Underground Corrosion Control 3:30
CP 1 – Cathodic Protection Tester Course Manual
© NACE International, 2000
02/01/05
You need to be aware that under some conditions such as the presence of
sulfides, bacteria, elevated temperatures, and dissimilar metals, these
criteria may not be sufficient. In well-aerated, well-drained soils, corrosion
protection may be achieved at less negative potentials. Also, on bare or
ineffectively coated pipelines, the measurement of a net protective current
(current flow toward the pipeline at predetermined current discharge points)
may be sufficient.
Your company probably has specific criteria for various situations. You
need to know what criterion is to be used in your work and to understand
that there are several criteria from which your design engineers have to
choose.
Underground or Submerged Aluminum and Copper Piping
These metals are covered in RP0169. The 100 mV cathodic polarization
criterion is used for both materials.
Aluminum is an amphoteric metal and can be damaged by excessive cathodic
protection. A polarized potential more negative than –1200 mV should not be
exceeded.
Dissimilar Metal Situations
RP0169 gives a criterion of a negative voltage to a stable reference
electrode equal to that required for protection of the most anodic metal
involved.
Offshore Platforms in Salt Water
RP0176 Corrosion Control of Steel Fixed Offshore Platforms Associated with
Petroleum
A negative (cathodic) voltage of at least –0.800 volts measured between the
platform surface and a silver-silver chloride (Ag-AgCl) electrode
contacting the water. Normally this voltage should be measured with the
protective current applied. This –0.800 criterion includes the voltage drop
across the steel/water boundary, but does not include the voltage drop in the
water.
There are other criteria and reference electrodes covered in this standard
along with a lengthy discussion of testing techniques.
To Next Page
Loading...