Oxidation behaviour of Nimonic 263 in high-temperature supercritical water

Abstract

The oxidation tests of the Nimonic 263 alloy exposed to deaerated supercritical water at 600-700°C under 25 MPa were carried out for up to 1000 h. Oxidation rate increased with an increase in temperature. The microstructure and phase composition of oxide scale were analysed by scanning electron microscopy/energy dispersive X-ray spectroscopy, X-ray diffraction and X-ray photoelectron spectroscopy. It can be seen that a complex oxide structure formed on the surface of Nimonic 263 including an outer layer of Ni-Fe/Ni-Cr spinel oxide, Ni/Co hydroxide and TiO 2 and an inner layer of a mixture of NiCr 2 O 4 and Cr 2 O 3 while the innermost layer is made up of Cr 2 O 3 . The MoO 3 can be observed at 600°C but disappeared with the increasing temperature. The growth mechanism of oxide scale was discussed.

Oxidation Processes and Involved Chemical Reactions of Corrosion-Resistant Alloys in Supercritical Water

“…As for Cr, Ni, and Fe, reaction can further be expressed by reactions –. ,, The second stage takes place in some of the reactions among the oxides and hydroxides mentioned. The two reaction stages can be explained by the solid-state growth mechanism and the metal dissolution/oxide precipitation mechanism . In general, the diffusion phenomenon plays an important role in the oxidation process, and the growth rate of oxide film is determined by the diffusion rates of metal ions and O 2– …”

“…Besides, NiO can also be formed by the reactions of Ni and H 2 O/oxygen based on reactions –. ,− NiO is stable in mildly basic oxidizing SCW and acts as a barrier to inhibit ion diffusion, whereas it is unstable in a neutral oxidizing SCW. Owing to the reduction of the oxygen partial pressure on the interface of oxide film and alloy substrate, the decomposition of NiO can occur via reactions and ., Ni 2+ can also react with high temperature water and OH – derived from water ionization to form Ni hydroxide through reactions –. ,, Herein, the ionization of water is realized via reactions and . However, Ni­(OH) 2 can be decomposed and reduced into NiO and Ni in SCW by reactions and , respectively. where n = 1, 2 The surface treatment process affects chemical reactions of elements in alloys.…”

“…Although Mo, Ti, Al, Si, and other elements have relatively low contents in corrosion-resistant alloys, they also undergo a series of chemical reactions in SCW. For example, Mo is oxidized at 600 °C (see reaction ) while it is absent on an alloy surface at 650 and 700 °C . Mo is also dissolved with the formation of hexavalent acids (i.e., H 2 MoO 4 ) in oxidizing SCW (see reaction ), thereby losing its protective ability .…”

“…With the development of oxidation, partial NiO further reacts with Cr 2 O 3 to form spinel NiCr 2 O 4 , which is more stable than NiO in SCW according to reaction . ,, Some other NiO reacts with Fe 3+ and OH – to form NiFe 2 O 4 via reaction in the outer oxide layer. Moreover, spinel can also be formed by reaction Spinel is likely derived from hydroxide reactions.…”

“…Formed Ni­(OH) 2 and Fe­(OH) 2 (see sections and ) precipitate on the alloy surface and then form the outer layer of nickel ferrite via reactions and . The instability of Ni­(OH) 2 results in its reaction with Fe 3+ and OH – to form spinel, as expressed by reaction At the same time, the Cr 2 O 3 inner layer can further react with Fe and/or Fe oxide to form Fe–Cr spinel FeCr 2 O 4 based on reactions –, ,,, −,,, so resulting in the decrease of the Cr 2 O 3 amount.…”