Page 574 - Corrosion Engineering Principles and Practice

536 C h a p t e r 1 3 C a t h o d i c P r o t e c t i o n 537

Active anode (magnesium or zinc)
buried in the ground and connected
to pipeline to provide protective current.

(a)

–
+ Driving voltage can be demonstrated
by connecting unprotected pipeline and
anode to voltmeter.
Typically pipeline could be 1.0 V
positive to magnesium anode
and 0.5 V to zinc anode.

(b)

FIGURE 13.12 CP with galvanic anodes. (a) active galvanic anode buried in
earth and connected to pipeline to provide protective current; (b) driving
voltage measured between unprotected pipeline and anode with voltmeter.

relatively low rates and thus permit designing groundbeds that can
discharge large amounts of current and still have a long life expectancy.
While scrap steel pipe, rail, rod, or other similar iron or steel material
may be used, such materials are consumed at the rate of about
44 kg/A-y (1 A flowing for 1 year). This means a relatively large
amount of material is needed to achieve the desired operating life for
an installation, something that was justified only when scrap steel
was available at a low cost. Additional details of ICCP anode materials
are provided later in the chapter.