Page 589 - Corrosion Engineering Principles and Practice

552 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 553

Measuring the potential of a buried structure is done by contact-
ing the earth (or other conducting environment) with a reference
electrode. Figure 13.24 shows how the measurement is usually taken.
It is not necessary to separate the two half-cell potentials since only
changes in the structure-to-earth measurements are of interest and
these changes show up well enough in the total value measured in
this fashion.
If we are talking about a steel structure, a value of −0.85 V or
more on the voltmeter of Fig. 13.24 would indicate full cathodic
protection. This is considered to mean that the structure potential is
−0.85 V with respect to a copper sulfate electrode and is based on the
value −0.80 V assigned to the most highly anodic steel found in
practical situations.
When metals other than steel are to be cathodically protected in
soils, different potentials to a copper sulfate reference may be used as
a criterion of protection. For lead-sheathed cables, a customary value
is −0.70 V. For aluminum, the potential should be held between the
limits of −1.00 and −1.20 V.
Notice the maximum negative limit given for aluminum. This is
because alkalinity, which is corrosive to aluminum, builds up at the
aluminum surface due to the forced cathodic current. Aluminum and

High impedance
voltmeter

V Good electrical
– + contact
Copper sulfate
electrode

Buried structure
to be protected

FIGURE 13.24 Pipe to earth potential measurement set-up.