Effect of Structure on the Electrochemical Oxidation Rate of Copper and Nickel Sulfides

Abstract

Synthesized samples were crystallized at 10 - 1000 K/sec (νcooling) to study the effect of sulfides structure on the electrochemical oxidation rates. The methods of X-ray, optical and electronic microscopy are used to analyze the phase composition, and the methods of voltamperometry and chronoamperometry are used to study the laws of electrochemical oxidation. Slow cooling of the samples with sulfide phase (Cu1,96S, Ni3S2) leads to crystallization of the metal (Ni, Cu). High cooling rate leads to crystallization of non-equilibrium phases, increasing of proportion of the sulfide phase and decreasing of metallic component up to complete disappearance. It is shown that electrochemical oxidation of copper and nickel sulfides proceeds stepwise: Me2S Me2-XS Me1+XS MeS Me2+ + S. Experiments have revealed the rates of electrochemical oxidation of granular sulfides that exceed the rates established for the slowly cooled samples. The rates of the electrochemical oxidation of Ni3S2 at potential of 1500 mV are 8.810-8 g/sec·mm2 (at vcooling = 10 К/sec) and 1.310-7 g/sec·mm2 (at νcooling = 1000 К/sec). The rates of the electrochemical oxidation of Cu2S at potential of 1000 mV are 2.810-8 g/sec·mm2 (at νcooling = 10 К/sec) and 4.310-8 g/sec·mm2 (at νcooling = 1000 К/sec). It is found that the forming of a passivation layer effects the rate of the electrochemical oxidation of copper sulfide and nickel sulfide. Non-equilibrium phase composition and refinement provide greater reactivity of sulfides in the conditions of anodic polarization. The oxidation and passivation of metallic phase along with sulfide phases proceed from the surface of Me2S Me alloys. As for the samples crystallized at 1000 K/sec only anodic oxidation of sulfides occurs due to a lack of metallic phase. The compositions of passivation films and potentials providing the extraction of sulfur in the elemental state from the sulfides crystallized at high rates were determined.