Multitechnique investigation of sulfur phases in the corrosion product layers of iron corroded in long-term anoxic conditions: From micrometer to nanometer scale
Résumé
The understanding of anoxic corrosion mechanism is essential to develop predictive models targeting to optimize extraction and conservation operations as well as aging behavior of industrial or cultural heritage ferrous objects. The corrosion product layers issued from carbonated anoxic environment of iron often contain iron sulfides, as pyrite FeS2, greigite Fe3S4, and mackinawite FeS, mixed at the submicron scale, and their presence strongly impacts the corrosion processes. The determination of their spatial arrangement in corrosion product layer directly informs about their origin (bacterial or inorganic) and, so, about the corrosion mechanisms. Previous studies have displayed that sulfur isotopic measurements using nano‐Secondary Ion Mass Spectrometry (SIMS) are particularly relevant to discriminate between biotic or abiotic origin of iron sulfides. But this technique does not enable to correlate this origin to the nature of sulfide compounds. In this work, an innovative approach combining μ‐Raman spectroscopy (structural identification) for sulfide compounds localization at micrometric scale and nano‐Auger spectroscopy (chemical diagnostic) at higher spatial resolution is presented. This multi‐technique methodology is detailed and its capabilities evaluated on an archeological system. Auger spectra of pure iron sulfides phases (pyrite, greigite, and mackinawite) are obtained and, by considering the derivative spectra of Fe‐MVV and of S‐LVV spectral regions, evidence‐specific signatures for each phase, and so, the possibility to differentiate them. This result enables to propose a complete determination of the iron sulfides mix by coupling nano‐SIMS and nano‐Auger to get a complete knowledge of the role of iron sulfides in the anoxic corrosion of ferrous objects.