Nace CP 1, Course Manual

Page: 409 / 535

RP0193-2001
2 NACE International
Oxidation: (1) Loss of electrons by a constituent of a
chemical reaction. (2) Corrosion of a metal that is exposed
to an oxidizing gas at elevated temperatures.
Piping: For the purposes of this standard, this term refers
to all piping associated with the transfer of products in and
out of storage tanks.
Reduction: Gain of electrons by a constituent of a chemical
reaction.
Reference Electrode: An electrode whose open-circuit
potential
is constant under similar conditions of
measurement, which is used for measuring the relative
potentials of other electrodes.
Stray-Current Corrosion: Corrosion resulting from current
through paths other than the intended circuit, e.g., by any
extraneous current in the earth.
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Section 3: Preliminary Evaluation and Determination of the Need for Cathodic Protection
3.1 This section outlines the information that should be
considered prior to designing a cathodic protection system
to protect on-grade carbon steel storage tank bottoms in
contact with an electrolyte.
3.2 Site Assessment Information
3.2.1 Prior to designing a cathodic protection system,
the following information should be obtained:
(a) Tank, piping, and grounding construction
drawings, including dimensions, etc.
(b) Site plan and layout
(c) Date of construction
(d) Material specifications and manufacturer
(e) Joint construction (i.e., welded, riveted, etc.)
(f)
Coating specifications
(g) Existing or proposed cathodic protection systems
in the area
(h) Location of electric power sources
(i)
Electrochemical properties of the tank bedding or
padding material
(j)
History of the tank foundation (i.e., whether the
tank has been jacked up/leveled, etc.)
(k) Unusual environmental conditions
(l)
Operating history of the tank, including leak
information (internal and external)
(m) Maintenance history of the tank
(n) Containment membranes/impervious linings
(o) Secondary bottoms
(p) Water table and site drainage information
(q) Liquid levels maintained in the tank
(r)
Nearby foreign structures
(s) Type of liquid stored
(t)
Operating temperature
(u) Electrical grounding
3.3 Predesign Site Appraisal
3.3.1 Determining the Extent of Corrosion on Existing
Systems
3.3.1.1 Information regarding the degree of
tank-bottom
corrosion is useful because
considerable bottom damage may require
extensive repairs or replacement prior to the
installation of cathodic protection.
3.3.1.2 Field procedures for determining the
extent of existing corrosion may include:
(a) Visual inspection
(b) Tank bottom plate-thickness measurements
(ultrasonic testing, coupon analysis, etc.)
(c) Estimation
of general corrosion rates
through the use of electrochemical procedures
(d) Determination
of the magnitude and
direction of galvanic or stray current transferred
to or from the tank through piping and other
interconnections
(e) Determination
of soil characteristics
including resistivity, pH, chloride ion
concentration, sulfide ion concentration, and
moisture content
(f)
Estimation of the degree of corrosion
deterioration based on comparison with data
from
similar facilities subjected to similar
conditions
3.3.1.3 Foundation characteristics are also
important factors in the assessment of the extent
of existing corrosion.
The pad material of
construction, thickness of ringwalls, and water
drainage should all be considered.
3.3.1.4 Data pertaining to existing corrosion
conditions
should be obtained in sufficient
quantity to permit reasonable engineering
judgments. Statistical procedures should be
used in the analysis, if appropriate.