Microbial-Influenced Corrosion (MIC) on an 18 in. API 5L X52 Trunkline

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C A S E H I S T O R Y — P E E R - R E V I E W E D

Microbial-Influenced Corrosion (MIC) on an 18 in. API 5L X52Trunkline

F. Elshawesh K. Abusowa H. Mahfud A. Abderraheem F. Eljweli K. Zyada

Submitted: 17 September 2007 / in revised form: 11 December 2007 / Published online: 29 January 2008

ASM International 2008

Abstract Analysis of failed sections from a 31 km long

pipeline show that premature failure was caused bymicrobial-influenced corrosion. This case history summa-

rizes the failure analysis and demonstrates the need for

extreme care when using untreated water to hydrotest a

pipeline.

Keywords Hydrotest Microbial influenced corrosion Trunkline

Background

The petroleum production environment is particularly

suitable for sulfate-reducing bacteria (SRB), partially

because it handles large volumes of oxygen free water,

which usually contains all the required nutrients to make

the microorganism thrive [1]. Pipelines are one of the most

vulnerable facilities in the oil industry to microbial corro-

sion because of the produced water, hydrostatic testing, and

shutdowns [1, 2].

The presence of microorganisms such as planktonic

SRB is not the real indication of microbial activity because

sessile SRB adhere to the metal surface, forming a biofilm.

These films modify the chemistry of metal solution inter-

face and can result in severe microbial corrosion [1].

Pitting caused by bacterial activity is a manifestation of

localized microbial-influenced corrosion (MIC) in iron

alloys. SRB are relevant anaerobic microorganisms related

to localized corrosion, because they are able to transform

the sulfate to hydrogen sulfide, which is a strong pitting

agent. The localized pitting attack is usually manifested bythe presence of small deep corrosion pits that are covered

with corrosion product [1–3].

The improper handling and management of hydrostatic

water used for pipeline hydrostatic testing can result in

MIC and detrimental effects on the pipeline integrity and

result in premature failure. Therefore, the sources of water,

water chemistry, biocide deployment, and oxygen scaven-

ger need to be considered before and after the hydrostatic

testing. Any water remaining within the pipeline needs to

be handled carefully, and complete removal is necessary

after test termination. There is an old adage that says

‘‘pump it up or drain it,’’ and this statement is very

applicable to any pipeline.

Case History

Two pipe joints in long trunkline (31 km) showed severe

corrosion and leakage after less than 2 years from com-

missioning. The leaking area around the two pipe joints

was removed and replaced with new pipe joints of the same

material.

Several samples with severe localized corrosion at both

the circumferential weld and the parent metal were sub-

jected to detailed failure analysis. The investigation goal

was to establish the main cause(s) of premature failure and

provide recommendations to prevent a recurrence of sim-

ilar failures.

The failed trunkline (Fig. 1), made of carbon steel grade

API 5L X52, was used to carry untreated crude oil (approx.

28,000 bbl/day) to the separation station before being

pumped through the main pipeline. The water cut was

reported to range between 6 and 7%. Both CO2 and H2S

F. Elshawesh (&) K. Abusowa H. Mahfud A. Abderraheem F. Eljweli K. Zyada

Libyan Petroleum Institute (LPI), P.O. Box 6431, Tripoli, Libya

e-mail: [email protected]

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J Fail. Anal. and Preven. (2008) 8:60–68

DOI 10.1007/s11668-007-9108-3



were present at concentrations of 350 and 1 to 4 ppm,

respectively. The corrosion inhibitor (oil soluble and water

dispersible) was injected to combat CO2 corrosion. Chlo-

ride ions levels were moderate, reaching 4200 ppm (mg/L).

No biocide was injected with the inhibitor. This was

because no bacterial growth had been detected during the

microbial analysis of water samples frequently collected

from the separators.

Frequent pipeline pigging (every month) was used to

reduce/remove the water and sand gathered at 6 o’clock

position and to make sure that the corrosion inhibitor is

spread over the entire internal surface of the trunkline.

Operating Condition and Water Chemistry

The operating conditions for the failed 18 in trunklinewere:

The pipe specification and operating parameters can be

given as:

• Trunkline (pipe) made of carbon steel as per API 5L

grade X-52

• Pipe nominal wall thickness: 0.375 in.

