X-ray Photoelectron Spectroscopy and Electrochemical Studies on the Interaction of Potassium Ethyl Xanthate with Metallic Copper R. M. Souto,* M. M. Laz, and S. Gonza ´ lez Department of Physical Chemistry, UniVersity of La Laguna, E-38205 La Laguna (Tenerife), Spain ReceiVed: July 17, 1996 X The adsorption of ethyl xanthate on copper specimens from moderately alkaline aqueous solutions has been studied combining polarization and X-ray photoelectron spectroscopic data. It was possible to derive structural information about the surface layers originated by either spontaneous adsorption or electrochemical anodization of the metal. Anodization of the metal in ethyl xanthate-containing solutions originates a surface highly resistant to the corrosive attack by chloride ions. XPS data suggest that copper protection is achieved through the growth of a cuprous diethyl dixanthogen layer on the metal in which the oxidation state +1 of copper becomes stabilized. Introduction The alkyl xanthate group of organics has been used widely in the recovery of heavy-metal sulfides for decades because of their ability to be adsorbed on the mineral surface with a change of its properties from hydrophilic to hydrophobic. In this way it is possible to separate the mineral from the ore by flotation. 1,2 Though long-chain xanthates can float both metallic sulfides and oxides, short-chain ones are effective only with sulfide minerals and lead to the selective flotation of sulfides from other gangue minerals. 3 Flotation processes are electrochemical in nature and are often studied with electrochemical techniques when metal sulfides are used as electrodes. These studies have proved that collector adsorption involves charge-transfer reactions which are highly sensitive to the applied potential. 4-7 That is, sulfide-collector interaction involves the formation of a hydrophobic species by an oxidation process involving electron transfer and the cathodic reduction of oxygen. 8 Three-dimensional phases are grown on the surface of the material which have been characterized by IRAS, ATR, and XPS in the case of the spontaneous adsorption of ethyl xanthate on copper and copper-activated zinc sulfide from aqueous solutions. 9 It was concluded that multilayer coverage of the metal leads to the formation of cuprous xanthate films. One of the most important methods in the corrosion protection of metals is the use of organic inhibitors to protect the metal surface from the corrosion environment. Since the electro- chemical reactions involved in mineral flotation are somewhat similar to those associated with metal passivity and corrosion, the applicability of xanthates can be considered as corrosion inhibitors. Indeed, in a preliminary investigation we found potassium ethyl xanthate (KEX) to be a promising inhibitor for copper corrosion in the presence of chloride ions. 10 This observation is in agreement with the finding that copper electrodissolution was partly inhibited in KEX-containing aqueous solutions as compared to plain NaCl solutions. 11 The objective of the present work was to study the corrosion behavior of copper in buffered moderately alkaline solutions (pH 9) with NaCl and potassium ethyl xanthate. The morphol- ogy and spectra of the passive films on copper were examined using scanning electron microscopy (SEM) and X-ray photo- electron spectroscopy (XPS), respectively. The inhibition efficiency of ethyl xanthate as a function of copper immersion and anodization also was studied. Experimental Section Specimens were made out of pure copper (99.9% purity). After mechanical polishing with silicon carbide paper up to 800 grit, samples were washed with distilled water, dried with cool air, treated thermally at 500 °C under 3 mm argon pressure for 2 h, and then electropolished in 85% H 3 PO 4 . 12 Passivation of copper was performed in 0.075 M Na 2 B 4 O 7 + 0.15 M H 3 BO 3 (pH 9) + 0.1 M NaCl + x mM KEX, with x in the range 0 e x e 50. Both dipped and anodized specimens in this solution were considered. All solutions were prepared from analytical grade reagents and twice-distilled water. Meas- urements were performed at room temperature (22 °C). Electrochemical measurements were performed with con- ventional electrochemical instrumentation in a three-electrode cell. Copper specimens were used as working electrodes. Each specimen was mounted at the end of a vertically movable cylindrical shaft, which made possible the contact between the copper disk surface and the solution through a hanging meniscus arrangement. 13 Potentials were measured against a saturated NaCl-calomel electrode (SSCE). A large cylindrical platinum gride around the specimen was used as the counter electrode. Deaeration of the solution was accomplished with argon prior to each experiment. A Hitachi S-450 scanning electron microscope operating with beam energies in the 20-25 kV range was used to study the topography of the copper surface after passivation and corrosion in the presence and the absence of KEX. X-ray photoelectron spectra were recorded on a VG Scientific ESCALAB 210 spectrometer equipped with a hemispherical electron analyzer and an Al KR X-ray excitation source (hν ) 1486.6 eV). 20-40 eV energy regions of the photoelectrons of interest were scanned several times to obtain adequate signal- to-noise ratios. Although sample charging was observed, X Abstract published in AdVance ACS Abstracts, December 15, 1996. 508 J. Phys. Chem. B 1997, 101, 508-511 S1089-5647(96)02144-X CCC: $14.00 © 1997 American Chemical Society