CHAPTER 1

INTRODUCTION

1.1 BACKGROUND OF STUDY

Corrosion of pipelines in the offshore oil industry has been a major problem for

years, and many industry leaders have tried to tackle the problem from various

angles. One way corrosion can be arrested is through the application of a barrier like

paint or a plastic lining, which would be able to separate the corroding surface from

the corrosive environment, thus reducing the potential for corrosion. Microbiological

influenced corrosion (MIC) can be defined as the deterioration of metals by natural

processes directly or indirectly related to the activity of microorganisms’ occurring

in the internal pipelines. Microbial Influence Corrosion affects many industries such

as petrochemical, ships and marine structures, power generating and water supply

distribution systems.

Since 1990’s, Microbial Influence Corrosion (MIC) has become a major issue which

affects the oil industry, particularly during the hydrocarbon extraction, transport and

storage. The activity and microorganisms’ growth in the internal pipelines steel may

cause surface modifications, which induce a more complex corrosion process [1].

The major problem with the inspection of pipelines is that they are usually difficult

to access, and therefore in order to obtain any useful information about the state of

the pipeline, expensive diving operations are needed. Therefore to save money and

to reduce the risk of accidents caused by the failure of corroded pipelines, it is

important to be able to predict the extent of corrosion in any specific pipeline

without having to inspect the pipeline manually.

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The presence of the Sulphate Reducing Bacteria (SRB) in the pipeline can cause a

major corrosion in the pipeline without the operator noticing it, thus it will effect oil

production and generate huge losses.

By doing MIC study, the bacteria in the pipeline can be detected and can be reduced

using suitable treatment methods, and thus it can prevent huge losses to the oil

industries.

1.2 PROBLEM STATEMENT

It has been estimated that 80% of failures occurring in production and pipeline

operations are caused by corrosion. One of the major causes of corrosion is

Microbial Influence Corrosion (MIC). The primary objective when failure occurs is

to establish that if corrosion is the cause, what are the specific reasons and how can it

be prevented in the future [2].

Sulphate Reducing Bacteria, SRB, are an assemblage of bacteria that can grow in

anaerobic medium by the oxidation of organic nutrients with sulphate being reduced

to H2S. The Sulphate Reducing Bacteria will cause corrosion in the offshore pipeline

silently undetectable by platform operator. The existence of bacteria is hardly

undetectable but using particular culture media growth the bacteria can be detected

1.3 OBJECTIVES AND SCOPE OF STUDY

1.3.1 Objectives

The objective of this project is to evaluate the corrosion rate of mild steel in the

presence and the absence of sulphate reducing bacteria (SRB). The evaluation will

be performed both in fresh and in ageing culture. The experiment will be carried out

on specific samples taken from offshore terminal.

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1.3.2 Scope of Work

For this project, the Microbial Influence Corrosion (MIC) study will be focusing on

the particular oil terminal around Malaysia region. The experiment will be done by

fabricating pipeline anaerobic condition in the lab. The water sample will be taken

from the well head form the offshore platform.

1.4 SIGNIFICANCE OF STUDY

Microbiologically influenced corrosion (MIC) refers to corrosion brought about by

the presence and/or activities of microorganism biofilms on the surface of the

corroding material.

In this study, the type of bacteria exist in the offshore pipeline can be detected and

we can know how critical the bacteria existence in the pipeline. The water sample

taken from the pipeline containing bacteria likes a Sulphate Reduce Bacteria (SRB)

and General Heterotrophic (GHB) will be detected using particular culture medium.

Then the corrosion rate cause by Sulphate Reduce Bacteria will be calculated to

show how critical the corrosion rate [3].

The focus will be more on the Sulphide Reduce Bacteria (SRB) by studying how

crucial it is in causing corrosion and how to treat the bacteria as MIC play an

important role in oil and gas industry.

Significant improvements in analytical, microbiological, electrochemical, and

microscopy techniques and instrumentation have allowed the development of new

methods for laboratory and field assessment of MIC in industrial systems. The

microbial influence on corrosion is now well established, although many of the

mechanisms are still not fully understood.

Microbiological influenced corrosion (MIC) is not a unique form of corrosion, but

rather, modified forms of localized corrosion that are enhanced by the action of

3

bacteria. Reviewing the literature it becomes evident that most of the information on

the MIC of stainless steels is based on post analysis of corroded specimens retrieved

from the field [4].

4

CHAPTER 2

LITERATURE REVIEW

2.1 Introduction of Microbial Influence Corrosion

In general, the study of MIC has been based upon microbiological tests and just a

few references mentioning alternative methods that can be used as criteria for their

evaluation in interpreting MIC problem.

Some knowledge of the behaviour of microorganisms in water is extremely

important since their presence can cause corrosion or plugging of equipment or the

injection wellbore. They are simply another source of plugging solids or conditions

which result in corrosion [5].

Most of the studies involving electrochemical techniques and Microbiological

Influence Corrosion tests have been done for short experimental times. Under these

conditions, mainly low corrosion rates can be observed. Usually, the experimental

time is established considering several aspects, such as:

• Kinetics growth of microorganisms in the electrolyte

• Sulphate consumption

• Sulphuric acid production.

However, a basic characteristic of the MIC process frequently is not considered:

biofilm formation on the metal surface. Biofilm is the layer that is caused by

Sulphide bacteria. Once the biofilm is formed, the corrosion damage on the metal

surface should be dependent mainly on the microorganisms.

5

2.2 Prediction of the risk of microbial Influenced Corrosion (MIC)

The ideal predictive model would furnish its user with a reliable estimate of the rate

of corrosion associated with a biofilm or an estimation of time to penetration of the

pipe wall, the locations along the pipe where the attack is most likely to occur and,

when corrosion will be initiated.

Unfortunately, biological interactions which characterise biofilm development and

activity, and the mechanism by which these interactions influence corrosion of the

underlying metal surface, are not only numerous but may also be interdependent,

making a description of the entire complement of reactions and process extremely

difficult.

The unpredictability of living organism is probably the greatest problem to be faced.

Indeed, for a pipeline of several kilometres in length, conditions may be such that a

number of different types of biofilm are supported at different locations along the

pipeline. The complexity is summarises schematically in Figure 1.

Figure 1: Microbial Influenced Corrosion in oil transport lines [6]

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It is generally accepted that biofilm formation is a key step by which

microorganisms become involved, both directly and indirectly, in corrosion

processes without a source of bacteria and suitable conditions for their survival

within a pipeline, no bio-film can become established.

For these reasons the first step in predicting MIC is to define the limitations for

microbial activity and biofilm formation within the selected pipeline. The potential

for microbial growth may be assessed by measuring a number of system parameters,

including water availability, water chemistry, pH and temperature.

Other factors influence the development of a biofilm, including flow characteristics,

metal composition and topology, surfaces scales or oil films, presence of production

chemicals and other operating practices, such as physical cleaning.

