Electrochemical interaction between silver and sulfur in sodium sulfide solutions

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I. INTRODUCTION

SILVER, like gold, is a precious metal that is widely used in many applications, including in photographic films and jewelry, and in the arts and other industries. Substantial amounts of silver are used in the jewelry industry. Silver has very specific characteristics and its application in industry has increased sharply in recent years. When silver or gold is subjected to corrosion under environments containing sulfur, the corrosion phenomenon is often referred to as tarnishing.[1] The tarnishing behavior of silver and gold or their alloys is developed over an extended period of time, especially when they are exposed to an environment containing sulfur. The tarnishing behavior appears often during the manufacturing stage of jewelry and causes many problems for manufacturers. The current practice uses cyanide solutions with oxidants to remove tarnished layers from jewelry metals and their alloys. Since cyanide is considered toxic, it is desirable to find effective substitutes. It is even more desirable is to find a means to prevent completely the tarnishing of these precious metals. Corrosion mechanisms for silver and gold under environments of selected relevant chemical variables, such as pH, and oxygen and sulfur potentials, have been studied by a number of investigators.[2–8] These investigations have established that silver and silver compounds have strong interactions with sulfur.[3] Many researchers have attempted to prevent the tarnishing phenomenon of silver through the use of a variety of substances, including benzotriazole, mercaptanes, and 1-phenyl-5-mercaptotetrazol.[9] However, the silver surface is not completely free of corrosion. In the current investigation, a study of polycrystalline silver corrosion behavior in sodium sulfide solutions has been carried out. Potentiodynamic polarization techniques were used to investigate the corrosion mechanism of silver under various conditions. The corrosion products formed J.I. LEE, former Graduate Student, is Research Assistant, Department of Materials Science and Engineering, University of Arizona, Tucson, AZ. S.M. HOWARD, J.J. KELLAR, and K.N. HAN, Professors, and W. CROSS, Research Scientist, are with the Department of Materials and Metallurgical Engineering, South Dakota School of Mines and Technology, Rapid City, SD 57701. Manuscript submitted January 15, 2001. METALLURGICAL AND MATERIALS TRANSACTIONS B

were characterized using Raman and X-ray diffraction (XRD) spectroscopies and scanning electron microscopy (SEM). II. THEORETICAL BACKGROUND Numerous investigators have studied the phase transformation from silver to silver sulfide (Ag2S).[4,10,11] Considering the Eh-pH diagram for the Ag-S-H2O system, it is apparent that silver can be oxidized to Ag2S. Further oxidation of this phase can produce silver oxides such as Ag2O and AgO. The following equations represent the phase transformation of silver to various corroded products in the presence of sulfur-bearing species. 2Ag ⫹ H2S ⫽ Ag2S ⫹ 2H⫹ ⫹ 2e E ⫽ ⫺0.03 ⫺ 0.059 pH ⫺ 0.0295 log PH2S 2