Comparing Two Different Types of Anaerobic Copper Biocorrosion by Sulfate- and Nitrate-Reducing Bacteria

WENJIE Fu, YINGCHAO LI, D AKE Xu,

AND TINGYUE Gu, Ohio University, Athens, Ohio

Biocorrosion (microbiologically influ­enced corrosion IMIC)) is caused by biofilms. There are two types of an­aerobic MIC. Type I involves utiliza­tion of extracellular electrons released by the oxidation of an energetic metal such as elemental iron (FeO) in a mi­crobe's cytoplasm. Type /I does not require this kind of electron transfer because the reduction of the secreted oxidant such as proton, occurs extra­cellularly. This work compared MIC mechanisms of copper by sulfate-re­ducing bacteria and nitrate-reducing bacteria.

It is said that 20% o r lIIorc of corrosion

losses can be attributed to microbiologi ­ca lly influ enced corrosion (MTC).! Gu t

classified a naerobi c MIC 1nto two types.

Type 1 is caused hy anaerobic respiration of

elemental m etal such as elemental iron

(Fell), Microbes such as sulfate- reducing bacLeria (SRn) utili ze Fell as an electron

dono r (fuel) when the normal e lectron

donor for metabolism (usually organic carbon) is unavailable underneath th e

biofilm . Because FeD in a carbon stee l (CS) matrix is inso lu ble. iron oxidation occurs

extrace ll u la rly. This m eans that in order to

utili ze t h e ext racellular electro ns. the

66 JUNE 2014 MATERIALS PERFORMANCE

biofilm must be ab le to transport them across the ce ll wa ll into th e cytoplasm,

where reduct ion of the oxidant sllch as

sulfate takes place under enzyme biocatal­

ysis. This kind ofbiofilms contain microbes

known as cl t!ctrogens, and thq· have bCt!fl used in microbial fuel ce ll s to gt!neratc

bioeleclricily. :i

Xu and Gu4 performed carbon starvation

lesls by subjecting mature bioHlms on CS

coupons to new culture media containing

different organic carbon concen tra tions.

They tound that. starved bioti hns were more aggressive provided that the hiofilms were

not compldely deprived (If organic carbon.

Their theory is alsu supporled by the pilus

(conduct ive nanowire) formalion by SRB

cells grown in an ol·ganic carbon-free medium for extracellular e lectron transfer

w hile no pilus form:l tion was observed for a medium with organic carhon hecause solu­

ble organic carbon ca.n donate electrons in t.he cytoplasm. 5

There are various electron trans fer

mecha nisms for bio fiL ms. They include

direct electron transfer and mediated elet;­

lron lransfer." Gu and Xu? udded ribuOuvin

to Dcsuifollibrio 1I11lgaris culLures and found that it p romoted MIC as expected because

ri bofl avin is II common electron mediator. Another common electron m edi ator is

molecular hydrogen (H2

) . In fact. th e

cathodic d epolarization t heory first pro-

NACE INTERNATIONAL: VOL 53, NO. 6

posed by von \Volzogen Kiihr and van der

Vlu gt in 1934 for MIC by hyd rogen ase­

posit ive SRRs is an examp le of Type T MTC

utili zing H ~ a" an electron mediator. A provi­

sional patent was filed by Ohio University on

the use and detection of an electron media­

tor in rvlIC t ests and forensics in :lOll.Y

Recently, a group of German researchers

presentt!d experimenLal evidence for wred

electron transfer in SRB MIC. to

Microbiologically Influenced Corrosion Thermodynamics

Iron oxidation

Elec on

{Peri plasm)

NO -3

Cytoplasm

N2 (or NH:>

Sessile NRB cell Gu, et aL" proposed a Simplified SilO

MIC m CL:hanism known as biocatalysis

calhudk sulfate rt!duction (DCSR) based on

the following two reactions: FIGURE 1 Electron transport and bioe ne rgetics of MIC against steel caused by NRB.l~

