METALS, MICROBES AND MIC – A REVIEW OF MICROBIOLOGICALLY INFLUENCED CORROSION

M Critchley* & R Javaherdashti** *CSIRO Manufacturing & Infrastructure Technology Clayton VIC **Monash University Clayton VIC Australia

Summary: Microbiologically influenced corrosion (MIC) is the deterioration of materials caused by the presence and activity of micro-organisms. It is a complex problem that presents huge costs to industry. This review will describe the processes and micro-organisms involved in MIC as well as detection techniques. Mitigation methods against MIC will also be addressed

Keywords: Biocorrosion, biofilms, microbiologically influenced corrosion

1 INTRODUCTION Microbiologically influenced corrosion (MIC), also known as biocorrosion, is the corrosion or deterioration of a material which is initiated and/or accelerated by the activities of micro-organisms. It affects not only metals but many materials including polymers, concrete and glass. MIC can manifest as pitting corrosion, crevice corrosion, stress corrosion, selective de- alloying and generalised corrosion, depending on the environment and micro-organisms present. It is a complex process and remains one of the least understood areas of corrosion science as it falls outside the traditional disciplines of both microbiology and corrosion. MIC presents huge costs for many industries. In the oil and gas industry, MIC causes pitting corrosion of pipelines and vessels and the plugging of reservoirs. In the water industry, MIC induces pipeline corrosion, pipe blockages and the corrosion of sewer mains. In the aviation industry, MIC can cause the corrosion of aircraft fuel tanks. MIC also causes the corrosion of heat exchangers, cooling towers and fire protection systems in the power industry. A recent NACE survey estimated the direct costs of corrosion to be 3.1% of US GDP, with indirect costs at 3% US GDP. MIC is estimated to account for at least 20% of corrosion, which calculates to approximately $US 55 billion/year (1).

2 BIOFOULING MIC is initiated by the fouling of surfaces, a process which naturally occurs in many environments (2). Biofouling is the undesirable formation of deposits on a surface. Biofouling comprises of both organic and inorganic components which include micro-organisms, precipitated materials, particulates and corrosion products. The biological component occurs from the formation of biofilms, where micro-organisms accumulate on surfaces and develop complex communities within a matrix of organic polysaccharides (2). Biofilms comprise of many different micro-organisms depending on the environment, including aerobic and anaerobic bacteria, fungi, algae and protozoa. The steps involved in the formation of biofilms are shown in Figure I (2). Organic conditioning films accumulate on surfaces upon exposure to the environment, changing the surface charge and hydrophobicity. Planktonic organisms associate with surfaces, with irreversible adhesion occurring through interactions with surface structures and charges. Attachment allows microbial colonies to develop where they can secrete extracellular materials and accumulate compounds from the bulk phase. Biofouling initially occurs on a micro-scale which allows the colonisation of very small areas such as crevices, weldments and surface imperfections. The structure of biofilms is complex. Biofilms contains structural and temporal variations such as gradients in pH, oxygen concentrations (Figure II) and nutrients (2). This creates microenvironments where the activity of certain micro-organisms may be enhanced. Underneath the biofilm, chemical properties such as the pH, dissolved oxygen and nutrient concentrations may be dramatically different from those in the bulk solution. This results in a shift in the open-circuit potential of passive metals in the noble direction (ennoblement). This has been well-documented for a range of metals and alloys, particularly stainless steel (3).

Corrosion & Prevention 2004 Paper 037 Page 1 Biofouling causes many problems for industrial systems. One of the most important problems is it increases the retention time and allows the proliferation of micro-organisms within an environment where they may not normally accumulate. Biofouling is an important contributor to microbial corrosion, however, it is important to note that MIC can also occur from suspended, unattached micro-organisms present in the planktonic phase.

