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CORROSION BEHAVIOR OF CARBON STEEL INFLUENCED BY SULFATE-REDUCING BACTERIA IN SOIL ENVIRONMENTS

SeonYeob Li*, YoungGeun Kim, YoungTai Kho

Korea Gas Corporation (KOGAS)

638-1, Il-Dong

Ansan, Kyunggi-Do, Korea

Tak Kang

Seoul National University (SNU)

Seoul, Korea

ABSTRACT

This work is a comprehensive study of microbiologically influenced corrosion (MIC) of carbon steel in

soil environments, induced by sulfate-reducing bacteria (SRB). Four experimental phases were

involved in this research: (1) field study, (2) anaerobic corrosion study, (3) localized corrosion study, (4)

mathematical modeling of cathodic protection (CP) current/potential distribution. Based on the field

survey and statistical approach, the predicting equation for the maximum corrosion depth of steel in soil

was presented and it was proved that the predicted values were well matched with the field data. The

contribution of microbial factors on soil corrosion was also discussed. It was also concluded that the

presence of SRB and the resultant biogenic iron sulfide film on steel surface greatly changed the

metal/electrolyte interface properties, and therefore the corrosion behavior of steel from anaerobic

study. Using the thin-film electrical resistance (TFER) sensors, it was possible to detect and monitor the

localized corrosion behavior induced by SRB. The mathematical model using the boundary element

method (BEM) is capable of predicting (1) local potential; and (2) local cathodic current density inside

crevice for a given applied holiday potential and soil resistivity. The model predicted a very short depth

of current penetration depending on the crevice geometry. From the results of modeling and the

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following regression analysis, the optimization plots for cathodic protection inside crevice under the

disbonded coating was proposed, which makes it possible to predict the current penetration depth.

Keywords: microbiologically influenced corrosion (MIC), sulfate-reducing bacteria (SRB), anaerobic

soil, thin-film electrical resistance (TFER) sensor, cathodic protection (CP), disbonded coating, current

distribution

INTRODUCTION

It is widely recognized that microorganisms attach to, and influence the corrosion of metals and alloys

exposed to the environments. Microorganisms are omnipresent in nature and their ability to grow and

reproduce at rapid rates accounts for their presence in soil environments. Buried steel pipeline is

designed to have a lifetime of about 30 to 50 years. In general, the protective organic coating and

cathodic protection (CP) are applied together. Despite these protection measures, however, failure

cases of underground pipelines due to corrosion have been continuously reported1-3. Microbiologically

influenced corrosion (MIC) has been identified as one of the major causes of corrosion failures of

underground pipelines4.

The first MIC case was identified in 19345, where sulfate-reducing bacteria (SRB) resulted in failure of

underground cast iron pipes. Since this observation, numerous works have been made concerning the

effect of bacteria, especially SRB, on MIC in soil environments. According to recent survey program

performed by authors, almost all corrosion of underground gas pipeline has occurred under the

disbonded coating6,7. The corrosion mechanism has been revealed mainly as the action of SRB.

SRB are known as one of the key microbes in the MIC process. SRB are obligately anaerobic bacteria

utilizing sulfate as a terminal electron acceptor and organic substances as carbon sources. During the

metabolic process, sulfate is reduced to sulfide. These biogenic sulfides react with hydrogen ions

produced by metabolic activities or by cathodic reaction of corrosion process to form corrosive

hydrogen sulfide and iron sulfide precipitate on metal surface. The detailed mechanism of corrosion by

SRB is reported elsewhere8,9.

The risk of MIC must be exactly evaluated and the proper control methods prepared to mitigate MIC

problems. The relationship between the activities of MIC-causing microbes, e.g., SRB, and various soil

environmental factors should be investigated and assessed. In view of economic and engineering

points, quantitative assessment of MIC is also important. The key point is, therefore, how much the

activity of SRB does contribute the entire amount of corrosion phenomena in soil environments, and

how the proper assessment techniques are set up.

