Time-resolved spinel evolution during an oxygenation transient on 304 L stainless steel in 300 °C water tracked by in-situ Raman spectroscopy

Oxide films on austenitic stainless steels in water-cooled nuclear power systems experience redox transients during operational transitions and oxygen ingress events. However, studies on the change of oxide chemistry with time in response to a dissolved oxygen (DO) excursion remain limited. In this study, in-situ Raman spectroscopy is used to track the time-resolved evolution of the 600–750 cm−1 spinel band on as-received and 30% cold-worked 304L stainless steel in high-temperature water (300 °C) under two protocols: (1) a low-DO baseline (DO <10 ppb) for 8 days, and (2) a controlled oxygenation step in which a 3-day low-DO period is followed by a high-DO period (8 ppm DO) for 3 days. Under low DO, measurable multi-day evolution persists, and band-shape descriptors register a gradual shift toward higher-frequency weighting even when Raman visibility is non-monotonic. Oxygenation drives a coupled change in the spinel band, increasing its intensity and shifting it to higher wavenumber; cold work magnifies and accelerates this response, with band-shape features reaching a transient maximum within 1 day of oxygenation. Overall, low DO yields slow changes in the spinel band metrics, whereas the high DO transient triggers a rapid, pronounced spectral response that is amplified by cold work.