Simulation of Biocorrosion in Pipe Flow Using an Electrochemical Glass Cell Bioreactor with a Rotating Cylinder Coupon

Microbiologically influenced corrosion (MIC) is a growing problem in the oil and gas industry that results in huge financial losses each year worldwide. Reservoir souring, equipment and pipeline failures due to MIC pitting attacks are of great concerns in field operations. A group of bacteria known as sulfate reducing bacteria (SRB) are often the culprits. Currently, the basic understanding of SRB biofilm growth and MIC under flow conditions is still surprising lacking. Mass transfer plays an important role in MIC and fluid flow is related to mass transfer and fluid shear. Fluid shear impacts biofilm attachment and growth that are directly linked to MIC. This work aimed at studying mass transfer and flow effects on MIC due to SRB. It is very inconvenient to use a pipe flow system to study MIC because of the very high requirement on pump flow rates. An electrochemical glass cell bioreactors with a rotating cylinder coupon was used to simulate pipe flows. The ATCC 7757 strain of Desulfovibrio desulfuricans (a common SRB) was used in this work. The carbon steel coupon's rotation speed could be mathematically correlated with the average linear velocity in pipe flow. The experimental results from this work help understand MIC behavior under stagnant and flow conditions. They may also provide criteria for using fluid shear as a potential non-biocidal mitigation method. Introduction Microbiologically influenced corrosion (MIC) causes significant financial losses in production cutback and equipment maintenance in the oil and gas industry (Costerton and Boivin, 1991). Most reports on MIC on the lab scale were done under non-flow conditions in anaerobic vials. However, MIC occurs not only in stagnant conditions, but also on surfaces with liquid flow. Lee and Characklis (1993) observed MIC at linear velocities of about 0.35 m/s on AISI 1018 mild steel. Mass transfer limited conventional corrosion can be accelerated by liquid flow (Silverman, 1984). In MIC research involving moving fluids, hydrodynamics and nutrient availability are two key factors influencing the biofilm growth. Hydrodynamic conditions can facilitate mass transfer but may also exert excessive shear that causes inhibition of cell adhesion and the detachment of biofilms (Stoodley et al., 1999). By using a recirculation loop with a vessel, Dunsmore et al. (2002) reported that the structure and material properties of SRB were influenced by the fluid shear. It is difficult to use a pipe flow system to study MIC because a large pump flow rate is needed to achieve the needed linear velocity on a coupon surface. A glass cell with a rotating cylinder coupon equipped with a potentiostat can be used in the lab to study the steel corrosion behavior. The shear stress on the surface of rotating cylindrical coupon can be calculated using following equation (Silverman, 1984): 2 30 . 0 ) ( Re 079 . 0 r cyl ω ρ τ − = (1) By assuming the surface to be hydraulically smooth for both pipe and rotating cylinder, Silverman (1988) also proposed an equation to correlate the rotation speed of the cylindrical coupon in glass cell with the average linear velocity in pipe flow: 4 / 5 0857 . 0 28 / 5 7 / 3 25 .