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Microbiologically influenced stress corrosion cracking failure of admiralty brass condenser tubes in a nuclear power plant cooled by freshwater
\
Corrosion Science\ Vol[ 39\ No[ 00\ pp[ 0710Ð0725\ 0887 Þ 0887 Elsevier Science Ltd[ All rights reserved[ Printed in Great Britain[ 9909Ð827X:87:,Ðsee front matter
Pergamon
PII] S9909Ð827X"87#99968Ð0
MICROBIOLOGICALLY INFLUENCED STRESS CORROSION CRACKING FAILURE OF ADMIRALTY BRASS CONDENSER TUBES IN A NUCLEAR POWER PLANT COOLED BY FRESHWATER T[ S[ RAO and K[ V[ K[ NAIR Marine Biology Programme\ Water + Steam Chemistry Laboratory\ Bhabha Atomic Research Centre Facilities\ IGCAR Campus\ Kalpakkam 592 091\ Tamil Nadu\ India Abstract*The failure of admiralty brass condenser tubes in a nuclear power plant was investigated[ Metallurgical analyses had indicated that stress corrosion cracking "SCC# could have caused the failure[ Studies were carried out to see if bacteria could have played a role in bringing about the conditions which resulted in SCC of the tubes[ Cooling water "Ranapratap Sagar lake\ Kota\ Rajasthan# quality and bio_lm formation were monitored by analysing various physical\ chemical\ biochemical and biological parameters[ Metal coupons were exposed online in the cooling water system to assess the microbial growth on the coupon surface by SEM[ Corrosion rates were assayed by weight loss and corrosion products were analysed by XRD and ESCA[ The study revealed the microbial reduction of nitrate to ammonia "estimated to be of the order of 4[7 mg l−0 at the substratumÐbio_lm interface# which resulted in SCC of the admiralty brass tubes[ Þ 0887 Elsevier Science Ltd[ All rights reserved
INTRODUCTION In aqueous environments\ microorganisms "bacteria\ algae\ fungi etc[# are attracted towards surfaces\ which they readily colonise resulting in the formation of bio_lms[0\1 Such microbial bio_lms are important in a wide spectrum of industrially relevant situations and lead to microfouling and biocorrosion[2\3 The implications of microfouling are energy losses due to increased ~uid frictional resistance and increased heat transfer resistance[4 Zelver et al[5 reported a 29) decrease in heat transfer rate due to biofouling in a fan cooler[ There have been instances of increased capital costs due to premature replacement of equipment caused by severe under!deposit corrosion[4 Moreover\ the fouling of service water systems in nuclear power plants is of concern because it reduces the heat transfer capacity during an emergency or an accident[6 The growth of microbial _lms "slimes# a few tens of microns thick in a condenser tube is su.cient to induce microbiologically in~uenced corrosion "MIC# and cause irreparable damage to the condenser tubes[3 The down!time costs to power plants due to condenser fouling are quite large[7 The temperatures prevalent inside the condenser of an operating power plant provide a conducive environment for the rapid Manuscript received 7 September 0886^ in amended form 17 April 0887 0710
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T[S[ Rao and K[V[K[ Nair
growth of microorganisms[ This results in a thick slime deposit\ which is responsible for heat transfer losses[ The slime layer forms a sticky surface which allows silt and other suspended particles to adhere to the condenser tubes\ thereby enhancing the aggregation of deposits on the material surface and inducing localised corrosion[4\7 An example quoted in the literature8 reports the failure of the alarming number of 3999 condenser tubes in a short span of 6 years of operation of a power plant "Marchwood\ Southampton\ UK#[ This resulted in the leakage of condenser cooling water into the feed water system and then into the boiler\ thereby accelerating boiler corrosion and failure[8 Most reported failures of condensers occur on the cooling water side of the tubes[09 Pope et al[\00 have documented MIC of cupronickel "89]09#\ admiralty brass and aluminum brass in systems cooled with freshwater and brackish water[ It was also reported that copper alloy condenser tubes had under!deposit corrosion due to the formation of slime deposits and ammonia[01 Ammonia producing bacteria were isolated from scale and organic material on admiralty brass tubes su}ering from SCC[01 Copper and its alloys are most commonly used in the fabrication of heat exchangers in cooling water systems[4 The alloys depend on their natural oxide for corrosion resistance[02 This oxide _lm "Cu1O# is a defective _lm with vacancies in the cuprous oxide lattice into which cuprous ions can migrate[03 As copper metal "Cu9# oxidizes to cuprous ion "Cu¦0#\ the cuprous ions move into the vacancies of the oxide lattice[ As the ions migrate through these vacancies\ they get oxidized to the cupric ion "Cu1¦#[ Copper corrosion occurs with the outward movement of the cuprous ion rather than the inward movement of oxygen[ Cu1O protective _lm can be disrupted by a variety of cooling water parameters which include pH\ water velocity\ chlorides\ ammonia and biofouling[ The addition of such elements as aluminum\ zinc\ tin\ iron and nickel to copper have been successfully used to modify the cuprous oxide _lm to make it more corrosion resistant[ These alloying elements essentially plug the holes in the corrosion product _lm\ making it di.cult for ions to migrate through the _lm[04 One of the most commonly used copper alloys in industry is admiralty brass "composition 69[74) copper\ 16[85) zinc\ 9[90) lead\ 9[91) iron\ 0[00) tin and 9[94) arsenic#[ In the present study\ the problem of condenser tube "admiralty brass# failure at Raja! sthan Atomic Power Station "RAPS# unit!II\ Kota\ Rajasthan\ India was investigated[ About 1499 tubes have been replaced over a span of 5 years[ The failure of the tubes lead to the leakage of cooling water into the boiler\ thereby violating the boiler water technical speci_cations[ The leak rate of cooling water ranged between 299 and 1099 l h−0[ Earlier metallurgical analyses of the failed tubes have revealed that the tubes were damaged due to stress corrosion cracking "SCC#[05\06 However\ no systematic study of the interaction between microbial bio_lms and metal has been attempted[ Moreover\ it would be interesting to see whether the initiation of a corrosion phenomenon\ as in bacterialÐhuman infections\ are associated with the adherent microbial community residing in the cooling system[ This present study was carried out to look into the environmental conditions:or causative agent which could have lead to the failure of the condenser tubes[ Comprehensive water quality analysis of the Ranapratap Sagar lake which is being used as the cooling medium for RAPS was carried out[ In addition\ bio_lms developed on perspex and metallic coupons were also characterised for various physical\ chemical\ biochemical and biological parameters in a time series study to understand the bio_lm parameters which could have contributed to the failure[
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MATERIAL AND METHODS Site description Rajasthan Atomic Power Station is located near Kota\ Rajasthan[ There are two units of Pressurised Heavy Water Reactors\ each of 129 MW capacity[ The power plant is cooled by drawing freshwater from an adjacent lake "Ranapratap Sagar lake# which was formed by damming of the river Chambal[ The catchment area of the lake consists largely of forest!land[ At the intake\ cooling water is drawn from a depth of 29 feet[ The heated e/uent is let o} at the surface of the lake[ During the course of the study no water treatment programme was in practice "Chlorination was in practice during the initial years of plant operation and later discontinued#[ The water samples from the pump house "intake# and outfall were analysed for various chemical parameters such as pH\ conductivity\ dissolved oxygen\ alkalinity\ calcium hardness\ total suspended matter\ total dissolved solids\ chlorine demand\ chlorides\ sulfates\ ammonia and nutrients "phosphate\ silicate\ nitrate and nitrites#[ Biological parameters such as chlorophyll content\ diatom counts\ total culturable bacteria "TCB#\ sulphate reducing bacteria "SRB# and nitrate reducing bacteria "NRB# were assayed[ Corrosion studies were also carried out using admiralty brass coupons[ Apart from weight loss measurements\ the coupons were also processed for SEM studies on microbial colonisation\ XRD and ESCA "Electron spectroscopy for chemical analyses or XPS*X!ray photoelectron spectroscopy# for corrosion product analysis[ Perspex panels "04×09 cm and 6×2 cm# were suspended in the lake at a depth of 0[4 m and retrieved after 13\ 37\ 61\ 85 and 019 h of immersion to study the sequential development of bio_lm[ Two stations were used] "0# at the intake point of the cooling circuit and "1# at the outfall site[ Bio_lm samples as well as samples of ambient water were simultaneously collected for analyses of various parameters as mentioned above[ In the laboratory\ the panels were gently rinsed with micro!_ltered "9[1 mm\ Millipore# and autoclaved "019>C for 09 min#[ Wet bio_lm thickness was determined microscopically "Carl Ziess# using the smaller coupons as per the method described by Bakke and Olson[07 Several "14# measurements of thickness were taken in order to give a reasonably accurate mean value of the thickness[ The bio_lm was scraped from the larger panels using a sterile nylon brush[ The bio_lm material was dispersed in a _xed volume "099 ml# using sterile lake water[ The bio_lm suspension was _ltered "9[1 mm\ Millipore#\ the _ltrate was made up to a known volume and used for nutrient "nitrate\ phosphate and silicate# analysis as per standard methods[08 Bio_lm nutrient values are expressed as mg per litre of bio_lm volume to facilitate comparison with ambient water whose values are expressed as mg per litre[ The bio_lm volume was calculated by multiplying the mean wet bio_lm thickness with area[ In general\ methods followed by other worker19Ð15 were used for di}erent biochemical and biological analyses[ Pre!ignited "349>C\ 3 h# _lter papers "Whatman GF:C# were used to _lter samples for biomass and particulate organic carbon analyses[ Diatoms were counted by haemocytometer[ Counts of heterotrophic bacteria\ nitrate reducing bacteria and sulphate!reducing bacteria "SRB# were made using standard microbiological methods[ SRB were enumerated as reported by Postgate[16\17 Nitrate reducing bacteria were assayed as described by Jetter and Ingraham18 using selective medium[ Proteins and hexose sugars were analysed following the methods of Lowry et al[29 and Dubois
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T[S[ Rao and K[V[K[ Nair
