Estimation of Oxidation Kinetics and Oxide Scale Void Position of Ferritic-Martensitic Steels in Supercritical Water

Abstract

Exfoliation of oxide scales from high-temperature heating surfaces of power boilers threatened the safety of supercritical power generating units. According to available space model, the oxidation kinetics of two ferritic-martensitic steels are developed to predict in supercritical water at 400 ∘ C, 500 ∘ C, and 600 ∘ C. The iron diffusion coefficients in magnetite and Fe-Cr spinel are extrapolated from studies of Backhaus and Töpfer. According to Fe-Cr-O ternary phase diagram, oxygen partial pressure at the steel/Fe-Cr spinel oxide interface is determined. The oxygen partial pressure at the magnetite/supercritical water interface meets the equivalent oxygen partial pressure when system equilibrium has been attained. The relative error between calculated values and experimental values is analyzed and the reasons of error are suggested. The research results show that the results of simulation at 600 ∘ C are approximately close to experimental results. The iron diffusion coefficient is discontinuous in the duplex scale of two ferritic-martensitic steels. The simulation results of thicknesses of the oxide scale on tubes (T91) of final superheater of a 600 MW supercritical boiler are compared with field measurement data and calculation results by Adrian's method. The calculated void positions of oxide scales are in good agreement with a cross-sectional SEM image of the oxide layers.

New insights into the oxidation mechanisms of a Ferritic-Martensitic steel in high-temperature steam

“…To reduce carbon emissions, it is desirable to improve the efficiency of the steam-generating power plants by increasing the operating steam temperature and pressure, such as in the ultra-supercritical coal-fired power plants [7,8] and supercritical water-cooled nuclear reactors [9][10][11]. With the increase of operating temperatures (>600 ºC), the growth of oxide film on the F-M steels is significantly enhanced [12][13][14][15][16][17][18][19][20][21][22][23][24][25]. The consequences of the thickening and failure of the surface oxides are of increasing concern in these steam-generating power plants.…”

Modelling and Analysis of the Corrosion Characteristics of Ferritic-Martensitic Steels in Supercritical Water

“…At lower temperatures (less than 400 °C), magnetite has a unit cell of 32 O 2− ions into which are placed 8 Fe 3+ ions at tetrahedral sites and 8 Fe 2+ + 8 Fe 3+ ions at octahedral interstices (i.e., inverse spinel structure), while a certain proportion of Fe 2+ goes into the tetrahedral interstices, more like a normal spinel, with an increase in temperature [46]. Generally speaking, the principal defects are cation vacancies and iron interstitials, which is the basic supporting theory for the off-stoichiometric structure, as well as for the metal transport within this bulk oxide, while the oxygen sublattice remains relatively fixed because its defects have higher energy [11,46,47,48]. However, for oxide formation at the metal/oxide interface, and noting that the oxide grows into the metal, which is correct for the growth of the inner layer, then oxygen vacancies are always produced at the metal/inner layer interface leading to the inward transport of oxide ions, and hence are always present in the barrier layer regardless of their energy [49,50].…”

“…The critical boundary oxygen partial pressure (pc,o2) generally increases with temperature. Taking magnetite as an example, which has been studied extensively as a function of oxygen partial pressure, the critical boundary oxygen partial pressure (pc,o2) is ~10 −6 atm at 1200 °C [48], ~10 −8.3 atm at 1000 °C [46], ~10 −20 atm at 600 °C [11], ~10 −25 –10 −22 atm at 500 °C [11,46], and ~10 −30 atm at 400 °C [11]. Previous studies have suggested that the rate-controlling step in the corrosion of ferritic-martensitic steels in SCW is the outward diffusion of iron and that the inner layer generally plays the role of the protective barrier layer [7,8,10,12,30,33,51].…”

“…Therefore, the so-called rate-limiting step, to be more precise, actually refers to the outward transport of cations through the inner layer which is the slowest among all transport processes occurred within the scale. In the inner layer, cation interstitials are generally predominant at lower oxygen partial pressure within the deeper oxide layer, such as the inner layer [11,54].…”

“…Because of relatively excellent properties, Ferritic-Martensitic (F-M) steels are primary candidates for the SCWRs design and are also currently used as common structural steels in fossil fueled thermal power plants [3,4]. The available literature reported a series of studies on the corrosion characteristics of F–M steels in SCW, from the oxidation kinetics [4,5,6,7,8], environmental parameters effects on corrosion [4,5,6,8,9,10], oxide scale formation and properties [8,11],and oxidation mechanisms [8,9,10,12]. For common F-M steels (~9–12 wt.% of Cr content) in SCW at 400–700 °C, the oxidation process generally follows near-cubic or near-parabolic rate laws [4,13].…”