Marine Engineering - General, Marine Engineering - Motor

Microbial Degradation of lubricating oils onboard

Microbial Degradation of lubricating oils onboard

One of the vessel I served as Second Engineer suffered a sudden microbial degradation and infestation which caused USD 20000000 on engine repair and a 30 days of laid up with further consequent losses.

The ship was about 20 years old with no history of Microbial attack on lube oil and the lube oil samples were sent ashore and “green” reply without any remarks was received from shore based just 2 weeks before the actual MD occurred.

What is a Micro – Organism?


  • There are three basic types of microorganisms that cause problems in the marine industry, these are bacteria, yeasts and moulds.


  • Bacteria can be subdivided into;
    • Aerobic Bacteria – Requires oxygen to survive.
    • Anerobic Bacteria Live in the absence of oxygen
    • Facultative Bacteria Live with or without oxygen
  • Bacteria is a highly dissimilar group of single celled organisms with firm cell walls. They may be rod like, spherical or spiral and many are actively travelling with a whip like attachment.
  • They can reproduce asexually and rapidly using binary fission with a doubling time of as low as 20 minutes. They are design to reproduce rapidly, some are able to produce extremely resistant bacteria’s which can withstand high temperatures and disinfectants.
  • Although they prefer neutral or slightly alkaline environments some can exist in the extremes of acid. They can emit partial breakdown products on which other forms of bacteria can feed.
  • In addition they can produce large amounts of extra-cellular slime which coats and stabilizes the living environment. This slime can protect against or deactivate biocides. This slime can prevent the diffusion of oxygen to the base of the growth and thereby promote Sulphate Reducing Bacteria which are particularly aggressive.


  • Yeasts are unicellular, being ovoid or spherical in shape, some may also produce simple filaments.
  • They reproduce by budding and growing off the parent until large enough to separate. This process may take several hours.
  • Unlike bacteria yeasts prefer slightly acidic environment.



  • Moulds are Multi-cellular with hard cell walls.
  • Moulds are usually found as branched hyphae forming a thick, tough entangled mat occurring mostly at oil/water interfaces. They reproduce by branching and can double their length in a few hours.
  • Moulds prefer slightly acidic conditions, using oxygen in their feeding process and produce by products suitable for other microbes to feed and an atmosphere suitable for Sulphate Reducing bacteria.
  • Moulds reduce complex hydrocarbons to simpler carbon compounds. Intensive corrosion can occur under the mat. Moulds are both sea water and temperature tolerant


Sulphate Reducing Bacteria (SRB)

  • SRB are a specific group of anaerobic bacteria with special growth requirements. They can only use simple carbon compounds therefore they require the presence of other microbes.
  • They will produce hydrogen Sulphide in the presence of sulphur containing compounds such as sulphates found in sea water.


Ideal Growth conditions

* Micro-organisms (Bacteria, yeast’s and mould) are both aerobes (require O2) and anaerobes (flourish when O2 are not present).

* Microbes thrive in small amounts of water (condensation quantities) at the oil-water interface, they dislike movement and most are unable to grow below 10oC or survive above 70oC.

* Ideal growth conditions are at temperatures of 25- 40oC when conditions are stagnant and there are ample nutrients; phosphorous and nitrogen. These are available from Bilge water, coastal waters, Lub oil / fuel oil additives, some corrosion inhibitors, sewage and food wastes.

* Microbes likes to grow on a relatively neutral or slightly alkaline pH between 8 – 9.

* Under ideal conditions bacteria can double in size & divide into “two” every 20 minutes, onset of attack can be a matter of just weeks.



  1. Corrosion due to:

(a)Oxygen deficient zones setting up corrosion cells.

(b)Production of acids which cause direct corrosion

(c)SRB, producing hydrogen sulphide and associated ions which are highly aggressive to steel and yellow metals. Once the aerobic strains have consumed all the dissolved O2, the metal corroding SRB (Sulphate reducing bacteria) is activated which start attacking steel work.

  1. Stabilization of water content (usually acidic) in oil making it very difficult to remove.
  2. Attack lub oil base stock and additives with consequent changes in oil viscosity, and properties which reduce lubrication and load bearing capabilities.
  3. Blocking of filters and restriction of flow in pipework.



  • Microbes can be found in, distillate fuel, lub oil, cooling systems, bilge’s/retention tanks and ballast water. Infection may be from: Bunkers (Distillate fuel and lub oil), contaminated coastal water in bilge’s and ballast systems.


  1. Stable water content after purification.
  2. Increased acidity of oils.
  3. Unusual smells (rotten eggs).
  4. More frequent filter and valve plugging.
  5. Heat exchanger performance falling off.
  6. Sliminess of the oil, apparent in crankcases with paint work discoloration / removal. (Deposits on tank surfaces are the texture of wallpaper paste)
  7. Black staining of white metal bearings & copper alloys.
  8. Corrosion of unprotected steelwork, journals, fuel pumps, injectors and pitting of tank surfaces (SRB produce deep shinny pits)


Good housekeeping which will include:

  1. Avoid water accumulation by regular draining of tanks.
  2. Regular movement of oils and avoidance of ideal temperature conditions. (Layup conditions must give consideration to this)
  3. Avoidance of contaminated water ingress with regular pumping & disposal of bilge water.
  4. Correct operation of purifiers and use of renovating tanks.
  5. Testing for contamination using representative samples from tank bottoms, purifier inlet (heater off) / outlet, renovating and storage tanks, before main filters, coolant systems etc.


Expert help is required to treat infections which may include use of biocides, heat treatment and complete manual cleaning of system components & pipe lines. Environmental considerations must be made when disposing of dead microbes and associated water.


Representative sampling

It is vital that the oil tests are carried out on a representative sample, i.e. one that closely matches the oil which is normally flowing into the engine. The following procedure is recommended under the CIMAC code of practice which will be used in conjunction with normal codes of safe practice to ensure oil spray onto hot surfaces does not occur.

  1. a) Same sampling location to be used every time
  2. b) Preferred location is the main supply line into the engine. The sample should be drawn over at least a five minute period then decanted into the sampling container
  3. c) Under no circumstances should sumps, purifier suction/discharge lines, or drain cocks of filters/coolers be used for sampling
  4. d) Only draw samples when the engine has been running at its normal operating temperature for at least five minutes
  5. e) The sampling container should be unused and specifically produced for the purpose of oil sampling
  6. f) The sample should be fully identified with the date, vessel name, running hours, type of oil, and sampling point identification.




High water content is a definite indication of a water ingress into the system which could be a leaky piston cooling pipe, leaky heat exchangers, Purifiers malfunctioning, Leaky liner which leaks past the cross head.

Due to water ingress the lube oil is attacked by bacteria and microbes which convert the lube to be highly acidic this is the reason for the deterioration of the TBN value.


– In the first place, source of water contamination should be traced and eliminated.

– Water should be removed from the system after stopping the engine allowing the circulating oil time to settle down.

– In case a drain plug or valve is fitted it may be possible to drain out the water. Alternatively, consideration may be given to transferring the oil, using a drain or transfer pump or even a portable pump, into a settling tank, applying heating steam, settling and draining the water out.

-Water may be removed by centrifugal in case if the separator is connected to the engine sump.

One thought on “Microbial Degradation of lubricating oils onboard

  1. linus says:

    Thanks for sharing the best information. I am really enjoying reading your well written articles.

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