Identification of microbiologically influenced corrosion (MIC) in

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Engineering Failure Analysis 14 (2007) 1500–1511

Identification of microbiologically influenced corrosion (MIC)in industrial equipment failures

J. Starosvetsky a,*, D. Starosvetsky b, R. Armon a

a Division of Environmental, Water and Agricultural Engineering, Faculty of Civil Engineering, Technion, Haifa 32000, Israelb Division of Corrosion and Electrochemical Processes, Faculty of Materials Engineering, Technion, Haifa 32000, Israel

Received 17 August 2006; accepted 6 January 2007Available online 2 March 2007

Abstract

The present study demonstrates different approaches towards MIC identification in three real cases of technologicalequipment failures.

In the first case the failure of carbon steel heat exchanger as a result of tubes, lids, tube sheets, and connection pipesclogging was investigated. Chemical analysis of cooling water and precipitates, as well laboratory screening of depositsfor bacteria, revealed that activity of iron-oxidizing bacteria present in cooling water led to heat exchanger blockage.

The second case is related to MIC detection on floating roofs made of magnesium–aluminum alloy following a 3-weekshydro-test. Corrosion tests carried out on the original and sterilized water used in hydro-test confirmed MIC process.

In the third case the potential of MIC occurrence in engine cooling system made of cast aluminum alloy and filled with20% ethylene glycol coolant solution was evaluated. The simulation tests allowed determining the real causes of the severecorrosion attack of examined system, including MIC high probability.� 2007 Elsevier Ltd. All rights reserved.

Keywords: MIC identification; Failure analysis; Simulation

1. Introduction

In the last two decades MIC was recognized as an important mechanism of metallic construction materialsdegradation. The heavy cost of microbial corrosion in various branches of modern industry is now well doc-umented [1,2]. However, ignoring this problem, taking as yet place in practice, gives rise new and new casehistories of metallic equipment failure due to participation of microbes in corrosion processes.

Uncovering of biological factor in corrosion processes that occurs in modern industry is quite an intricatetask, since MIC does not produce any unique type of localized corrosion. Even isolation of corrosion inducedmicroorganisms from a specific environment is not a sufficient argument to exclusively state that thesemicrobes were responsible in initiating or accelerating the corrosion failure. Successful identification of

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doi:10.1016/j.engfailanal.2007.01.020

* Corresponding author. Tel.: +972 4 9591160.E-mail address: [email protected] (J. Starosvetsky).

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J. Starosvetsky et al. / Engineering Failure Analysis 14 (2007) 1500–1511 1501

MIC having a multi-disciplinary nature requires an integrated approach. Chemical and microbiological anal-ysis of the milieu, examination of metal characteristics and its corrosion behavior, examination of corrosionproducts, etc. are necessary to understand the corrosion problem, however only corrosion process simulationmay exclusively confirm microorganisms contribution in the corrosion failure and assess their true role. In thiswork we present three typical investigations from our failure analysis practice devoted to assessing of micro-organisms involving in corrosion destructions occurred in Israeli industry.

2. Experimental

Water or coolant samples were collected in 1 L sterile plastic or glass containers. As a rule corrosion prod-ucts were sampled from the most damaged sites of the corroded surface by scrapping. For microbiologicalanalysis part of the collected corrosion products were resuspended in sterile phosphate buffer. Prior to inves-tigation the taken probes were stored at 4 �C. Chemical composition of water was determined by atomicadsorption method. Chemical composition and morphology of corrosion deposits were detected by meansenergy dispersive spectroscopy (EDS) and scanning electron microscopy (SEM). To estimate the role of micro-organisms in destructive processes several bacterial groups were selected as an index of biofouling and biocor-rosion potential: heterotrophic aerobic bacteria (HAB), sulfate-reducing bacteria (SRB), iron-oxidizingbacteria (IOB) and fungi. These bacterial groups are dominant and most important microorganisms associ-ated with bio-deposits formation and acceleration of corrosion processes. The isolation and count of themicrobes was performed according to Standard Methods [3] as it was described in [4–6].

