MIL-HDBK-1004/10
For a permanent bond, the resistor is adjusted while measuring the potential
of the foreign line at the crossing. The amount of resistance is correct when
the foreign line potential is equal to E fn (natural potential).
Bonding, as shown in Figure 27, is also used to insure continuity
of buried structures both for the prevention of interference and for the
proper operation of cathodic protection systems.
Control of Interference - Sacrificial Anodes. In some cases, the
5.2.1.5
method used to control interference is to install a sacrificial anode on the
foreign structure. This could bring the potential of the foreign structure in
the vicinity of the protected structure to the same or higher potential as the
protected structure at the crossing, providing current to the foreign line in
the opposite direction as the interference current. If the foreign line is
well coated the magnesium anode may negate all interference current. If the
foreign line has poor coating or is bare and the interference is large, the
anode at the crossing will reduce or negate the interference at the crossing,
but interference current will still flow from the foreign line at each side of
where the anode is connected. In summary, an anode on the foreign line at the
crossing will help and in some cases completely cure the interference problem,
but in other cases it will not completely solve the interference problem and a
resistance bond may still be needed. Use of a sacrificial anode to control
interference is shown in Figure 28. The anode should be sized to provide a
current in excess of the required bond current.
Effects of High Current Density. Excessive current density can
5.2.2
result in hydrogen embrittlement or coating disbondment (refer to para 3.7).
In many cases, improper location of the ground beds in an impressed current
system or interference from foreign structures can result in uneven current
distribution on the protected structure. In order to achieve protective
potentials in some areas of the structure, excessive potentials are
experienced in other areas of the structure. This situation can be corrected
by installing additional anodes or relocating the anode bed to achieve a more
even current distribution. In some cases, the structure can be separated into
sections and the current distribution improved by the installation of
resistive connections between the structure and the rectifier. Excessive
potentials are seldom experienced in sacrificial anode systems but if
experienced can usually be corrected through the installation of resistors in
selected anode lead wires.
Effects of Electrolyte pH. Aluminum and lead should not be
5.2.3
cathodically protected in some environments where the pH is greater (more
alkaline) than 8.0 (refer to paras. 3.3.1.2 and 3.3.2.1). In tight soils such
as clay, the alkali formed around the cathode by cathodic protection will
increase the pH to very high levels and cause severe chemical attack on lead
and aluminum. Where the electrolyte is water which is moving and exchanged,
the alkali formed by cathodic protection can wash away and the adverse effects
of cathodic protection on lead and aluminum are reduced. Some types of
protective coatings, particularly the alkyds, can also be damaged by highly
alkaline environments.
47
Previous Page
