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Indian Journal of Experimcntal Biology Vol. 4 I. September 2003. pp. 1023 -1 029

Control of metallic corrosion through microbiological route

S Maruthall1l1thll *, S Ponillari appan, S Mohanan, N Pal ani swalllY, R Palani appan l & N S Rengasw31llY

Elcctrochemical Protec tion & Biofou ling Scct ion. Corrosion Scicncc & Enginecring Division Central Elcc trochemica l Research Inst itute, Karaikudi. 630 006. India

I P.G. Dept of Microb iology. Sri Paramakalyani Co llegc. A lwa rkurichi. 627 4 12. India

In vo lvemcnt 0(' biof'i lm or microorganisms in corros ion proccsses is widely ack nowlcdged. Although majority of thc studies on microbiolog ica ll y induccd corros ion (MIC) have conccntrated on ae robic/anae robic bactc ri a. T here arc nUlllerous aerobi c b;lcteri a. which could hindcr thc corros ion process. T he Illi crobi ologica ll y produccd cxopoly illcrs prov idc thc structural framc work for the bio f'i I Ill . Thcsc polymers combinc w ith dissolvcd metal ions and rorm organometalli c complexes . Gcncra ll y hetcrotrophic bactcri a contribute to threc maj or processcs : ( i) synthes is of polymers ( ii ) accumulation of reservc Illaterials li ke po l y-~- hyd roxy butratc (iii ) producti on of high Illolccular wcight cx tracc llular polysacchari dcs. Poly- [3-hydroxy butyratc is a po lymcr or D( -)[3-hydroxy bu tratc and has a Illolecu lar we ight betwccn 60.000 and 2.50,000. Some cx trace llular polymcrs al so havc highcr molecular weight s. It sec ill s that highcr molccular weight polymer acts as biocoating. In the prescnt rev icw, rolc or biochemi stry on corros ion inhibiti on and poss ibiliti cs or corrosion inhibiti on by vari ous microbes arc di scussed. T he ro le of bacteri a on cu rrcnt demand during ca thodic protecti on is also debatcd. In add ition. some or the signi ficalll contribu tions madc by CECRI in thi s proilli sing arc;) arc highligh ted.

Keywords: Bioril lll, Corros ion contro l. Heterotrophic bacteri a. M eta lli c cormsion

Solid surfaces immersed in aqueous environments absorb organic matter from the surrounding environment. Bacteri a through their ex tra ce llular metabolites cause the formati on of an ex tremely complex micro layer called sl i me or biofi I m. The involvement of biofilm or microorganisms in corrosion processes is 'Nell known. The microorgani sms can promote corrosion in different ways by differential aerati on celli . extra ce ll u lar pol ymcri c substances2.3, or by their binding capability w ith metal i o n s~ -6. According to the more recent studi es by researchers, certain speci fic microorganisms which favour the fo rmati on of pass ive film on the metal surfacc could ac tu all y hinder the corros ion process. But no detailed study is available on microb iolog icall y induced corros ion control (MICC) systems. Thi s prov ides a vast scope for deve loping corros ion control techniques through microbio log ical route.

Process of corrosion control hy hacte.-ia A biofilm can contain diffcrcnt types o f bac tcri a,

viz. (a) heterotroph ic bacteria (b) sui phate reduci ng bacteri a, (c) thi osulphate ox idi zing bacteria and (d) manganese ox idi zing bacteria . The ac ti ve contri butor in a biol'ilill is heterotrophi c bactcri a. which utili ze encrgy 1'1'0111 carbon source. T hcse hetcrotrophi c bactcri a contri bute to three maj or processes.

