No abstract

Solid agar based gels have been investigated as an electrolyte system for the electrochemical study of soil corrosion, specifically microbiologically influenced corrosion (MIC) in soil. Traditional techniques for studying corrosion in soil include solutions and soil samples; however these systems do not reliably simulate the physico‐chemical properties of soil. Soils are complex environments with three phases critical to corrosion (solid‐soil, liquid‐water and gas‐oxygen). Therefore there is a need for a system which replicates and considers this complex environment and its effects on corrosion processes, while reducing inconsistencies and variations associated with moisture and oxygen content in soil. Open circuit potential (OCP) and potentiodynamic scans (PDS) were conducted on carbon steel exposed to solid agar electrolyte with varying oxygen concentrations. Results were analysed using Tafel extrapolation, consistency in anodic‐cathodic trends and optical microscopy of the exposed regions. For the conditions tested, a cathodic shift was seen with the observed corrosion potentials being notably lower than the measured OCPs. Consistency in the anodic‐cathodic trends of the PDS was observed with minimal oxygen conditions. Overall, an agar based gel system has potential as an electrolyte for soil based MIC studies, especially as an analogue for moist clay soils.

Buried steel infrastructure can be a source of iron ions for bacterial species, leading to microbiologically influenced corrosion (MIC). Localized corrosion of pipelines due to MIC is one of the key failure mechanisms of buried steel pipelines. In order to better understand the mechanisms of localized corrosion in soil, semisolid agar has been developed as an analogue for soil. Here, Pseudomonas fluorescens has been introduced to the system to understand how bacteria interact with steel. Through electrochemical testing including open circuit potentials, potentiodynamic scans, anodic potential holds, and electrochemical impedance spectroscopy it has been shown that P. fluorescens increases the rate of corrosion. Time for oxide and biofilms to develop was shown to not impact on the rate of corrosion but did alter the consistency of biofilm present and the viability of P. fluorescens following electrochemical testing. The proposed mechanism for increased corrosion rates of carbon steel involves the interactions of pyoverdine with the steel, preventing the formation of a cohesive passive layer, after initial cell attachment, followed by the formation of a metal concentration gradient on the steel surface.