The activity of biofilm within a line may be estimated from nutrients and inorganic

balances between the beginning and end of the line. The presence of bacteria and

corrosion products such as iron sulphide in internal pipeline scrapings is often used

to confirm that biofilm is the cause of corrosion within the line.

So, it is possible to determine the likelihood for biofilm to occur within the line.

Nevertheless, this information is not always sufficient evidence for the pipeline

manager to make a decision with respect to mitigation of biofouling.

The control of microbial activity and the build up of biofilm is frequently tackled by

the application of biocides, often in conjunction with physical cleaning using

pipeline scraping devices. But, the management equates such operations with

disruption of oil production, costs of biocides and, increasingly, with the problems

associated with the disposal of biocide-treated waters, which may not be

environmentally acceptable.

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2.3 Hydrogen Sulphide Corrosivity

Figure 2 below is from laboratory tests performed at low pressures and room

temperature. As such, specific values would have no relation with field operation

conditions. However, the rate of change shown can be considered reasonable

approximations of the change of the corrosion rate that could be anticipated for

similar changes in the variables in operations [6].

Figure 2: Corrosivity of Hydrogen Sulphide [7]

In considering the figure above, most sour corrosion will have less than 2000 ppm of

H2S and will be in the (5.0-6.5) pH range. Assuming an average pH of 6.25, an

increase from a trace of H2S to 2000 ppm (part per million) would increase the

corrosion rate by a factor of 4. The curve indicates that for an H2S content of over

100ppm the corrosion would be significant and it would be probably a pitting attack.

2.4 Bacteria Growth

8

The reason that bacteria can create so much trouble is that they can multiply with

incredible speed. Some can double their population in 20 minutes under ideal

conditions, which means that a single bacterium can become thriving colony of

millions of bacteria in a very few hours.

Bacteria can withstand an extremely wide range of temperature (-10 to 99°C), pH

values about (0-10.5), and oxygen concentrations (0-100%). However, in water

systems, they grow best in the ph range of 5-9 and at temperatures less than 82°C.

They also prefer fresh water, but can tolerate saltwater. They are extremely

adaptable and hardy.

Bacteria can live either in groups or colonies attached to solid surfaces or suspended

in water. Bacteria attached to a surface are called sessile bacteria. When they are

suspended in water, they are termed planktonic bacteria, or sometimes simply

swimmers or floaters.

The majority of bacteria are sessile. It has been reported that in a typical system,

there are 1000 to 10 000 times as many bacteria attached to a surface as there are

floating in the water [8].

It has also been shown that as sessile bacteria grow they produce a sticky substance

called a polysaccharide, which the bacteria utilize to cement themselves to a solid

surface. Continued production of the polysaccharide results in the formation of a

biofilm which surrounds and covers the bacteria.

The biofilm can become quite thick with 200-250 cells it can become 1mm

thickness. Within the layers of polysaccharide, there can be a whole community of

bacteria. Cells of one species often exist in their own protected state beside cells of

another, creating a mixed adherent population.

2.5 Types of Sulfate Reducing Bacteria

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About nine families of sulfate reducing bacteria are known. However, most SRB

corrosion problems are attributed to members of two families: Desulfovibrio and

Desulfotomaculum. Some of the species of each which are known to contribute to

corrosion are listed in Table 1.

Table 1: SRB families

Genus Species Shape

DesulfovibrioAfricanus

Desulfuricans

Salexigens

Vulgaris

Sigmoid rod

Vibrio

Vibrio

Vibrio

DesulfotomaculumNigrificans

Orientis

Rod

Curved Rod

The sulphate-reducing bacteria organisms most commonly detected in the oilfield

belong to the genus desulfovibrio. Desulfotomacuum can form spores. A bacterial

spore is a structure formed within the body of a bacterium. Spores are resistant to

temperature, acids, alcohols, disinfectants, drying, freezing and many other adverse

conditions. Spores may last for hundreds of years and then germinate in favorable

conditions. A spore has many of the characteristics of a seed but is not reproductive

structure.

2.6 Temperature, Pressure and pH

Sulphate reducing bacteria as a group are reported to tolerate temperatures from 4-

77°C, a pH range of about 5 to 9, and pressures of at least 14 500psi. However,

absolute values of temperature, pressure and pH required for the growth of sulfate

reducing bacteria in natural environments are impossible to state with any degree of

certainty. For example, sulfate reducers isolated from wells with bottom hole

temperatures in excess of 121°C have been cultured in the laboratory at lower

temperatures, but would not grow at temperatures greater than 88°C at atmospheric

pressure. Furthermore, the maximum temperature at which sulfate reducing bacteria

grow apparently increases with pressure [9].

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The following statements apply to growth in laboratory in artificial media:

Desulfovibrio: The optimum temperature range for growth is approximately

25-43°C, with an upper temperature limit of 49°C,

Desulfotomaculum nigrificans: The optimum temperature for growth is

54°C. They exhibit slow growth at 66-71°C, and can survive at 77°C,

Desulfotomaculum orientis: Exhibits optimum growth at temperature of 30-

38°C. They are killed when the temperature exceeds 42°C.

2.7 Bacteria Classification

Microbes fall into two basic groups, aerobic and anaerobic. These two groups are

based on the kind of environment they prefer, either with or without oxygen.

One method of classification of bacteria which is of interest in oilfield systems is

whether or not specific bacteria require oxygen to live. They fall into three

categories:

a) Aerobic bacteria - Require oxygen to grow,

b) Anaerobic bacteria - Grow best in absence of oxygen,

c) Facultative Bacteria - Grow in either the presence or absence of oxygen.

For aerobic bacteria, organic compounds react with oxygen producing water and

CO2 which are not to critical in causing corrosion. Figure 3 below shows the aerobic

process for the aerobic respiration with existence of oxygen.

Figure 3: Aerobic respiration

Aerobic BacteriaOrganic compounds+ 02 H2O+CO2

Oxygen ReductionO2 + 4H+

+ 4e- 2H2O

OrganicMaterial

CO2

Biomass

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Figure 4 below shows the anaerobic process for anaerobic respiration. For anaerobic

process, the organic compounds in the pipeline react with the Sulphate (SO4)

producing H2S, H2O and CO2. H2S and CO2 are two main elements causing corrosion

in pipelines.

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

microorganism involved in the corrosion processes. However, it is now well known

that bacteria are not found in isolation but biofilms in which many bacterial

communities exist [10].

2.8 Bacteria Role

Microbes tend to form colonies, with different characteristics from the outside to

inside region of the colony. On the outside, "slimers" may produce polymers (slime)

that attract inorganic material, making the colony look like a pile of mud and debris.

These aerobic organisms can efficiently use up all available oxygen, giving

anaerobic microbes (SRB's) inside the colony a hospitable environment, allowing

enhanced corrosion under the colony.