*Corrc,;pulldent author: [ -mail: lllarutila_lll @yahoo.colll

I. Synthes is o f polymers (anaboli sm) 2. Reserve material s (s torage) 3. Cataboli sm

More th an 95% of the cellular mater:al of E.coli and other microorgani sms consists of macromolecules. A typica l an alys is o r microbial ce ll s gil OOg o f dri ed ce ll s 7 is as fo llows : protei n 52.4 . polysaccharides 16.6, lipid 9.4, RN A 15.7 , DNA 3.2. These constitucnts havc their own di stinct role in controlling metalli c corrosion. Bacteri a do not accumulate lipids as reserve materi al but they conta in considerable amounts of lipids especiall y phospholipids and glycolipids as constituents of their membrane systems. These phospholipids are ncgati ve ly charged. Whil c g lucose serves as growth substrate for E.co/i , a nUlllber of hexose and pcntosc phosphates are intermediates in the brcakdown or thi s substrate. Phosphates arc also negati ve ly charged. Hence, it bccomes easier fo r the positi ve ly charged metal ions to combine w ith ph osphates and form protecti ve organometall ic complex .

Synthesis of polymers It has been mentioned that about 97% or the

ce llul ar matcri al are macromolecules. Three groups can be di stinguishcd, viz. (a) lipids. (b) peri od ic macromolecules Stich as peptidog lycan. polysaccharides and (c) macromolecules sllch as nucleic ac ids aile!

1024 INDIAN J EXP BIOL. SEPTEMBER 2003

proteins. Many microorganisms excrete polysaccharides which may be retained as part o f the ce ll structure. The macromolecules may become dispersed in the liquid phase or in the presence of a suitab le solid substrate. may adhere to immersed surfaces to form an organic film .

Lipids are not true macromolecules, as the monomers are not linked to one another by covalent bonds. However, in aqueous environment phospholipid molecules such as phosphatidyl choline associate in such a way that a double layered structure is formed. M embrane contains up to 60% of protein ; some are embedded in the lipid layers and have specific functions in the vari ous transport processes. Peptidog lycan is the ce ll wall wh ich is synthes ized from two building blocks: UDP - N - acetyl muramic ac id, pentapeptide and UDP - N - acety l glucosamine. The formation of these compounds in the cy top lasm is transferred to a membrane lipid carri er - undecapreny l phosphate. It is then linked to N - acety l glucosamine. Thi s di saccharide pentapeptide is transferred by the lipid carrier to the ex trace llu lar si te o f the cell membrane and used on a building block In peptidog lycan chain elongation. Bes ides these. molecules of fa tty ac ids are linked to glucosamine disaccharide of lipid throu gh ester and amide linkages.

Bacteria differ in the reserve material they accumulate under certain cond itions. Polyphosphate accumulati on is widespread among bacteria. It essentia ll y fu ncti ons as a phosphorus storage material and is utili zed for nucleic ac id and phospholipids synthesis under conditions of phosphate starvation .

Besides, glycogen and po l y- ~-hydroxy butyrate serve as energy storage compounds. Pol y-~-h ydroxy butyrate (PH B) is a typical prokaryo ti c storage materi al. It IS widespread in bacilli , in chemolithotrophic and phototropic bacteri a, and in Pseudolllonas sp. PHB is a pol ymer of 0 (-)~-hydroxy butyrate and has a molecular wei ght between 60,000 and 2.50,000. It is accumulated in the cell s of granul es surrounded by membranes. Under appropri ate conditions, bacterial ce ll s may accumu late

so much Poly-0-hydroxy butyri c acid (PHB) that accounts for approx imately 60% of their dry weight R. Alcaligenes sp accumulates up to 80% of its dry weight as polymer. In eubacteri a the synthes is of large amounts of PHB IS tri ggered by different environmental stimuli including limitat ion o f oxygen, nitrogen. phosphorus and potass ium individuall y or

collectively depending on the organism9.IO. Propionic and pentanic acids are required as substrates in the

production of poly -~-hydroxy butyri c co -~-hydrox y valoric acid. Since it has high molecular weight, it is possible that the compound may act as a very gooe! bio-coating. Pagel I has described the iso lation of a mutant strain of Azotobacter villelolldii, which rapidl y produces large amounts of PHB.

