Figure : STXM absorption image at Fe L edge and clustering analysis of absorption map on a thin flim of the corrosion product of a 400 years old nail. Purple : metal, yellow III+ iron phase, blue : II+ iron phase
Prediction of long term corrosion (on several hundred years) of low alloy steels is a crucial issue in several research domains: metallic cultural heritage conservation, civil engeeniring, etc. Recent results demonstrate that, despite the corrosion layers could reach several millimetre thicknesses, the kinetics can be controlled at the nanoscale (presence of barrier layer at the metal/oxide interface, influence of crystallinity in the electrochemical reactivity, etc). For that reason, in addition to micrometer scale investigation it is now necessary to characterise these complex systems at the nanometer scale. In addition to classical TEM methods, the appearance of new synchrotron based techniques as Scanning Transmission Xray Microsopy, opens the door to new kind of chemical (atoms environments) and structural investigations at the nanoscale, in the field of corrosion.
First investigations were conducted iron nails, coming from the 16th century archaeological site of Glinet. This site is a reference one for long term corrosion studies since about 10 years and present desaerated calco-carbonated water saturated soil. The metal oxide interface was investigated, focusing on the detection on specific barrier layers that could play a crucial role in the corrosion mechanisms..
After classical observations at the microscopic scale (SEM, µRaman spectroscopy,...) both samples sets, (were observed on thin films of the transverse sections prepared by FIB. A fine elementary and structural description of the interfaces between metal and corrosion products in the nails has been obtained by TEM- and STXM. Chemical and structural maps (atom environments) were provided at the nanoscale. STXM experiments have been performed at Pollux beamline at Swiss Light Source synchrotron on these alteration layers in order to determine the nature of the phases formed.