8.4 Manipulating microbial communities to mitigate corrosion

Changing environmental conditions may alter the composition of the microbial community, possibly favoring the growth of microbes that are less corrosive. For example, after 4 months of nitrate addition, the number and activity of sulfate-reducing bacteria in biofilm samples were substantially reduced (Thorstenson et al., 2002). This was accompanied by an increase in nitrate reducing bacteria. 32 months of nitrate additions reduced the number of sulfate-reducing bacteria 2000-fold and sulfate reduction 50-fold (Thorstenson et al., 2002). The corrosion rate of steel coupons decreased from 0.7 to 0.2 mm/year.

Microbially produced antimicrobials and surfactants can inhibit corrosion. For example, a genetically engineered Bacillus subtilis strain that secretes antimicrobials, inhibited the growth of sulfate-reducing bacteria with a substantial decrease in corrosion rates (Jayaraman et al., 1999). B. licheniformis excreting γ-poly-glutamate reduced the rate of aluminum alloy corrosion by 90% compared to treatment with non-antimicrobial-secreting B. subtilis (Ornek, Wood, Hsu, Sun, & Mansfeld, 2002). Some bacteria can secrete negatively charged corrosion inhibitors, such as polyaspartate and g-polyglutamate, that can impede corrosion (Mansfeld, Hsu, Ornek, Wood, & Syrett, 2002).

As noted earlier, in aerobic environments, abiotic metal corrosion with oxygen as the electron acceptor can be extensive. Under aerobic conditions, some microbes that do not have high corrosion capacities can colonize metal surfaces and reduce corrosion rates by several orders of magnitude compared to sterile controls. Microbes with this corrosion-inhibiting capability include: Escherichia coli (Javed, Stoddart, McArthur, & Wade, 2013; Jayaraman, Cheng, Earthman, & Wood, 1997), S. oneidensis (Dubiel, Hsu, Chien, Mansfeld, & Newman, 2002), Bacillus sp. (Ornek et al., 2002; Qu et al., 2015), Pseudomonas sp. (Gunasekaran, Chongdar, Gaonkar, & Kumar, 2004; Obuekwe, Westlake, & Plambeck, 1987; Rajasekar & Ting, 2011; San, Nazir, & Donmez, 2014), Pseudoalteromonas sp. (Wu et al., 2016), and Vibrio neocaledonicus (Moradi, Xiao, & Song, 2015). Microbial respiration consuming oxygen and preventing its contact with the metal surface is typically considered to be the major mechanism that inhibits corrosion in aerobic environments (Pedersen & Hermansson, 1989). However, there is no consensus on this topic. The availability of organic nutrients rather than oxygen concentrations was found to be the primary influence on corrosion rates of steel by E. coli (Javed et al., 2013). In studies with variants of Pseudoalteromonas lipolytica that had different capabilities for the production of exopolysaccharide cellulose, the strain with lower dissolved oxygen in the culture had higher rates of corrosion (Liu, Guo, et al., 2018).

Microbes can produce mineral oxides that protect the metal surface from corrosion. Pseudomonas flava formed a phosphate film on the surface of steel, slowing corrosion rates (Gunasekaran et al., 2004). Aerobic Bacillus megaterium and Pseudomonas strains deposited a phosphate salt layer on steel that inhibited corrosion (Rajasekar & Ting, 2011).

The ability of biofilms to protect against corrosion is not universal for all metals. Several studies noted that biofilms that protected some metals had no effect, or even accelerated corrosion of other alloys (Juzeliunas et al., 2006; San et al., 2014). An additional complication to designing strategies to prevent corrosion with biofilms is the fact that there are substantial contradictions in the literature over whether some aerobic microbes, such as Bacillus, Pseudomonas, and Pseudoalteromonas species protect against corrosion or accelerate it.

One of the most effective approaches to protect metallic materials from corrosion may be biomineralization (Guo et al., 2021, 2019; Liu, Guo, et al., 2018). In this process (Fig. 16) metal ions are electrostatically adsorbed to anionic cell surfaces of P. lipolytica and complexed in the exopolysaccharide surrounding the cells, providing nucleation sites for carbonate precipitation. The carbonate precipitates mainly as calcite and/or dolomite with trigonal rhombohedral structures, forming a protective film that prevents corrosion. Biomineralized films have the added potential of self-repair if damaged.

Ethylene Diamine Tetraacetate Acid Tetrasodium Salt (EDTA-4Na)

 

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Fig. 16. An exopolysaccharide (EPS)-producing strain of P. lipolytica induces calcite biomineralization on a steel surface. (A) SEM, (B) AFM, and (C) TEM images of the biomineralized film on day 14. Designated in panel (C) are nanometer-sized precipitates (labelled as c) embedded inside EPS that is attached to the bacterial cell wall (designated bc). (D) High-resolution transmission electron microscopy revealed that the nanometre-sized precipitates were polycrystalline with at least three planes: (012), (104), and (110). Selected area electron diffraction pattern (inset) showed that the nanometer-sized precipitates embedded in EPS were a mixture of polycrystalline and amorphous structures.

Anticorrosion strategies involving the manipulation of microbial communities that have been demonstrated in the lab will require significant additional investigation before they can be translated to field applications. The pleiotropic nature of microbes must be considered, microbes that inhibit corrosion under some conditions may accelerate corrosion under other conditions. Small environmental perturbations such as changes in water flow, temperature, pH, and nutrient concentrations might transform protective biofilms into corrosive biofilms. Although the deposition of biomineralized protective films onto metals may alleviate many of these concerns, they are susceptible to acidic conditions, somewhat limiting their scope.