Marcus P. Corrosion mechanisms in theory and practice

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Many microorganisms, bacteria and fungi, are able to use this reduction reaction.

Even the ferrous iron–oxidizing bacteria may be able to perform this reaction in

the event of oxygen shortage. The reactions are not restricted to mesophilic bacteria.

Thermophilic bacteria are known to participate in these reactions (oxidatively as

well as reductively).

As mentioned, sulfuric acid is produced concomitantly with the metal sulfide

oxidation. Sulfuric acid may react with metallic iron, Fe

, to give ferrous sulfate

and hydrogen. Both compounds may be oxidized biologically. Furthermore, ferric

iron may react with metallic iron to form ferrous iron, Fe

+ 2Fe

→ 3Fe

, which

may then be oxidized to ferric iron. Another reaction with iron results from the

sulfur cycle and demonstrates the intimate connection between both cycles.

Reduction of sulfate under anaerobic conditions, e.g., by SRB, results in the

formation of hydrogen sulfide. This may react with ferrous iron to form a precipitate

of ferrous sulfide and, if sulfide is in excess, finally pyrites (FeS

In summary, it becomes obvious that most reactions of materials with the

environment are influenced by microorganisms and are often even controlled by

them [36]. Thus, in the case of microbially influenced corrosion, the partici-

pation of microorganisms needs to be anticipated [54,95]. Furthermore, cases

are known in which excreted metabolic products, e.g., EPS free from microbial

cells, cause corrosion.

MECHANISMS OF BIODETERIORATION

Despite the vast diversity of microorganisms participating in various natural

cycles, the biological mechanisms contributing to or causing biodeterioration may be

summarized in a few main categories [48]. It needs to be pointed out, however,

that one microorganism may exert multiple detrimental effects. In addition,

under natural conditions pure cultures do not exist. Thus, mixed cultures called

biocoenoses are active by usually creating growth conditions for the corrosion-

causing microorganisms. In most cases microorganisms cause an attack resulting

from a chemical compound produced and excreted by metabolism. Thus, the basic

action will be a chemical reaction.

Excretion of Acid

Specialized bacteria are able to produce and excrete strong mineral acids. Usually

under aerobic conditions, thiobacilli oxidize inorganic sulfur compounds and sulfur

to sulfuric acid. The energy of this oxidation is coupled via special enzymes to

growth. Besides sulfur and sulfur compounds, the bacteria need only carbon dioxide

(cell mass). The genus Thiobacillus consists of several species that are able to

grow at moderately alkaline down to strongly acidic pH values. The species able

to grow and proliferate on alkaline materials are pioneers for the species growing

only under acidic conditions. After the buffer substance of, e.g., concrete (lime)

has been exhausted, the pH value in the surface water declines and the acidophilic

species start to proliferate. This finally causes severe biogenic sulfuric acid corrosion

[6,7,80,89,96].

Microbially Influenced Corrosion 573

The second group to be mentioned here is the nitrifying bacteria excreting nitric

acid. The nitrifying bacteria consist of two groups. The ammonia oxidizers convert

ammonia to nitrite, and the nitrite oxidizers subsequently form nitrate. Like sulfuric

acid, nitric acid can react with alkaline materials forming highly soluble salts

(in contrast, sulfates are much less soluble). Biogenic deterioration results

[7,64,66,76]. Nitrifiers are also lithoautotrophs. The energy source is a nitrogen

compound; cell carbon is derived from CO

The third important acid is produced by all life forms. Carbon dioxide is

excreted as the end product of metabolism. It reacts with water to carbonic acid,

which may dissolve, e.g., to carbonates forming soluble bicarbonates. In this way

the binding material of concrete, lime, may be dissolved.

The fourth group of microorganisms consists of those which during their

metabolism excrete organic acids such as oxalic, citric, malic, lactic, or acetic

acid, amino acids, uronic acids, etc. [25,34]. Usually, these acids are excreted

because of an unbalanced metabolic state. However, they may thus be taken up

again by the cells in later growth phases or be metabolized by other microorganisms

present in the biotope. This means that organic acids are usually available only

temporarily. Nevertheless, their presence may have caused transformations in the

crystal lattice. Organic acids may be excreted by almost all bacteria, cyanobacteria,

algae, lichens, and fungi. Sometimes the excretion is coupled to the uptake of

limiting cations into the cell.

