Biofilms And Corrosion

The main constituents of biofilm are the microbial cells, their extracellular polymeric substances (EPS), and inorganic precipitates. The latter originate from the bulk aqueous phase or are formed as corrosion products of the substratum. EPS, comprising macromolecules such as polysaccharides, proteins, nucleic acids and lipids, constitute the biofilm matrix and are often referred to as glycocalix or slime [7]. Microorganisms and/or their metabolites, e.g. enzymes active within the EPS matrix, organic and inorganic acids, as well as volatile compounds, such as ammonia or hydrogen sulphide, can alter electrochemical processes at the biofilm/metal interface [8].

2.1. Electrochemical corrosion

The physico-chemical interaction between a metallic material, and its environment that leads to changes in the material properties is termed corrosion [9]. The latter is an electrochemical process in which electrons are transferred from the metal, through a series of redox reactions, to an electron acceptor, often molecular oxygen, in contact with the metallic surface. As a result the metal is oxidized, which leads to its dissolution, and the electron acceptor is reduced. The most familiar form of corrosion is rusting of ferrous materials, which occurs when such materials are exposed to oxygen and water. The net chemical reaction of iron rusting can be expressed as follows:

The Fe (II) product of this reaction is further oxidized to Fe (III) and, under neutral conditions in the presence of oxygen, typically forms the amorphous solid Fe(OH)3. The latter can be converted to other iron oxides e.g. hematite (Fe203) and oxyhydroxides, e.g. goethite (a-FeOOH).

In most instances, the oxidation reaction slows to a low rate after a period of time because the oxidation products adhere to the metal surface and form an oxide/hydroxide layer that serves as a diffusion barrier to other reactants. These layers, referred to as passive layers, form a protective barrier to further oxidation of the underlying metal.

Environmental conditions can influence the equilibrium concentrations and diffusion rates of reactants and products of the oxidation reaction, metallic materials used for equipment fabrication are matched to the physical/chemical conditions under which the equipment is intended to operate. Changes in the environmental conditions can affect the stability of the passive layers and hence the susceptibility of the metal to corrosion. In general terms, any conditions that promote or retard metal oxidation will accelerate or inhibit corrosion, respectively.

2.2. Microbiaily-influenced corrosion

Due to the morphology of biofilms, which in most cases are present as a heterogeneous, non-continuous deposit or coating, on the metallic surface, MIC occurs as localized attack in form of pitting (Fig. 2). In water distribution systems, this type of corrosion often leads to extensive perforation of piping material.

Biofilm Corrosion

Fig.2. Scanning electron microscopy (SEM) micrograph demonstrating pitting corrosion of AISI 316 stainless steel tubing after the removal of 3 weeks old biofilm formed in the presence of bacterial consortium isolated from corrosion failure in cast iron potable water mains. The biofilm was grown in the laboratory under continuous flow conditions mimicking the flow regime, temperature and chemistry of water in the UK drinking water distribution system from which the bacterial consortium originated

Fig.2. Scanning electron microscopy (SEM) micrograph demonstrating pitting corrosion of AISI 316 stainless steel tubing after the removal of 3 weeks old biofilm formed in the presence of bacterial consortium isolated from corrosion failure in cast iron potable water mains. The biofilm was grown in the laboratory under continuous flow conditions mimicking the flow regime, temperature and chemistry of water in the UK drinking water distribution system from which the bacterial consortium originated

Although microorganisms detected within biofilms on corroded metallic materials include bacteria, fungi and algae, the majority of research efforts have focused on the role that bacteria play in corrosion processes. The main types of bacteria found on such materials are sulphate reducing bacteria (SRB), sulphur-oxidising bacteria (SOB), iron-oxidising/reducing bacteria (IOB/IRB), manganese-oxidising bacteria (MOB), as well as bacteria secreting organic acids and slime [4,8 and references therein]. Several recent reviews discuss in detail mechanisms by which SRB, SOB and MRB/IRB can promote corrosion of iron and ferrous alloys [10-12],

Bacteria representing all the groups listed above have been found in biofilms recovered from different types of piping material in water treatment and distribution systems [13, 14] and the deteriorating effect of these consortia on e.g. stainless steels, cast iron or copper has been demonstrated [15]. However, the contribution of any particular genera or species to the corrosion process has not been established. It is now acknowledged that bacteria form synergistic communities within biofilms that are able to catalyse electrochemical processes through co-operative metabolism in ways that a single species has difficulty to initiate and/or maintain [16]. It is, therefore, important to realize that in any given system, including drinking water distribution networks, biocorrosion is seldom linked to a single bacterial species or to a unique mechanism. Typically, both the aggressive and the inhibitory effects of bacterial populations on corrosion reactions are due to complex interactions, involving both biofilm and corrosion products on the material surface.