et al[\20 respectively[ Lipids were extracted and quanti_ed as per the method reported by Rao and Harbola[21 Hydrological analyses were carried out as per standard methods[08 Filter sterilised lake water "blank# was also analysed for all the relevant parameters and the values were subtracted from the bio_lm values[ All the analyses were carried out in duplicate and results are presented as mean values[ Scanning electron microscopy of the sample coupons was carried out after _xing the bio_lms with 1) glutaraldehyde and by dehydrating the sample in graded ethanol series[ Later\ the samples were vacuum dried\ sputter coated with goldÐpalladium and observed in a scanning electron microscope "Model] PSEM!490 Phillips make#[ Microfouling on metal coupons "1×1×9[1 cm# admiralty brass\ copper\ cupronickel "89]09 and 69]29#\ carbon steel\ stainless steel and titanium were also carried out[ The other materials "apart from admiralty brass# were tested for the sake of comparison in the light of the RAPS III + IV units\ which are being built at the same site[ The coupons were exposed for 439 h online using a corrosion test rack[ After retrieval\ the coupons were tested for corrosion rate and processed for SEM analyses[ One set of coupons was used for X!ray di}raction study for analysing the corrosion products[ The scraped!out corrosion products from the coupons were _nely powered using a pestle and mortar[ The powder was then dehydrated and compacted into a circular disc using a press[ XRD analysis was carried out using an X! # ray generator "Philips PW 0039:89 with a CuÐKa source\ wavelength used was 0[439487 A and a Siemens "Model] D!499#\ Theta!1 Theta Powder Di}ractometer[ From the D values obtained\ the various phases were identi_ed using online XRD data _les[ ESCA "Electron spectroscopy for chemical analyses# was performed in VG ESCALAB 199X "MK II#\ the X!ray source was AlÐKa at 0375[5 eV[ Hemispherical analyser\ 19 eV pass energy\ vacuum was maintained at 0[4×09−09 mbar[ Data interpretation was carried out with Fisons software[
RESULTS AND DISCUSSION The water quality data "Table 0# showed an increase in pH\ dissolved oxygen\ chlorine demand\ nitrite\ ammonia and bacterial numbers from intake to outfall[ An increase in nitrite and a decrease in nitrate from intake to outfall water indicated that a reduction of nitrate to nitrite was taking place within the cooling circuit[ The decrease in nitrates was also accompanied by an increase in ammonia content which indicates possible reduction of nitrate to ammonia[ Such a possibility was supported by an increase in pH "by 9[7 to 0[1 units observed during the course of this study#[ Decrease in conductivity and total dissolved solids was observed from intake to outfall "Table 0#[ Apparently\ the growth and mul! tiplication of microbes inside the cooling system could have resulted in their utilising the dissolved solids\ nutrients and organic matter[ Analyses of bio_lm samples showed that bio_lm thickness varied from 10 to 097 m and 06 to 196 m at intake and outfall water sites\ respectively\ during a 13 to 019 h exposure period at both the stations "Table 1#[ A gradual increase in bio_lm thickness was observed from 13 to 019 h at both intake and outfall[ Almost a two fold increase in bio_lm thickness was observed at the outfall as compared to the intake after 37 h[ A similar trend was also observed for bio_lm volume\ turbidity and biomass[ The _lm thickness was correlated with bio_lm biomass at both the stations "intake station R 9[86 and P 9[993^ outfall station R 9[83 and P 9[91#[ The bio_lm formed at the outfall was both qualitatively and
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Table 0[ Characteristics of Ranapratap Sagar lake water Kota\ Rajasthan Parameter
Unit
Intake
Outfall
Temperature pH Sp Conductivity Turbidity Total Alkalinity Total Hardness Total Suspended Matter Total Dissolved Solids Biomass BOD Dissolved Oxygen Chloride Sulfate Nitrate Nitrite Ammonia Silica as SiO2 Phosphate Chlorophyll a Particulate Organic Carbon Total Protein Total Hexose Sugar Diatom Count SRB NRB Total Culturable Bacteria
>C pH units mS cm−0 NTU ppm CaCO2 ppm CaCO2 mg l−0 mg l−0 mg l−0 mg l−0 mg l−0 mg l−0 mg l−0 mg l−0 mg l−0 mg l−0 mg l−0 mg l−0 mg m2 mg C l−0 mg l−0 mg l−0 cells ml−0 cfu ml−0 cfu ml−0 cfu ml−0
14 "13Ð15# 7[9 "6[7Ð7[0# 134 "124Ð144# 1[4 "1[9Ð2[9# 84"nil#"81Ð87# 86 "85Ð87# 0[9 "9[7Ð0[1# 037 "026Ð045# 00 "7Ð03# 9[3 "9[2Ð9[5# 4[4 "3[9Ð5[9# 03 "02Ð03# 00 "09Ð01# 195 "082Ð103# 09 "8[9Ð00# 03 "00Ð17# 6[4 "5[9Ð8[9# BDL 2[4 "1[9Ð4[9# 7[5 "6[7Ð8[3# 9[3 "9[24Ð9[34# 6[3 "5[7Ð7[5# 154 "149Ð175# 0×092 "SD 143# 3×091 "SD 34# 0×094 "SD 879#
29 "16Ð22# 7[6 "7[3Ð7[8# 127 "129Ð134# 5[4 "4[9Ð7[9# 77"4# "73Ð81# 78 "77Ð80# 6[9 "3[9Ð8[9# 024 "015Ð032# 19 "07Ð21# 0[1 "9[5Ð0[7# 7[6 "6[2Ð8[6# 02 "01Ð03# 6 "5[1Ð7[7# 51 "31Ð62# 06 "04Ð08# 67 "50Ð76# 7[4 "6[1Ð8[7# BDL 3[4 "3[9Ð4[9# 03[4 "02Ð05# 9[4 "9[3Ð9[5# 8[9 "7[9Ð00# 134 "139Ð145# 3×092 "SD 114# 8×093 "SD 399# 6×093 "SD 209#
Mean values^ Values given in the parenthesis are the minimum and maximum observed during the course of this study[ Values in " # are P!alkalinity values[
quantitatively richer[ SEM pictures "Plate 0# revealed the presence of dense algal and bacterial consortia on the metal coupons[ Chemical analyses "Table 1# of the bio_lm samples showed a concentration of nutrients in the bio_lm as compared to water[ It was observed that accumulation of nutrients took place in the initial stages of bio_lm formation[ Concentration factors for nitrate were relatively high[ A close examination of the nutrient data revealed a decreasing trend for nitrate and nitrite concentrations as the bio_lm aged\ indicating their possible reduction[ Ammonia values have shown an increase in concentration after 85 h of bio_lm growth\ otherwise the distribution was almost similar for the rest of the days[ Silicate values have shown an initial increase up to 37 h and\ thereafter\ decreased[ Ammonia levels were relatively high in the bio_lm formed at the outfall waters[ Anaerobic conditions prevailing at the bio_lmÐsubstratum interface could facilitate denitri_cation\ which generally occurs in anoxic conditions[ This is further corroborated by the presence of SRB in the bio_lm[ Hamilton22 reported that a bio_lm of 09Ð14 mm is enough to provide anoxic conditions in the bio_lm[ During this study\ it was noted that a bio_lm thickness of 06 mm is enough to support obligate anaerobic growth in the bio_lm "SRB 093 cfu cc−0 of bio_lm#[ The results of biochemical analysis "Table 1# showed that particulate carbon\ total
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T[S[ Rao and K[V[K[ Nair
Table 1[ Characteristic features of the various physical\ chemical\ biochemical and biological parameters of bio_lm formed at RAPS cooling circuit intake "I# and outfall "O#[ Parameter
Station
13 h
37 h
61 h
85 h
019 h
Water Temperature >C
I O I O I O I O I O I O I O I O I O I O I O I O I O I O I O I O I O I O
16 29 10 06 9[477 9[365 7[62 04[3 095 75 3[1 3[4 9[04 9[05 9[95 9[96 4 4 9[90 9[95 9[98 "09# 9[96 "3# 5[4 "21# 6[9 "055# 9[16 "23# 9[10 "29# BDL 0[6 "14# 2×092 1×092 0×093 2×093 0×095 0×096 1×091 0×094
15 29 16 48 9[645 0[541 00[14 32[6 042 120 02 15 9[07 9[08 9[0 9[5 6 06 9[91 9[04 9[04 "04# 9[16 "04# 09[2 "49# 0[5 "27# 9[04 "08# 9[97 "003# BDL 1[3 "23# 4×092 1×092 1×093 1×094 2×095 1×096 4×091 1×094
11 16 65 013 1[017 2[361 01[19 36[7 102 548 36 69 9[10 9[14 9[2 0[5 21 76 9[00 9[35 9[15 "14# 9[09 "5# 4[3 "16# 0[1 "18# 9[97 "099# 9[94 "60# 9[4 "23# 2[9 "32# 2×092 4×091 0×094 0×095 1×095 1×094 0×092 6×094
13 17 80 042 1[437 3[173 05[19 015[3 118 0645 54 097 9[18 9[20 9[3 2[7 36 048 9[05 9[51 9[98 "09# 9[03 "3# 1[1 "09# 0[9 "13# 9[96 "75# 9[93 "46# 9[8 "52# 4[7 "72# 3×091 0×092 1×094 2×095 7×094 6×094 1×093 0×095
14 20 097 196 2[913 4[799 07[39 88[1 161 1929 80 085 9[24 9[28 1[9 5[3 51 115 9[34 9[71 9[04 "04# 9[16 "04# 0[5 "7# 9[71 "08# 9[94 "52# 9[92 "32# 0[1 "75# 1[4 "25# 0×092 0×092 0×094 8×094 5×094 1×094 6×093 5×094
Bio_lm Thickness "mm# Bio_lm Volume "cc# Turbidity "NTU# Biomass "mg ml−0# Particulate Organic Carbon "mg C ml−0# Chlorophyll a "mg ml−0# Total Protein "mg ml−0# Hexose sugar "mg ml−0# Total Lipid "mg ml−0# Silicate "mg l−0# Nitrates "mg l−0# Nitrite "mg l−0# Ammonia "mg l−0# Diatom Count "cells ml−0# SRB Count "cfu ml−0# Total Culturable Bacteria "cfu ml−0# Nitrate Reducing Bacteria "cfu ml−0#
Values in parenthesis are concentration factors when compared with ambient water[
protein\ total carbohydrates and total lipids increased with time at the intake as well as at the outfall[ The rate of increase was found to be greater in the bio_lm formed at the outfall as compared to that at the intake\ indicating that the outfall "with a DT of 6>C# provides a conducive environment for rapid bio_lm growth[ The lake water was also high in chloro! phyll content "3Ð4 mg m−2# indicating dense algal population and also a high capacity for primary productivity[ This was also re~ected in the bio_lm\ where there was a pronounced growth of diatoms "diatom count# and Chlorophyceae species "chlorophyll content\ Table 0 and SEM pictures#[ Bacterial counts\ such as total viable counts "TVC#\ sulphate reducing bacteria "SRB#
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Plate 0[ "a# Scanning electron!micrograph showing microbial consortium on admiralty brass coupon exposed online in RAPS cooling circuit\ various morphologies "long rods\ short rods and coccoid shaped bacteria# can be seen "29 kV\ magni_cation 0421×#[ "b# Scanning electron!micrograph showing short\ rod!shaped bacteria and diatom "Fra`illaria Sp[ with exopolymeric substances# on admiralty brass coupon exposed online in RAPS cooling circuit "29 kV\ magni_cation 0421×#[ "c# SEM picture showing typical Fra`illaria Sp[ _lamentous growth "girdle view# on perspex coupons exposed at the intake of RAPS cooling circuit "29 kV\ magni_cation 0105×\ scale 0 mm#[ "d# SEM picture showing the presence of diatoms "Nitzchia Sp[ Gomphonema Sp[# on perspex coupon exposed at the outfall of the RAPS cooling circuit "29 kV\ magni_cation 1264×\ scale 0 mm#[
and nitrate reducing bacteria "NRB# have shown variations in their distribution in the bio_lm developed at both the stations "Table 1#[ Fungal counts at the outfall bio_lm were more "6×093 colonies per cm1#[ Fungi were detected after 61 h of bio_lm growth\ bio_lm formed at the intake site had very few fungi[ However\ the microbiological load was found to be higher in the bio_lm formed in the outfall water as compared to the bio_lm at the intake station\ particularly in the initial stages "13 h to 37 h#[ At the intake station\ bio_lm SRB has shown only a gradual increase with time[ At the outfall station\ however\ bio_lm\ SRB population has shown an increase of two orders of magnitude after 61 h of bio_lm growth and thereafter stabilised[ Relatively high counts of nitrate reducing bacteria were found in the lake water "093 cfu ml−0#\ which corroborate the view that denitri_cation could be the major energy!deriving pathway[ NRB were less at the intake bio_lm during the initial stages of bio_lm growth\ however\ after 85 h of bio_lm growth there was an increase of 1 orders of magnitude[ The outfall station bio_lm has a relatively high NRB population\ indicating the in~uence of temperature on NRB growth "see Table 1#[ The number of NRB in the bio_lm was signi_cantly correlated "r 9[72 and P 9[991# with ammonia