The reason that bacteria can create so much trouble is that they can multiply with

incredible speed. Some can double their population in 20 minutes in ideal conditions,

which means that a single bacterium can become a thriving colony of millions of

bacteria in a very few hours. A handful of slime from water may contain as many

bacteria as there are people in the world [11].

2.9 Sulphate Reduce Bacteria

Figure 4: Anaerobic Respiration

Sulphate Reducing BacteriaOrganic compounds+ SO4 H2S +H2O+CO2

Sulphate ReductionSO4

2- + 8H+ + 8e- S2- + 4H20

OrganicMaterial

CO2

Biomass

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Sulphate Reduce Bacteria are anaerobes that are sustained by organic nutrients.

Generally they require a complete absence of oxygen and a highly reduced

environment to function efficiently. Nonetheless, they circulate (probably in a

resting state) in aerated waters, including those treated with chlorine and other

oxidizers, until they find an "ideal" environment supporting their metabolism and

multiplication. Sulphate Reducing Bacteria reduce sulphate to sulphide, which

usually shows up as hydrogen sulphide [12].

For this study, the MIC study will be performed in a particular platform on the

Malaysia. It will cover a particular drilling platform and production platform. The

Dilution test SRB and GHB will be run on sample taken from particular well on the

platform.

Figure 5 below shows the chemical equation for oxidation process in the internal

pipeline surfaces that causes corrosion [13]. Iron oxidizing bacteria oxidize soluble

ferrous ions to less soluble ferric, Fe3+, ions. The lower Fe2+ activity increases the rate

of the anodic reaction.

Figure 5: Oxidation Process of H2S

Figure 6 below shows the SRB Growth Media used for culturing the media. This is

the media typically used for culturing bacteria. The water sample will be injected in

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the media and it will be monitored for 28 days. Any colour change will be observed

and recorded.

Figure 6: SRB Growth Media

2.10 Sulphate Reducer Effect

Sulphate reducers probably cause more serious problems in oilfield injection system

than any other bacteria. They can reduce sulphate or sulphite ions in the water to

sulphide ions, resulting in H2S as by product.

Four types of problems can result from sulphate reduce activity in an injection

system.

They can participate directly in the corrosion reaction and cause pitting

directly beneath the bacterial colony.

The generation of H2S by bacteria can increase the corrosivity of the water. If

the system is already sour, the additional H2S generated by the bacteria may

have little or no effect. However if the system was originally sweet, the

addition of H2S to the system by bacterial activity can substantially increase

corrosion rates and result in a pitting attack throughout the system.

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The presence of sulphate reducing bacteria in a system which was originally

free of H2S creates the possibility of sulphide cracking and blistering of

carbon steels.

Sour corrosion results in the formation of insoluble iron sulphide which is an

excellent plugging material. Sulphate reducing bacteria are most likely to be

found in stagnant or low velocity areas, and beneath scales or sludge.

Common place for bacterial activity in injection systems are tanks, filters and

the rat hole injection and water source wells.

2.11 Culturing Bacteria

Culturing bacteria is analogous to culturing flowers, potatoes, or green beans. The

object is to make them grow. Bacterial culturing in artificial growth media is the

standard technique for the estimation of bacterial populations.

A water sample thought to contain bacteria is placed in liquid known as a culture

medium which is a solution consisting of water and food that will make the bacteria

of interest grow and multiply. In addition, many media contain a growth indicator.

For example, culture media for sulphate reducing bacteria contain iron. When SRB’s

grows, they produce H2S, which react with the iron to create an insoluble black

precipitate, iron sulphide.

Different types of bacteria require different culture media, and some bacteria refuse

to grow at all in artificial media. However, most bacteria of interest can be cultivated

in a particular medium. The fact that media can be formulated in which only specific

types of bacteria will grow makes it possible to identify the bacteria simply but

noting the media in which growth occurred. Furthermore, by running the sample at

several different dilutions, the number of each type of bacteria can be estimated.

2.12 Extinction Dilution Technique

This is a field technique or sometimes called serial dilution technique, which can be

used to detect different kinds of bacteria. Detection of each class or type of bacteria

requires specific culture medium.

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The point of this technique is to dilute the sample to the point that the final 1ml of

solution that is injected into the last bottle has no bacteria in it, hence, the name

extinction dilution technique. This means that we have diluted the sample to the

point of extinction of any bacteria present.

The fact that the dilution is performed in a series of fixed dilution ratios allows you

to estimate the bacterial population of the original 1ml water sample. The rules of the

game state that we cannot transfer part of a bacterium from one serum bottle to

another. This means that when the withdrawal of 1ml from a serum bottle containing

10ml of liquid, there must be at least 10bacteria in a bottle, or an average of at least

one bacteria per ml, before a transfer is allowed.

For example, if there were only 8 bacteria in the bottle, the average population

would be 0.8 bacterium per ml and transfer cannot occur according to the rules.

Similarly, if there are 15 bacteria in a bottle, the average population is 1.5 bacteria

per ml. Since only whole number transfer are allowed, a one ml withdrawal of liquid

will net you 1 bacterium for transfer to the next bottle.

2.13 Types of culture

In practice, two types of culture media are normally utilized and, therefore, two

separate series of dilutions are: one for sulphate reduce bacteria and the other for

general bacteria.

2.13.1 Sulphate Reduce Bacteria

The SRB series utilizes a growth medium which is specific to sulphate reducing

bacteria. The bacteria counts obtained using this medium includes Desulfovibrio.

Desulfotomaculum may also be detected. However, if system conditions appear to be

favourable to the Desulfotomaculum, it may be desirable to inoculate an additional

media designed especially for their detection as an additional precaution.

Once the bottles have been inoculated, they are set aside and allowed to incubate for

a fixed time period. An incubation period of 28 days is recommended. However,

shorter incubation periods may be used when it can be demonstrated that all growth

occurs less than 28 days.

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A constant temperature is desirable during the incubation period and growth rates are

temperature sensitive. The cultures should be incubated at a temperature within 5°C

of the recorded temperature of the water at the time of sampling if possible. If not,

keep the temperature between 25-38°C.

Growth is indicated when the bottle turns black. The SRB media contain soluble

ferrous iron or a sterile iron nail. When SRB’s grow, they produce H2S which reacts

with the iron to form insoluble, black iron sulphide.

2.13.2 General Heterotrophic Bacteria

The general series employs a different growth medium which promotes the growth

of general heterotrophic bacteria as well as facultative bacteria. An incubation period

of 5days is common.

The general count includes the general aerobic bacteria, primarily the slime formers,

and can also include anaerobic bacteria as well as facultative bacteria. It does not

include iron bacteria, which are difficult to culture in an artificial medium. They are

usually detected by microscopic.

Three types of media are in common use for the detection of general heterotrophic

bacteria.

Standard Bacteriological Nutrient Broth.

Growth is indicated by the development of turbidity. The turbidity is caused by

the bacterial cells themselves, and is usually evident when the cell count

exceed 1 000 000 per ml.

