Encyclopedia of Industrial Biotechnology, Bioprocess, Bioseparation, and Cell Technology, 7 Volume Set (Part II)

2154 Pages·2016·225.09 MB·English

MICROBIOLOGICALINDUCED CORROSION sulfurbacteriawereresponsibleforthecorrosionofburied
AND INHIBITION ferrousmetals.
The formation of deposits in waterpipes by iron
PATRIZIAPEREGOand bacteria (in particular aerobic species Sphaerotilus
BRUNOFABIANO natans,Caulobacter,andGallionella)wasinvestigatedby
Harder(4).
DIChePChemicalandProcess
In 1921, Grant et al. (5) produced evidence that the
EngineeringDept.‘‘G.B.
rateofpittingofbrasswashighlyincreasedbyammonia,
Bonino’’,Universityof
whichcouldbeproducedbybacterialactivity.
Genoa,Genoa,Italy
In 1934, Von Wolzogen Kuhr and Van Der Vlugt (6)
proposed a theory on the direct role of sulfate reducing
INTRODUCTION bacteria in the corrosion mechanism, by their utiliza-
tion of cathodic hydrogen and stimulation of the anodic
Corrosion is a concept whose origin dates back to the dissolutionofiron.
mistsoftime:Herodotus(Alicarnasso484B.C.–Thurii425 In 1953, Uhlig reported on the role of microorgan-
B.C.)aGreekhistorian,suggestedtheuseoftintoprotect isms,suchasslime-formingbacteria,fungi,algaediatoms,
iron,whiletheRomanphilosopherGaiusPliniusSecundus and protozoa, attributing their corrosive action to the
(23–79 A.D.), well known as ‘‘Pliny the Elder’’ (author of formation and conservation of differential aeration and
the encyclopedia ‘‘Natural History’’), wrote about spoiled concentrationcells(7).
ironinhisessay‘‘FerrumCorrumpitar’’.Indeed,theorig- SeveralreviewsdealingwithMIChavebeenpublished
inal meaning of the term corrosion, from the Latin root since 1971 (8–11), with a particular attention given to
‘‘rodere’’,whichmeans‘‘actofgnawing’’,includesbiological researchduringtheso-calledenergycrisis.Infact,marine
activity. foulingonoffshore platformsand biofouling inindustrial
Microbial corrosion or biodeterioraton refers, in its cooling systems, as well as the problem of oil and gas
extensive meaning, to any corrosion caused by biological reservoirs’ contamination by sulfide enhanced the neces-
activity. Biological corrosion is an interdisciplinary sub- sityofadeeperinsightintotheroleofandthemechanism
jectinvolvingallbasicsciences,suchasphysics,chemistry, associatedwithbacterialcorrosionofmetal.
microbiology,biology,aswellasalldisciplinesofengineer- Bacterialadhesionandbiofilmformationarecommonly
ing, that is, civil, mechanical, electrical, chemical, and encountered both in natural environments and in indus-
metallurgicalengineering.Despitedifferentdefinitions,it trial processes (12). The heterogeneous biofilm and the
can be observed that corrosion is basically the result of associated bacteria form complex biological systems that
interactionbetweenmaterialsandtheirenvironment.Up can cause several chemical changes at the metal/biofilm
tothe1960s,thetermcorrosionwasrestrictedonlytomet- interface, such as the production of gradients in pH, dis-
als and their alloys and it did not incorporate ceramics, solvedoxygen,chloride,andsulfate(13,14).Underaerobic
polymers, composites, and semiconductors in its regime. conditions,themicrobialcolonizationusuallyleadstothe
The term corrosion now encompasses all types of natu- formationofdifferentialaerationandconcentrationcells,
ral and man-made materials including biomaterials and owingtothemetabolismofthebacterialcolony.Thegen-
nanomaterials,anditisnotconfinedtometalsandalloys eration of these concentration cells is widely recognized
alone. Corrosion and, consequently, biocorrosion cannot to be detrimental to the integrity of the passive film and
bedefinedwithoutareferencetoenvironment,evenifall to facilitate the initiation of pitting or crevice corrosion
environments are corrosive to some degree, for example, (15). Hence, metals that mainly depend on the forma-
soils,airandhumidity,marinewater,andsoon. tion of a stable oxide film for their corrosion resistance,
Before dealing with this subject, a short review of the such as stainless steel, are particularly susceptible to
milestones in its historical development seems appropri- MIC (14). MIC of stainless steel has been manifested
ate.Evidenceofanaerobiccorrosionwasobservedasearly in many forms of localized corrosion, which include pit-
as 1840, when Mallet noted sulfide corrosion products ting,crevice,underdepositcorrosion,andstresscorrosion
typical of anaerobic process on an ancient anchor found cracking(16).
at the bottom of the Seine River (1). The phenomenon of
microbialinducedcorrosion(MIC)wasreportedbytheend
MICROBIOLOGICALCORROSIONFEATURES
of the nineteenth century by Garret (2), who postulated
thatcorrosionofleadinwaterwascausedbymetabolites
Severalindustrialsectors,suchasoil,gas,nuclearpower,
produced by bacterial degradation of organic matter. He
shipping, process, and civil engineering suffer potential
suggestedthatbio-oxidationofnitritetonitrateenhanced
pollutionproblems,health,andsafetyhazards,aswellas
thecorrosiveactionofwater.
substantialfinanciallosses,asaresultofbiodeterioration
Beijerinckdiscoveredsulfate-reducingbacteriain1895,
and biofilm development (17). The latter phenomenon is
while Gainesin1910(3)indicatedthatironbacteriaand
consideredtobetheresultofcomplexprocessesinvolving
EncyclopediaofIndustrialBiotechnology:Bioprocess,Bioseparation,andCellTechnology,editedbyMichaelC.Flickinger
Copyright©2009JohnWiley&Sons,Inc.