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T[S[ Rao and K[V[K[ Nair
concentrations measured in the bio_lm "Fig[ 0#[ This indicates that ammonia generation was caused by NRB and not by mere accumulation from water[ The denitrifying bacteria observed during the course of this study were Alcali`ens Sp[\ Bacillus Sp[\ Micrococcus denitri_cans\ Pseudomonas aeru`inosa and Pseudomonas ~uorescens[ Total culturable bac! teria have not shown much variation in the intake bio_lm[ However\ at the outfall station bio_lm\ there was a decrease in the bacterial population by two orders of magnitude "096 to 094# after 37 h of bio_lm growth[ The decrease could be attributed to the presence of protozoans grazing on the microbial population\ which was observed after 61 h of bio_lm growth at the outfall station[ Similar results were observed earlier by Rao et al[23 in bio_lm characterisation studies carried out in a freshwater impoundment at Kalpakkam[ SRB are a diverse group of anaerobic bacteria which can survive in anaerobic micro! niches in aerated environments until conditions are conducive for their proli_c growth[ If the aerobic respiration rate within a bio_lm is greater than the oxygen di}usion rate\ at the substratum:bio_lm\ the interface becomes anaerobic and provides an environment for sulphide production by SRB[ Biocorrosion induced by SRB residing in bio_lm has been
Fig[ 0[ Regression analysis for nitrate reducing bacteria and ammonia production[
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particularly recognised as a serious problem in cooling systems[22\24 The presence of sulfate reducing bacteria in the intake water and their increase in the outfall water was a notable feature\ as it was reported by Postgate17 that SRB can also reduce nitrate to ammonia[ The diatom number in the bio_lm varied from 0×092 to 4×092 and from 499 to 1×092 cells cm−1 at the intake and outfall waters\ respectively\ during the period 13Ð019 h[ In the presence of light\ diatoms produce oxygen and change local oxygen concentrations at the metalÐbio_lm interface\ a}ecting the local chemistry to a great extent[25 The con! centration of ammonia "Table 1# in the bio_lm formed at the outfall station was found to be several times more than that in the bulk water "299 times^ 4[7 mg l−0#[ According to Dean and Lund26 and Berhens\27 0 mg l−0 of ammonia is su.cient to induce stress corrosion cracking in admiralty brass tubes "copper alloys#[ A possible mechanism for ammonia formation by the denitri_cation process has been proposed by Tiedje28 which involves four stages viz[^ reduction of nitrate to ammonia via nitrite\ hyponitrite\ and hydroxyl amine[ Nitrate reduction is driven by nitrate and nitrite reductase[ Since nitrate serves as an electron acceptor\ the growth rate of denitri_ers depends on nitrate concentration "055 times more in bio_lm when compared to bulk water#[ Denitri_cation can also occur in bio_lms despite the presence of relatively high levels of oxygen in the bulk water[39 Denitrifying bacteria require an electron donor to carry out the denitri_cation process which is served by organic matter[ The Ranapratap Sagar lake water is rich in organic matter "up to 6 mg l−00hexose sugar content#[ ¦ − NO− 2 ¦1H ¦1e:NO1 ¦H1O
"0#
¦ − 1NO− 1 ¦3H ¦1e:N1O1 ¦1H1O
"1#
¦ N1O− 1 ¦5H ¦3e:1NH1OH
"2#
NH1OH¦1H¦¦1e:NH2¦H1O
"3#
Copper alloys are susceptible to SCC fracture in many industrial environments[30 These environments include those containing ammonia\ citrate\ tartrate\ moist SO1\ sulfates\ nitrites\ and nitrates under certain circumstances[ SCC in copper alloys occurs most fre! quently in environments that contain ammonia or amines\ either in aqueous solutions or in moist atmospheres[ The presence of oxidising substances such as dissolved oxygen\ ferric iron and nitrate ions accelerates SCC of copper alloys in ammonical environments[ SCC is a failure process that occurs because of the simultaneous presence of tensile stress\ a conducive environment and susceptible material[ When the alloy stressed in tension is also exposed to a corrosive environment\ the ensuing localized electrochemical dissolution of metal\ combined with localized plastic deformation\ opens up a crack[30 Protective _lms that form at the tip of the crack rupture\ causing fresh anodic material to be exposed to the corrosive medium and SCC is propagated[ A preexisting mechanical crack or other surface discontinuity\ such as a pit or trench produced by chemical attack on the metal surface\ may act as a stress raiser and thus serve as a site for the initiation of SCC[31 In aqueous solutions\ pH has a strong in~uence on susceptibility to cracking[ Intergranular cracks occur most rapidly in neutral solutions[ The crack is transgranular in both alkaline and acidic solutions[09 Given that stresses are high enough at some locations on a Cu alloy tube\ the second prerequisite for SCC is that the environment be capable of inducing cracking[ The deleterious species responsible for service failures on the cooling water side have not been positively identi_ed in most cases[ Water side failures are frequently associated with
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T[S[ Rao and K[V[K[ Nair
deposits on the tubes\ presumably because the deposit provides a site for concentration of the deleterious species or local reactions which cause a pH change beneath the deposit to a value that favors SCC[ Rippon8 reported that admiralty brass condenser tubes in a power plant failed due to the SCC attributed to the ammonia produced by the bacteria[ Metallurgical studies carried out on the failed condenser tubes of RAPS unit!II05 have shown that the crack was initiated from the inside of the tube in an intergranular mode and subsequently propagated in a transgranular mode[ Di}erential aeration caused by corrosion products or bio_lm deposits could have resulted in a reduction in tube thickness of 9[65 mm as compared to 9[8 to 0[1 mm at other places[06 This loss in thickness could have been caused by di}erential aeration cells as the area under the deposit will be the anode and the outside the cathode which\ if large\ results in an e}ective galvanic cell[ Harrison and Kennedy04 reported that corrosion rates exceeding 0[9 m y−0 are considered very high for copper alloys[ In the present study\ corrosion rates "Table 2# of the admiralty brass coupons exposed online for 439 h were found to be relatively high\ 11[75 mm y−0 or 9[89 m y−0 "SD 9[94# for a set of three coupons[ In their studies\ Venugopal and Rawat06 reported that the pro_le of cracks observed on the inner side of the admiralty brass condenser tube was longer[ Both circumferential and longitudinal cracks were observed on the external surface of the failed tubes[ The cracks varied from 0 to 3[4 cm in length[ Pits were observed close to the cracked regions[ The etched samples showed that the propagation of cracks was transgranular in nature[ The nature of the crack indicated that the admiralty brass tubes had failed due to SCC[ The cracks were initiated from the pitted regions[ Generally\ pits act as stress raisers and\ when coupled with the residual stresses present in the tube due to manufacturing process\ can initiate SCC[ The results of the study point out that the water is rich in organic matter and nitrates\ since the water is drawn from a depth of 29 feet from the lake[ The decay of organic matter and\ thereafter the reduction of nitrate\ could have resulted in very high concentrations of ammonia "up to 299 times that in the water# in the bio_lm[ Phosphate was a limiting nutrient in the lake water[ Hence\ it can be emphasised that nitrate is the major energy deriving source of the microbial community of the lake[ As mentioned\ two metallurgical studies were carried out05\06 which reported that the tubes failed due to SCC[ Scrapings from the metal coupons "carbon steel and admiralty brass# exposed online have shown the presence of high levels of organic matter "16)#[
Table 2[ Corrosion rate of some heat exchanger materials exposed online in the cooling circuit of RAPS Material
Corrosion rate "mm y−0#
Admirality brass Carbon steel Copper Cupronickel "89]09# Cupronickel "69]29# Stainless steel "205L# Titanium
11[75 "SD 9[94# 054 "SD 9[25# 29[37 "SD 9[19# 01[6 "SD 9[92# 6[51 "SD 9[90# nil nil
Coupons exposed for 439 h[
Microbiologically in~uenced stress corrosion cracking failure
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According to Licina\6 if the organic content of the corrosion deposits is greater than 19)\ the corrosion process is likely to be biologically mediated[ SCC is initiated due to the formation of an active corrosion agent\ Cu"NH2#3"OH#1 "tetra amino copper"II# hydroxide#[ There is a possibility of developing relatively high concentration cells of ammonia at the metalÐbio_lm interface on account of the bio_lm matrix which can locally shield ammonia levels\ thereby preventing di}usion into the ~owing water[ When the oxide layer depleted metal surface comes into contact with an ammonical environment\ the anodic reaction triggers because the copper ions produced are known to complex with ammonia to form the highly soluble Cu"NH2#¦ 1 ions[ The overall cathodic reaction is oxygen reduction\ which could be achieved by an intermediate reaction\ the oxidation of Cu"NH2#¦ 1 ions to Cu"NH2#¦ 3 ions[ This reaction is rapid and can occur throughout the bio_lm wherever ammonia concentration cells are formed[ The Cu"NH2#¦ 3 ions are then reduced in the 09 cathodic reaction at the metal surface to reform the Cu"NH2#¦ 1 ions[ Secondly\ Syrett and 09 Coit also report that carbon!dioxide released by the respiring microbes in ammonical environments can signi_cantly increase the corrosion rates of copper alloys[ Khatak et al[05 reported that EDAX analysis of the failed condenser tubes at the intergranular region and at the bottom of the pit showed evidence of dezinci_cation[ Complete loss of zinc was also observed in the crevices[ In the present study\ XRD analysis of the admiralty brass corrosion products "Fig[ 1# revealed the presence of a copper ammonium complex as a major peak\ along with copper ammonium sulfate\ copper\ zinc\ copper nitrate\ and copper"II# oxide has minor peaks "Table 3#[ The presence of the copper ammonium complex indicates that the active corrosion agent has formed on the admiralty
Fig[ 1[ XRD spectrum of admirality brass corrosion products[
0721
T[S[ Rao and K[V[K[ Nair Table 3[ XRD data of admiralty brass corrosion products with reference to Fig[ 1[
S No[
1 Theta
Intensity ) d!spacing
Compound
0 1 2 3 4 5 6 7 8 09 00 01 02 03
