Effect of H₂/H₂O ratio on thermally grown chromia scales formed on Ni25Cr alloy in Ar–H₂–H₂O atmospheres at 1000°C

Abstract

The oxidation behaviour of a NiCr-base model alloy with a 25% chromium content is investigated at 1000 C in different low p(O 2 ) environments (Ar -H 2 -H 2 O) with the emphasis of how hydrogen andyor water vapour affects the oxidation behaviour of chromia forming alloys \in low p(O 2 ) environments containing H 2 and H 2 O. After equilibrium conditions are achieved, chromia growth on Ni25Cr in low p(O 2 ) type test gases containing H 2 and H 2 O is ruled by the concentration of Cr interstitials available for Cr outward transport. The chromia microstructure, therefore, consists largely of columnar shaped oxide grains. The chromia scales are growing predominantly from oxygen coming from the water vapour in the test gas. H-species may facilitate oxide growth in pores at the metal -oxide interface. H-defects affect the transport properties of chromia scales formed on Ni25Cr. At low p(O 2 ) the H-defects in the chromia scale are compensated by a lower concentration of Cr interstitials. Depending on the nature of the test gas, i.e. p(O 2 ), p(H 2 ) and p(H 2 O), as well as on the Cr activity in the metal substrate, the growth of chromia scales on NiCr alloys may be featured and shaped differently.

Effect of specimen thickness on chromia scaling of Ni25Cr in N2–O2–H2O test gases at 1000°C

“…For the case of water vapour containing gases several authors [18][19][20][21] proposed the classical defect equilibria to be affected by protons and/or hydroxyl ions in the chromia lattice. This was done to describe the effect of the gas composition on the chromia growth rate as well as on the electrical conductivity of chromia based scales.…”

Temperature Effect on Oxidation Behavior of Ni-Cr Alloys in CO2Gas Atmosphere

“…Six model alloys, 10,15,20,25 and 30 Cr (all in wt%) were prepared by arc melting pure metals Ni (99.99%) and Cr (99.995%) under a protective Ar-5%H 2 gas atmosphere, using a non-consumable electrode. Cast alloy buttons were annealed at 1100…”

“…[14][15][16][17] In air and oxygen environments, it has been generally accepted that binary Ni-Cr alloys with Cr contents lower than 10 wt% oxidize more rapidly than pure nickel does owing to the increased concentration of cation vacancies in the NiO scale. 9,11,13 However, a Cr level of approximately 20 wt% in nickel leads to greatly reduced oxidation rates, because of the formation of a continuous layer of Cr 2 O 3 at 900…”

“…15,16 It was proposed that this could be related to the differences in oxygen * Electrochemical Society Fellow.…”

“…14 The chromia scales on Ni-25Cr alloy after reaction in Ar-O 2 and Ar-H 2 O gas mixtures exhibited different morphologies, coarse and equi-axed grains in the former environment but fine and columnar grains in the latter. 15,16 It was proposed that this could be related to the differences in oxygen * Electrochemical Society Fellow.…”