An understanding of fabrication processes, metallurgy, electrochemistry, and microbiology is crucial to the resolution of microbiologically influenced corrosion (MIC) problems. The object of this effort was to use AC impedance spectroscopy to characterize the corrosion resistance of Type II anodized aluminum alloy 2219-T87 in sterile and biologically active media and to examine the corrosion resistance of 316L, alloy 2219-T87, and titanium alloy 6-4 in the welded and unwelded conditions. The latter materials were immersed in sterile and biologically active media and corrosion currents were measured using the polarization resistance (DC) technique

Walsh, Daniel W.

English

NASA Technical Reports Server

1991
N92-15898
NASA/ASEE SUMMER FACULTY FELLOWSHIP PROGRAM
GEORGE C. MARSHALL SPACE FLIGHT CENTER
THE UNIVERSITY OF ALABAMA
CHARACTERIZATION OF THE CORROSION RESISTANCE OF BIOLOGICALLY
ACTIVE SOLUTIONS - THE EFFECTS OF ANODIZING AND WELDING
Prepared By:
Academic Rank:
University and Department:
NASA/MSFC:
Laboratory:
Division:
Branch:
MSFC Colleague:
Daniel W. Walsh, Ph.D.
Professor
Cal Poly State University, SLO
Materials Engineering
Materials and Processes Laboratory
Metallic Materials Division
Corrosion Research Branch
Merlin D. Danford, Ph.D.
Contract No.: NGT-01-008-021
The University of Alabama in
Huntsville
Huntsville, Alabama
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https://ntrs.nasa.gov/search.jsp?R=19920006680 2020-03-17T14:19:25+00:00Z
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The study of microbiologically influenced corrosion (MIC) is an
inherently interdisciplinary effort. An understanding of fabrication
processes, metallurgy, electrochemistry and microbiology is crucial to
the resolution of MIC problems. The object of this effort was to use
AC impedance spectroscopy to characterize the corrosion resistance of
Type II anodized aluminum alloy 2219-T87 in sterile and biologically
active media and to examine the corrosion resistance of 316L, alloy
2219-T87 and titanium alloy 6-4 in the welded and unwelded conditions.
The latter materials were immersed in sterile and biologically active
media and corrosion currents were measured using the polarization
resistance (DC) technique.
MIC is recognized as a major problem in many industries. MIC is
ubiquitous; all alloy systems exhibit susceptibility to microbiologi-
cal attack. MIC of water treatment and delivery systems is particu-
larly insidious, MIC can occur during construction, testing, operation
or shutdown periods. In fact, extended periods of shutdown can be
particularly damaging. Researchers have developed some understanding
of environments that promote MIC, materials susceptible to MIC,
methods to detect MIC and rudimentary approaches to treat and prevent
MIC. However, little understanding of the metallurgical basis for
these processes has developed. Furthermore, little "hard data" on
corrosion rates of material systems in biologically active environ-
ments exists. In even fewer cases have researchers examined the
effects of fabrication processes on the MIC susceptibility of metals.
Microbes are living agents of corrosion. Their size provides
them access to material inhomogeneities at the microstructural level.
Microbes are motile, and able to move away from toxic chemicals and
toward food - they have specific chemical receptors for target chemi-
cals in the environment. Surfaces of any kind are attractive to most
waterborne bacteria. Surfaces, particularly metal surfaces, concen-
trate scarce resources. The surface concentration of protein and
polysaccharide molecules is much greater than that in the bulk solu-
tion. Microbes able to attach and establish themselves on such
surfaces will be at a competitive advantage in their environment.
Microbes are hardy, different varieties can withstand temperature
variations from -10°C to 99°C, pH variations from 0 to 10.5 and oxygen
concentrations from 0% to 100%. Microbes are prolific and able to
exist in large colonies. Many are able to produce extra-cellular
slime layers. The slime layer is used by the microbe to protect it
from toxins, collect food and anchor to the surface. It also is
critical to the development of complex microbial ecosystems, consortia
of many different microbes on the surface. These synergistic consor-
tia are able to accomplish complex chemical reactions at the material
surface and create aggressive chemical environments. The slime layer
is involved in the formation of oxygen depletion cells, it can seques-
ter metabolic byproducts (bacteria found in ECLSS waters are known to
produce organic acids and hydrogenase), it can harbor microbes that
depolarize cathodic sites (SRB have been found in large numbers in
ECLSS waters), thus it will accelerate corrosion. SRBs flourish in
anaerobic environments at the film-metal interface, rendering the
environment more chemically aggressive and providing several mecha-
nisms to depolarize the rate limiting cathodic reaction. Furthermore,
many bacteria can directly oxidize metal ions. At a minimum this will
lead to a passive co-accumulation of negative ions (typically Cl').
This may produce an acidic ferric chloride, cuprous chloride or magne-
W-i
sium chloride solution at flaw tips, producing an in-vivo stress
corrosion cracking environment more severe than many in-vitro tests.
Slime layers produce a crevice like anaerobic environment in
which passivating films damaged by abrasion or halide ion attack go
unrepaired. The biofilm can consume oxygen, and prevent oxygen in the
environment from reaching the metal surface and restoring passive
films. In addition, the lack of oxygen may also promote cathodic
reactions that do not involve oxygen.
Metal surfaces are extremely heterogeneous. Regional (on all
scales - from submicron to macroscopic) differences are so well
defined that local anodes and cathodes form and corrosion takes place
on individual pieces of metal. Surface condition, stress state,
microstructure, chemistry and inclusion size and distribution affect
local electrochemistry and MIC susceptibility. Welding changes the