3. Results and discussion

3.1. Case 1. Fouling of carbon steel heat exchanger caused by iron bacteria

A carbon steel (CS) heat exchanger was installed along a fresh water line at the reverse osmosis unit at theHaifa Refinery Plant, Israel. After 11/2 years from start-up the heat exchanger failed as a result of rust-coloreddeposits formation that clogged tubes, leads, tube sheets and connecting pipes. The goals of the study were todetect the causes of such fast clogging occurred under operation conditions, to estimate the bacterial effect onthe process, and to evaluate the countermeasures.

3.1.1. Field conditions

The heat exchanger was used to heat water from 15 to 25 �C during the cold season of the year, feeding areverse osmosis unit. During warm seasons, the heat exchanger was not active. Consequently, for severalmonths the internal water was subjected to very low flow conditions or stagnancy. Feed water to the reverseosmosis unit flowed through the tubes with steam on the shell side. The source water was a mixture of surfaceand potable well water stored in a large open air reservoir.

3.1.2. On-site observation

Visual observation revealed that practically all parts of tested heat exchanger were plugged with brown rustdeposits in the form of various dimension tubercles, which were strongly adhered to the steel surface (Fig. 1aand b). These deposits were accumulated from both sides of the heat exchanger (water inlet and outlet). Thesediments layer thickness ranged from 1 to 6 cm. Shallow and dip pits (up to 10 mm) were detected on tubesheets and led surfaces following removal of the tubercles. Both ends of the tubes were severely damaged bycorrosion.

3.1.3. Results of the investigations

The analyses of incoming water (Table 1) showed that this chemical combination could not provide anextreme milieu that would cause the observed fast accumulation of rust detected on the internal heat exchan-ger surface and high corrosion rate of CS.

Microbiological analysis conducted in water (planktonic bacteria) and in corrosion products scrapped fromthe corroded surface (sessile bacteria) revealed (Table 2) that most of the cultivable bacteria were HAB. The

Fig. 1. (a) Tube sheet ends damaged by corrosion beneath deposits. (b) Corrosion slime sediments formed on inside surface of the pipe.

Table 1Chemical composition of cooling water

Parameters Unit Value

pH – 6.9–7.4p-alkalinity CaCO3 (ppm) 0Total alkalinity CaCO3 (ppm) 80–190Cl� NaCl (ppm) 210–350SO�2

4 SO�24 (ppm) 40–48

Fe+2/Fe+3 (soluble) ppm 0.04–0.11Fe (total) ppm 0.1–06SiO2 ppm 5–10Total hardness CaCO3 (ppm) 210–230Ca hardness CaCO3 (ppm) 110–185Mg hardness CaCO3 (ppm) 100–125NO�3 ppm 8–11NHþ4 ppm 0.1–0.4COD MgO2/L 3800–4700Chlorine ppm 0.2–0.7TOC mg/L 2.8Conductivity lS/cm 900–1100Suspended solids ppm 1–4

1502 J. Starosvetsky et al. / Engineering Failure Analysis 14 (2007) 1500–1511

cooling water carried a high concentration of iron bacteria and SRB. As expected, in the rust slime sedimentsthe concentration of these bacteria was several orders of magnitude higher, especially for iron bacteria (IOB)and HAB.

Physicochemical analysis (EDS) of rust deposits sampled from internal surface of tested heat exchangershowed that the examined slime contained up to 18% of organic substances. Elevated concentration of organic

Table 2Bacterial count of tested bacteria in cooling water and slime sediments

Microbial group type Planktonic bacteria (CFUa/ml) Sessile bacteria (CFU/g)

HAB (TPC)b 2.2 · 104 4.5 · 109

SRBc (total count) 52 5.4 · 103

IOBd (total count) 4.0 · 102 4.9 · 107

a CFU-colony forming units (viable cells).b Heterotrophic aerobic bacteria (HAB), total plate count (TPC).c Sulfate-reducing bacteria (SRB).d Iron-oxidizing bacteria (IOB).


