This article focuses on the mechanisms of microbiologically influenced corrosion as a basis for discussion on the diagnosis, management, and prevention of biological corrosion failures in piping, tanks, heat exchangers, and cooling towers. It begins with an overview of the scope of microbial activity and the corrosion process. Then, various mechanisms that influence corrosion in microorganisms are discussed. The focus is on the incremental activities needed to assess the role played by microorganisms, if any, in the overall scenario. The article presents a case study that illustrates opportunities to improve operating processes and procedures related to the management of system integrity. Industry experience with corrosion-resistant alloys of steel, copper, and aluminum is reviewed. The article ends with a discussion on monitoring and preventing microbiologically influenced corrosion failures.

Six cases of failure attributed to microbiologically influenced corrosion (MIC) were analyzed to determine if any of the failures could have been avoided or at least predicted. The failures represent a diversity of applications involving typical materials, primarily stainless steel and copper alloys, in contact with a variety of liquids, chemistries, and substances. Analytical techniques employed include stereoscopic examination, energy dispersive x-ray spectroscopy (EDS), temperature and pH testing, and metallographic analysis. The findings indicate that MIC is frequently the result of poor operations or improper materials selection, and thus often preventable.

Aluminum 7075 aircraft wing tanks failed in the 1950s. Investigation (visual inspection, biological analysis, and chemical analysis) supported the conclusion that MIC was the cause of the failures. Water condensed into the fuel tanks during flight led to microbial growth on the jet fuel. Pitting attack occurred under microbial deposits on the metal surface in the water phase or at the water-fuel interface. Previously, exposure of aluminum 7075 to cultures of various isolates showed that 27 bacterial isolates and 3 fungi could seriously corrode the aluminum alloy over several weeks. No recommendations were made.

Several type 304 stainless steel fire truck water tanks developed through-wall leaks after being in service for approximately two years. One representative tank underwent a comprehensive laboratory analysis, which included metallographic examinations and chemical analyses. The examinations revealed a classic case of microbiologically influenced corrosion (MIC), which preferentially attacked the heat affected zones of the tank welds, resulting in the leaks.

Failures occurred in admiralty brass condenser tubes in a nuclear plant cooled by freshwater. About 2500 tubes had to be replaced over a span of six years. Investigation (visual inspection, chemical analysis, water chemistry (for both intake and outfall), and corrosion products in the operating system and on test coupons exposed to the operating environment) supported the conclusion that the failure was caused by microbe-initiated SCC. No recommendations were made.

A failure occurred in buried brass (92% Cu, 8% Zn) piping used to carry drinking water in wet clay soil after less than two years in service. Investigation (visual inspection, chemical analysis of both the pipe surface and water, and a comparison of the corrosion failure of power station condenser tubing cooled by seawater for two copper alloys, an aluminum brass alloyed with arsenic (UNS C68700, ASTM B111, or Cu-Zn-20Al DIN17660), and a cupronickel 70-30 alloy with iron added (C71500, ASTM B111, or Cu-Ni-30Fe DIN17665)) supported the conclusion that the failure was caused by microbial induced corrosion by sulfate-reducing bacteria. No recommendations were made.

Type 316L (UNS S31603) austenitic stainless steel piping was installed as part of a storm-sewer treatment collection system in a manufacturing facility. Within six months of start-up, leaks were discovered. Investigation (on-site current flow testing, visual inspection, water tests, and 5x/10x images etched in ASTM 89 reagent) supported the conclusion that the pitting in the austenitic stainless steel pipe was believed to be caused by damage to the passive layer brought about by a combination of MIC, high chloride levels, and high total dissolved solids. The low-flow and stagnant conditions present in the piping were primary contributors to the pit progression. Recommendations included replacing the pipe. Several alloys, nonmetallic materials, and lining materials were proposed for coupon testing to determine which would operate best in an environment with high levels of aerobic bacteria.

A closed-loop hot water heating system at a museum in South Carolina was the subject of failure evaluation. The system consisted of plain carbon steel pipes (Schedule 40) made of ASTM A 106 or A 53 (ERW or seamless). The supply and return lines were made of the same materials. The fittings were mechanically threaded assemblies. Temperatures ranged from 150 to 155 deg F (65.6 to 68.3 deg C). Leaks in the system had reportedly initiated immediately after the building had been placed in service. The cause of corrosion inside the steel pipes was attributed to tuberculation caused by oxygen concentration cells and oxygen-pitting related corrosion. Both types of corrosion are due to the poor quality of the water and the lack of corrosion control in the water system.

A shopping mall in South Carolina was originally constructed in 1988 and a second phase completed in 1989. The HVAC system inside the mall included an open, recirculating condenser water loop that served various fan coil units located within tenant spaces. The system had a recirculating capacity of about 44,000 gal (166,000 L) of water. It consisted primarily of steel pipes fitted with threaded connectors on the 2 in. (46 cm) pipes and bolted flanged couplings on the larger pipes. Seven years following the completion of the mall, corrosion problems were noted at the outer and inner surfaces of the pipe. Visual observations on the inner diametral surfaces revealed that the pipes were, in almost all cases, filled with corrosion products. A significant amount of base metal loss was documented in all of the samples. The cause of the observed corrosion was determined to be a lack of corrosion monitoring and poor water quality. Pipe replacement and a regular water testing program were recommended.

