microbiological corrosion

Lecture 26: MIC – Bacterial Transport, Attachment and Affected Materials NPTEL Web Course

1

Course Title: Advances in Corrosion Engineering

Course Co-ordinator: Prof. K. A. Natarajan, IISc Bangalore

Lecture 26

MIC-Bacterial Transport, Attachment and Affected

Materials

Keywords: Bacterial Adhesion, Biofilms, Structural Materials.

Bacterial transport to metal surface involve:

Fluid dynamic forces (currents in water bodies and eddy diffusion in

turbulent flow systems).

Flocculation or sedimentation.

Chemotactic response due to energy gradients.

Brownian motion (colloids).

Surface properties such as charge, free energy and roughness influence bacterial

adhesion. There can be reversible and irreversible adhesion. Many forces such as

electrostatic, chemical and hydrophobic forces may be involved in bacterial adhesion

mechanisms. The following stages can be visualised to understand a fully developed

biofilm on a metal surface.

Transport of organics from bulk.

Attachment and colonisation by bacteria.

Incorporation of higher organisms (fungi, algae, protozoa).

Build up of biofilms in thickness.


2



Besides its contribution towards MIC, biofilms can pose several other engineering

problems such as:

Reduction in heat transfer leading to energy loss (condenser tubes).

Reduction in mass and fluid transfer (water, oil, gas pipelines).

Structural failures (buildings, bridges, platforms and construction

materials)

Increased fuel and operating costs (ships and engines).

Several mechanisms and models have been proposed to understand biofilm

formation.

Aerobe-Anaerobe mutualism: Growth of aerobic bacteria such as Acidithiobacillus

and iron oxidizers utilizing oxygen and nutrients at the metal-solution interface

creating an anaerobic environment in the vicinity. Sulfate and other oxidised

metabolic products formed in the biofilm due to activity of such aerobes serve as

nutrients and energy source for anaerobic bacteria such as SRB which subsequently

proliferate in the anaerobic environment. Bacterial mutualism leads to the formation

and growth of a heterogeneous biofilm (often patchy and incoherent). Oxygen

concentration cells would be formed under such conditions as illustrated below in

Fig 26.1.


3



Fig. 26.1 Model for bacterial film formation on metals involving aerobic and anaerobic bacteria

Schematic representation of biofilm formation and consequent development of

differential aeration cells are shown in Fig. 26.2.

Fig. 26.2 Formation of differential aeration cells on metal surfaces due to biofilm growth.


4



Tubercles (massive bio – chemical deposits) can result with time. Extensive pitting

and cracks become visible under the biofilm. Anodic and cathodic reactions

pertaining to MIC of steels in marine or soil environments are illustrated in Fig. 26.3

and Fig. 26.4.

Fig. 26.3 Model for biocorrosion of ferrous alloys due to biofilm formation.

Fig. 26.4 Anodic and cathodic reactions in differential aeration cells formed on metal surfaces.


5



MIC of important structural materials

There are no known metals or alloys which can completely resist biofilm formation

and subsequent microbially-influenced corrosion. Behavior of various commonly

used metals and alloys in relation to microbially-influenced corrosion is outlined

below:

Copper and copper alloys. Commonly used in heat exchangers, pumps, valves and

condensers. 90-10 and 70-30 copper-nickel, brasses, aluminium bronzes and

admiralty brasses are used in marine environments. SRBs present in marine

environments contribute to localised corrosion of the above alloys. They are

susceptible to microbially-influenced corrosion of different kinds. Extra-cellular

polymers secreted by microorganisms can induce corrosion of copper-base alloys

through differential aeration, selective dissolution and cathodic depolarization.

Pitting, plug / dealloying and ammonia cracking of brasses and bronzes can occur.

Sulphate-reducing bacteria generate tubercles through formation of sulphide-rich

scales on copper alloys.

In spite of copper toxicity, copper and copper alloys are not free from biological

corrosion. Acidithiobacillus group of bacteria develop higher tolerance to copper

ions and dissolve the metal. Slime forming bacteria together with iron were isolated

from the corrosion products of copper-nickel alloy and monel tubes used in a nuclear

power plant. Sulphate reducing bacteria can corrode underground copper tubes and

pipes. Biologically generated ammonia is responsible for stress corrosion cracking

of several copper alloys. Corrosion of brass in heat exchanger tubes by ammonia

produced by bacteria is reported.

