Print this page Corrosion Mitigation through Electrical Design Cathodic Protection
A. Bhatia, B.E.
Course Outline
Corrosion occurs
on all metallic structures that are not adequately protected. The cost of replacing
a structure which may have been destroyed or weakened due to excessive corrosion
is substantial but avoidable, and means should be taken to consistently prevent
or mitigate this added cost through cathodic protection. This course will introduce
the concepts of sacrificial and impressed current cathodic protection with basic
details and examples to understand their field application.
This 4-hr course material is based entirely on the US Army Corps of Engineers
document EM 1110-2-3400, " Painting: New Construction and Maintenance "and
covers Chapter 2; "Corrosion Theory and Corrosion Protection" and
document TM 5-811-7, "Electrical Design, Cathodic Protection".
The course includes a multiple-choice quiz at the end, which is designed to
enhance the understanding of course materials.
The course also provide a link to the reference document ETL 1110-9-10(FR),
"Engineering and Design - Cathodic Protection System Using Ceramic Anodes"
for advanced reading, although it is not covered in a quiz.
Learning Objective
At the conclusion of this course, the reader will:
- Understand the basic types, forms and mechanisms of corrosion;
- Understand what happens to metal during the reduction step of the oxidation-reduction process;
- Learn the engineering design considerations for corrosion mitigation;
- Describe the fundamental relation that exists between sacrificial anode materials and the galvanic series;
- Describe the importance of cathodic protection to protect systems against corrosion;
- Explain the basic differences between sacrificial and impressed current cathodic protection;
- Describe the advantages and drawbacks of the sacrificial and impressed current cathodic protection design;
- Determine the type and design of cathodic protection system;
- Describe the common anode materials used in sacrificial and impressed current cathodic protection design;
- Understand the methods of soil resistivity measurement ;
- Understand the importance of current requirements; and
- Learn through
examples the quantity of anodes and step by step use of design procedure.
This course is aimed at students, engineers, designers, maintenance managers, material science and process engineers, H &S professionals, supervisors, and anyone who wants a basic understanding of corrosion protection.
Course Introduction
The basic principle
of cathodic protection is simple. A metal dissolution is reduced by applying
a protective electric current to the structure surface which is immersed and
in contact with an electrolyte. In the presence of certain other metals contacting
the electrolyte near the structure, this technique transforms the structure
into a cathodic electrode. A properly selected and designed cathodic protection
system can prevent surface corrosion of the structure, or drastically reduce
the rate at which it occurs.
Cathodic protection is often applied to coated structures, with the coating
providing the primary form of corrosion protection. The CP current requirements
tend to be excessive for uncoated systems.
In this course, you will learn about the concepts of sacrificial and impressed
current cathodic protection.
Course Content
In this course, you are required to study following US Army Corps of Engineers documents:
TM 5-811-7, "Electrical Design, Cathodic Protection"
ETL 1110-9-10(FR), "Engineering and Design - Cathodic Protection System Using Ceramic Anodes" (Optional advanced reading, not covered in the quiz)
You need to open
or download above documents to study this course.
Course Summary
Cathodic protection
enables to preserve the outer surface of buried or immersed steel structures
by guarding them against electrochemical attacks of the metal by the ambient
surroundings. The steel pipes are the main fields of use of this protection.
Even old and damaged steel networks can benefit from this technique in the most
economic conditions possible.
Sacrificial Cathodic Protection System: This type of system helps reduce
surface corrosion of a metallic structure immersed in an electrolyte by coupling
a less noble metal with the structure. Sacrificial CPS work through the sacrifice
of an anodic metal, i.e., one that has a negative electrochemical potential
relative to the protected ferrous structure. Sacrificial anodes for fresh water
applications typically are composed of zinc- or magnesium-based alloys. In the
past, installation of sacrificial anodes has often been done on an ad hoc basis,
relying largely on the installer's individual knowledge and experience.
