High-resolution and in-situ characterization of the material behavior of Fe-based CrNiMo alloys exposed to geothermal-type fluids

在腐蚀是关键因素的应用中，通常采用耐腐蚀且相对便宜的铁基CrNiMo合金。在这些材料中，奥氏体-铁素体双相合金尤其具有广泛的应用前景，并被认为是微观组织对腐蚀行为重要性的展示。提出的研究旨在研究双相合金，更简单的模型系统（奥氏体，铁素体）和元素参考材料（Cr, Mo）在岩土工程应用（如地热能利用，二氧化碳储存，核废料处理）相关条件下的行为。为了反映地热研究的现状，实验室模拟了温度高达300°C的腐蚀环境。合成了具有不同TDS（总溶解固体）的多组分流体（Na-K-Ca-Mg-Cl-SO4-H2O体系），与德国地热场的实际流体非常相似。这种腐蚀环境尚未得到充分研究，但对未来的地下活动至关重要。核心方面是：-测试测试合金在地热腐蚀环境中的稳定性极限-研究钝化/钝化分解的基本过程-不只是优先考虑宏观耐腐蚀性，而是特别强调界面反应性，这需要电化学（循环伏安法CV，阻抗谱EIS），光谱（原位表面增强拉曼光谱SERS），以及采用微观方法（原子力显微镜AFM）。CV和EIS用于确定测试合金的热力学域，并探测钝化膜的电子性能和保护性能。SERS可以准连续监测钝化膜的生长和修饰的动态，而样品则保持在恒电位或动态控制下。成分和结构的变化将揭示击穿机制（如氯化物引起的点蚀），从而导致高局部溶解速率。AFM和原位热液AFM （HAFM）用于亚纳米尺度的表面表征，HAFM能够在高达150°C的温度下实时成像。该项目采用跨学科的方法，涵盖了从基础研究到应用研究的一系列内容，为在与应用相关的温度范围内，接触不同TDS流体的CrNiMo合金的动态电化学和物理行为提供了有价值的见解。在深入了解界面动力学的基础上，评估了重要合金代表的适用性，有助于提高耐蚀性。由于原位技术的优越性和方法的协同组合，结果对材料评估具有很高的相关性，而不仅仅局限于本研究中考虑的合金和环境。

In applications where corrosion is a critical factor, typically corrosion-resistant and relatively inexpensive Fe-based CrNiMo alloys are employed. Among these materials, austeno-ferritic duplex alloys are particularly promising for a wide range of applications and are regarded as showcases for the importance of microstructure on the corrosion behavior. The proposed research aims at studying the behavior of duplex alloys, simpler model systems (austenite, ferrite), and elemental reference materials (Cr, Mo) under conditions relevant to geotechnical applications (e.g. utilization of geothermal energy, CO2 storage, nuclear waste disposal).Reflecting the current state of geothermal research, corrosion environments with temperatures up to 300°C are mimicked in the lab. Multi-component fluids (Na-K-Ca-Mg-Cl-SO4-H2O system) with varying TDS (total dissolved solids) that closely resemble real world fluids from geothermal sites in Germany are synthesized. Such corrosion environments have not yet been adequately researched, but are crucial for future subsurface activities.Central aspects are:- Testing the stability limits of the test alloys in geothermal corrosion environments- Studying the fundamental processes with regard to passivity/breakdown of passivityInstead of just prioritizing the macroscopic corrosion resistance, a special emphasis is laid upon the interfacial reactivity, which requires a combination of electrochemical (cyclic voltammetry CV, impedance spectroscopy EIS), spectroscopic (in-situ surface-enhanced Raman spectroscopy SERS), and microscopic methods (atomic force microscopy AFM) to be employed. CV and EIS are used to determine the thermodynamic domains of the test alloys and to probe the passive film with respect to its electronic properties and protectiveness. SERS enables to quasi-continuously monitor the dynamic of growth and modification of the passive film, while the sample is kept under potentiostatic or -dynamic control. Compositional and structural changes will reveal breakdown mechanisms (e.g. chloride-induced pitting) resulting in high local dissolution rates. AFM and in-situ hydrothermal AFM (HAFM) are employed for sub-nanometer scale surface characterization, HAFM being capable of real-time imaging at up to 150°C.Following an interdisciplinary approach, the project encompasses a range from fundamental to applied research and provides valuable insights into the dynamic electrochemical and physical behavior of CrNiMo alloys exposed to fluids with varying TDS in an application-relevant temperature range. On the basis of an deeper understanding of the interfacial dynamic, it assesses the suitability of important alloy representatives and helps to improve the corrosion resistance. Due to the superior nature of in-situ techniques and the synergistic combination of methods, the results have a high relevance for material assessment, not solely limited to the alloys and environments considered in this study.