Production of siderophores Escherichia coli develops four different Fe'+

chelator transport systems wh ich are termed siderophores. It occurs in all aerobic and facul tati ve anaerobic microorganisms. In gram negativc microorganisms, specific receptor proteins arc present fo r the siderophorcs in the outer membrane. It may ac t as inhibitor particularly in acid mediull1.

Enzymes Nitrate reducer is a molybdenulll containing

membrane bound enzyme which reduces nitrate to nitrite. Mol ybdenum is present in thi s cnzy me in the form of the so called molybdenum co factors (M o Co) in which molybdenum is bound to a protein moiety, the so ca lled molybdopterin Mo Co i - non cova lent ly bound to the protein and its func ti on is th at o f a prostheti c group. It is interes ting th at thi s co fac tor occurs in allmolybdoprotein except th e nitrogenase in which one type of subunit is a molybdenum iron-sulphur proteinH Besides nitrate reducers, molybdenum cofactor has been detec ted in format dehyd rogenase. ulfite ox idase, Xanth ine dehydrogenase, trimeth ylami ne - n-ox ide reductase, and co-ox ides . It is poss ible to inhibit the producti on o f hydrogenase enzyme by molybdate. I t is also well known that sulphate reducing bacteria can accclerate corrosion by conversion o f sulphate to sulphide in presence of hydrogenase enzyme. By USIn g appropriate nutrient, it is poss ible to control the production of hydrogenase enzyme thereby nulli fy ing the effect of sulphide conversion. On th e other hand, sulphate reducing bacteri a w ill be made to form protecti ve f ilm which wi ll be tenaciou s and adherent.

Metal ion and exopolymer interaction Microorgani sms growIng on water -immersed

metal surfaces form biofilms that are held together by extra cellular po lymeri c substances (E PS) or biopol ymers. Widespread ev idcnce indicates th at many ex tra cellular polymcrs produced by bacteri a arc acidic and contain fun cti onal groups that easil y binel

+

MARUTHAM UTH U et al.: CONTRO L OF M ET ALLIC CORROS ION 1025

metal ions in natural environment. T he naturall y formed biofilms can be strengthened by additi on of appropriate nutrients so as to fo rm a tenac ious and adherent protective film on the meta llic substrate. A direct benefi ci al effec t of thi s . approach will be a considerable reductio n in the current requirement of cathodicall y protected structures, since the tenac ious f ilm being less porous will reduce the cathodic area and lead to lesser current demand fo r cathodic polari zation. Thus thi s approach may prove to be an energy sav ing system in the case of cathod ic protection.

International status On ly few references are avail able w ith regard to

corros ion control by microorgani sms I2. IS. The protective action o f Serratia marcescens on alumini um has been reported . These po lymers combine with corrodib le meta l ions and fo rm organometa llic complexes. These meta llexopo lymer complexes in biofilm prov ide sites for d iffe rentia l aerati on cells 16 and anaerobi c zones fo r the growth of SRB 14 . Many workers have examined the ro le of bacteri a l exopolymers in meta l bindingS by many ways . Differenti al binding abi liti es he lp to es tabli sh io n concentratio n ce lls3.S.17 Bacte ri a l film can prevent d iffusion of corros ion species such as oxygen to the metal surface thereby reducing the corros ion rate. The similar reduction in corros ion of a luminium and copper obtained by the two di ffe rent biofilms (Bacillus brevis and Pseudomonas Jrag i) suggests that the protection of these meta l surfaces is a general phenomenon which occur due to oxygen removal. Pederson and Hermansson 18 fo und that protection was caused by cellul ar metabo li c activity. The gram positi ve bacteria excretes hydrophobic mycolic ac ids to its exterior, whereas the oute r membrane of the gram-negati ve consists mainl y of hydrophili c lipopolysaccharides, probably covered with prote in8 . Besides , these same gro ups produce "Vi vianite" formation which inhibits corros ion 19. Syrett el al. 20

reported that the genetica lly eng ineered bacteri a can release corrosion inhibiting compounds or antimicrobi al compounds. Ornek et al.2 1 observed that the Bacillus subtilis biofilm reduces the corros ion rate of the passive aluminium alloys at pH 6.5. Vorster and Wanders22 emphas ized the need fo r non tox ic environmentally beni gn compounds such as deri vatives of naturall y occurring po lysaccharides, acetyl amines that can protect meta l aga inst cOITosion. Corrosion inhibition by neutrali zatio n of corros ive

substances is re lated to the corrosIOn inhibition of mi ld stee l by aerobic bacteri al biofi Ims under flow conditions23 .

