Validating an Agar System to Mimic Soil for Microbiologically Influenced Corrosion of Pipeline Steel

Semi-solid agar has been developed as a substitute of soil for laboratory corrosion studies, in response to several identified weaknesses of traditional electrolytes for electrochemical analysis. Corrosion of metal in soil has often been studied using soil and simulated soil solution in laboratory. The former presents a large number of variables that cannot be controlled in laboratory to allow for systematic variation of crucial soil parameters. The latter is not an ideal replicate to soil, given the lack of unique characteristics, including microbial attachment to metal surfaces, biofilm formation and physical structure. The use of semi-solid agar as electrolyte has instead been shown to be a promising substitute for soil that links the agar system with clay-based soils. Prior research focused predominantly on developing an agar system as proof of concept, while this work has developed a more detailed understanding of agar as a substitute of soil through a systematic electrochemical investigation into several key corrosion parameters. Key influences of agar on corrosion measurements were established, notably effects of the agar matrix through adjustment of agar and oxygen concentration on surface corrosion characteristics of carbon steel, influenced by changes to local oxygen environments in agar, using surface polarisation resistance and other electrochemical analysis of the surface. Surface characterisation also showcased the influence of oxygen availability on the surface oxide formation characteristics and visual corrosion characteristics. Diffusion of ions and ferrous corrosion products were also found to be strongly influenced by the agar matrix. The essential mechanisms behind some of the key differences with changing agar concentration and oxygen concentration were identified in this analysis. In addition, this work has drawn strong correlations between the agar system and soil, highlighting the role of the pore structure in corrosion of testing metal. Direct comparison regarding electrochemical activity, diffusion characteristics, and corrosion mechanisms was performed on pipeline steel in aqueous saline solution, saline agar and saline sand to represent soil, which indicated similarities in oxygen diffusion for agar and sand media. Distinct corrosion characteristics and significantly weakened cathodic activity were identified as different to aqueous saline solution for agar and sand media. Corrosion in saline solution was also accelerated through extensive attack at defect sites by the high chloride ion diffusion rate, but such attack was reduced in agar and sand by the limited chloride ion movement and diminished driving force for anodic corrosion activity. The solid nature of agar outperformed aqueous electrolytes as soil replicate to explore soil-related corrosion responses at laboratory scale. There was also a detailed study of the MIC mechanisms in a semi-solid agar environment using P. aeruginosa commonly in soil. The BCNR theory that suggests accelerated MIC under carbon starvation did not occur, instead the opposite was more likely PYO-mediated respiration that accelerated MIC with more carbon sources. Reduced bacteria growth under carbon saturated conditions was also determined as the result of bacteria migration to rich nutrient media. Although high variability was observed between bacteria growth and corrosion for each condition, general corrosion inhibition was observed by thicker biofilm growth. However, bacteria growth was shown to accelerate corrosion under saturated nutrient agar conditions as a result of MIC by PYO-mediated respiration dominating the bacteria inhibition influence due to agar and sand structural differences. The outcomes demonstrated the care needed when considering MIC with the addition of nutrients to the agar system when representing corrosion in soil. In summation, the agar system provides confidence and guidance to conduct simplified laboratory study of metallic corrosion in a simplified environmental replicate to qualitatively and quantitatively predict the corrosion behaviour of metal in real service conditions, such as underground pipelines.