matter detected in slime rust indicated participation of isolated microorganisms in the process of slime forma-tion. Despite of the fact that concentration of HAB in examined rust was the highest, this type of bacteriacannot be assumed to be responsible for accumulation of the large amounts of iron oxides and hydroxides,which led to the heat exchanger clogging. The isolated both in water and corrosion products SRB, presumablyplayed certain role in localization of corrosion process occurred on heat exchanger surface under sediments,but also did not participated in slime deposits formation. Even sulfur compound traces were not found inscrapped deposits analyzed by EDS, despite the fact that H2S produced by SRB may react with metal ionspresent in water to form insoluble sulfides of metals.

The corrosion failure and rusty slime accumulation found on the inner surface of the heat exchanger may beattributed to iron bacteria activity. The EDS analysis revealed that sediments were comprised mainly of ironand oxygen (2:3 ratio). According to SEM examination, the rust deposit contained two different morpholog-ical forms: compact, dense particles, and crumbly, highly porous deposits with organic inclusions. After stain-ing the deposit with crystal violet solution, numerous filamentous bacteria cells were found among the rustparticles. The filaments resembled the classical form of iron-oxidizing (iron-precipitating) bacteria of Lepto-

thrix or Sphaerotilus genera. Microbiological analysis of iron bacteria mixed culture isolated from examinedrusty slime showed that the culture consisted only of Sphaerotilus genera.

It is well known that these bacteria may significantly participate in rusty slime formation, producinglarge amounts of iron oxides and hydroxides precipitates in a very short period of time. It was shownthat a biotic oxidation of rate of Fe(II) is at least one order of magnitude higher than an abiotic one.At a biotic oxidation of Fe(II) rusty deposits are usually very abundant in comparison with bacterial cells.Starkey [7] and Stephenson [8] calculated ratio iron-to-cell materials: it was 448–500:1. Iron–oxidizing bac-teria generate energy for growth by oxidation of ferrous ions to ferric ions. Some types of iron bacteriamay catalyze precipitation of preoxidized soluble ferric ions [9]. Iron bacteria were identified at high con-centration both in the cooling water and particularly in corrosion sediments (Table 2). According to Culli-mor and McCann [10], in running water iron bacteria proliferation could be observed if the ferrous ionconcentrations were to exceed 0.2–0.5 mg/l. Iron bacteria also may acquire iron from equipment materialsmade of iron or CS (pipes, heat exchangers, etc.) In the present case concentration of soluble ferrous ionsin tested cooling water (see Table 1) was insufficient (<0.2 mg/l) to assist proliferation of iron bacteria.Nevertheless, a high corrosion rate of CS in fresh water can provide a high rate of ferrous ions transferinto the water phase, which can be used as additional energy source to iron-oxidizing and iron precipitat-ing bacteria. Hence, it is fair to assume that the investigated water system had a high potential to carryiron bacteria activity, particularly through period of continuous flow. Under these favorable growth con-ditions, iron bacteria present in cooling water could stimulate deposition of Fe(III) on the heat exchangerinner surface, accelerating the corrosion rate of steel beneath deposits to cause plugging of the watersystem.

This assumption is supported by experimental results, such as:

– An high ratio 1:5.5 between organic matter and inorganic iron-containing compounds, indicating thatdeposits were most likely a result of microbial activity.

– High numbers of iron-oxidizing bacteria found in source water and particularly in rust sediments.


– The tubercle-shaped morphology of deposits, which are typical for rust slime formed due to iron bacteriaactivity.

Thus, on the base of conducted investigations it was concluded that failure of tested heat exchanger (clog-ging and severe localized corrosion) was directly related to the activity of iron-oxidizing bacteria detected inuse cooling water. Heat exchanger made of carbon steel most likely served as a nutrient source for iron bac-teria. Continuous leaching of ferrous ions into the running water coupled with good aeration of runningwater, resulted in corrosion of mild steel in fresh water encouraging aerobic iron bacteria growth and fast pro-duction of large amounts of biogenic rusts. Pitting corrosion observed under rust-slime layers (tubercles) onthe parts of heat exchanger (tubes, sheets etc.) was probably caused by action of concentration cells formed onmetallic surface due to creation of differential aeration zones under primary deposits and accumulation ofSRB, chlorides and other corrosive components in that zones.

