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Microbiological induced corrosion failure investigation peformed by MSi on a welded stainless steel vessel.

1. Based upon the performed metal corrosion analysis, it is our opinion that the failure of the submitted pressure vessel section was caused by a microbiologically-influenced corrosion (MIC) attack followed by a MIC-related stress-corrosion cracking (SCC) attack. The MIC attack was most likely triggered by specific microorganisms, either aerobic (e.g., sulfur-oxidizing bacteria), or anaerobic (e.g., sulfate-reducing bacteria).

2. The bacteria typically grow on assorted organic nutrients, deposited on the ID surface of the components during periods of exposure to a stagnant aqueous environment. The stagnant conditions were most likely produced by the internal structural component covering the affected area. MIC-based attack is known to be highly localized and aggressive, with the adjacent areas being unaffected. Most microbiologically-influenced corrosion failures occur within a short period of time.

3. The phenomenon of MIC on metal surfaces can be described as follows:

• First, the specific microorganisms that are present inside the system attach themselves to the areas coated with organic nutrients (slime, biofilm, etc.) and begin to grow. Microbial growth is most active at temperatures of 20-55°C (68-131°F).
• Second, the microbial metabolic activity results in the formation of a growing nodule. Nodule growth is promoted by the sticky by-products that capture organic and inorganic substances. The growing nodule forms a concentration cell by setting up electrochemical potential differences between the underlying metal and the adjacent, nodule-free metal. Consequently, the metal below the deposits becomes anodic and dissolves preferentially. Other by-products may contain sulfur-bearing compounds and organic acids, which would contribute to the localized, under-the-nodule metal corrosion.
• Finally, a combination of the concentration cell mechanism and the biological activity creates assorted pits. Weak organic acid by-products inside these pits can be converted to a strong hydrochloric acid, if chlorides are present in the water. At this point, the bacteria can survive only at the outer surface of the nodule and their contribution diminishes. Corrosion then continues predominantly as an electrochemical process.
• It should be noted that the pits tend to accumulate and concentrate impurities, including chlorides, even when these impurities are present in very small amounts. If the rising under-the-nodule chloride concentration reaches the critical level, it may trigger a localized stress-corrosion cracking (SCC) attack in susceptible materials. Such highly-localized SCC attack was observed at the bottom of the MIC pits in the submitted sample, with the adjacent areas being unaffected.

4. The phenomenon of stress-corrosion cracking (SCC) occurs in metals when three conditions are met simultaneously. These conditions are: (1) corrosive environment, (2) presence of residual, or service-applied tensile stresses in the metal, and (3) susceptible material. Removal of one of these conditions will prevent SCC from occurring.

5. The described corrosion failure mechanisms were confirmed by SEM/EDS and metallographic examinations. The affected metal exhibited no evidence of pre-existing plate fabrication defects, excessive nonmetallic inclusions, or any other detrimental material conditions that could have contributed to the failures.

6. Chemical testing identified the material of the submitted sample as a 300-series austenitic stainless steel similar to Type 316L, but with a slightly reduced molybdenum content. The slightly lower level of molybdenum was not considered to be a contributing factor to the corrosion attack.

7. Hardness testing results indicated that the sample was supplied in an annealed condition.

8. Once initiated, the MIC-related pitting and SCC will progress, due to the local electrochemical conditions that make the pitted areas anodic in relation to the adjacent, unaffected metal. Remedial maintenance can slow down the metal degradation process, but will not arrest it. To delay similar vessel failures in the future, we respectfully recommend the following measures:

• Inspect, clean and flush the vessel, to remove any organic and inorganic debris that could stick to the interior vessel surfaces.
• If practical, drain and dry the interior of the vessel during extensive shutdown periods.
• Identify and remedy areas where stagnant or low-flow conditions could develop. Stagnation allows debris and corrosion products to settle and attach to the metal, facilitating the formation and stabilization of under-deposit corrosion cells.
• Assess the feasibility of vessel repairs by removal and replacement of the affected wall sections, or by covering the affected areas with compatible stainless steel sheet and seal-welding the formed patch to the intact metal.