• Seam welded

• Total trunkline length: 31 km

• Installation and commissioning date: 2004

• Water used for hydrostatic test: sweet water treated

with oxygen scavenger

• Production rate: 28,000 bbl/day• Average pressure: 250 psi

• Temperature (min/max): (30/61 C)

• Pipeline pigging: once a month

The water chemistry was:

• Chloride ions: 4200 mg/L

• Carbon dioxide: 350 ppm

• Hydrogen sulfide: 1 to 4 ppm

• Water cut: 6 to 7%

• Crude type: very light with high API grade

• Corrosion inhibitor injected at rate of 15 L/day at

manifold upstream of the trunkline

• Biocides or chemical: none

• SRB not detected within water samples taken from pipe

end; however, was detected at the separators down

stream of the trunkline

• Flow regime: heterogeneous two-phase, turbulent flow

• Flow velocity: 1.15 ft/s

The first and second leaks were detected on 15th and

18th of November 2006. Two leaks occurred at 6 o’clock

position (5.0–7.0 o’clock positions). The pipe failure took

place approximately 1100 m downstream the pig launchers

support (PLS). The pipe failures were located at a low

elevation with respect to the land topography.

Flow regime was calculated using Petrochem. Software:

• Nominal pipe size: 18 in.

• Pipe schedule: standard

• Outside diameter: 18 in.

• Wall thickness: 0.375 in.

• Inside diameter: 17.25 in.

• Area of metal: 20.76 in.2

• Transverse internal area: 233.71 in.2

• Moment of inertia: 806.7 in.4

• Weight of pipe: 70.59 lb/ft

• Weight of water: 101.18 lb/ft of pipe

• External surface: 4.712 ft2 /foot of pipe

• Section modulus (2 9 I/OD): 89.6

Pressure drop calculations were:

Flow rate: 28,000 bbl/day

Density: 0.8151 gm/cm3

Viscosity, centipoises: 2.0

Pipe roughness: 0.00015

Velocity: 1.121 ft/s

Pressure drop: 0.009791 psi/100 ft

Reynolds number: 61,020

Friction factor: 0.0204

Fig. 1 (a) and (b) General view

for the failed 18 API 5L

trunkline. Magnification: 29

J Fail. Anal. and Preven. (2008) 8:60–68 61

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The results of calculations confirm that the flow within

the pipe is turbulent: the Reynolds number was calculated

to be 61,020. However, the flow of water at the 6 o’clock

position may be laminar and not turbulent particularly at

the lowest elevation of topography.

Visual Observation

Two samples of pipe sections, one with the circumferential

weld seam and one remote from the weld, were subjected

to metallurgical investigation. The sample remote from

weld seam showed severe localized corrosion attack, as

shown in Fig. 2(a) to (c). Two corrosion morphologies

produced by two different mechanisms seem to have

occurred. One form of attack was pitting that perforated the

pipe wall thickness as shown in Fig. 2(a). No sign of any

role of flow regime (erosion) was apparent.

The other form of attack was typical of flow-induced

localized erosion/corrosion (FILC), as shown in Fig. 2(b).

Both turbulent flow and high flow rate assisted by CO2 cor-

rosion have resulted in attack, as shown in Fig. 2(b) and (c).

The as-received samples were coated with a fragile/

brittle scale of iron carbonate. The scale was seen between

the 5 and 7 o’clock positions, as shown in Fig. 2(c). No

sign of a clear black scale of iron sulfide was noted. This

lack of iron sulfide scales can be attributed to the low level

of hydrogen sulfide.

The sample with circumferential weld showed severe

preferential corrosion of the weld seam, as shown in

Fig. 2(d). The localized corrosion attack was found to be

round and not elongated, as is usually seen in flow-induced

corrosion in a CO2-containing environment. Small deep

holes or pitting corrosion attack were visible within the

circumferential weld seam.

Macroscopic Examination

Sample without Circumferential Weld Seam

A detailed macroscopic examination was conducted on the

as-received pipe samples to assess the extent and mor-

phology of the corrosion attack. Fragile/brittle scale was

predominant on pipe samples, as shown in Fig. 3(a) and

(b). As mentioned previously, the pipe sample without

weld seam showed two different corrosion morphologies.

The deep corrosion pit that perforated the pipe wall

thickness showed a sloped wall typical of microbial cor-

rosion attack. Small tiny holes around this large pit were

seen, as shown in Fig. 4(a) to (f).

