Marcus P. Corrosion mechanisms in theory and practice

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corrosion processes. However, it is now well known that bacteria are not found in

isolation but rather in consortia or biofilms in which many bacterial communities

coexist. A good illustration of this is given in Figure 7 [21]. As already mentioned,

a biofilm is composed mainly of bacterial cells, extracellular polymers, inorganic

ions, and water. It influences the mass transfer of different species and in particular

dissolved oxygen from the solution to the metal, thereby creating a zone depleted

in oxygen at the metal–biofilm interface (Fig. 7a). In this part of the biofilm,

facultative and obligate anaerobic microorganisms such as sulfate-reducing bacteria

may find suitable conditions of growth (anaerobic conditions and low redox

potential). The result of this is the formation of microcolonies of SRB, which,

through their production of metabolite products, may generate conditions favorable

for other anaerobic bacteria to grow (Fig. 7b). This creates areas on the metal surface

with local gradients in, for instance, hydrogen sulfide and hydrogen concentrations

that may initialize localized corrosion on iron and steel (Fig. 7c). Many authors

[44–46], have reported similar synergistic effects of bacterial consortia on the rate

of corrosion of mild steel.

In addition to the ways already mentioned in which microorganisms influence

the corrosion of iron and mild steel, there are numerous reports that microorganisms

may cause hydrogen embrittlement, stress corrosion cracking, and corrosion

fatigue [35,132,133]. It has been shown that in pure cultures of hydrogen-producing

bacteria (hydrogen is produced as an end product of the fermentation process)

hydrogen embrittlement is enhanced. In natural microbial biofilms, the situation

may be described according to Figure 8 [133]. Consortia of hydrogen-producing

and hydrogen-consuming bacteria are formed at the metal surface, creating, for

instance, gradients of hydrogen [102,127]. Hence, depending on the primary

colonizers (hydrogen producers or hydrogen consumers) and their distribution on

the metal surface, hydrogen embrittlement may or may not occur.

It is also obvious that the production of H

S from SRB may accelerate

hydrogen embrittlement, stress corrosion cracking, and corrosion fatigue.

Microbially Influenced Corrosion 583

Figure 6 Schematic representation of the influence of the biofilm in the formation of

differential aeration cells.

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From these studies it seems clear that, even if studies of microorganisms in

isolation (i.e., in pure cultures) give valuable information concerning the mechanisms

of MIC of a particular species in a given environment, more work should be done

on the effect of mixed bacterial populations on the mechanisms of MIC.

Mechanisms of Microbially Induced Corrosion of Stainless Steels

and Titanium

There are large numbers of reported case histories of MIC on stainless steel in

water and aqueous waste systems. They are related to different industrial applications

such as freshwater storage and circulation systems in nuclear power plants [103,

113,116,142] and cooling water systems in chemical process industries [117,118].

There are basically three cases: (a) crevice corrosion under unexpected deposits, (b)

sensitivity of pitting and crevice corrosion to trace of H

S, and (c) crevice corrosion

in natural seawater. Most of these reports are not well documented concerning the

microorganisms involved in the process. However, some general features are

Microbially Influenced Corrosion 585

Figure 8 Diagrammatic representation of the effects of the spatial arrangement of hydrogen

producers and consumers in a natural biofilm on the embrittlement of metals (black circles

represent hydrogen producers, open ovals represent hydrogen consumers.) (From Ref. 133.)

Figure 7 Diagrammatic representation of the formation of microbial consortia and their

influence on the corrosion processes. (a) When the film becomes sufficiently thick its inner

part will be anaerobic with the possible development of SRB microcolonies (black cells).

(b) The SRB attracts secondary colonizers by its metabolic products and forms a consortium

with them. (c) The development of local areas with varying physicohemical parameters

leads to pitting corrosion. (From Ref. 21.)

common in almost all cases: (a) MIC is reported for the alloys with a relatively low

content of molybdenum; (b) pitting or crevice corrosion generally occurs at or around

welds (i.e., near the fusion line, or with sensitization in the heat-affected zones); (c)

discrete localized deposits are often found in direct connection, with or close to the

corrosion sites.

The higher sensitivity to MIC of welds compared with the unwelded metal

may be explained by a difference in surface roughness or/and chemical composition

that facilitates the colonization of the surface by microorganisms.

