MIC Corrosion: How Can Microorganisms Eat Holes in a Metal?
MIC is difficult to predict. However, proper sampling, assessment, mitigation/control and monitoring are critical to minimize its potential.
Microbiologically induced/influenced corrosion (MIC) is a major failure issue for pipelines and equipment buried in soils, or exposed to environments that contain microorganisms and bacteria. It is an insidious threat in a variety of industries, and it is commonly observed in cooling water systems, piping, vessels and storage tanks. MIC corrosion rate can cause relatively high through-wall penetration -- within weeks or even days -- especially in cases where fluids are stagnant and/or untreated (For more information about calculating corrosion rate, read: Corrosion Rate Conversion: Simple Ways to Convert Data Between Common Corrosion Units. For information on internal pipeline corrosion, read: Internal Corrosion of Pipelines Carrying Crude Oil.)
Pitting with subsurface loss of metal greater than the opening of the surface pitting is a typical sign of MIC. The pitting attack is often observed at the bottom of the pipe, particularly at areas of flow disruption or stagnant conditions, which favor the formation of biofilm.
What is Microbiologically Induced Corrosion?
MIC involves interaction between microorganisms, such as bacteria, fungi and algae, corrosive media and the construction material. Bacteria can be aerobic, requiring oxygen to become active, or anaerobic, requiring no oxygen for corrosion activity.
MIC is sustained by microbes attaching to a metal surface via an exopolymeric substance, which is the main constituent of the slime under which the biofilm forms at the solid-liquid interface. The metal surface, afterwards, corrodes due to the microorganism's metabolic action.
Why Does MIC Corrosion Occur?
Anaerobic sulfate-reducing bacteria is a common cause of MIC of mild steels. It metabolizes sulfate in the fluid and turns it into H2S, which accumulates in the biofilm (B. Little, R. Ray and R. Pope, Corrosion, 56, 433-443, 2000.)
Other common metabolites include organic and inorganic acids. Beneath the biofilm, localized corrosion initiates, causing a severe pitting attack with time. Iron- and manganese-reducing bacteria produce cations for redox reactions that accelerate the corrosion reactions. One study found iron-oxidizing/reducing bacteria produce oxygen concentration cells in the biofilm, which are cathodic relative to areas of low oxygen concentrations and initiate pitting (I. Beech and J. Sunner, Current Opinion in Biotechnology, 15, 181-186 (2004) DOI:10.1016/j.copbio.2004.05.001) (For more about pitting corrosion, read: Understanding Pitting Corrosion to Prevent Catastrophic Failures.)
Water chemistry at the interface could be significantly different than that in the bulk fluid, across which diffusive processes sustain. Factors that influence the formation process of biofilms include:
In operating conditions of 15 to 45oC, and a pH of 6 to 8, occurrence of MIC in an aqueous media is promoted.
MIC occurs at interfaces beneath scales or under solids that precipitate or settle out. Moreover, MIC could occur because of conditions or processes of:
- Pigging that might not completely reliably remove deposits.
- Dead legs that are not effectively drained.
- Low pipe velocity.
- Bottom of tanks on which biofilms could form, as some tank types can never be drained completely.
- After leakage for which a vessel or process line is hydro-tested with river or sea water and water residue remains because of incomplete drainage and/or improper cleaning (For more on hydro-testing, read: Integrity Management: How Ultrasonic Inline Inspection (ILI) Technology Enhances Safety.)
How to Identify MIC
Localized pitting attacks could be indicative of MIC, but for identifying the microorganisms, culture techniques with media kits are used (for microorganism growth). Samples taken to identify MIC should contain biofilms and fluids, as they contain the requisite microorganisms. The samples must be taken wet and prevented from drying.
Common molecular microbiological methods for sample analysis include quantitative polymerase chain reaction (qPCR), denaturing gradient gel electrophoresis (DGGE), and metagenomic sequencing. They require small sample sizes and microorganisms which do not necessarily need to be alive, such as when anaerobic bacteria are suspected to be the cause of MIC.
In molecular microbiological methods, the genetic material is extracted from the sample, an assay is performed and the types of microorganisms are identified. For internal pipeline surfaces, NACE TM0212 outlines the molecular methods for evaluating MIC, microscopic analyses, sampling, and chemical assays.
It is necessary to analyze water chemistry to determine the species that support microbial metabolism and growth. They include sulfate, acetate, or organic compounds, as well as abiotic factors, such as dissolved oxygen or sulfur. Simply measuring high concentrations of microorganisms is not enough to determine that MIC occurred, as the metabolic activity of each microorganism under similar conditions must be determined. Also, the presence or concentration of corrosive microorganisms in the bulk flow could be irrelevant to the susceptibility of MIC.
In many circumstances, MIC might be caused by several different organisms acting synergistically. Microscopic techniques can be used to determine the concentrations of microorganisms, and together with x-ray diffraction of the corrosion products, the level of synergy can be determined.
How to Prevent Microbiologically Induced Corrosion
A MIC mitigation strategy must involve the following:
- Sampling: Analyzing biofilms and water chemistry.
- Assessment: Procedures could be complex, but, at minimum, assess major types of metabolic agents, such as sulfur-reducing bacteria.
- Proper mitigation/control.
For example, for mitigation in a seawater cooling unit, continuous chlorination with periodic injection of a biocide is an effective method, after the equipment is cleaned to remove biofilms and corrosion products. Corrosion inhibitors can also be added, with concentrations and frequency of injection determined in correlation to water chemistry. In some cases, not injecting oxygen starvation inhibitors could promote the growth of undesirable sulfate-reducing bacteria. (For information on pipelines in seawater, read: Introduction to the Chemistry of Pipes in Seawater.)
Monitoring involves evaluating the effectiveness of MIC mitigation additives – which is necessary as changes in process conditions and water chemistry could significantly impact the efficacy of MIC mitigation additives.
MIC susceptibility must be considered in the practices of equipment design and materials specifications and selection. This includes properly considering locations of drainage of equipment and dead legs, and minimizing the number of locations of low-velocity flows. The possibility that fluid within a system may stagnate must also be minimized, such as during draining, cleaning, and pigging of the system.
Coating buried pipelines to prevent MIC from microorganisms in the soil is effective, as long as disbonded coating or pinhole areas are minimized, as these can initiate MIC. Cathodic protection applied with or without coatings could also help prevent/mitigate MIC. Good metallurgy plays a role in mitigating MIC, such as considering titanium. Finally, properly training personnel contributes to minimizing the possibility of an MIC occurrence in terms of detection and repair. (For information on ways to prevent corrosion in buried pipelines, read: Corrosion Prevention for Buried Pipelines.)
MIC corrosion is challenging to predict, control, mitigate, and prevent. But with clear, comprehensive mitigation strategies -- including detection, proper sampling, assessment, control, and monitoring, good design and materials selection, and the application of cathodic protection and effective coatings guided by effective education, a system's susceptibility to MIC can be made satisfactorily low.