Corrosion Deceleration Effect of Biofilms

Micro-organisms may not always enhance corrosion. The same bacterial species may show both corrosive and protective effects. For example, Hernandez et al. [56] reported the corrosive effects of two microbial species, one of which was Pseudomonas sp. By changing certain conditions, the very same micro-organisms were showing protective effects and slowing down corrosion. Those researchers also reported that in the presence of bacteria such as aerobic pseudomonades sp. and facultative anaerobic serratia marcescens in synthetic seawater, corrosion of mild steel is inhibited. The effect seemed to disappear with time in natural seawater. Jack et al. [57] reported monocultures of an aerobic Bacillus sp. that induced greater corrosion than that of an abiotic environment, but the rate of this corrosion decreased to that of a sterile control after 17 days. 

Iron-reducing bacteria (IRB) are a good example of the bacteria that can both accelerate and retard corrosion. These bacteria act by reduction of the generally insoluble Fe compounds to the soluble Fe , exposing the metal beneath a ferric oxide protective layer to the corrosive environment [58, 59]. Pseudomonas spp. are IRB species reported to have corrosive effects [60, 61]. However, there is an increasing body of evidence that IRB could actually slow down corrosion. 

Although Obuekwe had demonstrated the corrosivity of IRB, mainly on mild steel [31, 58, 65], other researchers [39, 51] found that some strains of pure IRB such as Shewanella could actually slow down the corrosion process. 

The core idea [63] here is that pure IRB can contribute to decelerating corrosion as the ferrous ions produced by the bacteria form a reducing shield that blocks oxygen from attacking the steel surface and acts like a protective coating. It 

Videla [67, see also [40], pp. 74-120, 193-196] has extensively reviewed probable mechanisms by which corrosion can be slowed down or inhibited by bacteria. In this respect, he addresses three main mechanisms that can be summarised as the following  

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However, IRB still have the power to surprise us Lee et al. reported that a mixed culture (biofilm) containing IRB (Shewanella oneidensis ) and SRB (Desul-fovibrio desuljuricans) that had been formed on mild steel could provide a shortterm (four days) protection to the steel [116]. As the authors put it, [t]he fact that an iron-reducing bacterium can inhibit corrosion when a corrosion-enhancing bacterium is present warrants future study with respect to its potential applicability to the design of biological corrosion-control measures . Such reports can lead us into another aspect of IRB a corrosion-inhibiting bacteria This matter is discussed in Section 5.2, Corrosion deceleration effect of biofilms of Chapter 5 and will not repeated here. 

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