STEPS IN BIOFILM FORMATION

BIOFILM FORMATION

ATTACHMENT GROWTH & ATTACHMENT

SURFACE COLONISATION CONDITIONING

Figure I. Steps involved in biofilm formation (Critchley 2001)

BULK LIQUID PHASE

O2 AEROBIC HETEROTROPHIC

BIOFILM ANAEROBIC HETEROTROPHIC ACTIVITY

OBLIGATE ANAEROBES

METAL

Figure II. Spatial relationships of micro-organisms within an established biofilm (Critchley 2001)

3 MICRO-ORGANISMS IMPLICATED IN MIC Micro-organisms can be classified in many ways including their cellular structure, morphology, source of energy and oxygen requirements (4). Numerous micro-organisms have been implicated in MIC, the most common are described briefly below:

Corrosion & Prevention 2004 Paper 037 Page 2 3.1 Sulphate reducing bacteria Sulphate reducing bacteria (SRBs) are naturally found in the environment in soil and surface waters. They are anaerobic, growing under very low oxygen or completely oxygen depleted conditions (5). The optimum conditions for their growth are temperatures of 25 to 35°C and a pH range of 6 to 9.5 (5). Some species of SRBs are thermophilic (ie. Desulfotomaculum) and can survive at temperatures up to 60°C. SRBs use hydrogen, alcohols, lactates and acetates as sources of energy. SRBs contain the hydrogenase enzyme which enables them to utilise hydrogen from the environment. For SRBs to flourish, sulphate concentrations of at least 50 to 100 ppm are required (5). SRBs reduce sulphates to sulphides which react with metals to produce metal sulphides. Corrosion by SRBs is usually detected by the hydrogen sulphide odour, blackening of waters or the presence of black coloured deposits (Plate 1). Common species of sulphate reducing bacteria include Desulphovibrio and Desulphotomaculum.

Plate I. Corrosion cause by sulphate reducing bacteria (source unknown)

3.2 Iron oxidising bacteria Iron oxidising bacteria require the oxidation of Fe2+ to Fe3+ to produce energy for metabolism (6). Common species of iron oxidising bacteria include Gallionella, Sphaerotilus, Siderocapsa and Crenothrix. They generally grow in filamentous clumps and can be visually detected by microscopy by the presence of a twisted helical/sheath that are excreted by the cells and encrust them with their growth (Plate 2). For this reason, corrosion by iron bacteria often manifests as tubercles (7). Chloride ions can accumulate within these tubercles, which combine with corrosion products to produce ferric chloride. Ferric chloride is extremely corrosive to stainless steel (7). Tubercles additionally allow the development of anaerobic conditions and can lead to the growth of SRBs (7). Iron oxidising bacteria prefer pH ranges from neutral to acidic, oxygen concentrations from 0 to saturated and temperatures up to 90°C. Bacteria that oxidise manganese have also been identified and implicated in the corrosion of stainless steel (8). In contrast to this, there are bacteria that can accumulate iron and manganese which also have been associated with corrosion (9).

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Plate II. SEM image of iron bacteria showing precipitated iron (Critchley 2003)

3.3 Acid producing bacteria Acid producing bacteria (APBs) generate acidic by-products with their metabolism that is corrosive to many materials. Clostridium spp. have been shown to produce acetic acids with fermentation and has been extensively isolated from industrial water systems (10). More importantly, bacteria which oxidise sulphur or sulphides including Thiobacillus and Beggiatoa can produce up to 10% sulphuric acid solutions (11). These bacteria have been widely implicated in the corrosion of reinforced concrete structures. Sulphur oxidisers are acid tolerant and are typical of microbes found in acid mine drainage systems. Sulphur oxidising bacteria are almost always found with SRBs, existing synergistically cycling sulphur. Biofilms allow the existence of both organisms due to the existence of microenvironments within its structure, where localised conditions are controlled.

3.4 Fungi Certain species of fungi are corrosive, mainly through the production of acids with metabolism. Hormoconis or Cladosporium resinae is a fungus notorious for corrosion in aluminium fuel systems (Plate 3)(12). It has presented major problems for the aviation industry as its growth in fuel systems causes significant corrosion and also operational problems such as errors in fuel gauging and filter blockages. Fungi have also been implicated in the corrosion of glass and more recently, compact discs (13). Fungi produce spores which enables them to survive environmental extremes only to regenerate when conditions are favourable. This makes the eradication of fungi extremely difficult. Other fungi that have been associated with corrosion include Aspergillus spp. (14)

