Formation of Copper Sulfide Artifacts During Electrolytic Dissolution of Steel
- PDF / 354,525 Bytes
- 4 Pages / 593.972 x 792 pts Page_size
- 65 Downloads / 209 Views
ch is commonly present as an impurity in recycled steel, has been reported to lead to precipitation of copper sulfide in cast steel products[1] and in steel welds.[2–4] The composition and morphology of sulfide inclusions are important because, as examples, manganese sulfide is known to promote pitting corrosion,[5] and copper sulfide has been suggested to affect acicular ferrite formation (during transformation of austenite).[4] Based on the observed positions of copper sulfides in transmission electron microscope (TEM) samples, it was concluded that copper sulfide precipitates on existing manganese sulfide inclusions.[1] This implies that copper sulfide is more stable than manganese sulfide (under some steel processing conditions). However, this is not what the available thermodynamic data show: In fact, manganese sulfide is much more stable than copper sulfide in steel. To illustrate the greater stability of manganese sulfide, Figure 1 shows the equilibrium constants for decomposition of CuS and Cu2S in the presence of pure Mn and Figure 2 gives the ratio of the activities of MnS and Cu2S for equilibrium in steel containing dilute Mn and Cu. Both of these figures show that manganese sulfide is by far more stable than copper sulfide; copper sulfide should not be expected to form in manganesecontaining steel. A similar conclusion was reached by Wrangle´n.[5] Similarly, Park et al. found low percentages of copper (assumed to be present as sulfide) in manganese
JIA TAN, Graduate Student, and P. CHRIS PISTORIUS, Professor, are with the Department of Materials Science and Engineering, Center for Iron and Steelmaking Research, Carnegie Mellon University, Pittsburgh, PA 15213. Contact e-mail: pistorius@ cmu.edu Manuscript submitted January 4, 2013. Article published online March 16, 2013. METALLURGICAL AND MATERIALS TRANSACTIONS B
oxysulfide silicates equilibrated with liquid Cu-Mn alloys; for example, equilibration with a 50 pct Cu alloy at 1523 K (1250 °C) yielded 0.8 pct CuS in an oxysulfide with the composition 53 pct MnS, 39 pct MnO, and 9 pct SiO2.[6] In apparent contradiction of the equilibrium considerations, copper sulfide in association with manganese sulfide has often been detected by energy dispersive X-ray (EDX) analysis of inclusions in TEM samples.[1–4] This paper presents a test of the suggestion that the detected copper sulfides were artifacts, which had formed during electrochemical dissolution of the steel (etching in an oxidizing acid or electropolishing). Copper sulfides can form during electrochemical dissolution of steel because of the low solubility product of copper sulfide in electrolytes. While elevated temperature chemical equilibrium between manganese, sulfur, and copper in steel greatly favors formation of manganese sulfide rather than copper sulfide, the situation is quite different for the reaction with ions in solution: Copper ions have a strong tendency to precipitate as copper sulfide in the presence of sulfide ions, much stronger than that of manganese ions.[10] This can be expressed as the solubility product, whi
Data Loading...
