Bacteria busting secrets

Microbiologically influenced corrosion (MIC) is an electrochemical corrosion process which, specifically in pipelines, forms the majority of internal corrosion. However good it may have been studied under laboratory conditions, it is not, however, that well-recognised and applied under real life conditions. In this respect, we have identified six important misunderstandings about MIC related to concepts and definitions used in MIC from a practical point of view – from the very basic definitions, to where to expect MIC in pipelines, to the link between cathodic protection (CP) and MIC, to name a few.

Background

When I was asked to participate in a project as a corrosion expert for managing MIC in a 320 km underwater pipeline, it was not my first experience in corrosion troubleshooting, particularly related to post-hydrotest MIC. However, this particular project was very different since the pipe had been hydrotested with seawater. The hydrotest fluid had not been treated with any particular chemical treatment (corrosion inhibitors or biocides), and the water had been left inside the pipe as a wet lay-up practice for about five months. MIC had therefore developed. Additionally, in the water inside the pipe, the number of planktonic sulphates reducing bacteria (SRB) had multiplied after application of a so-called ‘multi-purpose’ inhibitor – this meaning that it is both a corrosion inhibitor (to control non-MIC corrosion) and a biocide (to control MIC).

Normally, in the case of wet lay-up, this water must not be the same as the hydrotest water. And if it has to be (to cut down the costs or the interval between flooding and service exceeding one month), then this water must have been treated with a suitable cocktail of corrosion inhibitor and biocide(s).

Therefore, this case was exceptional in not only lacking the required initial chemical treatment, but also after the hydrotest had been done. Furthermore, addition of the selected corrosion inhibitor had increased the number of free-swimming SRB. I was told that the cost (including purchase, communications, application) of wrong selection of the inhibitor was at least US$700 000 or more at the time …

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