In Situ Growth-Stress Measurement of Cr<sub>2</sub>O<sub>3</sub> Scale Formed on Stainless Steels by Raman Spectroscopy

Abstract

Raman spectroscopy was conducted to evaluate mechanical stress in growing α-Cr2O3 scale upon oxidation of austenitic 25mass%Cr-20mass%Ni and ferritic 17mass%Cr stainless steels at 1173 K in air for up to 24 h. Sintered α-Cr2O3 pellet was heated to 373-1173 K and examined in order to obtain the temperature dependence of the wave length of the major Raman peak. For 1.2 mm thick 25mass%Cr-20mass%Ni steel specimen, compressive growth stress was indicated for α-Cr2O3 scale right upon oxidation and the stress increased until oxidation for 3 h, but it saturated and remained constant thereafter. The growth stress of α-Cr2O3 scale was 0.7 ± 0.1 GPa at 1173 K. For 1.2 mm thick ferritic 17mass%Cr steel specimen, mechanical stress was compressive, but the saturated growth stress was around 0.2 GPa, considerably smaller than the “strong” 25mass%Cr-20mass%Ni steel specimen. For 0.1mm thick austenitic 25mass%Cr-20mass%Ni steel specimen, the test results were similar. These were attributed to the different high-temperature strength of the metal substrate. Hence, for high-temperature oxidation of thin foils and/or ferritic steels of which high-temperature strength of the metal substrate is relatively poor, stress relaxation of protective α-Cr2O3 scale can result and the growth stress of α-Cr2O3 scale may be lowered by the “weak” metal substrate. Raman spectroscopy can offer useful information on the mechanical stress of protective oxide scale even at high temperatures.