lecture 24 microbially influenced corrosion (mic...

Lecture 24: MIC – Definitions, Environments and Microbiology NPTEL Web Course

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Course Title: Advances in Corrosion Engineering

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

Lecture 24

Microbially – Influenced Corrosion (MIC) – Definitions,

Environments and Microbiology

Keywords: Microbial Corrosion, Microorganisms, Biofouling.

Introduction

Microbially-influenced corrosion (MIC) occurs in environments such as soil, fresh

water and sea water and accounts for more than 30 percent of all corrosion damage

of metals, alloys and several building materials. Microorganisms of interest in MIC

belong to many types such as sulfur-sulfide oxidising, sulfate-reducing, iron

oxidising, acid producing, manganese fixing and ammonia and acetate producing

bacteria and fungi. The role of Sulphate Reducing Bacteria (SRB) in MIC has been

extensively studied. Microbial activities under natural conditions influence many

electrochemical reactions directly or indirectly. Microbe-metal interactions involve

initial adhesion, biofilm formation and colonisation, generation of polymeric

substances and inorganic precipitates and subsequent corrosion.

Microbiological as well as physico-chemical and electrochemical aspects of

microbially-influenced corrosion are analysed critically. Monitoring, diagnosis and

prevention of MIC is illustrated along with suggested remedial strategies.


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Seawater, fresh water and soil as corrosive media

Sea water is an aggressive corrosive medium for biofouling and microbially-

influenced corrosion (MIC). It contains about 3.4% salt and is a good electrolyte that

can lead to galvanic and crevice corrosion. The rate of corrosion in seawater is

influenced by oxygen content, temperature, velocity and microorganisms. Galvanic

series for metals and alloys in flowing seawater could be used to predict potential

corrosion involving metallic couples.

Similarly, fresh water and sub-soil environments are conducive for microbial life

leading to biofouling and MIC.

With reference to biofouling, copper and copper-base alloys are more resistant

compared to other ferrous alloys.

Definition and practical significance

The role of microorganisms in the deterioration and failure of materials can be

classified into Biofouling, Biodeterioration and Biocorrosion or Microbiologically-

influenced corrosion(MIC). The above terms could be complementary in their

ultimate consequences. Biofouling refers to adhesion of micro- and macro-organisms

onto material surfaces in marine, fresh water and soil environments leading to

formation of fouled layers. Deterioration of nonmetallic materials like glass,

concrete, cement, rubber, wood and plastics in the presence of microbes is termed

biodeterioration. Corrosion of metals and alloys induced by the activities of

microorganisms is defined as Microbially-influenced corrosion (MIC). The general

definition for corrosion can be invoked in this case also by adding the superimposed

microbiological forces.


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Microorganisms are omnipresent and grow and reproduce at amazingly rapid rates in

soil, water and air. The organisms exhibit extreme tolerance to hostile environments

such as acidic and alkaline pH, low and higher temperatures as well as pressure

gradients. Aggressive environments are generated by microorganisms, promoting

direct or indirect corrosion. As early as in 1891, corrosion of lead sheathed cables

was suspected to be caused by bacterial metabolites. Sulphur and iron sulphide

accumulation at the interior and exterior portions of water pipes were attributed to

the action of iron-sulphur bacteria during early 1900s. Anaerobic corrosion of

bacteria was first reported in 1931. Tubercle formation due to microbial growth and

reaction products has been reported almost forty years ago. However, a better

understanding of MIC processes based on microbiological and electrochemical

mechanisms, became available only since the last three decades.

The practical significance of microbial corrosion can be seen from Table 24.1, where

some industrial situations susceptible to microbial corrosion are listed. The extent of

microbial corrosion processes is evident from the fact that many of the commercially

used metals and alloys such as stainless steels, nickel and aluminium-based alloys

and materials such as concrete, asphalt and polymers are readily attacked by

microorganisms. Protective coatings, inhibitors, oils and emulsions can be

biodegraded.