Phenol Red Dextrose Broth

This medium contains sugar and phenol red, which is and acid base indicator

which turns from red to yellow when the ph of the culture medium drops below

6.6. When the sugar is fermented or oxidized by bacteria, various organic acids

are produced. The resulting acid-fermented or oxidized by bacteria, various

organic acids are produced. The resulting acidity causes the ph to drop, which

17

causes the colour of the medium to change from red to yellow. Growth is

indicated by the development of acid-producing bacteria.

Thioglycolate Medium

This medium is used to detect anaerobic heterotrophic bacteria. Growth is

indicated by the development of turbidity.

Once the bottles have been inoculated, they are set aside and allowed to incubate for

5days at the same temperature as the SRB bottles.

2.14 Bacteria Nutrition

Bacteria absorb their nutrients directly from the environment around them. A single

living cell contains hundreds of different enzymes, each of which is an effective

catalyst for a specific chemical reaction.

However, the enzymes work together in a coordinated manner to produce the

materials required for normal cell growth and metabolism. Although all enzymes are

initially produced in the cells, some are secreted through the cell wall and function in

the cell’s environment.

This type of enzyme enables the cell to assimilate large molecules by breaking them

down outside the cell into smaller molecules which can be absorbed through the cell

wall.

Sulfate reducing bacteria require a number of nutrients in order to sustain growth.

Some of the primary ones are as follows.

Carbon - Sulfate reducing bacteria are heterotrophic, meaning that all or most

of their cell carbon is derived from organic substances and that they generate

carbon dioxide when they grow. Apparently they cannot utilize petroleum

hydrocarbons.

Dissolved Iron - Sulfate reducing bacteria have an absolute requirement for

relatively high concentrations of dissolved iron.

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Sulfate, Sulfite, Bisulfite or Thiosulfate ions - Although the primary diagnostic

character of sulphate reducing bacteria is that they grow with sulphate,

reducing to sulphide, they can also grow with sulphite and other reduced

sulphur compounds.

A shortage of any of these materials can limit SRB growth. The oxygen scavengers

or phosphorus addition containing compounds such as scale inhibitors could enhance

growth if the concentrations of phosphorus or sulphate in the system are so low and

they are limiting growth. However, most injection waters contain sufficient nutrients

for abundant growth bacteria.

2.15 Biofilm Development

Biofilms are complex communities of microorganisms attached to surfaces or

associated with interfaces. Despite the focus of modern microbiology research on

pure culture, planktonic (free-swimming) bacteria, it is now widely recognized that

most bacteria found in natural, clinical, and industrial settings persist in association

with surfaces.

Furthermore, these microbial communities are often composed of multiple species

that interact with each other and their environment. The determination of biofilm

architecture, particularly the spatial arrangement of micro colonies (clusters of cells)

relative to one another, has profound implications for the function of these complex

communities. Biofilm development includes several stages before it is completely

matured. The phases include pre-maturation phase, maturation phase, structural

development phase and senescence phase as illustrated in Figure 7. In pre-maturation

phase, the cells will swim and attach to the pipeline surface. The cells interact with

each other and start to form a film.

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Figure 7: Pre – Maturation phase [13]. (a) Free-swimming cells alight on a surface

and attach; (b) New genes are expressed to synthesize matrix polymers; (c) Cells

coordinate by exchanging signalling molecules.

Stage 3(c)

Stage 1(a)

Stage 2(b)

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At this maturation phase stage, the bacteria start to multiply, doubling their

population and forming colonies. This is the phase where the anaerobic condition

occurred. The phase is shown in Figure 8. In order for corrosion to occur, the

bacteria will be go through a structural development phase where the bacteria will

obtain protection from antimicrobial agent during the corrosion process.

At this rate, the bacteria colonies will produce slime and it is completely matured.

The phase is shown in Figure 9. The final phase is senescence phase, where the

biological processes of a living organism approaching an advanced age. The process

is the combination of processes of deterioration which follow the period of

development of an organism. The phase is shown in Figure 10.

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Figure 10 below show the final phase of

Stage 2

Stage 1(a)

Stage 2(b)

Stage 1 Stage 2

Figure 8: Maturation Phase [13]. (a) Bacteria reproduce and form micro colonies; (b) Chemical gradients are established.

Figure 9: Structural Development phase [13]. (a) Variety of environmental niches promotes coexistence of diverse species; (b) Biofilm affords protection from antimicrobial agent.

22

Figure 10: Senescence [13]. (a) Cells dissolve matrix and are released.

Final Stage(a)

23

2.16 Bio Corrosion

Oxygen depletion at the surface also provides a condition for anaerobic organisms

like sulfate-reducing bacteria (SRB) to grow. This group of bacteria are one of the

most frequent causes for bio corrosion. They reduce sulfate to hydrogen sulfide

which reacts with metals to produce metal sulfides as corrosion products.

Aerobic bacteria near the outer surface of the biofilm consume oxygen and create a

suitable habitat for the sulfate reducing bacteria at the metal surface. SRBs can grow

in water trapped in stagnant areas, such as dead legs of piping. Symptoms of SRB-

influenced corrosion are hydrogen sulfide odour, blackening of concentrations, and

black deposits. The black deposit is primarily iron sulfide.

2.17 Specimen Cleaning

Care must be exercised in cleaning and removal of all corrosion products and foreign

matter from the surface of exposed specimens before the final weight can be

measured. Cleaning may be mechanical, chemical, or often both.

The ideal case, in which the cleaning operation removes adherent corrosion products

and leaves the underlying metal coupon unaffected, is seldom achieved. Instead,

gradual loss of the base metal continues during the process after initial rapid removal

of corrosion products.

Figure 11 below shows the schematic weight loss during specimens cleaning.

Extrapolation of the metal loss period, BC, back to the beginning of the beginning

operation at D gives the moss accurate final coupon weight. However, if the cleaning

operation can be timed to stop at point B, the error will be uniform and small when

extrapolation is omitted.

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Figure 11: Schematic weight loss during specimens cleaning [14].

2.18 Corrosion Rate Determination

Methods of exposure testing for corrosion measurement are fundamental in

corrosion engineering. The emphasis on measurement of uniform corrosion rates by

weight loss of specimen. Exposure test may be conducted in the laboratory or in

service. Laboratory tests are more flexible, less expensive, and can have any of the

foregoing objectives because modifications or interruptions of plant processes are

not required.

However, it is nearly impossible to simulate plant conditions exactly in the

laboratory. Time is usually at premium, and accelerating factors, such as increased

temperature, are often included. Thus, preliminary laboratory tests often required to

follow up with plant qualification test.

The calculation of corrosion rates requires several pieces of information and several

assumptions. The use of corrosion rates implies that all mass loss has been due to

WE

IGH

T L

OSS

NUMBER OF CLEANING CYCLES OR CLEANING TIME

D

25

general corrosion and not to localized corrosion, such as pitting or corrosion-

sensitized areas on welded coupons. When localized corrosion occurs, maximum pit

depth and pit density shall be reported.

