Broadly speaking, industrial coatings used in water-immersion conditions fail due to the following: Osmotic Blistering, Microbially Induced Corrosion (MIC), and Poor Surface Preparation and Application. In our previous blog post we explored Osmotic Blistering as a common cause for coating failure in water-immersion conditions. This blog post explains how MIC impacts the protective coating, how it can be avoided, and what are the inspection techniques to identify the problem. Causes of Microbially Induced Corrosion (MIC) Known also as microbial corrosion or microbiologically influenced corrosion, MIC is corrosion caused or accelerated by microorganisms and can lead to the deterioration of both coatings and underlying substrates. Broadly, these bacteria can be aerobic (requires oxygen) or anaerobic (does not require oxygen) in nature, including sulfate-reducing bacteria (SRB), iron-oxidizing and manganese-oxidizing bacteria. Often these organisms themselves do not consume metallic substances, but instead can create aggressive environments where highly localized corrosion can occur, leading to severe degradation in a short amount of time.The corrosion types commonly associated with MIC on carbon steel are pitting corrosion and crevice corrosion, while stainless steel also suffers from stress corrosion cracking from MIC. Effects of MIC on Coatings The effects of MIC on polymeric coatings and composites can manifest in a range of ways. Standard, immersion-grade polymeric coatings often can not handle the localized pH reductions associated with biofilm formation. Waste products for microbes range depending on the organism but commonly include organic acids and alcohols, two very aggressive exposure conditions for most polymers. Coating failures due to waste excretion from microorganisms often look like isolated areas of chemical attack, generally located in corners, low-flow areas and at coating pin-holes. Another way MIC can create failures on coatings is when anaerobic organisms are trapped behind the coating, in contact with metal. These organisms will degrade the metal, even without oxygen present, and cause localized delaminations from corrosion of the base metal. Substrates that have been exposed to microorganisms should be treated with bleach, peroxides or other disinfectants to ensure the organisms do not get trapped behind the coating system. Deep pitting and degradation of a tubesheet where copper/nickel tubes meet a steel tubesheet. Degradation of the concrete in a raw sewage storage tank. MIC is very common in wastewater plants because of the high bacteria levels. Deep pitting on the cone section at the bottom of a tank that is commonly associated with MIC. Produced through-wall failures along the weld seam. Severe MIC pitting on a large water pump after grit blasting. Prevention and Remediation of MIC Unfortunately, the presence of MIC causing microorganisms cannot be eliminated from most industrial processes, rather the corresponding corrosion has to be controlled. Three solutions have been used widely in industry. The first solution involves killing the bacteria causing the degradation. For this solution, an assay to determine the exact nature of the microorganism must be carried out, then an appropriate, commercially available biocide is chosen. Continuous injection of this biocide should reduce the influence of MIC; however, this requires a process modification including new tanks, injection ports, etc. The second solution is to eliminate any contact of the bacteria with the metallic substrate using an appropriate immersion-grade coating system. This is an excellent option if the degradation of the substrate is not severe, and the system can be taken offline and drained. It is critical to ensure any polymeric coating used in an active MIC environment is not susceptible to degradation from the waste products created by these microorganisms and does not trap anaerobic bacteria behind the barrier materials. The third widely-used solution is to rebuild or reinforce the substrate with a non-metallic repair option that is not susceptible to MIC. A composite material, like carbon-fiber-reinforced polymers, can be used to rebuild the structural strength of severely corroded substrates or to repair through-wall failures. Pipes can often be repaired externally by creating a structurally-independent pipe around your existing piping system, eliminating the need for internal access. The creation of a comprehensive standard for Non-Metallic Repairs of Pressurized Systems, ASME PCC-2 (2015) Article 4.1, “Non-metallic Composite Repair Systems: High-Risk Applications”, has helped regulate the composite repair options and ensure the quality of systems engineered and tested to comply with this standard. How Advanced FRP Systems Can Help There are many reasons for why coatings and the underlying substrate can fail in water-immersion conditions. Asset owners and maintenance staff should rely on a robust inspection program to proactively identify issues and ensure the condition of their protective coatings. If your facility is experiencing microbially induced corrosion, there are a number of long-term, maintenance-free repairs or remediation options available. The best option will depend on the specifics of your process and the extent of the damage. Advanced FRP Systems provides expert assistance in coatings assessments, thorough inspections, coating failure analyses, and detailed installation specifications. We do not believe in a “one-size-fits-all” approach to solving corrosion issues. Our team will work with your staff to identify the underlying causes and help formulate appropriate corrective actions to fit your process, timeframe, and budget. Reach out to an expert at Advanced FRP Systems and schedule a free consultation.