International Biodeterioration & Biodegradation 49 (2002) 235–243
www.elsevier.com/locate/ibiod
Microbially initiated pitting on 316L stainless steel
M. Geisera, R. Avcib, Z. Lewandowskic; ∗
aCenter for Bio
lm Engineering, Montana State University, PO Box 3980, Bozeman, MT 59717-3980, USA
bDepartment of Physics and Image and Chemical Analysis Laboratory, Montana State University, PO Box 3980, Bozeman, MT 59717-3980, USA
cDepartment of Civil Engineering and the Center for Bio
lm Engineering, Montana State University, PO Box 3980, Bozeman, MT 59717-3980, USA
Abstract
Pitting corrosion of 316L stainless steel ennobled in the presence of manganese-oxidizing bacteria, Leptothrix discophora, was studied
in a low-concentration sodium chloride solution. Corrosion coupons were 3rst exposed to the microorganisms in a batch reactor until
ennoblement occurred, then sodium chloride was added, which initiated pitting. The pits had aspect ratios (length divided by width) and
shapes closely resembling the aspect ratio and the shape of the bacteria, which suggested that the microorganisms were involved in pit
initiation. ? 2002 Published by Elsevier Science Ltd.
Keywords: Pit initiation; Leptothrix discophora; MIC; MOB; Localized corrosion; Manganese oxides
1. Introduction
It has been demonstrated that stainless steels and other
passive metals and alloys when exposed to natural waters
containing manganese-oxidizing bacteria can increase their
open circuit potential (OCP), a phenomenon termed enno-
blement (Dickinson et al., 1997; La Fond, 1999; Little et
al., 1998). The e=ect of microbial ennoblement on passive
metals is analogous to that caused by polarizing a metal
anodically using a potentiostat in which the open circuit
potential may reach the pitting potential. In arti3cial sea-
water (30 g NaCl=l) (Sedriks, 1996; Szklarska-Smialowska,
1986), 316L stainless steel has a pitting potential of ap-
proximately +300 mV, which means that the OCP must
reach +300 mV before pitting occurs. In natural waters,
316L maintains the OCP well below its pitting poten-
tial; therefore, the stainless steel should not pit. However,
manganese-oxidizing bacteria depositing manganese oxide
(MnO2) on the surface of the stainless steel (Boogerd and
Vrind, 1987) cause a positive shift in the OCP. This is be-
cause when manganese oxide is in direct electrical contact
with the stainless steel, the metal exhibits the following
equilibrium dissolution potential of the MnO2:
MnO2(s) + H+ + e− =MnOOH(s); E0 = +0:81VSCE;
E′pH=7:2 = +0:383VSCE: (1)
∗ Corresponding author. Tel.: +1-406-994-0211.
E-mail address: zl@erc.montana.edu (Z. Lewandowski).
MnOOH(s) + 3H+ + e− =Mn2+ + 2H2O;
E0 = +1:26VSCE; E′pH=7:2 = +0:336VSCE: (2)
The overall reaction is
MnO2(s) + 4H+ + 2e− =Mn2+ + 2H2O;
E0 = +1:28VSCE; E′pH=7:2 = +0:360VSCE: (3)
The standard potentials (E0) for Eqs. (1)–(3) were
calculated using the energies of formation: FG0f Mn
2+ =
−54:5 kcal=mol;FG0f MnOOH = −133:3 kcal=mol, and
FG0f MnO2 = −109:1 kcal=mol (Dickinson et al., 1996;
University Chemistry Data Tables, 1995). The formal poten-
tials (E′) were calculated at a pH of 7.2 and [Mn2+]=10−6.
It has been demonstrated that surface coverage of 6% by
manganese oxides (Dickinson et al., 1996) can increase the
resting potential of 316L stainless steel in a fresh water en-
vironment from −200 to +362 mVSCE, a 500 mV increase,
at a pH of 7.2 (Dickinson et al., 1997; Linhardt, 1998).
When studying the mechanism of ennoblement of 316L
stainless steels using pure cultures of manganese-oxidizing
bacteria, Leptothrix discophora, we have noticed oddly
shaped indentations in the passive layer on the metal sur-
face. We hypothesized that the bacteria were responsible
for these indentations and that these indentations were in
fact sites where corrosion pits initiated. To verify this hy-
pothesis, we set up experiments to (1) demonstrate that
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236 M. Geiser et al. / International Biodeterioration & Biodegradation 49 (2002) 235–243
Leptothrix discophora were responsible for the oddly
shaped indentations on the surface of the ennobled stainless
steel, and (2) to show that these indentations were sites
where pits were initiated.