Chelatization

Besides acid attack on materials, organic acids are chelating cations. Because of the

stability of the complexes, metals may be dissolved from a crystal lattice resulting in

a weakening of the structure. The action is sometimes deliberate because it allows the

cells to replenish limiting cation concentrations; e.g., pathogenic bacteria possess

special iron chelators to replenish growth-limiting ferric iron at the expense of the

host.

Organic Solvents

Many microorganisms are able to metabolize organic substances under anaerobic

conditions. If an electron acceptor such as nitrate, Fe

, Mn

, or sulfate is not

available, fermentation results [73]. This means that hydrogen (= redox equivalent)

is transferred from one to another organic compound. By this transfer, growth may

be possible because of a substrate phosphorylation. The result of a fermentation

process is thus usually another organic compound, sometimes carbon dioxide. The

organic compounds are in many cases organic acids, as mentioned before, or

organic solvents such as ethanol, propanol, or butanol. These solvents may react

with materials of natural and/or synthetic origin, causing swelling, partial or total

dissolution, etc., which finally results in deterioration.

Other Metabolic Compounds

An important compound for MIC is hydrogen sulfide. It is produced under anaerobic

conditions by the action of sulfate-reducing bacteria from sulfate, sulfite, and

sometimes sulfur [88]. It has also been shown that thiosulfate can by used by SRB

574 Thierry and Sand

[75]. As mentioned before, the SRB use the not fully reduced sulfur compounds

as electron acceptors in anaerobic respiration.

Hydrogen sulfide may act in several ways. Either it is reoxidized to sulfuric acid

under aerobic conditions [101] or in the presence of compounds such as nitrate or it

may react with cations to form metal sulfides that precipitate [50]. Further, H

S may

be oxidized anaerobically in the light by photosynthetic bacteria (mostly to sulfur,

sometimes to sulfate). In the case of materials containing acid-reactive substances,

the biogenic acid will become harmful. In the case of metals the presence of H

S may

cause MIC. Another source of H

S is not restricted to anaerobiosis. Amino acids

containing sulfur are degraded under aerobic conditions, giving rise to H

S (cysteine

and cystine).

Another important compound is ammonia. It may be generated as a result of

amino acid or urea degradation by microorganisms. Furthermore, ammonia (or

mainly ammonium salts such as sulfates or chlorides) is a major part of airborne

gases (or dust particles). By dry or wet deposition ammonia/ammonium salts

reach the surface of materials and, in the case of porous materials, enter the pore

system and become biologically available. Nitrifying bacteria as already described

may degrade ammonia/ammonium compounds. However, chemical reactions with

materials may also be possible.

Nitrogen oxides,N

O, NO, NO

, resulting from biological activity (nitrogen

cycle—oxidation of ammonia or nitrite reduction to N

) as well as from processes

such as the burning of fuels (heat and power generation, traffic), are a group of

compounds that may react with materials. NO

is a water-soluble gas dissociating

upon dissolution into nitric and nitrous acid. These acids may attack susceptible

materials. NO is less reactive but may be oxidized to NO

by light. N

O is not

known to interfere with materials.

Biofilm

Microorganisms tend to adhere to surfaces [48,56,106,141]. It is believed that

more than 90% of all microorganisms grow in this way [14]. Concomitantly,

exopolymeric substances are excreted. This may result in a slimy superficial layer

on materials and/or, in the case of porous materials, in total clogging of the free pore

volume [60]. The exopolymers are usually hydrated and may contain ionic groups

favoring water inclusion. Thus, one result may be an increase in water content in the

case of porous materials (a consequence is freeze–thaw attack; see the following).

Furthermore, the biofilm may act as insulation, reducing heat transfer. In the

case of heating systems, the efficiency may be considerably reduced [40]. Another

effect is the reduction of the cruising speed of ships due to increased friction caused

by the surfacial biofilm [5]. Finally, biofilm development may cause technical

processes to become susceptible to trouble. In paper machines, the inclusion of

exopolymers in the pulp may cause the paper web to break in the paper machine. The

coating of cans with resins may also be affected. Cans for food are coated on the

inside with a resin to protect the metal from corrosion by foodborne acids. If the metal

is covered at a few spots with biofilm (microcolonies), the coating process will not

be totally successful. Pores will remain in the resin at these spots, rendering the metal

accessible to acids from the food. Corrosion from the inside combined with

deterioration and spoilage of the contents will result.