There were some signs of a black, brown deposit/scale

on the examined samples, as shown in Fig. 3 and 4.

Localized corrosion attack underneath the deposit was also

visible. The corrosion was in form of deep corrosion pits

with a small mouth typical of microbial corrosion attack, as

shown in Fig. 4(a) to (f). Some of the large pits seem to be

formed through the coalescence of several small pits as

shown in Fig. 4(a).

Fig. 2 (a) to (d) General viewfor the severely corroded

pipeline samples. It can be

observed that the corrosion was

over and remote from the

pipeline circumferential weld

seam. Magnification: 39

62 J Fail. Anal. and Preven. (2008) 8:60–68

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Sample with Circumferential Weld Seam

Preferential corrosion attack of the circumferential weld

seam was seen on the as-received sample. The corrosion

attack initiated at the weld seam and spread over the heat-

affected zone (HAZ) and parent metal (PM). Small but

deep corrosion pits were seen over the weld seam, as

shown in Fig. 5(a) to (f). Some of these pits were covered

with the fragile scale and/or corrosion product, as shown in

Fig. 5(a) and (b).

The morphology of the corrosion attack is typical of

microbial corrosion. It is well established that the bacteria

Fig. 3 (a) and (b) Microscopic

view for the under-deposit

localized corrosion.

Magnification: 169

Fig. 4 (a) to (f ) Microscopic

view for the localized microbial

corrosion. Deep pit covered

with corrosion deposit is visible.

Magnification: 169


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prefer the weld seam because of factors such as residual

stresses, rough surfaces, and defects within the weld seam

(i.e., crevices, porosity).

Hardness Measurements

The hardness was measured over the parent and weld metalof the pipe samples using Vickers hardness testing Machine

at load of 10 kg to confirm that the carbon steel pipe had

the appropriate hardness. The results of average hardness

readings are shown in Table 1.

The weld hardness (cap filler) is higher than that of the

parent metal, and the hardness of the parent metal was

found to be typical of API 5L grade X-52.

Chemical Analysis

Samples from the received pipe sections were subjected to

chemical analysis using spark emission spectroscopy. The

results of chemical analysis are shown in Table 2. The

chemical analysis confirms that the pipe samples were

made of carbon steel as per API 5L standard specification

grade X-52.

Fig. 5 (a) to (f ) Microscopic

view for the localized microbial

corrosion encountered at the

circumferential weld seam.

Magnification: 209

Table 1 Average hardness readings (HV10) for parent and weld

metal samples taken from the failed 18 in. trunkline

Locations Weld Metal, HV10 Parent Metal, HV10

Average hardness readings *175 (cap) *155

*145 (root)

Table 2 Chemical analysis of API 5L X-52 carbon steel pipe using

spark emission

Composition, wt.%

C Mn P S Co V Ti

0.293 1.293 0.021 0.018 0.009 0.01 0.00


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Metallography

Several specimens from the received pipe section were cut

and prepared for metallographic examination. Samples

from the parent weld were examined, and the results are

shown in Fig. 6. The pipe microstructure was typical of the

ferrite and pearlite microstructure anticipated for the X-52

pipe, and the volume fraction of ferrite/pearlite is consis-

tent with carbon content.

X-ray Diffraction

Samples of corrosion product and scale were collectedfrom the failed pipe section and subjected to x-ray dif-

fraction. The results of analysis are shown in Table 3. The

collected samples were primarily iron carbonate. Other

elements such as Mg and Ca were also reported (MgCaF-

eCO3). This is expected in presence of CO2, which forms

carbonic acid when dissolved in water. The weak acid

reacts with steel surface, and the result is a FeCO3 scale.

The cohesiveness and coherence of the scale to the pipe

surface depends on the operating condition. The brittleness

of the scale is expected to accelerate the corrosion attack

where the broken areas of scale provide localized anodic

sites for corrosion process by CO2 and also trap bacteria. Inaddition, the flow regime that the pipe saw (high flow rate)

is expected to erode the metal surface and result in erosion

corrosion.