As a material to study, we used 316L stainless steel.
Coupons of the metal were polished to a very smooth tex-
ture, removing as many Maws from the surface as possible.
To identify sites of interest on the surface, small squares
(200 m × 200 m) were etched using ion milling on the
polished surface. Atomic force microscopy (AFM) was then
used to examine the surface bound by the squares. The
metal coupons were then exposed to Leptothrix discophora
in a batch reactor. Electrical potential of the metal, reMect-
ing progression of the ennoblement, was monitored versus
a saturated calomel reference electrode (SCE). To initiate
pitting, the ennobled coupons, once covered with microbial
deposits, were removed from the reactor and exposed to a
sterile solution of sodium chloride. The corrosion coupons
were then removed to compare pit morphology with the
shape of the bacteria Leptothrix discophora and their sur-
faces were examined with a scanning electron microscope
(SEM) and atomic force microscope (AFM).
2. Materials and methods
2.1. Stainless steel coupons
Stainless steel coupons 1:6 cm in diameter were cut from a
1 mm thick sheet of type 316L stainless steel purchased from
Reyerson in Spokane, Washington. Coupons were mounted
in polycarbonate holders with silicon gel (Dickinson et al.,
1997). The holders consisted of a hollow polycarbonate tube
10 cm long with an inner diameter of 9 mm and an outer
diameter of 19 mm (Fig. 1).
The coupons mounted in the holder were polished to pro-
vide a surface suNciently void of Maws for surface analy-
sis. They were wet-sanded with tap water on Buehler–Met
II metallographic grinding disks composed of silicon car-
bide grit of decreasing grit sizes: 120, 240, 360, 400, and
600. After the use of each grit size, the holder and coupon
were rinsed with running tap water to remove any remaining
grit. We then polished the coupons using Buehler aluminum
oxide powder and Buehler Micropolish II powder, each
suspended in water and applied with Buehler Microcloths.
We initiated polishing with suspended 5 m aluminum ox-
ide powder. The coupons were then rinsed with tap water.
Similarly, we used 0.5 and 0:05 m polishing powders to
make a mirror surface on the stainless steel, with rinses ap-
plied when polishing was complete with each powder size.
The coupons were then removed from the holders and
two squares (200 m×200 m), with a small number in the
corner of each for identi3cation, were etched on their smooth
surfaces (Fig. 2). The etchings were made by ion milling,
with a focused Ga+ ion beam emitted from a time-of-Might
secondary ion mass spectrometer (ToF-SIMS) for 7 min at
Fig. 1. Coupon holder.
Fig. 2. Corner of the etched squares with an identi3cation number on the
surface of a polished coupon.
∼ 1:5 A ion current at 22 keV impact energy (Pendyala,
1996). The etching produced a trench in the stainless steel
approximately 100 nm deep (Fig. 2). AFM was then used
to map the surface of the stainless steel.
2.2. Reactor
The reactor was a polycarbonate batch cylinder reactor
(Fig. 3) 10:2 cm tall and 11:1 cm in diameter. Nine pol-
ished stainless steel coupons with the etched squares were
mounted to their holders again, and the holders were at-
tached to the top of the reactor. The assembled reactor is
the same as the one used by Olesen et al. (2000), though
slightly modi3ed by the addition of the reference electrode
directly into the medium instead of using a salt bridge. This
M. Geiser et al. / International Biodeterioration & Biodegradation 49 (2002) 235–243 237
Fig. 3. Reactor.
Table 1
Composition of MSPV medium
(NH4)2SO4 0:24 g
MgSO4 0:06 g
CaCl2 · 2H2O 0:06 g
KH2PO4 0:02 g
Na2HPO4 · 7H2O 0:05 g
HEPES 1:15 g
FeSO4 10 mM 1:0 ml
Distilled Water 1000 ml
alteration gives a much more stable potential reading. Tubes
used for introducing air into the medium were mounted in
the reactor, and Pall–Gelman bacterial air vents were at-
tached to these tubes to prevent contamination of the reac-
tor. Stirring was provided by a magnetic stir bar placed at
the bottom of the reactor. The reactor was then sealed with
the same silicon gel and autoclaved on dry setting (depres-
surization method) at 123◦C and 1:2 atm for 30 min.