Copper piping in a closed-loop water heater system was found to be corroded with MIC and erosion of the weak oxide layer. Investigation (visual inspection, bacterial corrosion cultures, and 20x/400x micrographs) supported the conclusion that the corrosion was caused by microbiological attack, specifically sulfur-reducing bacteria. No recommendations were made.

During a routine start-up exercise of a standby service water pump, a threaded coupling that joined sections of a 41.5 ft (12.7 m) long pump shaft experienced fracture. The pump was taken out of service and examined to determine the cause of fracture. It was apparent early in the examination that the fracture involved hydrogen stress cracking. However, the nature of the corrosive attack suggested an interaction between the threaded coupling and biological organisms living in the freshwater environment of the pump shaft. The organisms had colonized on the coupling, changing the local environment and creating conditions favorable to hydrogen stress cracking. This paper describes the analysis of the fracture of the coupling and provides an example of how biologically induced corrosion can result in unexpected fracture of a relatively basic machine part.

Corrosion in a closed-loop cooling water system constructed of austenitic stainless steel occurred during an extended lay up of the system with biologically contaminated water. The characteristics of the failure were those of microbiologically influenced corrosion (MIC). The corrosion occurred at welds and consisted of large subsurface void formations with pinhole penetrations of the surfaces. Corrosive attack initiated in the heat affected zones of the welds, usually immediately adjacent to fusion lines. Stepwise grinding, polishing, and etching through the affected areas revealed that voids generally grew in the wrought material by uniform general corrosion. Tunneling or worm-holing was also observed, whereby void extension occurred by initiating daughter voids probably at flaws or other inhomogeneities. Selective attack occurred within the fusion zone, i.e., within the cast two-phase structure of the weld filler itself. The result was a void wall which consisted of a rough and porous ferritic material, a consequence of preferential attack of the austenitic phase and slightly lower rate of corrosive attack of the ferrite phase. The three-dimensional spongy surface was studied optically and with the scanning electron microscope.

A failure of an aboveground storage tank occurred due to external corrosion of the tank floor. The liquid asphalt tank operated at elevated temperatures (approximately 177 deg C, or 350 deg F) and had been in service for six years. Cathodic protection (rectifiers) had been installed since start-up of the tank operation. It was noted, however, that some operational problems with the rectifier may have interrupted its protection. Investigation (visual inspection, on-site examination and testing, EDS analysis of scale deposits, and MIC testing of the soil) supported the conclusion that corrosion may have been caused by an interruption in cathodic protection. The effectiveness of cathodic protection on established microbial deposits is questionable. Recommendations included ultrasonically testing the tank floor and replacing portions based on the remaining wall thickness. Doubling the wall thickness of the floor plates was also recommended.

This investigation involved two AISI 304L acid storage tanks and one AISI 304L spent solvent tank from a sewage treatment facility. After installation, these tanks were hydrostatically tested using sewage effluent. No leaks were found and after a year or two, the tanks were drained and filled with nitric acid in preparation for service. Three weeks later the two acid tanks were found to be leaking from the bottom. Samples from the spent solvent tank revealed that pitting was located in a depressed area near a suction hole, beneath a black residue. It was concluded that the acid tanks failed by chloride-induced pitting initiated by microbial activity. Further, the spent solvent tank failed by a similar, but anaerobic mechanism. The use of the effluent for the hydrostatic test and the failure to remove it and clean and dry the tanks was the primary cause of failure. Localized carbide segregation in the original plate served as preferential corrosion sites. Had the tanks been hydrostatically tested in a proper manner, the pitting may not have occurred.

This article focuses on the mechanisms of microbially induced or influenced corrosion (MIC) of metallic materials as an introduction to the recognition, management, and prevention of microbiological corrosion failures in piping, tanks, heat exchangers, and cooling towers. It discusses the degradation of various protective systems, such as corrosion inhibitors and lubricants. The article describes the failure analysis of steel, iron, copper, aluminum, and their alloys. It also discusses the probes available to monitor conditions relevant to MIC in industrial systems and the sampling and analysis of conditions usually achieved by the installation of removable coupons in the target system. The article also explains the prevention and control strategies of MIC in industrial systems.

The interior surface of a type 316L stainless steel trailer barrel used to haul various chemicals showed evidence of severe pitting after less than 1 year of service. Two sections were cut from the barrel and microscopically examined. Metallographic sections were also prepared at the weld areas and away from the weld zones. Terraced, near-surface pits with subsurface caverns and a high level of sulfur in the pit residue, both indicative of bacteria-induced corrosion, were found. No evidence of weld defects or defective material was present. Testing of the water used at the wash station and implementation of bacteria control measures (a special drying process after washing and use of a sanitizing rinse) were recommended.

Several hundred leaks were reported in the type 304 stainless steel pipelines, vessels, and tanks of a chemical plant at a tropical location within a few weeks after startup. Investigation of the failure involved a site visit, metallographic examination and analysis of the material, analysis of hydrotest waters, and microbiological examination of slime that had formed in certain pipework sections. It was determined that the failure resulted from microbially induced corrosion promoted by the use of poor-quality hydrotest water and uncontrolled hydrotesting practice. Use of appropriate hydrotesting procedures was recommended to prevent similar failures.