3.2. Case 2. MIC of aluminum alloy floating roofs for storage tanks during hydro-test

At Gadiv Petrochemical Company (Haifa, Israel) four steel tanks with aluminum alloy 5052 (USN A95052)floating roofs were erected to storage of aromatic solvents. Prior to use, these tanks were hydrostatically testedat ambient temperature for �3 weeks. The test revealed severe localized corrosion (up to full penetration) onthe underside of the roofs. In order to determine why tested roofs were subjected to such severe corrosion soquickly, this study was performed.

3.2.1. Field conditions

The tanks of 3000 m3 volume were made of mild steel. Test water came from the fire water piping withoutany pre-treatment. The tanks were filled and exposed to ambient temperature ranging from �12 �C (night) to25 �C (day) for 3 weeks. The floating roofs dimensions were 16.8 m in diameter and 0.48 mm. thick. Chemicalcomposition of the roof’s alloy was: (weight%): Mg 2.2–2.8; Si + Fe < 0.45; Cu max 0.1; Mn max 0.1; Al –remainder.

3.2.2. On-site observation and sampling

The roofs were inspected following drainage of water. Visual observation revealed that remarkable amountof white corrosion products had formed on the underside of the roofs (Fig. 2a). The entire roof surface in con-tact with water suffered severe localized corrosion. Shallow and deep pits, and even holes of various shapesand dimensions, were found under the white deposits (Fig. 2b). Corrosion products were scraped from variouslocations for chemical and microbiological composition. Water samples for chemical and microbiologicalanalysis were collected from the fire water piping at the same tap that supplied the hydro-test water.

3.2.3. Results of the investigations

Table 3 shows the chemical composition of water used for hydro-test. The susceptibility of aluminum andits alloys to localized corrosion is well known [11]. Chloride ions and pH are the main factors that determinethe corrosion resistance of aluminum and its alloys in water.

In the present study the pH of the water had a slightly basic value, which is not harmful to an aluminumalloy. The high chloride content could be considered a potential cause of pitting attack initiation of the roofs.However, the water parameters likely would not have caused the fast and severe corrosion attack observedduring the hydro-test.

The EDS analysis revealed that corrosion products taken from different parts of the corroded aluminumalloy roofs primarily contained aluminum and oxygen. Chlorides were detected in only one of the testedprobes. Analytical determination of chlorides in the corrosion products showed: their chloride content: was0.17%. The concentration of sulfur and sulfates was �0.24%. It was found only minute amounts of copper,silicon (alloying elements), carbon and calcium – and no traces of magnesium (the main alloying elementof the alloy) – in the corrosion products.

Water and corrosion product deposits were also tested for the presence of corrosion inducing microorgan-isms (Table 4).

Fig. 2. (a) Corrosion products formed on the underside of the roofs. (b) Severe localized corrosion of the roof ’s inside surface.

Table 3The chemical content of the fire-extinguishing water

Parameter Unit Value

pH 7.6–8.2p-Alkalinity ppm CaCO3 2–40Total alkalinity ppm CaCO3 150–200Cl� ppm 400–630SO2�

4 ppm 60–180Total hardness ppm CaCO3 250–300Ca hardness ppm CaCO3 110–160Mg hardness ppm CaCO3 130–140Iron ppm 0.5–16Oil ppm 1NO�3 ppm 10TOC ppm 3.0Conductivity S/cm 900–1300


Results of microbiological analysis showed, that water used in hydro-test was contaminated with varioustypes of microorganisms considered highly corrosive. The fire water contained high concentrations of fungi(1.7 · 102 cells/ml) and HAB and HAnB (>103cells/ml). SRB and iron-oxidizing bacteria were also isolatedfrom water samples, but T. thiooxidans was not detected. The number of isolated microorganisms in corrosionproducts was two to four orders of magnitude higher than detected in water.