To prepare the growth medium, 1 litre of ATCC Cul-
ture Medium 1917 MSPV (Table 1) was autoclaved on
liquid setting at 123◦C and 1:2 atm for 25 min. After the
medium was cooled to room temperature, we added 1 ml
of syringe-3ltered vitamin solution (Table 2) required by
the MSPV medium, 4 ml of syringe-3ltered 50 mmol man-
ganese sulfate solution, and 5 ml of syringe-3ltered 20%
sodium pyruvate solution. All chemicals were from Fisher
Scienti3c.
Manganese-oxidizing bacteria, Leptothrix discophora
SP-6 (ATCC 51168), were obtained from ATCC and stored
at −70◦C. To inoculate the reactor, 150 ml of the MSPV
medium containing vitamins, sodium pyruvate, and man-
ganese sulfate was poured into a sterile 250 ml Erlenmeyer
Mask with the stock culture of the bacteria and placed on a
Table 2
Composition of vitamin solution
Biotin 20:0 mg
Folic Acid 20:0 mg
Thiamine Hcl 50:0 mg
D-(+)-Calcium Pantothenate 50:0 mg
Vitamin B12 1:0 mg
RiboMavin 50:0 mg
Nicotinic Acid 50:0 mg
Pyridoxine Hcl 100:0 mg
P-Aminobenzoic Acid 50:0 mg
Distilled Water 1000 ml
shaker for 2 days. Then the broth was aseptically added to
the reactor along with 600 ml of the sterile medium.
Before mounting it in the reactor, the SCE reference elec-
trode was sterilized by soaking it in 99% ethanol for 1 h.
A sti= spring with a long conducting rod and stopper was
connected to the coupon and coupon holder, thereby 3n-
ishing the assembly (Fig. 1). The coupon assembly and
reference electrode were interfaced with a computer via a
Hewlett Packard 34970A Data Acquisition=Switch Unit (a
multiplexer) to monitor the potential of the coupons.
The reactor was operated with the stirrer bar rotating and
air bubbling through the medium until the potentials of the
coupons exceeded +200 mV (see Table 3 and Fig. 4) which,
according to our de3nition, indicated that the coupons were
ennobled: This process usually took usually 5 days. Two of
the nine coupons were removed from the reactor, sprayed
with deionized water to remove the attached bio3lm, and
air-dried. These two coupons were then used to describe the
surface of the coupons that were exposed to the microorgan-
isms but not exposed to the chloride solution. The remaining
coupons were also removed from the reactor, sprayed with
deionized water to remove the bio3lm, and then immersed
in a 0:2 M NaCl solution. Spraying removed the bio3lm,
but it did not remove the manganese oxides on the surface.
OCP of the coupons immersed in the NaCl solution was
monitored, and the coupons were removed one at a time at
pre-assigned intervals over the course of 2 days.
2.3. Surface analysis
Before the analysis, surfaces of all coupons were gently
wiped clean with acetone and a lab tissue paper to remove
the attached manganese deposits and remaining bio3lm.
Absence of the manganese oxides and the bio3lm was veri-
3ed with a light microscope, AFM, and SEM. Any remain-
ing attached silicon gel was gently removed or coated with
colloidal graphite from Ted Pella, Inc. to minimize any
charging that would occur in the SEM.
SEM and AFM were used to map the surface topogra-
phy of the following: (1) freshly polished sterile coupons;
(2) ennobled coupons after removing microbial deposits;
and (3) after exposure to the sodium chloride solution. We
used a Jeol JSM-6100 scanning electron microscope with
238 M. Geiser et al. / International Biodeterioration & Biodegradation 49 (2002) 235–243
Table 3
Highest potentials of the coupons reached. Sample 12 leaked and Sample 34 did not ennoble for an unknown reason
Sample ID 12 61 23 24 45 56 35 34 13
Potential (mV) Leaked 224.0 301.5 282.4 299.5 251.5 267.5 165.0 217.0
-0.20
-0.10
0.00
0.10
0.20
0.30
0.40
0 20 40 60 80 100 120 140
time (hr)
Fig. 4. Evolution of the OCP of 316L coupons exposed to manganese-oxidizing bacteria.
the beam voltage set to 15 kV. The features of interest were
photographed using a Polaroid camera, type 665, at a work-
ing distance of approximately 11 mm. Using a Digital In-
struments Dimension 3100 scanning probe microscope in
contact mode, the surface features identi3ed by the SEM
were then mapped using an AFM.