1
2 MICROBIOLOGICALINDUCEDCORROSIONANDINHIBITION
transport of organic and inorganic molecules and micro- (cid:129) interferenceinthecathodicprocessunderanaer-
bialcellstothesurfacewithinitialattachmentofcellsand obicconditionsbyobligateanaerobes;
subsequentnonreversibleadhesion,facilitatedbyproduc- (cid:129) sulfuricacidproduction.
tionofextracellularpolymericsubstances(EPS),referred 2. Onthebasisofthemechanisminvolved:
to as glycocalyx of slime (18). MIC is a many-sided prob- (cid:129) corrosion associated with the breakdown of pro-
lem occurring when the contact between microbial cells, tective films by metabolic processes of microor-
or products of their metabolism, such as EPS and the ganisms;
surface, is established, as documented by Gaylarde and (cid:129) corrosionbysulfate-reducingbacteria(SRB),gen-
Videla (19). Microbial extracellular polymers, including erallydevelopedinanaerobicregionsbut,insome
lipopolysaccharides, proteins, and nucleic acids are rec- instances,formedinaerobicbulkwater(31);
ognized as playing an important role in the process of (cid:129) corrosionbyacidproducers;
cell attachment to metal surfaces (20,21). In particular, (cid:129) corrosionduetoheavymicrobialgrowth,deposits,
EPS released in the bulk phase of surrounding liquid and discontinuous biofilms, creating conditions
can compete with bacterial cells for binding sites at the fornew/modifiedgalvaniccells.Eventhoughsuch
metal surface and therefore should be considered when classifications are useful, it must be stated that,
investigatingbiocorrosion(22). basically,thefeatureofMICisconnectedtothree
Microbial corrosion results from the formation of factors, that is, microorganisms involved, mate-
biofilms mainly consisting of water (94%w/w), car- rial,andenvironment(solutionandconditions).
bohydrates, and extrapolymeric proteins (6%w/w) on
steel surfaces (23,24). Under natural conditions, the Microorganisms
biofilms found in pipelines appeared to be very complex
In order to indicate proper defensive means against
microhabitats,owingtoawidevarietyofmicrobialgenera
corrosion of metal materials, an accurate study and a
(25,26). The formationof abiofilm takesplace due to the
deeperunderstandingoftheactionofthemicroorganisms
presenceofelectrostaticandadhesiveconditions,allowing
involvedareundoubtedlyofutmostimportance.
onemicroorganismtobindirreversiblytoasteelsurface,
Microorganisms from a wide range of genera and
further followed by the production of extrapolymeric
species are involved in corrosion problems. They may be
substances (EPS) by means of microorganisms. Mature
classified into three general groups: bacteria, fungi, and
biofilms formed on steel are composed of anaerobic
algae.
microorganisms close to the steel/biofilm interface, fac-
ultative aerobes in middle regions of the extrapolymeric
Bacteria. Themaintypesofbacteriathatarestudiedin
matrix, and, finally, aerobic genera in the surroundings
microbialcorrosionareasfollows(32):
of the biofilm/solution interface. Owing to the complexity
ofthisprocess,someauthorshavefocusedtheirattention
(cid:129) SRB
on studying the inner layer of the biofilm, that is, the
(cid:129) Sulfur/sulfide-oxidizingbacteria
steel–anaerobic microorganism interface (27,28). The
EPSproducedbysomebacteriawasshowntobindmetal (cid:129) Acid-producingbacteria
ions and hence can promote the corrosion of metals, (cid:129) Iron-oxidizingbacteria
such as copper, alloys, and steels. In addition to their (cid:129) Manganese-fixingbacteria
innate aggressivity toward metals, EPS also enhance
(cid:129) Acetate-oxidizingbacteria
corrosion by providing a matrix for the attachment of
(cid:129) Acetate-producingbacteria.
microbial cells to a metal surface. Close contact between
bacteria and metal was shown to be an important factor
ThetaxonomyandphysiologyofMIC-involvedbacteria
in determining the corrosion rate. One of the most
havebeenreviewedbyPringsheim(33)andStokes(34).
important groups of corrosion-causing organisms is the
The most important bacteria connected with the cor-
sulfate-reducing bacteria, a heterogeneous collection
rosion process are the ones whose metabolism is closely
of anaerobic heterotrophs, which can secrete various
relatedtothewell-knownsulfurcyclereproducedinFig.1,
organicsubstances,suchasextracellular(EPS)primarily
where S in amino acid represents the reduced organic
composed of polysaccharides, uronic acid sugars, and
sulfurinlivingmatter(e.g.,SHgroupofcysteine).
