Effect ofin-situgas changes on thermally grown chromia scales formed on Ni–25Cr alloy at 1000°C in atmospheres with and without water vapour

Abstract

Regarding oxidation resistance, most high temperature alloys rely on the formation of a protective chromia surface scale during service. In the present study, the oxidation behaviour of a Ni-25%Cr model alloy was investigated during single-and two-stage oxidation in Ar-O 2 and Ar-H 2 O gas mixtures at 1000uC. In the two-stage experiments, the test gas was changed after a predefined oxidation time from dry to wet gas, and vice versa, without intermediate specimen cooling. Oxidation kinetics were measured using thermogravimetry and post exposure characterisation was accomplished using scanning and transmission electron microscopy, focused ion beam and energy dispersive X-ray spectroscopy techniques. The single stage exposure to Ar-O 2 resulted in the formation of a voided, poorly adhering chromia scale exhibiting a coarse, equi-axed morphology. In Ar-7%H 2 O a fine, columnar grained oxide scale was formed which was free of microvoidage within the scale and at the scale/metal interface and therefore exhibited excellent adherence to the metallic substrate. During two-stage exposure to Ar-O 2 and subsequently to Ar-H 2 O, a fine grained, columnar shaped oxide developed on top of the coarse oxide scale formed in the first oxidation stage. The opposite effect occurred when the exposure started in the wet environment. The mechanisms which are responsible for the observed oxidation features are discussed on the basis of classical oxidation theory in combination with previous results on oxidation of chromia forming alloys in oxygen rich gases and water vapour containing, low p(O 2 ) environments.

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

“…The oxidation of various model Ni-Cr alloys in air and oxygen has been well investigated at temperatures between 800 and 1200 • C, [8][9][10][11][12][13] and the oxidation of these alloys in H 2 O-containing environments has also been reported. [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 • C. 10 Alloys containing 30 wt% or more of Cr are able to develop a protective chromia layer, and much lower oxidation rates result.…”

“…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 partial pressure ( p O 2 ) in the atmosphere and the incorporation of hydrogen into the scale. 17 In contrast, the oxidation behavior of binary Ni-Cr alloys in CO 2 -rich gases has received little attention.…”

“…[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.…”