Corrosion Prevention
06.13
Corrosion can be defined as the destruction or
deterioration of a material because of reaction with its environment. Corrosion
is a natural occurance and inevitable. Especially in seawater environment,
corrosion is a threat for carbon steel pipe (offshore pipeline). Corrosion will damage pipeline
and leads to pipe leak in which will be dangerous for the circumstances
surround. Petroleum industry spends a million dollars per day to protect its
pipelines. And so, there is urgency to protect and prevent pipeline from
corrosion.
There
are several methods that can be used to prevent and decrease the rate of
corrosion on offshore pipeline. These methods are:
MATERIAL SELECTION
This method is just simply selecting the best and
appropriate alloy carbon steel to a particular environment. For instance, the
use of nickel-based alloy steel allows pipeline to withstand seawater environment
without putting additional
sacrificial anodes or impressed current, yet it’s far more expensive than
having ordinary carbon steel with cathodic protected.
USE OF INHIBITOR
Sometimes
corrosion in offshore pipeline attacked from inside (compounds brought by the
fluid inside pipe e.g. sulphate). This can be helped by adding inhibitor.
Inhibitor is a substance that when added in small concentrations to an
environment, decreases the corrosion rate, such as chromate and nitrate.
CATHODIC PROTECTION
Cathodic protection is achieved by supplying
electrons to the metal structure to be protected. Basically, cathodic
protection has the pipeline become cathode, instead of anode, that way it won’t
be corroded. There are two ways to cathodically protect a stucture. Firstly, Impressed Current Cathodic
Protection (ICCP) and Sacrificial Anode Cathodic Protection (SACP).
1. Impressed Current Cathodic
Protection (ICCP)
ICCP
supplies electron by flowing electrical current from a power supply. This
method is suitable for large structures regarding cost. For
pipelines, anodes are arranged in groundbeds either distributed or in a deep
vertical holes depending on several design and field condition factors
including current distribution requirements.
2. Sacrificial Anode Cathodic
Protection (SACP)
This method is also known as Galvanic Coupling. In
the usual application, a galvanic anode, a piece of a more electrochemically
“active” metal, is attached to the vulnerable metal surface where it is exposed
to the corrosive liquid. Galvanic anodes are designed and selected to have a
more “active” voltage (more negative electrochemical potential) than the metal
of the target structure.
COATING
Relatively
thin coatings of metallic and inorganic materials can provide a satisfactory
barrier between metal and its environment. The chief function of such coatings
is to provide an effective barrier.
Coating can be in the form of, for example, cladding. Cladding
involves a surface layer of sheet metal put on by rolling two sheets of metal
together. For instance, a nickel and a steel sheet are hot-rolled together to
produce a composite sheet with, say, 1/8 inch of nickel and 1 inch of steel.
This way the steel are protected with its environment since nickel is layered
on the surface. Moreover, in the application for offshore pipeline, high density polyethylene(HDPE) and polypropylene layer
can be coated on pipe bare surface. Both HDPE and polypropylene coating have
low water permeation which will improve isolation of the pipe from seawater
surrounds. Coating for pipeline is illustrated as below:
Sumber:
http://en.wikipedia.org/wiki/Cathodic_protection
http://met-engineering.blogspot.com
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