A new solid‐state mode of hot corrosion at temperatures below 700°C

Abstract

Preliminary results on a single‐crystal nickel‐based superalloy indicated that hot corrosion can occur at temperatures as low as 550°C, where liquid formation, generally believed to be responsible for Type II hot corrosion, is not predicted. Additional tests were conducted on pure‐nickel samples at 650°C and below to more clearly elucidate the mechanism of this very low‐temperature hot corrosion. Environments in dry air and O2‐(2.5, 10, 100, and 1000) ppm SO2 were studied. Based on the results obtained, a solid‐state corrosion mechanism was inferred. The mechanism relies on the formation of a previously unreported compound phase, which was identified using transmission electron microscope analysis that indicated the stoichiometry of Na2Ni2SO5. Furthermore, it was nanocrystalline in structure and metastable. It was deduced that the Na2Ni2SO5 formation was responsible for the rapid nickel transport required for the observed accelerated corrosion process. Moreover, its eventual decomposition resulted in a mixed product of porous NiO with embedded particles of Na2SO4. Application of the proposed mechanism to nickel‐based alloys is discussed.

Surface Roughness Evolution to Identify Incubation Time for Hot Corrosion of Nickel-Base Superalloys: CMSX-4, CM247LC DS and IN6203DS at 550 °C

“…Type II attack may also occur after a similar reaction between CoO and SO 3 . This forms an even lower melting point sulphate system of Na 2 SO 4 :CoSO 4 , which has been cited as reaching 576 °C [7], 565 °C [1,9] or as low as 540 °C [4,5]. For types I and II hot corrosion with a liquid sulphate, a fluxing mechanism dissolves any protective scale from the component surface [1,2,4,5,7,8,10,11], thereby allowing rapid attack of the underlying superalloy as sulphur diffuses inwards [3,4,7].…”

“…Hot corrosion may also be experienced when deposits remain in the solid state [8,9,12,13]. This occurs when the temperatures experienced are below the melting point of the sulphate system that is present [9].…”

“…Hot corrosion may also be experienced when deposits remain in the solid state [8,9,12,13]. This occurs when the temperatures experienced are below the melting point of the sulphate system that is present [9]. One example of this is given in the literature [9] where hot corrosion exposures produced accelerated attack on the nickel-base superalloy PWA1484 (containing approximately 10 wt% cobalt) using Na 2 SO 4 deposits and a gaseous environment of SO 2 in oxygen at a temperature of 550 °C.…”

“…This occurs when the temperatures experienced are below the melting point of the sulphate system that is present [9]. One example of this is given in the literature [9] where hot corrosion exposures produced accelerated attack on the nickel-base superalloy PWA1484 (containing approximately 10 wt% cobalt) using Na 2 SO 4 deposits and a gaseous environment of SO 2 in oxygen at a temperature of 550 °C. From this example, owing to the various cited melting points of the Na 2 SO 4 :CoSO 4 sulphate system, there could be some debate as to whether the cobalt from the superalloy formed a melted sulphate system or not.…”

“…As such, for accelerated attack to happen with a solid sulphate on a nickel-base superalloy, solid-state diffusion must have taken place. Kistler et al [9] proposed that in such instances, nickel diffuses through porous, fine grained NiO in order to react with the Na 2 SO 4 . This produces a metastable nanocrystalline Na 2 Ni 2 SO 5 -mixed oxide with a structure that allows rapid nickel fluxing into the Na 2 SO 4 .…”