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. 1
Seawater, fresh water and soil as corrosive media Sea water is an aggressive corrosive medium for biofouling and microbiallyinfluenced 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 Microbiologicallyinfluenced 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. 2
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. 3
Table 24.1 MIC in industrial environments Nuclear and thermal power plants Cooling water tubes and pipes, sub-sea pipe lines, stainless steel and carbon steel, copperalloys, aluminium-alloys Subsoil pipe lines Steels On-shore, off-shore oil and gas processing. Steels, Aluminium alloys Chemical industries Pipelines, Tanks, Condensers, Joints, heat exchangers. Civil engineering Concrete in marine, fresh water and sub-soil conditions, bridges, buildings. Water treatment and metal working Heat exchangers and pipes, Breakdown of oils, emulsions and lubricants Aviation (Defence and Civil) Aluminium fuel tanks Mining and metallurgical operations Underground machinery and engineering materials. 4
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 Water storage tank Aluminium brass, 70:30 Copper-Nickel, 90:10 Copper-Nickel 316 stainless steel Pitting Rust, weld corrosion Water pipes 316 stainless steel weld Pitting Cooling towers Galvanised steel General corrosion Pumps Stainless steel Crevice, pitting 5
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 6
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 H 2 S, 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 + 4H 2 S = + 4H 2 O 2H + + S - - = H 2 S 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 7
Number of cells/ml sulphate concentration(g/l) E ESE in mv No.of Cells / ml Lecture 24: MIC Definitions, Environments and Microbiology Sulphur and ferrous iron-oxidising bacteria such as Acidithiobacillus thiooxidans and Acidithiobacillus ferrooxidans are acidophilic and aerobic promoting oxidation of sulfur and sulfides. 2H 2 S + 2O 2 = H 2 S 2 O 3 + H 2 O 5Na 2 S 2 O 3 + 8O 2 + H 2 O = 5Na 2 SO 4 + H 2 SO 4 + 4S 4S + 6O 2 + 4H 2 O = 4H 2 SO 4 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. 10 9 10 8 Fig 24.2 Bacillus subtilis 0 10 20 30 40 50 60 70 80 Time (hours) Fig. 24.3 Cell number as a function of time during growth of Bacillus subtilis 6x10 8 1.8 50 5x10 8 1.6 0 3x10 8 2x10 8 4x10 8 cell count E ESE Sulphate concentration 1.4 1.2 1.0-50 -100-150 8 1x10 8 0.8-200 0 0.6 0 20 40 60 80 100 120 140 160 Time (min) -250
Number of cells / ml ph Sulphate concentration (g / L) No.of cells / ml ph E SCE in mv Fe 2+ and Fe 3+ conc (g / L) Lecture 24: MIC Definitions, Environments and Microbiology Fig 24.4 Sulphate reducing bacteria Fig. 24.6 Acidithiobacillus Sp 2.0x10 8 1.6x10 8 1.2x10 8 8.0x10 7 4.0x10 7 Fig 24.5 Cell number, SO 4 conc and E SCE as a function of time during growth of Sulphate reducing bacteria 0 10 20 30 40 50 60 70 Time (hours) 2.1 Cell count ph 2.0 E SCE 2.5 2.4 2.3 2.2 1.9 Fig 24.7 Cell number, ph, E SCE as a function of time during growth of Acidithiobacllus sp 550 500 450 400 350 300 250 10 8 6 4 2 0 Fe3+ Fe2+ 0 10 20 30 40 50 60 70 Time (hours) Fig 24.8 Ferrous and ferric concentration as a function of time during growth of At.ferrooxidans 1.2x10 9 1.0x10 9 Cell count 2.1 1.8 28 24 8.0x10 8 6.0x10 8 4.0x10 8 2.0x10 8 0.0 0 50 100 150 200 250 300 Time (Hours) 1.5 1.2 0.9 0.6 0.3 ph Sulphate conc. 0 50 100 150 200 250 Time (Hours) 20 16 12 8 4 0 Fig. 24.9 Acidithiobacillus thiooxidans Fig. 24.10 Cell number as a function of time during growth of At. thiooxidans Fig. 24.11 ph & SO 4 conc. as a function of time during growth of At. thiooxidans 9
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. 24.13 Aspergillus spp Fig 24.14 Gallionella spp 10
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. 11
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. 25-30 0 C (some moderate thermophiles) Acidithiobacillus thiooxidans Acidithiobacillus ferrooxidans Sulphur and iron bearing minerals, soils and water Anerobic, ph2 4, 28 35 o C, oxidizes sulphur, sulphides producing sulphuric acid, Ferrous to ferric oxidation. Thiobacillus Thioparus Water, mud, sludge, sulphidic soils Aerobic ph 6-8, 30-35 0 C, 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. 12