7[9243 8[6439 01[1049 04[1118 07[2412 10[1369 11[1389 14[0289 14[6489 16[0939 24[5069 26[2699 32[2279 49[3529
01[4 099[9 04[5 03[3 02[1 06[1 08[3 08[1 03[1 08[1 00[6 01[4 64[2 14[9
Ammonium copper acetate "C5H02CuNO5# Copper ammonium complex "Cu"NH2#5 type# Copper nitrate hydroxide "Cu"OH#2NO2# Gerhardtite not detected not detected Copper ammonium sulfate hydrate "Cu"NH2#SO3H1O# Ammonium copper sulfate hydrate "1"Cu"NH3SO3#1#H1O Copper sulfate "CuSO3# Copper sulfate "CuSO3# Ammonium copper sulfate hydrate "1"Cu"NH3SO3#1#H1O not detected Copper "Cu# Zinc "Zn# Copper"II# oxide "Cu1O#
09[88301 8[95942 6[13994 4[70447 3[72924 3[06725 2[88139 2[42848 2[34467 2[17616 1[40755 1[39333 1[97504 0[79693
Compounds were identi_ed using Powder Di}raction File\ Inorganic phases*Chemical and Mineral Names\ International Centre for Di}raction Data\ USA "0879#[
brass surface[ The presence of the copper peak in the XRD analysis of the corrosion product attests that copper is getting leached by an anodic reaction triggered by bio_lm formation and zinc by the dezinci_cation process as reported by Khatak et al[05#[ XPS analysis "ESCA# of the corrosion products are presented in Fig[ 2\ a multiple chart showing the individual spectra of the four elements carbon\ copper\ nitrogen and oxygen which were observed in the corrosion product[ Tables 4\ 5 and 6 provide the ESCA data obtained for each element with their binding energies[ Cu9\ Cu¦1 and Cu¦0 oxides were used as the standard materials for characterisation of the corrosion products collected from the admiralty brass condenser tubes[ Table 4 shows the standard and satellite peak positions[ Generally\ the Cu1p2:1 principal peak is always associated with a satellite "broad# at around 8 eV above the main peak[ For CuO\ the broad satellite peak observed may be due to multiple shakeup transitions[ The small satellite associated with Cu¦0 could be due to the X!ray source AlÐKa line[ In Cu9\ the Cu peak was observed at 820[8 eV "0[8 eV FWHM#[ Table 6 shows that C0|s main peak appears at around 178 eV\ which may be due to charging\ because all peak positions appear at higher binding energies than their normal shift "chemical# position[ Assuming the shift contribution was mainly due to C1C type carbon "173[5 eV#\ the charge correction of 3[3 eV should be introduced for all the peak positions[ The other peak at 177[1 eV could be due to organic compounds present in the corrosion product[ After charge correction\ the Cu1p2:1 peak was observed at 822[7 eV with a satellite at 830[5 eV[ The satellite peak position and slope indicates that copper is present in the ¦1 state[ The presence of CuO or Cu1O was ruled out because the peak positions do not fully match with the reference peak values[ The N0s peak was observed at 288[0 eV\ which indicates that copper is present in the same CuÐN complex\ which could be a copperÐ amine complex[ The O0s peak was observed at 420[9 eV\ oxide peaks generally appear at 429[2 eV[ The mismatch could be due to the presence of some adsorbed organic compounds[ Based on the observations made above\ it can be concluded that SCC failure of admiralty brass tubes has been caused in an ammonical environment[ The cause could be the presence
Microbiologically in~uenced stress corrosion cracking failure
Fig[ 2[ ESCA spectra of the di}erent elements identi_ed in the admirality brass corrosion product[
0722
0723
T[S[ Rao and K[V[K[ Nair Table 4[ Binding energies of reference copper and copper oxides Element
B[E:FWHM "eV#
Satellite "eV#
Cu¦1 Cu¦0 Cu9
822[1 "2[4# 821[4 "1[7# 820[8 "0[8#
831[9 831[9 "small# nil
Table 5[ Binding energies of prominent peaks after charge correction along with FWHM Element
Binding energy "eV#
Cu C N O
827[1 178[9 392[4 424[3
822[9 "3[22# 173[5 "1[5# 288[9 429[9 "1[3#
Table 6[ The position of di}erent binding energies observed for elements in admiralty brass corrosion products Element
Binding energy "eV#
C0s Cu1p2:1 N0s O0s After charge correction "3[3 eV# C0s Cu1p2:1 N0s O0s
173[8 823[9 "shoulder# 392[4 418[6
178[9 "prominent# 827[1
181[5 835[9 "satellite#
422[7
424[3 "prominent#
173[5 "1[5# 829 "shoulder# 288[0 "broad# 418[3
177[1 822[7 "3[7 eV#
830[5
420[9 "1[3#
of bio_lms and the growth of copper tolerant microorganisms[ Complex bio_lms are ubiquitous at interfaces in nature[1 Glycolipids\ oligopeptides and polysaccharide con! centrations were reported in bio_lms developing on copper alloys and these compounds have been implicated in the corrosion process[32 Therefore\ it is essential to achieve a deeper understanding of microbial bio_lms and their role in corrosion processes and then implicate them in the corrosion mechanism[ Such studies will result in the devising of better control measures which are prudent in combating MIC[ So far\ to our knowledge\ this study is the _rst report of experimental evidence showing the role of nitrate reducers in inducing SCC in admiralty brass[
Microbiologically in~uenced stress corrosion cracking failure
0724
CONCLUSION The study showed that bio_lm formation in lake water is quite pronounced in terms of microbial _lm thickness\ biomass and enrichment of nutrients[ Ammonia is produced in situ bio_lm by the denitri_cation process\ which is corroborated by relatively high NRB counts[ Nitrate reduction could have lead to a build up of the average concentration of ammonia exceeding the threshold limit and causing SCC[ The presence of oxidising sub! stances as dissolved oxygen "from the high productivity of the lake water# and nitrate ions could also accelerate SCC of copper alloys in ammonical solution[ The detection of the copper ammonium complex in the scrapings of the admiralty brass coupons by XRD analysis and the evidence of the copperÐnitrogen organic complex by ESCA further support our hypothesis of microbially in~uenced failure of condenser tubes by SCC[ Acknowled`ements*The _rst author wishes to record his gratitude to Prof[ Kunthala Jayaraman\ Dean "Tech! nology#\ Anna University\ Chennai for her encouragement and keen interest in the work[ Dr[ V[P[ Venugopalan for fruitful discussions during preparation of this manuscript[ Dr P[K[ Mathur\ Head\ Analytical Chemistry Division\ BARC for necessary help and support during the course of this study[ Dr[ S[V[ Narasimhan\ Head\ WSCL and Dr[ S[ Bera for ESCA spectra and data interpretation and Dr[ G[V[N[ Rao\ MSD\ Indira Gandhi Centre for Atomic Research\ for XRD analyses[ Shri[ K[K[ Satpathy and R[ Rajamohan for water quality data[
REFERENCES 0[ Marshall\ K[C[ ASM[ News[\ 0881\ 47\ 191[ 1[ Costerton\ J[W[\ Lewandowski\ Z[\ Caldwell\ D[E[\ Korber\ D[R[\ Lappin!Scott\ H[M[ Ann[ Rev[ Microbiol[\ 0884\ 38\ 600[ 2[ Lapin!Scott\ H[M[ and Costerton\ J[W[ Biofoulin`\ 0878\ 0\ 212[ 3[ Bot\ T[R[ Corros[ Rev[\ 0882\ 00\ 0[ 4[ Bott\ T[R[\ Foulin` of Heat Exchan`ers[ Elsevier\ New York\ 0884[ 5[ Zelver N\ Flandreau J[R[\ Spataro W[H[\ Chapple K[R[\ Characklis\ W[G[ and Roe F[L[\ Analysis and monitoring of heat transfer fouling[ in 71!JPGC:Pwr!6\ ASME\ 0871[ 6[ Licina\ G[J[ MP\ 0878\ 17\ 44[ 7[ Characklis\ W[G[ and Marshall\ K[C[ "eds[#\ Bio_lms[ Wiley\ New York\ 0889[ 8[ Rippon\ J[E[\ UK biofouling control practices[ in EPRI symposium on biofoulin` in condensers\ Atlanta\ U[S[A[\ 0868\ p[ 0[ 09[ Syrett\ B[C[ and Coit\ R[L[ MP\ 0872\ 11\ 33[ 00[ Pope\ D[H[\ Duquette\ D[J[\ Johannes\ A[H[ and Wayner\ P[C[ MP\ 0873\ 12\ 03[ 01[ Little\ B[ and Wagner\ P[ Can[ J[ Microbiol[\ 0885\ 31\ 256[ 02[ Bo}ardi\ B[P[\ Power\ July 0874\ p[ 46[ 03[ North\ R[F[ and Pryor\ M[J[ Corros[ Sci[\ 0869\ 09\ 186[ 04[ Harrison\ J[F[ and Kennedy\ K[W[\ Advances in the control of copper and copper alloy corrosion in chlorinated cooling waters[ in American Power Conference\ Illinois Institute of Technology\ Illinois\ U[S[A[\ April 03Ð05 0875[ 05[ Khatak\ H[S[\ Gnanamoorthy\ J[B[ and Rodriguez\ P[ Pract[ Met[\ 0874\ 11\ 88[ 06[ Venugopal\ K[ and Rawat\ M[S[\ Report on Failure analyses of condenser tubes from RAPS!II turbine condenser[ BHEL R + D\ Hyderabad\ 0880[ 07[ Bakke\ R[ and Olsson\ P[Q[ J[ Microbiol[ Methods\ 0875\ 4\ 0[ 08[ American Public Health Association[ Standards Methods for the Examination of Water + Wastewater[ APHA\ Washington\ D[C[\ 0878[ 19[ Fletcher\ M[\ Comparative Physiology of Attached and Free Living Bacteria[ in Microbial Adhesion and A``re`ation\ ed[ K[C[ Marshall[ Springer\ Berlin\ 0873\ p[ 112[ 10[ Bhosle\ N[B[\ Nandakumar\ K[\ Venkat\ K[\ Dhople\ V[M[\ Sawant\ S[S[\ Wagh\ A[B[ and Kelkar\ P[G[ Proc[ Indian[ Natl[ Sci[ Acad[\ 0878\ B44\ 40[ 11[ Sharma\ M[O[\ Bhosle\ N[ and Wagh\ A[B[ Indian J[ Mar[ Sci[\ 0889\ 08\ 063[ 12[ Srivastava\ R[V[\ Gaonkar\ S[N[ and Karande\ A[A[ Proc[ Indian Acad[ Sci[ "Animal Sciences#\ 0889\ 88\ 052[
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T[S[ Rao and K[V[K[ Nair
13[ Liu\ D[\ Lau\ Y[L[\ Chau\ Y[K[ and Pacepaviciu\ G[J[ Water[ Res[\ 0882\ 16\ 250[ 14[ Madoni\ P[ Wat[ Sci[ Tech[\ 0883\ 18\ 52[ 15[ Vaisanen\ O[M[\ Nurmiaho!Lassila\ E[L[\ Marmo\ S[A[\ Salkinoja!Salonen\ M[S[ Appl[ Environ[ Microbiol[\ 0883\ 59\ 530[ 16[ Postgate\ J[R[\ The Sulfate Reducin` Bacteria\ 1nd edn[ Cambridge University Press\ Cambridge\ London\ 0873[ 17[ Postgate\ J[R[\ Genus Desulfovibrio[ In] Ber`eys Manual of Systematic Bacteriolo`y\ Vol[ I\ ed[ N[R[ Krieg and J[G[ Holt[ Williams + Wilkins\ Baltimore\ U[S[A[\ 0873\ p[ 552[ 18[ Jetter\ R[M[ and Ingraham\ J[L[\ The denitrifying prokaryotes[ in The Prokaryotes\ Vol[ I\ A Handbook on Habitat\ Isolation and Identi_cation of Bacteria\ ed[ M[P[ Starr\ H[ Stolp\ H[G[ Truper\ A[ Balows and H[G[ Schlegel[ Springer Verlag\ Berlin\ 0875\ p[ 802[ 29[ Lowry\ O[H[\ Rosebrough\ N[I[\ Farr\ A[L[ and Randall\ R[J[ Biol[ Chem[\ 0840\ 082\ 154[ 20[ Dubios\ M[\ Gille\ K[A[\ Hamilton\ J[K[\ Robers\ P[A[ and Smith\ F[ Anal[ Chem[\ 0845\ 17\ 229[ 21[ Rao\ T[A[ and Harbola\ P[C[ Int[ J[ Ani[ Sci[\ 0883\ 09\ 042[ 22[ Hamilton\ W[A[ Ann[ Rev[ Microbiol[\ 0874\ 24\ 084[ 23[ Rao\ T[S[\ Rani\ P[G[\ Venugopalan\ V[P[ and Nair\ K[V[K[ Biofoulin`\ 0886\ 00\ 154[ 24[ Iverson\ W[P[ Adv[ Appl[ Microbiol[\ 0876\ 21\ 0[ 25[ Little\ B[\ Wagner\ P[\ Ray\ R[\ Pope\ R[ and Scheetz\ R[ J[ Ind[ Microbiol[\ 0880\ 7\ 102[ 26[ Dean\ R[B[ and Lund\ E[\ Water Reuse] Problems and Solutions[ Academic Press\ London\ 0870[ 27[ Behrens\ D[\ Ammonia + ammonium hydroxide[ in Corrosion Handbook\ Vol[ 1[ VerlagÐAG\ Switzerland\ 0877\ p[ 36[ 28[ Tiedje\ J[M[\ Ecology of denitri_cation and dissimilatory nitrate reduction to ammonium[ in Biolo`y of Anaerobic Microor`anisms\ ed[ A[J[B[ Zehnder[ Academic Press\ New York\ 0877\ p[ 068[ 39[ Bitton\ G[\ Wastewater Microbiolo`y[ WileyÐLiss\ A John Wiley + Sons Inc[\ Publishers\ New York\ 0883\ p[ 40[ 30[ Mimaki\ T[\ Yamashita\ M[\ Hashimoto\ S[ and Miur\ S[ Mater[ Forum\ 0889\ 03\ 8[ 31[ Wilde\ B[E[\ Stress corrosion cracking[ in ASM Handbook\ Vol[ 00\ Failure analysis and prevention[ ASM International\ The Materials Information Society\ U[S[A[\ 0881\ p[ 192[ 32[ Thies\ M[\ Hinze\ U[ and Paradies\ H[H[\ in Microbial Corrosion\ ed[ A[K[ Tiller and C[A[C[ Sequeira[ The Institute of Materials\ U[K[\ 0884\ pp[ 06Ð37[