3.5 Heterotrophic slime forming bacteria Heterotrophic bacteria are aerobic and use organic carbon as a nutrient source. They are commonly found in the environment particularly within biofilms. Many can produce copious amounts of exopolysaccharides or slime which causes many operational problems as well as corrosion (15). Polysaccharides can contain acidic groups which directly promote corrosion. This can be enhanced if bacteria produce organic acids with metabolism, as has been suggested for the corrosion of copper in drinking water (16). In addition, these bacteria can consume oxygen which prevents the regeneration of protective oxide films. This slime can also neutralise some disinfectants, preventing penetration of the active component to the cells. Surface polysaccharides enable the development of anaerobic conditions within biofilms, which enables the existence of anaerobic species including SRBs. Heterotrophic aerobes can grow at pH ranges of 5 to 9 and at temperatures of 10 to 50 °C. Species of heterotrophic bacteria implicated in MIC include Pseudomonas. and Acidovorax.

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Plate III. Corrosion of aluminium caused by A. resinae in aviation fuel (Kobrin 1993)

3.6 Other micro-organisms Other micro-organisms involved in MIC include the methanogens. Methane producing bacteria are anaerobic and consume hydrogen promoting depolarisation reactions, similar to SRBs (17). The growth of other non-specific micro-organisms can also contribute to MIC by providing nutrients and an environment for the growth of more corrosive organisms.

4 MECHANISMS OF MIC The mechanisms causing microbial corrosion are varied. Corrosion can be influenced by the presence of micro-organisms (ie. production of acids through metabolism) or induced (ie. use of the material for energy). Mechanisms of MIC include the depolarisation of corrosion reactions. This is the mechanism of corrosion by SRBs and is shown in Figure III. SRBS consume hydrogen by their dehydrogenase enzymes, inducing cathodic depolarisation (5). Corrosion can be induced by biofilms from the formation of concentration cells between pH, chemical and oxygen gradients, as well as the production of both organic and inorganic acids (18,19). Biofilms may also degrade protective coatings allowing surfaces to be exposed to aggressive environments. This is particularly important if a coating leaches any compounds which can be used as a nutrient source, especially PVC (20). The presence of micro-organisms in a system can also cause corrosion by consuming corrosion inhibitors. For example, nitrogen based inhibitors can be consumed by nitrite oxidising bacteria (21). In many instances, these mechanism of MIC may be operating simultaneously to produce the corrosion observed. In contrast, micro-organisms can provide some protection against corrosion. Micro-organisms can decrease the corrosiveness of a medium by changing the bulk phase chemistry such as pH, promoting the formation of protective surface films, neutralising corrosive substances or consuming oxygen that is required for corrosion (22).

Corrosion & Prevention 2004 Paper 037 Page 5 SRB CORROSION MECHANISM

SRB 2- 2- SO4 + 8H → S + 4H2O

ANODE 4Fe → 4Fe2+ + 8e- CATHODE 8H+ +8e- → 8H

2- - 4Fe + SO4 + 4H2O → 3Fe(OH)2 + FeS + 2OH

Figure III. Mechanism of corrosion by sulphate reducing bacteria (Critchley 2001)

5 ENVIRONMENTS CONDUCIVE TO MIC Certain environments are considered conducive to microbial survival and hence MIC. MIC is often observed in stagnant environments with low flow rates, interrupted operation and “dead legs” of pipe work. MIC usually occurs at temperatures below 80°C and a pH level under 10. These are conditions that are optimum for the growth of a wide range of micro- organisms. MIC is also observed in areas containing organic materials and debris which can provide nutrients and shelter for microbes or anaerobic conditions.

