Technical Presentations at the October 2010 Meeting

1.1   Strategies for Suppressing Microbial Growth Causing MIC’, Edward Hill & Graham Hill, ECHA Microbiology Ltd

For the purpose of this short presentation, suppression of MIC will only be considered in ship and off shore lubrication systems, fuel systems, and cooling water systems, and in ship bilge and ballast water.  There are other specific locations where MIC can be occurring in the marine industry and the general issues discussed today will largely be applicable there also.

Theoretically, there are several categories of practical antimicrobial strategies, namely:

·        Good system design

·        Good housekeeping to prevent ingress of contaminating microorganisms

·        Prevention by creating an unfavourable environment for microbial growth and activity

·        The application of physical procedures to kill or remove microbes

·        The application of anti-microbial chemicals

·        Inactivation of corrosive agents involved in MIC

Good system design ensures that water and sludge can readily be removed, that there are no dead legs or surplus pipes/tanks where undisturbed growth could occur, and that clean and ‘dirty’ fluids can be segregated so that no cross contamination occurs.

Good housekeeping will vary according to particular systems and materials but could include regular and thorough removal of water from fuels and oils, prevention of stagnation and prevention of cross contamination.

Unfavourable environments can be created by avoiding the use of or by depleting essential microbial nutrients, and also by manipulating parameters such as pH, temperature, water activity (relative humidity) and Redox potential.  When MIC occurs spontaneously where it has not occurred before, it is usually because there has been an inadvertent change in one of these.  For example, if a lower concentration of anti-freeze is used in a coolant, this may change its water activity from one which suppresses microbial growth to one which actively sustains it.

Physical removal of microbes can be as simple as filtration (of microbial aggregates only), or by any cyclone, centrifuge, or settling device which can utilise the high density (c. 1.05 g/cm3) of microbes compared to water or mineral oils, to separate and segregate them.  Physical disinfection methods can be UV (water systems only), but are more often batch or in line heating.  Other exotic methods exist, such as hard irradiation and ultrasound, but are theoretical rather than practical on a large scale.

The use of anti-microbial chemicals introduces a host of technical and regulatory issues.  Technically they must be appropriate to the target organism, compatible with the fluids and materials in the system treated, have appropriate regulatory approvals, and endorsed by machinery and systems designers, and last but not least, capable of safe use and safe disposal.  There is no wonderful magic chemical bullet which kills all of the microbes all of the time, and active chemicals and appropriate concentrations and formulations of them, must be selected and applied according to particular circumstances.

Killing or removing microbes does not necessarily stop MIC and it mat be necessary to remove corrosive agents such as acids or hydrogen sulphide.

The choice of strategies may seem bewildering, more so if several strategies are deployed at the same time.  The choice will be discussed in relation to the marine MIC locations outlined above.  There is no guarantee of success and monitoring for microbes and MIC with on-site tests, to validate efficacy of the strategies implemented, should be an integral part of MIC suppression in systems at risk.

Experience indicates that the introduction of eco-friendly fuels, lubricants and additives is increasing risk of microbial growth and MIC and it should be anticipated that new problems will arise, as they have done several times over the last few years.

 

  1.2  Obvious and Not So Obvious Cost Savings Using Ship-borne ICCP’, Barry Torrance, Aish Technologies Limited

This presentation looked at the potential for savings in three main areas, by means of examples and case studies.  Causes and remedies for the corrosion-related signatures:  Power Frequency ELFE (Extremely Low Frequency Effect), Shaft-Related ELFE, Underwater Electric Potential and Corrosion-Related Magnetic were discussed.  Typical cost comparisons were made between 1-zone and 3-zone ICCP systems.

By specifying an ICCP system with a relatively large number of anodes and running the anodes at a relatively low maximum voltage, it is possible to enhance:

·        SURVIVABILITY by reducing corrosion-related signatures

·        AVAILABILITY by reducing the amount of time out of service due to corrosion

·        SUSTAINABILITY by reducing a vessel’s carbon footprint

OVERALL COST due to reduced running costs and corrosion-related maintenance/repair costs.

 

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  4.1  An Investigation on the Erosion-corrosion Mechanisms of UNS S31603 using Focused Ion Beam and Transmission Electron Microscopy’, Shiva Rajahram, University of Southampton

Erosion-corrosion causes problems to many industries due to the synergistic interaction between these processes.  A semi-empirical model developed at the University of Southampton has been used to evaluate erosion-corrosion of stainless steel UNS S31603.  It was found that the model predicted high synergistic interaction, indicating that the accelerated corrosion due to oxide film removal was not the only synergy mechanism present.  The aim of this work was to study the microstructure of UNS S31603 subjected to erosion-corrosion to inform the modelling process.   Electrochemical noise measurements were performed to study the effect of velocity, sand size and sand concentration on the corrosion current.  The rise in current levels during solid particle impact was due to the erosion enhanced corrosion synergistic effect.  

Post-test analysis was done using SEM, FIB and TEM.  SEM analysis on the surface revealed that each particle impact cuts into the material to form lips and craters and these features start to overlay each other after a short period.  The generation of surface roughness due to this process is believed to affect the adherence of the passive film and generates microgalvanic sites on the metal surface.  Micro-cracks were observed running from the surface into the subsurface along with embedment of particles and oxide film.  The density of cracks was observed to be significantly lower in the pure erosion sample compared to the erosion-corrosion sample, indicating that the corrosive fluid accelerates crack propagation. Physical models have been developed to explain these mechanisms.

[S.S. Rajahram*, T.J. Harvey, J.C.Walker, S.C.Wang, R.J.K. Wood; National Centre for Advanced Tribology, School of Engineering Sciences, University of Southampton, UK.  *: ssr1y07@soton.ac.uk]

 

 4.2   Corrosion of Copper-30% Nickel Submarine Seawater Cooler Tubes’, John Galsworthy & Robin Oakley, QinetiQ Ltd

The copper-30% nickel alloy has been used for submarine seawater cooler tubes for many years.  The alloy forms a thin, adherent, cuprous oxide when exposed to clean, oxygenated seawater which can offer corrosion resistance.  The deliberate formation of this protective surface layer on new or cleaned tubes prior to service is known as 'conditioning'.  Recent leaks in new and cleaned cooler tubes have been experienced after only short periods of operation.  These tubes have been found to contain deep, isolated, penetrating pits.  The effectiveness of conditioning treatments and the nature of the corrosion have been questioned.  Microbially Influenced Corrosion (MIC) has been suggested as the cause. This paper presents some of the studies conducted in support of this problem. It includes the evaluation of various conditioning treatments and other studies to improve the understanding of the observed problem.

© Copyright QinetiQ limited 2010 [John C. Galsworthy and Robin S. Oakley, QinetiQ ltd. Cody Technology Park, Farnborough, GU14 0LX, UK]

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