Corrosion Science\ Vol[ 39\ No[ 00\ pp[ 0710Ð0725\ 0887 Þ 0887 Elsevier Science Ltd[ All rights reserved[ Printed in Great Britain[ 9909Ð827X:87:,Ðsee front matter
Pergamon
PII] S9909Ð827X"87#99968Ð0
MICROBIOLOGICALLY INFLUENCED STRESS CORROSION CRACKING FAILURE OF ADMIRALTY BRASS CONDENSER TUBES IN A NUCLEAR POWER PLANT COOLED BY FRESHWATER T[ S[ RAO and K[ V[ K[ NAIR Marine Biology Programme\ Water + Steam Chemistry Laboratory\ Bhabha Atomic Research Centre Facilities\ IGCAR Campus\ Kalpakkam 592 091\ Tamil Nadu\ India Abstract*The failure of admiralty brass condenser tubes in a nuclear power plant was investigated[ Metallurgical analyses had indicated that stress corrosion cracking "SCC# could have caused the failure[ Studies were carried out to see if bacteria could have played a role in bringing about the conditions which resulted in SCC of the tubes[ Cooling water "Ranapratap Sagar lake\ Kota\ Rajasthan# quality and bio_lm formation were monitored by analysing various physical\ chemical\ biochemical and biological parameters[ Metal coupons were exposed online in the cooling water system to assess the microbial growth on the coupon surface by SEM[ Corrosion rates were assayed by weight loss and corrosion products were analysed by XRD and ESCA[ The study revealed the microbial reduction of nitrate to ammonia "estimated to be of the order of 4[7 mg l−0 at the substratumÐbio_lm interface# which resulted in SCC of the admiralty brass tubes[ Þ 0887 Elsevier Science Ltd[ All rights reserved
INTRODUCTION In aqueous environments\ microorganisms "bacteria\ algae\ fungi etc[# are attracted towards surfaces\ which they readily colonise resulting in the formation of bio_lms[0\1 Such microbial bio_lms are important in a wide spectrum of industrially relevant situations and lead to microfouling and biocorrosion[2\3 The implications of microfouling are energy losses due to increased ~uid frictional resistance and increased heat transfer resistance[4 Zelver et al[5 reported a 29) decrease in heat transfer rate due to biofouling in a fan cooler[ There have been instances of increased capital costs due to premature replacement of equipment caused by severe under!deposit corrosion[4 Moreover\ the fouling of service water systems in nuclear power plants is of concern because it reduces the heat transfer capacity during an emergency or an accident[6 The growth of microbial _lms "slimes# a few tens of microns thick in a condenser tube is su.cient to induce microbiologically in~uenced corrosion "MIC# and cause irreparable damage to the condenser tubes[3 The down!time costs to power plants due to condenser fouling are quite large[7 The temperatures prevalent inside the condenser of an operating power plant provide a conducive environment for the rapid Manuscript received 7 September 0886^ in amended form 17 April 0887 0710
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T[S[ Rao and K[V[K[ Nair
growth of microorganisms[ This results in a thick slime deposit\ which is responsible for heat transfer losses[ The slime layer forms a sticky surface which allows silt and other suspended particles to adhere to the condenser tubes\ thereby enhancing the aggregation of deposits on the material surface and inducing localised corrosion[4\7 An example quoted in the literature8 reports the failure of the alarming number of 3999 condenser tubes in a short span of 6 years of operation of a power plant "Marchwood\ Southampton\ UK#[ This resulted in the leakage of condenser cooling water into the feed water system and then into the boiler\ thereby accelerating boiler corrosion and failure[8 Most reported failures of condensers occur on the cooling water side of the tubes[09 Pope et al[\00 have documented MIC of cupronickel "89]09#\ admiralty brass and aluminum brass in systems cooled with freshwater and brackish water[ It was also reported that copper alloy condenser tubes had under!deposit corrosion due to the formation of slime deposits and ammonia[01 Ammonia producing bacteria were isolated from scale and organic material on admiralty brass tubes su}ering from SCC[01 Copper and its alloys are most commonly used in the fabrication of heat exchangers in cooling water systems[4 The alloys depend on their natural oxide for corrosion resistance[02 This oxide _lm "Cu1O# is a defective _lm with vacancies in the cuprous oxide lattice into which cuprous ions can migrate[03 As copper metal "Cu9# oxidizes to cuprous ion "Cu¦0#\ the cuprous ions move into the vacancies of the oxide lattice[ As the ions migrate through these vacancies\ they get oxidized to the cupric ion "Cu1¦#[ Copper corrosion occurs with the outward movement of the cuprous ion rather than the inward movement of oxygen[ Cu1O protective _lm can be disrupted by a variety of cooling water parameters which include pH\ water velocity\ chlorides\ ammonia and biofouling[ The addition of such elements as aluminum\ zinc\ tin\ iron and nickel to copper have been successfully used to modify the cuprous oxide _lm to make it more corrosion resistant[ These alloying elements essentially plug the holes in the corrosion product _lm\ making it di.cult for ions to migrate through the _lm[04 One of the most commonly used copper alloys in industry is admiralty brass "composition 69[74) copper\ 16[85) zinc\ 9[90) lead\ 9[91) iron\ 0[00) tin and 9[94) arsenic#[ In the present study\ the problem of condenser tube "admiralty brass# failure at Raja! sthan Atomic Power Station "RAPS# unit!II\ Kota\ Rajasthan\ India was investigated[ About 1499 tubes have been replaced over a span of 5 years[ The failure of the tubes lead to the leakage of cooling water into the boiler\ thereby violating the boiler water technical speci_cations[ The leak rate of cooling water ranged between 299 and 1099 l h−0[ Earlier metallurgical analyses of the failed tubes have revealed that the tubes were damaged due to stress corrosion cracking "SCC#[05\06 However\ no systematic study of the interaction between microbial bio_lms and metal has been attempted[ Moreover\ it would be interesting to see whether the initiation of a corrosion phenomenon\ as in bacterialÐhuman infections\ are associated with the adherent microbial community residing in the cooling system[ This present study was carried out to look into the environmental conditions:or causative agent which could have lead to the failure of the condenser tubes[ Comprehensive water quality analysis of the Ranapratap Sagar lake which is being used as the cooling medium for RAPS was carried out[ In addition\ bio_lms developed on perspex and metallic coupons were also characterised for various physical\ chemical\ biochemical and biological parameters in a time series study to understand the bio_lm parameters which could have contributed to the failure[
Microbiologically in~uenced stress corrosion cracking failure
0712
MATERIAL AND METHODS Site description Rajasthan Atomic Power Station is located near Kota\ Rajasthan[ There are two units of Pressurised Heavy Water Reactors\ each of 129 MW capacity[ The power plant is cooled by drawing freshwater from an adjacent lake "Ranapratap Sagar lake# which was formed by damming of the river Chambal[ The catchment area of the lake consists largely of forest!land[ At the intake\ cooling water is drawn from a depth of 29 feet[ The heated e/uent is let o} at the surface of the lake[ During the course of the study no water treatment programme was in practice "Chlorination was in practice during the initial years of plant operation and later discontinued#[ The water samples from the pump house "intake# and outfall were analysed for various chemical parameters such as pH\ conductivity\ dissolved oxygen\ alkalinity\ calcium hardness\ total suspended matter\ total dissolved solids\ chlorine demand\ chlorides\ sulfates\ ammonia and nutrients "phosphate\ silicate\ nitrate and nitrites#[ Biological parameters such as chlorophyll content\ diatom counts\ total culturable bacteria "TCB#\ sulphate reducing bacteria "SRB# and nitrate reducing bacteria "NRB# were assayed[ Corrosion studies were also carried out using admiralty brass coupons[ Apart from weight loss measurements\ the coupons were also processed for SEM studies on microbial colonisation\ XRD and ESCA "Electron spectroscopy for chemical analyses or XPS*X!ray photoelectron spectroscopy# for corrosion product analysis[ Perspex panels "04×09 cm and 6×2 cm# were suspended in the lake at a depth of 0[4 m and retrieved after 13\ 37\ 61\ 85 and 019 h of immersion to study the sequential development of bio_lm[ Two stations were used] "0# at the intake point of the cooling circuit and "1# at the outfall site[ Bio_lm samples as well as samples of ambient water were simultaneously collected for analyses of various parameters as mentioned above[ In the laboratory\ the panels were gently rinsed with micro!_ltered "9[1 mm\ Millipore# and autoclaved "019>C for 09 min#[ Wet bio_lm thickness was determined microscopically "Carl Ziess# using the smaller coupons as per the method described by Bakke and Olson[07 Several "14# measurements of thickness were taken in order to give a reasonably accurate mean value of the thickness[ The bio_lm was scraped from the larger panels using a sterile nylon brush[ The bio_lm material was dispersed in a _xed volume "099 ml# using sterile lake water[ The bio_lm suspension was _ltered "9[1 mm\ Millipore#\ the _ltrate was made up to a known volume and used for nutrient "nitrate\ phosphate and silicate# analysis as per standard methods[08 Bio_lm nutrient values are expressed as mg per litre of bio_lm volume to facilitate comparison with ambient water whose values are expressed as mg per litre[ The bio_lm volume was calculated by multiplying the mean wet bio_lm thickness with area[ In general\ methods followed by other worker19Ð15 were used for di}erent biochemical and biological analyses[ Pre!ignited "349>C\ 3 h# _lter papers "Whatman GF:C# were used to _lter samples for biomass and particulate organic carbon analyses[ Diatoms were counted by haemocytometer[ Counts of heterotrophic bacteria\ nitrate reducing bacteria and sulphate!reducing bacteria "SRB# were made using standard microbiological methods[ SRB were enumerated as reported by Postgate[16\17 Nitrate reducing bacteria were assayed as described by Jetter and Ingraham18 using selective medium[ Proteins and hexose sugars were analysed following the methods of Lowry et al[29 and Dubois
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T[S[ Rao and K[V[K[ Nair
