a JI | I s '
/ .... ,
rO 44 -. 4~o . -' 114
Dissolved Oxygen and pH Microelectrode
Measurements at Water-Immersed
Metal Surfaces*
Z. Lewandowski.* W C. Lee.* W G. Characklis. and B. Little'*
ABSTPR4CT
'Dissolved oxygen (DOt and pH were measurec at meta, aqif, c:a ton procranr and manr, other factors
seawater interfaces usmng microe ect!oces in boc ano abb3oc To understand mechanism of microbiaiv incicec
systems Measurements In a closed system proved that oresence is imperatve to describe exactly the conoit,,os a' the meta-v.a: o
of electrochemical and or biologica. reaction products substan- interface in terms of chemical composittoi Thus far tnere arc
tiaiy influence the conditions at the metal surface. For long-term many theoretical speculations regardng these conditions Trer_-.
studies. only open (e.g.. continuous flow) reactors should be cal calculations must be i...c'er. verified by direct meas.c
used. An open channe: pow -eacto' suitable both for microbiolog- ments The authors of this paper believe that tcols for suZ'- .e,'
icai and electrocnemicai measurements nas been constructed. cation already exist-microeectrodes Microeiectroce tecnnooc,
and successfully tested - " for analytical purposes has made considerable progress Mcrzc-
lectroc:s !or measurement o: m:, norcn:: :c: .'.:t:;es r
' INTRODUCTION microenvironments have been constructed and sjccessfu',
,__ _ _-._ _ _ _tested Microelectrndes have found application mainl. in te '..a
of physiology and medicine, but the same or a simila" const'u.:1,
'- Investigation of microbial corrosion requires incorporation of both can be applied to describe conditions at the meta! water i"te'a:c
electrochemical and microbiological methods Simple insertion of This paper describes the application of dissolved oxyger :P a-:
microorganisms into an electrochemical reactor may not yield use- pH microelectrodes for measurements at meta! surfaces irn--esez
ful or relevant information Inserting corrosion coupons into a mi- in water DO and pH are important factors for chemcal oesc "c
crobiological reactor may be srmlarly useless. Investigation of mi- of the metal-water interface Oxygen is the preferred electron a:-
crobial corrosion requires integration of methods from both ceptor for microbial respiration Under aerobic conditons o',s#ce
disciplines modified for specific measurements suitable for relevant concentration decreases as the microbiall colonized meta s,-
' experimental reactors face is approached Areas of oxygen depletion on meta' su'a-
Microbial colonization of a metal surface immersed in water are anodic relative to surrounding areas Reduction in oxie cc -
. changes the properties of the metal-water interface and influences centration also creates conditions conducive for sulfate-reduc ng
the corrosion rate because of microbial metabolic 3ctivity " The bacteria even if the bulk medium has measurable DO
difference between corrosion rates in the absence and in the pres- concentration 2 Microbial activity may also change the meta su-
ence of microorganisms is called microbia corrosion. The micro- face pH High metal surface pH promotes formation of calcareous
-€ bial corrosion rate is a function of microbial activity at the metal deposits while low pH causes dissolution of deposits and exoc-
surface The kind and intensity of microbially induced changes at sure of the metal surface ' Little. t al. .4 suggest that pH at the
the metal surface depends on many factors, and hence. generali- microbially colonized metal surface can. in some cases be as lov.