Table 4Bacterial count of microorganisms in fire-extinguishing pipelines water and corrosion products

Microbial group type Water (CFU/ml) Corrosion products (CFU/g)

HAB (TPC) 2.4 · 103 2.2 · 107

HAnBa (TPC) 5.1 · 103 4.5 · 107

SRB (total count) 2.2 6.7 · 102

IOB (total count) 3.2 1.4 · 104

T. thiooxidans Not detected Not detectedFungi (TPC) 7 · 102 8.0 · 106

a Heterotrophic anaerobic bacteria (HAnB).


Microorganisms accumulation in the corrosion products indicates that biofouling processes occurred onroof surfaces during the hydro-test. As known, biofouling may initiate and accelerate corrosion attack [1,2].

The availability of corrosion inducing microorganisms in water and even their adsorption to a surface isrequired, but it alone is not a sufficient condition for biocorrosion process development. This process heavilydepends on metal or alloy type, physico-chemical parameters of the medium and microbial composition. Avery important factor in biocorrosion initiation is bacteria metabolic activity; it depends on the suitabilityof the chemical compounds that are present in the water, which may serve as carbon and energy sourcesrequired for bacterial metabolism.

Consequently, the chemical and microbiological results did not yield a definite conclusion on the role ofmicroorganisms in the corrosion attack. At this stage, there was no way to separate the corrosion effect ofchloride concentration from that of microorganisms.

To clarify the role of microorganisms in this matter, laboratory corrosion tests were performed withuntreated and membrane-filtration sterilized waters. In these experiments coupons made of aluminum alloyAl 5052 (rings with diameter 30 mm, thickness 1.7 mm, and width 10 mm) and pure aluminum Al 1100(UNS A91100) (rectangular plates of 20 · 20 · 3 mm) were used. Chemical composition of tested aluminumwas: (weight% ) Cu 0.06; Al-remainder Table 5 shows the results.

No evidence of corrosion was detected on pure aluminum (1100) coupons exposed for 90 days in bothuntreated and sterilized by filtration waters. In addition, no signs of corrosion on Al 5052 aluminum alloy spec-imens exposed to sterilized water were observed during the entire experiment. The aluminum alloy couponstested in untreated water revealed visual corrosion on the 14th day of exposure. The corroded areas exhibiteda strongly localized character and were accompanied by the formation of flaky, white corrosion products (Fig. 3).

Corrosion tests showed conclusively that the microflora in the water used for the hydrates played a veryimportant role in producing corrosion on the roofs.

Data show that microorganisms may severely damage aluminum and its alloys in a fairly short time period– even in such relatively clean (from a microbiological point of view) but unsterile media. High susceptibilityof aluminum and its alloys to localized corrosion makes it particularly vulnerable to MIC [12]. Hence, it isreasonable to expect a higher risk of MIC initiation when untreated water is contaminated with corrosion-inducing microorganisms.

The experiments demonstrated that in bacterially polluted water the aluminum-magnesium alloy couponssustained strong corrosion attack, unlike pure aluminum. It should be emphasized that both waters had thesame chloride concentration; this comparison indicates that an alloying element such as magnesium reducesthe corrosion resistance of aluminum. Because the corrosion product probes detected no traces of magnesium,one can assume that magnesium forms soluble corrosion products. A likely scenario is that magnesium ionsemitted from the corroded surface interact with some organic metabolites excreted by biofilm bacteria, creat-ing soluble complex compounds. Pitting initiation on aluminum alloys is usually attributed to the presence ofchloride ions. The investigation revealed that the activity of microorganisms present in the surrounding

Table 5Results of corrosion tests carried out for 90 days in untreated and sterilized fire water pipelines

Metal Untreated water Sterilized water

Al 1100 No corrosion No corrosionAl 5052 Pitting corrosion on the 14th day of exposure No corrosion

Fig. 3. Picture of Al 5052 alloy coupons exposed to original (left flask) and sterilized (right flask) waters on 14th day of corrosion test.


medium may substantially accelerate the corrosion process. It was demonstrated that strong localized corro-sion of the aluminum alloy Al 5052 floating roofs that occurred during the hydrotesting was directly related tothe activity of microflora introduced with untreated water to tested tanks.

