Development of Analysis Method for Sulfide in Steel with Chelating Agent of Copper

Abstract

Copper sulfide (Cu_xS) has been frequently observed in steel samples, prepared using selective potentiostatic etching by electrolytic dissolution (SPEED). It is often the case that Cu_xS is detected unexpectedly from the precipitates extracted from steel samples by selective potentiostatic etching, although such Cu_xS formation during the heat treatment conducted is not anticipated by the thermodynamic equilibrium calculations.

In this study, we observed such artificial Cu_xS along with manganese sulfide (MnS) precipitates, which were extracted from steel materials by SPEED, using secondary electron microscope (SEM) equipped with energy dispersive X-ray spectroscopy (EDX) and Auger electron spectroscopy. These Cu_xS-MnS sulfide complex would be formed by the following mechanism: as the solubility of Cu_xS is far bigger (i.e. 10 times or more) than that of MnS, Cu²⁺ ion dissolved from steel matrix would be exchanged with Mn²⁺ ion on the MnS surface during the etching process, leading to a formation of Cu_xS-MnS sulfide complex.

In order to suppress the formation of such Cu_xS, we propose the use of following electrolyte: a non-aqaueous solutions of 4% methyl salicylate + 1% salicylic acid + 1% tetramethylammonium chloride (TMAC) + 5% Triethylenetetramine (TET) in volume fraction, in methyl alcohol (Cu ion selective hold etching by electrolytic dissolution, abridged as CUSH electrolyte). Then, this electrolyte was applied to precipitates in steel samples. It was effective to prevent the formation of sulfides in electrolyte, with the effect of metallic (Cu²⁺, Ag⁺, Pb⁺, etc.) chelating ability of TET.

Comparison of sulfides by SEM-EDX after SPEED etching with (a) 4% MS (b) 4%MS+5%TET.