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Table 24.1 MIC in industrial environments

Nuclear and thermal power plants

Subsoil pipe lines

On-shore, off-shore oil and gas processing.

Chemical industries

Civil engineering

Water treatment and metal working

Aviation (Defence and Civil)

Mining and metallurgical operations

Cooling water tubes and pipes, sub-sea pipe

lines, stainless steel and carbon steel, copper-

alloys, aluminium-alloys

Steels

Steels, Aluminium alloys

Pipelines, Tanks, Condensers, Joints, heat

exchangers.

Concrete in marine, fresh water and sub-soil

conditions, bridges, buildings.

Heat exchangers and pipes, Breakdown of oils,

emulsions and lubricants

Aluminium fuel tanks

Underground machinery and engineering

materials.


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A few cases of microbially-influenced corrosion reported more specifically in

systems or components in power plants are listed in Table 24.2.

Table 24.2 MIC in power plant materials

Heat exchanger tubing

Aluminium brass, 70:30 Copper-Nickel,

90:10 Copper-Nickel

Pitting

Water storage tank

316 stainless steel

Rust, weld

corrosion

Water pipes

316 stainless steel weld

Pitting

Cooling towers

Pumps

Galvanised steel

Stainless steel

General corrosion

Crevice, pitting


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Relevant Microorganisms

Microorganisms that are known to cause corrosion can be grouped as shown in Table

24.3.

Table 24.3 Microorganisms involved in MIC

1.

Bacteria

Sulphate Reducing Bacteria (SRB)

Desulfovibrio

Sulphur Oxidising and acid producing bacteria.

Acidithiobacillus

Iron Oxidising Bacteria (IOB) and metal

depositing bacteria

Gallionella, Crenothrix, Leptothrix

Metal reducing bacteria

Pseudomonas, Shewanella..

2.

Fungi

Cladosporium resinae

Aspergillus niger

Aspergillus fumigatus

Penicillium cyclospium

Paecilomyces varioti

3.

Algae

Blue green algae

4.

Microbial

consortia

Symbiotic activity among different groups of

microorganisms


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The sulphur cycle in nature is important to MIC. Sulphur and sulfide oxidising and

sulphate reducing bacteria (SRB) are involved in a number of biogenic redox

reactions leading to products such as H2S, metal sulphides and sulfoxy compounds.

All these microbially - intermediated processes participate in corrosion processes in

soils and aqueous environments. For example, sulphate reducing bacteria like

Desulfovibrio reduce sulphate to sulphide and hydrogen sulphide, under reducing

conditions.

SO=

4 + 4H2 S= + 4H2O

2H+ + S

- - = H2S

Sulphur (sulphide) oxidizing and sulphate reducing bacteria (SRB) involved in the

biological sulphur cycle in natural environments are shown in Fig. 24.1.

Fig. 24.1 Biological sulphur cycle in nature


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Sulphur and ferrous iron-oxidising bacteria such as Acidithiobacillus thiooxidans and

Acidithiobacillus ferrooxidans are acidophilic and aerobic promoting oxidation of

sulfur and sulfides.

2H2S + 2O2 = H2S2O3 + H2O

5Na2S2O3 + 8O2 + H2O = 5Na2SO4 + H2SO4 + 4S

4S + 6O2 + 4H2O = 4H2SO4

Fe++

= Fe+++

+ e

Acidithiobacillus bacteria can exist over a range of pH from acidic, to alkaline

conditions. For example, Thiobacillus thioparus could oxidise sulphur, sulphide and

thiosulphate at a pH of 6-10. Microbiological features of some thio-bacteria

involved in MIC are illustrated in Table 24.4.

Morphological features of some bacteria implicated in MIC along with typical

growth curves are illustrated in Fig 24.2 to 24.11.

All these bacteria are implicated in microbial corrosion processes and their growth

characteristics and metabolic reactions are important in understanding corrosion

mechanisims.