3.3. Case 3. Investigation of severe corrosion of aluminum parts from inoperative diesel engine cooling systems

Diesel engines were placed in long-term storage in an operational status in a low-humidity controlled atmo-sphere to prevent corrosion. The engines were serviced every two years, a process that included the replace-ment of engine coolant. Severe corrosion occurred on some of the aluminum components of the coolingsystems (Fig. 4). The analysis was done in order to determine the causes of the acute corrosion.

3.3.1. Field conditions and on-site observations

The coolant was a non-commercial, tailor-made formulation used for long-term storage. It was composedof 99.4% ethylene glycol (EG) (to prevent freezing) and several corrosion inhibitors that were intended to pro-

Fig. 4. Corrosion damage in cooling system components made of A03560 cast aluminum alloy. (a) Radiator elbow. (b) Thermostat cover.(c) Radiator housing.


tect the various metal components of the engine (aluminum, copper, steel, cast iron, etc.) against corrosion.The inhibitors consist of borates, nitrates, phosphates, silicates, and some supplementary organic inhibitors.The coolant is supplied by several vendors and each lot is tested according to ASTM D-1384 [13] require-ments. Because the coldest winter temperature in Israel is only near freezing, and also to cut maintenancecosts, the engine cooling systems were supplied with 20% coolant (diluted with tap water) instead of the usual50% concentration. Field experience suggested that the concentration of EG might be lowered to a range of20–30%.

During the scheduled replacement of the engine coolant, mechanics observed some leaks that were causedby corrosion attack on various aluminum A03560-T6 parts such as: elbows, pipes, covers, etc. (Fig. 4).

The causes of observed corrosion failures were investigated.

3.3.2. Supplementary experimental procedureThe coolant samples (duplicates) from ten systems were examined for presence of corrosion induced micro-

organisms. To evaluate the possible proliferation of micro-flora in the coolant, the activity and growth ofHAB, which are naturally found in tap water, were monitored in coolant of various concentrations dilutedwith tap water. Experiments were carried out in 1%, 5%, 10%, 20% and 50% (v/v) aqueous EG solutions pre-pared from a stock coolant. With the aim of accelerating the process, 500 mL experimental solutions weremaintained at 31 �C. The monitoring interval was 100 days.

The electrochemical behavior of cast aluminum alloy (A03560) specimens in coolant solutions was studiedin a standard 1 L electrochemical glass cell controlled by a M263 potentiostat/galvanostat (EG&G, USA). Thepotentials throughout the text refer to a saturated calomel electrode (SCE). A platinum wire was used as acounter electrode. These experiments were carried out on 15 · 15 · 3 mm flag-type specimens that were cutof the damaged parts. Prior to measurement, the specimens were degreased in acetone, washed in distilledwater, and air-dried. Potentiodynamic cathodic and anodic polarization curves were measured at a scan rateof 1 mV/s. The potentiodynamic measurements were initiated from an extra negative potential (�250 mV)compared to steady-state Ecorr value of a working electrode.

3.3.3. Results and discussionTo evaluate the possible microbial contribution to corrosion damage, a microbiological analysis of corro-

sion-inducing microorganisms was performed with coolant probes sampled from several cooling systems(Table 6). The coolant probes were originally contaminated with corrosive microorganisms capable of induc-ing biofouling and corrosion processes under appropriate conditions. MIC associated with contaminated die-sel engine coolants was previously reported [14].

