Microbially Induced Concrete Corrosion

Transcription

1 Lecture 37 Microbially Induced Concrete Corrosion Keywords: Concrete, Biocorrosion, Models, Prevention. Corrosion of reinforcing steel in concrete structures is one of the most expensive corrosion problems. Structural integrity of many bridges, buildings, terrestrial and ocean structures has been affected by microbial processes. Cement and concrete can be biodegraded and deteriorated through the action of various microorganisms. Compounds such as calcium carbonate, silicates and aluminates slowly react with water and bacterial metabolic products. Internal biocorrosion of reinforcing steel reduces the life of concrete structures. Biocorrosion products having larger space volume than the initial metal content initiate significant internal stresses leading to fracture. Microbial weathering of cement and concrete increases porosity enhancing water and corrodant penetration. Calcite precipitation and dissolution can be brought about by indigenous microorganisms. A beneficial aspect in this regard would be formation of calcium oxide (hydroxide) scales which can confer passive layer and also selectively plug up the pores. Biogenic cement and concrete can thus confer corrosion protection as well! This observation is of practical significance in curing of cements and concretes. Microbial precipitation of calcium carbonate and calcium hydroxide can lead to closure of pores, further increasing the corrosion resistance of reinforcing steel. Microbially induced concrete corrosion (MICC) has been known since 1900s. Concrete sewer pipes were observed to be degenerated into a pasty mass. Sulfur oxidizing bacteria could be identified and isolated from sewer pipes and channels. Bacterial sulfur cycle involving both sulfur-oxidizing and sulfate-reducing bacteria is relevant to MICC. SRB reduces sulfate present in sewage and soil materials to 1

2 sulfides leading to generation of gaseous H 2 S, dissolved H 2 S as well as reduced sulfur compounds such as hydrosulfides and polythionates. All the above sulfurbased reagents are corrosives to steel in cement and concrete structures. Bacterial oxidation of sulfur and sulfides lead to biogeneration of sulfuric acid which can corrode reinforcement steels and also dissolve cementaceous materials. Besides the above sulfur-oxidizing and sulfate-reducing bacteria, other neutrophilic heterotrophs of the genus, Bacillus and Pseudomonas present in sewer and soil environments generate organic acids, polysaccharides and proteins that can also corrode many metals and alloys. MICC can occur in different types of concrete systems such as Waste water treatment. Swimming pools and tanks. Cooling towers. Hydraulic structures, bridges and buildings. Rapid deterioration occurs in areas with elevated H 2 S and acid concentrations. A pictorial representation of microbial corrosion in sewage collection systems is illustrated in Fig

3 Fig Microbial corrosion of concrete pipes Reaction sequences: a) H 2 S generated by SRB in biofilms and sewerage-converted to various reduced-valence sulfur compounds. b) In presence of O 2, water and S-bacteria, sulfuric acid is produced. c) Acids and S-compounds react with binder in concrete-disintegration, porosity, structural damage and reinforced steel corrosion. Bio-and physico-chemical variables such as biomass (cell number), ph, Eh and concrete ingredients change with time during the above mentioned process sequences. The initial ph being neutral is rendered acidic through bacterial acidification. Biological succession and ph reduction lead to deterioration of concrete and corrosion of reinforced steel. 3

4 Colonization of concrete is an excellent example of bacterial succession. Concrete is generally alkaline with a ph of Environmental reagents such as CO 2, H 2 S, HS - etc can reduce the concrete ph to 9. Due to the availability of nutrients, bacterial attachment, growth and colonization on concrete surfaces take place. Neutrophilic Acidithiobacllus spp, such as T.thioparus can proliferate and the ph can drop to about 4-5 when other acidophiles take over. In concrete corrosion, the following Acidithiobacllus species were found to take part (Table 37.1). Table 37.1 Acidithiobacllus bacteria involved in concrete corrosion Bacteria ph range Substrate T. thioparus 4-10 S 2 O 3, H 2 S T.novellus 5-9 S 2 O 3 T. neapolitanus 4-9 H 2 S, S, S 2 O 3 T. intermedius S 2 O 3 At. Thiooxidans S 2 O 3, S Besides bacteria, acidophilic and neutrophilic fungi can also grow on concrete surfaces, reducing ph levels to highly acidic values (ph 2 and lower). Reaction of biogenic sulfuric acid with the cementatious materials in concrete lead to structural failure. Corroding layers consist of gypsum and moisture. Ettringite (3CaO. Al 2 O 3. CaSO 4. 12H 2 O) produced due to bacterial action can induce internal stresses leading to cracking, pits and fissures, which facilitates further acid penetration. Corrosion rates up to 4-5 mm/year can be realized under the above conditions. 4

5 A model for concrete biocorrosion is shown in Fig Fig Biocorrosion of concrete Corrosion potential of steel in reinforced concrete is measured relative to a reference electrode (Cu CuSO 4 ) in contact with concrete surfaces using high impedance voltmeters as shown in Fig Fig Potential measurements in reinforced concrete 5

