Multimodal Spectroscopic and Microscopic Analyses of Biofilm-Metal Interactions on Stainless Steel in Simulated Repository Groundwater

Understanding the behaviour and mechanisms of microbially induced corrosion (MIC) in low and intermediate-level nuclear waste (LILW) repositories is critical for securing the long-term integrity of engineered barrier systems. In the context of Finland’s deep geological repository strategy, understanding the impact of microbial activity on corrosion-resistant materials such as stainless steel (SS) is important. However, investigating the interface between heterogeneous biofilms and the underlying metal surface remains challenging due to the chemical and structural complexity of both the biofilm and the metal, as well as limitations in analytical detection techniques. This study applies a multimodal spectroscopic and microscopic analytical approach to investigate the biofilm-metal interface on EN 1.4404 steel in oxic simulated repository groundwater, both in the presence and absence of microbial activity.

The SS coupons were incubated for two weeks under abiotic (microbial-absence) and biotic (microbial-presence) artificial groundwater conditions, designed to replicate the geochemical environment of the Olkiluoto repository in Finland. Pseudomonas putida was selected to represent microbial activity relevant to the early post-closure phase of the repository.

The samples were subsequently characterized using focused ion beam-scanning electron microscopy (FIB-SEM), time-of-flight secondary ion mass spectrometry (ToF-SIMS), and X-ray photoelectron spectroscopy (XPS). The multimodal spectroscopic and microscopic analyses provide a more detailed understanding of the metal interface beneath the biofilm relevant to SS under LILW repository water conditions. By integrating high-resolution chemical imaging (ToF-SIMS), morphological and structural characterization of both the biofilm and metal substrate (FIB-SEM), and surface chemical state analysis of oxide layer composition (XPS), this study provides insight into the bacteria-surface interactions. These surface-level insights are also supported by electrochemical measurements, which reveal the changes of the SS under both abiotic and biotic groundwater conditions. Optionally, short-term electrochemical tests were conducted, including in situ and ex-situ measurements, to assess the corrosion behaviour of the alloy under abiotic and biotic conditions.

Building on the results of the presented study, our future work will apply multimodal spectroscopic and microscopic analyses to MIC and corrosion research, with a particular focus on ToF-SIMS measurements. This method is expected to provide new insight into the corrosion processes and MIC mechanisms in metals and their affected zones under varying conditions that simulate the evolutionary stages of an LILW repository.