|
Corrosion
Analysis
At the conclusion
of the field investigation, data is analyzed and correlated to determine
the present condition of the pipe or structure under study and the
expected remaining life. Data
can be used in life cycle cost analysis to delineate how the economics
of management of leaks costs verses management of failure prevention
costs are justified. As
critical operation and reliability of the structure increases, the
importance of corrosion assessment/definition and control increases.
This type of
study may be conducted concurrently with other investigations, or
independently of other investigations depending on what is being
considered for the structure.
There may be other considerations independent of corrosion
requiring structure modification or replacement.
Corrosion
Mitigation
Once the corrosion
mechanisms are defined and the pipe condition is understood, the
decision for continued operation can be made.
Corrosion mitigation solutions would depend on what was determined
in the initial phases of the study.
Poorly engineered corrosion solutions will not perform as
required in the long run, thereby falling short of expectations.
The goal of this phase of the study, is to implement solutions
that facilitate management of the operating water system, not continued
management of failures and leaks.
Corrosion mitigation
solutions may involve a number of applications, often applied in
combination. Which
corrosion mitigation solution(s) are to be applied is dependant
on material, coating quality, electrical continuity, exposure from
structure corrosion failure and sensitivity due to the location
of the structure with respect to other facilities, communities or
environments. Generally,
the corrosion mitigation solutions for existing infrastructure include
some or all of the following:
These options
are also utilized extensively in new design of infrastructure with
the additional consideration of the materials of construction.
Corrosion problems can be eliminated through the utilization
of other materials, based on the corrosion survey and analysis performed
during the design phase of a project.
Cathodic
Protection (Impressed Current or Sacrificial Anode)
Application
of cathodic protection is a proven electrochemical method for arresting
corrosion on metallic structures.
Cathodic protection converts all active anode sites on the
structure, the areas that corrode, into cathode sites that do not
corrode. New anode
sites are provided through the installation of anode ground beds.
It is important
to understand that corrosion is only mitigated on the structure
metallic surfaces that are in contact with the electrolyte.
In the case of a water storage tank, only the portions below
the water line would receive cathodic protection.
The roof and wall sections out of the water receive corrosion
protection through the protective coating system.
For underground piping, cathodic protection would provide
protection to the bare metal in contact with the soil.
Where there is intact, well bonded coating isolating the
metal from soil contact, no cathodic protection is required.
Therefore, cathodic protection requirements and efficiency
is dependant on the structure coating system.
Anode size,
quantity, location and other operating parameters, need to be designed
to ensure proper operation. Much of the field data obtained in the
earlier phases of the investigation would be used in the system
design. These include
soil resistivity, structure electrical continuity and stray current
exposure.
Energy for an
impressed current system is provided by a power supply or rectifier.
This is an electrical device which converts AC power to DC
power. The rectifier
provides a positive current supply to the anodes and a negative
current return from the structure.
For this circuit, Ohms law applies: E=IR, where E is the
driving voltage of the rectifier output voltage, I is the current
magnitude that results from the resistance of the circuit R.
Proper system design seeks to minimize the resistance of
the circuit through anode ground bed design.
Impressed current systems are capable of small to very large
energy output levels through proper design.
This range of ability allows protection possibilities for
poorly coated pipe, large structures, automatic control and other
options in design and operation.
Sacrificial
anode cathodic protection provides protection in the same manner
as the impressed current system, except there is no power source
or rectifier. Sacrificial
anodes are provided through the system design to corrode, thereby
protecting the steel structure of interest.
PAGE
1 2 3 4 5 6 7 8 9 next
|