Analytical Chemistry

Samples of corrosion product and scale collected from the

failed pipe were subjected to detailed analytical chemistry

as per ASTM standard D 800-91. The analysis was to

detect the presence of chloride ions, carbonates, and sul-

fates. The results of analysis are shown in Table 4. These

results are consistent with the x-ray diffraction showingthat the main compound in the analyzed scale/corrosion

products was iron carbonate.

Microbiological Analysis

Samples from the dry corrosion deposits and scale were

collected from the pipe section and subjected to microbi-

ological analysis to detect any presence of microorganism

(bacteria) that might induce/influence the corrosion pro-

cess. The test included evaluations for sulfate-reducing

bacteria (SRB), iron-related bacteria (IRB), and acid-pro-ducing bacteria (APB). The analysis was made using

BART medium as selective media and conducted as per

test method SM 9215 B/IRB-SRB-BART TM.

The results of analysis are shown in Table 5. Both the

iron and sulfate-reducing bacteria were detected within the

collected corrosion product, as shown in Fig. 7(a) and (b).

The dangerous SRB was detected when 1 mL from grown

subculture in BART (iron-related bacteria) was centrifuged

at 20,000 rpm for 30 min. After that, 0.5 mL of superna-

tant was transferred into API sulfate agar. After 24 h, SRB

were found, as shown in Fig. 7(b). The presence of SRB

was also confirmed microscopically, as shown in Fig. 8(a)

and (b). The detection of SRB is consistent with the results

of visual and macroscopic examination that indicated that

Fig. 6 Typical microstructure of the received pipeline samples.

Magnification: 1509

Table 3 XRD results for the corrosion products/scale collected from

the 6 o’clock position of 18 in. trunkline

Compounds

Mg, Ca, FeCO3 (calcium magnesium iron carbonate)

Table 4 Results of the chemical analysis for the collected corrosion/

scale product

Concentration, wt.%

Chloride ions (Cl-) Carbonate (FeCO3) Sulfate ðSO24 Þ

0.28 52.2 0.15

Table 5 Results of microbiological analysis of corrosion products/

scale collected from trunkline bottom

Bacteria Detected Results and Remarks

Iron-related bacteria

(IRB)

Present with high count 540,000 cfu/mL

was detected after 24 h from inoculation

Acid-producing

bacteria (APB)

Not detected

Sulfate-reducing

bacteria (SRB)

Detected


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the corrosion attack was typical of microbial-influenced

corrosion caused by the presence of SRB bacteria.

To confirm these results, two water samples from the

separators were also subjected to microbiological analysis.

The results confirm the absence of SRB and acid-producing

bacteria (APB). The iron bacteria (IRB) were detected in

pronounced amount. This test usually assesses the presenceof planktonic bacteria and not the sessile bacteria that may

have been attached to the metal surface and well protected

by biofilms.

The technical data along with the evaluations of the

pipeline samples indicate the presence of SRB in the sep-

arators downstream in the trunkline. This means SRB are

present within the trunkline, and their presence is consis-

tent with the results from the corrosion deposits. Generally,

the iron bacteria first attach to the pipe surface and sub-

sequently oxidize the steel. This results in consumption of

oxygen underneath the biofilm and creates a safe haven for

SRB to grow and produce its by-products (hydrogen sul-

fide), which will stimulate the corrosion process.

On-Site Microbial Activity

Based on the investigation, the role of microbial activity

was expected to be vital to the corrosion processes;

therefore, a field trip was arranged to assess the microbial

growth/activity at different locations within the failed

trunkline (Fig. 9), separator, water wells, and manifold.The microbial analysis was conducted on the water sam-

ples collected from various sites as well as on the corrosion

product and water after the pigging operation. The sample

locations and results of analysis were:

Sampling locations:

• Pig biomass (water and sludge)

• Water source well

• Separator and dehydrator

• Manifold

Fig. 7 General view for the

bacteria kit test results where

both sulphate reducing bacteria

(SRB) and iron bacteria (IB)

were detected

Fig. 8 Microscopic view for

the detected SRB within the

collected corrosion deposit as a

function of time. Magnification:

1509

Fig. 9 Biomass sample and water samples collected from trunkline

receiver end


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The following test method was applied in the microbi-

ological biomass and water analysis:

• BART medium

• Commercial medium

• B 326 sulfate API medium

Results on the water and pig biomass samples (Table 6)

confirm that a high count of SRB and IB are present. Thisis in complete agreement with the results obtained from the

corrosion products collected from the failed pipe section.

