Corrosion deceleration

IRB and Their Corrosion-Decelerating Effect A very interesting feature related to IRB is that under certain conditions, they can actually decelerate corrosion instead of accelerating it. This feature of IRB, while it is certainly worthwhile to be noticed for research, may not appear to be that useful from an industrial application point of view. 

Therefore corrosion deceleration could be the result of either one or a combination of these mechanisms. These three mechanisms can successfully explain most of the cases mentioned here. Considering the possibility of having one or more of these mechanisms in place, it seems the bacteria can play a different role in corrosion. 

Works by researchers on the slowing down of corrosion by IRB cultures [39, 68] postulate that for batch culture of IRB there is a chance for corrosion deceleration instead of acceleration, due to an increased number of ferrous ions thus produced because of the reduction of ferric ions by these bacteria. These ferrous ions can also combine with oxygen to form more ferric ions and mean-... 

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. 

Transfer of Momentum Deceleration of one fluid (motivating fluid) in order to transfer its momentum to a second fluid (pumped fluid) is a principle commonly used in the handhng of corrosive materials, in pumping from inaccessible depths, or for evacuation. Jets and eductors are in this categoiy. 

The effect of impurities in either structural material or corrosive material is so marked (while at the same time it may be either accelerating or decelerating) that for rehable results the actual materials which it is proposed to use should be tested and not types of these materials. In other words, it is much more desirable to test the actual plant solution and the actual metal or nonmetal than to rely upon a duphcation of either. Since as little as 0.01 percent of certain organic compounds will reduce the rate of solution of steel in sulfuric acid 99.5 percent and 0.05 percent bismuth in lead will increase the rate of corrosion over 1000 percent under certain conditions, it can be seen how difficult it would be to attempt to duplicate here all the significant constituents. 

Furthermore, the electroosmotic outward flow of water molecules, which follows the anodic hydrogen ion transport, counteracts the inward diffusion of water molecules into the occluded solution. The dehydration of the occluded solution will then occur as the corrosion progresses. Since metal dissolution requires water molecules for metal ions to hydrate, the depletion of water molecules will finally result in the deceleration of metal corrosion. The cation-selective rust layer, therefore, will be preventive of the corrosion of underlying metals. 

Particularly noteworthy is a marked deceleration in the corrosion rate in a sterilized soil as 1,87x10 m/Ms of corrosion rate was reduced to 1x10 m/Ms, which clearly indicating the contribution of lOB and/or SOB to accelerated corrosion. 

We have been using the term corrosion-related bacteria throughout this chapter without really defining it. It must be noted that bacteria do not always accelerate corrosion, but under certain circumstances, as mentioned earlier, bacteria can decelerate corrosion. That is why we preferred the term corrosion-related bacteria to address both corrosion accelera-tion and deceleration by these microorganisms. 

This chapter will deal with MIC, its definition and importance, and how historically both our understanding of and research methods for the study of MIC have evolved. We will then have a look at the parameters that can be used for categorising bacteria, and also the steps involved in biofilm formation. After discussing the ways by which biofilms can both accelerate and decelerate corrosion, we will look at three examples of bacteria that are involved in corrosion, the well-known SRB (sulphate-reducing bacteria), the rather shy , infamous IRB (iron-reducing bacteria) and almost unknown magnetic bacteria. 

To understand how biofilms can accelerate or decelerate corrosion, an understanding of the structure of biofilms is necessary. Several models have been proposes to explain biofilm stmctures. Some models are described very briefly below. 

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... 

The inhibitory effect of halogenide ions on the active dissolution of iron was first reported in 1930 by Walpert, who found that addition of halides to sulfuric acid solutions decreased the corrosion rate, the decelerating action increasing with the halide concentration. A summary of the experimental results reported by various authors since that time is given here. 

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