Cathodic Protection
305
with 3 or 4 parts gypsum to 1 part sodium chloride. The consumption of the
anode is reduced somewhat because the coke backfill carries some of the
current. Backfill is not required when the anode is immersed in a riverbed,
lake, or ocean.
9.2.2.3 Testing for Completeness of Protection
Once the system has been installed, it must be tested for completeness of
protection. The preferred method is to take potential measurements. By
measuring the potential of the protected structure, the degree of protection,
including overprotection, can be determined. The basis for this determination is the fundamental concept that cathodic protection is complete when
the protected structure is polarized to the open-circuit anodic potential of
the local action cells.
The reference electrode is placed as close as possible to the protected structure to avoid and to minimize any error caused by internal resistance (IR)
drop through the soil. For buried pipelines, a compromise location is directly
over the buried pipe at the soil surface because cathodic protection currents
flow mostly to the lower surface and are minimum at the upper surface of
the pipe buried a few feet below the surface.
The potential for steel is –0.85 V vs. the copper-saturated copper sulfate
half-cell, or 0.53 V on the standard hydrogen scale. The theoretical opencircuit anodic potential for other metals may be calculated using the Nernst
equation. Several typical calculated values are shown in the table:
Metal
E o (V)
Solubility
Product M(OH) 2
OH 2 Scale
(V)
O vs. Cu-CuSO 4
Reference (V)
Iron
0.440
1.8 × 10 −15
−0.59
−0.91
Copper
–0.337
1.6 × 10 −19
0.16
−0.16
Zinc
0.763
4.5 × 10 −17
−0.93
−1.25
Lead
0.126
4.2 × 10 −15
−0.27
−0.59
Overpotential of steel structures, to a moderate degree, does not cause any
problems. The primary disadvantages are waste of power and increased consumption of auxiliary anodes. When overprotection is excessive, hydrogen can
be generated at the protected structure in sufficient quantities to cause blistering
of organic coatings, hydrogen embrittlement of the steel, or hydrogen cracking.
Overprotection of systems with amphoteric metals (e.g., tin, lead, aluminum, zinc) will damage the metal by causing increased attack instead of
reduced corrosion. This stresses the need for making potential measurements of protected structures.
There are several ways that the effectiveness of protection can be checked.
The first two methods are qualitative and do not provide data about
305
with 3 or 4 parts gypsum to 1 part sodium chloride. The consumption of the
anode is reduced somewhat because the coke backfill carries some of the
current. Backfill is not required when the anode is immersed in a riverbed,
lake, or ocean.
9.2.2.3 Testing for Completeness of Protection
Once the system has been installed, it must be tested for completeness of
protection. The preferred method is to take potential measurements. By
measuring the potential of the protected structure, the degree of protection,
including overprotection, can be determined. The basis for this determination is the fundamental concept that cathodic protection is complete when
the protected structure is polarized to the open-circuit anodic potential of
the local action cells.
The reference electrode is placed as close as possible to the protected structure to avoid and to minimize any error caused by internal resistance (IR)
drop through the soil. For buried pipelines, a compromise location is directly
over the buried pipe at the soil surface because cathodic protection currents
flow mostly to the lower surface and are minimum at the upper surface of
the pipe buried a few feet below the surface.
The potential for steel is –0.85 V vs. the copper-saturated copper sulfate
half-cell, or 0.53 V on the standard hydrogen scale. The theoretical opencircuit anodic potential for other metals may be calculated using the Nernst
equation. Several typical calculated values are shown in the table:
Metal
E o (V)
Solubility
Product M(OH) 2
OH 2 Scale
(V)
O vs. Cu-CuSO 4
Reference (V)
Iron
0.440
1.8 × 10 −15
−0.59
−0.91
Copper
–0.337
1.6 × 10 −19
0.16
−0.16
Zinc
0.763
4.5 × 10 −17
−0.93
−1.25
Lead
0.126
4.2 × 10 −15
−0.27
−0.59
Overpotential of steel structures, to a moderate degree, does not cause any
problems. The primary disadvantages are waste of power and increased consumption of auxiliary anodes. When overprotection is excessive, hydrogen can
be generated at the protected structure in sufficient quantities to cause blistering
of organic coatings, hydrogen embrittlement of the steel, or hydrogen cracking.
Overprotection of systems with amphoteric metals (e.g., tin, lead, aluminum, zinc) will damage the metal by causing increased attack instead of
reduced corrosion. This stresses the need for making potential measurements of protected structures.
There are several ways that the effectiveness of protection can be checked.
The first two methods are qualitative and do not provide data about