0 20 40 60 80 100 120 140 160

0

1x108

2x108

3x108

4x108

5x108

6x108

Nu

mb

er

of

cells

/ml

Time (min)

cell count

-250

-200

-150

-100

-50

0

50

EE

SE i

n m

v

Sulphate

concentration

0.6

0.8

1.0

1.2

1.4

1.6

1.8

su

lph

ate

co

ncen

trati

on

(g/L

)

EESE

0 10 20 30 40 50 60 70 80

108

109

No

.of

Cells / m

L

Time (hours)

Fig. 24.3 Cell number as a function of time

during growth of Bacillus subtilis Fig 24.2 Bacillus

subtilis


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Fig. 24.6

Acidithiobacillus Sp

0 10 20 30 40 50 60 70

4.0x107

8.0x107

1.2x108

1.6x108

2.0x108

No

.of

cells / m

L

Time (hours)

Cell count

250

300

350

400

450

500

550

ESCE

ES

CE

in

mV

1.9

2.0

2.1

2.2

2.3

2.4

2.5

pH

pH

0 10 20 30 40 50 60 70

0

2

4

6

8

10

Fe 2

+ a

nd

Fe

3+ c

on

c (

g / L

)

Time (hours)

Fe3+

Fe2+

Fig 24.7 Cell number, pH, ESCE as a function of time during growth of

Acidithiobacllus sp

Fig 24.8 Ferrous and ferric concentration as a function of time during growth of

At.ferrooxidans

Fig 24.4 Sulphate reducing bacteria

Fig 24.5 Cell number, SO4 conc and ESCE as a

function of time during growth of Sulphate reducing

bacteria

Fig. 24.10 Cell number as a function of

time during growth of At. thiooxidans

Fig. 24.11 pH & SO4 conc. as a

function of time during growth

of At. thiooxidans

0 50 100 150 200 250 300

0.0

2.0x108

4.0x108

6.0x108

8.0x108

1.0x109

1.2x109

Nu

mb

er o

f cells / m

L

Time (Hours)

Cell count

0 50 100 150 200 2500.3

0.6

0.9

1.2

1.5

1.8

2.1

pH

Time (Hours)

pH

0

4

8

12

16

20

24

28

Su

lph

ate

co

ncen

trati

on

(g

/ L

)

Sulphate conc.

Fig. 24.9 Acidithiobacillus

thiooxidans


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Fig. 24.12 to Fig. 24.14, illustrate typical morphological features of fungi such as

Cladosporium and Aspergillus besides those of an iron and manganese oxidizing

bacteria.

Fig. 24. 12 Cladosporium

resinae

Fig 24.14

Gallionella spp

Fig. 24.13

Aspergillus spp


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Morphological features of Aspergillus,SRB and Acidithiobacllus are more

revealingly illustrated in Fig. 24.15.

Fig. 24.15 Morphological features of Aspergillus fungal network, SRB with flagellum, Acidithiobacillus and SRB

colonizing a steel surface.


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Table 24.4 Microbiological features of some thio-bacteria

Organism

Environment

Activity

Desulfovibrio desulfuricans

(Sulphate reducing)

Mud, sewage oil wells,

subsoil

Anerobic, sulphate

reduction, pH 6-7.5,

Temp. 25-300C (some

moderate thermophiles)

Acidithiobacillus thiooxidans

Acidithiobacillus ferrooxidans

Sulphur and iron

bearing minerals, soils

and water

Anerobic, pH2 – 4,

28 – 35oC, oxidizes

sulphur, sulphides

producing sulphuric acid,

Ferrous to ferric

oxidation.

Thiobacillus Thioparus

Water, mud, sludge,

sulphidic soils

Aerobic pH 6-8,

30-350C, oxidises

thiosulphate and sulphur

to sp.

From the sulfur-bacteria cycle, bacterial oxidation and reduction cycles involving

sulfur species are evident. Both these redox concepts are important in MIC

mechanisms.

lecture 24 microbially influenced corrosion (mic...

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