Microbially Influenced Corrosion 575

Salt Stress

The biogenic reactions that have been mentioned result in the generation and,

generally, accumulation (except in an aquatic environment) of salts as reaction

products. Because salts are hydrophilic they are usually hydrated, resulting in an

increase in the water content of a porous material. This may increase the

susceptibility to physical attack by freezing and thawing because of the volume

change in water/ice crystals.

Furthermore, upon desiccation, salt crystals may develop, causing the surfacial

removal of layers of material, e.g., deterioration destroying wall paintings on natural

stone. The third detrimental effect of salts results from the formation of large crystals

causing a swelling attack. A well-known example is the formation of ettringite

from gypsum crystals destroying concrete and bricks.

Exoenzymes and Emulsifying Agents

Besides exopolymers of a lipopolysaccharidic nature, microorganisms excrete

lipoproteins and proteins. The latter include exoenzymes for the degradation of high-

molecular-weight compounds such as cellulose. Cellulases degrade the insoluble

cellulose fibrils to soluble molecules such as cellobiose (dimer) and glucose,

which may be taken up by the cells for metabolism. Similar exoenzymes exist for

other materials, e.g., esters, amines, and waxes. Other molecules, although not as

large as cellulose, may also be insoluble and, thus, only poorly degradable.

In some cases microorganisms excrete emulsifying agents to increase the

solubility of hydrophobic substances [112]. In the case of elemental sulfur, which

is highly insoluble due to its hydrophobic nature, the excretion of such emulsifying

agents causes an increase in dispersibility from 5 to 20,000 μg/L [114]. This holds

for other hydrophobic materials as well. Water-insoluble hydrocarbons become water

soluble because of microbial production of emulsifying agents and consequently

become degradable.

These microbial deterioration mechanisms represent the main categories of

action. Several mechanisms are usually active jointly, and because these mechanisms

may also influence other types of physical or chemical attack, the microbial share

of the total attack can rarely be determined. Much too often, studies of deterioration

mechanisms suffer from inadequate microbiological analysis (if it is done at all). If

corrosion occurs in the range where life is possible, a microbial contribution needs

to be taken into account and, hence, a microbiologist should be consulted [54,84].

MICROBIAL CORROSION OF CONSTRUCTIONAL MATERIALS

Metallic Materials

Metallic materials are an important group of construction materials. Microbially

influenced corrosion may occur for these materials for many industrial applications,

as listed in Table 1 [27]. Microbially induced corrosion of metallic materials

does not involve any new form of corrosion. Thus it is necessary to discuss the

electrochemical nature of corrosion briefly before continuing with the mechanisms

of MIC for different construction metals.

576 Thierry and Sand

The corrosion of metals in the presence of water is of an electrochemical

nature. This includes corrosion in water-containing solutions, atmospheric corro-

sion due to the presence of a humidity film at the metal surface, and corrosion in

soils due to the humidity of the soil. A detailed discussion of electrochemically

induced corrosion is outside the scope of this chapter. For more details, a large

number of good comprehensive reviews may be consulted (for instance, Ref. 63).

Hence only some basics will be given here insofar as they may help in understanding

MIC of metals.

The anodic reaction involves the oxidation of the metal into metal ions,

whereas the cathodic reaction involves the reduction of some component in the

corrosive environment:

Microbially Influenced Corrosion 577

Table 1 Industrial Applications for Which Microbial Corrosion Has Been Reported

Source: Ref. 27.

Cathodic reactions may be summarized as follows:

The rates of the anodic and cathodic reactions must be balanced in order to preserve

electroneutrality.

The nature of the cathodic reaction depends on the pH of the solution and

on the presence of dissolved oxygen or other oxidizers such as dissolved CO

For instance, in the pH range between 4 and 10, the diffusion of dissolved

oxygen to the metal surface controls the rate of corrosion of iron. In this pH range

and in the absence of oxygen, very low corrosion rates should therefore be

expected.

The term “uniform corrosion” implies that the anodic and cathodic sites are

virtually inseparable, whereas “localized corrosion” implies that macroscopic

anodic and cathodic sites are physically separable.

As already stated, MIC of metallic materials does not involve any new form

of corrosion. The mechanisms involved in the biodeterioration of materials have

already been given in detail. The main ways in which microorganisms may

enhance the rate of corrosion of metals and/or the susceptibility to localized corrosion

in an aqueous environment are given below in relation to known corrosion

mechanisms (for each case some examples are also given):

Formation of concentration cells at the metal surface and in particular oxygen

concentration cells. This effect may occur when a biofilm or bacterial growth

develops heterogeneously on the metal surface. Concentration cells are

also associated with the tubercles formed by iron-oxidizing bacteria, such

as Gallionella. Certain bacteria may also trap heavy metals such as copper

and cadmium within their extracellular polymeric substance, resulting in the

formation of ionic concentration cells.