et al[\20 respectively[ Lipids were extracted and quanti_ed as per the method reported by Rao and Harbola[21 Hydrological analyses were carried out as per standard methods[08 Filter sterilised lake water "blank# was also analysed for all the relevant parameters and the values were subtracted from the bio_lm values[ All the analyses were carried out in duplicate and results are presented as mean values[ Scanning electron microscopy of the sample coupons was carried out after _xing the bio_lms with 1) glutaraldehyde and by dehydrating the sample in graded ethanol series[ Later\ the samples were vacuum dried\ sputter coated with goldÐpalladium and observed in a scanning electron microscope "Model] PSEM!490 Phillips make#[ Microfouling on metal coupons "1×1×9[1 cm# admiralty brass\ copper\ cupronickel "89]09 and 69]29#\ carbon steel\ stainless steel and titanium were also carried out[ The other materials "apart from admiralty brass# were tested for the sake of comparison in the light of the RAPS III + IV units\ which are being built at the same site[ The coupons were exposed for 439 h online using a corrosion test rack[ After retrieval\ the coupons were tested for corrosion rate and processed for SEM analyses[ One set of coupons was used for X!ray di}raction study for analysing the corrosion products[ The scraped!out corrosion products from the coupons were _nely powered using a pestle and mortar[ The powder was then dehydrated and compacted into a circular disc using a press[ XRD analysis was carried out using an X! # ray generator "Philips PW 0039:89 with a CuÐKa source\ wavelength used was 0[439487 A and a Siemens "Model] D!499#\ Theta!1 Theta Powder Di}ractometer[ From the D values obtained\ the various phases were identi_ed using online XRD data _les[ ESCA "Electron spectroscopy for chemical analyses# was performed in VG ESCALAB 199X "MK II#\ the X!ray source was AlÐKa at 0375[5 eV[ Hemispherical analyser\ 19 eV pass energy\ vacuum was maintained at 0[4×09−09 mbar[ Data interpretation was carried out with Fisons software[
RESULTS AND DISCUSSION The water quality data "Table 0# showed an increase in pH\ dissolved oxygen\ chlorine demand\ nitrite\ ammonia and bacterial numbers from intake to outfall[ An increase in nitrite and a decrease in nitrate from intake to outfall water indicated that a reduction of nitrate to nitrite was taking place within the cooling circuit[ The decrease in nitrates was also accompanied by an increase in ammonia content which indicates possible reduction of nitrate to ammonia[ Such a possibility was supported by an increase in pH "by 9[7 to 0[1 units observed during the course of this study#[ Decrease in conductivity and total dissolved solids was observed from intake to outfall "Table 0#[ Apparently\ the growth and mul! tiplication of microbes inside the cooling system could have resulted in their utilising the dissolved solids\ nutrients and organic matter[ Analyses of bio_lm samples showed that bio_lm thickness varied from 10 to 097 m and 06 to 196 m at intake and outfall water sites\ respectively\ during a 13 to 019 h exposure period at both the stations "Table 1#[ A gradual increase in bio_lm thickness was observed from 13 to 019 h at both intake and outfall[ Almost a two fold increase in bio_lm thickness was observed at the outfall as compared to the intake after 37 h[ A similar trend was also observed for bio_lm volume\ turbidity and biomass[ The _lm thickness was correlated with bio_lm biomass at both the stations "intake station R 9[86 and P 9[993^ outfall station R 9[83 and P 9[91#[ The bio_lm formed at the outfall was both qualitatively and
Microbiologically in~uenced stress corrosion cracking failure
0714
Table 0[ Characteristics of Ranapratap Sagar lake water Kota\ Rajasthan Parameter
Unit
Intake
Outfall
Temperature pH Sp Conductivity Turbidity Total Alkalinity Total Hardness Total Suspended Matter Total Dissolved Solids Biomass BOD Dissolved Oxygen Chloride Sulfate Nitrate Nitrite Ammonia Silica as SiO2 Phosphate Chlorophyll a Particulate Organic Carbon Total Protein Total Hexose Sugar Diatom Count SRB NRB Total Culturable Bacteria
>C pH units mS cm−0 NTU ppm CaCO2 ppm CaCO2 mg l−0 mg l−0 mg l−0 mg l−0 mg l−0 mg l−0 mg l−0 mg l−0 mg l−0 mg l−0 mg l−0 mg l−0 mg m2 mg C l−0 mg l−0 mg l−0 cells ml−0 cfu ml−0 cfu ml−0 cfu ml−0
14 "13Ð15# 7[9 "6[7Ð7[0# 134 "124Ð144# 1[4 "1[9Ð2[9# 84"nil#"81Ð87# 86 "85Ð87# 0[9 "9[7Ð0[1# 037 "026Ð045# 00 "7Ð03# 9[3 "9[2Ð9[5# 4[4 "3[9Ð5[9# 03 "02Ð03# 00 "09Ð01# 195 "082Ð103# 09 "8[9Ð00# 03 "00Ð17# 6[4 "5[9Ð8[9# BDL 2[4 "1[9Ð4[9# 7[5 "6[7Ð8[3# 9[3 "9[24Ð9[34# 6[3 "5[7Ð7[5# 154 "149Ð175# 0×092 "SD 143# 3×091 "SD 34# 0×094 "SD 879#
29 "16Ð22# 7[6 "7[3Ð7[8# 127 "129Ð134# 5[4 "4[9Ð7[9# 77"4# "73Ð81# 78 "77Ð80# 6[9 "3[9Ð8[9# 024 "015Ð032# 19 "07Ð21# 0[1 "9[5Ð0[7# 7[6 "6[2Ð8[6# 02 "01Ð03# 6 "5[1Ð7[7# 51 "31Ð62# 06 "04Ð08# 67 "50Ð76# 7[4 "6[1Ð8[7# BDL 3[4 "3[9Ð4[9# 03[4 "02Ð05# 9[4 "9[3Ð9[5# 8[9 "7[9Ð00# 134 "139Ð145# 3×092 "SD 114# 8×093 "SD 399# 6×093 "SD 209#
Mean values^ Values given in the parenthesis are the minimum and maximum observed during the course of this study[ Values in " # are P!alkalinity values[
quantitatively richer[ SEM pictures "Plate 0# revealed the presence of dense algal and bacterial consortia on the metal coupons[ Chemical analyses "Table 1# of the bio_lm samples showed a concentration of nutrients in the bio_lm as compared to water[ It was observed that accumulation of nutrients took place in the initial stages of bio_lm formation[ Concentration factors for nitrate were relatively high[ A close examination of the nutrient data revealed a decreasing trend for nitrate and nitrite concentrations as the bio_lm aged\ indicating their possible reduction[ Ammonia values have shown an increase in concentration after 85 h of bio_lm growth\ otherwise the distribution was almost similar for the rest of the days[ Silicate values have shown an initial increase up to 37 h and\ thereafter\ decreased[ Ammonia levels were relatively high in the bio_lm formed at the outfall waters[ Anaerobic conditions prevailing at the bio_lmÐsubstratum interface could facilitate denitri_cation\ which generally occurs in anoxic conditions[ This is further corroborated by the presence of SRB in the bio_lm[ Hamilton22 reported that a bio_lm of 09Ð14 mm is enough to provide anoxic conditions in the bio_lm[ During this study\ it was noted that a bio_lm thickness of 06 mm is enough to support obligate anaerobic growth in the bio_lm "SRB 093 cfu cc−0 of bio_lm#[ The results of biochemical analysis "Table 1# showed that particulate carbon\ total
0715
T[S[ Rao and K[V[K[ Nair
Table 1[ Characteristic features of the various physical\ chemical\ biochemical and biological parameters of bio_lm formed at RAPS cooling circuit intake "I# and outfall "O#[ Parameter
Station
13 h
37 h
61 h
85 h
019 h
Water Temperature >C
I O I O I O I O I O I O I O I O I O I O I O I O I O I O I O I O I O I O
16 29 10 06 9[477 9[365 7[62 04[3 095 75 3[1 3[4 9[04 9[05 9[95 9[96 4 4 9[90 9[95 9[98 "09# 9[96 "3# 5[4 "21# 6[9 "055# 9[16 "23# 9[10 "29# BDL 0[6 "14# 2×092 1×092 0×093 2×093 0×095 0×096 1×091 0×094
15 29 16 48 9[645 0[541 00[14 32[6 042 120 02 15 9[07 9[08 9[0 9[5 6 06 9[91 9[04 9[04 "04# 9[16 "04# 09[2 "49# 0[5 "27# 9[04 "08# 9[97 "003# BDL 1[3 "23# 4×092 1×092 1×093 1×094 2×095 1×096 4×091 1×094
11 16 65 013 1[017 2[361 01[19 36[7 102 548 36 69 9[10 9[14 9[2 0[5 21 76 9[00 9[35 9[15 "14# 9[09 "5# 4[3 "16# 0[1 "18# 9[97 "099# 9[94 "60# 9[4 "23# 2[9 "32# 2×092 4×091 0×094 0×095 1×095 1×094 0×092 6×094
13 17 80 042 1[437 3[173 05[19 015[3 118 0645 54 097 9[18 9[20 9[3 2[7 36 048 9[05 9[51 9[98 "09# 9[03 "3# 1[1 "09# 0[9 "13# 9[96 "75# 9[93 "46# 9[8 "52# 4[7 "72# 3×091 0×092 1×094 2×095 7×094 6×094 1×093 0×095
14 20 097 196 2[913 4[799 07[39 88[1 161 1929 80 085 9[24 9[28 1[9 5[3 51 115 9[34 9[71 9[04 "04# 9[16 "04# 0[5 "7# 9[71 "08# 9[94 "52# 9[92 "32# 0[1 "75# 1[4 "25# 0×092 0×092 0×094 8×094 5×094 1×094 6×093 5×094
Bio_lm Thickness "mm# Bio_lm Volume "cc# Turbidity "NTU# Biomass "mg ml−0# Particulate Organic Carbon "mg C ml−0# Chlorophyll a "mg ml−0# Total Protein "mg ml−0# Hexose sugar "mg ml−0# Total Lipid "mg ml−0# Silicate "mg l−0# Nitrates "mg l−0# Nitrite "mg l−0# Ammonia "mg l−0# Diatom Count "cells ml−0# SRB Count "cfu ml−0# Total Culturable Bacteria "cfu ml−0# Nitrate Reducing Bacteria "cfu ml−0#
Values in parenthesis are concentration factors when compared with ambient water[
protein\ total carbohydrates and total lipids increased with time at the intake as well as at the outfall[ The rate of increase was found to be greater in the bio_lm formed at the outfall as compared to that at the intake\ indicating that the outfall "with a DT of 6>C# provides a conducive environment for rapid bio_lm growth[ The lake water was also high in chloro! phyll content "3Ð4 mg m−2# indicating dense algal population and also a high capacity for primary productivity[ This was also re~ected in the bio_lm\ where there was a pronounced growth of diatoms "diatom count# and Chlorophyceae species "chlorophyll content\ Table 0 and SEM pictures#[ Bacterial counts\ such as total viable counts "TVC#\ sulphate reducing bacteria "SRB#
Microbiologically in~uenced stress corrosion cracking failure
0716
Plate 0[ "a# Scanning electron!micrograph showing microbial consortium on admiralty brass coupon exposed online in RAPS cooling circuit\ various morphologies "long rods\ short rods and coccoid shaped bacteria# can be seen "29 kV\ magni_cation 0421×#[ "b# Scanning electron!micrograph showing short\ rod!shaped bacteria and diatom "Fra`illaria Sp[ with exopolymeric substances# on admiralty brass coupon exposed online in RAPS cooling circuit "29 kV\ magni_cation 0421×#[ "c# SEM picture showing typical Fra`illaria Sp[ _lamentous growth "girdle view# on perspex coupons exposed at the intake of RAPS cooling circuit "29 kV\ magni_cation 0105×\ scale 0 mm#[ "d# SEM picture showing the presence of diatoms "Nitzchia Sp[ Gomphonema Sp[# on perspex coupon exposed at the outfall of the RAPS cooling circuit "29 kV\ magni_cation 1264×\ scale 0 mm#[
and nitrate reducing bacteria "NRB# have shown variations in their distribution in the bio_lm developed at both the stations "Table 1#[ Fungal counts at the outfall bio_lm were more "6×093 colonies per cm1#[ Fungi were detected after 61 h of bio_lm growth\ bio_lm formed at the intake site had very few fungi[ However\ the microbiological load was found to be higher in the bio_lm formed in the outfall water as compared to the bio_lm at the intake station\ particularly in the initial stages "13 h to 37 h#[ At the intake station\ bio_lm SRB has shown only a gradual increase with time[ At the outfall station\ however\ bio_lm\ SRB population has shown an increase of two orders of magnitude after 61 h of bio_lm growth and thereafter stabilised[ Relatively high counts of nitrate reducing bacteria were found in the lake water "093 cfu ml−0#\ which corroborate the view that denitri_cation could be the major energy!deriving pathway[ NRB were less at the intake bio_lm during the initial stages of bio_lm growth\ however\ after 85 h of bio_lm growth there was an increase of 1 orders of magnitude[ The outfall station bio_lm has a relatively high NRB population\ indicating the in~uence of temperature on NRB growth "see Table 1#[ The number of NRB in the bio_lm was signi_cantly correlated "r 9[72 and P 9[991# with ammonia