The microbiological test results showed no bacterial

activity in the oil wells with highest water cut, the water

source well, and in the water well used in the hydrostatic

testing. It was reported during the site visit that the hy-

drotest water was transported by trucks before being

injected into the trunkline. This result suggests that the

source of bacteria that led to the corrosion damage may be

the water transported by the trucks.

In addition, the transported hydrotest water was not

treated with the biocide before being injected into the

trunkline. The only treatment was the injection of an

oxygen scavenger to reduce the overall corrosion activity.

However, the oxygen scavenger should create a favorable

condition for microorganisms such as SRB to grow and

establish colonies on the pipe surface.

Discussion

The results of the investigation confirm the fact that the

corrosion and premature failure of pipeline samples was

caused by localized corrosion attack. Two different cor-

rosion mechanisms namely microbial corrosion and CO2

corrosion have acted together and resulted in severe attack

and failure of trunkline. However, the role of bacteria is

expected to be the most pronounced.

The calculation of flow regime confirms that the flow

within the trunkline was generally turbulent with a calcu-

lated Reynolds number around 61,000. However, the water

flow at the surface of the pipe in the trunkline was laminar

and not turbulent. The two-phase flow is expected to be

pronounced over places with low trunkline elevation where

the corrosion rate is maximum.

The presence of microorganisms, biofilms, and CO2 in

the water at the 6 o’clock position accelerated the corrosion

process. The CO2 in water results in formation of weak

carbonic acid. This acid reacts with the metal surface,

producing iron carbonate scale (FeCO3). The scale is

expected to be noncoherent and porous due to low workingtemperature. This results in easy breakdown and removal

of the scale. The locations where the scale has failed will

act as a potential anodic site inducing corrosion (dissolu-

tion) by the weak carbonic acid. However, the rate of the

localized corrosion is expected to be slow due to low CO2

concentration. Nevertheless, the role of flow was seen on

the examined samples. Clear grooves, an erosion-like

corrosion attack, as a result of turbulent flow were visible.

This is not always the case since the flow most likely is

laminar over the area near the pipeline surface. In all cases

the extent of the attack was influenced by the presence of

bacteria.Pipeline samples with a circumferential weld showed

severe corrosion attack over the weld seam between the 5

and 7 o’clock positions. The corrosion-induced pit was

round and not elongated as usually seen when the flow

velocity is high.

Based on these facts, microbial-influenced corrosion in

addition to CO2 and water chemistry (i.e., high chloride

ions) was considered.

The microbiological analysis of the corrosion products

confirm the presence of iron-related bacteria (IRB) and the

sulfate-reducing bacteria (SRB). It is worth mentioning that

there was some difficulty in detection of SRB using normal

test procedure; however, the bacterial activity was con-

firmed by the microbiological analysis of pig biomass

(removed water and sludge). The presence of SRB was also

confirmed using special optical microscope.

The microscopic examination of the pipeline samples

confirm the presence of deep tunneled and bottleneck type

of microbial corrosion over the welded and nonwelded

areas. Most of the observed corrosion pits were deep with

small mouth covered with thick reddish brown corrosion

products. The low flow velocity at the pipeline surface

assisted in establishment of the biofilm, which in turn

produced an environment that acted as a safe haven for the

sulfate-reducing bacteria to grow (sessile bacteria). The

hydrogen sulfide produced by the SRB produced an envi-

ronment that caused severe localized corrosion attack. The

method used to monitor the biological activity within the

pipeline and around facilities has aggravated the problem

by failing to detect the bacteria. Frequency of pigging

(once a month) was not enough to clean the pipeline sur-

face from bacteria, and the lack of biocides provided a safe

haven for bacterial growth.

Table 6 Results of microbial analysis collected from different

locations

Sample Description SRB Bacteria Iron-Related Bacteria (IRB)

Pig biomass, sludge

and water

104 cfu/mL 540,000 cfu/mL, water

and sludge samples

Separators 103 cfu/mL 14,000 cfu/mL

Dehydrator 10

3

cfu/mL 14,000 cfu/mLManifold Nil Nil

Water well source Nil Nil


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In conclusion, the source of the bacterial activity in the

current case was not the produced water but the contami-

nated water used during the hydrostatic testing. On-site

visits and microbiological analysis suggest that the

untreated water (no added biocides) remaining inside the

pipe during the hydrostatic testing and before commis-

sioning was the main source of microorganisms (i.e., SRB

and IB).

