The effect of alloying and surface roughness on biofouling of stainless steels in Baltic Sea brackish seawater

In stainless steel (SS), the sulfide derivatization resulting from the microbial sulfate-reduction processes and ennoblement of corrosion potential are two microbiological processes frequently discussed in literature concerning biofouling and microbially induced corrosion (MIC). In the brackish Baltic Sea, these phenomena may exhibit variations compared to high-saline oceans. Therefore, studies have been conducted to investigate the initial stages of microbial colonization and their effects on the ennoblement of commonly used SS industrial equipment.

This study aimed to define the behavior of two SS grades when exposed to brackish seawater to assess the influences of alloy composition and surface types on biofouling and ennoblement within a natural setting. Particularly, the study focused on grades EN 1.4404 and EN 1.4162, chosen for their alloy compositions and single-duplex phase microstructures. Also, surface finishes 2R (cold rolled and bright annealed surface) and finish 1D (hot rolled, heat treated, and trimmed surface) were selected for examination.

The experimental setup involved immersing the samples in aquariums containing naturally flowing seawater from the Baltic Sea. To comprehensively evaluate biofouling and ennoblement phenomena, a multidisciplinary assessment pipeline was employed. This included electrochemical measurements such as open circuit potential, electrochemical impedance, and anode-cathode polarizations. Additionally, various steel surface characterization techniques were utilized such as optical and scanning electron microscopy. Molecular biological analyses, including quantitative polymerase chain reaction (qPCR) and amplicon sequencing, together with integrated DNA fluorescence in situ hybridization bioimaging method, were also employed to provide insights into microbial colonization and its effects on the SS surfaces.

The findings from electrochemical measurements indicated that the observed phenomena of ennoblement, pitting corrosion, and cathodic reaction are likely associated with the development of surface biofilm and the adherence of diatoms, which are recognized as potential factors in MIC initiation. Furthermore, diversity in microbial communities and their distributions were observed, particularly concerning surface roughness, PREN value, and microstructural differences. These insights will be further discussed in conjunction with electrochemical and microbiological methods, and microscopy results.