2.4. Other tests
2.4.1. Imaging surfaces of electrochemically polarized
corrosion coupons
An essential part of the project was to quantify the mor-
phology of the pits initiated on the coupons that had been
ennobled and exposed to the sodium chloride solution. The
working hypothesis was that the microorganisms were in-
volved in the pit initiation. To verify this hypothesis, we
compared the morphology of pits generated in the pres-
ence of manganese-oxidizing bacteria with the morphology
of pits generated by anodic polarization in sterile media.
The 316L stainless steel coupons were polished as previ-
ously described and placed in an electrochemical cell with
the MSPV medium, vitamins, sodium pyruvate, and man-
ganese sulfate. Sodium chloride was then added to make a
0:1 M solution. An EG & G Princeton Applied Research
Potentiostat=Galvanostat, Model 273A, was used to anodi-
cally polarize the coupons. The potential was increased at
a rate of 10 V=h from −0:5 to +0:8VSCE using a graphite
counter electrode. Care was taken to ensure that crevice
corrosion did not occur near the edge of the coupon holder
(Kelly, 1995). After the applied potential exceeded the
pitting potential, the coupon was removed, rinsed with
deionized water, dried, and analyzed with SEM and AFM.
The corrosion pits were located on the surface, and their
morphology (size and aspect ratio) was quanti3ed.
2.4.2. Imaging surfaces of sterile corrosion coupons
exposed to sodium chloride
To justify our conclusions, it was necessary to show that
the observed indentations in the passive layer did not form
spontaneously in the sodium chloride solution. To show this,
a 316L stainless steel coupon was polished, as previously
described, and then cleaned with acetone and a laboratory
wipe. Two squares with the same dimensions as those used
in previous experiments were etched on the surface of the
coupon by the same ion milling procedure. The surface of
the coupon was thoroughly examined with AFM, and then
the coupon was exposed to 0:2 M NaCl solution (prepared
with deionized water at room temperature) and aged for 2 12
days. The coupon was then removed, rinsed with deionized
water, dried, and again analyzed with AFM.
2.4.3. Imaging Leptothrix discophora attached to the
surfaces of corrosion coupons
To compare the morphology of the bacteria with the
morphology of the corrosion pits, we took images of the
Leptothrix discophora attached to surfaces of corrosion
coupons. Six 316L stainless steel coupons were polished to
the described speci3cation. Two 250 ml Erlenmeyer Masks
each had three polished coupons placed in them. The Masks
were then sealed, autoclaved for 25 min on the dry setting
at 123◦C and 1:2 atm, and cooled to room temperature. Two
batches of MSPV medium were prepared in the same fash-
ion as the previous experiments and autoclaved for 25 min
on the dry setting at 123◦C and 1:2 atm. Both batches had
syringe-3ltered solutions of sodium pyruvate and vitamins
added, but only one had manganese sulfate added to it to
make the same concentration as in the previous experi-
ments. One sterile Mask had 125 ml of the MSPV medium
with manganese sulfate solution added aseptically. In the
other Mask, 125 ml of the MSPV media without manganese
M. Geiser et al. / International Biodeterioration & Biodegradation 49 (2002) 235–243 239
sulfate was aseptically added. The two Masks were inocu-
lated with Leptothrix discophora and shaken at room tem-
perature. A coupon from each Mask was removed and dried
after 6, 8, and 10 h of bacterial growth. The coupons were
gold=palladium-coated to a thickness of 15 nm and studied
with the SEM.
3. Results and discussion
The potentials of the coupons in the reactor were contin-
uously monitored against an SCE reference electrode, see
Table 3 and Fig. 4.
When the ennobled coupons were placed in the sodium
chloride solution, their potentials 3rst Muctuated, indicating
formation of metastable pits, then dropped, indicating that
active pitting was in progress (Fig. 5).
-0.05
0.00
0.05
0.10
0.15
0.20
0.25
0 500 1000 1500 2000 2500 3000
Time (min)
Fig. 5. Potentials measured versus SCE as a function of time.
