A study of breakaway oxidation of 9Cr–1Mo steel in a Hot CO₂ atmosphere using Raman spectroscopy

Abstract

The microstructure of the oxide scale and metal bulk in ferritic 9Cr-1Mo steel was observed in order to explore the oxidation and carburisation mechanisms upon exposure to a CO2 gas environment at high temperature and high pressure. An experimental 9Cr-1Mo steel sample that had been oxidised at 580ºC for more than 165000 hrs in the coolant gas consisting primarily of CO2 gas was analysed. In order to elucidate the oxidation characterisation, scanning Raman spectrometry was used to analyse the oxide close to the metal/oxide interface. Carbon was found to be deposited in the spinel layer. The microstructure and the distribution of elemental chemical composition were examined and analysed using optical and high-resolution scanning electron microscopy combined with energy dispersive X-ray analysis. Raman signals from the oxide scale were analysed in order to explore the possibility of deriving strain information close to the oxide-metal interface. The results are discussed with relation to understanding the mechanisms of oxidation and carburisation which aim to underpin extension of the service life of components fabricated from this steel.

A comparison of two high spatial resolution imaging techniques for determining carbide precipitate type and size in ferritic 9Cr-1Mo steel

“…The HS-AFM image from sample B (Figure 4d) which includes cored carbide precipitates reveals that the height is greater at the core indicating a higher hardness of the inner core than the outer layer carbide. As discussed by Liu et al [25], the Cr concentration of the inner core and the shell carbide are between 40 and 80, and 30 and 50 wt%, respectively, indicating that the higher Cr concentration increases hardness for the carbide precipitates. Thus HS-AFM imaging of cored carbides is based upon the relative hardness of these three features: inner core > outer layer carbide > matrix.…”

Corrosion behaviour of Ni-based alloy Inconel 740H in supercritical carbon dioxide at 650–700°C

“…Figure 9 shows the distribution of elements in the oxide scale and the metal matrix after exfoliation. The Cr-rich oxide layer is rapidly formed on the surface of Inconel 740H and the area after exfoliation, which is different from the breakaway corrosion of T92, T91 and other 9Cr steels [7]. However, multiple oxide layer exfoliation may occur during long-term oxidation, which may lead to the depletion of Cr in the matrix below the critical value, resulting in a series of loose Ni oxides on the surface [30].…”

“…It is worth noting that the carburising behaviour of metals also occurs in S-CO 2 . The amorphous carbon formed in the substrate/scale interface and substrate will lead to chromium deficiency in the matrix [4][5][6], and the accumulation of amorphous carbon at the substrate/scale interface will also impair the adhesion between the inner oxide scale and substrate [7,8]. Owing to the different thermal expansion coefficients between the scale and the substrate, the oxide scale can quickly produce exfoliation when the temperature changes [9,10], which in turn leads to the exfoliation of the oxide scale.…”

Carbide precipitation associated with carburisation of 9Cr–1Mo steel in hot CO2 gas

“…Changes in the mechanical and corrosion properties occur as a result of thermal ageing, oxidation and carburisation in the coolant gas containing CO2 with traces of CO, H2, H2O and CH4. The consideration of service life extension of AGRs has revived the interest in 9Cr-1Mo steel and prompted the study of the mechanisms of concurrent oxidation and carburisation [6] [7] [8]. The thermodynamic equilibrium reaction governing carburisation is shown by equation 1 [6] [9]:…”