Anodic reaction Fe ---* Fe2' + 2e- (-E"' = ,147 mV)

(extracellular iron oxidation)

Cathodic reaction sot + YW+ge- --+ HS- +IJ HP

(BCSR in cytoplnsm)

(I)

(2)

Tn Reaction (2) , HS- rather than S2-

s hould be u sed. Reaction en has a n

extremely small equiUbdum constant bern.'een 25 and 37 QC.I~ Thus, S~- is signifi­

cant only at very high pH that is usually n ot

suilable for microbial grow thY

(3)

Formation of hydrogen sulfide (H2S) is

facilitaLed b y an acidic pH lhrough

Reaction (4):

(4)

~GO' = - 1,165 kJ / mul nitrate. Thus, it is rea­

sunable Lo speculate LhaL electrogenic

nitrate-reducing bacteria (NRB) can be cor­

rosive when th ey switch from an organic

carbOll to FeD as an electron donor.

Nitrate injection has heen successfully

practiced in reservoir souring mitigation hy

suppressing sulfate reduction. A few pub­

lished wurks demunstraled NRI3 curro­

sion. I:; Thus, nitraLe injection should be

metcred carefully to prevent nitrate from

getting into transport pipel ines.

Xu, e\: al. ls performed a n experiment: by

growing a Bacillus licheniformis biofilm on

C10 18 (UNS (;10180) CS coupons in a NRH

cul ture medium using nitrate as the term i­

nal electron acceptor. B. liclzell(formis is a

ubiquHous microbe in nature and is fo und

in oilfield biufilm consortia. It. Currosion

products analyzed by Xu, et al. suggest that

in addition to N~, ammonium is also formed

:l J\'Og - /N~ has a reduction potential of due to t he following ni trate reduction reac-

En' = +760 mV that is much greater than lion:

-2 17 mV for sulfate rcdudiun. 14 Thus,

nitraLe is a more polenL oxidant:

2NO.l

+ 12H ' + lOe- ---7 N2 + 6Hp (E"' = +760 ITIV)

The coupling of Reactions (I) and (5)

produces a redox reaction with a ce ll

potential of filE'" = + 1,207 m V in which the

apostrophe denotes pH 7. This leads to

N03

+ 8c- + lOH~ --+ NH 4+ + 3Hp

(E"' = +358I11V) (6)

This provides a cell potential of fIlEo' =

+805 m V w hen coup led ,vith Reaction ( 1), an d leads to fIlGo' = - 621 kJ/molnitrate.

Figure 1 illu strates th e role of e lectron

t r ansport and bioenergetics in thi s kind of

NKB attack against steel.

TABLE 1. MEDIUM FOR NITRATE·

REDUCING BACTERIA"

Sucrose 10g

K,HP04 13.9 9

KH P04 2.7g

NaCI 1 9

Yeast extract 1 9

NaNO, 2.5 9

MgSO, 0.25 9

Distilled water 1L

Composition of trace element stock solution

MnCl, ·4H,o 180mg

CoCli 6H,o 270mg

H;B0 3 50 mg

CuCi ·2H ° 24mg

NaMoO .. 2H,o 23 mg

ZnCl, 19m9

Distilled water 100 mL

':A)Ten mL of a trace element stock solution

was added to 1 L of the culture medium .

The SRB and NRB MIC mechanisms dis­

c lissed above are fUlld umentaJly similar.

They both belong to Type T MTC that is very

different from Type n T\HC. Type n MTC is

cau sed by secreted corrosive metaholites

NACE INTER NATIONAL: VO l. 53, NO. 6 MATERIALS PERFORMANCE JUNE 2014 67

MATERIALS SELECTION & DESIGN

FIGURE 2 Top: Copper coupons exposed to D. vulgaris for seven days after cleaning with 20% H

2S0

4, Bottom: Abiotic control coupons.

'" ... .8 OJ MH no " . H'l ' " 10 2' .. , ... '" .0 13 17 78

..