6 IDENTIFICATION OF MIC Corrosion is often attributed to microbial involvement due to a lack of understanding of the process. An accurate identification of MIC involves replication of the corrosion under controlled laboratory conditions, problematic as many environments can not be replicated and microbial activity can change with experimentation. The identification of MIC based solely on the presence of micro-organisms is misleading as they do not always have detrimental effects. In some instances, microbial populations can be anticipated by the examination of physicochemical environments. However, micro-organisms survive in complex consortiums, existing synergistically where their relative activities support each other. This often makes testing for specific micro-organisms insufficient in defining whether one specific microbe is contributing to the corrosion observed. In the field, MIC is usually implicated if micro-organisms are detectable on a corroding surface and the application of biocides or antimicrobials reduce the effects. In some instances the morphology of corrosion, and to a very limited extent, pit morphology, can indicate the potential presence of micro-organisms. Hence, by observing the type of corrosion and associated deposits the potential involvement of micro-organisms be determined. Corrosion by SRBs is suggested by the presence of sulphides or black deposits in anaerobic environments. A shiny metal surface is generally observed after the removal of the corrosion products, with a pit-in-pit morphology (especially in stainless steels). A simple field test of treating corrosion products with diluted hydrochloric acid will produce the characteristic “rotten egg” smell when sulphides are present. Similarly for iron oxidising bacteria, the presence of tubercles and ferric chloride or ferric manganic chloride is observed. Analysis of corrosion products can be performed using various chemical techniques such as XRD, EDXS, XPS or ICP to determine elements present. Non destructive test methods such as radiography or ultrasound can also be used to indicate the severity and extent of problems. There are numerous methods for detecting micro-organisms. Culture techniques are extensively used, however have disadvantages as they are time consuming and not all organisms can be cultured. Microscopy techniques are also useful in microbial identification/enumeration as they are rapid and culture independent. SEM can quickly and accurately provide information on corrosion, provided samples are preserved prior to imaging so microbial populations are preserved (Plate 4)(23). Numerous test kits for SRBs and other microbes are available, such as MICKIT™ and Rapidchek™. These kits are either culture based or biochemical assays detecting specific proteins. While every method of detection has its relative

Corrosion & Prevention 2004 Paper 037 Page 6 advantages and disadvantages (ie. sensitivity, detection of viable populations, detection limit), consistency in the type of test method should be adhered to so errors are minimised.

Plate 4. Detection of micro-organisms involved in copper corrosion using SEM imaging. Note – background is filter paper used for concentrating sample. (Critchley 2004)

7 MITIGATION The prevention and management of MIC involves minimising conditions conducive to the survival and proliferation of micro- organisms, as well as restricting their access to surfaces (24). This includes changes to equipment design, the application of protective coatings, the use of antimicrobials/biocides, as well as changes to the environment. There are essentially three ways to combat against MIC:

7.1 Physical treatments This involves physically removing the fouling, dirt and debris to achieve a cleaner environment. Methods may involve brushing, hydroblasting, pigging or similar techniques. Physical methods are highly effective in removing biofouling on a surface. However, inaccessible areas such as weldments and crevices are difficult to clean and need to be considered. Also, regular physical treatments are often needed to maintain long term control.

7.2 Chemical and biocidal treatment. Chemical treatment involves removing the corrosion products, debris and biofilms from a system. Treatments have been developed which chemical clean corrosion products from surfaces. Numerous biocides, surfactants and dispersants are also commercially available. Biocides should be selected to suit the environment and the micro-organisms targeted (25,26). The biocide must also be compatible with the system ie. not cause corrosion of materials itself. Chemical treatments generally require some stagnation time within the system to be effective, which may be problematic. For example, the treatment of aircraft with biocide to prevent fungal corrosion requires 72 hours standing time which is often unachievable in this industry.

7.3 Management of the system and the problem. Management of systems is highly important in prevention and management on microbial corrosion. Monitoring of corrosion can be performed by inspection and various electrochemical and non destructive techniques. Monitoring of biofouling can be performed by analysing microbial populations or frequent biocidal dosing. The use of metals as antimicrobial materials needs to be treated with some caution. There are large amounts of evidence demonstrating microbial resistance to copper and more recently, to chromate. Cathodic protection can also be used to prevent corrosion in systems anticipated to experience MIC. The above principles should be modified according to the industry and system of concern.

Corrosion & Prevention 2004 Paper 037 Page 7 The management of MIC is not and can not be limited to experts in the area. MIC should be addressed by applying principles of corrosion management, which are detailed at length elsewhere (27). MIC is a complex problem, however a considerable proportion of damage can be prevented if detected in time and concepts of treatment and prevention are applied.

8 CONCLUSIONS MIC is a complex problem affecting a wide range of industries. MIC results primarily from biological fouling of surfaces. By understanding the micro-organisms involved and the causative mechanisms, appropriate treatment and prevention strategies can be applied for the management of this problem.

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