Oxidation mechanisms and scale evolution in a novel Cr-Ni-Mn-based valve steel (X30CrNiMnNbN25-12)

X30CrNiMnNbN25-12 steel is a recently developed Cr-Ni-Mn-based alloy for intake and exhaust valve applications, designed to withstand elevated thermal and mechanical loads under aggressive service environments. This study provides a comprehensive assessment of its high-temperature oxidation behavior between 800 and 1000°C under laboratory air conditions for up to 200 h. At 800 and 900°C, oxidation proceeded through an initial transient stage characterized by the formation of Mn-, Cr-, Fe-rich oxides and the preferential oxidation of Nb-rich carbides, which contributed to the initial mass gain. A subsequent parabolic regime developed, with an apparent activation energy of ∼205 kJ/mol, and oxidation rate increased by approximately one order of magnitude with each 100°C increment. At 1000°C, parabolic behavior persisted for only 25 h. After that, severe oxide scale spallation occurred during cooling. Prolonged exposure resulted in the development of a multilayered oxide scale composed of an inner Cr 2 O 3 layer, an intermediate MnCr 2 O 4 spinel layer, and an outer Mn 2 O 3 . Additionally, Si enrichment near the metal/oxide interface at 900°C contributed to reduced oxidation kinetics. These findings demonstrate that the steel exhibits stable oxidation behavior and good scale integrity up to 900°C, confirming its suitability for exhaust valve applications operating below this temperature.