scale was referenced by setting the hydrocarbon peak maximum in the C 1s spectrum to 285.0 eV. Typical pressures in the analysis chamber during spectral acquisition were 10 9 Torr. SSI data analysis software was used to calculate the elemental compositions from the peak areas and to peak fit the high resolution spectra. The SIMS mass surveys of the Cu were obtained with a Perkin-Elmer 6600 dynamic SIMS system using a 1.5 keV Cs primary ion beam and negative secondary ion detection. In the SIMS experiment we are especially interested in fragments containing nitrogen since The objective of this work was to investigate the efficiency of imidazole derivatives for corrosion inhibition of copper in 0.5 M hydrochloric acid. Corrosion inhibition was studied using impedance spectroscopy. Imidazole and its derivatives 4-methylimida-zole, 4-methyl-5-hydroxymethylimidazole, 1-phenyl-4-methylimidazole, 1-(p-tolyl)-4-methylimidazole were investigated. These studies have shown that 1-(p-tolyl)-4-methylimidazole is the best inhibitor in this series and that it acts as mixed inhibitor. The nature of the chemical interaction between these molecules and the copper surface was investigated for Cu exposed to solutions having two very different pH values: 0.5 M HCl and unbuffered purified water. X-ray photoelectron spectroscopy and secondary ion mass spectrometry were used to explore the nature of the interaction. Possible mechanisms of corrosion inhibition for these molecules are discussed.