zation of mechanisms for microbial corrosion is difficult. Microbial as 0 6
( corrosion rate depends on the kind of microorganisms that colo- The performance of DO and pH electrodes were tested , a
nize the metal surface, the electron donor and the electron accep- system that allowed comparison of measured and theoretical re-
tor in the microbial respiration chain, rate of microbial metabolic suits Cathodic polarization was chosen as a suitable system Djir-
activity, physical and chemical properties of the bulk water, hydro- ing cathodic polarization, DO is reduced at the metal surface and
dynamic flow regime at the metal surface, metal composition a stoichometrically predictable amount of hydroxyl ion is release'
physical and chemical properties of metal surface. corrosion inhibi- Thus an increase in applied cathodic potential would cause DO
concentration at the metal surface to decrease and pH iv,
increase This test. conducted under abiotic conditions was under-
Submitred for publicaton Decerbe' 1967 revised April 198e Paper No 93 taken to verify the applicability of the micro 'lectrodes for descric-
presented at CORROSION'a8 tion of conditions at the metal surfe. Positive evaluation of
Bz emi. M o , n It n a -, , d7 ' . ,-,, I I, t' tia,', i- '- :b., microelectrode performance in an abiotic System (measurec re-
Nozen MontaRa 59717 eTSoMs suie correspondirg to theoretical prediction) can ustfy ther
• "Navat Ocean Research and Develomeni Actay NSTL Station MississipD1 application in a biotic system where results are hard to predict
39529 4
Reprinted from-CROSION. Vol 45 No 2 pp. 92-98 (1989) February
Covricht 1989 by the National Association nr Corrosion Engineers P 0 Box 218340 Houston Texas 77218
ASHIELDED CABLE
A
PLATINUM
,TePeX
FIGURE 1. DO mcroelectrode
EXPERMENTAL PROCEDJ-PE
Dissolved Oxygen Electrode
The DO electrode. des:gneo b, Ciark. el a- was reduced to
microelectrode size using a procedure described by Revsbecn and
Jorgensen." The electrode was made of a 0.1-mm. high-purty
(99 99%) platinum wire etched electrochemically with one end in
KCN to a tip diameter of -2 pm The wire was rinsed with con-
centrated HCI and ethanol and covered with soda-lime glass The
tip of the platinum wire was exposed oy grinding on a rotating
wheel covered with diamond paste The exposed platinum tip was
subsequently etched in KCN to yield a recess of -- 2 iirr, The op-
eration was performed with a microscope with a mounted TV cam
era and was observed on a video screen. The tip of the electrode
was covered with a polymer (TePeX " I. serving as the oxygen-
permeable membrane The electrode was calibrated in 3,5% In-
stant Ocean' solution by aeration and subsequent purging with FIGURE 2. Microphotograph of a pH microelectone
pure nitrogen. The current in the measuring circuit was measured
with a picoammeter with output to a linear recorder a sketch of a
DO electrode is presented in Figure 1
0 1 M NaCl buffered to pH 6 with 0 1 M citrate buffer The sii\e-
pH Electrode sil'vr chloride (Ag'AgCI) wire was prepared by coating a silve,
The pH microelectrode was of a recessed type The wire (cleaned with nitric acid) with silver chloride by making it the
construction was a modification of tha: described by Thomas' A -!rcde in d-'tp hvdrnchinnc acid Thp wire was sealed inside the
electrode shaft with silicone rubber. The recessed construction
micropipette made of lead glass (Corning t 01201 served as insula- permits the electrode to touch a metal surface without breaking
The. cpeit made ofnive glass .(Conige5aled as ae membranes the pH-sensitive membrane The difference in potential between
The capillary made of pH-Isensiive glass. sealed on one end, was thmesrenadrerncAggIlcroewsesud
inserted into the insulating lead glass pipette almost to the end the measurement and reference Ag/AgCI electrode was measuredApplication of pressure to the pH-sensitive glass capillary, along using an electrometer connected with an amplifier of 10"2 ohms
with careful heating of the sealed end with a heating loop impedance The electrode was calibrated using standard pH buffer
expanded the pH-sensitive glass inside the insulating glass. solutions.