It is equally important to consider the fact that documenting the presence of microorganisms on a corroded metallic surface, even if they are species known to produce metabolic by-products aggressive toward metals, is not sufficient evidence for their contribution to the corrosion process [17]. Similarly, the number of biofilm microorganisms detected at a corroded site does not necessarily correlate with the extent of corrosion [18], It has been argued that the active metabolic capability of the microbes is most likely the key contributing parameter. To date, no direct evidence of a relationship between specific microbial metabolic rates and observed corrosion rates has been demonstrated, although emerging data strongly indicate that such a relationship exists [19-20],

2.2.1. Latest hypothesis of MIC

Studies of bacterial interaction with metallic materials led to the formulation of a unifying electron-transfer hypothesis of biocorrosion, using MIC of ferrous metals as a model system [11]. According to this hypothesis, biocorrosion is a process in which metabolic activities of microorganisms supply insoluble products, which are able to accept electrons from the base metal. This sequence of biotic and abiotic reactions produces a kinetically favored pathway of electron flow from the metal anode to the universal electron acceptor, oxygen. Although convincing and based on sound scientific evidence, this theory does not take into account (i) the part the EPS matrix plays in electron transfer nor (ii) the involvement of ultimate electron acceptors other than oxygen. Indeed, the theory has recently been challenged based on a study of marine biocorrosion of carbon steel under anoxic conditions [21]. A model demonstrating the likely involvement of biofilm EPS in electron transfer has also been proposed [8].

2.3. Biocorrosion viewed as a biomineralization process

A current trend in biocorrosion research focuses on biomineralization processes. Investigations of cell interactions with mineral surfaces is of obvious importance to MIC, as passive layers of oxides and oxyhydroxides formed on metallic surfaces, as well as abiotically and biologically produced corrosion products, including metal sulphides formed in the presence of SRB, are prime examples of such minerals. A number of recent studies report the use of state-of-the-art analytical techniques to elucidate the importance of biogenically formed nanocrystals in electron transfer processes, as well as in sequestering metal pollutants such as e.g. Pb (II) [22-24 and references therein].

Of particular interest to corrosion are processes leading to the dissolution of protective metal oxide/hydroxide films on metallic surfaces due to microbial dissimilatory reduction reactions i.e. using metal reduction as a mechanism for generating energy. The impact that bacterial metal reduction has on corrosion of iron and its alloys has been extensively reviewed [10, 25]. In the case of stainless steels, passive layers can either be lost or replaced by less stable reduced metal films that allow further corrosion to occur. Numerous bacteria are known to promote corrosion of iron and its alloys through dissimilatory reduction reactions [26], In contrast to assimilatory iron reduction, which allows incorporation (assimilation) of iron into proteins, dissimilatory iron reduction involves electron transfer to iron as part of the fermentative or respiratory pathways. Iron reducing bacteria are readily recovered from cast iron pipes in water mains, for example. One of the best examples of a microorganism causing biocorrosion due to iron reduction is Shewanella oneidensis, formerly classified as S. putrefaciens. This is an extensively studied, Gram-negative, facultatively anaerobic bacterium. It oxidizes various carbon substrates by reductively dissolving Fe(III)-containing minerals, such as ferrihydrite, goethite and hematite. The biocorrosion of steel in the presence of S. oneidensis has been documented [10]. The corrosion rate was first measured by Little et al. [27], who also showed that the rate depended on the type of oxide film under attack. In a later study it was confirmed that iron oxides, such as hematite, accumulated a higher density of S. oneidensis cells and showed a greater accumulation of Fe(II) than did magnetite [20].

2.4. Biocorrosion and bacterial species specificity

The study of Neal et al. [20], as well as other investigations, clearly demonstrated that in the presence of identical bacterial populations under the same growth conditions, different materials including metallic substrata, are colonised to a varied extent [4] (Fig. 3).

Furthermore, it is now accepted that the composition of the primary biofilms that develop on material surfaces depends on physico-chemical properties of the material and that different bacterial species preferentially colonize different materials [28 and references therein]. Studies have shown that biofilms developed more quickly on iron pipe surfaces than on plastic polyvinyl chloride (PVC) pipes. Fast colonization on iron occurred, despite the fact that adequate corrosion control was applied, i.e. the water was biologically treated to reduce assimilable organic carbon (AOC) levels and chlorine residuals were consistently maintained [29, 30]. It has also been reported that in water distribution systems, iron pipes supported a more diverse microbial population than did PVC pipes [31].

It is now recognized that the pipe surface itself can influence the composition and activity of biofilm population. Therefore, the choice of materials in water distribution systems is of paramount importance when one aims to reduce fouling and corrosion problems. A variety of materials in contact with water may leach materials that support bacterial growth. For example, pipe gaskets and elastic sealants (containing polyamide and silicone) can be a source of nutrients for bacterial proliferation. Pump lubricants should be non-nutritive to avoid bacterial growth in treated water [32], Coating compounds for storage reservoirs and standpipes can contribute organic polymers and solvents that may support growth of heterotrophic bacteria. Liner materials may contain bitumen, chlorinated rubber, epoxy resin or tar-epoxy resin combinations that can promote bacterial growth [33].

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