6 Models have been proposed to predict microbial corrosion of concrete taking into consideration the following parameters. a) Sulfide generation. b) Sulfide flux to the pipe wall. c) Rate of acid reaction with concrete. Prevention strategies Corrosion effects of H 2 S and H 2 SO 4 can be reduced by creating conditions that prevent or minimize their generation. This may not be that easy always. Proper treatment of sewer or modification of concrete could be useful. Concrete protection methods include modification of concrete mix and design, coatings or paintings on concrete surfaces or use of appropriate impermeable liners. Increasing and maintaining alkalinity could prevent acid corrosion. In sanitary constructions, appropriate specifications of cement need be used. Control of concrete sewer corrosion by the crown spray process has been reported. A high ph mixture is sprayed into the crown area of sewer. Deactivation and sterilization of acid generating bacteria along with acid neutralization is aimed at. Activity of sulfate reducing and sulfide-oxidizing bacteria in the environment and formation of biofilms need be curbed. Magnesium hydroxide slurries of ph about 11 can be used to neutralize acid. Control of dissolved oxygen levels, sulfate concentrations and organic levels in the sewer environment could deactivate the corrosion-causing microbial load. Modification of concrete materials used in structures is another strategy. Closure of leakages and pores in the pipe lines through socket-seals would prove effective. Microbial concrete is a novel strategy to restore damaged structures through biomineralization of calcium carbonate using microbes such as Bacillus sp. 6

7 Microbiologically induced calcite precipitation can generate highly impermeable calcite layer over the surface of already existing concrete layers. Bacterial concrete can be made by embedding bacteria that are capable of calcite precipitation in the concrete. Bacteria such as Bacillus subtilis and Bacillus pasteurii can induce calcite deposition. The use of microbial concrete enhances durability of cement materials, sealing of decayed concrete structures and improves corrosion resistance of buildings and structures. Reduction in permeability through bioprecipitation of calcite in situ can prevent penetration of corrosion reactants. 7

8 References and recommended reading (Lectures 24 37) 1. J.D.A Miller, Microbial Aspects of Metallurgy, Medical & Tech. Pub. Co, Lancaster (1971). 2. S.W.Borenstein, Microbiologically influenced corrosion handbook, Wood head Pub. Ltd., Cambridge (1994). 3. C.C. Gaylarde and H.A. Videla, (Eds), Bioextraction and Biodeteriortion of metals, Cambridge Univ. Press, Cambridge (1995). 4. M.G. Fontana, Corrosion engineering, Mc Graw Hall, New York (1987). 5. D.A Jones, Principles and Prevention of Corrosion Prentice Hall Inc, New York (1996). 6. H. A. Videla and L. K. Herrera, Microbiologically influenced corrosion: looking to the future. Research Review. International Microbiology Vol.8, pp (2005). 7. T.R. Jack, Biological corrosion failures. 8. I.B Beech, C.C. Gaylarde, Recent advances in the study of biocorrosion an overview, Rev. Microbiol 30, pp 1-22 (1999). 9. K.A. Natarajan, Microbially influenced corrosion, chapter 3, in CORROSION SCIENCE AND TECHNOLOGY, (Eds) U. K. Mudali and Baldev Raj. Narosa, New Delhi (2008). 10. K.A. Natarajan, Microbes, Minerals and Environment, Geological survey of India, Bangalore (1998). 11. G.Manivasagam, D.Dhimasekharan and A.Rajamanickam, Biomedical Implants: Corrosion and its prevention. A Review, Recent patents on corrosion science, 2, (2010). 12. D.C. Hansen, Metal Corrosion in the human body. The ultimate biocorrosion scenario. The Electrochemical society interface, pp ( 2008). 8

9 13. D.J Roberts, D. Nica, G. Zuo, J. L. Davis, Quantifying microbially induced deterioration of concrete: initial studies. International Biodeterioration and Biodegradation, 49, (2002). 14. A.K. Parande, P.L. Ramasamy, S.Ethirajan, C.R.K Rao and N. Palanisamy, Deterioration of reinforced concrete in sewer environments, Proc. Inst of civil engg. Mucipal Engineer, 159, pp 11-20, (2006). 15. V.Achal, A.Mukherjee and M.S.Reddy, Microbial concrete: A way to enhance durability of building structures, Int. conf. sustainable construction materials and Technologies,. Ancona, Italy June (2010). 16. N.Eliaz, Biomaterials and corrosion, chapter 12, in CORROSION SCIENCE AND TECHNOLOGY. (Eds, U. K. Mudali and Baldev Raj, Narosa, New Delhi (2008). 17. U. K. Mudali, T. M. Sridhar and Baldev Raj, Corrosion of bio implants, Sadhana, 28, pp (2003). 9