JANUARY 31 2003
TROUBLESHOOTING GALVANIC (SACRIFICIAL) CATHODIC
PROTECTION SYSTEMS. Galvanic cathodic protection is inherently maintenance-free.
The current is merely a result of the potential difference of the two metals. Recurring
maintenance checks are performed to ensure continued satisfactory performance.
Galvanic anodes sacrifice themselves to protect the structure. They normally consume
themselves at a constant rate and failure can be predicted by current measurement
Common Problems. The most common problem in sacrificial anode
systems are shorts around or failure of dielectrics on isolated protected structures. Due
to the very limited voltage, sacrificial anodes usually cannot supply sufficient current to
protect the structures if isolation is lost. On well-coated structures, the contact
resistance to earth is high. Other metals in the earth that are not coated have a very
low contact resistance, providing a low resistance path for anode current. Maintaining
the dielectrics in an isolated system is essential to continued satisfactory performance
of sacrificial anode systems. See paragraph 7-7 for detailed procedures to locate failed
dielectrics. One failed dielectric can result in loss of protection for the entire system.
Lead Wires. Failure of the anode lead wires is uncommon, since copper
exposed by nicks or insulation defects are cathodically protected by the anodes.
However, these wires can be cut by extraneous excavations. Exercising control over
digging permits in the areas of the anode ground beds may ensure that if the wires are
cut, they can be repaired on site, before backfilling occurs. Troubleshooting to locate
the break at a later date is usually not successful, and replacement of a prematurely
failed anode is more economical in almost all cases. A sudden zero anode current
output reading indicates probably failed lead wires.
Anode Consumption. When sacrificial anode systems reach the end of
their useful life, potential, current, and voltage measurements begin to change. When
performing recurring maintenance, a significant drop in anode current indicates
imminent failure of the anode. Potential measurements over the protected structure will
begin to show dips or drops in the areas of failed anodes. A significant drop in anode
potential indicates a failed anode. Anode current may actually reverse after failure, due
to the copper center tap of the anode being cathodic to the protected structure. When
drops in the potential of the protected structure begin to occur, a closer inspection
should be made to determine the extent of the damage to the anodes.
Improper Use. Except on small or extremely well coated structures, such as
underground storage tanks or short pipelines with butyl rubber/extruded polyethylene
coatings, it is normally not economical to replace a distributed galvanic anode system.
When galvanic anodes begin to fail on a distributed system, impressed current cathodic
protection should be considered.
. Although impressed current cathodic protection