The Na2 SO 4 ‐ Induced Corrosion Properties of Ni‐Base Superalloy Phases

Abstract

The separate phases present in a Ni-base superalloy may offer different resistance to hot corrosion attack. Ni-Cr, Ni-A1, and Ni-Cr-A1 alloys of composition corresponding, respectively, to the nominal ,y matrix, the ~' phase, and a system of both these two phases, were coated with Na2SO4 and exposed to oxygen or H2S-H2 gas mixtures in the temperature range 850~176 The corrosive attack was monitored with a recording microbalance. The results indicate that the degree and mechanism of corrosion of the above phases is markedly influenced by the nature of the environment and the composition of the oxide scales formed at the alloy surface, in general, the ~,' phase appears to be a weak link in the over-all hot corrosion process.

The Effects of Lead on the Hot Corrosion of Nickel‐Base Alloys

“…The Dy ~+ ions cannot be excited effectively by these discharges which emit mainly at ~ > 250 nm (corresponding to the wave number region ~ < 40.000 cm-1). The lowest charge transfer level of Dy 3+ in oxides is situated at energies corresponding to ~ > 50.000 cm -1 (10) and the same applies to the lowest 5d level of Dy ~+ (11). Excitation of Dy 3+ can therefore occur only by 4f-4f transitions.…”

“…Under these conditions, alloys with aluminum concentrations in the range 5-20 w/o were vigorously attacked by Na2SO4 alone, the corrosion becoming catastrophic at an alloy composition of Ni-13 A1 (Ni3A1). Alloy coupons of this composition were reduced to a gray porous mass after exposure to the molten salt for a period of only 2 hr (10). The presence of PbO exerted very little influence on the very high corrosion rates of these alloys.…”

“…It seems likely that the presence of 1% PbO in N a~SO4 alters the corrosion mechanism for Ni in the following way. Following melting of the salt mixture and the formation of homogeneous solution, Ni oxidizes by reducing PbO Ni(s) + PbO(soln) --NiO(s) -}-Pb(1) [10] A large thermodynamic driving force exists for this reaction AG----AG ~ + RTlna(PbO)…”

Burner Rig Hot Corrosion of a Single Crystal Ni-48Al-Ti-Hf-Ga Alloy

“…In comparison, much less work as been performed on NiA1 or NiAI-X alloys. Early studies found 13-NiAI relatively resistant to Na2SO4 attack although the lower-Al, y' Ni3A1 phase was not [6][7][8][9]. Kaufman concluded that "major attack on NiAI is preceded by transformation of a surface layer to Ni3A1 due to A1 loss to the otherwise protective m1203 scale" [6].…”