CECRI's contribution on corrosion inhibition 1 74 2S h · d Maruthamuthu et a .-. suggested t at Improve

pass ivity of meta ls could be attributed to negatively charged cell wa ll s of bacte ri a . 1t was shown that reducti on in anodic current (ip) is accomplished by pass ivato rs present within the biofilm. The actua l concentrati on o f potenti al pass i vato rs such as phosphates and nitrate have been measured in the bi o fi 1m. Bes ides, Mohanan el al. 26 observed t.hat biofilm reduces the growth of inorgani c phase but promotes the formatio n of organo meta llic complex thus improv ing the pass ivity of metal. Recently Po nmari appan et al.27 noticed that Pseudomonas maltophilia and Serratia sp. inhibited (communicated) mild stee l corros ion by about 5 times. It can be seen fro m Table I that CEC RI has identi fied ] 0 bacteri al species whi ch can confe r e ffective corrosion protecti on and ra ise the durab ili ty of stee l substrate by diffe rent fac tors rang ing fro m 1.1 7 to 5.3328 . T hey have a lso establi shed for the first time that gram negati ve stra ins have mo re inhibiti ve capab ili ty than gram positive stra ins (Tabl e ] ). All the bacteri a studied have corros ion inh ibiting property but to va ry ing degree. The reason fo r the more inhi bitive capac ity by negati ve strains is the presence of phospho lipids in the ir cell wa ll when compared to positi ve strains. Hence, it can be concluded that it is poss ible that improvement of cell wa ll may inhib it the corros ion. The corros ion rate is acti vation contro lled and not diffusio n controlled. Hence it can be concluded that the inhibiti ve bacteri a act as "anod ic inhibitor". Since the literature ava ilable o n the ro le of enzymes in biologica lly inhibiting corrosion can be said to be nearl y scarce, investigati on has been done

Tab le I - Corrosion Inhi biti ve properly of various st rains in 1000 ppm ch loride en v i ronment2~

S.No. Bacterial spec ies Corrosion rate Durability (mpy) factor

I. Actionobacillus lignieresii 0. 1597 3.44 2. Actinobacter calcoaceticus 0. 1303 3.83 3. AcinelObacter radioresistens 0. 1030 5.30 4. Bacillus arnyloliquefaciens 0.3299 1.87 5. Bacillus brevis 0.5283 1.1 7 6. Escherichia coli 0. 11 72 4.60 7. Kluyvera cryocrescens 0. 1383 4.76 8. Pseudomonas aureofaciell s 0.2022 3. 10 9. Salmonella typhimuriwll 0. 11 52 4.78 10 Xanthomonas campestris 0. 103 1 5.33

1026 INDIAN J EXP BIOL, SEPTEMBER 2003

by using a-amy lase enzyme in presence of Pseudomonas pUlida and Bacillus amyloliquefaciens. Polari zation study indicated that both Pseudomonas pUlida and Bacillus allly /oliquefacieJl s act as ca thod ic inhib itors. The impedance behav iour of mild steel with and without vari ous bacterial species is shown in Figs 1-6.

E N

300 0--01 st day D--

MARUTHAM UTHU el al.: CONTROL OF METALLIC CO RROSIO N 1027

inhibition has also been observed in presence of Vibrio sp. which leads to solubili zati on of inorganic phosphate by the producti on of organic acid and conversion of the same as free phosphate ions which may get adsorbed on the materi a l and inhibit the corrosion29 . The immobili zed strains o f BaciLLus brevis, Xanthomonas campestris have also been identified as good corros ion inhibitive spec ies.