Moreover, MIC is a localized, pitting-type corrosion in most cases. Therefore, proper test methods for

localized corrosion are needed. However, the understanding the risk of MIC is only half the solution.

The other is to provide suitable protection of control method. Generally, underground pipelines are

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protected from corrosion by CP as mentioned above. It is well known that CP could prohibit the activity

of SRB and therefore SRB-induced corrosion effectively if the protection potential is maintained under -

0.95V/CSE10-12. However, for coated steel pipeline, all SRB-corrosion occurs under the disbonded

coating where CP current cannot penetrate and remains as free-corroding conditions6. Therefore, it is

important to study the effect of CP under various conditions, such as disbonded area, resistivity and the

degree of applied CP current, etc.

In this study, various field survey, laboratory techniques and mathematical tools for the monitoring,

detection, assessment and the protection of SRB-induced corrosion are briefly summarized.

FIELD STUDY

An attempt was made to investigate the effect of environmental factors on corrosion of underground

steel pipeline. Field survey was carried out, and the relationship between maximum pit depth (Pmax) and

the measured environmental factors were analyzed statistically. The field survey of soil parameters and

the analysis of sampled soil around the cathodically protected, polyethylene (PE) coated steel pipes

(API X65 Gr.E) was performed. Detailed procedures are found elsewhere13. These survey results

provide the useful information on the risk of MIC. Chemical and microbiological analysis of soil samples

and the combination of the statistical methods such as multiple regression analysis are useful tools for

this purpose.

It is well known that the rate at which corrosion pits grow in the soil under a given set of conditions

tends to decrease with time and follows a power-law equation (1)14.

nktP = (1)

Where P is the maximum pit depth in time t, and k and n are constants.

If k and n can be determined using proper statistical methods, it becomes possible to predict the

progress of corrosion in depth. Therefore, this equation (1) was adopted as the basic model. The

predicting equation was obtained by multiple regression analysis. Figure 1 shows the dependence of

the maximum corrosion depth on the key environmental factors.

Considering the effect of environmental factors and the burial time, the final predicting equation for the

maximum corrosion depth of steel pipeline was obtained as follows15;

372.0

ccalmax, tP573.1P = (2)

wherein Pmax,cal: the predicted value of Pmax )(LogpH014.0ClayE050.0)Cl(Log203.0S/P749.0)SRB(Log069.0700.0LogP hc r--+++= - (3)

SRB: the population of SRB (cells/g of soil)

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P/S: the pipe-to-soil potential (V/CSE)

Cl-: the level of chloride (ppm)

Eh: the reduction-oxidation potential (V/NHE)

Clay: the clay content (%)

?: the soil resistivity (O?cm)

Figure 2 shows the scatter diagram of Pmax and Pmax,cal. The correlation coefficients between P0 and

P0,cal was 0.947. Data fell closely onto the straight line with the slope of 0.860.

It is also found in this study that the underground corrosion of steel was affected mainly by three factors

as shown in equation (3): (1) chemical factors such as Cl-, pH Log (?), (2) biochemical (microbial)

factors such as Log (SRB) and Eh Clay and (3) CP factors such as P/S. This means that the

corrosivity of soil can be evaluated quantitatively by analyzing the chemical and biochemical properties

separately if ignoring the interaction effect of each factor. Figure 3 shows the relative contributions of

each factor. The contribution of microbial, chemical, CP factors are in a range of 45 to 75%, 20 to 45%

and 2 to 7%, respectively. It is evident that the contribution of microbial factor (about 45 to 75%) is most

important in anaerobic environment as shown in Figure 3.

Therefore, it can be concluded that the risk of underground corrosion can be successfully predicted

quantitatively and the main cause for the corrosion is the activity of anaerobic microorganism, i.e., SRB.

