Use of the 34S/32S isotopic ratio for the study of the bioorigin of the sulfur phases located in the corrosion product layer of iron or low alloy-steel samples corroded in anoxic conditions

International audienceThe presence ofSulfate-Reducing Bacteria (SRB) may influence the corrosion rate offerrous objects by inducing iron sulfides precipitation [1]. The same phases are formed by biotic or abiotic ways. An analytic method for detecting bacterial interventions in corrosion pro cesses is required. Yet, biotic iron sulfides are supposed to be depleted in heavy isotopes of sulfur relative to the starting sulfate [2]. The aim ofthis study is to setup a methodology based on 34S/32S isotopic ratio determination to evidence the origin of sulfides in the corrosion product layers of "long term systems": ferrous archaeological samples studied as analogues oflong-term corrosion of nuclear waste package and steel samples issued from anoxic corrosion experiments in contact with claystone. In paraUel "model systems", iron coupons corroded in anaerobiosis with or without SRB, are studied to compare the sulfur isotopic fractionation resulting from a bacterial or an inorganic sulfide source. Complementary techniques were used to characterize the sulfur-bearing compounds in the samples: FESEM-EDS for their distribution, I-l-Raman and nano-Auger spectroscopy for th e nature of phases. Then, the 34S/32S isotopic ratio was detennÏned by NanoSIM.S and compared to the one of the sulfates from the surrounding solution. As a result, d34Ssultàte_sulfide reaches values of ~ 10 %0 to ~60 %0