Influence of fresh water heterotrophi c bacteria on rei nforced concrete has a lso been studied30 . whereas the presence of HB in the surrounding medi um adversely affects the compressive strength of concre te due to the destructi ve action of g lucose derived ,fro m bacterial act ion, polarization study (Figs 7 & 8) revealed that microbes improves the passiv ity of

-500

-> E .700 -"

-900

Fig. 7 - Tafel '. plOl showing relation of cun'ent(l) with potential (E)

80r---------------~---------,

o

-80

> -160 ~:::~~~e

E ·240 .3' ~·320 .... z W·400 .... o 0. .4eO

~60

-640

0·1 1-0 10 CURRENT,)lA

100

Fig. 8 - Potentio dynamic pol ari zation for reinforced steel in presence and absence of Heterotrophic bacteria

re inforcing steel by adsorpti on of organic species on steel which has been confirmed by XRD observation. The XRD pattern of mild stee l in presence of bacterial spec ies is shown in Fig. 9.

Underground pipelines normall y traverse through diffe rent so il strata which might be aerob ic or anaerobic in characte r. Cathodic protection is one of the well known techniques of protecting pipeli nes against so il corrosion. Various standards (NACE standard , DN V standard) suggest that maintaining the potentia l of pipeline at hi gher negati ve va lue (-950 m V vs C U.CUS04) is needed in anaerob ic environment. Recently CEC RI has estab li shed that Pseudomonas sp. and Vibrio sp. dramatically reduces the current demand at va rio us potenti als during cathodic protectio n. These bacteria are organi c nutrients utili zing bacte ri a , while attaching on struc ture, the first attached cell s may became less acti ve ce ll s because oxygen reduction may be hi gher on cathodi c surface. At the same time, these groups consume hydrogen formed at cathodic surface (Table 2) . The possible routes of corros io n inhibiti on through the microb io logically secreted products is graphica ll y represented in chart form.

..... C)

JC -r:1 ~ .. 0.

'C 0

C) 0 ~ L- 0 &l ~

10 30 D iffracti on angle 29 /deg.

Fig. 9 - XRD patterns o f thin film s on re inforced steel speci men

Table 2 - Current demand during cathodic protection in presence of bacteri al spec ies (Ponmariappan el {//.) .1 1

Control

Potenti al (mY vs SC E)

-700

-800

-900

- 1000

- 11 00

1000 ppm (chloride

a lone) (~lA. cm '2 )

15 ± 2

28 ± 3

32 ± 3

28 ± 2

68 ± 4

3% nutrient s 1000 ppm chloridc

().lA.cn,-2)

16 ± 2

26 ± 2

30 ± 3

30 ± 3

69 ± 5

Bacte ri al species

Pseudomo nas Vibrio sp. ().lA.c m·2) ().lA .cm,2)

2 ±O. I 2.0 ± 02

3 ± 0 .1 4.0 ± 0.5

5 ± 0 .1 5.5 ± 0.5

7 ± 0 .2 6.0 ± 0.5

25 ± 2.0 16 ± 2.0

1028 INDIAN J EXP BIOl, SEPTEMBER 2003

l CORROSION ) I

I

Microbiologically

J

Microbiologically Induced corrosion (MIC) inll uenced corrosion

control (MICC)

I ..L ,

Manganese Iron Acid SRB Heterotrophic bacteria oxidising bacteria ba cteria Producing

Bacteria

I

l Organometallic com plexes

l Reserve mat~rial l Extracellu lar polysa ccharides 1 l Capsular J l Slime Polysaccharide

J ( Polyphosphate ) ( Glycogen PHB Polysaccharide

L, ~ I ~ ( I

l Biocoaling J Chart 1- Microbiological route for corrosion control

Conclusion The present review clearly shows that there is vast

scope for developing newer technologies of controll ing metallic con'osion through microbiological route. Heterotrophic bacteria as a class are capable of developing a tenacious and adherent biocoating on the meta l surface which can confer efficient protect ion. In thi s regard, production of high molecular weight polysaccharide becomes very important. The fact that molecular weight can be as high as 2 ,50,000 makes thi s approach quite promising.