thereby providing a fused seal between the two glasses (Figure 2). rectot
All manipulations were performed using a light microscope The reactors:
electrodes were subsequently immer,, in lialllled wa-" and 1. A closed. abi l c reactor for describing conditions at a ca-
",'ij Muilea gent:y for -i n lt nyor . ';ie pH-sensfwve glass thodically polarized stainless steel (SS) coupon, and
Then. the distilled water in the microelectrodes %as replaced with 2 An open channel reactor for measuring the DO profile in
an artificial biofilm attached to SS corrosion coupons
Artificial seawater (3.5*0 Instant Ocean) and 3 5%. sodur
Product Of SOi Limied Englan chloride (NaCI) were used as the medium Chemical composilo r
'Reg.sred iracniame of the Instant Ocean is presented in Table 1
CORROSION-Vol. 45, No. 2 93
4w 5 I II I
COiROC'n:. S'ClE\,,2
TABLE 1 Open Biotic Reactor
Ionic Cmi.ositon r Measurements in the presence of microorganisms were con-
3.5% Instant Oceani ducted in an open channel flow reactor with dimensions 1.00 x
Ion ppm 0.15 m x 0.30 m (Figure 4). Twenty-four corrosion coupons madePPM__ 
_ of AIS1' 4 ' 304 SS (1.59-cm inside diameter) were fixed in the reac-
CiloroE ( 1 78E tor using sample holders and were connected to potentiostat Cor-
Sod,'m (Na-' 10424 rosion coupons were prepared by polishing with 600-grit powder
Suate (SOI 256- degreasing with 100% ethanol, and rinsing with acetone Twelve
• Ma~gnevirr, 1Wc] 1265
Calcium [ca- 396 coupons were maintained as sterile control, twelve additional cou-
Potassium [K 31 pons were inoculated with mixed culture of microorganisms A
Bartornale (BCO- 14 drop of activated sludge was placed on the coupon surface and
Borate [BO' 28
Phospiate [P0, 14 covered with 0.5% agar Subse4uently, the agar-microbe mix was
covered with 2% agar. The 2% agar prevented penetration of m,-Solas Tota 33 99- A croorganisms from the bulk medium to the metal coupons The
pore size of 20 agar had been estimated to be 44 nm " The
present measurement (Figure 5) showed the pore size of 2% agar
to be in the range of 10 to 60 nm Consequently. bacteria fro,-, the
Closed Abiotic Reactor liquid phase cannot penetrate through the 2% agar to tne metal
Measurements in an abiotic. closed reactor we ,: onducted surface On the other hand, the mixed culture originally immob-
to evaluate the electrode performance under well-defined condi- lized remains trapped on the metal surface The medium 3 5-%
lions at the metal surface The influence of cathodic polarizaton artificial seawater (instant Ocean) with 100-mgL-' yeast extract
on interfacial DO concentration and pH was measured Measure- was supled contnuously and flowed over the reactor surface
ments were conducted in a 1-L volume PARCL2 flask equipped creating a suitable shear stress The oxygen and nutrients wee
witn two graphite counter electrodes a SCE and an Allegheny transported from the bulk medium by diffusion through the aca'
Ludlum' 3 6X (SS) working electrode (Figure 3) Cathodic polarza- layer to the immobilized cells The conditions at the corrosion co -
tion potentiai voltages were dete-mined ana maintainec constan' pons were evaluated by measuring the DO profile n the adar laye-
using a PARC 273 potentiostat-galvanostat Applied potential was during cathodic polarization of the corrosion coupons The first
varied from 0 to -1 0 V(SCE) in 0 1-V intervals Circular AL6X measurements were conducted just after inoculation. ther,
electrodes with a surface area o' 1 c"- were cut from a 0 65-mm repeated after 14. 38 ano 52 n of reactor ooeratio r After 3E r- o'
metal sheet The workinc electroE was rnsed Witi-, acetone prior reactor operation. qlutaraldehyde was introduced for 14 r, in tie
to mounting in the PARC-flat electrode holder DO and pH at the reactor influent at 500 mgL
cathodicaliy polarized meta surfaces were measured using micro- The purpose of the glutaraldehyde treatment was to mact -
electrodes positioned at ine surface ol the working electiode using vate microorganisms at the metal surface anc to comoare co c
a micromanipulator Measurements of DO and pH were made lions (microbial activity) with those at the beginning o' reacto
against a standard Ag AgCI electrode Durno the measurements eration After 52 n of operation (14 h of Contact with dlutarace
the reactor iquid was purged with air Measurements were cor- hyde, the DO profile an cathodic polanzat,o measu..eme-ts
ducted in 3 5% artificial seavwate-, Instant Ocean and 3 5% so- were repeated
dium chloride
nPr,nceto, A~ppe, Researc c: -,:e -- -______
'AieGhen, Uac Co-c rls, rcC- Pe s a Ame-car iro- anc Stee irhtrie AIS Was--crc- DC
IRECORDER H  MECTERO AMPLIFIE POTENTIOSTAT
RECORDERJ-
FLIrOW METER
GAS
SATURATED
CONE ELEOCT EL g.p"U L .