In some reports, a higher sensitivity to MIC of the delta-ferrite in duplex welds

has been reported [12,67], and in others both the austenite and delta-ferrite phases

have been shown to be sensitive to MIC [11]. There is also evidence that surface

treatment of welds, such as solution annealing and pickling or polishing and grinding,

may result in welds that are less susceptible to microbial corrosion [11,134].

The observation of localized deposits is often associated with the presence of

slime-forming bacteria such as Pseudomonas and iron bacteria. These bacteria

consume oxygen diffusing into the deposit and thus create a differential aeration

cell that may initiate crevice corrosion. Under the deposit, the environment

becomes anaerobic with the subsequent possibility of growing SRB in a scenario

similar to that described for microbial biofilms. Besides the effect of concentration

cells, the specific role of metal-concentration/oxidizing bacteria such as iron- and

manganese-oxidizing bacteria in the initiation of localized corrosion is often men-

tioned [29]. By controlling the redox potential of Fe

/Fe

and/or Mn

/Mn

these bacteria may polarize the metal surface at a potential at which Fe

and/or

may exist. Together with the accumulation of chloride ion (due to the

necessity of preserving electroneutrality), this leads to the formation of FeCl

and

MnCl

solutions that are aggressive to stainless steels. It is well known that both

situations facilitate the breakdown of the passive film with crevice corrosion and/or

pitting corrosion as a result.

Sulfate-reducing bacteria have also been reported to be responsible for pitting

corrosion on stainless steels in aqueous environments. The mechanisms proposed are

mostly related to iron and steel. However, a different mechanism has been proposed

in which the role of thiosulfate in the microbial pitting of stainless steel has been

emphasized [137]. The same authors have also demonstrated clearly that SRB-

induced pitting corrosion of stainless steel is unlikely to occur in a uniformly

anaerobic SRB medium, whereas it will occur when the anaerobic sites are coupled

to an oxygen cathode [136].

This discussion is related almost solely to alloys with a relatively low

molybdenum content, but the high molybdenum alloy stainless steels may also be

susceptible to MIC. 904L, AL-6X, and 254 SMO were attacked by bacteria in pure

cultures of Desulfovibrio desulfuricans and Hyphomicrobium indicum. The corrosion

attack seems to be localized at the grain boundaries [108]. However, MIC on highly

alloyed stainless steels has never clearly been reported in natural environments.

Besides the reports of MIC described above that are mainly related to water

with relatively low salt content such as fresh water, an increase in the corrosion

potential has also been reported for stainless steels exposed to natural seawater. The

ennoblement of the corrosion potential has been observed for various stainless steel

compositions (i.e., austenitic, ferritic, duplex, and superaustenitic stainless steels)

exposed in natural seawater with different salinities and at different temperatures

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[3,28,30,61,62,82]. It is well known that in a given steel/environment system,

localized corrosion occurs on stainless steel when the corrosion potential becomes

greater than the pitting potential. Hence, the result of the potential ennoblement is

that there is a higher probability of localized corrosion of stainless steel in a natural

environment than in artificial seawater. This effect has been connected with the

presence of a biofilm on the metal surface as shown in Figure 9. The corrosion

potential of 21Cr-3Mo steel increases as a function of time in natural seawater to

a value of about 400 mV/ECS, whereas it remains unchanged in sterilized seawater.

When a specific inhibitor of respiratory activity (sodium azide) is added to both

natural and sterilized seawater, a rapid decrease in the corrosion potential is

observed in the case of natural seawater but it remains unchanged in sterile

seawater [109]. The increase of the corrosion potential on stainless steel has

been correlated with an increase of the rate of the cathodic reaction, e.g., the

reduction of oxygen. This is shown in Figure 10. During the development of

the biofilm at the stainless steel surface, an important depolarization of the

oxygen reduction reaction occurs. This will lead to a higher rate of propagation

of localized corrosion on stainless steel. Hence the effect of the biofilm settlement

on stainless steel may be summarized as follows:

1. The corrosion potential of stainless steel increases in the noble direction,

which increase the probability of localized corrosion.

2. A higher corrosion propagation rate is observed once localized corrosion

develops.

3. Higher galvanic currents between stainless steel coupled to less noble

materials are measured.

The study of the mechanisms of the ennoblement of stainless steel is complex,

as seawater constitutes an environment in which numerous parameters can act.