LABORATORY STUDY

Corrosion potential and DC polarization techniques

Because of its simplicity, the measurement of the corrosion potential (Ecorr) and linear polarization

resistance (LPR) method has been used for MIC studies for many years16. A rapid and easy

interpretation of the results is achievable using these methods together. Figure 4 shows the trends for

Ecorr and uniform corrosion rate of carbon steel obtained by LPR method exposed to anaerobic soil for

148 days in laboratory.

It is apparent that the existence of SRB greatly influences the corrosion behavior of carbon steel. In

case of active growth of SRB, the potential increased slightly for the first 6 days and then maintained

around -740 mV/SCE, but the potential fluctuated -600 mV to -800 mV/SCE after 50 days until the

experiment ended. The potential in control (biocide-added) case was around -600mV and always more

positive than that in SRB-active cases.

One of the mechanisms for SRB-induced MIC of carbon steel has been thought to be the galvanic

action between precipitated iron sulfides and metal2. After 50 days exposure, Ecorr of carbon steel in

SRB-active soil fluctuated until the experiment ended with the continuous increase of uniform corrosion

rate in Figure 4. It seems that this potential fluctuation was attributed to the repetitious production and

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breakdown of iron sulfide film and is indicative of the onset of localized corrosion.

Figure 5 is the Tafel polarization curves of carbon steel exposed to anaerobic soil. The growth of SRB

had reduced continuously the polarization resistance of carbon steel, i.e., the increase of corrosion rate

as shown in Figure 4.

Electrochemical impedance spectroscopy (EIS)

The electrochemical impedance spectroscopy (EIS) is particularly useful in the presence of non-

conducting and semi-conductive films such as organic coating and inorganic precipitates. Many of

biofilms adsorbed on metal surface are non-conductive and EIS techniques are potentially useful in

their presence16. EIS plot in Figure 6 also shows the decrease of polarization resistance in the

presence of SRB in anaerobic soil.

In the first 7 days, it is thought that only one mechanism dominated the reactions in both cases from the

phase angle-vs-frequency curves. After 148 days, however, there were significant changes in SRB-

active case. It was observed that an increase in phase angle to a positive value, the appearance of a

new maximum value of phase angle at a lower frequency (

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SEM/EDXA analysis.

LOCALIZED CORROSION STUDY

As mentioned above, MIC is in general localized, pitting-type corrosion. The authors have applied multi-

lined thin-film electrical resistance (TFER) probes for the study of this localized corrosion induced by

SRB.

The ER method is widely used as corrosion monitoring techniques especially in industrial field

environments. At constant temperature, corrosion of the probe, which causes a decrease in cross-

sectional area of probe, can be monitored by periodically measuring the electrical resistance. The

corrosion rate can be determined over very short time16.

In general, this technique is used for the measurement of uniform corrosion rate. However, TFER probe

shown in Figure 8 can be used to assess the localized corrosion due to its multi-sensing line. The

details of fabrication, applications are previously described 20. Figure 9 shows the response of thin-film

ER probes immersed in various electrolytes. Nitrogen was purged in all cases to maintain anaerobic

environments.

It is apparent that the responses of each probe were quite different. The corrosion behavior of carbon

steel can be divided as three categories in anaerobic condition, i.e., (1) anaerobic inorganic corrosion

which depends on the ability to utilize the cathodic reactants, which may be water or hydrogen ion. As

shown in Figure 9, the anaerobic, inorganic corrosion rates are in a range of 0.016 mm/y to 0.026

mm/y, and showed the uniform corrosion characteristics regardless of the types of electrolytes. (2) the

precipitation of protective films (carbonate/bicarbonate); in this case, hard and protective film

precipitated on the metal surface and remained for a long time, which resulted in no decrease of

electrical resistance, i.e., no start of corrosion, (3) MIC induced by SRB; initially transient protective iron

sulfide film prohibited the progress of corrosion for about 4 days, however, the film became ruptured

and the localized corrosion developed (stepwise increase of resistance).