Modification of corrosion inhibitors. To this group belong microorganisms that

may destroy corrosion inhibitors such as bacteria that transform nitrite

(corrosion inhibitor for iron and mild steel) to nitrate, or nitrate (corrosion

inhibitor for aluminum and aluminum alloys) to nitrite and ammonia and N

Production of corrosive metabolites. Bacteria may produce different metabolites,

such as inorganic acids (e.g., T.thiooxidans), organic acids (almost all

bacteria, algae, and fungi), sulfide (sulfate-reducing bacteria), and

ammonia, that are corrosive to metallic materials.

Destruction of protective layers. Various microorganisms may attack organic

coatings, and this may lead to corrosion of the underlying metal.

Stimulation of electrochemical reactions. An example of this type of action is

the evolution of cathodic hydrogen from microbially produced hydrogen

sulfide.

Hydrogen embrittlement. Microorganisms may influence hydrogen embrittlement

on metals by acting as a source of hydrogen or/and through the production of

hydrogen sulfide.

In summary, all known cases of microbial corrosion of metals can be attributed

to known corrosion mechanisms. In the following sections, the mechanisms of

microbially induced corrosion for different metals and alloys are discussed.

578 Thierry and Sand

Mechanisms of Microbially Induced Corrosion of Iron and Mild Steel

Corrosion Under Anaerobic Conditions Considering the electrochemical

reactions discussed earlier, very low corrosion rates are expected for iron and steel

in near-neutral conditions and in the absence of oxygen. However, a large number

of case histories in the literature, particularly for buried pipes and marine

structures, report that the corrosion may be very severe and can be several orders

of magnitude higher than that experienced under normal aerobic conditions [47].

It is well accepted that this is due to sulfate-reducing bacteria and their ability to

produce sulfide. However, despite the large body of literature available, the exact

mechanism(s) responsible for the corrosion in such environments is still the

subject of discussion. Several good review articles present this subject in more

detail than is attempted here [50,123,124,128].

The first attempt to explain the anaerobic corrosion of iron was that by

Von Wolgozen Kühr and van der Vulgt [130]. These authors proposed as early as

1934 that sulfate-reducing bacteria were responsible of the pitting corrosion they

observed on buried cast-iron pipes through their ability to remove the hydrogen from

the metal surface via their enzyme hydrogenase for the dissimilary reduction of

sulfate according to the following reactions:

Microbially Influenced Corrosion 579

Reaction (9) represents the ability of SRB to remove the adsorbed hydrogen

from the metal surface through the enzyme hydrogenase. This reaction was referred to

by the authors as “depolarization.” This term was used only to underline that there was

an undefined change in the electrochemical response of the system studied [33]. This is

an alternative reaction path compared with the classical hydrogen evolution reaction:

Reactions (14) and (15) are the rate-determining steps (i.e., a higher activation

energy is required for these reactions compared with the discharge of hydrogen

ions) for many metals and alloys, including iron in deaerated aqueous solutions.

The rate-determinating steps may, however, be different in the case of complex

corrosion layers. Sulfate-reducing bacteria, by removing adsorbed hydrogen to

form sulfide according to reaction (9), lower the activation energy for the desorption

steps. As these steps are rate determinating, this may lead to higher corrosion rates

if hydrogen evolution is thermodynamically possible (i.e., if the immunity potential

is lower than the redox potential for H

This theory, also referred to in the literature as the classical depolarization

theory, suggests that only sulfate-reducing bacteria that are hydrogenase positive

(i.e., bacteria in which the enzyme hydrogenase is present) are responsible for the

anaerobic corrosion of iron. Even though there are data supporting this view

[9,57], it has been shown in many other works that this is not the case and that a

high corrosion rate may be obtained with hydrogenase-negative strains [10,81,92].

In addition, several other important factors are not taken into account in the classical

depolarization theory: (a) the effects of sulfide, bisulfide, and hydrogen sulfide

produced from the sulfate reduction on the anodic reaction; (b) the effect of

hydrogen sulfide on the cathodic reaction; (c) the effect of elemental sulfur from

the biotic or abiotic oxidation of sulfur; (d) fluctuations in the environmental

conditions between anaerobic and aerobic conditions [47]; (e) the production of

other corrosive metabolites [128].

