Effect of hydrogen from steam on external and internal oxidation processes of Fe- and Ni-base alloys

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Metallic materials for high-temperatures rely on the formation oxide scales for protection from the environment. The critical alloy chromium or aluminium concentration required to achieve formation of a protective surface scale is known to be affected by the presence of water vapour. A high number of investigations have revealed that the mechanisms responsible for this technologically important effect differ between various alloys. Recenttudies indicate that, in the higher temperature range, hydrogen present in the gas or released by the reaction of water vapour with the metal surface affects the selective oxidation of the protective-scale-forming element. This may be attributed to a direct effect of hydrogen on the internal oxidation process and/or growth rate of the transient, non-protective oxide. For the latter (Fe-rich oxides in the case of Fe-Cr base alloys and Ni-rich oxides in the case of Ni-Cr(-Al) base alloys) may be related to hydrogen doping of the oxide lattice or molecular transport of gaseous species (water vapour and/or hydrogen) through the scale. One major drawback in elucidating the underlying mechanisms has been the ability to acquire a depth-resolved quantitative analysis of hydrogen in the surface scale and in the internal oxidation zone. In the proposed project the effect of water vapour on protective scale formation at temperatures around 900°C will be studied using three model alloy systems, FeCr- and NiCr-base and NiCrAl alloys. For this purpose in-situ determination of oxidation kinetics will be combined with microstructural characterization using SEM, EDX/WDX, TEM, XRD and depth profiling by SNMS and GDOES. The alloy compositions will be chosen in such a way that the effect of water vapour on internal oxidation kinetics can be studied with or without presence of an external oxide scale of the base elements Fe or Ni, respectively. Additionally, the effect of gas composition on the conditions for protective chromia- or alumina-scale formation will be determined and correlated with the effect of water vapour on internal oxide formation. Emphasis will be put on using a new approach for depth-resolved detection of hydrogen in the outer oxide layer, the internal oxidation zone and the unaffected alloy. For this purpose NRA and PIXE analysis in well-defined sputter craters produced by GDOES will be utilized. For excluding possible effects of background hydrogen signals, some of the exposures will be carried out in gases containing D2 and/or D2O rather than H2 and/or H2O. For elucidating contributions of molecular transport of gaseous species via microdefects induced by oxide growth stresses, in-situ studies using acoustic emission will also be employed. A modelling approach will be developed to modify existing oxidation theories to account for the presence of water vapour and/or hydrogen on the conditions for obtaining protective external scale formation in case of FeCr-, NiCr- and NiCrAl-base high temperature alloys.