Test your knowledge of Microbiologically influenced Corrosion(MIC)-Part 2

 Following the warm  reception of “Test your knowledge of Microbiologically influenced corrosion” article. I decided to add a "Part 2" to it. This part 2 is also coinciding with the release of our new book on the assessment of Hydrotesting (The image at the top of this note shows our book , published by CRC Press, USA, 2017). As before, first read the question and try to answer without looking at the answer! Good Luck !Hope to see you all for Part 3!

1.  As soon as a biofilm forms, it will be leading to Corrosion. True/False.

False. It normally takes a while for the formed biofilm to become  corrosive. For example, when its thickness reaches, a few micrometres, the under-biofilm conditions become anaerobic and thus suitable for anaerobic CRB such as SRB. In addition based on whether the seawater has been physically/chemically treated or not, it may take (on the average) from at least one week to one month or more for a biofilm that has been formed to become an aggressive , corrosive one.

2.  All MMM (Molecular microbiology methods) can detect all bacteria/Archaea related to corrosion (CRB/CRA). True/False.

False. While MMM-or also alternatively known as “Culture independent Methods”-are certainly more precise and thus more reliable than culture-dependent methods such as MPN (Most probable number), they also have their own limitations. For example, DGGE (Denaturing Gradient Gel Electrophoresis) cannot detect microorganisms such as Methanogenic Archaea

3.  When you are using seawater as hydrotest medium, you can take this water from anywhere/any depth  of the sea. True/False.

False. It is advised to take it from about 15 m below the seawater surface (to minimise the chances of getting enough nutrients) and about 15 m above the seabed surface (to minimise the chances of getting water with too high TDS)

4.  MIC always occurs as under-deposit corrosion. True/False

False. The term “Under-deposit corrosion” is in fact a misleading term as it just says “where” corrosion is happening not that by “which mechanisms” it is happening. If by "deposit" we mean “biofilm + corrosion products” , we must  also note that while a significant share of corrosion (typically about 90%) comes from these “deposits”, there is still an approximately 10% contribution to corrosion from planktonic bacteria as well. Therefore, it is prudent to monitor both planktonic (freely swimming) and sessile (motionless, stuck) bacteria.

5.  If you can keep water moving, the likelihood of MIC will decrease. True/ False

False. While o prevent biofilm formation, having a velocity above 5 ft/s (1.5 m/s) is necessary , it is not enough. In fact, relatively high speeds may actually help transferring nutrients to where bacterial colonies need it, thus increasing the risk of MIC.

6.  Copper is toxic to micro-organisms. True/False.

False. It is toxic if and only if it gets into the “body of the CRB and not entangled within the biofilm fabric, thus being prevented from being taken in by the bacteria.

7.  If SRB produces Fes (assuming that Stumper Model holds true), then solving  galvanic effect between the corrosion product and the substrate will not suffice to solve the corrosion problem. True/False.

True. Stumper Model (1923) states that Galvanic corrosion between the iron sulphide film and underlying steel explains the high corrosion rates observed. Nevertheless, this mechanism is not that valid anymore (in the new versions of MIC theory, EMIC, it is the bacteria itself that will play the role of cathode). There are two sets of  corrosion reactions (biological generation of FeS and the galvanic couple between the  iron sulphide and the steel substrate), and they are “series corrosion  reactions” with respect to each other. Here, the galvanic corrosion is the result of biological activity. Therefore, by applying a suitable biocide (or any of the other three MIC treatment  mechanisms), the microbiological part needs to be removed to solve the overall corrosion problem. Thus as long as there is no CRB that will produce FeS (assuming that CRB are the only source of iron sulphide production) no corrosion will happen.