surface texture, and produces local stress fields. More importantly,
it alters the size, shape, distribution and amount of microstructural
constituents in the fusion zone and in the heat affected zone. In our
study welding accelerated and aggravated corrosive attack.
Corrosion implies the existence of anodic sites on a metal
surface, where oxidation of insoluble metal atoms to soluble metal
ions takes place. Cathodic regions on the metal surface balance the
reaction. In systems where external current is not supplied, anodic
and cathodic currents must be equal. Reactions at the anode and the
cathode can be treated as "half-reactions" whose sum is the total
corrosion reaction. Corrosion potentials are thermodynamic quanti-
ties, measured at equilibrium, and as such indicate what reactions are
possible. Corrosion currents are kinetic values and reflect dynamic,
nonequilibrium processes at electrodes. Typically there is a single
anodic reaction, the dissolution of metal ions. However, there may be
several cathodic reactions; which is favored depends on the chemistry
of the environment. In our systems, the pH is near neutral and the
oxygen and water reactions are possible. In biologically active
media, other reactions are possible, the chemistry at the metal-film
interface is drastically different from the bulk chemistry.
Polarization resistance techniques were used to study the unco-
ated samples, AC impedance techniques were used to study the anodized
materials. These methods have been described by Danford (Ref. 1,2,3).
The corrosive media used was mild corrosive water for the bare A1 2219
samples and 3.5% NaCl solutions for all other samples. All media were
sterilized in an autoclave. Bioactive solutions were made by inocu-
lating these sterile media with 20 ml of ECLSS water. Samples Icorr
and E were monitored over a four week period. Samples were exam-
ined _yr scanning electron microscopy and optical microscopy. Water
chemistry and microbial ecology were monitored.
Figure 1 shows the corrosion current measured in A1 2219 over a
three week period. Note that initial measurements do not indicate the
average current over longer periods. The A1 2219 (welded and un-
welded) corrodes at a much lower rate in the sterile media. Similar
results were observed in parallel tests with 316L and Ti 6-4. Figure
2 shows the charge transfer resistance measured for anodized samples
over a four week period. This value is inversely proportional to the
corrosion current. Note the precipitous drop in the resistance for
the bioactive solution after several days exposure. The morphology of
corrosive attack for the bioactive solutions was also different,
microbes aggregated at weak spots in the coating (Fig. 3) and grossly
V
%f
VV- 2
enlarged pits. Figure 4 shows microbes located between weld ripple
marks on 316L material. Microbes also aggregated at pores and flaws
in oxide coatings. The microbial attachment density was lower on the
unwelded base material.
The results of this testing indicate that ECLSS waters are
microbe rich, it contains large numbers of SRBs, Enterobacter, Pseudo-
monas and Klebsiella - that the AC impedance technique and the DC
polarization resistance techniques are well suited to MIC investiga-
tions - that microbes attach to al___!materials studied - that microbes
increase Ic0rr and decrease Ec0rr in all samples for all treatments (in
the media studied A1 2219 was most susceptible, Ti 6-4 least suscepti-
ble) - that anodized materials in sterile media maintain coating
integrity for longer periods than typically measured - that ECLSS rich
media aggravate corrosive attack of anodized materials and change the
morphology of attack - that microbes localize corrosion and fix anodes
at microstructural features - that welding increases Icorr - that
microbes randomly attach to a surface, proliferate at opportune
locations, develop a biofilm and form complex consortia on the materi-
al surface that inhibit subsequent colonization by other bacteria -
that microbes are at a minimum catalysts for the corrosion reaction,
but are often directly involved in the corrosion process - that
enhanced corrosion in A1 2219 is caused by the promotion of the more
rapid hydrogen evolution reaction at the cathode, anaerobic films and
SRBs foster this reaction.
The list of people who have helped me this summer reads like the
Huntsville phone directory. Thanks to all in the corrosion, micro, FA
and metallography groups. Special thanks to Merlin Danford whose
quietly correct and kind approach makes working and learning so easy
and to Jeff Sanders for field emission effects. Special thanks to Joe
Montano, Barry Moody and Carlla Hooper. Special thanks to Tim Huff for
all his help and interest. Finally thanks to Kevin Buford - good luck
next year at school.
ANODIZED ALLOY 2219
55T"
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4- .................... ;" -- \ ' ",,i ....... \ "
\ / M •
4,_ 3i ..................................................................... ___- /\_ :
o 5 1o 1'5 2o _ 3o
DAYS EXPOSURE
ALLOY 2219 CORROSION
3
24 ................ .,".,.--. '.
24 - i "
.- //j' ................... - ..... ,¢ -%< io,4 :' ........... .
0 6!0.3"I: !2Z--_ZI
0 I , , , , - , , , ,
0 2 4 6 8 10 12 14 16
DAYS EXPOSURE
Sterik --4-- ECLSS ]
i--
-e- ECLSS -+- STERILE -_:- ECLSS W -e- STERILE W
Figure i. Figure 2
VV- 3
Figure 3. Localized MIC attack at weak spot in anodized coating.
Figure 4. Microbes at weld ripple marks on 316L
References
I. Danford, M. D., Equivalent circuit Models For AC Impedance Data
Analysis, NASA Technical Memorandum TM-100402, June 1990 i2. Danford, M. D., and Knockemus, W. W., The Corrosion Mechanisms
For Primer Coated 2219-T87 Aluminum, NASA Technical Paper 2715, April,
1987.
3. Danford, M. D., The Corrosion Protection of 2219-T87 Aluminum By
Anodizing, NASA Technical Memorandum TM-I03540, June 1991.
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ORIGINAL PAGE
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Characterization of the corrosion resistance of biologically active solutions: The effects of anodizing and welding

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