Impressed current Cathodic Protection System: This type of system uses
direct current applied to an anode system from an external power source to drive
the structure surface to an electrical state that is cathodic in relation to
other metals in the electrolyte. A number of impressed current anode materials
and geometries are used. Materials include mixed metal oxides, precious metals
(e.g. platinum-clad titanium, niobium), and high-silicon chrome-bearing cast
iron. The most common geometries are slab or button anodes, rods, and strings.
Any anode mounted on the structure must be isolated with a dielectric shield
to assure effective current distribution.
Cathodic Protection System Selection: When selecting which type of system
to use, the designer should consider the size of the structure to be protected
and past project experience in operating and maintaining both types of systems.
Early in the selection process, if practical, it is useful to perform a current
requirement test to help define the total amount of electrical current needed
to protect the structure. For large structures with significant expanses of
bare or poorly coated metal, where the total current requirement tends to be
very high, a properly maintained impressed current system can provide 10 to
30 years of effective corrosion protection. Where current requirements are lower
and the structure's protective coatings are well maintained, sacrificial anode
systems can be very effective. Improved modern coating systems and maintenance
practices today allow for a wider use of sacrificial cathodic protection. For
both types of systems, preliminary design estimations and comparisons of costs,
current output, and overall design life should give an adequate indication of
which system is preferable for the specific application. Other factors such
as future maintenance needs, reliability, accessibility, and impact on operations
may also warrant consideration.
Basis for selecting
a sacrificial anode system:
1) External power
source is not required
2) Installation is less complex since an external power source, including rectifier,
is not required
3) System works very well when electrolyte resistivity is low, surfaces are
well coated, structure is easily accessible, and significant deterioration of
the coating is not expected within 5 to 10 years
4) System is easier to install on moving complex structures such as tainter
valves where routing of cables from an impressed current system could present
a problem.
Basis for not
selecting a sacrificial anode system:
1) Current output
per anode is low and may not be sufficient to protect large structures with
significant expanses of uncoated or poorly coated bare metal
2) System generally cannot be economically justified where large surface areas
of a poorly coated metallic structure require protection
3) Anode replacement expenses and/or the number of anodes required can be high
compared with impressed current systems for structures with high current requirements
4) Current output cannot easily be adapted to seasonal changes in water resistivity
or to unexpected changes in coating coverage caused by weathering, routine wear,
or impact damage due to debris, ice, or aquatic vessels
5) Due to the buildup of algae, silt, or other deposits on sacrificial anodes,
current output to the structure may be reduced
6) Monitoring system operation in accordance with NACE criteria is labor-intensive
and inconvenient because it requires that structure-to-electrolyte potential
measurements be taken in the field.
Basis for selecting
an impressed current system:
1) Can be designed
for a wider range of voltage and current applications
2) Higher total capacity (i.e., ampere-years) can be obtained from each installation
3) One installation can protect an extensive area of the surface of a metallic
structure
4) Voltage and current can be varied to meet changing conditions, providing
operational flexibility that is very useful to increase protection of the surface
coating
5) Current requirement can be read and monitored easily at the rectifier
6) System can be designed to protect bare or poorly coated surfaces of metallic
structures
Basis for not
selecting an impressed current system:
1) First costs
for design, acquisition, and installation are high.
2) Installation is complex due to the need for an external power supply, cabling,
and numerous electrical connections
3) Maintenance costs can be high
4) System can create stray currents that may potentially corrode other nearby
ferrous structures
5) If an excessive amount of current output is used, hydrogen gas may form between
the substrate and coating, causing paint blistering or possible hydrogen-embrittlement
of high strength steel.
Quiz
Once you finish studying the above course content, you need to take a quiz to obtain the PDH credits.
DISCLAIMER: The materials contained in the online course are not intended as a representation or warranty on the part of PDH Center or any other person/organization named herein. The materials are for general information only. They are not a substitute for competent professional advice. Application of this information to a specific project should be reviewed by a registered architect and/or professional engineer/surveyor. Anyone making use of the information set forth herein does so at their own risk and assumes any and all resulting liability arising therefrom.