The isolated microorganisms were presumably introduced into the system through non-sterile tap waterused to dilute the coolant. Chemical composition of the water was very close to that given in Table 1. An addi-tional aspect was revealed during bacterial counting of the experimental probes: the higher the dilution ofcoolant, the higher the number of HAB detected. This phenomenon occurs when certain dilution of the cool-ant diminishes the biocidic effect of present inhibitors. The original combination of parameters such as: (a)

Table 6Composition of corrosion-inducing microorganisms in the coolant probes (CFU/100 ml)

Number pH HAB SRB Iron Bacteria Fungi and Yeast

1 8.40 48 N.D.a N.D. 82 8.20 1.2 · 105 N.D. N.D. 1.8 · 102

3 8.90 1.1 · 107 N.D. 57 3.0 · 103

4 8.50 26 1.8 · 102 N.D. 25 8.61 22 5.5 · 102 N.D. N.D.6 8.44 2.5 · 105 N.D. 2.3 · 102 1.3 · 105

7 8.43 3.6 · 102 N.D. N.D. N.D.8 8.46 1.1 · 103 N.D. N.D. N.D.9 8.78 2.4 · 106 N.D. 9.5 · 102 1.0 · 103

10 8.46 2.0 · 102 90 N.D. N.D.

a N.D. – not detected.


basic pH (8.5) of coolant that is not optimal for microorganism’s activity, (b) the presence of borates (withbiocidic features), and (c) high concentrations of EG lead to the inhibition of bacterial proliferation. Furtherexperiments done were to assess the role of these microorganisms and whether they could proliferate at dif-ferent EG solution concentrations. The growth potential of HAB that were originally seeded into 1%, 5%,10%, 20% and 50% (v/v) EG-containing solutions through dilution with non-sterile tap water was monitoredfor a 100 days interval.

As shown in Fig. 5, during the first two weeks of exposure in EG-containing solutions (concentrations 1%,5%, and 10%) a sharp increase in HAB concentration was observed (two to four orders of magnitude) com-pared with the control (tap water) that revealed ‘‘regrowth’’ of only two orders of magnitude. Following thisinitial ‘‘regrowth’’ period, the number of HAB in control continuously decreased (due to luck of nutrients),while in EG solutions (1%, 5%, and 10%) bacterial cells continued to grow.

Around the 20th exposure day, the increase in the number of bacteria in dilutions 1% and 5% EG solutionscame to an end and persisted at the same level during the remaining exposure period. At 10% EG solution theHAB growth was slower, but it reached the same number as 1% and 5% solutions after 100 days. At EG con-centrations of 20% and 50%, a decrease of one order of magnitude during the 14 days was observed, followedby an increase in HAB numbers of 3 1/2 orders of magnitude for the remaining time interval (approx. 85days).

The initial HAB number decrease may likely be attributed to the higher concentration of biocides in thesesamples. In due course, however, bacterial adaptation to toxic media can explain the re-growth and levelsachieved. In this experiment, the kinetics of HAB growth revealed that bacteria could most probably utilizecomponents of EG solutions for their metabolism and cell production.

This conclusion is confirmed by the viable bacterial number reduction in the control sample (tap water withvery low organic carbon content and without coolant solution).Throughout the preparation process of EGsolutions with tap water, the formation of large white precipitates was observed. This result was not observedwhen experimental solutions were prepared with double distilled water.

To determine whether the sediment formation affects the coolant corrosiveness on aluminum specimens,comparative electrochemical measurements were conducted in 20% EG solutions (field concentration, as men-tioned earlier) diluted with tap and distilled water (Fig. 6). The potentiodynamic polarization curves showedthat in EG solution diluted with distilled water, the aluminum was characterized by deep passivity in a widepotential range (up to 1 V). No evidence of pitting corrosion was detected in this kind of solution. In contrast,the aluminum demonstrated high susceptibility to pitting corrosion with a breakdown potential around –0.05 V (SCE) in EG solution diluted with tap water. These results demonstrate that in tap water dilutions,the efficiency of the coolant’s inhibitors sharply decreased compared to dilutions with distilled water. Thiseffect could be attributed to chemical reaction between inhibitory compounds and bivalent ions such asCa+2, Mg2+, and some other ions present in tap water that caused insoluble salts formation. Furthermore,Cl� ions from tap water promote pitting corrosion.