proteins, containing functional groups, which could be
Aerobic bacteria of the Thiobacillus genus, known
acidic and capable of binding metal ions. The metal ion
as sulfur-oxidizing bacteria, operate the upper part of
chelating properties of EPS have been reported as a
the cycle—that is, the oxidation of sulfur to sulfuric
mechanismofcorrosion(29).
acid—while the central part of the cycle, that is,
ThereareseveralwaystoclassifyMIC,forexample:
reductionofsulfatetosulfide,iscarriedoutbyanaerobic
bacteria—of Desulfovibrio and Desulfomaculatum
1. Onthebasisofthemetabolismofimplicatedmicroor- genus—indicatedasSRB.
ganisms(10,11,30): Some 20% of the annual corrosion damage of metals
(cid:129) absorption of nutrients by microbial growth in and nonmetalsiscaused bymicrobialactivities, ofwhich
adhesiontothemetalsurface; asignificantpartisduetoanaerobiccorrosioninfluenced
(cid:129) evolutionofcorrosivemetabolitesorendproducts by SRB. Their activity often causes the formation of
offermentativegrowth; biofilms on iron and steel and tends to promote and
MICROBIOLOGICALINDUCEDCORROSIONANDINHIBITION 3
n S (cid:129) canexistinbothmarine(saltwater)andfreshwater
SulBfieCdgherg iooatximodaati,a tiuTomhi, oCbDhaelcsoilrullfouobsi viubrmio ChrBomegagtiiuamto,aS C,u Thlfhuloiror oobbxaiicduiallutios n m(3i9cI,r4roo4eg(on4)nrr,-2agvo,adaix4rsuni3odai)ntsi.lhzmmlyienesygbnaedtrsbeecaposcconootsvmeiterthmiraaitroetondonalrytohmsryoefadr-frceeriarsonrlhxleeeiddwdsettasormtaeaeiersxnttaecroalna-fuvdcsieeSriplRonloungBslmiaMtcreianlInyCngt.
m
Most of them generate energy for growth by oxidation
Thiobacillus of ferrous ions to ferric ions, which afterward precipitate
S2− SO42− as Fe(OH)3. Several types of iron bacteria may catalyze
Desulfovibrio Escherichia precipitation of preoxidized soluble ferric ions (45). In
aerobacter general,itisacknowledgedthatironbacteriamaysignifi-
Escherichia pareoErtoesbcuahscetreicrhia AsNpeeurrgiollsupsora, Aspergillus cap(vb1aimeeenroowoitrxoulyido2ndf.ttitoBazshrekiodedoefeteitiprnrocsaoesonrorutxgfopiiexmdnptiaildarcytegusiotnsiphnntiretyveruoecasdlinltvpeieemei)rtdotgaefhy(tfleFaoarsnerermd(gqItueuIha)riaetriicmneoaamgnobn,uaieobpnnvtertiteocssrmdoyfououfnsrcicehrihgnoo(rgnr4hot6iwlmgta)i.thrumhgIes)neert,
Fe(OH) depositsareusuallyinlargeramountscompared
3
S in amino acid to the bacterial biomass. This phenomenon poses sev-
eral problems to consumers of the various water supply
Protein systems: besides poor water quality and equipment clog-
Proteolysis
synthesis
ging,biofoulingmayalsocauseseriouscorrosionproblems.
Severalcasesofcorrosiondamagecausedbyoxidizingbac-
S in protein
teria were observed in different carbon steel equipment,
Figure1. Thesulfurcycleandmicroorganismsinvolved. suchasheatexchangers(tubes,lids,tube-sheets,connec-
tionpipes),extinguisherpipelines,andotherpartsofthe
watersystem(47).Theroleofiron-oxidizingbacterialies
intheformationofcondensedoxygenzonesandpartition
pit them (35). By SRB, we mean any organism that
of the metal surface into small anodic sites (beneath a
metabolicallyreducessulfatetoH S.SRB,andespecially
2
dense deposit of iron hydroxides and biomass) and large
one of its species, known as Desulfovibrio, which can
surroundingcathodicarea(48).Accordingtothishypothe-
grow in the pH range 5–10 and in the temperature
◦ sis,enhancementofthecorrosionprocessbyironbacteria
range 5–50 C, though certain SRB grow well at high
◦ can be expected on stainless steels and other passivated
temperaturesaround100 C(32)uptoapressurelimitof
metalsthatarepronetocrevicecorrosion,ratherthanon
about500atm(36)ormore(37).
activelydissolvedcarbonsteel.Accelerationofcarbonsteel
Thereasonwhyinvestigatorsandengineersareinter-
corrosioninwateriscommonlyassociatedwiththeactivity
ested in SRB is that they are regarded as the most
of other microorganisms, such us the already-mentioned
trouble-producing organisms in industry (38), although
SRB or acid-producing bacteria (APB). The dense rust
some researchers do doubt that SRB are worth studying
depositsformedbyiron-oxidizingbacteriamayalsocreate
so much and regard their unique importance in MIC as
oxygen concentration zones and initiate crevice corrosion
‘‘myths of MIC’’ (39). SRB can be found everywhere; it is
oncarbonsteel.
