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Microbially Influenced Corrosion (MIC)

Microbially Influenced Corrosion (MIC)

  • Bacteria can be classed into the below categories:
    • Aerobes (Grow in the presence of Oxygen) e.g. Pseudomonas
    • Facultative anaerobes (Can grow without Oxygen) e.g. E. Coli
    • Anaerobes (Grow in the absence of Oxygen) e.g. Sulfate Reducing Bacteria (SRBs)
  • All natural water sources, including tap water,  may contain bacteria which can multiply rapidly in suitable conditions – This is why appropriate water treatment is required
  • Pseudomonads are often used as an indicator of the biological quality of water as when Pseudomonad levels are high in a system, the risk of problems such as blockages and sludge formation (& therefore MIC) increases
  • Certain metals such as mild steel, stainless steel and copper may be more prone to MIC
  • Low velocity & stagnation allows sedimentation – Accumulation of deposits increases the likelihood of under deposit corrosion, biofouling and MIC

2 types of MIC

Direct MIC

  • Direct MIC occurs when bacteria are in direct contact with the metal surface (usually under biofilm)
  • Examples include:
    • Oxygen depletion corrosion
      • Where a large amount of biofilm is present, an area of Oxygen depletion can occur between the metal & biofilm. The different Oxygen potentials allows electrolytic corrosion
    • Stainless steel corrosion
      • A film of stable oxide on the surface of stainless steel makes it corrosion resistant, however, in low oxygen conditions under a biofilm, the oxide layer breaks down, allowing corrosion
    • Corrosion due to SRBs
      • SRBs are one of the most frequent causes of MIC
      • They are an anaerobic bacteria, so can thrive in the absence of oxygen under a biofilm
      • They reduce sulfate from fill water to hydrogen sulfide, which produces metal sulfides as corrosion products when it reacts with metal surfaces

Indirect MIC

  • Examples include:
    • MIC due to Nitrite Reducing Bacteria (NRB) presence
      • NRBs can rapidly decrease levels of nitrite-based inhibitor in a system, making the inhibitor less effective and increasing the rate of corrosion
      • The degradative products can also have detrimental effects e.g. ammonia – Which can increase corrosion of copper & brass systems
    • Interference with the action of treatment chemicals
      • Inhibitors generally reduce corrosion by coating exposed metal surfaces
      • If biofilm is present, this prevents the inhibitor from reaching the metal surface, therefore allowing corrosion to occur
    • Degradation of glycols
      • If the glycol level in a system is less than 20%, bacteria can use it as a nutrient and grow

Reducing the risk of MIC

  • The addition of an appropriate biocide can significantly reduce the risk of MIC by controlling the levels of bacteria in a system
  • The risk of corrosion due to SRBs can be reduced by limiting the nutrients they require (e.g. Phosphorous, Nitrogen & Sulfate). A system filled with reverse osmosis or deionised fill water also has fewer issues with SRBs
  • Flushing / Cleaning
  • Removal of deadlegs

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