Fig. 6. AFM images of the control coupon with the etched square. Left—before experiment, Right—after experiment. Conditions: bacteria, absent;
manganese, absent; chloride, 0:2 M.
The control run using the abiotic sodium chloride solu-
tion and sterile corrosion coupons showed that the solution
of sodium chloride alone did not initiate pits on the surface
of the 316L coupons as was expected (Fig. 6). Actually, the
surfaces of the coupons exposed to abiotic sodium chloride
became slightly smoother and had fewer scratches. The time
for which the coupons were exposed to the abiotic sodium
chloride exceeded the time for which the microbially colo-
nized 316L coupons were exposed to the solution of sodium
chloride. Therefore, it can be said that the sodium chloride
solution alone did not initiate pitting on the surface.
AFM and SEM images (Figs. 7 and 8) illustrate pits
that were formed on the surfaces of microbially colonized
coupons after they were placed in sodium chloride solution.
These pits formed highly organized groups of small inden-
tations oriented in long narrow rows with smooth walls and
bottom. Twenty-3ve of these images showed the depth of
240 M. Geiser et al. / International Biodeterioration & Biodegradation 49 (2002) 235–243
Fig. 7. Correlation between AFM and SEM of microbially initiated pits: SEM image, left; AFM image, right; Conditions: bacteria, present; manganese,
present; chloride, 0:2 M.
Fig. 8. A single line of organized pits in a long narrow row as seen by AFM and SEM. SEM image, left; AFM image, right. Conditions: bacteria,
present; manganese, present; chloride, 0:2 M.
the pits to be 44±20:51 nm, with dimensions of 12 m long
by 1:4 m wide. Fifteen of these pits could be located with
both AFM and SEM.
Fig. 9 shows SEM images of corrosion pits on anodically
polarized 316L stainless steel. The pits were round and typ-
ically larger and deeper than pits initiated by bacterial col-
onization. Pits initiated by anodic polarization were bowl
shaped with a thin metal sheath covering their mouths as
previously described (Geesey et al., 1996). Our pits were on
average 60:7 m long and 50:2 m wide. We could not use
AFM to describe their shape because their depth exceeded
6:7 m, the maximum depth the instrument could read.
Surprisingly, the two coupons removed from the reactor
prior to the addition of the sodium chloride solution had
similar indentations as those in Figs. 7 and 8 (Fig. 10). They
were shallower, around 6 nm deep, which is approximately
equal the thickness of the passive layer on stainless steels
(Cunha Belo et al., 1977). Using SEM we perceived that the
morphologies of pits formed in the presence of the bacteria
with and without sodium chloride were similar.
To verify our hypothesis that the pits were formed at the
sites occupied by the microorganisms, we took SEM images
of the bacteria. To enhance visibility and prevent formation
of manganese oxides, we grew the bacteria in the absence
of manganese. We provide some background concerning the
bacteria that were used, Leptothrix discophora is a cylin-
drical bacterium. It connects to other Leptothrix discophora
end-to-end in a chain. Characteristically, these bacteria form
a sheath of proteins and polysaccharides around them a
long, narrow protective layer (Emerson and Ghiorse, 1993).
The groups of bacteria in Fig. 11 are approximately 10 m
long by 1 m wide. The agglomerates of bacteria in Fig. 11
have the same shapes as the pits on the surface of the metal
(Figs. 7 and 8).
Surprisingly, even though the conditions of this test were
designed to map the microorganisms, and we did not add
chloride to the solution, we found indentations in the pas-
sive layer to be of similar shape to those we found in
the presence of chloride (Fig. 12). These indentations had
the same shape as those in Fig. 7 but were quite shallow.
M. Geiser et al. / International Biodeterioration & Biodegradation 49 (2002) 235–243 241
Fig. 9. SEM images of pits initiated by anodic polarization. Left, groups of holes in the thin metal sheath covering the pit’s mouth; Right, small holes
surrounding a large one indicating a thin metal sheath covering the pit. Conditions: bacteria, absent; manganese, present; chloride, 0:1 M.
Fig. 10. Indentation on the edge of a square made by ion milling initiated by the microorganism without addition of chloride. SEM image, left; AFM
image, right. Conditions: bacteria, present; manganese, present; chloride, absent.
Fig. 11. SEM images of Leptothrix discophora grown in the absence of manganese on 316L stainless steel coupon. Conditions: bacteria, present;
manganese, absent; chloride, absent.