knownthatSRBareresponsiblefortheblacksands,which
Many of the oil refinery products on the one hand
givetheBlackSeaitsname.IntheUnitedKingdom,they
possess strong inhibitory characteristics and are, on the
havebeenencounteredatadepthof71minclay(40).
other hand, an excellent carbon source for heterotrophic
A list of the SRB with their main characteristics is
bacteria(49).Oilrefineryplantssufferhighriskofwater
giveninTable1(modifiedfromRef40).
contaminationwiththeseproducts.Asanexample,inan
Recent research has enlightened how a wide variety
oil-refinery plant in Haifa, it was supposed that corro-
of bacteria (from newly found genera) have the ability
sion of the carbon steel heat exchanger of cooling water
to reduce in size, as a survival strategy during starva-
wasinducedthroughtheabove-proposedmechanism(50).
tion; these features indicate that many types of bacteria,
Referringtotheoilindustry,astudyonatropicalcountry
includingSRB(41):
pipelineevidencedthatB.cereus ACE4iscapableofcon-
vertingtheferricandmanganeseonmetalintooxidesand
(cid:129) are able to penetrate reservoirs previously thought
accelerates severe pitting attack on the surface of steel
tohaveverysmallporesizestoallowthepassageof
API5LX,indieselenvironment(51)
normal-sizedcells;
The combination of SRB and IOB was shown to con-
(cid:129) are not detected using the media and procedures tribute sinergically to the highest corrosion rate of 316L
routinelyusedformonitoring; stainless steel; in aquatic environments, these microor-
(cid:129) possiblyhave adifferent sensitivity toward biocides ganisms attach to surfaces, altering the rates of partial
thannormalcells; reactions in corrosion processes and shifting corrosion
4 MICROBIOLOGICALINDUCEDCORROSIONANDINHIBITION
Table1. ListofSRB(Note:fromref.40)
Genus Temperature Metal
◦
orSpecies pH [ C] Conditions Influenced Mechanism
Desulfovibrio H2utilizationtoreduceSO42– toS2–
andH2S;promotionofsulfidefilm
formation;nosporeformation
Dv.africanus
Dv.desulfuricans Ironandsteel, Growthonpyruvateorcholinein
stainless sulfate-freemedia
Dv.gigas 4–8 10–40 Anaerobic steel,aluminium,
Dv.salexigens zinc,copperalloys Obligatesalt-waterspecies(Cl–)
Dv.vietnamensis
Dv.vulgaris
Desulfotomaculum 10–40 ReductionofSO42– toS2– andH2S;
sporeformation
Dt.nigrificans 6–8 Optimum55 Anaerobic Ironandsteel, Hydrogenaseactivityvariable;not
stainlesssteel coupledtosulfatereduction;growth
onpyruvateinsulfate-freemedia
Dt.orientis 10–40 Hydrogenaseactivityapparentlyabsent
Dt.rumnis 10–40 Growthonpyruvateinsulfate-free
media
Desulfobacter 6–7 20–40 Anaerobic Iron,steel,stainless
steel
Desulfomonas 6–8 10–40 Anaerobic Ironandsteel ReductionofSO42– toS2– andH2S
Desulfonema 7–8 10–40 Anaerobic Ironandsteel
Table2. ListofOtherBacteriaRelevanttoMicrobialCorrosion
Genus Temperature Metal
◦
orSpecies pH [ C] Conditions Influenced Mechanism
Sphaerotilus 7–10 20–40 Aerobic Ironandsteel
stainlesssteel Fe2+oxidationtoFe3+;
manganousoxidationto
manganic;
S.natans Aluminiumalloys tuberculeformationpromotion
Pseudomonas 4–9 20–40 Aerobic Ironandsteel,
stainlesssteel
SomestrainsreduceFe3+to
Fe2+
P.aeruginosa 4–8 Aluminiumalloys
Thiobacillus 0.5–8 Ironandsteel, Sulfurandsulfideoxidationto
thiooxidans H2SO4;
stainlesssteel protectivecoatingdamage
Thiobacillus 1–7 20–40 Aerobic Ironandsteel Fe2+oxidationtoFe3+
ferrooxidans
Gallionella 7–10 Ironandsteel, Fe2+oxidationtoFe3+;
stainlesssteel manganousoxidationto
manganic;
tuberculeformationpromotion
mechanism(52).Therealroleofiron-oxidizingbacteriain steel covered with sediments, as well as the role of these
carbon steel corrosion is still unknown. Corrosion accel- sedimentsincorrosionacceleration.
eration by iron bacteria through creation of concentrated A list of other bacteria relevant to biological corrosion
oxygenzonesisapparentlyonlyoneofthepossiblescenar- isgiveninTable2.
ios. It is difficult to state, owing to lack of experimental
evidence,thatthesuggestedmechanismdescribesthereal Fungi. The fungi associated with corroding metal are
situationofsteelcorrosioninwatercontainingironbacte- generally filamentous or yeastlike. In contrast to some
ria. At present, adequate information is not available on bacteria,fungirequireoxygenforgrowth.
the role of iron bacteria in corrosion initiation and prop- They obtain energy for growth through aerobic oxi-
agation during the cathodic process taking place on mild dation of an organic substrate, which releases organic
MICROBIOLOGICALINDUCEDCORROSIONANDINHIBITION 5
acidmetabolitesasanendproduct,orthroughanaerobic For the corrosion to continue, these electrons need to
fermentation. be consumed by reduction reactions that take place on
Thefungicanutilizehydrocarbonsasasourceofcarbon; the same metal on so-called ‘‘cathodic’’ sites. Generally,
theymaybeactiveinhydrocarbonenvironmentsandhave theoxygenreductiontohydroxylionisthemostcommon
beenprovedtobetheagentsinthecorrosionofaluminium cathodereactioninneutralenvironment:
aircraftfueltanksinthepresenceofcondensedwater.
