Oxidation of X20 in Water Vapour: The Effect of Temperature and Oxygen Partial Pressure

Abstract

The oxidation behaviour of X20 in various mixtures of water, oxygen, and hydrogen was investigated at temperatures between 500°C and 700°C (time: 336 h). The samples were characterised using reflected light microscopy and scanning electron microscopy equipped with energy dispersive spectroscopy. Double-layered oxides developed during oxidation under all conditions. The morphology of the oxide layers was strongly influenced by temperature, whereas the influence of the oxidising environment appeared to be less pronounced, as long as it contained water vapour. The inner layer consisted of converted M 23 C 6 embedded in Fe-Cr spinel after oxidation at 500 and 600°C, while alternating layers of Cr-rich and Cr-poor oxide were observed after oxidation at 700°C. An internal oxidation zone developed during oxidation at 500 and 600°C, with its depth influenced by the oxidising environments. The results are discussed based on the various hypotheses of the accelerating effect of water vapour that have been put forth in the literature.

Microscopical investigation of steamside oxide on X20CrMoV121 superheater tubes

“…This was also observed for T92 [4] -and showed preferential presence of tungsten, however for X20CrMoV121 there is no preferential presence of Mo in this area, although Mo segregations are present within the inner part of the oxide (shown as white particles within the oxide in Figure 5). The presence of a Fe oxide within the Fe -Cr oxide may be due to cracks in the oxide such that the partial pressure of oxygen increases so it is similar to that present within the outer oxide layer resulting in the formation of magnetite.…”

“…Figure 7 is X20 exposed in the laboratory to steam at 700 C for 334 h. The outer oxide is not as thick as the inner oxide due to spallation. At this temperature bands of chromium rich and chromium poor oxide are present similar to Figure 4, however, they were not present at 600 C [4].…”

“…For X20CrMoV121 it has been shown that the outer magnetite and haematite grows by Fe ion diffusion outwards and the inner oxide grows from diffusion of oxygen bearing species inwards [4], such as transport of OH À species into the oxides [3]. This paper compares steamside oxides formed at two temperature ranges from plants with laboratory studies.…”

“…The steamside oxide thickness and morphology has been investigated with light optical and scanning electron microscopy with EDS analysis. The laboratory investigations and experimental set-up are described in detail in [4]. Oxidation experiments were performed at 500, 600, and 700 C for 336 h using argon as carrier gas.…”

“…X20CrMoV121 does not contain tungsten and therefore is a interesting steel to compare with HCM12 to reveal whether tungsten is responsible for the faster oxidation rates observed after long term exposures. Direct comparison with laboratory exposures of X20CrMoV121 in steam [4] is also undertaken to try to bridge the knowledge and understanding of laboratory and plant exposures.…”