The use of corrosion rates also implies that the material has not been internally

attacked, such as by intergranular corrosion. Internal attack can be expressed as a

corrosion rate, if desired. However the calculations shall not be based on mass loss,

which is usually small, but on micro sections that show depth of attack.

2.19 The Corrosion Rate Units and calculation

The corrosion rate in mils (1 mil = 0.001-in) penetration per year (mpy) may be

calculated from:

Where W is weight loss in milligrams in density in grams per cubic centimetre, A is

area in square inches, and T is time in hours. Units of penetration per unit time are

most desirable from an engineering standpoint, but weight loss per unit area per unit

time per day (mdd), are sometimes used in research. For conversion, 1 mpy = 1.44

(mdd) / specific gravity.

The unit mpy continues as the most popular for corrosion rate in the United States,

despite increased use of metric units in recent years. The range of practical corrosion

rates are expressed conveniently in terms of small whole integers from 0 to 200 mpy

for ferrous alloys in a time period (one year) useful for engineering purposes.

Conversions to equivalent metric penetration rates are: 1 mpy = 0.0254 mm/yr =

2.90 µm/yr = 2.90nm/h = 0.805 pm/s , where 1 meter = 103 millimetre (mm), 106

micrometer or micron (µm) 109 nanometre (nm), and 1012 picometer (pm).

Table 2 compares mpy with competing metric units. Equivalent mm/yr gives

fractional numbers, and µm/yr gives large integers. Desirable small whole integers

mpy = 534 W DAT

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are produced in terms of nm/h and pm/s, but the time units, hours and seconds, are

insignificant from an engineering standpoint.

Metric units of penetration rates will probably see still further use. The most

promising appear to be mm/yr and µm/yr for high and low corrosion rates,

respectively. The proportionality constant, 534 in previous equation varies

depending on the units required for corrosion rate and used for the parameters in the

equation:

µm/yr = 87600 W a and

DAT

Where W, D, and T have the same units for mpy but area, A is measured in square

centimetres .

Source: M.G Fontana, Corrosion Engineering, McGraw Hill [15].

mm/yr = 87.6 W DAT

µm/yr = 87600 W DAT

RelativeCorrosionResistance mpy mm/yr µm/yr nm/h pm/sOutstanding <1 <0.02 <25 <2 <1Excellent 1-5 0.02-0.1 25-100 2-10 1-5Good 5-20 0.1-0.5 100-500 10-50 20-50Fair 20-50 0.5-0.1 500-1000 50-150 20-50Poor 50-200 1-5 1000-5000 150-500 50-200Unacceptable 200+ 5+ 5000+ 500+ 200+

Table 2: Comparison of mpy with equivalent Metric-Rate Expressions

27

CHAPTER 3

METHODOLOGY

3.1 The Work Flow

For this Microbial Influence Corrosion (MIC) study, the methodology is divided to

four main steps. The steps are:

a) Field Water Sampling: Water sample will be collected at sampling

point from the particular platform,

b) Run Media Test: Run GHB & SRB test on the water sample with

standard procedure,

c) Result Monitoring: The Media (SRB & GHB) will be put incubator

with optimum condition to be monitored for 28 days,

d) Corrosion weight loss measurement experiment.

Figure 16 below shows the work flow of the Microbial Influence Corrosion Case

Study. The main components of the work flow will be discussed in subsequent

section.

28

Weight Loss Experiment

Preparation seminar on MIC

Overview

Report Findings

Analyze and Interpret Data

Final Report

Study Research

Preliminary report

Run SRB & GHB test

Result Monitoring at

incubator

Field Water Sampling

29

3.2 Field Water Sampling

One of the first items of interest in water handling is to sample it and determine its

composition. The importance of good sampling practices cannot be overemphasized.

The field area sampling procedure will be summarizing as follow.

1. On arrival at location notify operations personnel of the locations where

sampling is proposed and establish whether there any operational procedures due

to take place could impact upon the sampling.

2. The size of obtained fluids sample to be taken must be sufficient to ensure that a

minimum of 600 ml of produced water is obtained. The produced fluid sample is

placed in a separating funnel and allowed to settle until the water has separated.

3. The pH and temperature of the sample is measured and recorded and the sample

is subdivided as follows :

a) 250 ml of water is placed in a plastic bottle, sealed, labelled and

submitted to laboratory for ionic analysis,

b) 250 ml is placed in a glass bottle containing a sodium hydroxide pellet,

sealed, he bottle agitated until the sodium hydroxide has dissolved,

labelled and submitted to Analytical Services Company for measurement

of the level of volatile fatty acids present,

c) Serial Dilution tests for SRB and GHB are carried out on the remaining

water and the bottles transported to the laboratory where they are placed

in the incubator.

The incubator temperature is set at 20 degree Celsius in order to run the

SRB/GHB test.

Figure 16: Work Flow

30

3.3 Sulphate Reduce Bacteria SRB Test Procedure

To detect the SRB from the water sample, there is a test procedure that needs to be

followed. The test procedure follows the NACE Standard TM0194-94 [16].

3.3.1 Purpose

This method presents procedures for determining the numbers of sulphate-reducing

bacteria (commonly referred to as SRB) in a range of industrial samples (with the

exception of food and potable water) which might be collected in the field or

supplied to the laboratory for testing. This might include aqueous fluids, oils,

sludges, metal coupons, etc. The systems may be operated at a range of temperatures

from < 10°C to 100°C.

3.3.2 Summary of method

The numbers of SRB are determined by serial dilution in Modified Postgate B

Medium (alternative broth media may also be used - American Petroleum Institute

(API) Medium or SRB/2 Medium (in-house formulation)). The broths are incubated

for (minimum) 28 days (+ 2days) at 30°C ± 1°C, or other appropriate temperature

dependant on the system under investigation. Depending on the nature of the sample

(fluid or solid) a suspension may have to be prepared in Phosphate Buffered Saline

(PBS) or appropriate diluents.

3.3.3 Apparatus

They are requirement need to be followed to ensure the smoothness of the test. The

apparatus need to be set up as below before starting the test.

Incubator set at 30°C (or appropriate temperature) ± 1°C,

Modified Postgate B Broth/American Petroleum Institute (API)

Medium/SRB/2 Medium in 9 ml ±0.1 ml in crimp sealed serum vials,

1 ml syringes and hypodermic needles (25 gauge, 1.5 inch).

31

3.3.4 Test Procedure

To obtain data for the existence of the SRB, the following procedure is required to

be followed to ensure the results obtained are accurate.

a) Determine whether four (<1.4x103 cells per ml) or eight successive dilutions

(<1.4x107) are required and set out sufficient packets of Modified Postgate B

Broth/American Petroleum Institute (API) broth/SRB/2 broth media, syringes

and needles.

b) Ensure sample is thoroughly mixed, prior to inoculation, unless otherwise

stated.

c) Draw 1 ml sample into a sterile syringe and needle and inoculate bottle 1A.