Aa/xii-Cl ftFEK ft ECTf WR K ELECTRODO
FIGURE 3. Exper,menta, apparatus for measurements of conditions at a cathodcaiy polarized meta,
surface
94 CORROSION-February 1989
ELEC 
CA ®E LCrl, ICE
MICOELCIODE'~EkLIB~L. CALQtEL ELECTRODE SE
I ~ol'EfI CHA141i
yFLOW REACTORAL
NUT RiENS----RIGELCRD
FILTEREDI P U MP
0 RAIIN
FIGURE 4. The open channel flow reactor.
(Figure 5) with a JEOL 6 100 CX eletori mr.crosrnno using a-
ASiD 7-40 scanning attachment
EXPERIMPATAL RESULTS
Abiotic Experiment
Interfacial 00 concentration at a cathodtcally polarizez AL6X
SS surface in aerated instant Ocean measured (Figure 6; Resu:!5
indicate that increasing applied potential to - 0 3 VISCE, did o*.
change the DO at the metal surface Further increase in applie-
potential resulted in rapid decrease in DO at the metal surface
Oxygen concentration reached zero when the applied potentil
reached -0.8 VISCE). The curve reaches zero oxygen ccncentra-
tion asymptotically.
pH- was measurcc! at "ic- ca*hodicniiy poilrized metal surface
in 3.50v artificial seawater and 3.5%. sodium chloride solution
(Figure 7), pH reaches a plateau between -0.6 and -0 8 VISCEI
FIGRES SE mirogapt ofAceobeteracei i 2%aga marix applied potential for artificial seawater and NaCI solution. The re-FIGUE S SE mirogaphof cetbectr aetiin 0,4aga marix sults correspond with DO consumption (Figure 6) Increase in ap-
(thesizeof ar i eqivalnt o 20 nmplied potential over -0.8 V(SCE) in both cases resulted in further
increased pH.
Agar Pore Size Measurement Biotic Experiment
The pore size of 2% agar was estimated by scanning elec- Current density as a function of cathodic polarization was
tron microscopy (SEM). Agar discs colonized with Acetobacter measured using corrosion coupons in the open channel flow reac-
acet, were fixed for 8 h in 2% glutaraldehyde diluted with filter tor (Figure 8). Cathodic potential against a SCE was varied from
sleilized artificial seawater. The agar discs were dehydrated in a corrosion potential E, to -0.6 VISCE). The current density re-
graded series of ethanol (30 min in ethanol 30 to 100%). The agar sulting from a specific imposed potential decreased considerably
discs were prepared for electron microscopy by critical point drying with run time. Glutaraldehyde treatment reversed this tendency
using a Balzersisi CPD-020 critical point dryer Samples were
sputter-coated with Au-Pd alloy and were examined at 30,OOOX________
________________ *JEOL. Japan
laiZers Hudson, Ne Hampshire 'Product of JEOL
CORROSION-Vol. 45, No. 2 9
CL)R R 0SO ', c S CE:
10. 16
-@I- PWged I/ er-0- 0 h,
I-- 14 hr.
aU 12 - - 38 hr.
0~
Ce U& E
V Z
-. 6 -. -4 -. 3 -. 2 -
0-1 ~ ~ 4 - Cathodic potentials V(SCE)
Cathodic potentials V(SCE) FIGURE 8. The results of cathodic polarization measr)ennents a'
corrosion? coupons in an o4en) channel flow reacfc-
FIGURE 6. Metal surface DO as a functor of appliec catnodic
Potential in 3.5% artificiaf sea ware- purged wti, a;,
I1 -sC.
10. a -1 500
-20004
7-
Dissolved oxygen concentration (ppm)
FIGURE 9. DO profiles in agar films on corrosion~ coooo-s -m
absence (_-) and presence (0, oi , nicroorga nisms afte, 4 reprfc,
operation.
