Low pH (acidic) conditions can also cause problems with both
corrosion and cathodic protection. In soils with a pH lower than 3.0, current
density requirements for the protection of steel can be up to 10 times as high
as those required for protection of steel in neutral soil. These excessive
current requirements make cathodic protection in such environments difficult
and costly. The use of high quality coatings in conjunction with cathodic
protection in such environments is essential.
Hazards Associated with Cathodic Protection. Cathodic protection
systems, like all other electrical systems, can be dangerous under some
circumstances. They can, however, be safely operated under most conditions if
the proper precautions are taken in their design, installation, operation, and
Explosive Hazards. In areas where flammable liquids or explosive
gasses may be present, consideration should be given to the proper design of
both impressed current and sacrificial anode cathodic protection systems.
Such areas include, but are not limited to, fuel storage farms, fuel terminals
and fueling areas, refineries, ammunition depots, and manholes (sewer gas).
As the protective currents used for cathodic protection flow through the
environment without inherently causing arcing or appreciable resistive
heating, cathodic protection can be safely used in such environments.
However, cathodic protection rectifiers to be used in such applications should
be special oil-immersed explosion-proof types, or should be located outside of
the hazardous area. In addition, all connections should be made in explosion-
proof housings. In addition to explosive liquid and gas situations, currents
from impressed current systems can be picked up on blasting leads and could
cause detonation of the blasting caps. Impressed current cathodic protection
systems should be turned off whenever there is blasting in the vicinity.
Bonding for Electrical Safety. Electrical bonding is often
required for safety where ships, vehicles, or aircraft are fueled or loaded.
Any voltage gradient in the soil can result in a potential difference between
structures located at different points. These potential differences can
result in dangerous arcing. Cathodic protection systems can increase the
level of such voltage gradients. The normal electrical bonding used in such
circumstances is sufficient to mitigate this hazard.
The voltage gradients produced by cathodic protection systems can
also cause arcing when, during pipeline repairs, the pipeline is severed. A
temporary bond installed across the pipeline before cutting should be used to
prevent this hazard.
Interference caused by docking a vessel in the vicinity of a
cathodically protected pier or quaywall can cause accelerated attack on the
vessel (see Figure 29). This problem is normally prevented by using a direct
bond between the protected structure and the vessel (see Figure 30).