The Influence of Water Vapour on the Oxidation Behaviour of High-temperature Materials Microstructure investigation using analytical electron microscopy
Doctoral thesis, 2004
This thesis covers the analytical electron microscopy work carried out on samples exposed to high-temperature oxidation in O2 atmospheres with the intention of investigating the effect of oxygen and water vapour. SEM/EDS, TEM/EDS and diffraction techniques, complemented by FIB precision milling, were employed to characterise the formed oxide scales as well as the material directly under the scale in order to understand the mechanisms that influence the corrosion process. Two distinct categories of high-temperature materials - Fe-Cr stainless steels and MoSi2-based refractory composites - were studied in this research, both showing behaviour affected by the selective volatilisation of species from the oxide scale by oxygen and water vapour.
On Fe-Cr stainless steels, water vapour heightened Cr vaporisation (through the formation of volatile CrO2(OH)2) from the originally Cr-rich oxide scale even at temperatures as low as 600°C. Below a critical Cr concentration, transition to a non-protective oxide scale occurred, followed by rapid breakaway oxide growth. The severity of corrosion was a function of the degree of Cr vaporisation (intensified by an increase in pH2O and oxidant gas flow rate) and the Cr supply from the underlying steel. The latter, in turn, depended on the Cr content and the microstructure near the steel surface. Comparing type 304L (18Cr10Ni) and 310 (25Cr19Ni) austenitic stainless steels, the higher Cr content of the latter provided it with the capability to withstand a higher degree of Cr vaporisation. As austenitic steel grains are relatively large (~ 50 µm), the breakaway tended to be localised, occurring far away from high diffusivity paths (i.e. efficient Cr supply) such as steel grain boundaries. The local breakaway resulted in oxide nodules, each consisting of an outward growing Fe-rich island and an inward growing multi-phase subscale crater region. In contrast, the X20 (CrMoV 11 1) ferritic/martensitic steel microstructure, with more rapid bulk diffusion and high grain boundary density, was more efficient at maintaining the Cr concentration in the oxide scale initially, only to suffer rapid and widespread breakaway later due to its lower overall Cr content.
MoSi2-based refractory composites oxidised in O2/H2O in the 400-600°C range, producing MoO3 nanocrystals in amorphous SiO2. Mo vaporisation resulted in a more open structure in the silica scale, hence facilitating oxygen access. The oxidation rate peaked at around 500°C; at higher temperatures, silica healing could take place. Above 600°C, the quicker MoO3 removal and silica healing resulted in a protective pure silica scale, thus preventing further oxidation. The presence of water vapour expedited Mo vaporisation through the formation of more volatile MoO2(OH)2. It was thus detrimental at lower temperatures, but beneficial at higher temperatures where silica healing was possible. The oxidation of (Mo,W)Si2 produced WO3 in addition to MoO3 and SiO2. The more sluggish vaporisation kinetics of W meant that the peak oxidation temperature was higher (around 750°C) and protective silica scale formation was possible only above 950°C.