It is therefore now widely accepted that, even if the so-called depolarization

of the cathodic reaction by the enzyme hydrogenase occurs, it plays no more than

a secondary role in the pitting corrosion of iron in anaerobic conditions. This theory

is therefore reported here more for historical interest than as a potential mechanism

for pitting corrosion of iron in presence of SRB.

More recent theories have been proposed in which the role of the biogenic

sulfide, the formation of a galvanic cell between the metal and the iron sulfide film

formed, the role of elemental sulfur, the role of iron phosphites, and the local

acidification of anodes have been discussed. The alternative theories to the classical

depolarization theory are presented briefly in Table 2.

It has also been shown that the nature of the iron sulfide film plays an important

role in the initiation of pitting corrosion. Figure 5 shows the different biogenically

and chemically formed iron sulfide films [93]. When protective films such as

mackinawite and siderite are formed, the corrosion rate may be very low. However,

with changes in the environmental conditions these films may be transformed to iron

sulfides that are less protective, such as pyrrhotite, with the possible initiation of pits

as a result. The importance of the environment for the initiation of localized corrosion

by SRB as been mentioned by several authors [24,51]. Of particular importance is

the ability of SRB to regulate pH, the ionic composition, the composition of Fe

aerobic/anaerobic cycles, and the presence of other microorganisms, as they may

lead to conditions resulting in passivation or uniform corrosion or localized

corrosion. This is probably the main reason why the severe pitting corrosion

experienced in the field is only seldom reproduced in the laboratory using growing

cultures of SRB, where uniform corrosion is generally observed. Indeed, in practice

SRB are not found in isolation but in consortia of microorganisms or biofilms

in which many physicochemical parameters such as pH and dissolved oxygen

580 Thierry and Sand

Microbially Influenced Corrosion 581

Table 2 Alternative Mechanism to the Classical Depolarization Theory

Figure 5 Chemical and biological interrelationships between iron sulfides [93].

concentration change both with time and within the thickness of the biofilm. This

dynamic picture of microbial corrosion is discussed in more detail later.

Corrosion Under Aerobic Conditions Corrosion may occur under aerobic

conditions through the production of sulfuric acid by bacteria of the genus

Thiobacillus [119]. The sulfuric acid is produced through the oxidation of various

inorganic sulfur compounds, as illustrated in Table 3 [22]. Some of these bacteria

can tolerate a concentration of sulfuric acid up to 10–12%. Under these conditions,

iron and mild steel are heavily corroded.

Corrosion of iron and steel may also occur in aerobic conditions due to the

colonization of bacteria at the metal surface and the formation of an uneven patchy

biofilm. Nonuniform biofilms or bacterial colonization results in the formation of

differential aeration cells in which the anodic areas are found under respiring

colonies or in the thick part of the biofilm (zones depleted in oxygen) whereas the

cathodic areas are found in noncolonized areas or in the areas where the biofilm

is thin (regions rich in oxygen). This is schematically illustrated in Figure 6. The

iron-oxidizing bacteria such as Gallionella ferruginea, Crenothrix, and Leptothrix

are often associated in the literature with this form of corrosion, particularly in the

internal surfaces of water pipes. These bacteria are aerobes and are believed to

oxidize ferrous ions into ferric ions with the resulting formation of heavy deposits

of ferric oxide or hydroxide, also referred to as tubercles. However, even if

iron-oxidizing bacteria are the best-documented case, many other organisms may

be involved in this kind of microbially influenced corrosion.

Finally, it should be mentioned that microorganisms may cause the degradation

of corrosion inhibitors such as aliphatic amines, nitrite, and phosphate-based

corrosion inhibitors used, for instance, for steel pipes in cooling water systems. This

results in a higher demand for corrosion inhibitor and an increase in the bacterial

population, which leads to high corrosion rates, if the system is not controlled

[120,121].

Microbial Biofilms and the Interactions Between Aerobic and Anaerobic

Populations In the preceding discussion, the mechanisms of microbial corrosion

have been divided in the traditional way into anaerobic and aerobic mechanisms,

which refer to the living conditions of the microorganisms involved in the

582 Thierry and Sand

Table 3 Oxidation Reactions of Thiobacilli and Other Aerobic Sulfur-Oxidizing Bacteria

Source: Ref. 22.