0717
T[S[ Rao and K[V[K[ Nair
concentrations measured in the bio_lm "Fig[ 0#[ This indicates that ammonia generation was caused by NRB and not by mere accumulation from water[ The denitrifying bacteria observed during the course of this study were Alcali`ens Sp[\ Bacillus Sp[\ Micrococcus denitri_cans\ Pseudomonas aeru`inosa and Pseudomonas ~uorescens[ Total culturable bac! teria have not shown much variation in the intake bio_lm[ However\ at the outfall station bio_lm\ there was a decrease in the bacterial population by two orders of magnitude "096 to 094# after 37 h of bio_lm growth[ The decrease could be attributed to the presence of protozoans grazing on the microbial population\ which was observed after 61 h of bio_lm growth at the outfall station[ Similar results were observed earlier by Rao et al[23 in bio_lm characterisation studies carried out in a freshwater impoundment at Kalpakkam[ SRB are a diverse group of anaerobic bacteria which can survive in anaerobic micro! niches in aerated environments until conditions are conducive for their proli_c growth[ If the aerobic respiration rate within a bio_lm is greater than the oxygen di}usion rate\ at the substratum:bio_lm\ the interface becomes anaerobic and provides an environment for sulphide production by SRB[ Biocorrosion induced by SRB residing in bio_lm has been
Fig[ 0[ Regression analysis for nitrate reducing bacteria and ammonia production[
Microbiologically in~uenced stress corrosion cracking failure
0718
particularly recognised as a serious problem in cooling systems[22\24 The presence of sulfate reducing bacteria in the intake water and their increase in the outfall water was a notable feature\ as it was reported by Postgate17 that SRB can also reduce nitrate to ammonia[ The diatom number in the bio_lm varied from 0×092 to 4×092 and from 499 to 1×092 cells cm−1 at the intake and outfall waters\ respectively\ during the period 13Ð019 h[ In the presence of light\ diatoms produce oxygen and change local oxygen concentrations at the metalÐbio_lm interface\ a}ecting the local chemistry to a great extent[25 The con! centration of ammonia "Table 1# in the bio_lm formed at the outfall station was found to be several times more than that in the bulk water "299 times^ 4[7 mg l−0#[ According to Dean and Lund26 and Berhens\27 0 mg l−0 of ammonia is su.cient to induce stress corrosion cracking in admiralty brass tubes "copper alloys#[ A possible mechanism for ammonia formation by the denitri_cation process has been proposed by Tiedje28 which involves four stages viz[^ reduction of nitrate to ammonia via nitrite\ hyponitrite\ and hydroxyl amine[ Nitrate reduction is driven by nitrate and nitrite reductase[ Since nitrate serves as an electron acceptor\ the growth rate of denitri_ers depends on nitrate concentration "055 times more in bio_lm when compared to bulk water#[ Denitri_cation can also occur in bio_lms despite the presence of relatively high levels of oxygen in the bulk water[39 Denitrifying bacteria require an electron donor to carry out the denitri_cation process which is served by organic matter[ The Ranapratap Sagar lake water is rich in organic matter "up to 6 mg l−00hexose sugar content#[ ¦ − NO− 2 ¦1H ¦1e:NO1 ¦H1O
"0#
¦ − 1NO− 1 ¦3H ¦1e:N1O1 ¦1H1O
"1#
¦ N1O− 1 ¦5H ¦3e:1NH1OH
"2#
NH1OH¦1H¦¦1e:NH2¦H1O
"3#
Copper alloys are susceptible to SCC fracture in many industrial environments[30 These environments include those containing ammonia\ citrate\ tartrate\ moist SO1\ sulfates\ nitrites\ and nitrates under certain circumstances[ SCC in copper alloys occurs most fre! quently in environments that contain ammonia or amines\ either in aqueous solutions or in moist atmospheres[ The presence of oxidising substances such as dissolved oxygen\ ferric iron and nitrate ions accelerates SCC of copper alloys in ammonical environments[ SCC is a failure process that occurs because of the simultaneous presence of tensile stress\ a conducive environment and susceptible material[ When the alloy stressed in tension is also exposed to a corrosive environment\ the ensuing localized electrochemical dissolution of metal\ combined with localized plastic deformation\ opens up a crack[30 Protective _lms that form at the tip of the crack rupture\ causing fresh anodic material to be exposed to the corrosive medium and SCC is propagated[ A preexisting mechanical crack or other surface discontinuity\ such as a pit or trench produced by chemical attack on the metal surface\ may act as a stress raiser and thus serve as a site for the initiation of SCC[31 In aqueous solutions\ pH has a strong in~uence on susceptibility to cracking[ Intergranular cracks occur most rapidly in neutral solutions[ The crack is transgranular in both alkaline and acidic solutions[09 Given that stresses are high enough at some locations on a Cu alloy tube\ the second prerequisite for SCC is that the environment be capable of inducing cracking[ The deleterious species responsible for service failures on the cooling water side have not been positively identi_ed in most cases[ Water side failures are frequently associated with
0729
T[S[ Rao and K[V[K[ Nair
deposits on the tubes\ presumably because the deposit provides a site for concentration of the deleterious species or local reactions which cause a pH change beneath the deposit to a value that favors SCC[ Rippon8 reported that admiralty brass condenser tubes in a power plant failed due to the SCC attributed to the ammonia produced by the bacteria[ Metallurgical studies carried out on the failed condenser tubes of RAPS unit!II05 have shown that the crack was initiated from the inside of the tube in an intergranular mode and subsequently propagated in a transgranular mode[ Di}erential aeration caused by corrosion products or bio_lm deposits could have resulted in a reduction in tube thickness of 9[65 mm as compared to 9[8 to 0[1 mm at other places[06 This loss in thickness could have been caused by di}erential aeration cells as the area under the deposit will be the anode and the outside the cathode which\ if large\ results in an e}ective galvanic cell[ Harrison and Kennedy04 reported that corrosion rates exceeding 0[9 m y−0 are considered very high for copper alloys[ In the present study\ corrosion rates "Table 2# of the admiralty brass coupons exposed online for 439 h were found to be relatively high\ 11[75 mm y−0 or 9[89 m y−0 "SD 9[94# for a set of three coupons[ In their studies\ Venugopal and Rawat06 reported that the pro_le of cracks observed on the inner side of the admiralty brass condenser tube was longer[ Both circumferential and longitudinal cracks were observed on the external surface of the failed tubes[ The cracks varied from 0 to 3[4 cm in length[ Pits were observed close to the cracked regions[ The etched samples showed that the propagation of cracks was transgranular in nature[ The nature of the crack indicated that the admiralty brass tubes had failed due to SCC[ The cracks were initiated from the pitted regions[ Generally\ pits act as stress raisers and\ when coupled with the residual stresses present in the tube due to manufacturing process\ can initiate SCC[ The results of the study point out that the water is rich in organic matter and nitrates\ since the water is drawn from a depth of 29 feet from the lake[ The decay of organic matter and\ thereafter the reduction of nitrate\ could have resulted in very high concentrations of ammonia "up to 299 times that in the water# in the bio_lm[ Phosphate was a limiting nutrient in the lake water[ Hence\ it can be emphasised that nitrate is the major energy deriving source of the microbial community of the lake[ As mentioned\ two metallurgical studies were carried out05\06 which reported that the tubes failed due to SCC[ Scrapings from the metal coupons "carbon steel and admiralty brass# exposed online have shown the presence of high levels of organic matter "16)#[
Table 2[ Corrosion rate of some heat exchanger materials exposed online in the cooling circuit of RAPS Material
Corrosion rate "mm y−0#
Admirality brass Carbon steel Copper Cupronickel "89]09# Cupronickel "69]29# Stainless steel "205L# Titanium
11[75 "SD 9[94# 054 "SD 9[25# 29[37 "SD 9[19# 01[6 "SD 9[92# 6[51 "SD 9[90# nil nil
Coupons exposed for 439 h[
Microbiologically in~uenced stress corrosion cracking failure
0720
According to Licina\6 if the organic content of the corrosion deposits is greater than 19)\ the corrosion process is likely to be biologically mediated[ SCC is initiated due to the formation of an active corrosion agent\ Cu"NH2#3"OH#1 "tetra amino copper"II# hydroxide#[ There is a possibility of developing relatively high concentration cells of ammonia at the metalÐbio_lm interface on account of the bio_lm matrix which can locally shield ammonia levels\ thereby preventing di}usion into the ~owing water[ When the oxide layer depleted metal surface comes into contact with an ammonical environment\ the anodic reaction triggers because the copper ions produced are known to complex with ammonia to form the highly soluble Cu"NH2#¦ 1 ions[ The overall cathodic reaction is oxygen reduction\ which could be achieved by an intermediate reaction\ the oxidation of Cu"NH2#¦ 1 ions to Cu"NH2#¦ 3 ions[ This reaction is rapid and can occur throughout the bio_lm wherever ammonia concentration cells are formed[ The Cu"NH2#¦ 3 ions are then reduced in the 09 cathodic reaction at the metal surface to reform the Cu"NH2#¦ 1 ions[ Secondly\ Syrett and 09 Coit also report that carbon!dioxide released by the respiring microbes in ammonical environments can signi_cantly increase the corrosion rates of copper alloys[ Khatak et al[05 reported that EDAX analysis of the failed condenser tubes at the intergranular region and at the bottom of the pit showed evidence of dezinci_cation[ Complete loss of zinc was also observed in the crevices[ In the present study\ XRD analysis of the admiralty brass corrosion products "Fig[ 1# revealed the presence of a copper ammonium complex as a major peak\ along with copper ammonium sulfate\ copper\ zinc\ copper nitrate\ and copper"II# oxide has minor peaks "Table 3#[ The presence of the copper ammonium complex indicates that the active corrosion agent has formed on the admiralty
Fig[ 1[ XRD spectrum of admirality brass corrosion products[
0721
T[S[ Rao and K[V[K[ Nair Table 3[ XRD data of admiralty brass corrosion products with reference to Fig[ 1[
S No[
1 Theta
Intensity ) d!spacing
Compound
0 1 2 3 4 5 6 7 8 09 00 01 02 03
7[9243 8[6439 01[1049 04[1118 07[2412 10[1369 11[1389 14[0289 14[6489 16[0939 24[5069 26[2699 32[2279 49[3529
01[4 099[9 04[5 03[3 02[1 06[1 08[3 08[1 03[1 08[1 00[6 01[4 64[2 14[9
