CHAPTER 11
CATHODIC PROTECTION
This chapter contains a discussion about the
Agueous-- Pertaining to water; for example, an aqueous
cathodic protection systems installed in naval ships.
solution is a water solution.
there are two systems--the sacrificial anode system
Base-- A solution that contains an excess of hydroxyl
and the impressed current system. The two systems are
ions and exhibits a pH above the neutral value of 7.
different in both construction and operation.
Cathode-- The electrode of an electrochemical cell at
which reduction is the principal reaction. The
LEARNING OBJECTIVES
electrode where corrosion usually does not occur
unless the electrode metal is amphoteric.
Upon completion of this chapter, you will be able to
do the following:
Cathodic corrosion-- Corrosion of a metal when it is a
cathode. Cathodic corrosion occurs on metals, such
1. Recognize the purose of cathodic protection.
as Al, Zn, Pb, when water solution turns strongly
2. Identify the two major types of cathodic
alkaline as a result of the normal cathodic reactions.
protection available.
It is a secondary reaction between the alkali
generated and the amphoteric metal.
3. Identify the major components of both types of
cathodic protection systems.
Cathodic polarization-- The change of the electrode
potential in the negative direction due to current
4. Define various terns used to descibe cathodic
flow.
protection components and processes.
Cathodic protection-- A technique or system used to
reduce or eliminate the corrosion of a metal by
TERMS
making it the cathode of an electrochemical cell by
means of an impressed direct current or attachment
Several terms used with this chapter may be
of sacrificial anodes, such as zinc, magnesium, or
unfamiliar to you. Because you'll need to understand
them, they are defined at this point in the chapter.
aluminum.
Cell-- A circuit or system consisting of an anode and a
Acid-- A solution that contains an excess of hydrogen
cathode in electrical contact in a solid metal or liquid
ions and exhibits a pH below the neutral value of 7.
conducting environment.
Active-- A state in which a metal tends to corrode
Corrosion-- The reaction between a material and its
(opposite to passive); freely corroding.
environment that results in the loss of the material
Amphoteric metal-- A metal capable of reacting
or its properties. For example: when used as a
chemically either as an acid or as abase.
material of construction, the transformation of a
metal from the metallic to the nonmetallic state.
Anode-- The electrode of an electrochemical cell circuit
where corrosion occurs, and metal ions enter the
Corrosion potential-- The potential that a corroding
solution.
metal exhibits under specific conditions of
concentration, time, temperature, or velocity in an
Anodic polarization-- The change of the electrode
electrolytic solution and measured relative to a
potential in the noble (positive) direction due to
reference electrode under open-circuit conditions.
current flow.
Corrosion product-- A product resulting from
Antifouling-- The prevention of marine organism
corrosion. The term applies to solid compounds,
attachment or growth on a submerged metal surface
gasses, or ions resulting from a corrosion reaction.
through chemical toxicity. Achieved by the
Corrosion rate-- The speed at which corrosion
chemical composition of the metal, including toxins
progresses. Frequently expressed as a constant loss
in the coating or by some other means of distributing
or penetration per unit of time. Common units used
the toxin at the areas to be kept free of fouling.
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