UFC 3-570-06
JANUARY 31 2003
7-4.2
Water Storage Tanks. The coatings used in water storage tanks are the
most prone to this damage. This type of coating disbonds very easily as compared to
coatings used on underground structures. It is not uncommon for elevated or ground
level water storage tanks to have the coatings bubbled or blown off from excessive
cathodic protection. It is essential on these tanks to maintain the level of current at a
safe level. The accuracy of permanent reference electrodes used with automatic
systems should be of concern when performing the CP System Check. The ON
potentials of coated water storage tanks have many errors in the measurement
(paragraph 7-2.3). ON potential measurements over -1.10 volts DC to a copper/copper
sulfate reference electrode should be suspected of coating damage and instant-OFF
potentials taken. Coating damage should be expected when the potential measurement
is over -1.50 volts DC to a copper/copper sulfate reference electrode and instant-OFF
potentials must be taken. The instant-OFF potentials should not exceed -1.00 volt DC
and must never exceed -1.10 volts DC to a copper/copper sulfate reference electrode.
7-4.3
Underground Structures. Coatings for underground structures are
generally resistant to this damage. The ON potentials of underground structures also
have many errors in the measurement (paragraph 7-2.3). ON potential measurements
to a copper/copper sulfate reference electrode should be suspected of coating damage
if over the potentials listed in Table 23 "SUSPECTED" column, and instant-OFF
potentials should be taken. Coating damage should be expected when the potential
measurement is over the potential listed in Table 23, "EXPECTED" column, and instant-
OFF potentials must be taken. This figure assumes an IR drop error and is given for
information only. The only true way to measure this possible damage is with an error-
free measurement (paragraph 7-3). Instant-OFF measurements should be used
whenever possible. Instant-OFF measurements that are over approximately -1.22 volts
DC are not theoretically possible. If instant-OFF readings are significantly over -1.22
volts DC, other DC current sources are present. Synchronous interruption of all current
sources must be accomplished.
7-4.3.1
For fusion-bonded coatings, the instant-OFF potentials should not exceed -
1.07 volts DC and must never exceed -1.12 volts DC to a copper/copper sulfate
reference electrode.
7-4.3.2
For coal tar coatings, the instant-OFF potentials should not exceed -1.12
volts DC and must never exceed -1.20 volts DC to a copper/copper sulfate reference
electrode.
7-4.3.3
For plastic tape coatings, the instant-OFF potentials should not exceed -1.02
volts DC and must never exceed -1.07 volts DC to a copper/copper sulfate reference
electrode.
7-4.3.4
For other coatings, refer to specifications for cathodic disbondment
properties compared to above coatings.
7-18
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