Ammonium copper acetate "C5H02CuNO5# Copper ammonium complex "Cu"NH2#5 type# Copper nitrate hydroxide "Cu"OH#2NO2# Gerhardtite not detected not detected Copper ammonium sulfate hydrate "Cu"NH2#SO3H1O# Ammonium copper sulfate hydrate "1"Cu"NH3SO3#1#H1O Copper sulfate "CuSO3# Copper sulfate "CuSO3# Ammonium copper sulfate hydrate "1"Cu"NH3SO3#1#H1O not detected Copper "Cu# Zinc "Zn# Copper"II# oxide "Cu1O#
09[88301 8[95942 6[13994 4[70447 3[72924 3[06725 2[88139 2[42848 2[34467 2[17616 1[40755 1[39333 1[97504 0[79693
Compounds were identi_ed using Powder Di}raction File\ Inorganic phases*Chemical and Mineral Names\ International Centre for Di}raction Data\ USA "0879#[
brass surface[ The presence of the copper peak in the XRD analysis of the corrosion product attests that copper is getting leached by an anodic reaction triggered by bio_lm formation and zinc by the dezinci_cation process as reported by Khatak et al[05#[ XPS analysis "ESCA# of the corrosion products are presented in Fig[ 2\ a multiple chart showing the individual spectra of the four elements carbon\ copper\ nitrogen and oxygen which were observed in the corrosion product[ Tables 4\ 5 and 6 provide the ESCA data obtained for each element with their binding energies[ Cu9\ Cu¦1 and Cu¦0 oxides were used as the standard materials for characterisation of the corrosion products collected from the admiralty brass condenser tubes[ Table 4 shows the standard and satellite peak positions[ Generally\ the Cu1p2:1 principal peak is always associated with a satellite "broad# at around 8 eV above the main peak[ For CuO\ the broad satellite peak observed may be due to multiple shakeup transitions[ The small satellite associated with Cu¦0 could be due to the X!ray source AlÐKa line[ In Cu9\ the Cu peak was observed at 820[8 eV "0[8 eV FWHM#[ Table 6 shows that C0|s main peak appears at around 178 eV\ which may be due to charging\ because all peak positions appear at higher binding energies than their normal shift "chemical# position[ Assuming the shift contribution was mainly due to C1C type carbon "173[5 eV#\ the charge correction of 3[3 eV should be introduced for all the peak positions[ The other peak at 177[1 eV could be due to organic compounds present in the corrosion product[ After charge correction\ the Cu1p2:1 peak was observed at 822[7 eV with a satellite at 830[5 eV[ The satellite peak position and slope indicates that copper is present in the ¦1 state[ The presence of CuO or Cu1O was ruled out because the peak positions do not fully match with the reference peak values[ The N0s peak was observed at 288[0 eV\ which indicates that copper is present in the same CuÐN complex\ which could be a copperÐ amine complex[ The O0s peak was observed at 420[9 eV\ oxide peaks generally appear at 429[2 eV[ The mismatch could be due to the presence of some adsorbed organic compounds[ Based on the observations made above\ it can be concluded that SCC failure of admiralty brass tubes has been caused in an ammonical environment[ The cause could be the presence
Microbiologically in~uenced stress corrosion cracking failure
Fig[ 2[ ESCA spectra of the di}erent elements identi_ed in the admirality brass corrosion product[
0722
0723
T[S[ Rao and K[V[K[ Nair Table 4[ Binding energies of reference copper and copper oxides Element
B[E:FWHM "eV#
Satellite "eV#
Cu¦1 Cu¦0 Cu9
822[1 "2[4# 821[4 "1[7# 820[8 "0[8#
831[9 831[9 "small# nil
Table 5[ Binding energies of prominent peaks after charge correction along with FWHM Element
Binding energy "eV#
Cu C N O
827[1 178[9 392[4 424[3
822[9 "3[22# 173[5 "1[5# 288[9 429[9 "1[3#
Table 6[ The position of di}erent binding energies observed for elements in admiralty brass corrosion products Element
Binding energy "eV#
C0s Cu1p2:1 N0s O0s After charge correction "3[3 eV# C0s Cu1p2:1 N0s O0s
173[8 823[9 "shoulder# 392[4 418[6
178[9 "prominent# 827[1
181[5 835[9 "satellite#
422[7
424[3 "prominent#
173[5 "1[5# 829 "shoulder# 288[0 "broad# 418[3
177[1 822[7 "3[7 eV#
830[5
420[9 "1[3#
of bio_lms and the growth of copper tolerant microorganisms[ Complex bio_lms are ubiquitous at interfaces in nature[1 Glycolipids\ oligopeptides and polysaccharide con! centrations were reported in bio_lms developing on copper alloys and these compounds have been implicated in the corrosion process[32 Therefore\ it is essential to achieve a deeper understanding of microbial bio_lms and their role in corrosion processes and then implicate them in the corrosion mechanism[ Such studies will result in the devising of better control measures which are prudent in combating MIC[ So far\ to our knowledge\ this study is the _rst report of experimental evidence showing the role of nitrate reducers in inducing SCC in admiralty brass[
Microbiologically in~uenced stress corrosion cracking failure
0724
CONCLUSION The study showed that bio_lm formation in lake water is quite pronounced in terms of microbial _lm thickness\ biomass and enrichment of nutrients[ Ammonia is produced in situ bio_lm by the denitri_cation process\ which is corroborated by relatively high NRB counts[ Nitrate reduction could have lead to a build up of the average concentration of ammonia exceeding the threshold limit and causing SCC[ The presence of oxidising sub! stances as dissolved oxygen "from the high productivity of the lake water# and nitrate ions could also accelerate SCC of copper alloys in ammonical solution[ The detection of the copper ammonium complex in the scrapings of the admiralty brass coupons by XRD analysis and the evidence of the copperÐnitrogen organic complex by ESCA further support our hypothesis of microbially in~uenced failure of condenser tubes by SCC[ Acknowled`ements*The _rst author wishes to record his gratitude to Prof[ Kunthala Jayaraman\ Dean "Tech! nology#\ Anna University\ Chennai for her encouragement and keen interest in the work[ Dr[ V[P[ Venugopalan for fruitful discussions during preparation of this manuscript[ Dr P[K[ Mathur\ Head\ Analytical Chemistry Division\ BARC for necessary help and support during the course of this study[ Dr[ S[V[ Narasimhan\ Head\ WSCL and Dr[ S[ Bera for ESCA spectra and data interpretation and Dr[ G[V[N[ Rao\ MSD\ Indira Gandhi Centre for Atomic Research\ for XRD analyses[ Shri[ K[K[ Satpathy and R[ Rajamohan for water quality data[
REFERENCES 0[ Marshall\ K[C[ ASM[ News[\ 0881\ 47\ 191[ 1[ Costerton\ J[W[\ Lewandowski\ Z[\ Caldwell\ D[E[\ Korber\ D[R[\ Lappin!Scott\ H[M[ Ann[ Rev[ Microbiol[\ 0884\ 38\ 600[ 2[ Lapin!Scott\ H[M[ and Costerton\ J[W[ Biofoulin`\ 0878\ 0\ 212[ 3[ Bot\ T[R[ Corros[ Rev[\ 0882\ 00\ 0[ 4[ Bott\ T[R[\ Foulin` of Heat Exchan`ers[ Elsevier\ New York\ 0884[ 5[ Zelver N\ Flandreau J[R[\ Spataro W[H[\ Chapple K[R[\ Characklis\ W[G[ and Roe F[L[\ Analysis and monitoring of heat transfer fouling[ in 71!JPGC:Pwr!6\ ASME\ 0871[ 6[ Licina\ G[J[ MP\ 0878\ 17\ 44[ 7[ Characklis\ W[G[ and Marshall\ K[C[ "eds[#\ Bio_lms[ Wiley\ New York\ 0889[ 8[ Rippon\ J[E[\ UK biofouling control practices[ in EPRI symposium on biofoulin` in condensers\ Atlanta\ U[S[A[\ 0868\ p[ 0[ 09[ Syrett\ B[C[ and Coit\ R[L[ MP\ 0872\ 11\ 33[ 00[ Pope\ D[H[\ Duquette\ D[J[\ Johannes\ A[H[ and Wayner\ P[C[ MP\ 0873\ 12\ 03[ 01[ Little\ B[ and Wagner\ P[ Can[ J[ Microbiol[\ 0885\ 31\ 256[ 02[ Bo}ardi\ B[P[\ Power\ July 0874\ p[ 46[ 03[ North\ R[F[ and Pryor\ M[J[ Corros[ Sci[\ 0869\ 09\ 186[ 04[ Harrison\ J[F[ and Kennedy\ K[W[\ Advances in the control of copper and copper alloy corrosion in chlorinated cooling waters[ in American Power Conference\ Illinois Institute of Technology\ Illinois\ U[S[A[\ April 03Ð05 0875[ 05[ Khatak\ H[S[\ Gnanamoorthy\ J[B[ and Rodriguez\ P[ Pract[ Met[\ 0874\ 11\ 88[ 06[ Venugopal\ K[ and Rawat\ M[S[\ Report on Failure analyses of condenser tubes from RAPS!II turbine condenser[ BHEL R + D\ Hyderabad\ 0880[ 07[ Bakke\ R[ and Olsson\ P[Q[ J[ Microbiol[ Methods\ 0875\ 4\ 0[ 08[ American Public Health Association[ Standards Methods for the Examination of Water + Wastewater[ APHA\ Washington\ D[C[\ 0878[ 19[ Fletcher\ M[\ Comparative Physiology of Attached and Free Living Bacteria[ in Microbial Adhesion and A``re`ation\ ed[ K[C[ Marshall[ Springer\ Berlin\ 0873\ p[ 112[ 10[ Bhosle\ N[B[\ Nandakumar\ K[\ Venkat\ K[\ Dhople\ V[M[\ Sawant\ S[S[\ Wagh\ A[B[ and Kelkar\ P[G[ Proc[ Indian[ Natl[ Sci[ Acad[\ 0878\ B44\ 40[ 11[ Sharma\ M[O[\ Bhosle\ N[ and Wagh\ A[B[ Indian J[ Mar[ Sci[\ 0889\ 08\ 063[ 12[ Srivastava\ R[V[\ Gaonkar\ S[N[ and Karande\ A[A[ Proc[ Indian Acad[ Sci[ "Animal Sciences#\ 0889\ 88\ 052[
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T[S[ Rao and K[V[K[ Nair
13[ Liu\ D[\ Lau\ Y[L[\ Chau\ Y[K[ and Pacepaviciu\ G[J[ Water[ Res[\ 0882\ 16\ 250[ 14[ Madoni\ P[ Wat[ Sci[ Tech[\ 0883\ 18\ 52[ 15[ Vaisanen\ O[M[\ Nurmiaho!Lassila\ E[L[\ Marmo\ S[A[\ Salkinoja!Salonen\ M[S[ Appl[ Environ[ Microbiol[\ 0883\ 59\ 530[ 16[ Postgate\ J[R[\ The Sulfate Reducin` Bacteria\ 1nd edn[ Cambridge University Press\ Cambridge\ London\ 0873[ 17[ Postgate\ J[R[\ Genus Desulfovibrio[ In] Ber`eys Manual of Systematic Bacteriolo`y\ Vol[ I\ ed[ N[R[ Krieg and J[G[ Holt[ Williams + Wilkins\ Baltimore\ U[S[A[\ 0873\ p[ 552[ 18[ Jetter\ R[M[ and Ingraham\ J[L[\ The denitrifying prokaryotes[ in The Prokaryotes\ Vol[ I\ A Handbook on Habitat\ Isolation and Identi_cation of Bacteria\ ed[ M[P[ Starr\ H[ Stolp\ H[G[ Truper\ A[ Balows and H[G[ Schlegel[ Springer Verlag\ Berlin\ 0875\ p[ 802[ 29[ Lowry\ O[H[\ Rosebrough\ N[I[\ Farr\ A[L[ and Randall\ R[J[ Biol[ Chem[\ 0840\ 082\ 154[ 20[ Dubios\ M[\ Gille\ K[A[\ Hamilton\ J[K[\ Robers\ P[A[ and Smith\ F[ Anal[ Chem[\ 0845\ 17\ 229[ 21[ Rao\ T[A[ and Harbola\ P[C[ Int[ J[ Ani[ Sci[\ 0883\ 09\ 042[ 22[ Hamilton\ W[A[ Ann[ Rev[ Microbiol[\ 0874\ 24\ 084[ 23[ Rao\ T[S[\ Rani\ P[G[\ Venugopalan\ V[P[ and Nair\ K[V[K[ Biofoulin`\ 0886\ 00\ 154[ 24[ Iverson\ W[P[ Adv[ Appl[ Microbiol[\ 0876\ 21\ 0[ 25[ Little\ B[\ Wagner\ P[\ Ray\ R[\ Pope\ R[ and Scheetz\ R[ J[ Ind[ Microbiol[\ 0880\ 7\ 102[ 26[ Dean\ R[B[ and Lund\ E[\ Water Reuse] Problems and Solutions[ Academic Press\ London\ 0870[ 27[ Behrens\ D[\ Ammonia + ammonium hydroxide[ in Corrosion Handbook\ Vol[ 1[ VerlagÐAG\ Switzerland\ 0877\ p[ 36[ 28[ Tiedje\ J[M[\ Ecology of denitri_cation and dissimilatory nitrate reduction to ammonium[ in Biolo`y of Anaerobic Microor`anisms\ ed[ A[J[B[ Zehnder[ Academic Press\ New York\ 0877\ p[ 068[ 39[ Bitton\ G[\ Wastewater Microbiolo`y[ WileyÐLiss\ A John Wiley + Sons Inc[\ Publishers\ New York\ 0883\ p[ 40[ 30[ Mimaki\ T[\ Yamashita\ M[\ Hashimoto\ S[ and Miur\ S[ Mater[ Forum\ 0889\ 03\ 8[ 31[ Wilde\ B[E[\ Stress corrosion cracking[ in ASM Handbook\ Vol[ 00\ Failure analysis and prevention[ ASM International\ The Materials Information Society\ U[S[A[\ 0881\ p[ 192[ 32[ Thies\ M[\ Hinze\ U[ and Paradies\ H[H[\ in Microbial Corrosion\ ed[ A[K[ Tiller and C[A[C[ Sequeira[ The Institute of Materials\ U[K[\ 0884\ pp[ 06Ð37[