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

高温金属材料依靠地层氧化层来保护其不受环境影响。形成保护性表面水垢所需的临界合金铬或铝浓度已知会受到水蒸气存在的影响。大量的研究表明，造成这种技术上重要影响的机制在不同的合金之间是不同的。最近的研究表明，在较高的温度范围内，气体中存在的氢或由水蒸气与金属表面反应释放的氢影响保护结垢元素的选择性氧化。这可能归因于氢对内部氧化过程和/或瞬态非保护性氧化物生长速率的直接影响。对于后者（Fe-Cr基合金中的富铁氧化物和Ni-Cr（-Al）基合金中的富镍氧化物）可能与氧化晶格中的氢掺杂或气体物质（水蒸气和/或氢）通过水垢的分子传递有关。阐明潜在机制的一个主要缺点是无法获得表面尺度和内部氧化区氢的深度分辨定量分析。在提议的项目中，将使用三种模型合金系统（FeCr-和nicr -基和NiCrAl合金）研究900°C左右温度下水蒸气对保护垢形成的影响。为此，氧化动力学的原位测定将与微观结构表征（SEM, EDX/WDX， TEM， XRD）以及SNMS和GDOES深度分析相结合。合金成分的选择将使水蒸气对内部氧化动力学的影响可以分别在有或没有碱性元素Fe或Ni的外部氧化尺度存在的情况下进行研究。此外，气体成分对保护性铬垢或氧化铝垢形成条件的影响将被确定，并与水蒸气对内部氧化物形成的影响相关联。重点将放在使用一种新的方法深度分辨检测氢在外氧化层，内氧化区和未受影响的合金。为此目的，将利用NRA和PIXE分析对gdo产生的明确的溅射陨石坑进行分析。为了排除背景氢信号的可能影响，一些暴露将在含有D2和/或D2O的气体中进行，而不是在含有H2和/或H2O的气体中进行。为了阐明由氧化物生长应力引起的微缺陷对气体分子传输的贡献，声发射的原位研究也将被采用。将开发一种建模方法来修改现有的氧化理论，以解释在获得保护性外部结垢的条件下，在FeCr-， NiCr-和nicral -基高温合金中存在水蒸气和/或氢气。

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.