Nace CP 1, Course Manual

Page: 311 / 535

RP0285-2002
NACE International 7
4.5.2 Chloride Ion: Chloride ions are depassivating
agents and cause pitting corrosion. ASTM D 512
11
is
used to measure chloride ion concentration.
4.5.3 Sulfide Ion: The presence of sulfide ions in the
soil indicates anaerobic conditions.
Under these
conditions, sulfate-reducing bacteria can greatly
accelerate the rate of corrosion. The test procedure
shall satisfy the requirements of Method 4500
12
for
sulfide, sulfite, and sulfate, and Method 9240 D
13
for
sulfate-reducing bacteria.
4.5.4 Moisture Content
4.5.4.1 The moisture content is a significant
parameter in determining the corrosiveness of a
soil
environment. When soil samples are
collected, they shall be immediately sealed in
sample
containers to prevent evaporation and
contamination. The moisture content of the
samples should be determined using ASTM D
2216.
14
4.5.4.2 Test borings allow for a determination of
the variations in moisture content with depth.
When encountered, the depth of the water table
should also be noted on the boring logs.
4.6 Data Analysis
4.6.1 This subsection outlines the analysis that may
be performed prior to designing a CP system to protect
existing USTs.
The results of tests determine the
extent and type of CP system to be installed. The
analysis of these data is valuable in determining
unusual conditions (e.g., stray current, dissimilar
metals,
and large corrosion cells), native-state
characteristics needed for commissioning the
completed CP system, and electrical continuity.
4.6.2 Results of other tests (soil resistivity, structure-
to-soil potential, stray current, current requirements,
electrical continuity, pH, chloride ion, sulfide ion,
moisture, etc.) aid in determining the type and extent of
the CP system to be considered.
Interpretation of
these results should consider seasonal variations.
4.6.3 Investigation of all previous repairs to the UST
shall be performed to determine the probability of
severe corrosion damage to these facilities.
All
necessary repairs shall be completed before the
installation of a CP system is considered (see API RP
1631
8
for additional information).
4.6.4 All UST systems may not be good candidates for
a CP system.
If tests reveal critically damaged
components, consideration should be given to UST
system replacement or removal from service.
______________________________________________________________________________
Section 5: Criteria for Cathodic Protection
5.1 General
5.1.1 This section lists criteria for cathodic protection
which, when complied with either separately or
collectively, indicate that adequate cathodic protection
of a metallic UST system has been achieved.
5.1.2 The objective of using CP is to control the
corrosion of UST systems.
5.1.3 The selection of a particular criterion depends, in
part, on prior experience with similar structures and
environments in which the criterion has been used
successfully.
5.1.4 The criteria in Paragraph 5.2 were developed
through laboratory experiment or were determined
empirically
by evaluating data obtained from
successfully operated CP systems. It is not intended
that people responsible for corrosion control be limited
to these criteria if it can be demonstrated by other
means that the control of corrosion has been achieved.
5.1.5 Voltage measurements on UST systems are to
be made with the reference electrode located on the
electrolyte surface as close as possible to the UST
system. Consideration must be given to voltage (IR)
drops other than those across the structure/electrolyte
boundary, the presence of dissimilar metals, and the
influence of other structures that may interfere with
valid interpretation of voltage measurements.
All
readings shall be taken with reference electrodes that
are in contact with the electrolyte. Readings shall not
be taken through concrete or asphalt. Soil contact may
be established through at-grade openings, by drilling a
small hole in the concrete or asphalt, or by contacting a
seam of soil between concrete and asphalt.
NOTE: Consideration is understood to mean the
application of sound engineering practice in
determining the significance of voltage drops by
methods such as:
(a) Measuring or calculating the voltage drop(s).
(b) Reviewing the historical performance of the CP
system.
(c) Evaluating
the physical and electrical
characteristics of the UST system and its
environment.
(d) Determining whether or not there is physical
evidence of corrosion.