The effect of the hydrogenotrophic methanogens on the phosphate production of copper alloys: a structural and electrochemical study

The interaction between microorganisms and metallic materials is one of the primary focuses of the corrosion sector [1]. Over the past few years, there has been a shift in focus regarding MIC towards the Archaea domain, specifically targeting methanogens. These microorganisms have a detrimental effect on the pipelines within the natural gas network, resulting in damage to both the supply and distribution systems. In environments lacking oxygen, as copper gas piping systems, these microorganisms are one of the most common species detected, but rarely considered as possible promoters of MIC via methanogenesis. In CO2-rich conditions and with H2 available they use the metallic substrate (i.e. copper or its alloys) as a source of electrons to produce methane. Phosphates can thus precipitate as a byproduct of the methanogenesis as sometimes reported in literature studies on carbon steel alloys: they create a barrier that protects the metal from further corrosion. However, this inhibition mechanism (MICI) is still not well understood and never reported for copper alloys so far. The purpose of this study is therefore to characterize, at a laboratory level, the role of a distinct group of hydrogenotrophic methanogens of the Archea domain on the corrosion behavior of two copper alloys (electrolytic Cu and duplex 60:40 brass), specifically related to the precipitation of phosphates. Through replicated two-week tests, we documented the corrosion caused by methanogens-enriched media compared to sterilized media. To conduct our investigations, we utilized a three-electrode cell comprising a working electrode (representing the test alloy), a reference electrode (3M Ag/AgCl electrode), and a counter electrode (titanium mesh). Our research involved monitoring the open circuit potential (OCP) and performing electrochemical impedance spectroscopy (EIS) to distinguish the effect of microorganisms on the single time constants of the corrosion process. To detect the nature of corrosion products and possibly distinguish the composition of the phosphates produced, SEM and micro-Raman spectroscopy (μRS) were utilized. To identify target microorganisms as Metanobacter present in the microbial communities for each case, molecular analysis using next-generation sequencing (NGS) of 16S RNA was performed. Results not only evidenced a different electrochemical behavior according to the considered alloy (Mic of copper and MICI for duplex 60:40 brass), but also the presence of different phosphates according to the alloy composition and evolution of the pH at the interface metal/solution.