Another aspect is the infl uence of gram negative strains on the durabi lity factor. CECRI has identified some species which can give a durabi lity of five and more in 1000ppm ch loride. This observation cou ld be quite usefu l in extending the life of the water pipelines . The future R&D efforts should concentrate on ensuring sustainability.

CECRI'S observation that certa in bacteria can actually reduce the current demand for catho lic protection is bound to have a benefic ial impact on cost of cathodic protection. Large scale fie ld data are to be generated for assess ing the actua l economic impact.

Acknowledgement Thi s research was supported by the Department of

Science and Technology, New Delhi. The authors are thankful to the Director, Central Electrochemical Research In stitute, Karaikud i for giving perm ission to commun icate the rev iew paper. One of the authors

(S.Ponmarippan) expresses his sincere thanks to CSIR, New Delhi for the award of SRF.

References I Iverson W P. Microbial corrosion of metal s. Adv Appl

Microbiol . 32 ( I) ( 1987). 2 Beech B, Gay larde G.C, Smith J J & G G Geesey, Extra

ce ll ular po lysaccharides from Desulfovibrio deslI!furicalls and Pseudomonas j1uorescel1s in the presence of mild s teel and stainless steel, Appl Microbiol Biolechnol, 35 ( 1997) 65 .

3 Geesey G , Mittleman W .W, Iwakoa Tek i & Griffith R, Rolc o f bacteria l exopolymers in the deterio ration of metalli c copper surfaces, MaIre Pelf ( 1986) 25.37 .

4 Geesey G, Jang l, Jo lley J G, Hankins M R & lovaokat Griffiths P R, Bindi ng of metals io ns by extra cellu lar polymers by biofilm bacteria . Waler & Waler Microbio!' 20 ( 1988) 3 13.

5 Geesey G & Jang l in Binding of meta l ions and bacteria, ed ited by T Betroidge, R Doy le (John Wi kcy and Sons, Ncw York) 1988, 325.

6 Madigan M T , Mattwiko J M & Parker J, Biology of Inicroorgallisms (Prenti ce Hall International Inc. Great Brita in) 1989.

7 Stouthamer A H, Energy-y ie lding pathways, In The bac teri a, Vo1.6, edited by L N Ornston and J R Sokatch (Academic Press, New York) 1978,389,462.

8 Gottschalk G , Bacte ria l metabolism (Springer- Verlag) 1988. 1-34 1.

9 Suzuki T, lamane T & Sh imizu S, Mass production of po ly ~-butyric ac id by fu ll y au tomat ic fed batch cu I lUre of methylotroph, Appl Microbiol Biolechl/ol, 23 ( 1986) 322.

10 Dawes E A & Senior P J, The role and regulat ion o f energy reserve polymers in mi croorgani sm, Adl' Mirrobiol Phvsiol, 10 ( 1973)35.

II Page W .J & Huyer H, Derepress ion of the A:% baCler vinelandii siderophore sys te m, us ing iron -containing mineral s to limit iron repletion , J Bac/eriul. 158 ( 1984) 496.

MAR UTHAMUTHU et al.: CONTROL OF METALLIC CORROSION 1029

12 yidela H A & Guiamet P, In Biodeterioration research, vol.l , edited by G Liewellyn and C 0 Rear (Plenum Press, New York) 1987, 272.

13 Thomas C J, Edyvean R G J & Brook R, Biologically enhanced corrosion fatigue . Bi%uting, ( 1) (1988) 63.

14 Ford T, Maki J S & Mitche ll R, Invo lvement of bacterial exopolymer in biodeterioration of metals, In Biodeterioration 7, edited by D R Smith, R N and Eggins HOW (Elsevier Applied Science, London) 1988, 378.

