Deterioration Due to Biofilms

7/30/2019 Deterioration Due to Biofilms

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Biofilms are a collection

of microorganismssurrounded by the slime

they secrete, attached toeither an inert or living

surface.


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Surface associated microbial

growth, like biofilms, influencethe physico-chemical interactionsbetween metal and environment,

frequently leading to deteriorationof the metal such as corrosion in

stainless steel piping systems


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STEPS IN BIOFILM DEVELOPMENT

The instant a clean

pipe is filled withwater, a biofilm beginsto form.


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Surface Conditioning First substance to associated with surface is trace

organic.

Almost immediately after the clean pipe surface comesinto contact with water, an organic layer deposits onthe water

form a conditioning layer


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Adhesion of pioneer bacteria some of theplanktonic (freefloating) bacteria will

approach the pipe wall

entrained within the boundary layer, the pipe wallwhere flow velocity falls to zero


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Glycocalyx or slime formationBiofilm bacteria excrete extracellularpolymeric substances,or sticky

polymers, which hold the biofilmtogether and cement it to the pipe wall.

As nutrients accumulate, the pioneercells proceed to reproduce.


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In a mature biofilm, more of the

volume is occupied by the looselyorganized glycocalyx matrix thanby bacterial cells

Glycocalyx matrix holds a lot of

water, a biofilm-covered surface isgelatinous and slippery


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Secondary colonizersThe glycocalyx net also snares

other types of microbial cellsthrough physical restraint andelectrostatic interaction


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Fully functioning biofilmThe mature, fully functioning biofilmis like a living tissue on the pipesurface.

As the film grows to a thickness thatallows it to extend through theboundary layer into zones of greater

velocity and more turbulent flow, somecells will be sloughed off.


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Iron pipes in a

potable water distribution system


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MICROBIOLOGICAL

INFLUENCEDCORROSION (MIC) or

BIOCORROSION


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Figure 2. The colonization by bacteria results in differential aeration cells. Thisschematic shows pit initiation due to oxygen depletion under a biofilm.(Borenstein 1994)


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Stainless steels Protective Film Oxygen depletion at the surface of stainless steel can

destroy the protective passive film.

Corrosion occurs when the oxide film is damaged oroxygen is kept from the metal surface bymicroorganisms in a biofilm.


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Sulfate-reducing Bacteria Oxygen depletion at the surface also provides a

condition for anaerobic organisms like sulfate-reducing bacteria (SRB) to grow.

They reduce sulfate to hydrogen sulfide whichreacts with metals to produce metal sulfides ascorrosion products.

Aerobic bacteria near the outer surface of thebiofilm consume oxygen and create a suitablehabitat for the sulfate reducing bacteria at themetal surface.


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SRBs can grow in water trapped in stagnant areas, such

as dead legs of piping. Symptoms of SRB-inf luenced corrosion are hydrogen

sulfide (rotten egg) odor, blackening of waters, andblack deposits.

The black deposit is primarily iron sulfide.

Deterioration Due to Biofilms

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