Formation of Iron Sulfides on Carbon Steel in a Specific Cement Grout Designed for Radioactive Waste Repository and Associated Corrosion Mechanisms.

Carbon steel coupons were buried in a specific low-pH cement grout designed for radioactive waste disposal and left 6 months in anoxic conditions at 80 °C. The corrosion product layers were analyzed by µ-Raman spectroscopy, XRD, and SEM. They proved to be mainly composed of iron sulfides, with magnetite as a minor phase, mixed with components of the grout. Average corrosion rates were estimated by weight loss measurements between 3 and 6 µm yr-1. Corrosion profiles revealed local degradations with a depth up to 10 µm. It is assumed that the heterogeneity of the corrosion product layer, mainly composed of conductive compounds (FeS, Fe3S4, and Fe3O4), promotes the persistence of corrosion cells that may lead to locally aggravated degradations of the metal. New cement grouts, characterized by a slightly higher pH and a lower sulfide concentration, should then be designed for the considered application.