Using the same syringe and needle inoculate bottles 1B and 1C.

d) Discard the needle and syringe carefully, and shake the bottles vigorously to

ensure good mixing.

e) Check the level of liquid in each vial to ensure they are all equal ie a dilution

has not been missed.

f) With a new syringe and needle, draw 1 ml of the broth from bottle 1A and

inoculate bottle 2A.

g) Using the same syringe and needle inoculate bottles 2B and 2C with the broth

from 1B and 1C respectively. Discard the needle and syringe carefully, and

shake the bottles vigorously to ensure good mixing.

h) Check the level of liquid in each vial to ensure they are all equal ie a dilution

has not been missed.

i) With a new syringe and needle, draw 1 ml of the broth from bottle 2A and

inoculate bottle 3A.

j) Using the same syringe and needle inoculate bottles 3B and 3C with the broth

from 2B and 2C respectively. Discard the needle and syringe carefully, and

shake the bottles vigorously to ensure good mixing.

k) Check the level of liquid in each vial to ensure they are all equal ie a dilution

has not been missed.

l) With a new syringe and needle, draw 1 ml of the broth from bottle 3A and

inoculate bottle 4A.

32

m) Using the same syringe and needle inoculate bottles 4B and 4C with the broth

from 3B and 3C respectively.

n) Discard the needle and syringe carefully, and shake the bottles vigorously to

ensure good mixing. Check the level of liquid in each vial to ensure they are

all equal ie a dilution has not been missed.

o) If further dilution is required, label another packet of media from 5-8 and

repeat step 6.5 for the5th, 6th, 7th and 8th set.

p) Incubate the broths for a minimum of 28 days (+2 days) at 30°C ± 1°C (or

appropriate temperature). If an incubation temperature of 60°C ± 1°C or

above is being employed, incubate an uninoculated packet of media vials of

the same batch number along with the inoculated vials to allow comparisons

of the media after incubation at the higher temperatures.

q) Broths which become black or show a black precipitate are read as positive.

3.3.5 Health and Safety

During the test, for safety precautions, the personnel need to wear personnel

protective equipment which includes wearing a lab coat, gloves, mask and safety

glasses.

3.4 General Heterotrophic Bacteria (GHB) Test Procedure

To detect the GHB from the water sample, there is a standard test procedure that

needs to be followed. The test procedure follows the NACE Standard TM0194-94

[16].

3.4.1 Purpose

This method presents procedures for determining the numbers of aerobic and

facultative anaerobic heterotrophic bacteria in a range of industrial samples (with the

exception of food and potable water) which might be collected in the field or

supplied to the laboratory for testing. This might include aqueous fluids, oils,

sludge’s, metal coupons, etc. The systems may be operated at a range of

temperatures from < 10°C to 100°C.The NACE Standard refers to General

Heterotrophic Bacteria (GHB) and recommends three different types of media for

their enumeration:

33

3.4.2 Summary of Method

The media type use in this test procedure is Phenol Red Dextrose Broth (PRD) for

enumeration of aerobic and facultative anaerobic acid producing heterotrophic

bacteriaThe bacteria existance is determined by serial dilution in Standard

Bacteriological Nutrient Broth / Phenol Red Dextrose Broth or Thioglycolate Broth.

The broths are incubated for (minimum) 7 days (+ 1 day) at 30°C ± 1°C, or other

appropriate temperature dependant on the system under investigation. Depending on

the nature of the sample (fluid or solid) a suspension may have to be prepared in

Phosphate Buffered Saline (PBS) or appropriate diluents. The most applicable

method is selected from written procedures.

3.4.3 Apparatus

They are requirement need to be followed to ensure the smoothness of the test. The

apparatus need to be set up as below before starting the test.

Incubator set at 30°C (or other appropriate temperature) ± 1°C.

Standard Bacteriological Nutrient Broth/ Phenol Red Dextrose Broth /

Thioglycolate Broth in 9 ml ± 0.1 ml in crimp sealed serum vials.

1 ml syringes and hypodermic needles (25 gauge, 1.5 inch).

3.4.4 Test Procedure

To obtain data for the existence of the SRB, the following procedure is required to

be followed to ensure the result obtained is accurate.

a) Determine twelve successive dilutions are required and set out sufficient

packets of Standard Bacteriological Nutrient Broth or Phenol Red Dextrose

Broth or Thioglycolate Broth media, syringes and needles.

b) Ensure sample is thoroughly mixed prior to inoculation, unless otherwise

stated.

c) Draw 1 ml sample into a sterile syringe and needle and inoculate bottle 1A.

Using the same syringe and needle inoculate bottles 1B and 1C.

34

d) Discard the needle and syringe carefully, and shake the bottles vigorously to

ensure good mixing. Check the level of liquid in each vial to ensure they are

all equal ie a dilution has not been missed.

e) With a new syringe and needle, draw 1 ml of the broth from bottle 1A and

inoculate bottle 2A.

f) Using the same syringe and needle inoculate bottles 2B and 2C with the broth

from 1B and 1C respectively. Discard the needle and syringe carefully, and

shake the bottles vigorously to ensure good mixing.

g) Check the level of liquid in each vial to ensure they are all equal ie a dilution

has not been missed.

h) With a new syringe and needle, draw 1 ml of the broth from bottle 2A and

inoculate bottle 3A.

i) Using the same syringe and needle inoculate bottles 3B and 3C with the broth

from 2B and 2C respectively.

j) Discard the needle and syringe carefully, and shake the bottles vigorously to

ensure good mixing.

k) Check the level of liquid in each vial to ensure they are all equal ie a dilution

has not been missed.

l) With a new syringe and needle, draw 1 ml of the broth from bottle 3A and

inoculate bottle 4A.

m) Using the same syringe and needle inoculate bottles 4B and 4C with the broth

from 3B and 3C respectively.

n) Discard the needle and syringe carefully, and shake the bottles vigorously to

ensure good mixing. Check the level of liquid in each vial to ensure they are

all equal ie a dilution has not been missed.

o) If further dilution is required, label another packet of media from 5-8 and

repeat step 6.5 for the 5th, 6th, 7th and 8th set.

p) Incubate the broths for a minimum of 7 days (+ 1 day) at 30°C ± 1°C (or

appropriate temperature). If an incubation temperature of 60°C ± 1°C or

above is being employed, incubate an uninoculated packet of media vials of

the same batch number along with the inoculated vials to allow comparisons

of the media after incubation at the higher temperatures.

35

For Aerobic & facultative anaerobic heterotrophs, the positive media Phenol Red

Dextrose Broth the indication of positive growth colour will change from red to

yellow.

3.4.5 Health and Safety

During the test, for safety precautions, the personnel need to wear personnel

protective equipment which includes wearing a lab coat, gloves, mask and safety

glasses.

