13C isotopic labeling to decipher the iron corrosion mechanisms in a carbonated anoxic environment

Abstract

A two-step corrosion experiment was performed on a ferritic steel (Armco) in a synthetic solution representing the Callovo–Oxfordian at 120°C. After the development of a carbonated corrosion product layer (CPL) during the first 15 days of the experimental step, corrosion front progression was investigated using ¹³C marked carbonate species during the second 15 days experimental step. CPL was characterized at each step, in terms of morphology (scanning electron microscopy), composition (energy-dispersive spectroscopy), and structure (µ-Raman). ¹³C corrosion product locations were analyzed by time-of-flight secondary ion mass spectrometry. Results evidenced that after a step of generalized corrosion, iron corrosion continues locally at the metal/CPL interface. These results suggest that although a protective siderite layer formed on the iron surface after 15 days, a local dissolution of the carbonate layer at the M/CPL interface occurred. A galvanic effect is developed between the bared surface (anode) and the covered one (cathode). This activates iron oxidation. The precipitation of carbonate corrosion products to the metal/CPL interface is possible by the diffusion of ¹³CO₃²⁻ ions from the bulk through the siderite layer.