The Attack of Copper-Gold, Silver-Gold, Nickel-Copper, and Silver-Copper Alloys by Sulfur at Elevated Temperatures

Abstract

Copper-gold alloys involving 10 and 33 mole % Au and Ag-Au alloys involving 12 mole % Au are attacked by liquid or gaseous sulfur above 400~ very rapidly with formation of a composite scale consisting of an outer homogeneous sulfide layer and an inner two-phase layer involving Au-rich alloy and sulfide in accord with theoretical considerations regarding the instability of a plane alloy-sulfide interface. The tendency to form a rugged alloysulfide interface decreases with higher Au contents especially at lower temperatures and the rate of attack by sulfur decreases accordingly. Silvercopper alloys involving 40 mote % Cu are also rapidly sulfidized at 400~ and yield a single sulfide layer of uniform thickness. Nickel-copper alloys sulfidized at 400~ yield a scale consisting of an outer layer which is supposedly digenite Cul.sS, and an inner layer which is supposedly a solid solution of Cu~S in nickel sulfide.

Kinetics and Mechanism of Nickel-Sulfur Reaction

“…The extent of this plastic flow is limited by the dimensions and the size of the specimen. For geometrical reasons the deformations of the scale are not observed at edges and curved parts of the sample (2,15,22,23). At some critical scale thickness which depends on the temperature of the reaction, on the plasticity of the scale, and on the geometry of the sulfurized metal specimen the decrease in the volume of the metal cannot be further compensated by the plastic flow of the scale; the contact between the scale layer and the metal becomes partially broken and longitudinal cracks are formed.…”

Corrosion in Aqueous Solutions and Corrosion in Gases at Elevated Temperatures – analogies and disparities

“…Copper and silver react preferentially with sulfur, whereas gold remains in essence in the metallic state. Thus analogous structures with a highly rugged alloysulfide interface are found (22).…”

The sulphidation of metals and alloys

“…If we consider the pressure of sulphur (p s 2) to be I atm, then expression (41) simplifies to where X is the mole fraction of element B in alloy AB, and therefore (I -X) is the mole fraction of A. The derivation of expression [43] also implies that the two sulphides AS and BS are not soluble in one 165 ... [43] From [42] it is clear that as the concentration of B increases so does its affinity for sulphur. In particular, the affinities of elements A and B for sulphur are equal (both metals sulphidise) wheñ…”