3.5 Weight Loss Experiment Procedure

3.5.1 Summary of Method

The method will include three medium which is represent by medium X that will act

as a constant, medium Y as medium contain organic nutrient and medium Z

containing Sulphate Reducing Bacteria. The coupon of mild steel will be immersed

in each medium the corrosion rate will be calculated in period of week 1, 7, 14, 21

and week’s 28.The experimental procedure will follow the NACE Standard

TM0194-94.

Experimental is shown below.

(1) The experiment that fabricated the pipelines condition will be fabricated in

the lab. The metal sheet sample with composition C 0.71, Mn 0.50, Si 0.30, S

0.015, P 0.02% Fe bal provided.

(2) Steel sheet samples were first abraded with emery paper from No.2 up to No.

0, and then polished with alumina from 1µm up to 0.05µm, then degreased

with an electrolytic solution at 70°C, washed with distilled water and finally

etched in HCI 7.4% for 30min and dried with ethanol and cool air.

(3) The microorganism studied was the Desulfovibrio desulfuricans. Solutions

with composition given in table 3.The Ph was adjusted to 7.6.All experiments

were carried out at 37°C. Table 1 below show the composition of 1 litre of X,

Y and Z.

36

Medium Compounds Amounts Concentration (mol.dm-3)

  NH4Cl 2g 3.74x10-2

  MgSO4.7H2O 2g 8.0x10-3

X K2HPO4 0.5g 2.9x10-3

  Na2SO4 4g 2.8x10-2

  FeSO4.9H20 0.010g 3.6x10-6

  CaCl2 0.2g 1.8x10-3

  Na2S.9H2O 0.25g 1.1x10-3

 Modified Wolfe’s

Minerals 1ml    Medium X 987.5ml 8.7x10-2

Y Sodium Lactate 60% 12.5ml    Yeast extract 1g 1.6x10-3

  Cysteine-HCL 0.25g  Z Medium Y 900ml  

  Bacterial Culture 100ml  

(4) Prior to each experiment the solutions need to be purged with nitrogen to

ensure anaerobic conditions. All the media will be sterilised at 120°C.

(5) A batch of steel samples treated as above will accurately weighed and then

place in a transversal position in bottles of about 30ml volume and sterile at

120°C for 30 min.

(6) Afterwards the bottles were completely filled with the solutions X or Y

previously well deaerated and sterilised under the same conditions.

Table 3: Composition of 1 Litre of X, Y and Z

37

(7) Inoculation, to obtain solution Z, will be made at this time. The bottles then

sealed with black screw caps (to maintain anaerobic conditions and to

prevent unwanted contamination). Finally they were incubated at 37°C.

(8) The bottles then are exposing for periods of 28 days than the steel sample is

taken from the respective bottle. The sample then is re-weighing.

All the data gained will be entering in the Table 4 and the graph corrosion rate Vc as

a function of exposure time and medium.

Table 4: Vc (mgcm-2d-1) of steel samples immersed in X, Y and Z media

Time(Days) Medium X Medium Y Medium Z

1

7

14

21

28

38

CHAPTER 4RESULTS AND DISCUSSION

4.1 RESULT

The number of bacteria present in the original 1 ml of water injected into the first

bottle can be estimated using table below.

Table 5: Bacteria Growth Interpretation

Bottle

Showing

Growth

Dilution

Factor

No of Bacteria

Indicated

(Bacteria/ml)

Number of

bacteria

reported

(Bacteria/ml)

1 1:10 1 to 9 10

2 1:100 10 to 99 100

3 1:1000 100 to 999 1000

4 1:10000 1000 to 9999 10000

5 1:100000 10000 to 99999

100000

6 1:1000000 100000 to 99999

1000000

For example, if bottles 1, 2 and 3 show growth, but bottles for through 6 remain

clear, then the water contains 100-999 bacteria per millilitre, and a count of 1000 per

ml is reported. If only one bottle show growth and the rest remain clear, then the

water contains 1-9 bacteria per millilitre, and a count of 10 bacteria per ml is

reported.

For the result, it’s expected that it would show a differences of weight before and

after the exposure sample to the medium. These differences of weight loss will be

calculated for each sample.

39

The variation of the corrosion rate as a function of exposure time for each medium

will be presented in graph form. From the graph, it would be expected that the living

bacterial medium is the most aggressive during the first day of exposure.

The result obtained by monitoring the SRB / GHB test will be entered in the SRB /

GHB result form. Table 6 is on the next page show the result for the SRB

monitoring. For the GHB result, the result shown in table 7.

40

Table 6: SRB Result Form

Platform : Drilling Production Platform pH: 7.08 Pipeline No: Well 15L Temperature: 30.7Sample Date: 4/9/2008Sample Location: Drilling Production Platform

41

Table 7: GHB Result Form

Platform : Drilling Production Platform pH: 7.08 Pipeline No: Well 15L Temperature: 30.7Sample Date: 4/9/2008Sample Location: Drilling Production Platform

42

4.2 Corrosion Rate Determination

Assumption Made

Assuming that localized or internal attack is not present, the corrosion rate expressed

as millimetres per year can be calculated by equation:

mm/y = ____mass loss x 87.6___ (Area)(Time)(Metal density)

Where test specimen mass loss is expressed in mg, area in cm2 of test specimen

surface exposed, time in hours exposed, and the metal density in g/cm3.The corrosion

rate expressed as mils per year can be calculated by:

mpy = mass loss x 534.57 (Area)(Time)(Metal density)

Where test specimen mass loss is expressed in mg, area in2 of metal surface exposed,

time in hours exposed, time in hours exposed, and the metal density in g/cm3.

Table 4: Vc (mgcm-2d-1) of steel samples immersed in X, Y and Z media

Time(Days) Medium X Medium Y Medium Z

1 0.7479 0.6033 1.0376

7 0.1261 0.1515 0.1100

14 0.1140 0.0874 0.0195

21 0.1352 0.0744 0.0169

28 0.1102 0.0819 0.0110

Mass loss = Weight Before - Weight After

43

Figure 18 below illustrate the specimen in transversal position in three different

medium. The specimens will be immersed in the medium for 28 days to show

different corrosion rate for each specimen. Each mild steel 1cm x 4cm sample

weighs 20 g with density of 7.86g/cm3 .

Figure 18: Three different culture media with specimen in transversal position

Corrosion rates evaluated from the difference in weights before and after exposure

are given in Table 2. The variation of the corrosion rate as function of time for each

medium is represented in Figure 2 below:

Figure 19: Plots of corrosion rate Vc as a function of exposure time and medium

Medium XConstant 35 ml

Medium Y Organic Nutrient 35 ml

Medium ZSRB 35 ml

44

From Table 4 above it clearly show that organic nutrients which are medium Y cause

only a slight decrease in the corrosion rate of mild steel: 0.6033 against 0.7479 mg

cm-2 after one day of exposure and 0.0819 against 0.1102 mg cm-2 after 28 days.