O +2H O+4e−→4OH−
2 2
Algae. Algaeareaheterogeneousgroupofchlorophyll-
In neutral environment, the overall reaction is forma-
containingplantsfoundinseaandfreshwaters.Theyare
tion and possibly subsequent precipitation of insoluble
autotrophicorganisms.
products formed by the reaction of the ferrous ions with
Algaearecommonlyfoundincoolingwatersystems.
the hydroxyl ion (and, often, oxygen) in solution. As a
They act similarly to fungi. By adhering to the metal
rule,reactionsoccurevenlyoverthemetalsurface.Anode
surface, they encourage the formation of differential aer-
sitesmaybedeterminedeitherbyheterogeneousmaterial
ationandconcentrationcellstherebyacceleratingalready
(stressareas,inclusions,etc.)orbypreexisting,unevenly
existing corrosion processes. The corrosive effect of algae
distributed oxide films. The overall corrosion rate may
attachedtometalsurfaceswasinvestigatedbyTerryand
depend on quite a number of factors: anode and cathode
Edyvean(53),whodemonstratedthatmicrofoulingorgan-
reactions,however,mustremainbalancedintheprocessin
isms attach readily to unprotected steels, cathodically
ordertofullymaintainelectroneutrality.Lackingoxygen,
protected steels, and painted steels, within 100 days of
the cathode reaction that may replace the above-written
exposure,causingalternatingpHvaluesatnightandday
reactioninaqueousambient,soastosustainthecorrosion
andenhancingpittingcorrosion. process,isreductionoftheionH+ asfollows:
Evidence of siliceous skeletons of diatoms causing
defects in electroplated coatings of metals was shown by 2H++2e−→H
2
Ebnethetal.(54).
The reaction, however, is quickly checked in neutral
Material solutions,sothatthecorrosionprocessisrapidlystopped.
Atlower pH values, hydrogen evolution usually prevails,
Thebehaviorofthematerial,particularlymetalandalloys,
bothinthepresenceandintheabsenceofoxygen.Besides
in the solution environment depends on the chemical
these basic reactions, there are, however, other factors
composition, protective film stability, metallurgical and
affecting the whole process as, in particular, the metal
processing parameters, and the effectiveness of possible
surface microgeometric characteristics and the ambient
protectivemeasures.Microorganismsrevealastrongten-
oxygen concentration variation that creates differential
dencytowardadsorptiononmaterialsurfaces.Themicro-
aeration cells. Cathode sites are in this case the highest
bialcolonizationonthesubstratumsurfacecandrastically
oxygenation areas. Similar concentration cells may occur
modifythecorrosionresistanceofawiderangeofmateri-
on metal in case of contact with soils or solutions where
als (20,55–57), even those considered for employment on
theconcentrationofaggressiveionsisnonhomogeneous.
theinternationalspacestation(58).Dealingwithmetals,
Manyarticles(10,11,60,61)onthistopichaveoriginally
microstructure andmetallurgicalfeatures,suchasdefor-
assumedthat,inordertoparticipateinthecorrosionpro-
mationofthegrainstructure andinclusionpresence,are
cess, microorganisms ought to be in a vegetative phase
reportedtoactassitesofdecreasedresistancetoMIC(59).
and produce oxidizing agents through their metabolism,
Metal corrosion in aqueous environment is recognized
or else directly intervene in one or both the metal elec-
as an electrochemical phenomenon in which the anode
trochemical reactions. It has always been known that all
reactiontakesplacewhenpartofthesubstratum(metal)
bacterial colonies (not only the ones consisting of vegeta-
isoxidizedandtransferredintosolutionandthebalancing
tivecells)mayberesponsiblefordifferentialaerationcell
cathodereactionconsistsinthereductionofcomponentsin
formation on metal surfaces. Microhabitats generated by
thecorrosiveenvironment,whichtakesplaceatthesame
thesecoloniesoftencreateanaerobicconditionsthatstim-
time, to fully preserve electroneutrality. Metal ions in
ulate,inturn,theactivityofSRB.Examinationofprotein
solution might subsequently precipitate as free insoluble
profilesinextrapolymersofmarine–SRBisolatesrevealed
productsorfirmlyadheretothemetalsurface,wherethey
theappearance ofnewbandsinEPSharvested fromcul-
can have a protective effect. In favorable circumstances
tures grown on mild steel surfaces, thus confirming the
and under natural conditions, these films may have a
effectofsurfaceoncellmetabolism(62).
protectivefunction;atypicalexampleofthisfunctionare
archaeologicalfindingsconsistingofironhandwork,which Environment
is well preserved even though found in corrosive ground.
