Understanding the Dual Impact of Under-Deposit Corrosion and Microbiologically Influenced Corrosion in Electrically Conductive Environments
Corrosion is an age-old and persistent problem, especially for industries like oil and gas, where it results in substantial economic losses and environmental damage. Pipeline corrosion is particularly problematic because it leads to failures that have the potential to cause catastrophic leaks or ruptures. Corrosion, in this context, can occur due to abiotic factors, such as the presence of certain solid deposits, or biotic factors, such as microbial activity near metal surfaces.
One of the most commonly encountered forms of corrosion is under-deposit corrosion (UDC). UDC occurs when deposits like scales, corrosion products, sand, or schmoo accumulate on metal surfaces, forming barriers that shield the metal from the environment. This results in localized corrosion beneath the deposits, creating an environment for severe damage over time.
Furthermore, microbiologically influenced corrosion (MIC), a type of corrosion accelerated by microbial activity, plays a critical role in pipeline degradation. The native microorganisms, including sulphate-reducing bacteria (SRB), can accelerate corrosion by consuming metals and producing corrosive by-products such as hydrogen sulfide (H2S), which contributes to the deterioration of the pipeline.
However, until now, research on the combined effect of electrically conductive deposits and microbial activity on carbon steel corrosion has been limited. This study attempts to bridge this gap, shedding light on how conductive minerals like magnetite (Fe3O4) and troilite (FeS) influence both UDC and MIC, as well as their interactions with native microbial communities.
The Role of Electrically Conductive Deposits in Corrosion
This study focuses on three types of deposits commonly found in oil and gas pipelines: magnetite (Fe3O4), troilite (FeS), and silica (SiO2), with the latter serving as an inert control. The deposits' impact on carbon steel corrosion was measured under both abiotic (no microorganisms present) and biotic (microorganisms present) conditions.
Magnetite (Fe3O4) and its High Conductivity Impact
Magnetite is an iron oxide mineral that is highly electrically conductive. In the study, it was shown that the magnetite-containing reactor exhibited the highest corrosion rates, with a uniform corrosion rate of 0.110 mm/year under abiotic conditions. Magnetite's high electrical conductivity promotes the formation of galvanic cells, where the metal surface acts as an anode, and the conductive deposit forms the cathode, leading to accelerated corrosion. Magnetite’s conductive properties allow it to facilitate electron transfer between the metal and its surrounding environment, creating a localized corrosive electrochemical environment.
This process results in uniform corrosion, which refers to the uniform loss of metal across a surface. The significant corrosion rate found with magnetite under abiotic conditions highlights its role as a key factor in corrosion acceleration in CO₂-rich environments, which are common in oil and gas pipelines.
Troilite (FeS) and its Moderate Conductivity Effects
Troilite, another iron sulfide mineral, has moderate electrical conductivity, and in the study, it led to a lower corrosion rate than magnetite, with a measured corrosion rate of 0.017 mm/year. The less conductive nature of troilite means that it is not as efficient at promoting galvanic corrosion. However, it still contributes to corrosion, albeit to a lesser extent, as it provides a medium for localized corrosion to occur. Troilite’s conductive properties allow it to facilitate some degree of electron flow, leading to mild corrosion.
Silica (SiO2) and its Lack of Conductivity
Silica, an inert material, was used as a control in the experiment. Silica has negligible electrical conductivity, and as expected, the corrosion rate in the silica deposit reactor was the lowest at 0.006 mm/year. However, under biotic conditions, this seemingly inert deposit still contributed to pitting corrosion, suggesting that microbial activity could significantly influence corrosion processes, even in the absence of conductive minerals.
Microbiologically Influenced Corrosion (MIC): The Biotic Factor
The second component of this study focused on microbiologically influenced corrosion (MIC), which can exacerbate the corrosion rates found in pipelines. MIC occurs when microorganisms, such as sulphate-reducing bacteria (SRB), thrive near metal surfaces, producing corrosive by-products like hydrogen sulfide (H2S), which directly damages the metal. In this study, the researchers explored the impact of microbial consortia on UDC and under-deposit microbial corrosion (UDMC) in the presence of the three deposits: magnetite, troilite, and silica.
Magnetite: The Most Corrosive Deposit in Biotic Conditions
In biotic conditions, the study found that magnetite led to the highest average corrosion and pitting rates, confirming that it is the most corrosive deposit under both abiotic and biotic conditions. The higher corrosion rates observed in the magnetite reactor under microbial conditions can be attributed to microbial activity. In particular, the presence of sulphate-reducing bacteria (SRB) in magnetite’s environment accelerated pitting corrosion. This process occurs when localized areas on the metal surface develop deep pits due to the interaction between microorganisms and metal.
Silica and Troilite in Biotic Conditions
While troilite produced moderate corrosion in abiotic conditions, it exhibited lower corrosion rates compared to magnetite under biotic conditions. However, interestingly, silica showed higher pitting corrosion than troilite under microbial influence. This anomaly can be explained by the higher microbial activity in the silica deposit. The microbial consortia in the silica-based reactor were able to colonize and metabolically interact with the metal surface more actively, thus accelerating pitting corrosion despite the deposit's lack of electrical conductivity.
Electrical Microbial Corrosion (EMIC): A Synergistic Effect
An important finding of this study is the identification of Electrical Microbial Corrosion (EMIC). This phenomenon occurs when microorganisms, particularly sulphate-reducing bacteria (SRB), directly interact with the metal surface, utilizing the metal as an electron donor for their metabolic processes. Magnetite, with its high electrical conductivity, facilitates this electron transfer, enhancing microbial corrosion in a synergistic manner. The result is an accelerated corrosion process where both the conductive properties of the deposit and the microbial activity interact to enhance corrosion rates.
Implications for the Oil and Gas Industry
The findings of this study have profound implications for the oil and gas industry, where pipeline corrosion is a major cause of equipment failure, environmental disasters, and high repair costs. The study highlights that the electrical conductivity of deposits, like magnetite and troilite, significantly influences both under-deposit corrosion (UDC) and microbiologically influenced corrosion (MIC). The interaction between these conductive deposits and microbial communities should be considered in corrosion mitigation strategies.
Key Takeaways:
• Magnetite (Fe3O4) is the most corrosive deposit due to its high electrical conductivity, leading to the highest uniform corrosion and pitting corrosion rates under both abiotic and biotic conditions.
• Troilite (FeS), with moderate conductivity, results in a lower corrosion rate compared to magnetite but still contributes to significant localized corrosion.
• Silica (SiO2), though inert, still promotes pitting corrosion under microbial conditions due to microbial activity.
• Sulphate-reducing bacteria (SRB) accelerate pitting corrosion, particularly in the presence of conductive minerals like magnetite, which enables microbial cells to utilize the metal as an electron donor.
• The interplay between electrical conductivity of deposits and microbial activity introduces a complex environment that accelerates corrosion and needs to be taken into account when designing corrosion prevention strategies for pipelines.
