Development of a High-pressure Scanning Probe Microscope for the study of in situ corrosion mechanisms of metallic mater
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Development of a High-pressure Scanning Probe Microscope for the study of in situ corrosion mechanisms of metallic materials Christophe Harder1, L. Berlu1, B. Reneaume1 1 CEA/DAM Centre de Valduc, 21120 Is-sur-Tille, FRANCE ABSTRACT Corrosion mechanisms take place at the extreme surface of materials before spreading in the bulk. In this way, in situ surface characterization techniques as scanning probe microscopy (Scanning Tunneling Microscopy (STM) and Atomic Force Microscopy (AFM)) allow the observations of the very initial reaction steps. To achieve that goal, an environmental cell has been designed ; it is able to integrate either an atomic force microscope (AFM) or a scanning tunneling microscope (STM). This cell can resist to internal pressures ranging from 10-5 to 20 atm. Heterogeneous “solid – gas” reactions that only occur with pressures above several atmospheres, can then be studied. This could be achieved by following the topographical evolution of samples reacting with gaseous species. Identification of the surface defects at the origin of corrosive attacks as well as proposition of reaction mechanisms will be describe in future works. The present work shows first in situ measurements that validate this new and unique experimental “HP-AFM” (High Pressure Atomic Force Microscope). The impact of the atmosphere’s composition as well as the pressure values on the topographical measurements recorded by the AFM system is especially studied. In this way, a calibration standard is used to detect a potential working drift of the AFM system (scanner head displacements, optical detection …) that could lead to eventual distortions of pictures recorded and misinterpretation of observations. This sample has been studied under several experimental conditions and the results have shown an identical behaviour of the AFM used ex situ and in situ under Ar or He up to 1.5 atm as well as a good stability during long recording acquisitions (up to 90 min) necessary for kinetic studies. INTRODUCTION Several interactions between two phases occur at the interface during solid-gas heterogeneous reactions. These types of reactions take place in numerous industrial fields such as automotive (exhaust systems), energy or nuclear fuel storages, but they also occur in natural way, especially with atmospheric corrosion phenomena. An oxydo-reduction between a solid (metal, alloy) and gaseous corrosive species usually occurs. In this case, the kinetic of degradation is a function of several parameters such as contact area between the two phases [1], temperature [2] or gas pressure [3]. Impact of the latter parameter is studied in the present work. Understanding the initial corrosion process can allow the identification of the predominant factors that drive the reactions and can lead to identify technical solutions able to prevent the initiation phenomenon. Collecting physico-chemical data from gas-metal interface with a nanometric scale resolution can be of a major interest to achieve this goal. In this way, scanning probe microscopy (SPM)
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