15 Pedersen A & Hermansson M, The effec ts on metal corros ion by Serratia marcescens and a Pseudomonas sp, Biojouling , I (1989) 3 13.

16 Geesey G G, What is biocorrosion 2, In BiojOlllillg and biocorrosion in industrial water systems, edited by H C Flemming & G G Geesey (Springer, Berlin) 1991,1 55.

17 Jayaraman,A, Cheng E T, Earthman J C & T K Wood, Appl Mircobiol & Biolechnol, 11 ( 1997) 48.

18 Pedersen A & Hermansson M, Bi%lliing, 3 (1) 1-9( 199 1). 19 Yolk land H.P, Harms H, Knopf K, Wanner 0 & Zehnder

J.B, Corrosion inhibitio n of mild stee l by bacteri al, Biojouting, 15 (4) (2000) 287.

20 Syre tt B C, Earthman J C , Wood T K, Arps P .J & Mansfeld F B, Corrosion control using regenerati ve biofilms (CCURB) - An update, Proc.of Int.Cong. Corrosion in Refinery petrochemical and power aeration plants, (2000) 387.

21 Ornek D, Jayaraman A, Syrett B C, Hsu C H, Mansfeld F B & Wood T K, Appl Microbiol Biolechnol, 58 (2002) 65 1.

22 yorster S W & Wanders F B, Biopolymeric inhibition of mild steel corrosion, Proc. 14th Int. Corrosion Cong o (ICC), South Africa, (1999) 3 18.

23 Ismail K L .M, Gehrig T , Jayaraman A, Wood T K, Trandem K, Arps P J & Earthman J C, Corrosion control o f mild steel

by aerobic bacteria under continuous fl ow conditi ons, Corrosion , 58 (5) (2002) 41 7.

24 Maruthamuthu S , Raj agopal G, Sathiyanarayanan S, Eashwar M & Balakri shnan K, A photoelec trochemical approach to the ennoblement process: Proposal of an adsorbed inhibitor theory , Biojouling, 8 ( 1995) 223.

25 Maruthamuthu S, Rajagopal G, Sathiyanarayanan S, Angappan S, Eashwar M & Balakrishnan K, Contribution of oxide film and bacterial metabolism on ennoblement process: A novel mechanism, CII ,.,. Sci, 7 I (1996) 315.

26 Mohanan S, Maruthamuthu S, Mani A & yenkatachari G. Anticorrosion Methods & Materials, 43. (5) ( 1997) 23.

27 Ponmariappan S, Maruthamuthu S, Mohanan S, Palani appan R, Palani swamy N & Ragavan M, Application of microbial technology to inhibit the deterioration of mild steel, J Chem Techno/ Biotechnol, 2003 (communicated).

28 Corrosion cont ro l by microorganisms, Fi nal report. Department of Science & Technology , New Delhi , Indi a, 2000.

29 Ponmariappan S, Selvakumar G , Jeya Prabha C, Maruthamuthu S, Palani appan R & Re ngaswamy N S, Corrosion and scale inhibiti on by Vibrio sp. in cooling water system, ProC. of Eighth National Cong o on Corrosion cont rol, (1998) 4.6.1 -4.6. 15.

30 Maruthamuthu S , Sarasvathi V, Mani A, Kalyanasundaram R M '& Rengaswamy N S. Influence of freshwater Heterotophi c bacteria on reinforced concrete structures. Bi% uling, 11 (4) (1997)313 .

3 1 Ponmariappan . S , Kandaswamy P, Mathiyarasu J, Mohanan S, Maruthamuthu S, Pa lani appan R & Palaniswamy N, Effec t o f Pseudomonas and Vibrio sp. on cathodic protec tion. Proc . of 4th Int. EFC workshop on Microbial corros ion, Eds C.A.C. Sequeria, Portugal, EFC No: 29, ( 1999) 219.