FIGURE 3 EDS analysis of corrosion products on a copper coupon exposed to D. vulgaris for seven

days before the corrosion products and biofilm were removed,

such as prolons and un dissociated organic

acids that can serve as proton reservoirs. In

the absence of all exogenous electron

acceptor such as su lfat.e and nitrate, fer­

m entative anaerohes produce their own

electron acceptors that are products from

organic carbon oxidation. Acid-producing bacteria (APB) arc a typical example of fcr­

mentative microbes. The reduction of the

corrosive metabolites occurs extracellu ­

larly as shown helow using proton aR an

example:

68 JUNE 2014 MATERIALS PERFO RMANCE

2f-f++2e--7 1-i , (proton reduction) (7)

T)11e I and Type II J\:IIC mechanisms are

fundamentally different. Type I M Ie pit mor­

phology tends to he pit in pit or terrace-like

while the pits in Type II j\'lIC are expected to

be more shalluw amI widespread. Nitrate is

a far more poLenL uxidanllhan sulfaLe (+ 760 mV 'o's. - 217 IllV for r'). The coupling ofeul)

oxidation with nitrate reduction produces

rather positive !1F.°' values of +240 mV and

+420 111\~ respectively. Thus, Type T MTC hy

NRB can proceed fonvard .

.Elemental copper (Cu~) corrosion by SH.ll

in sprinkler sys tems and heat exchange

tubes has been well knmvn. BCSR is not a

valid theory for CUll currosion by SRD

because CuI) is not an energetic metal ",,·hen

sulfate is used as the oxidant. This is

retlected by th e rather positive EO' values for

Cu'/Cu (+520 mY) and Cu 2 -1 /Cu (+340 mY)

in the follmving reactions.

CU-7CU++ C- (- E"' =-520mV) (8)

eu ---7 C1I2• + 2c- (_EM == - 340 mV) (9)

The redox reactions of Cuo oxidations in

Heact ions (8) and (':J ) coupled with sulfate

reduction obviously have negative cell poten­

Hals (l~e' = - 737 mV and - 557 mV, respec­

li .... ely). Thus, direct oxidalion uf CUll by

sulfate jg Lhermodynamically Lwfa-.!orable.

A very different mechanism is needed to

expJaul Cuo corrosion by SRB. Puigdomenech

and Taxen l7 found that the following reac­

tion is favored thermodynamically,

2ClI(cq .. t olj + s2-- + 21-1 - -7

CulS{"'J""I} + H2(g l (10)

Th.is suggests that tJle suJtide secreted by

SRR and the proton cau se the corrosion

rather than fl ulfate reduction. The t erminaJ

electron acceptor is proton. This is Type II

lVue by SHlJ rather than Type j NIlC.

Experimental Conditions D. vulgaris (ATeC 7757t) was u scd to

represent SRR. The cu lture m edium used to

grO\.\I the SRR was the same as the one used

by Xu and Gu.4 B. licheniformis (ATCC

14580-t ) was cultured using a Nf{K culture

tTrnde name.

NACE INTER NATIONAL: VOL. 53, NO. 6

Comparing Two Different Types of Anaerobic Copper Biocorrosion by Sulfate- and Nitrate-Reducing Bacteria

medium (Tahle 1). Sucrose served as the

carbon source and nitrate as the external

electron I:u..:ceptur. After sterilization, buth

culture m edia were deoxygenated using fil ­

tered nitrogen gas sparging for 45 min. Copper Alloy llO (UNS ClJOOO) (99,98 wt%

Cu and 0.02% 0) coupons were used . The

di sk coupon s we re s li ce d off a ~/R -in

(9-mm) diameter rod and sequenti ally pol­ished with # 180, #400, and #600 grit abra­

sive papers. Coupons were th en cleaned

with isopropanol for 10 s.