Cathodic potentials V(SCE)
FIGURE 7. a/- as a function of applied cathodic potential in 3.5% DISCUSSION
artificial seawater (AS) and 3.5%o sodium chloride INaCl), purged
with air. Abiotic Experiment
K'croelectrodes have been used to measure DO and pH at a
and the resulting cathodic polarization curve after treatment is es- cathodically polarized metal surtaces. The results can be analyzed
sentially the same as at the beginning of the experiment, based on chemical reactions that occir on cathodically polarized
The DmO pivfgi~ in the agar layer reflect the microbial respira- metal surfaces, Application of cathodic potential causes reduction
tion activity during the experiment (Figures 9 and 10). Tl._ agar of oxygen and release of hydroxyl ion according to reaction
layer without microorganisms does not show any significant oxy-
gen consumption after 14 h (Figure 91. The biotic agar layer exhib- 11202 -H 2 0 - 2e =20H (1)its significant oxygen consumption, which reSL!'S in anaerobic con-
ditions at the metal surface, After glutaraldehyde treatment, the
D0 profile is similar to the profile in the abiotic agar layer. A slight Reduction of oxygen occurs at the expense of increasiric pH
consumption of oxygen has been detected in agar layer after glu- Once the applied potential is more negative than the potentiai Of
taraldlehyde treatment, which suggests that a small number of mi- the reversible hydrogen electrode, Reaction (1) is followed by
croorganisms survived the treatment and were still respiring
(Figure 10). 21-20 -2e- = H, - 20H (2.
96 CORROSION -February 1989
0 -. onate in seawater is respon'sible for the shape of the pH vs ap-
plied potential curve
E -0 Theoretical calculations of pH at a cathodically polarized
Z- oo I metal surface generally neglect the buffering capaciy 9 Theoretica!
, -surface pH calculations should include not only the flux of nydroxyl
0-1500 ions towards the bulk medium but also the flux of the bufferna
constituents from bulk toward the metal surface
-2000 - The influence of buffering capacity is very relevant for reac-
tors used for microbial corrosion investigations. Experiments in the
6 -2-00 presence of microorganisms often last many days. If the products
of electrochemical or microbial reactions are not removed from the
-3000. system continuously, they can influence water buffering system
considerably. This, in turn. influence-, the conditions at the meta'
:o surface. Continuous removal of reaction products is necessary for
microbial corrosion investigations. Presence of microorganisms
0- 4000 producing acid or hydroxyl ions as metabolic products at the metal
surface does not necessarily cause changes in pH. Before consl-
- 2 4 1 IS erable changes in pH can be observed the water buffering systen
Dissolved oxygen concentration (ppm) has to be destroyed It is possible to speculate that such a situa-
tion can exist inside a corrosion pit covered with biofilm. This stu-
FIGURE 10. DO profiles in aga films or corrosion coupons in the ation, however, can also be created artificially by experimentai
presence of microorganisms before (Uj and after (_) glutaral, conditions neglecting the proper replacement of water in a reacto
dehyde treatment. Experiments conducted in closed reactors, which last many days
especially in the presence of microorganisms. may result in cra-
matic changes in the water buffering system. This. in turn nfi-
ences condtions at the metal-water interface and corrosion pro-
which causes further increase in p cesses. Thus, open continuous flow reactors are desirabie O'ne
The DO consumption curve ,Figure 6, behaves accordingly such reactor, an open channel flow reacto r was constr,;ctec 'o
Increasing the applied potential dec'easec the metal surface DO future investigations
concentration The asymptotic approach of DO to zero suggests
that transport of oxygen from the bulk water may have been rate
limiting. Otherwise. the curve would be inear Oxygen diffusion
rate from bulk to surface increases wlt.r DO at the metal surface Biotic Experiment
decreases. Turbulence in the water niase caused by aeration in The open channel flow reactor proved its usefulness to n, -
the PARC flask influences the DO flux into diffusion layer crobial corrosion investigations The reactor contained 24 circila-