Microbially Influenced Corrosion 587

Figure 9 Corrosion potential as a function of exposure time for 21Cr-3Mo steel in natural

seawater (black circles) and in artificial seawater (white squares). Time of addition of sodium

azide is indicated by arrows. (From Ref. 109.)

A literature review of the subject has been published [16]. It has been shown that

the potential ennoblement in the presence of biofilms is probably due to an

enhancement of the cathodic reduction of oxygen by (a) a decrease in pH at the

metal-biofilm interface, (b) the formation of macrocyclic organometallic catalysts,

such as porphyrins and phtalcyanins, (c) bacterially produced enzymatic catalysis, or

(d) the production of hydrogen peroxide within the biofilm or ie. the catalyse of the

oxygen reduction by manganese. More recent literature on the subject seems to

confirm the role of enzymes in the depolarization of the oxygen reduction reaction on

stainless steel [1,32]. A possible mechanism in which the potential ennoblement

results from enzymatic competition between the production of hydrogen peroxide

and acids by oxidases and the enzymatic consumption of these two chemicals has

been proposed. This is schematically shown in Figure 11. Based on a large amount of

biofilm growth on stainless steel at different European locations, it has been shown

that enzymes entrapped in the EPS matrix of the biofilm could be responsible for the

ennoblement of the corrosion potential on stainless steel [110]. Finally it should be

noted that potential ennoblement of stainless steel has also been reported for stainless

steel exposed to river water [49]. The mechanisms are, however, similar to that of

seawater.

According to a literature review, there is no reported case of MIC on titanium

[72].

Mechanisms of Microbially Induced Corrosion on Aluminum

and Aluminum Alloys

Case histories of MIC on aluminum and aluminum alloys have been mainly

reported for aircraft fuel storage tanks and heat exchanger tubes using water with

different salinities. In all cases, pitting corrosion occurred.

Even though these cases are not as well documented as MIC on iron and steel,

the possible mechanisms and microorganisms involved have been given for the

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Figure 10 Oxygen reduction current density on stainless steel exposed to natural

seawater and continuously polarized at fixed cathodic potentials. The envelopes of curves

1 and 2 correspond to the set of data on clean and already fouled stainless steel surfaces,

respectively [83].

microbial contamination of aircraft aluminum fuel tanks [17,52]. Some of the

most predominant bacteria and fungi are Pseudomonas aeruginosa, Aerobacter

aerogenes, Desulfovibrio, and Cladosporium species. The corrosion occurs mainly

in the water phase of the fuel-water mixture at the bottom of the tanks and at the

fuel-water interface. The mechanisms involved are the same as those already

given for iron and steel: (a) the production of corrosive metabolites by bacteria

and fungi (organic acids and hydrogen sulfide), (b) the creation of differential

aeration cells, (c) the transformation of nitrate (corrosion inhibitor for aluminum)

to nitrite, and (d) extracellular activity resulting in the removal of major or minor

metallic atoms from the basic structure of the alloy [92].

Mechanisms of Microbially Induced Corrosion on Copper

and Copper Alloys

As in the case of aluminum and aluminum alloys, the MIC of copper and its

alloys is not very well documented. This is probably due to the general belief

that copper is toxic to microorganisms, which is not, however, the case for all

microorganisms. As an example, bacteria of the genus Thiobacillus may tolerate

copper concentrations up to 6%.

Case histories of MIC on copper and its alloys have been reported for piping

systems and heat exchangers. Whereas heat transfer problems are observed with

growing biofilms, the corrosion increases after the death of the microorganisms

within the biofilm. This is believed to be due to the production of ammonia and

carbon dioxide upon the death of cells that may result in pitting corrosion and/or

stress corrosion cracking for copper and its alloys [72,92].

Microbially Influenced Corrosion 589

Figure 11 Hypothetical mechanism of the stainless steel ennoblement in natural

seawater [32].

The production of hydrogen sulfide by sulfate-reducing bacteria is also

believed to lead to pitting corrosion and stress corrosion cracking on copper

and copper alloys. In this case the corrosion is due to the formation of a thick

nonadherent layer of chalcocite (Cu

S) or covellite (CuS

1–x

). Pitting corrosion

may occur where the copper sulfide film has been removed, with the cathodic

reaction taking place on the intact copper sulfide film [79].

