Pipe failures resulting from microbiologically influenced corrosion (MIC) have been widely recognized in petrochemical, gas and nuclear power industries, but only recently has this phenomenon been associated with failures in fire protection systems (FPS). MIC results in mechanical blockages of piping and sprinkler heads, as well as through-wall penetration of ferrous and non-ferrous metals. FPS are designed for the life of the structures in which they reside; however, reports of new systems developing MIC-associated through-wall leaks within months of installation are becoming more prevalent.
Causes of Blockage and CorrosionFire protection systems represent a particularly complex challenge for biological fouling prevention and control. Mechanical blockages and MIC have been implicated as significant problems in FPS, both in carbon steel and copper-based systems, because these water distribution systems are usually composed of carbon steel of decreasing diameters. Fluid flow is nearly always stagnant, and the piping conduits are not designed to facilitate routine cleaning operations. As a result, the presence of reduced corrosion products coupled with long residence times diminishes available chlorine, which would otherwise control biological fouling activities.
Pitting corrosion occurring under deposits in FPS can be initiated or propagated by these microbial activities. Through-wall penetration of carbon steel and copper has been reported within months after a new pipeline has been brought into service. This extensive tuberculation can cause occlusion of pipelines, sometimes completely blocking flow in six-inch diameter pipelines. These problems become more critical as pipe diameter decreases, posing a potential threat to proper sprinkler head mechanical functioning.
In addition, FPS make-up waters are typically stagnant, soft (relatively low in hardness), acidic and devoid of antimicrobial agents such as the sodium hypochlorite that is used for microbial control in potable waters. These characteristics predispose FPS to biological fouling and MIC. Regulatory requirements that dictate periodic testing can also contribute to development of MIC in FPS when make-up waters are replaced with oxygenated and nutrient-rich waters. MIC-associated microorganisms can use these nutrients as growth sources, leading to fouling of affected systems.
The most serious consequence of MIC in FPS is mechanical blockage of piping and sprinkler heads. MIC-associated organisms can attach to the metallic surfaces of FPS, forming corrosion deposits that are termed tubercles (Figure 1). Tubercles can completely occlude pipes, and more significantly, these deposits can break off and block sprinkler head flow channels. Localized pitting-type attack can also occur underneath tubercles, resulting in through-wall penetration. The resulting acid production, hydrogen sulfide generation and development of differential aeration cells can lead to the loss of essential metallic properties of mild steel, copper, stainless steel and other ferrous and non-ferrous metals.
Causes of CorrosionA number of investigators have described possible mechanisms for the corrosion of stainless and mild steels and copper. These mechanisms include development of differential oxygen cells, under-deposit (biofilm) chloride concentration, sulfide-mediated attack, acidic dissolution of corrosion products and cathodic depolarization of protective hydrogen films. While corrosion of copper piping may present an environmental and economic threat in both fresh water and seawater systems, relatively little is known of the role microorganisms play in initiation or propagation of corrosion events. It is known, however, that bacterial biofilms (Figure 2) are the initiating and/or propagating agents of MIC; microbial corrosion does not occur in the absence of biofilms. Biofilms provide the localized environmental conditions (e.g., decreased pH; differential oxygen cells) for initiating or propagating corrosion activities.
MIC-associated bacteria are grouped on the basis of their mode of attack on ferrous and non-ferrous metals. The most common MIC groups include sulfate-reducing, iron-oxidizing, acid-producing, sulfur-oxidizing and nitrate-reducing bacteria. Acid production, hydrogen sulfide generation, tubercle formation and the subsequent development of differential aeration cells can lead to deterioration and failure of mild steel, copper, stainless steel, and other ferrous and non-ferrous metals used as materials of construction. The only material found todate that has been shown to be impervious to MIC attack is titanium. It has been suggested that the thick TiO2 passive film that forms in aqueous environments is protective in this regard.
Sulfur bacteria obtain energy by reducing or oxidizing inorganic sulfur compounds that are present in feed waters. The bacteria most often associated with MIC in water systems belong to the anaerobic sulfate-reducing (SRB) group, which includes Desulfovibrio desulfuricans (Figure 4). Direct attack of ferrous and non-ferrous metals by their hydrogen sulfide metabolic by-product is a significant problem in many industries.
Sulfur oxidizing bacteria, such as Thiobacillus spp., are aerobic microorganisms that can produce sulfuric acid. This group of organisms often lives in close association with SRB.
Nitrate reducing bacteria (NRB) can utilize nitrogen containing organic compounds in feedwaters, producing significant quantities of ammonia. In addition to odor problems, ammonia production is associated with stress corrosion cracking of copper alloys. Nitrite-based corrosion inhibitors may be a source of nitrogen for this group of MIC bacteria.
The presence of standing water is the single-most important determinant of MIC activity in dry-type fire sprinkler systems (Figure 5). In the presence of water, the combination of MIC and exposure of wet surfaces to a significant oxygen headspace can greatly accelerate corrosion rates. In the absence of water, microbial activity ceases, and there is no possibility of MIC or other corrosion activity. While current fire codes specify dry-type systems are self-draining--in order to prevent blockage from accumulated ice--standing water is often observed in these systems. Proper drainage combined with on-line dehumidification will prevent MIC attack in dry fire protection systems.
Treatment of MICDifferentiation of microbial from abiotic corrosion is an essential first step in developing effective treatment and preventative maintenance programs for FPS and for other water circuits. Appropriate techniques for sampling, sample preservation and analysis are similarly important. A combination of microbiological analyses, chemical testing, metallurgical assessment and non-destructive examination techniques (e.g., borescope, ultrasonic, etc.) are used in root-cause failure investigations.
It is essential that an appropriate suite of analytical tests be used to confirm the presence of MIC bacteria. Table 1 lists the key microbiological tests that are used to assess MIC activity. Problem resolution in affected fire protection pipeline systems requires an integrated approach involving microbiologists, metallurgists, mechanical engineers, electrochemists and non-destructive testing specialists. These individuals use materials testing, engineering analysis, and on-site inspection resources to develop remedial treatment plans, as well as cost-effective programs for pipeline replacement and/or design modifications.
While it is not possible to estimate the overall contribution of MIC to corrosion in sprinkler systems (both pitting and generalized), it is clear that microbial activity can cause significant damage to ferrous and non-ferrous metals. Where water chemistry, pipe joining techniques, and/or electrolytic processes have been eliminated as possible causes for corrosion events, MIC activity should be suspected. If laboratory testing confirms the presence of MIC-associated microorganisms, antimicrobial treatments (e.g., appropriate application chlorine or other biocides) of potable and industrial waters should be considered.
PreventionMIC represents a growing problem for the fire protection industry. The application of thinner-walled piping (e.g., Schedule 10 vs. Schedule 40) may influence the rate of through-wall penetration. Changes in feedwater quality and failure to control corrosion activity through effective inhibitors may also be contributing factors.
While the mechanisms associated with MIC aren't completely understood, there are a number of common-sense approaches to prevention. These include elimination of water from dry sprinkler systems, recharging of systems with water containing low concentrations of nutrients (e.g., purified water), critical cleaning of newly installed piping systems to remove organic and inorganic debris, and the addition (where appropriate) of safe and compatible corrosion inhibitors. Effective testing, proper maintenance, and prompt treatment can also stop MIC activity and prevent irreversible damage to sprinkler piping and blockage of critical system components. Understanding those factors that contribute to microbiological growth and associated corrosion activities is the key to MIC prevention.