Stress and Adhesion of Protective Oxide Scales on Stainless Steels and RE Effects

• Published in: ISIJ International Vol. 59; no. 9; pp. 1642 - 1649
• Main Authors: Kitamura, Kazuyuki, Nishiyama, Yoshitaka, Fujimoto, Shinji, Otsuka, Nobuo
• Format: Journal Article
• Language: English
• Published: The Iron and Steel Institute of Japan 15.09.2019
• Subjects: acoustic emission; breakaway oxidation; in situ measurement; mechanical stress; Raman spectroscopy
• ISSN: 0915-1559; 1347-5460
• DOI: 10.2355/isijinternational.ISIJINT-2019-037

The stress and adhesion of a protective oxide scale formed on 25Cr-20Ni steel were characterized by in situ acoustic emission (AE) and Raman spectroscopy in order to examine breakaway oxidation during cyclic oxidation and the effects of various reactive elements (RE), such as Y, La, and Ce. Scale failure, such as spalling and/or cracking, was detected from AE measurements during cooling after isothermal oxidation at 1273 K. The internal stress of the scale during cooling was evaluated using in situ Raman spectroscopy to determine the stress increase that arises due to mismatch between the thermal expansion coefficients of the scale and the steel. Stress relaxation induced by scale failure or creep deformation in the steel substrate was also examined. Comparing the two measurements, we introduced the maximum permissible stress, σmax, as an index describing scale failure resistance. It is considered that scale failure occurs when the internal stress of the scale exceeds σmax during cooling. Scale failure makes the scale more susceptible to oxidation, resulting in increased Cr consumption from the substrate. The steel oxidizes drastically if the Cr content decreases below a critical concentration, exhibiting breakaway oxidation. The addition of RE improves scale adhesion and suppresses the oxidation rate, delaying breakaway oxidation.