Observations on Accelerated Oxidation of a Ferritic Stainless Steel Under Dual Atmosphere Exposure Conditions

Abstract

Iron-base alloys, conventionally used for the fabrication of cell-to-cell interconnects, undergo localized oxide scale overgrowth when exposed to a bipolar atmospheric condition, in which one side of the metal is exposed to a reducing gas and the opposing side is exposed to an oxidant. The phenomenon, coined “dual atmosphere corrosion”, is prevalent in many electrochemical and thermochemical systems where separation of fuel and oxidant gas streams is required. It is apparent that hydrogen exposure and the existence of a dual atmosphere plays a key role, as metals exposed to single atmospheres of reducing or oxidizing gases do not show the extent of oxide scale overgrowth. The exact role that hydrogen plays in accelerated iron oxide growth on the air-exposed side of metals, however, remains largely unknown. Experimental results from oxidation tests conducted on a select ferritic stainless steel under dual atmosphere exposure conditions are presented. After 50h in dual atmosphere, the ferritic steel had an extensive iron oxide scale with a particular needle-like growth morphology. In comparison, the same steel in dry air for 50h only showed uniform scale growth and absence of iron oxide nodules. These results along with thermodynamic driving forces are discussed in regard to metal oxidation and active species involved in oxidation. Current hypotheses regarding the role of hydrogen in dual atmosphere corrosion are also discussed.