Nace CP 1, Course Manual

Page: 310 / 535

RP0285-2002
6 NACE International
4.4.3 When required, soil samples should be extracted
from the bore holes and placed in sealed sample
containers for analysis to include:
(a) resistivity;
(b) pH;
(c) sulfide ion concentration;
(d) chloride ion concentration; and
(e) moisture content.
4.4.4 Tests that should be included in the investigation
for the evaluation of corrosion on USTs and the design
of CP systems include the following:
4.4.4.1 Soil Resistivity: Low-resistivity soils are
usually more corrosive than high-resistivity soils;
however, serious corrosion can also be associated
with high-resistivity soils, particularly when the soil
composition is not uniform. Variations in resistivity
indicate variations in soil composition, which is
conducive to galvanic corrosion.
Accepted soil
resistivity tests include the Wenner four-pin,
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soil
box, and single-probe methods.
4.4.4.2 Structure-to-Soil Potential
4.4.4.2.1 Structure-to-soil potentials are used
to evaluate the corrosion activity associated
with UST systems. If properly interpreted and
correlated
with other measurements,
structure-to-soil potentials should give an
indication of the severity of both galvanic and
electrolytic corrosion cells.
4.4.4.2.2 Measurements should be taken
with a high-input impedance voltmeter.
4.4.4.2.3 Saturated copper/copper sulfate
reference electrodes (CSEs) are used for
underground corrosion testing because they
are stable, rugged, and yield reproducible
results.
Electrode placement is important
when collecting the data. Proper notation of
electrode
location for each reading is
required. When test borings are made, the
reference electrode should be placed near the
tank/soil interface. Typically, the test hole is
drilled 0.3 m (1 ft) from the edge of the tank to
a depth of 0.6 m (2 ft) below the tank. Tank-
to-soil potentials are recorded at various
depths to establish the potential profile from
grade level to below the tank.
4.4.4.3 Stray direct current (DC) can emanate
from the operation of DC transit systems, CP
rectifiers, DC welding equipment, and DC motors.
When discharged from the surface of a steel tank,
these currents consume approximately 9 kg/A-y
(20 lb/A-y) of metal.
Concentrated electrolytic
corrosion can cause rapid deterioration of UST
systems.
The presence of stray current is
detected through the use of structure-to-soil
potential, current flow, and potential gradient
measurements.
4.4.4.4 Current requirement: Tests simulating the
effects of a permanently installed CP system can
be evaluated to determine the DC requirements for
protection. Temporary CP anodes are installed in
the surrounding soil and connected to the positive
terminal of a DC power source; the negative
terminal of the power source shall be connected to
the UST system under test.
Structure-to-soil
potentials are then measured at accessible
locations. Data, including the polarization effects
over time, can then be extrapolated to determine
the requirements for CP current.
The status of
electrical isolation of the structure under study is
considered
in the evaluation of current
requirements testing.
4.4.4.5 Electrical continuity testing is necessary to
determine whether all tanks and piping are
electrically continuous and whether continuity with
other structures exists. Temporary anodes shall
be energized by a DC power source that is cycled
“on” and “off.” A CSE or other suitable reference
electrode shall be placed in a stationary position,
and structure-to-soil potentials recorded with the
structure connection moved from one location to
the next. At each point of structure connection,
both “on” and “instant off” potential readings can
be observed with the cycling of the DC power
source.
The CSE must remain at the same
location for the duration of each continuity test.
Electrical continuity is indicated when the potential
measurements
and changes in potential
measurements with the applied current are
approximately equal, regardless of the point of
connection to the structure.
Differences in the
structure-to-soil potentials and changes in
potential indicate the lack of electrical continuity
between the points of contact.
4.5 Laboratory Testing
4.5.1 pH: For a given resistivity, acid soils (pH less
than 7) are more conducive to ferrous corrosion. At pH
values below 4, the rate of corrosion accelerates
rapidly. At pH values above 10, the environment tends
to passivate the steel. All pH testing should be done in
accordance with ASTM
(4)
G 51.
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