ItiswellknownthatMICresistanceofamaterialstrictly
These well-preserved items were covered with a slightly
dependsalsoonthenatureoftheenvironmentandopera-
compact, tightly adherent thin black film of ferric and
tiveconditions.Theseverityofmicrobialcorrosionvaries
ferrousphosphate.
considerablyfromoneplacetoanother.Knowledgeofthe
When the metal is submersed in water, wet soil, or
corrosivityoftheenvironmentandofcorrosionresistance
any other aqueous system, the initial reaction is metal
ofametalinagivenenvironmentisofutmostimportance
dissolutiontocation,whichinvolvesfreeingofelectrons:
for its successful industrial application. The most impor-
Fe→Fe+++2e− tant requirement for microbial growth is water presence
6 MICROBIOLOGICALINDUCEDCORROSIONANDINHIBITION
environment, and oilfield environment. Few hours’ expo-
sureofanymaterialtoseawatercausesthedevelopmentof
anonlivingorganicconditioningfilmonthesurface.Inthe
following days, the accumulation of bacteria, fungi, and
microalgae and their secretions create a bacterial slime
filmovertheconditioningfilm,knownasbiofilm.Biofilm
is capable of maintaining environments at the interface
that are radically different from the bulk fluid in terms
of pH, dissolved oxygen, and other organic and inorganic
species, thus facilitating reactions not predicted thermo-
dynamicallyonthebasisofthebulkmediumchemistries
(64).Anindustrialplantcansufferhazardsfrombiocorro-
Figure2. Schematicmechanismoflocalizedcorrosionattackby sionandbiofoulinginseveralsections,forexample,storage
pittingonametalsurfacemodifiedbybiofouling.
tanks,pipelines,waterinjectionlines,wastewatertreat-
mentsystem,filtrationsystem,open/closedcoolingsystem,
heat exchanger system, and potable water distribution
system. Differenttypesofbiofouling, enhancing localized
alterations of ions, pH, and oxygen level are connected
to the different environments in terms of microbiological
components aswell as abiotic constituents content. Basi-
cally,differentbiofoulingtypesandtheircombinationscan
occur in the following ways: (i) biological fouling due to
micro or macroorganisms, (ii) chemical fouling connected
to chemical reactions not involving the metal structure,
(iii) corrosion fouling due to the reaction of the metallic
substratum with the liquid environment (iv) particulate
foulingduetothetransportandsubsequentsedimentation
of particulate solids sedimenting by gravity on the metal
surface,and(v)precipitationfoulingresultingfromprecip-
itationofdissolved substances onthe metal(65).Itmust
be remarked that the interaction of biofouling and inor-
ganiccorrosionproductsinfluencesthepassivebehaviorof
thestructuralmaterial(66).Theextentofthispromoting
effect can vary widely. Localized corrosion attack by pit-
tingonametalsurfacebiologicallymodifiedbybiofouling
is schematically reproduced in Fig. 2 (after Ref. 67). It
mustbenoticedthatkineticgrowthisdifferentforplanck-
tonicmicroorganismsandsessilemicroorganisms,thatis,
those established at the metal surface. Once the biofilm
is formed, MIC damage depends mainly on the sessile
microorganismsratherthanontheplancktoniconesthat
arenotincontactwiththemetal.Visualappearanceofa
Figure3. Biofilm and corrosion products over a steel surface
showingtypicaldiscontinuityinthebiofilm/corrosionproductthat biofilm and corrosion products formed on a steel sample
possiblyleadstopitformation. are given in Fig. 3: a localized attack by pitting can be
observedduetotheactionofmicroorganismsembeddedin
thebiofilm.Theformationofabiofilminagivenenviron-
mentcandetermineaninterestingandstillnotcompletely
and availability, so MIC is common in aqueous environ- understood phenomenon known as ennoblement, that is,
ments such as fresh water, seawater, sewage, oil, and the increase in the open-circuit potential E . Ennoble-
corr
humid environments. All bacteria require metal ions for ment was mainly observed for stainless steel exposed to
their growth. The availability and the type of ions (type naturalseawater.Variousattemptsweremadetoaccount
ofmetalspeciesandreactivity,expressedasmetaloxida- forthisphenomenon.JohnsenandBardalsuggestedthat
tion state) depend on the environment and are likely to ennoblement is connected to a change in the cathodic
exert a significative effect on the colonization of a metal propertiesofthestainlesssteelasaresultofmicrobiolog-
surface (63). Water plays a predominant role in the cor- ical activity on the surface (68). Eashwar et al. observed
rosionprocess,beinganelectrolyte,whichisanessential that ennoblement in seawater is due to the production
component of a corrosion cell. In presence of salts, acids, of inhibitors by bacteria that are present in the biofilm,
and dissolved gases, such as carbon dioxide, hydrogen as confirmed by the fact that E measured in natural
corr
sulfide, and oxygen, the degree of corrosivity of water is seawater can exceed the pitting potential E in sterile
pit
significantlyenhanced.ThecommonMICaggressiveenvi- seawater, without pitting occurrence (69). A special case
ronments are mainly seawater environment, industrial ofennoblementwasobserved inadifferentenvironment,
MICROBIOLOGICALINDUCEDCORROSIONANDINHIBITION 7
that is, natural river water by Linhardt (70). Stainless A case history with specimens from a discontinuous
steel components of a turbine in a hydroelectric plant reactor employed for yeast biomass industrial produc-
were severely corroded by pitting caused by biomineral-
tion, showing uniform generalized corrosion, is depicted
izedMnoxide,asrevealedbythelargeamountsofMnO
2 in Fig. 6. Microbiological corrosion on a reactor made of
andMnOOHfoundoncorrodedsurfaces(consideringthat
steel, with no addition of particular alloy element and
MnO is characterized by an E close to the observed
2 o
belongingtoFE33commercialcategory,canbesomarked
ennobled E of stainless steel). In relation to the envi-
corr
ronment,thosematerialsthataresusceptibletocreviceor underaerobicconditionstobedetectablebysimpleoptical
underdeposit corrosion appear to be more susceptible to microscope.Apictureofbiofilmandcorrosionproductsis
microbialcorrosion. showninFig.7,wherethedarkregionsaremainlyformed
by inorganic corrosion products (with high content of Fe)
TYPESOFMICANDCASEHISTORIES andthelightareas(inthepresenceofNandC)aremainly
formed by bacteria and exopolymers. The corroded sur-
In view of industrial application, the mechanism of MIC face, examined under scanning electron microscope and
and the ways of its recognition are discussed in the fol-
reproduced in Fig. 8, shows how the difference in molar
lowing,makingreferencetoapracticalclassification into
volumes existing between oxide flakes and metal causes
aerobic or anaerobic conditions. Some case histories are
theprocessofspalling.
