Abstract
Fundamental insights into the mechanism of breakaway oxidation in Fe9Cr1Mo steel are deduced, through advanced characterisation and modelling. Degradation at 600 °C/∼42 bar CO2 for ∼20,000 h is emphasised: conditions relevant to components such as the finned superheater tubes used for advanced gas-cooled nuclear reactors. It is shown that such conditions are sufficient to cause carbon saturation of the metallic substrate, as confirmed by direct observation of extensive carbide precipitation but also numerical analysis of the carbon transport. Thus the observation of graphite precipitation close to the scale/metal interface is rationalised. Nonetheless, the activity of carbon at the scale/metal interface does not reach unity – with respect to graphite – at time zero. A modelling method is proposed which accounts for this kinetic retardation of the attack; this can be used to interpolate across the regimes within which breakaway oxidation is prevalent. It is a plausible model for extrapolation to the lower temperatures relevant to service conditions and is suitable for lifetime estimation – so-called ‘remnant life analysis’ – of such safety-critical components when prone to this form of attack.
Highlights
Ferritic-martensitic 9Cr steels (P91, P92) possess good creep strength and adequate corrosion resistance in CO 2 environments at medium temperatures (400-700 C)
A specimen exposed to 600 C was chosen for detailed high-resolution microstructural characterisation, since a transition between protective/breakaway oxidation was observed, see Figure 1(b); the specimen was exposed for 19,924 h while the first breakaway initiation of two fins was observed at 18,921 h
Energy dispersive X-Ray (EDX) chemical mapping was conducted at the internal oxide zone (IOZ) using a Zeiss Merlin field emission gun scanning electron microscope (FEG-SEM), with a beam of 5 keV energy and a probe current of 500 pA
Summary
Ferritic-martensitic 9Cr steels (P91, P92) possess good creep strength and adequate corrosion resistance in CO 2 environments at medium temperatures (400-700 C). An important application is in CO 2 gas-cooled nuclear reactors, typically in the temperature range 480 to 520 C [1] Corrosive attack in these circumstances is characterised by an outer, duplex scale consisting of magnetite and Cr-rich spinel layers, accompanied by precipitation of Cr-rich carbides within the underlying steel [2]. Whilst the kinetics are close to parabolic at first, increasing carburisation leads to the onset of rapid linear kinetics This ‘breakaway’ corrosion phenomenon is of practical significance, as it limits reactor lifetimes. It is of fundamental significance, as the processes whereby a low carbon activity gas can carburise steels and produce graphite which mechanically disrupt the protective scale and potentially triggers the ‘breakaway’ kinetics are not well understood [3, 4, 5]. A method is proposed to allow for the extrapolation of the time to breakaway data from higher to lower temperatures, which will be of significant practical value
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