chapter 1 - universiti teknologi petronasutpedia.utp.edu.my/3742/1/final_dissertation.doc · web...
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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
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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].
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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.
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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
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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
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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.
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Figure 10: Senescence [13]. (a) Cells dissolve matrix and are released.
Final Stage(a)
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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
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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
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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.
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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
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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
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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).
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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.
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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:
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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.
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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.
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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.
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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
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(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
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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.
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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.
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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
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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
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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
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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
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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.
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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
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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
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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
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[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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