Each 1:l0-rnL anaerobk vial cuntaining

100-JIlL of culture medium was inoculated in a glovcbox fi lle d with filtered nitrogen

gas byintroduchlg 1 ml. of seed cu lture. The

initial cell concentration in the vial immedi­

at ely after inoculation. was ", 106 cell s/mL

for D. vulgaris and 2 x 10(' cells/mL for

lJ. licheniformis. After inoculation, all vials were sealed and then incubated aL~7 °C for

scvcn days beforc coupons \y(!rc removed

for analysis. Abiolic contruls had lhe same contenls in lhe vials and sam e incubation

time, but they were not inoculated. To remove biotilm and corrosion prod­

ucts, a 20% (v/v) sulfuric acid so lu tion was

m ed to clean the copper coupon surface for 10 s, exposing the bare Cuo metal. llefore

viewing a biofilrn image on acoupon surface under a scanning elec tron microscope (SEM), the biofilm was prcpar(!d wi th a

method described by "Ven, et al. 1K SEt",I ami energy dispersive spectroscopy (EDS) were

conducted u sing a JEOL JSM-6390~ SEM. X-ray diffraction (XRD) all a lysi s was per­

form ed ",.'ith a Rigaku Ultima TV'I" x-ray dif­

fractometer. An Alicona ALC13t profilome­

tel' 'was used to obtain pit depth prufiles.

Results and Discussion The copper coupons retrieved from a

seven -d ay SRB culture all showed corro­

sion, as shown in Figure 2. During the seven-day test, the bulk liquid pH remained

betv\.'een 6.8 to 6.9. Figure:2 shows that the

scv(!n -day abiotic coupons (controls) were

relatively dean while the SRn corruded coupons all sho\ved corrosion products that were flaky. The SRB corrosion rate cal­

cu lated from th e copper COUpOll weight

loss of CJA m g/em :! was O.5!i mm / y. much

greater than the 0.01 mm/y for the abiotic control coupons,

Figure 3 shows Lhe EDS elemenl analy­sis results of an SItB corroded copper cou-

NACE INTER NATIONAL: VOl. 53. NO. 6

400

350

300

@ 250 Q.

S,l ~ 200 '. c ~ .E

150

100

50

o ~-5 25

A

A

45

2-Theta (")

A: Elemental copper S. Chalcocite (Cu2S)

65 85

A

i .- ...

FIGURE 4 X-ray diffraction patterns of corrosion products on a copper coupon exposed to D. vulgaris for seven days before the corrosion products and biofilm were removed.

~. t,· I. '

. ,

.'

"

\ ~i ,",,: / 1 " , '~/ I /rJj. i ., I .",.. '. .t .- "'/ i ' '/ ./ j,.}:, ~/' .'. r

, ! '''/~ ,'I " '.' J,

.' .;: I ;. ..' tl ~ , ~.,,!II"r.: I '>" " .. i~ .'.4', ~- .,' . f "

I :/".1' ;", .l'·~i t I . I.' .~, J

~m (b)

/'\ o V (" 'Y \ - 2 +

t " ."'.'/.'_/ '+ i '( -4 --- +

l i

.;' / •..... \ il ,.. "".

; ,rl !~,''''', t, ,'" '. t.,~. ~'

J !. ',' " • -,'" • J •

/;"', ",f' .f! I~;' ./.'

-6 .c-- t -40 -20 o 20 40 j.lm

FIGURE 5 Surface analysis of a cop per coupon after seven days of exposure to NRB. A 7-~m deep pit was found on the co upon surface after it was cleaned with 20% sulfuric acid.

pon surface hefore the corrosion layer was removed. Th e S/Cu ratio indicated that the

corro sion laye r containe d s ignificant

amount s of either copper sulfide (CuzS) (chalcudte) or cuppt!r monosulfide (ellS).

Further analysjs hi Fjgure 4 shows lhalthe

corrosion layer was mostly Cu2S. The unfa­

vorable thermodynami cs of CUD oxidation

coupled with su lt~'1te r ed uction under an

SRR biofilm prevented Type T MTC . This proves that Type IT MTC of CUD by NRB

occurred v,"ith Cu2S as the main corrosion

product.

