Studying corrosion induced by sulfide species in iron archaeological artifacts in soil

Significance 

Most archaeologists have been determined to study and understand the human origin, life, and culture over the years. They have employed the use of archaeological artifacts, that is, the remains of the various objects and tools used by man over the past, through which they have been able to approximate the age of such artifacts and the possible generation of man that used them. This has however been applied in the various fields especially that of nuclear waste systems, to obtain the corrosion behavior of mild steel over long periods of time.

Mild steel due to its distinctive properties has numerous applications. For instance, its use is envisioned for long term geological disposal and storage of nuclear wastes globally. The advantage is that you can select a site with the similar sample as those of your interests for the study. A good example is the use of archaeological nails obtained from Glinet Site, which typically represent a mild steel found in the soils for centuries.

Researchers at University of La Rochelle in France led by Professor Philippe Refait, in collaboration with CEA/University Paris-Saclay researcher Delphine Neff, investigated the corrosion process induced by sulfide species and how it is influenced by previously formed corrosion layers. In this study, the authors used archeological iron nails of the 16th century to represent the mild steel structures primarily used in the storage of nuclear wastes. Their work is published in the journal, Corrosion Science.

The research team commenced their experiments by immersing the archaeological nails in sulfide anoxic electrolyte, at room temperature for two months. They after that conducted chemical analysis to categorize the corrosion product layers by combining µ-Raman spectroscopy, scanning electron microscopy and energy dispersive spectrometry. A comparison of the corroded nails in the sulfide-containing electrolyte and the sulfide-free electrolyte was then made.

The authors observed that addition of sulfide species results in the formation of mackinawite (FeS) at the interface of the product layer and the soil. The pre-existing corrosion product layer that consisted of mainly iron carbonate acted as a protective barrier.

According to the authors, mackinawite was formed as a result of the dissolution of iron carbonate (FeCO3) in the outer part of the dense corrosion product layer (DCPL). FeCO3 then played a significant role in the corrosion process by preventing the reaction between the sulfide species and the metal surface. However, they also pointed out that large cracks, especially on the siderite layer, should be capable of overcoming the protective layer imposed by FeCO3.

Moreover, the detailed characterization of the DCPL demonstrated that it contained a Fe3C network lamellas. The network is responsible for the electrical connection of the environment interface to that of the metal surface. If oxygen could reach the vicinity of the DCPL, as a consequence of any external interference, the Fe3C network could act as cathode and facilitate the resumption of corrosion.

Reference

Rémazeilles, C., Neff, D., Bourdoiseau, J., Sabot, R., Jeannin, M., & Refait, P. (2017). Role of previously formed corrosion product layers on sulfide-assisted corrosion of iron archaeological artefacts in soilCorrosion Science, 129, 169-178.

 

Go To Corrosion Science

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