Nace CP 1, Course Manual

Page: 443 / 535

RP0290-2000
NACE International 1
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Section 1: General
1.1 Background
1.1.1 Reinforcing steel is compatible with concrete
because of similar coefficients of thermal expansion
and because concrete normally provides the steel with
excellent corrosion protection. The corrosion protection
is the result of the highly alkaline portland cement that
allows a stable, corrosion-mitigating passive oxide film
to form and be maintained on the surface of the
encased steel. If the film does not form, or is
weakened or destroyed so that it does not protect the
steel, corrosion can occur. The protective oxide film
does not form or is destroyed if (1) excessive amounts
of chloride or other aggressive ions are present, (2)
alkalinity is lost by reaction with aggressive gases, or
(3) the concrete does not fully encase the steel.
1.1.2 Corrosion occurs as a result of the formation of
an electrochemical cell. An electrochemical cell
consists of four components: an anode, where
oxidation occurs; a cathode, where reduction occurs; a
metallic path, where the electric current is electron flow;
and an electrolyte (concrete), where the electric current
is ion flow in an aqueous medium. The anodic and
cathodic areas occur as a result of coupling dissimilar
metals and/or exposure to differential environmental
conditions. If any one of the four elements of the
electrochemical cell is eliminated, corrosion can be
prevented.
1.2 Cathodic Protection
1.2.1 The basic principles of corrosion can be used to
understand the theory of cathodic protection. Cathodic
protection is defined as a technique to reduce the
corrosion of a metal surface by making that surface the
cathode of an electrochemical cell.
1.2.2 Cathodic protection is a proven technique for
controlling corrosion of steel in existing chloride-
contaminated concrete structures. However, cathodic
protection will neither replace lost steel nor return
corroded reinforcement to its original section. There
may be areas of the reinforced concrete structure that
appear sound by traditional inspection techniques that
are, in fact, corroding and experiencing corrosion-
related tensile stress near the rupture levels. Such
distressed areas may crack, spall, or delaminate
subsequent to the application of cathodic protection.
NOTE: For further information, see a glossary of terms
in Appendix A.
1.3 Scope and Limitations
1.3.1 The provisions of this standard should be applied
under the direction of a registered Professional
Engineer or a person certified by NACE International
as a Corrosion Specialist or certified as a Cathodic
Protection Specialist. The person’s professional
experience should include suitable experience in
cathodic protection of reinforced concrete structures.
Under certain circumstances, a cathodic protection
system may either become a structural component or
significantly affect the serviceability and structural
performance of a reinforced concrete structure;
therefore, review of such impact by the cathodic
protection system should be made by a qualified
registered Structural Engineer or the equivalent.
1.3.2 The guidelines presented here are limited to
impressed current cathodic protection systems for new
or existing atmospherically exposed reinforced
concrete and are not applicable to prestressed
concrete.
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Section 2: Criteria
2.1 The criteria in this section serve as a guide for
achieving cathodic protection and providing corrosion
control for reinforcing steel embedded in atmospherically
exposed concrete. Compliance with these criteria is
dependent on analysis of representative data in each
situation. These criteria are applicable only with impressed
current systems. The number and locations of
measurements made during data collection should be
commensurate with the complexity of the structure being
protected. Sampling plans should be in accordance with ASTM
(1)
E 105.
5
Sample size should be determined in
accordance with ANSI
(2)
/ASQC
(3)
Z1.4,
6
with the unit of
product typically being 0.836 m
2
(1.00 yd
2
) of protected
metal surface area. For structures in which cathodic
protection systems are divided into discrete zones, testing
inspection lots should be defined. Acceptable quality and
confidence levels should also be defined. Potentials of
reinforcing steel or other embedments measured against
portable reference electrodes shall be obtained in
accordance with the techniques described in ASTM C 876.
7
____________________________
(1)
American Society for Testing and Materials (ASTM), 100 Barr Harbor Dr., West Conshohocken, PA 19428-2959.
(2)
American National Standards Institute (ANSI), 11 W. 42nd St., New York, NY 10036.
(3)
American Society for Quality (ASQ), 611 East Wisconsin Ave., Milwaukee, WI 53201-3005.