Definition of damage

Microbial corrosion is a type of corrosion caused by living organisms such as bacteria, algae, or fungi. This type of corrosion is often accompanied by tubercles or slimy organic matter.

Materials susceptible to this type of corrosion

Many materials used in the construction of equipment include carbon and low-alloy steels, 300 and 400 series stainless steels, aluminum, copper, and some nickel-base alloys.

Critical factors

1. Microbial corrosion is often found in aquatic environments or services where water is present permanently or periodically. It is worth noting that stagnant or low-flow conditions increase the growth rate of microorganisms.
2. Because there are so many types of microorganisms, these organisms can survive and grow in harsh conditions such as the absence of oxygen, light or darkness, high salinity, a pH range of 0 to 12, and temperatures of 0 to 235 degrees Fahrenheit (-17 to 113 degrees Celsius).
3. Systems may become resistant to organisms that multiply and spread if properly controlled.
4. Various organisms grow in the presence of inorganic substances such as sulfur, ammonia, H2S, hydrocarbons, and organic acids. In addition, all organisms require a source of carbon, nitrogen, and phosphorus for their growth.
5. Leakage of process contaminants, such as hydrocarbons or H2S, may lead to a sharp increase in biofouling and corrosion rates.

Vulnerable equipment and units

1. Microbial corrosion often occurs in heat exchangers, bottom water of storage tanks, pipes that have low or stagnant flow, and pipes that are in contact with certain soils.
2. Microbial corrosion is also found in equipment from which hydro test water has not been removed or in equipment that has been left unprotected.
3. Product storage tanks and cooling water heat exchangers in any unit where the cooling water is not properly treated can be vulnerable to this type of corrosion.
4. Fire water systems can be affected by this type of corrosion.

Appearance or morphology of the injury

1. Microbial corrosion is usually observed as localized pits under the sediment or tubercles that cover the organisms.
2. This damage is characterized by cup-shaped cavities within the pores of carbon steel or subsurface cavities of stainless steel (Figures 1 to 7).

Prevention/reduce

1. Microbes need water to grow. In systems that contain water (such as cooling water, storage tanks, etc.), biocides such as chlorine, bromine, ultraviolet light, or other specific compounds must be used.
2. Proper use of biocides can control microbes but will not eliminate them. Therefore, continuous use of biocides is essential.
3. Maintain flow rates above minimums, minimizing areas with low or stagnant flow rates.
4. Systems that are not designed to contain water must be kept clean and dry.
5. Drain the hydro test water as quickly as possible, dry it, and prevent moisture from penetrating.
6. Cathodic coating and protection have been an efficient and effective method for preventing microbial corrosion of buried structures.
7. Effective reduction of organisms requires complete removal of sediments and organisms using a combination of pigging, blasting, chemical cleaning, and biocide application.
8. Add biocide to the water phase in storage tanks.
1. Maintain the interior lining of storage tanks.

Inspection and monitoring

1. In cooling water systems, the effectiveness of the operation is monitored by measuring residual biocide, microbial count, and appearance.
2. Special probes are designed to monitor the presence of fouling.
3. A decrease in the operational efficiency of a heat exchanger may indicate the presence of fouling and the potential for microbial corrosion.
4. A bad smell from the water may be a sign of a problem.

Related mechanisms

Cooling water corrosion

Figure 1 - Microbial corrosion on the bottom of a diesel tank

Figure 2 - Pitting corrosion in a 6-inch carbon steel pipe after 2.5 years of service. The pits are approximately 1 to 2 inches wide.

Figure 3 - Pitting corrosion in the pipe of Figure 2

Figure 4 - Oil pipeline with microbial corrosion under tubercles

Figure 5 - Oil pipeline similar to Figure 4. Hemispherical holes caused by microbial corrosion are visible after removal of sediments.

Figure 6 - 304 stainless steel exchanger tubes have deteriorated due to pitting corrosion on the shell side in the cooling water service after 2.5 years without the use of biocide.

Figure 7 - Cross-section of the tube in Figure 6 that has undergone subsurface tunneling due to microbial corrosion.

Resources:

1. D.H. Pope and J.G. Stoecker, “Process Industries Corrosion - The Theory and Practice,” NACE - International, Houston, TX, 1986, pp 227-235.
2. T.J. Tvedt, Jr., “Cooling Water Systems,” NACE Course Book on Corrosion Control in the Refining - Industry, NACE International, Houston, TX, 1999.
3. S.C. Dexter, “Biologically Induced Corrosion,” NACE Proceeding s of the International Conference on - Biologically Induced Corrosion, June 10 –12, 1985, NACE International, Houston, TX, 1986.