pH at the cathodically protected meta' surface has been mea- coupons. Twelve of the coupons were covered with an artificial
sured in two different systems. (1) sodium chioride and (2) artificial biofilm prepared by immob!itzing microorganisms in a gel majr'
seawater. The metal surface PH varies with applied cathodic po- Immobilization of microorganisms at the surface of corrosto- co-
tential in different ways for the two solutions Increasing applied pons offers several advantages, including (1 microorgasns im-
potential increased the measured PH in botn cases. which is con- mobilized at the metal surface can be chosen according to exper-
sistent with theory [Equation (1)1 and meta! surface DO consump- menta goals. (21 a 2% agar layer prevents bacterial contamain: ",c
tion (Figure 6). The shape of the pH curves (Figure 7). as a func- from the water, and (3) the agar layer prevents wash out o' inoc.-
tion of applied potential in the range zero to -0.8 V SCE), reflects lum from the metal surface. Nutrient supply to the mlcrooganis.s
the changes in DO in the same rarie cf applied potential (Figure and cnemical inhibition are accomplished by transport Inrouch th
6). Further increase in potential below -08 V(SCE) causes re- agar layer The relatively high number of coupons in the rea:c',,
lease of hydroxyl ions [Equation (2)] Results clearly indicate that atlows replications and destructive sampling e o cat hooc pola'-
the ionic constituents influence the pH obtained at a given DO ization). A constant supply of nutrients and constant removal o'
concentration products avoids the problems arising in a closed system because
The flux of hydroxyl ions toward the bulk liquid from the metal of changing water quality.
surface during cathodic polarization is not the only factor that influ- The results of cathodic polarization measurements in the
ences pH at the metal surface An increase in cathodic potential presence and absence of microorganisms on the corrosion cou-
causes oxygen reduction and release of hydroxyl ions This is. in pons confirmed expectations i resence of microorganisms
principal, similar to titrating the metal-water interface with strong decreased measured current density resulting from consumption o'
base. The change in pH as a function of applied potential in so- oxygen by microorganisms thus decreasing its availability for ca-
dium chloride is sigmoidal, while in artificial seawater, the chance thodic processes (Figure 8). Direct measurements of the DO Iro-
is minimal. For sodium chloride, the titration ' = for a strong acid file through the agar film confirmed this result. In the presence of
with a strong riase. In the case of artificial seawater, the carbonate microorganisms, DO was zero at the metal surface (Figure 9)
buffering system acts as a weak acid Glutaraldehyde treatment was undertaken to prove that the ob-
served phenomena was caused by the microorganisms presence
CO2 - H2 O - H2CO, (3) After 14 h of glutaraldehyde treatment, the cathodic polarization
curve was almost identical to that at the beginning of the experi-
HCO, - H- - HCO, (4) ment (Figure 8). The microorganisms were inactivated and oxygen
was again available for cathodic processes Measurements of the
DO profiles in agar layer in the presence of microorganisms before
HCO3 - H - CO;-
2  (51 and after glutaraldehyde treatment confirm this result The DO pro-
file in the agar layer with immobilized microorganisms after gluta-
Hydroxyl ions. released as a result of cathodic polarization raldehyde treatment was similar to that in the agar layer without
(Equations (1) and (2)] react with hydrogen ions (Equations (4) microorganisms present. A slight oxygen gradient suggests that
and (5)) and shift the reaction balance toward carbonate formation some microorganisms survived the glutaraldehyde treatment and
The titration is for a weak acid with strong base. Thus, the bicar- were still respiring (Figure 10)
CORROSION-Vol. 45, No. 2 97
CORROSIC', SC!E -! a
Consumption of oxygen by microorganisms colonizing metal enced by the buffering system of the electrolyte
surfaces has implications for cathodic protection systems If DO
concentration at the mete' surface is loA. cathodic protection will N Buffered electrolytes can reduce n.tal surface cH fror tneore;-
not provide the expected increase in pH [Equation t11) From a cal expectations during cathodic polarization Metal surface rncrc
