Development of a High-pressure Scanning Probe Microscope for the study of in situ corrosion mechanisms of metallic materials

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.