Burning ambition to prevent microbiologically influenced corrosion in fire extinguish pipe systems

As MIC is usually present in combination with other corrosion mechanisms, it is therefore recommended to further investigate the system by means of an in-line camera inspection and/ or destructive testing to determine the full extent of the damage.

Nanni Noël-Hermes

Microbiologically influenced corrosion, also called MIC, is an extremely fast form of local corrosion caused by micro-organisms. MIC is a known phenomenon in various industries where metal has been (at least temporarily) in contact with non-sterile water.

PITTING CORROSION IN FIRE EXTINGUISH PIPE SYSTEM

During regular maintenance, pitting corrosion was found in the tank of a fire extinguishing pipe system. To identify the cause of this damage, the ENDURES MIC team sampled tubercles in the damaged areas for further examination in the laboratory. In addition, water samples were taken at various points in the fire extinguishing system. Further inspection revealed tubercles on the inside of the pump flange. Similar tubercles were present on the inside of the pipes. After removing the tubercles, the underlying pitting corrosion was exposed, as in the water tank. These spots were also sampled for further research. In-line camera inspection and destructive testing was in this case not an option. Samples were then analysed in the lab for presence of MIC. Analysis included: DNA- and growth-related quantification of MIC-related microorganisms, SEM-EDX investigation of corrosion products, and chemical-physical and biological water analysis

TESTING POSITIVE FOR MIC

All corrosion product samples as well as water samples tested positive for various MIC related microbial groups including sulphate reducing bacteria (SRB), iron reducing bacteria (IRB) and acid producing bacteria (APB). The numbers of MIC-related micro-organisms, in particular SRB, were lower in the examined water samples than in the corrosion products. This indicates the formation of a local biofilm, accumulating on the metal surface and actively growing in the system. SEM analysis showed a high sulphur peak in the corrosion products. This confirms the presence of active SRB’s, as these can lead to the formation of black iron sulphide when in contact with iron. Comparing replenishing water with water in the system, reveals differences in numbers of MIC related organisms. Particularly in the part of the system with no flushing, high numbers of MIC-related organisms can be found. Apparently, these conditions are

favourable to allow biofilm formation under which accelerated corrosion occurs, leading to pitting. This explains the damage found at the pump and pipe near the water tank. Chemical-physical analysis of the water sample and SEM-EDX analysis of the corrosion products indicate that metals dissolve in the system. In addition to iron, also zinc, copper, aluminium, and nickel were determined in significant amounts. Zinc was found both in water and corrosion product samples, indicating active dissolution in the system. Zinc can originate from the galvanised pipes or from selective corrosion of zinc containing brass alloy (zinc content >15%) valves. In addition, relatively high concentrations of nickel measured in the water typically originate from coppernickel alloys (Cupronickel), a material often used in extinguishing systems. The MIC problem can most likely be traced back to the flushing process, in which nearby river water containing MIC-related organisms, is used. This way, the organisms enter the system, multiply under favourite conditions and distribute further in the system. In between flushing cycles, the stagnant water provides the organisms the opportunity to build their own micro-environment in which they can settle and grow, thus leading to accelerated corrosion.