It appears, therefore, that the pits are initiated when the bac-
teria are present, and the presence of chloride only acceler-
ates their progression.
To compare the dimensions of the pits with those of the
bacteria, we used an aspect ratio, the length divided by the
width. To compare the sizes of the indentations in the pas-
sive layer with the sizes of microbial aggregates, we quan-
ti3ed the dimensions of the groups of pits (see Fig. 8 as an
example). We found that the 15 pits initiated by anodic po-
larization had an aspect ratio of 1:28 ± 0:27, meaning they
242 M. Geiser et al. / International Biodeterioration & Biodegradation 49 (2002) 235–243
Fig. 12. Shapes with the same dimensions as those pits initiated by bacterial colonization of the surface. Left, single group of shapes; Right, groups of
indentations. Conditions: bacteria, present; manganese, absent; chloride, absent.
were almost round. In contrast, 71 groups of pits found on
the surface of the coupons colonized by the bacteria had an
aspect ratio of 9:97 ± 5:50. Twenty groups of bacteria we
measured had an aspect ratio of 10:0±3:7. Both the bacteria
and the pits were ten times longer than their width.
In summary, the coupons of freshly polished 316L stain-
less steel did not pit in an abiotic solution of sodium chlo-
ride solution. However, the same material pitted in the same
chloride solution if previously microbially colonized by
Leptothrix discophora. The formation of pits in the presence
of bacteria did not require the presence of chloride. When
we used the manganese-oxidizing bacteria, the pits were
initiated with or without the presence of manganese. This
indicates that it is the presence of the microbes, not the mi-
crobially deposited manganese oxides, that initiates pitting.
The depth of the pits depends on the concentration of the
chloride solution and on the time of exposure. When no
chloride was present, the pits were small, and their depth
was comparable with the thickness of the passive layer.
When chloride was added, the pits were deeper. The pit
aspect ratio (10 ± 5) and size (10 m long by 1 m wide)
were the same for the agglomerates of bacteria and the
groups of pits initiated by bacterial colonization. The stan-
dard deviations of the sizes and the aspect ratios are large
for both agglomerates of the bacteria and the pits.
The manganese oxides deposited on the surface ele-
vate the potential, creating an environment where the pits
initiated by microbes cannot repassivate. In this light, it
appears that the bacteria initiate the pits, and the micro-
bially deposited manganese oxides stabilize the growth of
the pits by maintaining a high potential. Further experi-
ments will be conducted to determine the rate of the pit
growth.
Finally, it is possible that pit locations are not random
but pertain to features of the underlying metal substratum
such as grain boundaries or crystalline phases. Geesey et al.
(1996) showed a similar correlation between grain bound-
aries and microbial attachment that resulted in localized
substratum changes. The hypothesis that the locations of
microbially initiated pits are correlated with the grain bound-
aries will be veri3ed in future experiments.
4. Conclusions
(1) Pits in 316L stainless steel were initiated in the pres-
ence of bacteria Leptothrix discophora.
(2) The presence of chloride ions made the pits deeper
but was not required for the initiation of the pits. The
presence of the bacteria was suNcient.
(3) We did not see any evidence of pitting when a sterile
316L stainless steel coupon was immersed in a sterile
solution of sodium chloride.
(4) Pits formed in the presence of bacteria had the same
sizes and aspect ratios as the agglomerates of the
bacteria.
(5) Pits formed in the presence of bacteria had morpholo-
gies di=erent from those initiated by anodic polariza-
tion of the material in the same solution.
(6) The evidences presented here indicate that the bac-
teria were involved in pit initiation on 316L stainless
steel. However, this conclusion is based on indirect
evidences—corrosion pits and microbial aggregates
had the same morphology.
Further tests are needed to verify these conclusions and to
determine the mechanism by which the microbes inMuence
the integrity of passive layers.
Acknowledgements
This work was supported by United States ONce of
Naval Research, contract number N00014-99-1-0701 and by
Cooperative Agreement EEC-8907039 between the Na-
tional Science Foundation and Montana State University,
Bozeman, MT, USA. We would also like to specially thank
the ICAL facility of Montana State University for the use
of the SEM, AFM, and ToF-SIMS.
M. Geiser et al. / International Biodeterioration & Biodegradation 49 (2002) 235–243 243
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