Microbially induced pitting corrosion has also been observed in isolated

locations in Europe, the United States, Australia, and New Zealand in potable water

reticulation systems plumbed by copper [131,138]. This results in a contamination

of the water such that the water is in breach of the maximum copper levels permitted

by the U.S. EPA lead-copper rule (i.e., 1.3 ppm) [126]. Generally a biofilm is

observed under a black deposit of copper (II) oxide and basic copper salts.

Although the link between the pitting corrosion of copper and the microbial activity

has been clearly established, the exact mechanism by which microorganisms

increase the probability of initiation of pitting corrosion on copper is not known

at the present time. However, it has been suggested that the role of the biofilm

could be to oxidize copper (I) oxide to copper (II) oxide [15]. Finally it should be

noted that an increase in the bicarbonate level of the water as well as a change in

the cholride/sulfate ratio with increasing chloride content yielded a decrease in the

initiation and propagation of pitting corrosion copper, respectively [131].

Other Construction Materials

Mineral Materials

Concrete is a mixture of a coarse-grained aggregate, hydraulic binding agents

(cement, gypsum, lime, asphalt, sulfur, and resin), and water (except in the case of

sulfur and resin, where water is not needed). For a limited period after preparation,

the mixture can be molded into different shapes until hardening occurs as a result

of chemical reactions between the components.

Ceramics are products made from fired clay–containing masses such as

kaolin. The products are distinguished by the grain size of the clay and additives

used and the firing temperature.

Glass is a transparent, sometimes colored, inorganic material, which is fragile

and predominantly noncrystalline. It has no well-defined melting point, but with

continuous heating it changes from being viscous to a soft and finally thin fluid

state (i.e., an undercooled melt without cyrstallization).

Natural stone originates from rocks obtained from quarries in natural deposits.

Rocks are defined as natural, solid formations of the earth’s crust. They consist of

a mixture of minerals or a single mineral and may contain organic residues.

Characteristics are the composition of primary (main), secondary, and auxiliary

minerals, the structure, and the stratification. Three main groups need to be

distinguished. Magmatic rocks are molten masses in the earth’s crust (basalt, granite,

etc.). Sedimentary rocks contain grains (often quartz) held together by a binding

material (often calcareous or siliceous, such as gypsum, limestone, sandstone,

schist). Metamorphic rocks are formed by transformation from magmatic or

sedimentary types by heat and pressure (quartzite, marble, crystalline schist).

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All these materials are characterized as nonmetallic mineral materials. A

biogenic attack can be caused by almost all microorganisms and by the mechanisms

described above [7,25,34,68,71,80,85,86,90,91,97–100]. Only the action of

exoenzymes, emulsifying agents, and organic solvents seems to be negligible for

these materials. However, if mixtures of mineral materials with organic polymers are

used to improve the properties, as in resin-modified mortars or in resin- or sulfur-

bound concrete, the latter mechanisms may contribute to some extent.

Organic Polymers of Natural and Synthetic Origin

To the group of construction materials [54] belong, besides stone, those that have

been used since humans began construction work. Wood is the oldest known con-

struction material. Because of its unique characteristics, it is still in use today.

The main components of wood are cellulose, polyoses (hemicelluloses), and lignin.

The proportions are 40–50%, 15–35%, and 25–35%, respectively. Extractable com-

pounds such as terpenes, waxes, or tannins may account for 1–3% and mineral

compounds for 0.1–0.5%. The tensile strength of wood is determined by the cellulose

cell structure. The cellulose molecule contains from 10,000 up to 14,000 β-(1,4)-

glycosidically bound glucose units and represents the valuable part of wood for

further products such as paper, cardboard, foils, films, and fibers.

Polyoses are also polysaccharides, but with lateral chains and ramifications.

Between 50 and 200 saccharide molecules are polymerized. Galactoglucomannan

or 4-O-methylglucuronoxylan is the main constituent. The polyoses connect the

polysaccharides with the lignin in the cell wall. They determine the swelling and

shrinkage of wood.