The responses of TFER sensor buried in real soil environments is presented in Figure 10, which shows

the relationship between the time to the onset of localized corrosion, that is, the time to the first

resistance-decrease step detected, and the population of SRB; the larger the population of SRB, the

faster the initiation of localized corrosion. Some scattering of data might be attributed to the incubation

time for the activation of MIC or the influence of other factors such as oxygen or stray current detected

at some sites. This result also confirms the applicability of TFER sensor in real situations. Though

quantitative information on the corrosion is insufficient, TFER sensor can be used for the assessment of

MIC risk, especially in terms of localized corrosion behavior within a relatively short period.

EFFICIENCY OF CP

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CP effectively inhibits the progress of MIC in soil environments. This may be due to the increase of pH

induced by hydroxyl ion produced by cathodic reaction suppresses the growth of microbes. It is more

important, however, that the efficiency of CP should be considered. Figure 11 is the typical example of

the disbondment of heat-shrink sleeve applied at the girth weld joint of carbon steel pipeline and the

resultant corrosion morphology under the disbonded sleeve4,6. It is very difficult for CP current to

penetrate inside this disbonded region because of defect geometry.

Table 1 is the measured potential inside this disbonded coating by introducing the reference electrode

(saturated copper/copper sulfate electrode) and platinum (Pt) electrode. CP current could not penetrate

into the disbonded area, which resulted in the pipe surface exposed to anaerobic soil under the

disbonded region being in the freely corroding condition (-610 mV to -550 mV/CSE) even though the

pipe-to-soil potential was in a range of -1200 mV - -1400 mV/CSE.

Eh under the disbonded area was about -160 mV/NHE. When SRB is cultivated in culture medium, the

recommended initial Eh should around -100 mV/NHE [12]. Therefore, the suitable anaerobic condition

for the active growth of SRB was provided under the disbonded coatings and MIC occurred in this

region mostly.

This insufficient CP condition was confirmed by numerical modeling of current/potential distribution

inside the crevice under the disbonded sleeve, calculated by two-dimensional boundary element

method (BEM) at various applied CP potential and the soil resistivity15. Figure 12 is the schematic

representation of the model simulating the disbondment case in Figure 11. Figure 13 is the general

trend for the CP potential distribution expressed in constant equipotential lines at the resistivity of 3000

Ocm, P/S = -1.2 V/CSE. A sharp potential drop was observed adjacent to the opening region of

crevice, which results in the poor potential condition inside crevice. Figure 14 shows the

potential/current distribution at metal/solution interface for crevice formed under the disbonded sleeve

at the crevice opening potential (E0) of -0.85V/CSE. It can be seen that a significant shift on the crevice

potential to more positive values occurred as the distance from the holiday opening (position = 20 cm)

along the crevice increased, which results in the freely-corroding condition at the crevice center

(position = 0 cm). This trend is related to the current density available along the crevice as shown in

Figure 14. Of course, as the soil resistivity decreases and the applied CP potential shifts to more

negative values, the effective penetration depth which satisfies the CP criteria inside crevice increases

as shown in Figure 15. Considering the overprotection problems such as hydrogen embrittlement21,22,

however, the optimization plot for CP of the crevice inside the disbonded sleeve is given in Figure 16.

As shown in Figure 16, the optimum applied opening potential should be maintained in a range of -0.85

V/CSE to -1.2 V/CSE, i.e., the hatched areas should be avoided because of underprotection or

overprotection. Therefore, the effective penetration depth of CP into the crevice is in a range of 1 cm to

12 cm depending on the soil conductivity (resistivity) and applied potential (or current) in this case and

there is a limitation of CP to protect the crevice region, which result sin the MIC in the crevice under the

disbonded coating. This disbonded coating should be prepared as quickly as possible to avoid the

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breakdown of the pipeline.