practical standpoint, this means that the conditions at the metal bial activity has probably the same effect
surface may not be so conducive for calcareous deposition, as is
expected from theory. This modifying effect can be magnified by Ii Reactors for microbial corrosion investigations must provide for
high water buffering capacity. Decreases in buffering capacity continuous removal of electrochemical and microbial reaction
make the metal-water interface sensitive, to PH changes that may products. Accumulation of reaction products in the system may
adversely influence the system from a corrosion standpoint when substantially influence water puality and change conditions at tie
the metal surface is colonized with microorganisms. Microbial ac- metal surface, possibly causing artifactual results. ContiuoJs s-o
tivity at the metal surface is generally "patchy." thereby resulting
in nonuniform distribution of pH at the metal surface The nonuni- ply of nutrents at relevant concentratons to the reactor is arso
form pH, in turn. creates local action cells High water buffering preferred
capacity protects against this mechanism The higher the water
buffering capacity. the more resistant the system is to changes ir, bP An open channel flow reactor with microorganisms 1ocuiate: a
p, which equalizes pH at the metal surface Tnis scenario .s con- the surface of corrosion coupons fulfills many of the requirements
sistent with observations by Pisigan and Singley that maximum for microbial corrosion investigations
corrosion rate is associated with minimum water bufiering capac-
ity. Stumm" also attributed increased corrosion rates to decrease ACK!\DWLEDGMENTS
in buffering capacity The advantage of high bufferng capacti is
that the system is more resistant to changes in the metal surface
pH caused by the presence of microorganisms Tne doisadvantage Tne project was sponsored by National Science Foundatic-
of high buffering capacity is that cathooc protecton will no: Project No. CTB-8420785 "An Investigation of Mechansms fc-
increase the metal surface pH and calcareous oeposits may no: Mcrobially Assisted Corrosion.' and by NORDA Program Eieme:
form as expected 61153 N in suppot of the Defense Research Program The a. -
thors are indebted to A Stone. Johns Hopkins University icr cs-
The hypotheses are consistent wit'n tnose in otner lfteatlure cussio and valuable suggestions R Varsanik Calgon, Corro is
Dhar. et ai. * investigated use o' caTod"ic proiectio- to, deceas- gratefully acknowledged for contribiting the electrocnenica eq,
ing the number of bactera adsorbe2 on metals in seawater The ment used dung the experimentation Tne PA lnojstra Asso:
authors expected that bacte:a woud oe adversel,. affectec ,oe- ates are acknowledged for their SUpon of tnis wa.
cause of in-situ electrochemical reduction o
, 
of o H O. and
OH ' They observed that the decrease in bacterial numbers at
an electrode :mmersed in sodium cnio,,oe resulting from aoplyno
-0.3 ISCE) cathodic potentia! was similar to applying - 06
V(SCE) in seawater In conclusion they stated that Ine observed 1 F; E Ta-a, M~ae- Pe- Vo 2: N: 8 3: 19c
differences may have been caused by 'the fact that tne seawater 2 5 Ma.e' . ae Pe, vo 26 Nc 1 r 5- !9sE
has organic impurities that may be partly oxidized by H;O, anc S S Ee* A C C se.-: A Ma:,a, C.Co S v - 2E
also the possibility that. because o tne considerable bufferino ca- , P vace C D..ene 1 .' :0:: 20 cac  a: :
za: o Pape NC . CO OSION - NtAOE csiz e. 5-pacity of seawater. the lethal effect of oH at the electrode wouloc a! a
' L C C~a,- R Wb D G~anoe- A,. Taj:" . Ap ",;) = : _- P -
be minimal . The second hypothesis has beer confirmed by re- E N P Re snec- b E Jio,'eise , M Coee:-zoi( Te. se ':. a
SuIts presented in this work og, AZacec - M,C.a Eco og Vc .- K C 'a-s'a E_ ,e. C,,
,q 0c %" 196
P C Tromas or.Sers'.e 'oeCe .:-' eeI.OeC 5:8a--.
CONCLUS(O-S
e G Ac,e, r L :ee, - o er Boo,: A,,,& V: 5 .
Direct DO and pH measurement a: a metal surface immersec s A PQS a V- E Soge, V, . C: R AP.,ga-J J E Sng, ; W¢, \ ?= Z. g-
in water can be conducted with microelectrodes i w s5:- J Sa Err 0. ASCE BE 1 -
12 r Da. D A io,o e ,O .,Bo:,-s. Ee:".C',e- S:
1 pH at a cathodicaly protected metal surface is strongly nfl- 2!78 158-
98 Printed in u S A CORROSION-February 1989