Lignin, either guaiacylpropane solely or combined with syringylpropane, is a

complex molecule responsible for the pressure resistance of wood. The extractable

substances such as terpenes provide the resistance against degrading microorganisms,

e.g., bacteria and/or fungi. From these two groups and lichens, wood-degrading

microorganisms originate. The main action is due to exoenzymes degrading the

highly polymerized substances into water-soluble monomers or dimers. Similar

mechanisms are involved in the degradation of wood-based materials such as

laminated timber, blockboard, plywood, chipboard, and fiberboard. Plywood and

chipboard may be especially endangered by biogenic attack because the binders

used for these materials may increase their susceptibility. Binders may be

urea-formaldehyde resins, phenol-formaldehyde resins, cresol-formaldehyde

resins, isocyanates, etc., which may serve as a nitrogen source.

Besides wood, construction materials are made from plastics. These are

materials consisting mainly of macromolecular organic compounds obtained

synthetically or by modification of natural products. On application of heat and

pressure, synthetics melt and become formable. Three main groups of synthetic

polymers may be distinguished.

The term polycondensates covers phenolic, urea, thiourea, melamine,

unsaturated polyester, alkydic, and allyl resins as well as silicones, polyimides,

and polybenzimidazoles in the group of duroplasts. To the category of thermoplasts

belong polyamides, polycarbonates, polyesters, polyphenylene oxides, polysulfones,

and polyvinyl acetals.

Polymerizates are generally thermoplasts (polyethylenes, polypropylenes, poly-

1-butenes, poly-4-methyl-1-penteneionomers, polyvinyl chlorides, polyvinylidene

Microbially Influenced Corrosion 591

chlorides, polymethyl methacrylates, polyacrylonitriles, polystyrenes, polyacetals,

fluorosynthetics, polyvinyl alcohols, polyvinyl acetates, and poly-p-xylylenes).

Polyadducts include duroplasts such as epoxidic resins and cross-linked

polyurethanes as well as thermoplasts such as linear polyurethanes and chlorinated

polyethers.

Although natural compounds are believed to be degradable by microorganisms

in general—microbiological omnipotence—this does not always apply to synthetic

polymers. These substances belong at least partially to the xenobiotica, which

are known to be resistant to biological attack. For a few synthetic resins,

experience of biodegradability has been available for nearly a century. Perspex has

been demonstrated to be biologically not degradable. The same holds for

nonflexible PVC. The main groups of resins may be characterized with regard to

their biodegradability as follows. Polyurethanes with molecular weights of 10,000

and above are generally believed to be easily biodegradable. However,

introducing substituents into the molecule may modify this. Polyethylenes with

molecular weights of 1500 to 7000 range from resistant to easily biodegradable.

Polypropylenes (MW 25,000 to 500,000) also vary from resistant to easily

biodegradable. Polymethyl methacrylates (MW 500,000 to 1,000,000) and

polyamides (MW 10,000 to 100,000) are comparable to those already mentioned.

Polystyrenes (MW 200,000 to 300,000) are generally believed to be resistant to

biological attack. The same holds for polyvinyl chlorides (MW 30,000 to 520,000)

and polytetrafluoroethylenes (MW 500,000 to 5,000,000).

Besides the basic polymers, the final products often contain a variety of

additives to adapt a resin to the intended purpose. This includes softeners (may

make up to 60% of the final product), stabilizers, antioxidants (0.01–2%), antistatics,

light-protecting agents (0.1–1%), optical brighteners, anti-ignition agents (up to

30%), microbicides (0.3–5%), lubricating agents, filling agents (up to 30%),

accelerators, and pigments (0.02–5%).

It is not possible in the available space to mention all the compounds used. It is

important that the majority of these additives are organic compounds, which are often

easily biodegradable. As a consequence, a synthetic polymeric material may not be

biodegradable with respect to the polymeric backbone, the synthetic polymer, but it

becomes biodegradable because of the additives used. A result may be an

embrittlement of the material (e.g., flexible PVC after being used several times). Even

if a synthetic material is not attacked structurally, depending on the use, microbial

growth may cause deterioration associated with, e.g. aesthetically intolerable color

changes. A variety of microorganisms produce pigments (mainly black, some also

blue or red) that may diffuse into the resin, causing a change in appearance.

Rubber, of either natural or synthetic origin, also contains because of its method

of production many biodegradable compounds. It is used for purposes in which it

needs to maintain its elasticity for a long time. Generally, synthetic rubbers are more

stable than natural ones. In any case, rubbers may be attacked by microorganisms.

COUNTERMEASURES

A large number of approaches may be used to prevent or minimize the

microbiological degradation of construction materials. The choice of the proper

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