CONCLUSIONS

(1) It is possible to assess the activity of SRB, i.e., the risk of MIC from the relationship with key

environmental factors qualitatively and this risk is directly related to the corrosion rate by field survey

and the statistical approach. It was also found that the main factor affecting underground corrosion is

the action of SRB.

(2) Microbial colonization of carbon steels can drastically alter their corrosion behavior. Electrochemical

measurements can be made under MIC condition and showed good results. However, all

electrochemical methods are averaging techniques that work best when the chemical and

electrochemical conditions on the metal surface are uniform and steady state and provide no

information on the localized corrosion.

(3) The thin film ER probe test makes it possible to shorten the experimental periods and to study the

localized corrosion behavior, which is a characteristic of MIC.

(4) The optimization plot for the CP of crevice under the disbonded coating was also presented using

the multiple quadratic regression analysis by considering the numerical modeling results and the

overprotection problems. It is possible to predict and evaluate the CP status under the disbonded

coating using this plot. According to the optimization plot for CP, it was inevitable to avoid MIC under

the disbonded sleeves and therefore the proper and quick repair should be made.

(5) The study for MIC of steel in soil environments requires a multi-disciplinary approach comprising

different combination of microbiological, electrochemical and microscopic measurement as well as the

consideration of CP that may be the only protective measure in soil environments.

ACKNOWLEDGEMENT

The authors gratefully acknowledge the Korean Ministry of Science and Technology for their financial

support through the National Research Laboratory (NRL) program.

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(2002)

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TABLE 1. RESULTS OF POTENTIAL MEASUREMENTS

Types Potential

Pipe-to-Soil Potential -1430 - -1200 mV/CSE

Potential inside disbonded region -610 - -550 mV/CSE

Reduction-Oxidation Potential (Eh) -160 mV/NHE

FIGURE 1 - Relationship between P0 (Pmax/Pmax_average) and environmental factors

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FIGURE 2 - Relationship between Pmax and Pmax,cal

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FIGURE 3 - Relative contribution of environmental factors to underground corrosion

FIGURE 4 - Effect of SRB on the corrosion potential and uniform corrosion rate of carbon steel

exposed to anaerobic soil in laboratory

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FIGURE 5 - Polarization curves for steel exposed to SRB-active soil for specified periods

FIGURE 6 - Comparison of EIS plot obtained after exposure to anaerobic soil

(a) for 7 days, (b) for 145 days (A: SRB-active, B: control)

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A

20m

(a)

B 20 m

(b)

2m

(c)

15

C D

(d)

FIGURE 7 - Surface morphology and the results of EDXA of carbon steel exposed to SRB-active

soil; (a) for 14 days, (b) for 148 days, (c) SRB mixed with iron sulfide film after 148 days, (d) pit

morphology after removal of surface deposit

FIGURE 8 - Schematic diagram of thin film electrical resistance (TFER) probe

Insulation layer

Sensing area

Lead wire

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FIGURE 9 - The responses of TFER probes in anaerobic environments

FIGURE 10 - Time to the onset of localized corrosion vs. the population of SRB detected using

TFER sensor

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(a) (b)

FIGURE 11 - Typical example of (a) disbonded sleeves at the girth weld joints, (b) morphology of

corroded area

SLEEVE

STEEL

CREVICE

40cm

5cm x

y

GROUND

SOIL

2m

40cm 1cm

2m

u=P/S

q = 0 q = 0

q = 0 q = 0

FIGURE 12 - Schematic representation of the model simulating the disbondment in Figure 11.

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FIGURE 13 - Typical potential distribution expressed in constant equipotential lines at the

resistivity = 3000 Ocm (0.00033 S/cm), P/S=-1.2V/CSE

FIGURE 14 - Potential and current profiles for crevice formed under disbonded coating at 3000

Ocm and crevice opening potential of 0.85V/CSE

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(a)

(b)

FIGURE 15 - (a) Response surface and (b) contour plot for the effective penetration depth of CP

current

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FIGURE 16 - Optimization plot for CP of crevice under the dibonded sleeve