0 20 40 60 80 100101

102

103

104

105

106

107

Tap water1% EG5% EG

10% EG20% EG50% EGB

acte

ria n

umbe

r (

CF

U/m

l)

Time (day)

Fig. 5. Growth kinetics of HAB in different concentrations of EG in tap water at 36 �C.

-1.0

-0.5

0.0

0.5

10-7 10-6 10-5 10-4

A- 20% EG in dist. water B- 20% EG in tap water

B

A

Current (A/cm2)

Po

tent

ial

(VS

CE

)

Fig. 6. Aluminum potentiodynamic polarization curves measured in 20 v% EG solution diluted with double distilled water (A) and tapwater (B). Scanning rate: 1 mV/s.


Thus, through simulation of different parameters that might be responsible for the strong corrosion attackon the experimental cooling systems, four major failure factors were determined:

(1) Depletion of applied inhibitor additives because of the chemical reaction between inhibitors and ionspresent in tap water (i.e. Ca+2, Mg+2), leading to formation of insoluble salts.

(2) Initiation of pitting corrosion by Cal� ions introduced into coolant through tap water as diluent.(3) MIC by the proliferation of corrosion-inducing caused by three factors: (a) the dilution of the original

coolant with tap water containing local bacteria and contamination, (b) the reduction in inhibitory com-pounds through dilution and (c) the successful utilization of EG or inhibitors (at lower concentrations)by introduced bacterial population.

(4) Higher coolant concentrations (>20%) is less favorable for the development of corrosion-inducingmicroorganisms and, consequently, to MIC development, and they can be used as a threshold to preventcorrosion damages in such systems.

4. Conclusions

The present study demonstrates that uncovering of MIC in technological equipment failures requires anindividual approach of each case. These investigations also revealed that in certain cases, assessment of micro-organisms present in surrounding medium role in corrosion destruction is possible only by simulation of thecorrosion parameters found in the field, which have a real effect on the process.

Acknowledgements

This work was accomplished with financial support and facilities of the Oil Refineries, Ltd. (Israel). J.S. andD.S were partially supported by the Ministry of Adsorption.

References

[1] Videla HA. Manual of biocorrosion. Boca Raton, New York, London, Tokio: Lewis Publishers; 1996.[2] Borenshtein SW. Microbiologically influenced corrosion handbook. Cambridge, England: Woodhead Publishing Limited; 1994.[3] Standard Methods for the Examination of Water and Wastewater, APHA, 19th ed., Washington, D.C., 1996.[4] Starosvetsky J, Armon R, Groysman A, Starosvetsky D. Mater Perform 1999;38:55.[5] Starosvetsky J, Armon R, Groysman A, Starosvetsky D. Mater Perform 2002;41:50.[6] Starosvetsky J, Armon R, Groysman A, Starosvetsky D. Mater Perform 2005;44:56.[7] Starkey RL. Science 1945;102:523.[8] Stephenson M. Bacterial metabolism. New-York: Longmann, Green & Co.; 1950.[9] Ehrlich H.L. Geomicrobiology. M. Dekker, editor. New-York-Basel-Hong-Kong, 1996.


[10] Cullimor DR, McCann AE. Aquatic microbiology. In: Skinner FA, Shewan JM, editors. London, New York, San Francisco:Academic Press; 1977.

[11] Szklarska-Smialovska Z. Pitting corrosion of metals. An official NACE Publication; 1986.[12] Schmitt CR. Corrosion-case histories in biologically influenced corrosion. In: Dexter SC, editor. NACE, Houston, TX. 1986.[13] ASTM D1384-04. Standard test method for corrosion test for engine coolants in Glasswarem. Annual book of ASTM Standards,

West Conshohocken, PA: ASTM International, 2004.[14] Genner C, Hill EC. Br Corros J 1980;15:95.

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Identification of microbiologically influenced corrosion (MIC) in