Case Study-Corrosion Protection
Case Study-Corrosion Protection
Case Study-Corrosion Protection
PIPELINE CORROSION
How Do We Control Pipeline Corrosion?
• CATHODIC PROTECTION
• MATERIAL SELECTION
• COATING (Metallic Spray Coatings, Coal Tar Enamel Coatings,
Polyolefin, Epoxy (Fusion-Bonded))
• CLEANING ( Black Oxide, Wax, Sand,etc)
PIPELINE PIGGING ( PIPELINE INSPECTION GAUGE)
CATHODIC PROTECTION
• Cathodic protection (CP) is one of the few methods that successfully
mitigates corrosion. It can be applied in any situation where the
environment surrounding the metal acts as a conductor for electric
current. It has been successfully applied to offshore structures, ships,
boats, propellers, moorings, pipelines, storage tanks, piers, jetties,
bridges, aquaria, instrumentation etc.
STRUCTURE
CP design begins with a thorough understanding of the structure to be
protected. This includes the following information:
• Metal type(s)
• Operating conditions
• Dimensions and surface area
• Coatings
• Data from previous structures and CP systems
• Design life
LOCATION
• The environmental conditions are determined by the location of the
proposed installation. Factors such as climate, electrolyte conductivity
and chemistry, physical loading, and biological activity, all impact CP
requirements. These factors are generally allowed for in the CP
current demand and polarization potential criteria.
CP Criteria
• Potential measurements are the most commonly used criteria to
ascertain the level ofCP afforded to metals and alloys.
• CP potential values vary according to the metal and the environment.
Corrosion is likely to occur at potentials which are more positive than
the protected value. Damage may also occur if the metal is
overprotected (i.e. The potential too negative).
• The most common error associated with potential measurements is a
result of IR drop. This is the displacement of measured metal
potential due to current flow through the electrolyte. High electrolyte
resistivity and high current densities can cause significant differences
between the measured and actual metal potential.
CATHODIC PROTECTION CURRENT DEMAND
• For pipelines with the following coating systems, another coating breakdown factor is
calculated.
⇒ asphalt + concrete weight coating
⇒ fusion bonded epoxy + adhesive + polyethylene or polypropylene
⇒ polychloroprene rubber
⇒ equivalent coating systems based on an inner layer dedicated to corrosion
protection and one or more outer layers for mechanical protection.
Concrete
• It is now recognized that cathodic protection of concrete reinforcing
steel is necessary to ensure the long term integrity of the structure.
Also, any CP system designed to protect metallic appendages and
components must be designed to allow for current drain from CP to
the reinforcement. The cathodic current density is determined by
transport of oxygen to the steel by capillary action of pore water
driven by evaporation in the atmospheric zone and internal dry
compartments. The current densities are, therefore, dependent on
depth and climatic conditions.
Current Requirements for Pipelines in Soils of
Different Types
• The current demands for steel pipelines are determined by the soil
type (conductivity, pH, moisture, temperature) and the condition of
the steel (coating type). An example of typical CP current demand for
a pipeline with different coating conditions is presented as follows
(next slide)
Range of current required to protect 10 miles of 36" diameter pipe in soil with average resistivity of
1000 ohm-centimeters. Current required is that needed to cause a 0.3 Volt drop across the effective
resistance between pipeline and remote earth. [from A.W.Peabody, of Pipeline Corrosion,NACE, 1967]
Cathodic Protection Types
• The CP type determines how the cathodic current is supplied to the
structure.
• CP can be applied by either an impressed current system or by a sacrificial
anode system.
• Impressed current CP systems use an external DC current source and a
variety of anode materials to supply the cathodic current.
• Sacrificial anode CP systems generate the cathodic current from the
corrosion of metals less noble than the metal to be protected.
• The choice between impressed and sacrificial cathodic protection depends
many factors and may be just personal preference.
• There are, however, situations where one or the other provides the correct
choice.
Anode Selection
• Anodes, for both impressed current and sacrificial anodes, are
selected according to their size and chemical composition. This
determines the current output and design life.
Impressed current anodes
Sacrificial Anode Type
Anode Size, Weight, Number, Distribution and
Design Life
• The calculations for impressed current CP systems are relatively
simple. In this case, it is only necessary to match the number of
anodes of known current output to the total current demand of the
structure, and to be sure that the anode distribution insures an even
and well balanced current distribution.
• The calculations for sacrificial CP systems are a little more complex.
Not only must the number of anodes satisfy the current demand of
the structure, but they must also have sufficient mass to provide
electricity for the design life of the structure