Living SRB, in medium Z, cause a big increased in the corrosion rate during the first

day with a VC =1.0367 against VC = 0.6033 mgcm-2 d-1 in medium Y. But after 7 days

it is reduced to Vc = 0.1100 mgcm-2 d-1 in medium Z and Vc = 0.1515 mgcm-2 d-1 in

medium Y .

These data show that the corrosion rate of mild steel drops steeply to a very low

value due to the presence of SRB (94% in medium Z compared with 7% in medium

Y).

Medium Y contains the organic nutrients, increasing the complexity of the system.

The presence of yeast extract may explain the decrease in the corrosion rate observed

over long periods of exposure after two weeks. Medium Z suffers change in the

concentration of SO42-, S2- and protons as a consequence of bacterial growth.

4.3 DISCUSSIONS

There are a number of common occurrences which can interfere with interpretation

of the results obtained using the SRB test techniques. Common problem as discussed

below.

All Bottles Show Growth: If all bottles show growth, then it is not possible to

estimate the population. For example, if all 6 bottles in a series are positive,

you would have to report the population as “equal to or greater than 1 000

000 per mL”.

Bottle Skipping: Sometimes one of the bottles in the middle of a series

remains clear while bottles on either side of it show growth. If this occurs,

the skip should be noted and the numbers of bacteria corresponding to the

highest numbered bottle in the series which shoed growth should be reported.

45

The SRB and GHB test method permits the estimation of the number of planktonic

bacteria floating in the water. Since most of the bacteria in a system will be a sessile

bacteria attached to solid surfaces, it is a poor way to asses the number of bacteria

actually living in the system.

The corrosion rate is significant in systems where uniform corrosion occurs.

However, it is relatively meaningless in pitting system because all the weight loss is

occurring in a few isolated spots. The number, depth and diameter of the pits should

be noted.

Specimen exposure times vary depending on the corrosivity of the system. Corrosion

rates usually start out high on the fresh metal surface of the coupon and very short

exposures may give unrealistic high rates. Exposure periods of 30-90 days are very

common involving laboratory test.

In the determining the relative corrosion resistance, Table 2 below will be used as

the comparison of mpy with equivalent Metric Rate Expressions.

To determine the mpy, the equation below will be used:

RelativeCorrosionResistance mpy mm/yr µm/yr nm/h pm/sOutstanding <1 <0.02 <25 <2 <1Excellent 1-5 0.02-0.1 25-100 2-10 1-5Good 5-20 0.1-0.5 100-500 10-50 20-50Fair 20-50 0.5-0.1 500-1000 50-150 20-50Poor 50-200 1-5 1000-5000 150-500 50-200Unacceptable 200+ 5+ 5000+ 500+ 200+

Table 2: The comparison of mpy with equivalent Metric Rate Expressions

mpy = 534 W DAT

46

D = Density in grams per cubic centimetre/cm-3

A = Area in square inch in2

T = Time in hours

The mpy will be determined from the results of corrosion rate obtained from the first

day of exposure. Mass loss, area and mpy obtained from each specimen in medium

X, Y and Z is listed in Table 8 below.

Table 8: mpy data for specimen X, Y and Z

Specimen X Specimen Y Specimen Z

Mass Loss 1 Mass Loss 2 Mass Loss 3

0.6mg 0.48mg 5mg

Area 1 Area 2 Area 3

6.2x10-3 in2 0.014 in2 0.038 in2

mpy mpy mpy

42.4 33.9 372.4

From the table above, the mpy obtained after calculation for medium X is 42.4 mpy.

By referring to Table 2, the mpy value is in between range 20-50. The condition of

the material for medium Z is in fair condition. For specimen Y, the mpy value is

33.9 mpy. Same like specimen X, the range fall in fair condition that is in between

20-50.

For specimen Z, the obtained value for mpy is 372.4, the mpy value fall out of range

200 + and the relative Corrosion resistance is unacceptable. From the comparison, its

clearly show that the specimen Z, which is immersed in Sulphate Reduce Bacteria

medium shows it has the highest corrosion rate and the resistance of the material in

unacceptable.

CHAPTER 5

CONCLUSION

47

5.1 CONCLUSION

In this study, two different laboratory techniques, the Sulphate Reduce Bacteria Test

(SRB) and General Heterotrophic Bacteria (GHB) Test were used, in order to

determine the existence of SRB and GHB Bacteria. The weight loss of corrosion

cause by SRB was also calculated by a running the weight loss measurement

experiment. At the end of the experiment, different culture media would give

different corrosion rate for each specimen. The culture media which containing

Sulphate Reduce Bacteria clearly show that it is significant in causing corrosion.

The first step in selecting control method is to find out what is causing the corrosion.

It is very important in selecting the biocide. After the bacteria existence was

measured and identified, a suitable biocide could be determined to eliminate the

bacteria’s existence. Usually the biocide would not totally terminate the bacteria but

at least it could reduce the quantity which then reduces the corrosion activities.

Prevention of MIC requires frequent mechanical surface cleaning and treatment with

biocides to control populations of bacteria. Biocide treatments without cleaning may

not be effective because organisms sheltered beneath deposits may not be reached by

the injected chemicals.

REFERENCES

48

[1] Howard J. Endean “Oil Field Corrosion Detection and Control”

[2] Dr. Charles C. Patton “Applied Water Technology Book”Second edition

[3] Offshore Pipelines Book   By Boyun Guo

[4] Scotto, V; Di Cintio, R; Marcenaro, G “The Influence of Marine Aerobic Microbial Film on Stainless Steel Corrosion Behaviour”

[5] B.J. Little, P.A. Wagner, F. Mansfeld, Microbiologically Influenced Corrosión <http://www.sciencedirect.com/ >

[6] H.A. Videla, Manual of Biocorrosion, CRC Lewis Publishers, USA <http://www.corrosionsource.com/technicallibrary/ >

[7] J.R. Postgate, The Sulphate Reducing Bacteria, Cambridge University Press, Cambridge

<http://scielo.isciii.es/ >

[8] Oil and Gas Management (M) Sdn Bhd Procedure

[9] A.K Tiller,Microbial Corrosion,1983,The metals Society,London

[10] R. Cord-Ruwisch,W.Kleinitz and F.Widdel,’Sulphate-Reducing Bacteria and their activities in Oil Production,’J. Petroelum Technol,1987

[11] W.A.Hamilton,’Sulphate reducing Bacteria and anaerobic corrosion, Ann.Rev.Microbiol’ 1985

[12] J.R Postgate,The Sulphate-Reducing bacteria,2nd Edn,Cambridge University Press,Cambridge,UK,1984

[13] T.Ford and R.Mitchel,’The Ecology of Microbial Corrosion,’Adv. Microb.Ecol.

[14]Patton,C.C”Petroleum Production Stringent Corrosion Control Procedures Key to extende life,”

[15]Method can improve Corrosion Evaluation,” Oil and Gas J.(Nov.12,1984) 144

[16]

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