alsoreportedinordertoprovidethereaderwiththevisual
appearance of different MIC phenomena and practical Spalling of the corrosion products and superficial pit-
waysofidentification. tingduetotheactionoflocalizedcellulardepositsisshown
inFig.9.
BacterialCorrosionunderAerobicConditions
Aparticulartypeofaerobiccorrosionisduetomicroor-
Alarge numberofaerobicmicroorganisms arecapable of ganismsbelongingtotheThiobacillusgenus.Thesechemi-
affectingthecorrosionprocess.Somemicroorganismsare olithotrophicmicroorganismsdrawtheenergyneededfor
responsible for surface cell-deposit formation (bioscaling
their vital processes from oxidation of inorganic sub-
phenomena). In marine environment, the biofouling phe-
stances. The nutrition functioning of these microorgan-
nomenonismostlyduetocirripeds,mussels,andsoon.The
isms, in particular the Thiobacillus ferrooxidans, was
corrosionprocess,insuchcases,consistsintheformation
ofadifferential-aerationcellduetooxygenassumptionby investigated; various models have been proposed among
the microbial colonies tending to exhaust oxygen concen- which one of the most feasible is that iron as a com-
tration under such mass. Poorly aerated surfaces behave plex is absorbed through the microorganism’s cell wall
asanode,whilebetter-aeratedsurfaces—fartherfromthe and gives the electron away to the respiratory tract. The
deposit—provide the cathode balancing of the reaction.
energythat is liberated during the subsequent metabolic
Thesimplemechanismofaerobiccorrosionisschematized
phasesisemployedtoformATP,whiletheferricironand
inFig.4(Ref10).
waterformedduringtheprocessareexpelledfromthecell
Formationofoccludedareasonmetalsurfaceisamech-
anism potentially involved in MIC. It is based on the and colloidal ferric hydroxide is formed (71). The energy
observation that, when microorganisms form colonies on made available through the ferrous iron oxidation reac-
the surface of a metal, they do not constitute uniform tion(about6.6kCalpergram-atom)is so scarce thatthe
layersbutlocal‘‘colonizationsites,’’relatedtosuchmetal- organismiscompelledtooxidizevastquantitiesofironto
lurgicalfeaturesasroughness,inclusions,surfacecharge,
obtainwhatisneededforgrowing.
orsuperficialdefects.Thedevelopmentofanoccludedcell
Another type of microorganism, such as Thiobacillus
formedbystickypolymersisreproducedinFig.5;biolog-
thiooxidans, is capable of producing sulfuric acid. The
ical and nonbiological (e.g., metals and chloride) species
relevant mechanism has been studied in detail by Booth
tend to aggregate to the colony site, as evidenced in the
specimentakenafter12monthsofexercise,fromacarbon (72)andPurkiss(73):thegenerallyacceptedpathinvolves
steelpipe,inanenvironmentwithhighchloridecontent. thefollowingreactionsthatmaydependonthesubstrate
Water solution
Tubercle
O2 4Fe(OH)2 O2
2H2O + O2 + 4e− 4OH− 4OH− 4e− + O2 + 2H2O Biofilm
4Fe++
Cathode Cathode
Pit
4e− Fe 4e−
Metal Figure4. Aerobiccorrosionmechanismbydif-
Anode
ferentialaerationcell[afterRef.10]).
8 MICROBIOLOGICALINDUCEDCORROSIONANDINHIBITION
Figure5. AISI 304 (500X). Development of occluded cell under Figure7. InorganiccorrosionproductsandbiofilmovertheFE33
aerobicconditions. surface(300X).
Figure6. FE33 (120X). Uniform generalized aerobic corrosion
phenomena.
andtheenvironment:
2H S+2O →H S O +H O
2 2 2 2 3 2 Figure8. FE33(500X).Morphologyofaerobiccorrosionproducts,
5Na2S2O3+4O2+H2O→5Na2SO4+H2 SO4+4S showingspallingphenomenon.