Steels. Tubercle formation with pitting underneath is encountered in steel pipes and

tubes, resulting in hampered flow and plugging problems. Carbon steels are used for

water, oil and gas transport under sub-soil and marine environments. Aerobic


6



bacteria such as Gallionella, Leptothrix and Acidithiobacillus contribute to MIC

resulting from differential aeration cells. These organisms oxidize ferrous to ferric

resulting in the deposition of ferric oxyhydroxides. Anaerobic bacteria such as

Sulfate Reducing Bacteria inhabit the tubercles.

Aerobic bacteria can bring about MIC through formation of slimes, oxidation of iron

and sulphides and generation of acidic metabolites. Hydrated slimes coat the metal

surfaces, creating differential aeration cells. Iron oxidising bacteria listed in Table

26.1, oxidise ferrous ions to less soluble ferric ions, leading to the formation of

insoluble tubercles, which consist of hydrated ferric oxides and biological slimes.

Steel water pipes are prone to such attack. Massive tubercle formation inside steel

pipes, hinders fluid flow, and creates severe corrosion problems, such as extensive

pitting, fissures and crevices.

Table 26.1 Role of slime – forming bacteria in metallic corrosion

Organism

Action

Gallionella Sp

Aerobic,

Iron & Steels, Tubercle

formation

Sphaerotilus Sp

Aerobic,

Iron & Steels, Ferrous oxidation

and tubercle.

Pseudomonas Sp

Aerobic,

Iron & Steels, (Some iron

reducing)

P.aeroginosa

Aerobic,

Aluminium alloys (pitting)


7



Stainless steels. Stainless steels are used in nuclear power plants in sea water

environments. Iron oxidising and depositing bacteria induce MIC of stainless steels

characterized by pitting, usually adjacent to weldments. SRB can attack stainless

steels, super stainless steels such as duplex steels and molybdenum steels. Slimes

formed by bacteria can create sites for initiation of pits in stainless steels in sea water

or fresh water. Destruction of passive films in stainless steels is observed through

reducing environments created by SRBs.

Nickel-based alloys. Monels and inconels are susceptible to MIC. Nickel-based

alloys used in nuclear power plants corrode due to microbial attack under marine

environments.

Aluminium and its alloys. Protective oxide (passive) films present on aluminium

and its alloys could be disrupted and destroyed through biological attack.

Aluminium and 2024, 7075 alloys used in aircraft and fuel storage tanks are

susceptible to MIC in the presence of hydrocarbons (fuels). The generation of water-

soluble organic/inorganic acids by bacteria and fungi lead to corrosion of aluminium

and alloys (pitting and intergranular corrosion). Aluminium-magnesium (5000

series) alloys used in marine applications are susceptible to pitting, intergranular

corrosion, exfoliation and stress corrosion through microbial interaction.

Aircraft fuel tanks and sea water components of aluminium and its alloys are

corroded by organisms such as Pseudomonas, Leptothrix, Sulphate Reducing

Bacteria and fungi. The fungus, Cladosporium resinae can proliferate on kerosene

or paraffins as sole carbon sources, developing pinkish brown colonies. Fuel tanks

of especially ground aircrafts are affected by fungal growth.


8



The following microorganisms had been observed in an aircraft tank sludge.

Pseudomonas aeruginosa.

Aerobacter aerogenes.

Clostridium,

Bacillus,

Desulfovibrio,

Fusarium,

Aspergillus,

Cladosporium and

Penicillium.

Titanium. Titanium is susceptible to biofouling. SRBs and acid-producing bacteria

may generate differential aeration cells leading to destruction of passive films.

Titanium and its alloys used in marine environments are susceptible to biofilm

formation involving manganese and iron oxidising bacteria as well as sulfate

reducing halophiles. Surface passive films on titanium could be disrupted in the

presence of anaerobes, leading to ennoblement and pitting.

microbiological corrosion

Documents