Conclusions

• Premature failure of 18 inch trunkline can be attributed

to the presence of micro-organisms such SRB and IRB.

The presence of low partial pressure of CO2 may assist

the corrosion process; however, its contribution is not

considered essential.

• Although the flow regime is turbulent (Re & 61,000)

inside the trunkline, the water flow at the 6 o’clock

position was laminar. This has created a favorablecondition for microorganisms to grow, attach to the

metal surface, form biofilms, and build safe places for

SRBs to grow on the pipeline surface.

• Maximum corrosion attack is expected to be over

locations of lowest pipe elevation because the location

maximizes the collection of water.

• Weak, porous, and noncoherent iron carbonate scale on

the trunkline has created proper anodic sites for

corrosion attack by CO2 and helps bacteria

colonization.

• Lack of proper microbial tests, the lack of biocides, and

failure to detect microorganisms in early stages haveaggravated the problems.

• Lack of techniques to properly clean the water and any

other material attached to the pipeline surface after

termination of the hydrostatic testing resulted in

conditions detrimental to the pipeline integrity.

• Proper chemical and microbial analysis should be made

on the collected material from the pipe pigging. This

will help detect the microbial activity. The analysis of

water samples may not be sufficient to obtain full

information on the bacterial activity.

• Lack of proper treatment of the water used to hydrotest

the pipe may have created favorable conditions formicroorganisms such as SRB and IB to grow and

establish inside the trunkline.

Recommendations

• Immediate action needs to be taken to assess the extent

of the corrosion attack within the trunkline (31 km),

particularly over areas conducive to water collection.

However, caution needs to be taken when dealing with

the microbial corrosion attack where the corrosion pits

are not open. In fact, the corrosion attack is in form of

small tunneled pits with small open mouth (bottleneck

corrosion attack), thus making it difficult to treat the

existing pipeline.

• The trunkline needs to be properly cleaned using a

special type of pig. The cleaning shall ensure propercleaning or removal of corrosion deposits and disrupt or

destroy the biofilm on the pipe surface (5 to 7 o’clock

positions). An extensive pigging operation using brush

pigs followed by preferably a slug of biocide between

two isolating pigs needs to be done two to three times.

If the slug of biocide is not possible, dose the pipeline

after each pig run and repeat for three to five times

(every week) for 3 to 5 weeks. Analysis of the fluids

during the pigging operation near the time the pig

arrives should provide good indication of the level of

bacteria.

• Pigging operation should be conducted using brush pigsand not sphere or disk pigs. Alternatively, use a brush

pig in addition to the sphere or disk pigs.

• Trial tests should be conducted for several types of

biocides and select the best performers to reduce the

microorganism activity to a minimum. Chock (at the

beginning) and batch treatment need to be considered.

For cleaning purposes, a high dose of biocide (depend-

ing on type of biocide) needs to be injected for 2 h and

alternately between the two cleaning pigs.

• Biocide treatment should consider the biofilm disrup-

tion and kill as much as possible from the sessile

bacteria colonies. Slug of biocide mixture between two

pigs needs to be considered. The brush pigs will disrupt

the biofilm, the biocide will kill the bacteria, and the

pigs will remove the biomass.

• A corrosion-management program should be imple-

mented that includes corrosion monitoring (i.e., probes,

coupons etc.).

Acknowledgments The authors would like to thank Dr. Khalifa

Esaklul for the fruitful technical discussion and extend their thanks to

the microbiology lab staff, particularly Mrs A. ElQadawy and A.

Benhaliem for their patient and endless help and assistance in order to

complete part of microbiological investigation successfully.

References

1. Kobrin, G.: In: Dexter S.C. (ed.) Proc. International Conference on

Biological Induced Corrosion, p. 32. National Association of

Corrosion Engineers, Houston, TX (1986).

2. Failure Analysis and Prevention, vol. 11, 9th edn. Metals

Handbook. ASM International, Materials Park, OH (2002).

3. During, E.D.D.: Corrosion Atlas. Elsevier Science, Amsterdam,

The Netherlands (1997).


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Documents

Microbial-Influenced Corrosion (MIC) on an 18 in. API 5L X52 Trunkline