The Effect of O\(_2\) Traces in H\(_2\)/H\(_2\)O Atmosphere on Oxide Growth on Iron and Crofer 22 APU at 550 and 600 \(^{\circ }\)C

Abstract

The effect of O\(_2\) traces on the oxidation behavior of iron and a Fe–Cr steel interconnect material (Crofer 22 APU) in a 5\(\%\) H\(_2\) + 3\(\%\) H\(_2\)O (bal. Ar) environment at 550 and 600 \(^{\circ }\)C has been investigated. The reaction of O\(_2\) with H\(_2\) in the gas at 550 and 600 \(^{\circ }\)C is slow, allowing O\(_2\) traces to reach the samples, despite an excess of H\(_2\) in the gas. Traces of unreacted O\(_2\) resulted in increased oxidation rate as well as hematite formation on iron. The rate of iron oxidation increased with the level of O\(_2\) in the range of 2–550 ppm. The acceleration of oxide growth by O\(_2\) traces is attributed to a greater oxygen activity gradient across the iron oxide scale. To remove O\(_2\) from the gas, a nickel component was positioned upstream from the samples which allowed the gas to equilibrate. Consequently, iron oxidized without hematite formation. Moreover, the use of the nickel component greatly improved the reproducibility of results for both iron and Crofer 22 APU. The use of a catalyst for the O\(_2\) + H\(_2\) reaction in H\(_2\)/H\(_2\)O exposures proved essential because it provides better control of experimental conditions and thereby more reliable experimental outcomes.