Over the past decades MIC or Microbiologically Influenced Corrosion was recognized as a separate corrosion form besides general corrosion, galvanic corrosion, crevice corrosion, pitting, intergranular corrosion, erosion corrosion and stress corrosion. Today MIC should be considered as an extra parameter, a biological element amplifying the abiotic electrochemical corrosion process. MIC refers to the influence of microorganisms on the kinetics of the corrosion processes of metals. This accelerated type of corrosion may not be associated with one specific organism but with a collection of bacteria co-existing at the same time at the same place forming a microbial consortium. The main type of bacteria generally associated with corrosion or iron or steel are sulfate reducing
bacteria (SRB), Sulphur-oxidizing v-bacteria (SOB), iron-oxidizing/reducing bacteria (IOB/IRB), manganese oxidizing bacteria and bacteria secreting organic acids and extracellular polymeric substances (EPS) or slime.
The classical mechanisms for microbial influenced corrosion can be reviewed as follows:
1. Metabolic production of aggressive compounds
2. Oxygen concentration cell formation
3. Acceleration of anodic or cathodic reactions by depolarization effect
4. Hydrogen embrittlement (depolarization).
Ballast water discharged by ships is generally identified as a major pathway for introducing species to new environments. The effects of the introduction of new species have in many areas of the world been devastating. The upcoming IMO ballast water management convention (2004) (maybe in force this year?) will try to call a hold to this explosive situation. Today, ships exchange their ballast water in the middle of the ocean but in the future, all ships will have to install a ballast water treatment
system on board to rule out these organisms effectively. One of the possible techniques is to sterilize the ballast by means of UV light. The D-2 ballast water treatment system shall have an efficacy of;
• not more than 10 viable organisms per m³ ≥50 micrometers in minimum dimension, and
• not more than 10 viable organisms per milliliter < 50 micrometers in minimum dimension
and ≥10 micrometers in minimum dimension.
Indicator microbe concentrations shall not exceed:
• toxicogenic vibrio cholerae: 1 colony forming unit (cfu) per 100 milliliter or 1 cfu per gram of zooplankton samples;
• Escherichia coli: 250 cfu per 100 milliliter
• Intestinal Enterococci: 100 cfu per 100 milliliter
Bacteria are smaller than the organisms discussed above and technically are not taken into account when evaluating the efficiency of a D-2 ballast water management system. However, we think and expect, that by means of this experiment we will be able to prove that the bacteria causing MIC are killed or rendered infertile by a ballast water management systems using UV. If this is indeed the case UV ballast water treatment systems will stop or at least retard MIC. Four groups of bacteria involved in MIC will be cultivated in a standard Postgate "B" medium and diluted with seawater before being pumped through an experimental set-up consisting out of a glass tube spiraling around a UV lamp. The exposure time will be regulated by varying the transit time. The flow-through will be passed over a steel coupon, which will then be cultivated for several weeks in artificial, non-shaken conditions. Biofilm formation will be followed up.
In a second similar set-up the glass tube will be coated with a titanium dioxide film. TiO2 will act as a catalyst on the UV radiation effect. If this can be established, it should be possible to reduce the exposure times and realize and economize a lot of energy in the future ballast water management systems.
The "tour de force" of this experiment is that with one system, UV radiation of ballast water, two problems may be solved, MIC of ballast tanks and the carriage of invasive species via ballast water.