Development of a novel test system for fundamental studies of Microbial Influenced Corrosion (MIC) of carbon steel in a soil environment
Creators: name: Spark, Amy
Publisher: name: RMIT University
Year: 2024
Subjects: classificationCode: 340604
subject: Electrochemistry
classificationCode: 400499
subject: Chemical engineering not elsewhere classified
classificationCode: 410699
subject: Soil sciences not elsewhere classified

description: The corrosion of buried steel water pipelines is of great importance across the world because of its impact on community health and related costs, as well as the high costs of maintenance and replacement to infrastructure. The majority of corrosion that leads to leaks and bursts, and thus failure of water pipelines, is localised corrosion from the external surface of the water pipeline. Localised corrosion in the soil-steel environment is often due to the action of microbes accelerating corrosion in a process known as microbiologically influenced corrosion (MIC). As yet little is understood of the mechanisms of MIC and how it occurs in soil.<br><br>Commonly, MIC in soil is studied in solutions or in soil samples extracted from sites of interest. Both of these experimental scenarios have significant limitations due to the nature of soil corrosion as it is highly dependent on the physical structure of soil. Solution systems cannot replicate the porous nature of soils, though they can replicate the chemical components of soil. Extracted soil samples lose their structure and it is difficult to obtain reproducible results using this system. To overcome these limitations, a novel analogue for soil for electrochemical investigations has been developed using semi-solid agar.<br><br>Initial investigations in this thesis used solid agar, with an agar concentration of 15 g/L. In this phase of experimental work it was determined that oxygen control was crucial to obtain replicable results and to prevent crevicing of the metal samples. Significant difficulties in obtaining replicable electrochemical results due to the lack of movement of oxygen through solid agar led to the introduction of semi-solid agar as an analogue for soil. Semi-solid agar had an agar concentration of 4 g/L and has many qualities that make it an ideal medium to form the framework of a soil analogue. These include the ability for gases, nutrients and bacteria to move through the gel; the porous nature of the tertiary structure of the gel; and the wide range of temperatures at which the gel is stable.<br><br>Experimental work with semi-solid agar formed the basis of phase two of this thesis. The electrochemical limits of agar and how the introduction of peptide based nutrients changed the interactions between steel and agar were investigated. Peptide nutrients were shown to have an inhibitive effect on the corrosion of steel. This finding changes the way in which MIC results should be analysed. The effect of the nutrients included to support the growth of bacteria and to replicate a given environment have not been duly considered in the past when determining the mechanisms of corrosion.<br><br>Bacteria were introduced to the semi-solid agar system in the third experimental phase. This allowed the metal-bacteria-agar environment interactions to be systematically studied. Taking into consideration the findings from the work in solid and semi-solid agar, Pseudomonas fluorescens was selected as a trial bacteria species to increase understanding of the capabilities of the semi-solid agar system. P. fluorescens is a gram negative aerobic bacteria that is capable of forming a biofilm in isolation. The findings from this phase of work were twofold. Under the conditions examined, P. fluorescens was shown to have a corrosive effect on carbon steel. A mechanism for this was determined based on the production of siderophores that bind Fe(III) by the bacteria, which is then taken up by the cells and reduced to Fe(II) thus accelerating corrosion. In addition, both corrosion product and P. fluorescens were shown to migrate through the semi-solid agar, demonstrating a unique capability of the semi-solid agar. Due to the composition of semi-solid agar and the addition of 2,3,5-Triphenyltetrazolium chloride (TTC), a dye which allows the movement of bacteria to be tracked, the semi-solid agar system was able to visually show interactions between corrosion products and bacteria as well as give a rapid indication of the viability of bacterial cells at the conclusion of an accelerated electrochemical test.<br><br>In summation, a novel semi-solid agar system for corrosion testing has been developed. The semi-solid agar system has been found to be electrochemically stable within a given potential range, gives reproducible results, can be used at a range of temperatures, the chemical composition and stiffness are readily adaptable and the system behaves in a manner representative of clay based soils. In the course of this project, the mechanisms of carbon steel corrosion in agar have been determined and understood under a variety of environmental conditions. This in turn has enabled the interactions between carbon steel, P. fluorescens and the agar environment to be systematically studied leading to a greater understanding of how P. fluorescens interacts with metallic iron and iron oxides. A mechanism for the accelerated corrosion of carbon steel in the presence of P. fluorescens has also been determined. Given the ability of the system to indicate the movement of bacteria and corrosion product and how they interact with one another, there are a wide range of potential applications for the system including furthering the understanding of iron reducing bacteria, bioremediation and cathodic protection in cases of MIC.