Nitrate is a far more p ot ent oxidant than sulfate and thus 'I'ype I IVII C of Cuo oxi­

dation coupled with nitrate reduction is

thermodynamically favorable. Figure [;

shows charactt!rislic Type 1 .MIC pits on NRB corroded copper surface. A 7 -IlID dee p

pit \'\'as found. The pH value during incuba­

tion remained bet'ween 5.8 to 7.2 ill all the via ls. The coupon s in side the R. Uchen­

iformis cultures looked dull whil e the

abiotic control coupons appeared shiny.

The weight loss of the copper coupons after seven days in the NRB culture \-\Tas very

M ATERIALS PERFO RM A NCE JUNE 2014 69

MATERIALS SELECTION & DESIGN

small, ",0.5 mg/cm 2, corre sponding to

0.02 m m /y uniform corrosion rate.

Conclusions Thermodynamics d etermines wh ether

a MTC mechani sm is possible or not. Tn thi s

work, bioen erge tics was used to pro be the

th ermodyn ami cs ofSRB and NRB attacks in

Typ e I MIC (electr o n t ra n sfe r M IC) a n d

Type II MIC (m etaholite MI C). Pavorahl e

thermodynamics must b e assisted by kinet­

ic s to muve a corru sion mech a nism fo r­

ward a t a n a ppreciable rale. This work

rcvic\.vcd some of the charac teristics of the h vo types of anaerobi c J\H C. It a lso pre­sented experim enta l data of SRR and NRR

a ttac ks 011 e lem en t al copper to comp"l e­

men t published dat a on SHE and NRB cor­

rosion orcs for a systematic comparison of MIC m echanism s among different systems.

This kind of academic study will improve

lhe fundamen Lal umlersLand..ing of various MIC m echanism s. The SRB copper corro­

sion analysis in th is work may help explain t he so-call ed "toxic drf\\'a ll " corros ion of

copper due to its re lease ofH~S.

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WENJIE FU is a M.B.A. graduate student at the Univers ity of Denver, 2199 S. University Blvd., Denver, CO 80208, e -m a il : mi s sfuwe njie@gmail.com. Sh e received her M.S. degree in chemical engineering at Ohio University in 2013. Her thesis was on MIC mechanisms and its mitigation.

YINGCHAO LI is a Ph ,D, student at Ohio University, 171 Stocker Center, Athens, OH 45701, e-mail: yI23709@ohio.edu. He received a B.Sc. degree in 2007 and an M.S. degree in 2010, both in materials scie nce, from the Unive rsity of Science and Technology, Beijing, China. He is currently involved with MIC research at Ohio University 's Department of Chemical and Biomolecular Engineering. He has co-authored three journal papers and two book chapters about MIG. He is a member of NACE Inte rnational.

DAKE XU is an associate professor at the In stitute of Metal Research, Chinese Academy of Sciences, 72 Wenhua Rd., Shenyang, Liaoning 110016, China, e-mail: xudake @imr.ac .cn. He earned a Ph.D. in chemical engineering from Ohio University in 2013. His research interests include MIC and antimicrobial metallic materials . He won second place for the best student poster in applied corrosion technology (Ha rvey Herro category) at CORROSION 2011 in Houston, Texas, He is the co-corre­sponding author of this article and is a member of NACE.

TINGYUE GU is a professor at Ohio Unive rsity, De partment of Chemical and Biomolecular Engineering with affiliation with the Institute for Corrosion and Multiphase Technology, 167B Stocker Center, Athens, Ohio, 45701, e-mail: gu@ ohio.edu. He earned a Ph.D. in chemical engineering from Purdue University in 1990. His research interests include detec­tion and mitigation of MIC mechanisms and fore nsics . He has served as a consul­tant for severa l companies and is the co-corresponding author of this article, He is a 1 O-year member of NACE. /tIP

70 JUNE 2014 MATERIALS PERFORMANCE NACE INTER NATIONAL: VOL. 53, NO. 6