To begin with, before
giving detailed information about the reasons of corrosion there is a need to
stress that pipeline is not consists of pipes only. Pipeline is just
combination of all facilities joining to the main line: all parts, including
branches, physical facilities by means of which all types of hydrocarbon fluids
are transported, namely pipe, valves, compressor units, pump stations, metering
stations, regulator stations, delivery stations holders and fabricated
assemblies which comprises pipeline altogether.

There are many factors which can not be
undermined, lead to corrosion, so in order to minimize or prevent the
destructive power of corrosion, considerable steps should be taken. The factors
that lead to corrosion are the material itself and environment for it called
medium (sea, air, soil) where it is expected to take place. It can also be
associated with mechanical, chemical or environmental factors as well as human
activities. The other considerable reasons that may lead to corrosion are
facility age, poor functional performance because of deterioration and not
properly design as well as the nature of materials which it contains.

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When the material used for preparation of facility is
near or at the last stage of its demanded useful life, corrosion is highly
probable to occur because at this time materials’ properties are deteriorated because
of long usage.

One of the main factors that affects the corrosion
rate is the nature of the material, since some materials are more vulnerable
and less resistant to other material properties (corrosivity, salinity, oxygen
content etc.) so in terms of their usage, the stage at which corrosion occurs
would be defined. The physical and chemical factors of place should also be
considered where an oil and gas facility is located; so while discussing about
corrosion, material selection 1 should be considered carefully regarding with
the material’s mechanical and chemical resistance. For example, before
construction pipeline in seawater, the factors below should be considered:

·        
General and
localized corrosion rate under steady state condition

·        
Possibility of
crevice corrosion occurrence and pitting especially under stagnant moving
condition

·        
Resistance to
stress corrosion cracking

·        
Effect of seawater
composition, such as salinity, oxygen content and other pollutants

In addition to necessary
medium-environment (air, water, soil), corrosion is created by mainly the
reasons given below associated with production or transportation of
hydrocarbons.

Vibration and Cavitation

The amount of vibration within a facility
created as a result of fluid flow during production and transportation causes
wear and tear through its continuous effects, especially when these effects are
not seriously checked through the providing of shock absorbers or expansion
bellows if necessary. Even when there are wedges or stacks on pipes, if there
is even slight displacement of fluid movement due to continuous flow of oil and
gases, those points will wear out.  In addition
to this, when the fluid flows under turbulent effect due to high velocity of
corrosive fluids some parts of pipeline, namely elbows of pipes and pump
propellers are wearing out which is called cavitation due to destructive effect
of corrosive elements. The cavitation of elbows and other parts of facilities
deteriorate the facility as a result of which corrosion rate increases. 2

Sweet Corrosion – Carbon dioxide (CO2)

While
considering chemical issues as one of the significant factors leading to
corrosion, during pumping crude oil from reservoir, which actually may flow as
associated or non-associated hydrocarbons, the chemicals containing in that
fluid flow react different from each other within diverse facilities. Carbon
dioxide can be taken as an example which is one of the natural components of
hydrocarbon that together with other components such as helium, nitrogen,
hydrogen sulfide and water, it would be dissolved in water and become corrosive
3. After carbon dioxide dissolved in water iron carbonate (siderite scale) is
produced and although it is a protective corrosion product, because of it,
pitting and other forms of metal loss may occur contingent on flow condition
through pipeline. Siderite is soluble scale in its turn. The contributing
factors that enables siderite to form and remain in certain place are increased
temperatures, high pH value which is encountered in bicarbonate-bearing waters
and lack of turbulence. Turbulence is considered as a critical factor during
generation of iron carbonate film. Since the siderite does not have
conductivity it does not allow galvanic corrosion to occur. That’s why when
siderite mineral has not formed it enables corrosion to occur. After forming of
carbonic acid crevice and pitting corrosion may take place. Due to carbon
dioxide, embrittlement may form which in its turn leads to stress corrosion
cracking. Corrosiveness of CO2 depends on partial pressure of it as
given below:

·        
P(CO2)>30psi – corrosive

·        
3psi

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