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Microbiological contribution to accelerated low water corrosion of support piles

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Author(s): Dr. Reza Javaherdashti, Senior Corrosion Engineer, Extrin Consultants, Perth, Australia, (on behalf of Nace International)

Introduction
Accelerated low water corrosion (ALWC) is a complex type of corrosion as it not only involves purely electrochemical factors, it also is greatly affected by microbiologically influenced corrosion (MIC). While some definitions of ALWC do consider MIC as an integral part of the definition, emphasising the importance of the role played by corrosion-enhancing bacteria, it is still open to discussion how MIC can enhance severity of this type of corrosion.

ALWC has been reported to cause severe corrosion of mainly steel piles in ports and jetty structures all around the world such as USA, Europe and Australia. In many cases of ALWC, microbial corrosion manifest itself as an orange mass known as ‘orange bloom’ that schematically can be shown in Figure 1. Figures 2a to 2d show real life examples of the orange blooms. Figures 3a and 3b show two more examples of orange blooms as appearing under-water with the deteriorating effects of orange blooms on the steel material of the piles. The metal underneath the orange blooms can be highly deteriorated. Figures 4a and 4b show such instances.

It has been suggested that some groups of corrosion-enhancing bacteria such as sulphate reducing bacteria (SRB) and Iron reducing bacteria (IRB) may play an important role in ALWC problems, Figure 5.

However, there are some issues that when taken to consideration, may further increase the complexity that MIC can induce to ALWC. Of these issues is possible involvement of other types of corrosion-enhancing bacteria such as iron bacteria (IB) as their growth and corrosion activity can be supported by biochemical nature of seawater. The other issue is the rather false reputation that routine laboratory-based methods for investigation of numbers and intensities of corrosion-enhancing bacteria have created among professionals who have little hands-on experience on MIC-assisted issues.

This ‘over-relying’ on these methods may br ing about technical misunderstandings and ‘myths’ that would result in misinterpretations and thus wrong synopsis of the corrosion-case…

 


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