4S+6O +4H O→4H SO
2 2 2 4
oxygen-deficient environment, because, under these con-
ditions,neitherofthetwocommoncathodereactionscan
BacterialCorrosionunderAnaerobicConditions
takeplaceatasustainedrate.This,however,isnotalways
Accordingtotheabove-mentionedconsiderationsconcern- true and, when a corrosion phenomenon takes place in
ingelectrochemicalreactions,acorrosionprocessinvolving suchconditions,itismuchmoreseverethanunderaerobic
ferrousmetalswouldseemunlikelyinanear-pH-neutral, conditions.
MICROBIOLOGICALINDUCEDCORROSIONANDINHIBITION 9
Several research works carried out in recent years
(74,75)seemtosupporttheassumptionofhydrogeninac-
tivationandsubsequentsulfatereduction.Morerecently,
somestudiespointtoanalternativeprocessinvolvingsul-
fide and the iron sulfide film (76,77). It is interesting to
note that laboratory studies employing cultures of SRB
are unlikely to produce heavy pitted corrosion and that
normal iron oxide and hydroxide corrosion products are
nearly always absent. A similar phenomenon also occurs
in the presence of elementary sulfur, which often forms
undernaturalconditionsintheperipheryofpittedareas,
whileitveryseldomdoessounderlaboratoryconditions.
Acombinationoftheseandotherobservationsrelatedby
KingandMiller(77)hasledtoamorecriticalapproachto
theclassictheory.
Inviewoftheparticularlycomplexproblemsinvolved,
we deemed it advisable to analyze the different mecha-
nismsseparately:
1. Classic Model: The classic theory is based on the
factthatsomeSRBstrainspossessthehydrogenase
enzyme, which is capable of catalysing both the
reversiblereactionsinvolvinghydrogen,thatis:
Figure9. FE33 (300X). Spalling of aerobic corrosion products
andsuperficialpittingduetolocalizedcellulardeposit. H↔2H↔2H++2e
as well as sulfide reduction by means of molecular
hydrogenaccordingtothefollowing:
The main factor responsible for this problem is the
activity of SRB as well as their sulfured metabolic prod- SO2−+4H →S2−+4H O
4 2 2
ucts. Buried metals corroded by this sort of process are
an example of the heavy economical losses and damages Hydrogenase-positive strains of alophile, ther-
causedbythistypeofcorrosion. mophile, and mesophile bacteria were shown by
The process of anaerobic corrosion is characterized by Booth and Tiller (78) to be capable of cathode
acoatingoncorrodingsteelconsistingofstronglyreduced depolarizing, while hydrogenase-negative bacteria
black-sulfide-containing corrosion products. Graphitiza- proved completely inactive in this process. Thus,
tiontakesplaceincastironandlittlesuperficialevidence strains such as Desulfovibrio desulfuricans and
of corrosion can be noticed; the iron migrates from the Desulfovibriovulgaris,hydrogenase-positive,utilize
materialleavingasoftresidue,mainlycarbonmade. thecathodehydrogenforsubstratereduction,while
In connection with bacterial involvement in the corro- hydrogenase-negativestrainssuchasDesulfomacu-
sionprocess,theexplanationproposedbyWolzogenKuhr latum orientis, were found to play no role at all in
thismechanism.
and Van der Vlugt (6) is based on the assumption that
Among other hypotheses, a relation, sustained by
cathode hydrogen may be utilized for sulfate reduction.
evidences in a limited field of experimental condi-
Theequationsdescribingtheprocessareasfollows:
tions, was supposed to exist between hydrogenase
4Fe→4Fe2+8e−-anodicreaction activityinthemicroorganismandcorrosionrate.
8H O→8H++8OH−-dissociationofwater However, subsequent experiments employing
2
active cell cultures of several different strains
8H++8e−→8H-cathodicreaction(*)
of SRB indicated that the corrosion rate did not
SO24++8H→S2−+4H2O-cathodicdeplorization(*) depend on the hydrogenase activity. And further,
Fe2++S2− →FeS-corrosionproduct hydrogenase-negativebacteriaprovedtobeatleast
3Fe2++6OH→3Fe(OH) -corrosionproduct as aggressive as the Desulfovibrio species. This
2
clearlyindicatesthatotherfactorsmustbeinvolved.
Theoverallreactioncanbewrittenas 2. AlternativeTheories:Studiesoncathodedepolariza-
tion always refer to the acknowledged property of
4Fe+SO24H O→3Fe(OH) +FeS+2OH− sulfides, both in solution and in solid state, as, for
4 2 2
instance,elementalsulfur,toactascorrosiveagents.
Throughthereactionsmarkedby(*),abiologicalaction Further,itmightbenotedthatmildsteelcorrosion
develops,bringingabouthydrogenactivationthat,inturn, rate increases by up to 230% when in contact with
causessulfatereduction. ironsulfide.Seriouscorrosionsinacidenvironments,
However, there is disagreement between the workers likewise,werefoundtobeimputabletothepresence
onthemechanismdeterminingthecorrosionprocess. ofsulfuredcompoundsandofelementalsulfur,with